Oxygen fugacity at the base of the Talkeetna arc, Alaska

The origin of the more oxidized nature of arc magmas as compared to that of mid-ocean ridge basalts (MORB) is debated, considered to be either a feature of their mantle source, or produced during crustal transit and eruption. Fe3+/FeT ratios (Fe3+/[Fe3+ + Fe2+]) in arc volcanic rocks and glasses and thermodynamic oxybarometry on mantle xenoliths from arc lavas indicate elevated magmatic oxygen fugacity (fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document}), whereas, redox-sensitive trace elements ratios and abundances in arc volcanic rocks have been used to suggest that arcs have source regions with fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} similar to the MORB source. Here, we take an alternative approach by calculating the fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} of the uppermost mantle and lowermost ultramafic cumulates from the accreted Jurassic Talkeetna arc (Alaska). This approach allows us to quantify the fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} of the sub-arc mantle and of primary arc magmas crystallizing at the base of an island arc, which have not been affected by processes during crustal transit and eruption which could affect their fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document}. Implementing olivine–spinel oxybarometry, we find that the upper mantle (harzburgites and lherzolites) and ultramafic cumulates (clinopyroxenites and dunites) crystallized between + 0.4 and + 2.3 log units above the fayalite-magnetite-quartz buffer, consistent with previous studies suggesting that the sub-arc mantle is oxidized relative to that of MORB. In addition, the Talkeetna paleo-arc allows us to examine coeval lavas and their redox-sensitive trace element ratios (e.g., V/Sc). The average V/Sc ratios of high MgO (> 6 wt%) lavas are 6.7 ± 1.6 (2σ), similar to that of MORB. However, V/Sc ratios must be interpretted in terms the degree of partial melting, as well as, the initial V/Sc ratio of the mantle source in order to derive information about fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} of their mantle source. The V/Sc ratios of Talkeetna lavas are consistent with the elevated fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} recorded in the sub-arc mantle and primitive cumulates (olivine Mg# [Mg/(Mg + Fe)] × 100 > 82) if a depleted mantle source underwent 15–20% melting. Our results suggest that the arc mantle is, on average, more oxidized than the MORB source and that V/Sc ratios must be interpreted in the context of a partial melting model where all model parameters are appropriate for arc magma genesis. This study reconciles V/Sc ratios in arc volcanic rocks with fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{{\text{O}}_ }}$$\end{document} of primary arc basalts and the sub-arc mantle from the same locality.

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title: An experimental test of the spinel peridotite oxygen barometer
abstract: It has recently been proposed that the Fe3O4 content of spinel coexisting with olivine and orthopyroxene may be used as an oxygen barometer for spinel peridotites (O'Neill and Wall, 1987; Mattioli and Wood, 1988). This barometer relies on the equilibrium: 6 Fe2SiO4. (olivine) +O2 = 3 Fe2SiO6 (orthopyroxene)+2 Fe3O4 (spinel). The two calibrations, both based on a regular solution model for olivine (WFeMg = 8.36 kJ/mol), ideal two-site model for orthopyroxene, and different activity data for spinel, generally give results within 0.7 log fo2 units of each other for mantle compositions. These calibrations have been tested, together with a recent spinel model of Nell and Wood (1990) by reequilibrating mixtures of olivine (Fo89) orthopyroxene (En90) and spinel of varying composition at known T and fo2 at 1 atm pressure. Experiments were performed at 1200°C and oxygen fugacities between fayalite-Magnetite-quartz buffer (FMQ) and 1.4 log units below FMQ. The equilibrium Fe3O4 content of spinel was reversed in experiments of 5–7 days duration at ratios of Cr/Cr+Al (spinel) of 0.13–0.53. Phases were analyzed by electron microprobe with Fe3+/total Fe of spinel being determined using spinel microprobe standards of known ferric iron content. The oxygen barometer was then tested by comparing oxygen fugacities calculated from the compositions of experimental products with the actual fo2 of the experiment. The Mattioli-Wood version of the barometer gives fo2 values which average close (−0.15 log units) to the true values but which show a dependence on (Cr/Cr+Al) in the spinel. The O'Neill-Wall version gives average values which are 0.65 log units low but which show no compositional bias. The Nell-Wood version gives composition-independent values which are closest to the true values, averaging 0.35 log units low. All three versions are sufficiently accurate to be petrologically useful and can be corrected, using these experimental data, to approximate the true values within a few tenths of a log unit in fo2.

title: The redox states of basic and silicic magmas: a reflection of their source regions?
abstract: At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni−NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2Ma contain about 2 wt% FeO(total), and typically phenocrysts of fayalite, quartz and Fe−Ti oxides. Despite extensive exchange of the 18O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H2O to H2, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H2O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria disturbed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe2O3, and their pressure derivatives ϖV/ϖP, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.

title: Aspects of magma genesis in arcs
abstract: Geochemical analysis of magmas developed above subduction zones in combination with chemical, phase equilibria and physical modeling of petrogenetic processes have shown that the mantle wedge overlying the subducted lithosphere is volumetrically the most significant source of the majority of island arc magmas. Locally and at distinct episodes in the history of some arcs, direct melts of the subducted lithosphere may be erupted essentially unmodified or become involved as components of wedge-derived magmas. A flux of hydrous fluid from the subducted lithosphere is the trigger for partial melting of the overlying wedge, a source of oxidation of the magmas produced, and a transporting agent for some trace elements that are typically enriched relative to elements of equal crystal-melt incompatibity but non-hydrophilic character, in arc magmas compared with ocean ridge settings. For some hydrophilic trace elements (e.g., B, Be), the subducted lithosphere is volumetrically the largest supplier in arc magmas. In addition however, leaching of some of these elements (e.g., Sr) from the wedge by fluid infiltration prior to melting significantly augments the eventual magmatic abundances. Wedge compositional variability can be demonstrated on a global basis. A number of processes have probably led to this variability including concurrent and/or past magma extraction in backarcs, localised past input of hot-spot/ocean island magmas, global variability of sediment input, and incorporation of ancient subcontinental mantle domains. In the case of the Bonin arc, a remarkably stable environment of magma generation has perisisted for the past ∼45 million years since arc inception, with no evidence of secular increases in alkalinity. Similarly, a consistent trend of increasing alkalinity with evolution of the Mariana arc is absent. In both examples, extension of the lithosphere concurrent with magmatism may have maintained vertically extensive melting domains between arc lithosphere and the subducted slab. There is no convincing support for production of greater arc crustal thicknesses than backarc crust for either the Bonins or Marianas, indicative of partial melting percentages in arcs⩽backarcs. For most estimates of the percentages of melting beneath arcs, mass balance of Fe2O3/FeO in the mantle wedge requires addition of oxygen (via H2O). Although the melting relations of amphibole are critical in arc magma genesis, it is likely that this phase does not persist to the mantle wedge domains where the majority of high-Mg island arc basalts with eruption temperatures ⩾ 1200°C are formed.

title: Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions
abstract: V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘ see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluidmobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO2 during melting. However, for the most part, the average V/Sc-inferred fO2s of arc basalts, MORB and peridotites are remarkably similar ( 1 25 to þ0 5 log units from the FMQ buffer) and disagree with the observation that the barometric fO2s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth’s mantle may be strongly buffered in terms of fO2. The higher barometric fO2s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithosphericmantle.

title: Water and the Oxidation State of Subduction Zone Magmas
abstract: Tracing Mantle Oxidation The chemical composition of the Earth's mantle varies with tectonic setting. For example, basaltic melts near subduction zones are more oxidized than magma near divergent plate boundaries. Kelley and Cottrell (p. 605; see the Perspective by Hirschmann) examined melts formed in different tectonic environments, using highly sensitive synchrotron-based analytical methods. The oxidation state of Fe increased with water content and mobile trace elements concentrations. Thus, fluids released from wet subducting plates drive mantle oxidation above subduction zones, which may help to explain the spatial differences in oxygen fugacity of the mantle. Oxidation of Earth’s mantle at subduction zones is caused by fluids released from the melting of subducting plates. Mantle oxygen fugacity exerts a primary control on mass exchange between Earth’s surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe3+/ΣFe), determined with Fe K-edge micro–x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H2O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe3+/ΣFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H2O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

title: Oxidation state of subarc mantle
abstract: The subarc mantle is a primary control on the composition of arc magmas and the formation of arc-related ore deposits. Elevated ferric iron contents in arc lavas have been interpreted as a record of subarc mantle that is oxidized relative to mid-oceanic ridge basalt (MORB), but this conclusion is controversial. Measurements of spinel compositions in primitive arc lavas imply an arc magma source 1-4 log units more oxidized than MORB and ocean island basalts analyzed using the same technique. Samples from seven arcs show a significant correlation (P < 0.0005) between redox budget, subduction zone convergence rate, and subduction zone age. These results support the notion of oxidized arc lavas in the mantle source zone, but resolution of contradictory evidence regarding subarc mantle redox state requires further work.

title: Oxidation states of mid-ocean ridge basalt glasses
abstract: Precise new analyses of ferrous and total iron for 78 hand-picked mid-ocean ridge basalt (MORB) glasses constrain the redox states of MORB magmas, and the systematics of those redox states with respect to geography and chemistry. The data indicate that MORB magmas at the point of eruption include some of the most reduced terrestrial lavas yet analyzed. Irrespective of their tectonic setting, geographic setting or chemical type, quench glasses from the outer surfaces of MORB lavas have Fe3+/ΣFe ratios of 0.07 ± 0.03 (2σ), equivalent to relative oxygen fugacities 1–2 log10 units below the fayalite-magnetite-quartz buffer (FMQ). These reduced values contrast markedly with those of cogenetic whole rocks, even for samples from the same pillow. Pillow cores (including virtually all published whole rock analyses) typically have Fe3+/ΣFe close to 0.15, equivalent to oxygen fugacities close to FMQ. The post-eruptive oxidation responsible for this contrast apparently occurs rapidly, while the lava is still partially molten, most likely as a result of hydrogen loss. The striking contrast between glass and whole rock data suggests that two common assumptions with respect to treatment of MORB are inappropriate: (1) assumption of Fe3+/ΣFe close to 0.15 for calculating norms and magnesium numbers; and (2) assumption of FMQ conditions for experimental studies of MORB crystallization. Oxygen fugacities in the mantle source regions of MORB have also been assumed to be close to FMQ. It is likely that, redox states determined from wholly or partially crystalline lavas are generally not representative of magmatic values. The new data reported herein define a new and rigidupper limit for the oxidation state of the sub-oceanic mantle.

title: The Oxidation State of Fe in MORB Glasses and the Oxygen Fugacity of the Upper Mantle
abstract: Micro-analytical determination of Fe{sup 3+}/{Sigma}Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure ({mu}-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe{sup 3+}/{Sigma}Fe ratios of 0.16 {+-} 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the quartz-fayalite-magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe{sup 3+}/{Sigma}Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or < 10%) of the difference between Fe{sup 3+}/{Sigma}Fe ratios determined by micro-colorimety and XANES can be attributed to the Moessbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe{sup 3+}/{Sigma}Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe{sup 3+} behaving incompatibly in shallow MORB magma chambers. MORB Fe{sup 3+}/{Sigma}Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na{sub 2}O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulkmore » partition coefficient of Fe{sup 3+} may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at {approx} QFM. Both explanations, in combination with the measured MORB Fe{sup 3+}/{Sigma}Fe ratios, point to a fertile MORB source with greater than 0.3 wt.% Fe{sub 2}O{sub 3}.« less

title: The influence of magmatic differentiation on the oxidation state of Fe in a basaltic arc magma
abstract: Subduction zone basalts are more oxidized than basalts from other tectonic settings (e.g., higher Fe{sup 3+}/{Sigma}Fe), and this contrast may play a central role in the unique geochemical processes that generate arc and continental crust. The processes generating oxidized arc magmas, however, are poorly constrained, although they appear inherently linked to subduction. Near-surface differentiation processes unique to arc settings might drive oxidation of magmas that originate in equilibrium with a relatively reduced mantle source. Alternatively, arc magmas could record the oxidation conditions of a relatively oxidized mantle source. Here, we present new measurements of olivine-hosted melt inclusions from a single eruption of Agrigan volcano, Marianas, in order to test the influence of differentiation processes vs. source conditions on the Fe{sup 3+}/{Sigma}Fe ratio, a proxy for system oxygen fugacity (fO{sub 2}). We determined Fe{sup 3+}/{Sigma}Fe ratios in glass inclusions using {mu}-XANES and couple these data with major elements, dissolved volatiles, and trace elements. After correcting for post-entrapment crystallization, Fe{sup 3+}/{Sigma}Fe ratios in the Agrigan melt inclusions (0.219 to 0.282), and their modeled fO{sub 2}s ({Delta}QFM + 1.0 to + 1.8), are uniformly more oxidized than MORB, and preserve a portion of the evolution of this magma from 5.7 to 3.2 wt.%more » MgO. Fractionation of olivine {+-} clinopyroxene {+-} plagioclase should increase Fe{sup 3+}/{Sigma}Fe as MgO decreases in the melt, but the data show Fe{sup 3+}/{Sigma}Fe ratios decreasing as MgO decreases below 5 wt.% MgO. The major element trajectories, taken in combination with this strong reduction trend, are inconsistent with crystallization of common ferromagnesian phases found in the bulk Agrigan sample, including magnetite. Rather, decreasing Fe{sup 3+}/{Sigma}Fe ratios correlate with decreasing S concentrations, suggesting that electronic exchanges associated with SO{sub 2} degassing may dominate Fe{sup 3+}/{Sigma}Fe variations in the melt during differentiation. In the case of this magma, the dominant effect of differentiation on magmatic fO{sub 2} is reduction rather than oxidation. Tracing back Agrigan melts with MgO > 5 wt.% (i.e., minimally degassed for S) along a modeled olivine fractionation trend to primary melts in equilibrium with Fo{sub 90} olivine reveals melts in equilibrium with the mantle beneath Agrigan at fO{sub 2}s of {Delta}QFM + 1 to + 1.6, significantly more oxidized than current constraints for the mantle beneath mid-ocean ridges.« less

title: A re-assessment of the oxidation state of iron in MORB glasses
abstract: The oxidation state of Fe, Fe / 3 + Σ Fe (where Σ Fe = Fe 2 + + Fe 3 + ), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) ( n = 42 ). This value is lower than that reported previously by XANES, 0.16 ( 1 ) ( n = 103 ), but consistent with the most recent value determined by redox titrations, 0.11 ( 2 ) ( n = 104 ), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of Mossbauer spectra and the resulting Fe / 3 + Σ Fe values of the standards. Our alternative interpretation removes the problem of resolving Fe / 3 + Σ Fe values / 3 + Σ Fe values that are consistent with the thermodynamically expected dependence on oxygen fugacity ( f O 2 ). Fe / 3 + Fe 2 + is related to f O 2 for our synthetic MORB composition by the temperature independent expression Δ QFM = 4 log ⁡ ( Fe 3 + / Fe 2 + ) + 4.23 ( 5 ) , where ΔQFM is the f O 2 in log units relative to the quartz–fayalite–magnetite buffer. The average f O 2 of natural MORB was estimated to be QFM+0.1.

title: Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB
abstract: To improve the accuracy of X-ray absorption near-edge structure (XANES) calibrations for the Fe3 +/ΣFe ratio in basaltic glasses, we reevaluated the Fe3 +/ΣFe ratios of glasses used as standards by Cottrell et al. (2009), and available to the community (NMNH catalog #117393). Here we take into account the effect of recoilless fraction on the apparent Fe3 +/ΣFe ratio measured from room temperature Mossbauer spectra in that original study. Recoilless fractions were determined from Mossbauer spectra collected from 40 to 320 K for one basaltic glass, AII_25, and from spectra acquired at 10 K for the 13 basaltic glass standards from the study of Cottrell et al. (2009). The recoilless fractions, f, of Fe2 + and Fe3 + in glass AII_25 were calculated from variable-temperature Mossbauer spectra by a relative method (RM), based on the temperature dependence of the absorption area ratios of Fe3 + and Fe2 + paramagnetic doublets. The resulting correction factor applicable to room temperature determinations (C293, the ratio of recoilless fractions for Fe3 + and Fe2 +) is 1.125 ± 0.068 (2σ). Comparison of the spectra at 10 K for the 13 basaltic glasses with those from 293 K suggests C293 equal to 1.105 ± 0.015 (2σ). Although the 10 K estimate is more precise, the relative method determination is believed to be more accurate, as it does not depend on the assumption that recoilless fractions are equal at 10 K. Applying the effects of recoilless fraction to the relationship between Mossbauer-determined Fe3 +/ΣFe ratios and revised average XANES pre-edge centroids for the 13 standard glasses allows regression of a new calibration of the relationship between the Fe XANES pre-edge centroid energy and the Fe3 +/ΣFe ratio of silicate glass. We also update the calibration of Zhang et al. (2016) for andesites and present a more general calibration for mafic glasses including both basaltic and andesitic compositions. Recalculation of Fe3 +/ΣFe ratios for the mid-ocean ridge basalt (MORB) glasses analyzed previously by XANES by Cottrell and Kelley (2011) results in an average Fe3 +/ΣFe ratio for MORB of 0.143 ± 0.008 (1σ), taking into account only analytical precision, and 0.14 ± 0.01(1σ), taking into account uncertainty on the value of C293. This revised average is lower than the average of 0.16 ± 0.01 given by Cottrell and Kelley (2011). The revised average oxygen fugacity for MORB based on the database of Cottrell and Kelley (2011) is − 0.18 ± 0.16 log units less than the quartz-fayalite-magnetite buffer of Frost (1991) at 100 kPa (∆ QFM = − 0.18 ± 0.16).

title: The oxidation state, and sulfur and Cu contents of arc magmas: implications for metallogeny
abstract: Global data for measured Fe2O3/FeO ratios and Cu contents in unaltered volcanic and intrusive arc rocks indicate that, on average, they are slightly more oxidized than other magmas derived from depleted upper mantle (such as MORB), but contain similar Cu contents across their compositional ranges. Although Cu scatters to elevated values in some intermediate composition samples, the bulk of the data show a steady but gentle trend to lower concentrations with differentiation, reaching modal values of ~ 50–100 ppm in andesitic rocks. These data suggest that Cu is mildly compatible during partial melting and fractionation processes, likely reflecting minor degrees of sulfide saturation throughout the magmatic cycle. However, the volume of sulfides must be small such that significant proportions of the metal content remain in the magma during fractionation to intermediate compositions. Previous studies have shown that andesitic magmas containing ~ 50 ppm Cu can readily form large porphyry-type Cu deposits upon emplacement in the upper crust. A review of the literature suggests that the elevated oxidation state in the asthenospheric mantle wedge source of arc magmas (ΔFMQ ≈ + 1 ± 1) derives from the subduction of seawater-altered and oxidized oceanic crust, and is transmitted into the mantle wedge via prograde metamorphic dehydration fluids carrying sulfate and other oxidizing components. Progressive hydration and oxidation of the mantle wedge may take up to ~ 10 m.y. to reach a steady state from the onset of subduction, explaining the rarity of porphyry deposits in primitive island arcs, and the late formation of porphyries in continental arc magmatic cycles. Magmas generated from this metasomatized and moderately oxidized mantle source will be hydrous basalts containing high concentrations of sulfur, mainly dissolved as sulfate or sulfite. Some condensed sulfides (melt or minerals) may be present due to the high overall fS2, despite the moderately high oxidation state. These sulfides may retain some highly siderophile elements in the source, but are unlikely to be sufficiently voluminous to significantly affect the budget of more modestly sulfide-compatible and more abundant elements such as Cu and Mo. These primary magmas can therefore be considered to be largely Cu-Mo-undepleted, although highly siderophile elements such as Au and platinum group elements (PGE) may be depleted unless no sulfides remain in the source. The latter condition seems unlikely during active subduction because of the continuous flux of fresh sulfur from the slab, but may occur during post-subduction re-melting (leading to potentially Au-rich post-subduction porphyry and alkalic-type epithermal systems). Lower crustal differentiation of main-stage arc magmas results in some loss of Cu to residual or cumulate sulfides, but again the amount appears to be minor, and does not drastically reduce the Cu content of derivative intermediate-composition melts. Fractionation and devolatilization affect the oxidation state of the magma in competing ways, but, while crystallization and segregation of Fe3 +-rich magnetite can cause reduction in reduced to moderately oxidized evolved magmas, this effect appears to be outweighed by the oxidative effects of degassing reduced or weakly oxidized gaseous species such as H2, H2S, and SIVO2, and preferential solvation and removal of Fe2 + in saline hydrothermal fluids. Consequently, most arc magmatic suites show slight increases in oxidation state during differentiation, reaching typical values of ΔFMQ = + 1 to + 2. This oxidation state is significant, because it corresponds to the transition from dissolved sulfide to sulfate dominance in magmas. It has been shown that Cu and Au solubilities in silicate magma increase up to this level (ΔFMQ ≈ + 1), but while Cu solubility continues to increase at higher oxidation states, Au shows a precipitous drop as sulfide, which solvates Au in the melt, is converted to sulfate. This may explain the somewhat restricted distribution of Au-rich porphyry Cu deposits, but the general association of porphyry Cu deposits with relatively oxidized magmas. Exsolution of a saline, high temperature aqueous fluid enables metals to partition from the magma into a highly mobile volatile phase. Sulfur also partitions strongly into this fluid phase, predominantly as SO2 at ΔFMQ = + 1 to + 2. However, as the fluid cools below ~ 400 °C, SIVO2 disproportionates to form reduced H2S− II and oxidized H2SVIO4. The H2S bonds with metals in solution to precipitate as Cu- and Mo-sulfides, while the H2SO4 (and HCl) generates progressively acidic wallrock alteration (phyllic, argillic, advanced argillic). Gold may precipitate with early Cu/Mo-sulfides, but some may also stay in solution as bisulfide complexes, eventually reaching the epithermal environment. Thus, three components, [S], [H2O], and fO2 work together throughout subduction and arc magmatic processes to transport chalcophile and siderophile metals from the mantle into the upper crust, where they may be concentrated by hydrothermal processes to form ore deposits. These processes are far from 100% efficient, and metals (especially highly siderophile elements such as Au and PGE) may be left behind at various stages of the passage of arc magmas through the lithosphere, where they may form potentially metalliferous source rocks for partial melts and subsequent magmatic-hydrothermal ore deposits generated during later tectonomagmatic events.

title: Peridotites from the Izu-Bonin-Mariana Forearc (ODP Leg 125): Evidence for Mantle Melting and Melt-Mantle Interaction in a Supra-Subduction Zone Setting
abstract: Drilling Program Leg 125 recovered serpentinized harzburgites and dunites from a total of five sites on the crests and flanks of two serpentinite seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu–Bonin forearc. These are some of the first extant forearc peridotites reported in the literature and they provide a window into oceanic, supra-subduction zone (SSZ) mantle processes. Harzburgites from both seamounts are very refractory with low modal clinopyroxene (<4%), chrome-rich spinels (cr-number = 0.40–0.80), very low incompatible element contents, and (with the exception of amphibole-bearing samples) U-shaped rare earth element (REE) profiles with positive Eu anomalies. Both sets of peridotites have olivine–spinel equilibration temperatures that are low compared with abyssal peridotites, possibly because of water-assisted diffusional equilibration in the SSZ environment. However, other features indicate that the harzburgites from the two seamounts have very different origins. Harzburgites from Conical Seamount are characterized by calculated oxygen fugacities between FMQ (fayalite–magnetite–quartz) – 1.1 (log units) and FMQ + 0.4 which overlap those of mid-ocean ridge basalt (MORB) peridotites. Dunites from Conical Seamount contain small amounts of clinopyroxene, orthopyroxene and amphibole and are light REE (LREE) enriched. Moreover, they are considerably more oxidized than the harzburgites to which they are spatially related, with calculated oxygen fugacities of FMQ – 0.2 to FMQ + 1.2. Using textural and geochemical evidence, we interpret these harzburgites as residual MORB mantle (from 15 to 20% fractional melting) which has subsequently been modified by interaction with boninitic melt within the mantle wedge, and these dunites as zones of focusing of this melt in which pyroxene has preferentially been dissolved from the harzburgite protolith. In contrast, harzburgites from Torishima Forearc Seamount give calculated oxygen fugacities between FMQ + 0.8 and FMQ + 1.6, similar to those calculated for other subduction-zone related peridotites and similar to those calculated for the dunites (FMQ + 1.2 to FMQ + 1.8) from the same seamount. In this case, we interpret both the harzburgites and dunites as linked to mantle melting (20–25% fractional melting) in a supra-subduction zone environment. The results thus indicate that the forearc is underlain by at least two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. They also demonstrate that both types of mantle lithosphere may have undergone extensive interaction with subduction-derived magmas.

title: The redox state of subduction zones: insights from arc-peridotites
abstract: Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (fO2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by 57Fe Mossbauer spectroscopy and resultant oxygen fugacities. Geochim.  Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between fO2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between fO2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between fO2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe2O3. This amount of Fe2O3 in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H2O is required which is considerably less than the 0.25% H2O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. ], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in fO2 of the source region. Although it is unclear which model is correct the maximum fO2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher fO2 values of primitive arc lavas than arc-peridotites.

title: Stability of arc lower crust: Insights from the Talkeetna arc section, south central Alaska, and the seismic structure of modern arcs
abstract: One process for the formation of continental crust is the accretion of arc terranes at continental margins. A longstanding problem with this model is that although the composition of the continental crust is andesitic, the majority of arc lavas are basaltic. Moreover, those arc lavas that are andesitic tend to be evolved (lower Mg #) compared to the continental crust. Continental crust can be produced through mixing of basaltic and silicic arc lava compositions, assuming that mafic cumulates formed during generation of the silicic component are removed. If these cumulates are denser than the underlying mantle, removal can occur via foundering of lower arc crust. Indeed, field observations of the Talkeetna arc section in south central Alaska, combined with modeling of fractionation in primitive arc magmas, suggest that large amounts of primitive gabbronorite and pyroxenite are missing from the lower crust. Using rock compositions from the Talkeetna section and the free energy minimization program Perple_X, we calculated equilibrium mineral assemblages for a range of gabbroic and ultramafic compositions at P, T, oxygen fugacity (fO2), and H2O contents appropriate for arc lower crust. The quartz-olivine-garnet-free mineral assemblage found in the Talkeetna gabbronorites (and in the similar Kohistan section in Pakistan) defines a narrow range of fO2 centered on NNO+2 (±1 log unit). Predicted mineral assemblages calculated under these conditions were used to estimate the density and seismic structure of the arc lower crust. We find that the missing gabbroic and ultramafic rocks from the Talkeetna section were likely denser than the underlying mantle, while the gabbronorites that remain are either neutrally or slightly positively buoyant. Generalizing, we show that lower crustal Vp > 7.4 km/s in modern arcs is indicative of lower crust that is convectively unstable relative to the underlying mantle. However, most lower crust in modern arcs is observed to have Vp < 7.4 km/s, implying that gravitationally unstable material must founder rapidly on geologic timescales, or high Vp plutonic rocks crystallize beneath the Moho.

title: The redox state of arc mantle using Zn/Fe systematics
abstract: Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source. Here we show that Zn/FeT (where FeT = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/FeT. Primitive arc magmas have identical Zn/FeT to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/FeT during early differentiation involving olivine requires that Fe3+/FeT remains low in the magma. Only after progressive fractionation does Fe3+/FeT increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

title: Ferric iron in peridotites and mantle oxidation states
abstract: 57Fe Mossbauer spectroscopy is employed to estimate Fe3+ΣFe ratios and the Fe2O3 content of 28 well characterized pyroxenite and peridotite xenoliths from central France, southeastern Australia, northern Tanzania, southern Africa and Siberia. The Fe2O3 contents of various mantle regions were assessed, and found to range from 0.1 to 0.4 wt%. With an appraisal of the likely mantle inventories of C, H and S, the results demonstrate that these volatile elements and Fe all have similar buffering capacities for oxygen in peridotites. For this reason, the oxidation states of upper mantle peridotites should not generally be controlled by any particular buffer system. 
 
New estimates for the oxidation states of cratonic garnet peridotites indicate that the upper mantle beneath cratons is within 1 log unit of the fayalite-magnetite-quartz (FMQ) buffer, but may have been progressively oxidized since its original formation in the early Archean.

title: Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation
abstract: Copper-Bottomed Crust The formation of volcanic arc chains near subduction zones brings large amounts of magma from the upper mantle to the crust, contributing to the formation of island chains in the ocean and adding material to continents. Over time, arc magmas also contribute indirectly to the composition of the oceans and atmosphere through outgassing and weathering of volcanic minerals; however, it is unclear what determines the oxidized nature of arc magmas themselves. Lee et al. (p. 64) measured Cu contents in a range of arc-derived volcanic rocks as a proxy for arc magma redox states. An overall depletion of Cu, which is sensitive to reduced sulfur contents, in global continental crust suggests that there is a hidden reservoir of copper-rich sulfides deep in Earth's interior. The copper contents of magmas imply that the formation of sulfide-bearing cumulates under reducing conditions is a critical step in the formation of continental crust. Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper’s (Cu’s) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

title: Iron isotopes may reveal the redox conditions of mantle melting from Archean to Present
abstract: High-precision Fe isotopic data for 104 samples, including modern and ancient (≥ 3.7 Ga) subduction-related magmas and mantle peridotites, are presented. These data demonstrate that mid-ocean ridge and oceanic-island basalts (MORBs and OIBs) have on average small, but distinctly (~+ 0.06‰) higher ^(56)Fe/^(54)Fe ratios than both modern and Eoarchean boninites and many island arc basalts (IABs) that are interpreted to form by large degrees of flux melting of depleted mantle sources. Additionally boninites and many IABs have iron isotopic compositions similar to chondrites, fertile mantle peridotites, Eoarchean mantle peridotites, and basalts from Mars and Vesta. The observed variations are best explained by the bulk silicate Earth having a near-chondritic iron isotopic composition, with ~ + 0.3‰ equilibrium isotope fractionation between Fe^(3+) and Fe^(2+) and preferential extraction of isotopically heavier, incompatible Fe^(3+) during mantle melting to form oceanic crust (as represented by MORBs and OIBs). A quantitative model that relates the iron isotopic composition of basaltic magmas to the degree of partial melting, Fe^(3+)/Fe^(2+) ratio, and buffering capacity of the mantle is presented. The concept that redox conditions may influence iron isotopic fractionation during melting provides a new approach for understanding the redox conditions of magma genesis on early Earth and Mars. Experimental and theoretical work is required to establish iron isotopic fractionation as an oxybarometer of mantle melting.

title: Trace element mineral/melt partitioning for basaltic and basaltic andesitic melts: An experimental and laser ICP-MS study with application to the oxidation state of mantle source regions
abstract: Understanding magmatic processes such as crystallization and melting recorded in natural samples requires calibration of mineral–melt trace element partition coefficients (D) and their dependence on temperature, pressure, oxygen fugacity (fO2) and chemical composition. However, few experimental studies have focused on measuring trace element partition coefficients for a large number of trace elements, in the various minerals present in basaltic rocks, and under diverse conditions, particularly of variable fO2. Twenty-seven 0.1 MPa experiments provide partition coefficients for major elements and Sc, Ti, V, Mn, Co, Ni, Ga, Sr, Y, Nb, Ba, Ce, Nd, Eu, Gd, and Yb for the mineral phases olivine, plagioclase, orthopyroxene and clinopyroxene. The experimental conditions range from 1150 to 1190 °C with oxygen fugacities from QFM to NNO+2. Run products were analyzed by laser-ablation ICP-MS.

The new partition coefficients, combined with previously published data, can be used to model crystallization processes at low pressure. Partitioning of multivalent cations V, Fe and Eu varies as a function of the redox conditions, consistent with previous work, and can be used to constrain oxidation states of magmatic source regions. The V/Yb ratio is shown to be a useful proxy for oxidation state. The V/Yb ratio varies during mantle melting as a function of oxidation state of the mantle source, and it is not modified during fractional crystallization of olivine ± plag ± cpx. V/Yb increases from MORB, BABB to arc lavas, suggesting a progressive increase of fO2 from QFM to NNO+2. Apparent fO2 of arc lavas, however, is quite variable. These results demonstrate that sub-arc mantle displays a larger range of redox conditions toward a more oxidized mantle than the MORB mantle.

title: The Border Ranges ultramafic and mafic complex, south-central Alaska: cumulate fractionates of island-arc volcanics
abstract: A discontinuous, elongate zone of mafic and ultramafic plutonic rock crops out in south-central Alaska for a distance of more than 1000 km. Intermediate- and detailed-scale geologic mapping, petrographic study, and compositional data suggest that the plutonic rocks are compositionally, petrologically, and mineralogically distinct from rocks in mid-ocean ridge and back-arc basin ophiolites. The mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc.The mafic–ultramafic zone, referred to as the Border Ranges ultramafic and mafic complex (BRUMC), is composed of ultramafic cumulates, gabbronorite cumulates, and massive gabbronorites. A very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the BRUMC. A thick sequence of andesitic volcanic rocks, the Talkeetna Formation of Early Jurassic age, lies to the north of and structurally above the mafic–ultramafic zone. Voluminous calcalkaline plutons composed of quartz diorite,...

title: Examination of the deep levels of an island arc: Evidence from the Tonsina ultramafic-mafic assembla
abstract: The Tonsina ultramafic and mafic assemblage, located in the northern Chugach Mountains along the Trans-Alaska Crustal Transect (TACT), is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc. It is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the Peninsular terrane along the northern fringe of the Chugach Mountains in southern Alaska. These masses, the roots of a mid-Jurassic island arc, have been uplifted along the north dipping Border Ranges fault system and crop out sporadically for 700 km from Kodiak Island to eastern Alaska. The Tonsina ultramafic-mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite (1 km thick) overlain by a narrow zone of websterite and clinopyroxenite. Above these ultramafic rocks is a 5- to 6-km thick layered gabbro that is garnet bearing at its base. Textural relationships and chemical trends throughout the series of dunite, pyroxenite, garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous, tholeiitic parent magma at deep levels, i.e., at the base of a mature island arc before being emplaced along the Border Ranges fault. The rocks were crystallized from a melt at pressures between 9.5 and 11 kbar (28- to 33-km depth) and equilibrated at temperatures of 800° to 90O°C based on phase relationships in the cumulus assemblages, pyroxene thermobarometry, and olivine-spinel thermometry. Transitional sequences from ultramafic to mafic rocks are characterized by increasingly AL-rich pyroxenes and spinels (or garnet) and the absence of olivine + plagioclase assemblages. Deep-level fractionation of Al-poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of Al in the evolving magma. As crystallization continued, the phases recorded this alumina concentration and the resultant gabbros are Al-rich. This observed trend supports the theory of high-alumina basalt generation by fractional crystallization of olivine and pyroxene. Comparison of the Tonsina rocks with Aleutian lower crustal xenoliths as well as other exposures of arc-related lower crustal magma chambers in Japan, Pakistan, and western North America indicates that this process is a common mechanism for island arc magma evolution at depth.

title: Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska
abstract: The Trans-Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra-mafic-mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid- to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid-Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite-pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc-derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time. A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism.

title: High-Mg, low-Al bulk composition of the Talkeetna island arc, Alaska: Implications for primary magmas and the nature of arc crust
abstract: Major- and trace-element mass-balance calculations were used to determine the bulk composition of the Jurassic Talkeetna island arc (Talkeetna Formation and Border Ranges mafic and ultramafic complex) of south-central Alaska. A representative section of the arc was chosen near Tonsina, Alaska, where a cross section of the arc exposes 18% ultramafic cumulate rocks (dunites, wehrlites, and pyroxenites), 16% lower gabbroic (garnet- and spinel-bearing) rocks, 34% upper gabbroic to tonalitic rocks, and 32% volcanic rocks (tuffs and flows). Mass-balance calculations for the Talkeetna arc yield a high-Mg basaltic bulk composition with REE abundances approximately 8x chondrites and La/Yb ∼1.8. The bulk composition is high in mafic components (MgO = 11.3 wt. %, Ni = 247 ppm, Cr = 723 ppm) and is compatible with derivation by partial melting of a mantle-wedge source. It is too mafic and too poor in Al 2 O 3 (15 wt. %) to have been derived by partial melting of subducted oceanic crust. High-alumina basaltic compositions are present within the arc as fractionates after crystallization of dunite and pyroxenite; however, they do not represent primary magmas parental to the arc. This finding supports the idea that the mass flux of magma from mantle to crust in island arc regions is high Mg, not high Al, in composition. The arc is stratified with respect to composition and density, with a dense ultramafic to mafic cumulate lower crust, a mafic to intermediate plutonic middle crust, and a mafic to silicic plutonic and extrusive upper crust. The presence of a highly dense, deformed, ultramafic, and garnet-bearing lower crust in this arc suggests that lower crustal delamination may be a viable mechanism for removing a highly mafic component from island arc crust.

title: 4.21 – One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust
abstract: This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.

title: 4.21 – One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust
abstract: This chapter has four main aims. Provide a comprehensive picture of the composition of volcanic rocks from subduction-related magmatic arcs. Review evidence in favor of the existence of andesitic, as well as basaltic primary magmas in arcs. Present new data on the composition of arc lower crust, based mainly on our work on the Talkeetna arc section in southcentral Alaska. Summarize evidence from arc lower crustal sections that a substantial proportion of the dense, lower crustal pyroxenites and garnet granulites produced by crystal fractionation are missing.

title: Arc‐parallel flow within the mantle wedge: Evidence from the accreted Talkeetna arc, south central Alaska
abstract: Residual mantle exposures in the accreted Talkeetna arc, Alaska, provide the first rock analog for the arc-parallel flow that is inferred from seismic anisotropy at several modern arcs. The peridotites exposed at the base of the Jurassic Talkeetna arc have a Moho-parallel foliation and indicate dislocation creep of olivine at temperatures of ∼1000° to >1100°C. Slip occurred chiefly on the (001)[100] slip system, which has only rarely been observed to be the dominant slip system in olivine. Stretching lineations and olivine [100] slip directions are subparallel to the long axis of the Talkeetna arc for over 200 km, indicating that mantle flow was parallel to the arc axis. The alignment of the olivine [100] axes yields a calculated S wave anisotropy with the fast polarization direction parallel to the arc. Thus (1) the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog; (2) arc-parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones, fluid-filled pockets, or glide on the (010)[001] H2O-induced slip system; (3) seismic anisotropy beneath modern arcs may be due to slip on (001)[100] with a horizontal foliation rather than slip on (010)[100] with a vertical foliation; and (4) the observed dominance of the (001)[100] slip system may be due to high H2O concentrations, suggesting that strain in the oceanic upper mantle may be accommodated dominantly by (001)[100] olivine slip.

title: Stratigraphic and geochemical evolution of an oceanic arc upper crustal section: The Jurassic Talkeetna Volcanic Formation, south-central Alaska
abstract: The Early Jurassic Talkeetna Volcanic Formation forms the upper stratigraphic level of an oceanic volcanic arc complex within the Peninsular Terrane of south-central Alaska. The section comprises a series of lavas, tuffs, and volcaniclastic debris-fl ow and turbidite deposits, showing signifi cant lateral facies variability. There is a general trend toward more volcaniclastic sediment at the top of the section and more lavas and tuff breccias toward the base. Evidence for dominant submarine, mostly mid-bathyal or deeper (>500 m) emplacement is seen throughout the section, which totals ~7 km in thickness, similar to modern western Pacifi c arcs, and far more than any other known exposed section. Subaerial sedimentation was rare but occurred over short intervals in the middle of the section. The Talkeetna Volcanic Formation is dominantly calc-alkaline and shows no clear trend to increasing SiO 2 up-section. An oceanic subduction petrogenesis is shown by trace element and Nd isotope data. Rocks at the base of the section show no relative enrichment of light rare earth elements (LREEs) versus heavy rare earth elements (REEs) or in meltincompatible versus compatible high fi eld strength elements (HFSEs). Relative enrichment of LREEs and HFSEs increases slightly up-section. The Talkeetna Volcanic Formation is typically more REE depleted than average continental crust, although small volumes of light REE-enriched and heavy REE-depleted mafi c lavas are recognized low in the stratigraphy. The Talkeetna Volcanic Formation was formed in an intraoceanic arc above a northdipping subduction zone and contains no preserved record of its subsequent collisions with Wrangellia or North America.

title: A detailed geochemical study of island arc crust : the Talkeetna Arc section, south–central Alaska
abstract: Author Posting.  This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 47 (2006): 1051-1093, doi:10.1093/petrology/egl002.

title: U-Pb geochronology of volcanic rocks from the jurassic talkeetna formation and detrital zircons from prearc and postarc sequences: Implications for the age of magmatism and inheritance in the talkeetna arc
abstract: Six samples collected from pre-, syn-, and post-Talkeetna arc units in southcentral Alaska were dated using single-grain zircon LA-MC-ICP-MS geochronology to assess the age of arc volcanism and the presence and age of any inherited components in the arc. The oldest dated sample comes from a volcanic breccia at the base of the Talkeetna Formation on the Alaska Peninsula and indicates that initial arc volcanism began by 207 ? 5 Ma. A sedimentary rock overlying the volcanic section in the Talkeetna Mountains has a maximum depositional age of 467 Ma. This is in agreement with biochronologic ages for the top of the Talkeetna Formation, suggesting that the Talkeetna arc was active for ca. 40 m.y. *!%mail, Amato: amato@nrnsu.edu. Present address, Rioux: Department of Earth, Ahnospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA. Amato, J.M., Rioux, M.E., Kelemen, P.B., Gehrels, G.E., Clift, P.D., Pavlis, T.L., and Draut, A.E., 2007, U-Pb geochronology of volcanic rocks from the Jurassic Talkeema Formation and detrital zircons from prearc and postarc sequences: Implications for the age of magmatism and inheritance in the Talkeetna arc, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O'Neill, J.M., eds., Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska: Geological Society of America Special Paper 431, p. 253-271, . For permission to copy, contact editing@geosociety.org.

title: Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska
abstract: The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ϵNd(t) = 6.0–7.8 and 87Sr/86Srint = 0.703379–0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5–168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ϵNd(t) = 5.6–7.2 and 87Sr/86Srint = 0.703383–0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ϵNd(t) = 4.0–5.5 and 87Sr/86Srint = 0.703656–0.706252] and contain inherited xenocrystic Carboniferous–Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

title: Reconstruction of the Talkeetna intraoceanic arc of Alaska through thermobarometry
abstract: The Talkeetna arc is one of two intraoceanic arcs where much of the section from the upper mantle through the volcanic carapace is well exposed. We reconstruct the vertical section of the Talkeetna arc by determining the (re)crystallization pressures at various structural levels. The thermobarometry shows that the tonalites and quartz diorites intruded at ∼5–9 km into a volcanic section estimated from stratigraphy to be 7 km thick. The shallowest, Tazlina and Barnette, gabbros crystallized at ∼17–24 km; the Klanelneechena Klippe crystallized at ∼24–26 km; and the base of the arc crystallized at ∼35 km depth. The arc had a volcanic:plutonic ratio of ∼1:3–1:4. However, many or most of the felsic plutonic rocks may represent crystallized liquids rather than cumulates so that the liquid:cumulate ratio might be 1:2 or larger. The current 5- to 7-km structural thickness of the plutonic section of the arc is ∼15–30% of the original 23- to 28-km thickness. The bulk composition of the original Talkeetna arc section was ∼51–58 wt % SiO2.

title: Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating
abstract: As one of two well-exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu-Hf and Sm-Nd garnet ages, Ar-40/Ar-39 hornblende, mica and whole-rock ages, and U-Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U-Pb ages establish the main period of arc magmatism as 202-181 Ma in the Chugach Mountains and 183-153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu-Hf and similar to 182 Ma Sm-Nd garnet ages indicate that 25-35 km deep sections of the arc remained above similar to 700 degrees C for as much as 15 Myr. The Ar-40/Ar-39 hornblende ages are chiefly 194-170 Ma in the Chugach Mountains and 175-150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U-Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from similar to 180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are similar to 137 to 129 Ma, indicating no Cenozoic regional heating above similar to 180 degrees C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One-dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories.  Hacker, B. R., P. B. Kelemen, M. Rioux, M. O. McWilliams, P. B. Gans, P. W. Reiners, P. W. Layer, U. Soderlund, and J. D. Vervoort (2011), Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating, Tectonics, 30, TC1011, doi:10.1029/2010TC002798.

title: Role of Arc Processes in the Formation of Continental Crust
abstract: We review data and recent research on arc composition, focusing on the relatively complete arc crustal sections in the Jurassic Talkeetna arc (south central Alaska) and the Cretaceous Kohistan arc (northwest Pakistan), together with seismic data on the lower crust and uppermost mantle. Whereas primitive arc lavas are dominantly basaltic, the Kohistan crust is clearly andesitic and the Talkeetna crust could be andesitic. The andesitic compositions of the two arc sections are within the range of estimates for the major element composition of continental crust. Calculated seismic sections for Kohistan and Talkeetna provide a close match for the thicker parts of the active Izu arc, suggesting that it, too, could have an andesitic bulk composition. Because andesitic crust is buoyant with respect to the underlying mantle, much of this material represents a net addition to continental crust. Production of bulk crust from a parental melt in equilibrium with mantle olivine or pyroxene requires processing of igneou...

title: Origin and age of the Border Ranges Fault of southern Alaska and its bearing on the Late Mesozoic tectonic evolution of Alaska
abstract: Local geologic relationships in the western Chugach Mountains, together with regional considerations, suggest that in the Early Cretaceous a subduction zone was formed along the southern edge of the Wrangellia-Peninsular-Alexander composite terrane (Talkeetna superterrane of Csejtey et al., 1982) of southern Alaska. The effects of this event include (1) a shattering of older (pre-Cretaceous) crystalline rocks along a complex fault system, (2) emplacement of a tectonic melange beneath the shattered crystalline rocks, and (3) a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks. This event created a regionally mappable structural contact between broken crystalline rocks and the melange; the Border Ranges fault of MacKevett and Plafker (1974). Regional stratigraphy as well as radiometric ages of rocks predating and postdating the Border Ranges fault appear to bracket the age of the Border Ranges fault and its associated deformational effects to the interval between about 135 and 120 Ma. Further regional tectonostratigraphic associations suggest that the deformation along the Border Ranges fault represents the nucleation of a north and (or) east dipping subduction zone beneath the Talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the Gravina-Nutzotin belt of southeast Alaska. Mismatches in the distribution of different elements of this Early Cretaceous arc-trench system are probably a result of Late Cretaceous or early Tertiary strike slip motion, but Early Cretaceous ridge-trench interaction (suggested by the occurrence of Early Cretaceous near-trench plutons) may have played a role as well. The Talkeetna superterrane is generally thought to be an exotic block that collided with the Cordillera in the middle Cretaceous (Coney et al., 1980). The age data commonly cited as evidence for a middle Cretaceous age for the collision are, however, misleading. A model is proposed here in which the initiation of convergence along the trailing edge of the Talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle Cretaceous deformation recognized along the leading edge of the Talkeetna superterrane (Csejtey et al., 1982) records destruction of a syncollisional flysch basin during the final phase of the collision.

title: Pre-Cretaceous crystalline rocks of the western Chugach Mountains, Alaska: Nature of the basement of the Jurassic Peninsular terrane
abstract: A narrow belt of Mesozoic and(or) Paleozoic crystalline rocks exposed immediately north of the Border Ranges fault of southern Alaska is the southern (trailing) edge of the Peninsular terrane, an exotic micro-continental block that collided with North America during the Cretaceous. The tectonic significance of this crystalline belt has been debated because previous K-Ar geochronology suggested that the belt contains both Early Jurassic high P/T metamorphic rocks—an apparent subduction complex—and Early Jurassic intermediate plutonic rocks—an apparent magmatic arc. Three major observations from detailed mapping in the western Chugach Mountains pertain directly to this apparent problem of mixed rocks association. First, despite major postmetamorphic faulting along the Border Ranges fault, all of the metamorphic rocks have been subjected to a regional, apparently low P/T metamorphism at the greenschist-amphibolite facies transition. The metamorphism is imprinted on rocks ranging from nearly undeformed ultramafic rocks to highly deformed amphibolites, yet regardless of structural histories of fault-bounded slices, the culmination of metamorphism appears to postdate deformation. Second, the metamorphic rocks are locally an argillite matrix melange in which main-phase structural fabrics appear to have formed at relatively low grade but are overprinted by growth of postkinematic, transitional greenschist-amphibolite facies minerals. Third, despite ambiguities in correlation across faults and equivocal absolute age relationships—presumably due to Cretaceous resetting of K-Ar ages—the transitional greenschist-amphibolite facies rocks are intruded by plutons that yield Early Jurassic K-Ar ages. Thus, the metamorphic protolith is presumably pre-Jurassic. In addition, the close association of low P/T metamorphic rocks with a large plutonic complex implies that the regional metamorphism is probably Early Jurassic in age and is associated with extensive plutonism.

New age data from the western Chugach Mountains imply that a belt of Early Jurassic plutonism (and associated low P/T metamorphism) probably extends throughout the southern edge of the Peninsular terrane. The entire terrane probably represents a long-lived magmatic arc, because this Early Jurassic magmatic arc is separated from a parallel belt of Early to Middle Jurassic plutons by an Early Jurassic andesitic-volcanic pile. In the western Chugach Mountains, emplacement of alpine ultramafic bodies and development of chaotic rock bodies is premetamorphic, presumably pre-Jurassic in age. These observations of structural style, together with lithologic associations and regional occurrences of blueschist-facies rocks, suggest that a pre-Jurassic subduction complex may have formed parts of the basement for the Peninsular terrane arc. As a corollary of this conclusion, the blueschists of the southern Peninsular terrane are probably pre-Jurassic in age and represent scraps of the pre-Jurassic basement, not parts of an Early Jurassic high-pressure metamorphic belt.

title: Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low-velocity middle crust in modern arcs
abstract: Seismic profiles of several modern arcs have identified thick, low-velocity midcrustal layers (Vp = 6.0–6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U-Pb zircon dates, radiogenic isotope data, and whole-rock geochemical analyses that provide the first high-precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion–thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 ± 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from ɛNd(t) = 5.2 to 9.0 and 87Sr/86Srint = 0.703515 to 0.703947 and are similar to age-corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole-rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low-velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs.

title: The Fe3+/ΣFe ratios of MORB glasses and their implications for mantle melting
abstract: The Fe3+/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe2+ analyses and electron microprobe FeOtotal measurements. The data provide a new estimate for the MORB oxygen fugacity (fO2) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe3+/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO2 estimate is 0.8 log units more oxidized than the average fO2 proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe3+/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe2+ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe3+/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe3+/ΣFe relationships with partial melting proxies such as Na8.0. Based on the Fe3+ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe3+ during partial melting, and in the relatively constant Fe3+/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe3+/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe2O3 systematics can be accounted for by using a fO2 of FMQ-1 that is equivalent to the average fO2 reported for abyssal peridotites.

title: Upper mantle oxidation state: Ferric iron contents of Iherzolite spinels by 57Fe Mössbauer spectroscopy and resultant oxygen fugacities
abstract: The ferric iron contents of spinels from 30 spinel Iherzolite xenoliths have been measured by 57 Fe Mossbauer spectroscopy. The samples are widely dispersed in geographic and tectonic setting, coming from Southwest United States (San Carlos, Kilbourne Hole), Japan (Ichinomegata), Massif Central, France (Mont Briancon) and Central Asia ( Plateau). The total range of Fe 3 O 4 contents of the spinels is from 1.7 to 5.2 mol% with an uncertainty of 0.2 to 0.3 mol%. These data were used to calculate the oxygen fugacities recorded by the spinel Iherzolites using the oxygen thermobarometer 6 Fe 2 SiO 4 + O 2 = 3 Fe 2 Si 2 O 6 + 2 Fe 3 O 4 . olivine orthopyroxene spinel The Fe(III) contents of the spinels translate to oxygen fugacities which, at 15 kb, range between 1.7 log units below and 1.2 log units above FMQ using either the Mattioli and Wood (1988) or O'neill and Wall (1987) version of Fe 3 O 4 activity. There are distinct regional differences ƒO 2 , the specimens from SW U.S.A. and Central Asia exhibiting values from slightly above FMQ to 1.5 log units below FMQ. At an estimated pressure of 15 kb, these values overlap with the ƒO 2 - T field of MORB glasses, indicating, in agreement with trace element abundances, that many of these samples are related to the MORB source region. Samples from Ichinomegata and Mont Briancon are all above the MORB range, however, suggesting progressive oxidation related to subduction processes. All of our samples give oxygen fugacities more than 2 log units above IW, implying that CHO fluids in the upper mantle are dominated by CO 2 and H 2 O and that CH 4 is a minor ( fX Fe 3 O 4 ) but, in general, too inaccurate for oxygen thermobarometry. Use of Mossbauer-analyzed spinels as microprobe standards enables accuracy to approach precision, however, and appropriate standards are available on request.

title: Oxygen thermobarometry of abyssal spinel peridotites: the redox state and C-O-H volatile composition of the Earth's sub-oceanic upper mantle
abstract: The authors have applied the spinel peridotite oxygen barometer to abyssal spinel peridotites from the mid-Atlantic, central Indian, southwest Indian and American-Antarctic ocean ridge systems. The results indicate that the oxygen fugacity (F{sub O{sub 2}}) of the suboceanic mantle is on average 0.9 {plus minus}0.7 (n = 33; {plus minus} 1sd) log units below the Fayalite-Magnetite-Quartz (FMQ) f{sub O{sub 2}} buffer, in excellent agreement with f{sub O{sub 2}} estimates of MORB glasses (FMQ {minus}1.20 {plus minus} 0.63, n = 87; {plus minus} 1 sd). The agreement between MORBs and their mantle source region suggests that the rapidly quenched liquids have not undergone significant oxidation (by hydrogen degassing, for example) during their ascent and eruption. Their results also show that the suboceanic mantle is more reduced than the subcontinental mantle, for which the average value of log f{sub O{sub 2}} lies approximately at FMQ (log f{sub O{sub 2}} = FMQ + 0.24 {plus minus} 0.5, n = 54; {plus minus} 1 sd). Abyssal spinel peridotites from the Islas Orcadas fracture Zone (FZ), near the Bouvet Island hotspot, are the most reduced samples in our suite (log f{sub O{sub 2}} = FMQ {minus}1.67 to {minus}2.32), and they are compatible with a graphite-saturatedmore » fluid being present in this source region. In general, however, upper mantle f{sub O{sub 2}}'s are too high for graphite to be stable, and they estimate an activity of carbon relative to graphite of about 0.05 under the approximate conditions of MORB generation (P {approximately} 10 kb, T {approximately} 1,325{degree}C, log f{sub O{sub 2}} {approximately} FMQ {minus}0.9). Under these conditions an average CO{sub 2} content in the mantle of {approximately} 215 to 545 ppm would be consistent with the fluid compositions of MORB glasses.« less

title: Constraining the cooling rate of the lower oceanic crust: a new approach applied to the Oman ophiolite
abstract: Knowledge of the cooling rate of newly formed lower oceanic crust is critical in constraining models of crustal accretion and hydrothermal circulation at mid-ocean ridges. In this study we calculate the cooling rate of the lower oceanic crust based on the closure temperature of down-temperature diffusive exchange of Ca from olivine to clinopyroxene in gabbros from the Oman ophiolite. Modelling of the closure temperatures shows that the cooling rates for the lower oceanic crust decrease with depth, changing rapidly in the upper third of the gabbros but more slowly in the lower two-thirds. There is several orders of magnitude variation in cooling rate between the top and bottom of the lower crust. The cooling rate versus depth profile closely matches that of a conductive cooling model, or a model in which conductive cooling is enhanced by a constant factor due to hydrothermal circulation.

title: Cooling rates in the lower crust of the Oman ophiolite: Ca in olivine, revisited
abstract: Debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models. The thermal profile of the crust, specifically the roles of hydrothermal circulation and cooling rate, is a key component in distinguishing between these two models. Results of this study show no systematic variation of cooling rate with depth in the lower crust of the Khafifah section in the Wadi Tayin massif of the Oman ophiolite. Additionally, very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust. While the conclusions presented here do not rule out the possibility of accretion by gabbro glacier, they do contradict the initial tenet of the gabbro glacier model – convective heat loss by hydrothermal circulation can penetrate to Moho depths and crystallization is not, by necessity, constrained to the upper crust. On the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust.

title: Electron microprobe analysis of Ca in olivine close to grain boundaries: The problem of secondary X-ray fluorescence
abstract: Analysis of Ca in olivine grains close to boundaries with Ca-rich phases is inherently problematic because of the effects of secondary X-ray fluorescence of Ca from the adjoining Ca-rich phase. Commercial fluorescence-correction methods assume chemical homogeneity throughout the entire interaction volume and yield enhanced Ca concentrations for the olivine in such cases. We have determined the effects of Fe content of the olivine and Ca content of the adjoining phase on the magnitude of secondary fluorescence effects during electron microprobe analysis of Ca in olivine. Significant errors in Ca determination were observable at horizontal beam-interface distances ranging from 30 μm for an Fe-free olivine-diopside couple to 70 μm for an olivine (Fo90.7)-calcite couple. In general, the beam-interface distance required to suppress secondary fluorescence of Ca X-rays from the adjoining phase increases with increasing Fe content of the olivine and Ca content of the neighboring phase. Analysis of an olivine (Fo90.7) wedge mounted on a standard thin-section glass slide (5.06 wt% Ca) showed significant secondary fluorescence of Ca to a wedge depth approaching 20 μm. It is recommended that olivine grains be separated and remounted in a Ca-free matrix for the determination of Ca concentrations in olivine by electron microprobe analysis.

title: Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotites
abstract: Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity (fO2) of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe3+/∑Fe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe3+/∑Fe ratios that relies on a reported correlation between the difference in Fe3+/∑Fe ratio by Mössbauer spectroscopy and by electron microprobe (ΔFe3+/∑FeMöss-EPMA) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mössbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe3+/∑FeMöss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe3+/ZFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = 0.031 and corrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe3+/ZFe ratios despite the absence of a correlation between ΔFe3+/∑FeMöss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe3+/∑Fe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive fO2. Because there is a logarithmic relationship between the activity of magnetite and logfO2, the uncertainty in fO2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe3+/∑Fe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe3+/EFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative fO2. We also document and correct errors that appear in the literature when formulating fO2 that, combined, could yield errors in absolute fO2 of greater than 0.75 log units—even with perfectly accurate Fe3+/∑Fe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe3+/∑Fe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of fO2 from spinel peridotite mineral compositions.

title: Petrology of recrystallized ultramafic xenoliths from Merelava volcano, Vanuatu
abstract: Xenoliths in primitive olivine tholeiite lavas from Merelava Volcano, Vanuatu, include recrystallized wehrlites and harzburgites characterized by extremely fine grain size (0.02–2 mm) and equigranular textures. The harzburgites display mineral segregations, have highly variable ratios of ol: opx, minor clinopyroxene and accessory Cr-spinel, and are interpreted as the residues of high degrees of melting of upper mantle peridotite. Annealed Cr-spinel aggregates in harzburgite sample # 31564B enclose numerous small inclusions of sodic sanidine and minor plagioclase, attributed to infiltration of the harzburgite by a residual melt derived from an earlier period of island arc magmatism. The recrystallized wehrlites have high ol/cpx ratios and depleted REE patterns compatible with a cumulus origin. The refractory nature of the phases in both groups of recrystallized xenoliths compares closely with phases in ‘Alpine-type’ peridotites and primitive arc lavas, and is incompatible with compositions of abyssal peridotites. The recrystallized wehrlites give equilibration temperatures of 1070–1130° C and are interpreted as cumulates derived from an earlier period of Vanuatu Arc magmatism. The range of composition displayed by phases in the harzburgites is greater than phase variation in the wehrlites, and reflects a more complex thermal history. Textural, mineralogical, and geothermometric considerations indicate the harzburgites underwent cooling to 800°/900° C before being re-heated to 1000–1100° C by the current magmatic regime. A shallow crustal origin for these xenoliths is indicated by gravity data and tectonic considerations which strongly imply the presence of an ophiolite body beneath Merelava, representing the northward extension of the Pentecost Ophiolite. These interpretations are compatible with a published model for generation of the host basalts by partial melting at the crust/mantle boundary (ca. 17 km). Sr isotopic data show that the harzburgites are incompatible with residues of ocean-floor magmatism, or with residues of Merelava and Central Chain magmatism, but suggest an affinity with Vitiaz Arc magmatism of Eocene-lower Miocene age. Both groups of xenoliths were apparently entrained from wall rocks during ascent of the host magmas.

title: Peridotite xenoliths from Grenada, Lesser Antilles Island Arc
abstract: Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~≤22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒO2) from primary values 1–2 log ƒO2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO2 18–20 wt% Al2O3 and 3–4 wt% Na2O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.

title: An improved experimental calibration of the olivine-spinel geothermometer
abstract: The calibration of the olivine-spinel geothermometer by Fabriès (1979) is commonly adopted by a number of petrologists. But the temperatures calculated in this way for ultramafic rocks are significantly lower than those obtained by the pyroxene geothermometers. These O1-Sp temperatures are also lower than those measured experimentally in the natural system (four-phase lherzolite). Different rates of cation diffusion cannot fully account for these differences. The temperature deviation is actually related to the inconsistencies between natural and experimental data which support the calibration. A re-evaluation of the calibration is proposed on the basis of a set of new experimental data.

title: Experimental calibration of the roles of temperature and composition in the Ca-in-olivine geothermometer at 0.1 MPa
abstract: The calcium content of olivine in equilibrium with a low-Ca pyroxene and clinopyroxene has previously been calibrated in several studies at elevated pressure as a geobarometer. However, the calcium content of olivine in such systems is also dependent on temperature and the Mg# (100*Mg/Mg + Fe) of the system which is represented here by the olivine forsterite (Fo) content. To isolate these variables a series of experiments has been performed at 0.1 MPa between 1170 °C and 1322 °C in both, the simple CaO–MgO–Al2O3–SiO2 (CMAS) system and in Fe–Na–Ti–K bearing compositions. In the CMAS system the Ca content of pure forsterite coexisting with low-Ca pyroxene and clinopyroxene is strongly temperature dependent, decreasing by > 0.1 wt.% CaO between 1322 °C and 1254 °C. Addition of Fe to the system leads to a substantial increase in the Ca content of olivine coexisting with a low-Ca pyroxene and clinopyroxene at a given temperature. Empirical fitting of these data gives a geothermometer applicable at low pressure and in the compositional range Fo70–Fo100: T ° C = − 12368 2032 ln X Mo − 6.395 1.79 + 3.235 0.51 Fo − 273 where the values in parentheses are the standard error on the parameter. This reproduces all experimental data within 20 °C.

title: Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kb with applications
abstract: The equilibrium Ca content of olivine coexisting with clinopyroxene was experimentally determined in natural lherzolitic compositions (Mg/(Mg + Fe)ol ≈ 0.9). The experiments from 2–60 kb and 900–1400°C cover most of the crystallization conditions of upper mantle peridotites. 
 
The Ca content of olivines in the experimental products was determined by electron microprobe in the WDS mode with a precision of ±5 rel% (1σ). In a few instances an ion microprobe was used (±2 rel%). Accuracy was tested with olivine standards analyzed for calcium by isotope dilution analysis. 
 
The solubility of Ca in olivine showed a non-linear change at around 1100°C. Therefore, we have fitted our experimental data to two equations, given here in the form of barometers: P[kb] = (−T∗ln DCa − 11982 + 3.61∗T)/56.2 for T ≥ (1275.25 + 2.827∗P)[K]P[kb] = (−T ln DCa − 5792 − 1.25∗T)/42.5 for T ≤ (1275.25 + 2.827∗P)[K] where Dca = Caol/Cacpx(Caol and Cacpx = atomic proportions of Ca in structural formulae of ol and cpx based on 4 and 6 oxygens, respectively). These equations reproduce experimental conditions to ±1.7 kb(1σ).∗ 
 
This is the first barometer which can be applied successfully to spinel peridotites. Calculated P, T conditions for spinel peridotite nodules from southwestern USA plot within the spinel stability field and along predicted geotherms for this region. Application to garnet peridotites from southern Africa generally shows good agreement with the Al-opx/gt barometer, but some significant deviations also exist. They are most likely caused by short-lived thermal events immediately before eruption to the surface. These are recorded by the Ca contents of olivines but not in the associated clinopyroxenes, since the diffusion coefficient for Ca in olivine is 3 to 4 orders of magnitude higher than in clinopyroxene. The tendency is that olivines from “high-temperature” peridotites record heating events while olivines from peridotites plotting along a shield geotherm record cooling events. High Cr spinel peridotites from southern Africa plot in the P, T field of “low-temperature” peridotites from the same occurrences. Pressures calculated for spinel-bearing “fertile” harzburgites (with exsolved cpx) from southern Africa are excessively high, indicating intense cooling with reequilibration of olivines to lower temperatures. Careful investigation of zoning patterns of minerals, especially of Ca in olivine will help to constrain the magnitude of cooling or heating these rocks have experienced. 
 
Due to the high diffusion coefficient of Ca in olivine, Ca contents can also be used as a “speedometer” for geologically very brief thermal events such as heating of mantle xenoliths by their host magmas or cooling in meteorites.

title: The olivine — clinopyroxene geobarometer: experimental results in the CaO-FeO-MgO-SiO2 system
abstract: The exchange of Ca and Mg between olivine and clinopyroxene has been studied in the CFMS system. Experiments were conducted in a piston-cylinder apparatus in the temperature range of 1,100–1,300° C and the pressure range 10–30 kbar. Results confirm the previous suggestion (Adams and Bishop 1982) that this reaction has a sufficiently largeΔ V° to be used as a geobarometer in several basic and ultrabasic systems. Experimental results were corrected for compositional effects using recent activity-composition models for quadrilateral pyroxenes and olivines. The corrected results indicate that the exchange reaction has aΔH1 bar of 34,900 J, a ΔS° of -7.25J/deg, and a ΔV° of -0.489 J/bar. Corrected results agree well with calculations based on the thermodynamic properties of the endmembers.Application of the olivine-clinopyroxene geobarometer to many systems will require additional calibration of non CFMS components. Preliminary pressure estimates based on simple assumptions about the activity relations of these components have been made for spinel lherzolites from southwestern United States and coarse and porphyroclastic garnet lherzolites from southern Africa. A geotherm calculated from spinel lherzolites near the Rio Grande rift is consistent with a geophysical geotherm based on near-surface heat-flow measurements of 100 mW/m2 or greater. Results on garnet lherzolites yield a southern African geotherm with no inflection which falls at somewhat higher temperatures than pyroxene geotherms calculated for the same area.

title: Chronometry and Speedometry of Magmatic Processes using Chemical Diffusion in Olivine, Plagioclase and Pyroxenes
abstract: The magmatic processes that fuel volcanism, crustal growth, ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales, from millions of years to just a few seconds. For example, the production of new oceanic crust at mid-ocean ridges is a near-continuous process that can operate in any one ocean basin on timescales of more than 100 m.y. However, the driving force for such processes is the spreading of the ocean plates that happens on a cm/yr timescale. At the other end of the spectrum, explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m/s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes. In this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure. At intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit, while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years. Although each of these processes has a characteristic, time-averaged timescale on which it operates, this is typically the end result of one or more natural processes that operate on much shorter timescales. For example, mid-ocean ridges do not extrude magma continuously onto the ocean floor, mineralising fluids do not discharge continuously through the shallow crust, and granitic magmas do not dribble continuously into evolving batholithic chambers. In some cases it is the long-term timescales that are important, for example the spreading rate of ocean basins, in others it is the short-term timescales that are important, for example the episodic growth of lava domes at active volcanoes. Although …

title: High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle
abstract: AbstractSynthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under controlled oxygen fugacity (fO2). fO2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO2-C, Ni−NiO, and Fe3O4−Fe2O3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe3+/ΣFe in spinel is a linear function of fO2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog(fO2), but decreases slightly with increasing Cr content in spinel. The Fe3+/ΣFe ratio falls with increasing pressure at given Δlog(fO2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing fO2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from the fayalite-magnetite-quartz (FMQ) buffer: 
$$\Delta log (f_{O_2 } )^{FMQ} = 0.27 + 2505/T - 400P/T - 6 log(X_{Fe}^{olv} ) - 3200(1 - X_{Fe}^{olv} )^2 /T + 2 log(X_{Fe^{2 + } }^{sp} ) + 4 log(X_{Fe^{3 + } }^{sp} ) + 2630(X_{Al}^{sp} )^2 /T.$$

The oxygen barometer is applicable to the entire spectrum of spinel compositions occurring in mantle rocks and mantle-derived melts, and gives reasonable results to temperatures as low as 800° C.

title: Thermometers and Barometers for Volcanic Systems
abstract: Knowledge of temperature and pressure, however qualitative, has been central to our views of geology since at least the early 19th century. In 1822, for example, Charles Daubeny presented what may be the very first “Geological Thermometer,” comparing temperatures of various geologic processes (Torrens 2006). Daubeny (1835) may even have been the first to measure the temperature of a lava flow, by laying a thermometer on the top of a flow at Vesuvius—albeit several months following the eruption, after intervening rain (his estimate was 390°F). In any case, pressure ( P ) and temperature ( T ) estimation lie at the heart of fundamental questions: How hot is Earth, and at what rate has the planet cooled. Are volcanoes the products of thermally driven mantle plumes? Where are magmas stored, and how are they transported to the surface—and how do storage and transport relate to plate tectonics? Well-calibrated thermometers and barometers are essential tools if we are to fully appreciate the driving forces and inner workings of volcanic systems.

This chapter presents methods to estimate the P-T conditions of volcanic and other igneous processes. The coverage includes a review of existing geothermometers and geobarometers, and a presentation of approximately 30 new models, including a new plagioclase-liquid hygrometer. Our emphasis is on experimentally calibrated “thermobarometers,” based on analytic expressions using P or T as dependent variables. For numerical reasons (touched on below) such expressions will always provide the most accurate means of P-T estimation, and are also most easily employed. Analytical expressions also allow error to be ascertained; in the absence of estimates of error, P-T estimates are nearly meaningless. This chapter is intended to complement the chapters by Anderson et al. (2008), who cover granitic systems, and by Blundy and Cashman (2008) and Hansteen and Klugel (2008), who consider additional methods …

title: Calcium zoning in olivine and its relationship to silica activity and pressure
abstract: Microprobe analyses of olivine phenocrysts from nephelinite and basanite lavas show strong zoning toward calcium enrichment. Tholeiitic and other more siliceous lavas show little or no such calcium enrichment, but more extensive magnesium to iron zoning. Consideration of reactions between clinopyroxene and olivine indicate that silica activity as well as pressure is an important variable. Zoning trends toward calcium enrichment in olivine could be interpreted as a response to pressure release during crystallization. A normal, magnesium to iron, trend with no increase in calcium would, then, reflect crystallization with stable pressure conditions.

title: Primary textures and mineral associations in the ultramafic zone of the Stillwater complex, Montana
abstract: -------------~---------------------------Introduction __________ ·,___________________________ _ Fieldwork, acknowl~dgments, and scope of report_ __ General geology of the complex.-----------------Composition of the rocks ___________________________ _ ~1ineralogY-------~---------------------------Rock types in the Ultramafic zone _______________ _ Lithologic similaritiEis and contrasts ______________ _ Textures of the rocks ___ .:. ___________________________ _ Primary precipitate~ ___________________________ _ Shape of the settled crystals _________________ _ Euhedra __ ~---------~-----------------Subhedra and anhedra __________________ _ Discussion:. _______ ~ ___________________ _ Distribution of: the settled 'crystals ___________ ~ Basal zon~----------~-----------------Ultramafic zone, Peridotite member ______ _ Ultramafic zone, Bron~itite member ______ _ Discussion ____________________________ _ Internal character of the settled crystals ______ _ Inclusions~:-_______________________ .. __ Oriented intergrowths __________________ _ Zoning ___ ~ _________________ · __________ _ Fractures-~---------------------------Undulatory extinction.,. _________________ _ Grain size aild grain-size distribution ___ · ______ _ Grain size_; ______________ :... _____________ _ Sorting ___ ~---------~-----------------Size-density relations ___________________ _ Discussion·~ ___________________________ _ Orientation of the settled crystals ____________ _ Bronzite __ ·~ _________________________ ~ __ Olivine _____________ :... _________________ _ Discussion.:. _________ ~ _________________ _ Relations of the pri~nary preci~itate to· the interprecipitate materiaL~ ___________________________ _ Reaction replacement ____ ,_ _________________ _ Olivine--~ronzite ____ . __________ ~· ______ _ Olivine--6hrorriian augite ____________ ~ __ Bronzite__;..chromian ~ugite _____________ _ Volume relations _____ · __________________ _ Discussion _______________________ .,. ____ _ Secondary enlargement ________________ ~ ____ _ Poikilitic harzburgite and dunite ________ _ Bronzitite _____________________________ _ Chromitit~--_____________ "" _ ""________ _ Olivine chromitite arid poikilitic chromite harzburgite _______ · __________________ _ Olivine bronzitite and granular harzburgite_ Volumetric relations ____________________ _ Stratigraphic and areal distribution ______ _ Discussion ____ :. ____ ~ ____ :.: __ ~ _______ .. __ _ Page 1 Textures of the rocks-Continued 2 Relations of the primary precipitate-Continued 2 Initial porosity ____________________________ _ 2 Bronzitite _____________________________ _ 4 Poikilitic harzburgite _____ ~-------------4 Chromitite ____________________________ _ 5 Granular harzburgite_...: _________________ _ 8 Olivine chromitite and poikilitic chromite 11 harzburgite _______ -·~ _ ~: _____________ _ 13 Discussion ____________________________ _ 13 Interprecipitate materiaL ----------------------13 Shape of the interstitial materiaL ____________ _ 13 Shape of the interstices _________________ _ ~3 Shape of the poikilitic crystals ___________ _ 13 Discussion ____________ ~ __________ · _____ _ 14 Distribution of the interprecipitate materiaL __ _ 14 Poikilitic harzburgite: ___ ~ ____ : --·-------14 Granular harzburgite ___________________ _ 14 Bronzitite _____________________________ 15 Chromitite __ · ____ --.:-__________________ _ 15 Olivine chromite and poikilitic chromite 17 harzburgite ____________ · _____ _------18 Stratigraphic distributio~______________ _ 18 Discussion __ .., _________________________ 18 Internal character of the interprecipitate ma20 teriaL ___ ~ ________ ~ _____________________ _ 21 Inclusions _____________________________ _ 28 Oriented intergrowths __________________ 30 Zoning _______________________________ _ 35 Size and size distribution of poikilitic crystals __ _ 37 Grain size ___ :_.:_______________________ -39 Size distribution_~ _______________ ------42 Discussion ____________ ~ _______________ _ 45 Orientation of poikilitic crystals ________ c-. ____ _ De~elopment of dominant textures in the -Ultramafic 47 zone _______________________________________ _ 4 7 Automorphic-poikilitic texture,.. _______________ 47 Hypautomorphic texture ____________________ _ 4 7 Xenomorphic texture_~ ___ ~ _________ ~ _______ _ 50 Development of the Ultramafic zone __ ~ ______________ 50 Conditions of crystallization and deposition ________ 52 Predepositional conditions ___________________ 52 The magma ____________ -._____________ 54 Primary crystallization products _________ 54 Depositional conditions _______ ~ ___ ._________ 54 Agents of transportation and deposition __ Origin of the layering_ ~ __ · _________ --58 Postdepositional conditions ________________ -58 Character of the crystal mush __ · ________ -58 Interprecipitate magma _________________ _ 60 Crystallization of. the interprecipitate ma61 terial ___________________ · __ -

title: Postcumulus and subsolidus equilibration of chromite and coexisting silicates in the Eastern Bushveld Complex
abstract: Analyses of chromite and coexisting olivine or bronzite in various textural associations in the Critical Zone of the Eastern Bushveld Complex are examined for evidence of the extent and nature of postcumulus or subsolidus equilibration of chromite and Fe-Mg silicates. Sub-solidus equilibration of chromite inclusions in bronzite and olivine is indicated, and postcumulus or subsolidus equilibration of chromite with postcumulus bronzite is indicated in mottled anorthosite of the H unit. Compositional zoning of cumulus bronzite is inferred to be due to equilibration with cumulus chromite during the postcumulus stage. The main pattern of variation in Mg/Fe ratio and A12O3 content of chromite with modal per cent chromite is inferred, however, to have been produced during the cumulus stage. The data are consistent with control of chromite precipitation and composition by the oxygen fugacity of the magma, but the effects of changes in pressure or other variables of the magma system on chromite stability and composition are still largely unknown.

title: The cumulus and post-cumulus evolution of chrome-spinels in ultrabasic layered intrusions: evidence from the Cuillin Igneous Complex, Isle of Skye, Scotland
abstract: Chrome-spinels from the layered Peridotilte Series of the unmetamorphosed, anorogenic 60 Ma Cuillin Igneous Complex, Isle of Skye, display a wide variety of compositions. Cumulus (within seams) chrome-spinels from the lowest exposed portion of the Peridotite Series exhibit features indicative of textural equilibrium, are rich in Al and Mg, and have low values of the ratio Cr/(Cr+Al). Cumulus chrome-spinels from higher up in the series are different from these: particularly, textural disequilibrium is evident, intercumulus plagioclase and olivine are present, and the chrome-spinels are rich in Cr, Fe and Ti, with high values of the ratio Cr/(Cr+Al). Intercumulus (dispersed) chrome-spinels tend towards anhedral forms and define enrichment trends towards Fe (both Fe2+ and Fe3+) with decreasing Mg, Cr and Al, and towards Al, with decreasing Fe2+ and Cr (and increasing Mg). Individual crystals are completely homogeneous and are devoid of reaction rims. The observed textural characteristics and compositional data of the chrome-spinels documented here suggest that the semi-quantitative peritectic reaction: aluminous chrome-spinel + melt→plagioclase + olivine + chromian chrome-spinel, is responsible for the observed parageneses, and that both the environment of crystallization (eumulus or intercumulus) and the role of plagioclase ±olivine crystallization are critical parameters for this geochemical trend in spinels within upper crustal magmatic systems. The effects of pyroxene crystallization on the development of this geochemical trend are also considered. This investigation highlights the need to consider the role of post-cumulus mineral-melt reactions and their influences upon the final compositions of major oxide and silicate phases within layered intrusions.

title: Postcumulus processes in layered intrusions
abstract: During the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1 %) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

title: The role of compositional convection in the formation of adcumulate rocks
abstract: The origins of cumulate rocks are re-examined in the light of new concepts in the fluid dynamics of crystallising systems. We propose that during the formation of adcumulate rocks, compositional convection allows continuous exchange of melt in the pores of a cumulus pile with the main magma reservoir. We consider this explanation to be physically more plausible than that proposed in the conventional cumulus theory. We present an experimental study of crystallisation in a porous medium to illustrate the process, and a theoretical analysis to demonstrate its applicability to magma chambers. We also consider factors which may inhibit convection and thus lead to the formation of meso- and orthocumulates. Finally we suggest how these ideas may be used to understand the distribution of these different rock types in layered intrusions.

title: Crystallization and compositional convection in a porous medium with application to layered igneous intrusions
abstract: Growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components, and vigorous convective motions in the fluid can result. We report a quantitative experimental study, in which saturated aqueous solutions of CuSO4 and Na2SO4 in porous media of either glass balls or CuSO4•5H2O crystals were crystallized by cooling from below. The crystals growing in the pore space released light, chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above. We explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium, the cooling rate, and the initial composition of the solution. This type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile. Adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front. We scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions. When our porous media had a small permeability, compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate. Trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically. Small, shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur.

title: Compositional convection in a reactive crystalline mush and melt differentiation
abstract: Fractional crystallization and partial melting involve relative motion of liquid and solid phases and chemical and thermal interactions between them. To elucidate some physical principles of thermosolutal convection in a reactive porous medium, we describe experiments on the directional solidification of aqueous ammonium chloride solutions. The addition of small amounts of a polymerizing agent permits variation of the solution viscosity independently of thermal conditions, the phase diagram, and permeability. Solutions were cooled from below, and crystallization developed at the base of the tank generating at first a field of thin plumes of light residual fluid, released from the boundary layer at the top of the “mush.” The interstitial fluid within the mush also became unstable eventually, the onset of convection occurring when the porous medium Rayleigh number of the mush reached a critical value. This threshold value was found to be 25 at low initial superheat and to decrease with increasing superheat. Local thermodynamic equilibrium between crystals and liquid within the mush coupled the evolution of temperature, composition, and porosity. Convective motions locally caused dissolution and precipitation, and hence fluctuations of porosity developed. Dissolution occurred preferentially in the central parts of upwellings, and the upflow gradually focused, being ultimately channelized into narrow “chimneys” devoid of crystals. All else being equal, the areal density of chimneys was greater the lower the viscosity. Chimney diameter increased with increasing solution viscosity. In the liquid above the growing mush, the convective plumes were very similar to salt fingers. Depending on solution viscosity and temperature gradient, they exhibited a phenomenon of collective instability such that vertical motion was disrupted by wave instabilities. The base plate temperature chosen was above the eutectic, and hence the total amount of crystals at the end of experiments with the same initial composition and the same final temperature was a constant fixed by the phase diagram. The spatial distribution of crystals and the final porosity of the mush were, however, determined by the strength of compositional convection as measured by the porous medium Rayleigh number. When no convection occurred, mush thickness finally became equal to the initial layer thickness, and the system was homogeneous. When compositional convection occurred, the overlying reservoir underwent chemical evolution, and mush growth slowed dramatically. In experiments with progressively lower solution viscosity (and hence more vigorous convection), final mush thickness was progressively less and final porosity lower. Final mush thickness was found to scale with solution viscosity to the power +0.33. During fractional crystallization of magma the effects of compositional convection can be recorded in the chemical and mineralogical features of cumulate rocks. We speculate that fossil chimney structures can be found in the Lower Zone of the Bushveld ultramafic complex in the form of iron-rich, platinum-bearing dunite pipes. This could explain the adcumulate nature of the Lower Zone rocks and the chemical and mineralogical similarities between the pipes and the overlying Merensky Reef; the Merensky Reef would be interpreted as a hiatus in mush growth. The results may also have applications to the flow structure in regions of partial melting and in the Earth's core.

title: Assessing the Role of Compaction in the Formation of Adcumulates: a Microstructural Perspective
abstract: The formation of adcumulates necessitates the continued growth of primocrysts down to low porosities. Gravitationally driven viscous compaction at the base of a crystal mushy layer on the magma chamber floor, driven by the weight of the mushy layer itself, is commonly suggested as a significant process acting to drive out interstitial liquid and promote adcumulate formation. Compaction necessitates viscous deformation, by either dislocation creep or diffusion-controlled processes such as pressure-solution: many studies suggest that the foliations preserved in cumulates are a consequence of recrystallization during compaction, completely overprinting primary magmatic fabrics. We test the compaction hypothesis by looking for microstructural evidence of viscous deformation. A detailed examination of cumulates from the Skaergaard intrusion, East Greenland, demonstrates only limited crystal plastic deformation, with no correlation between the extent of dislocation creep and the calculated volume fraction of trapped liquid left in the cumulates. Although the evidence for diffusion-controlled deformation is often cryptic, there is an anti-correlation between apparent aspect ratio of plagioclase and the extent of adcumulate crystallization, contradicting previous hypotheses involving transposition of original magmatic fabrics by dissolution–reprecipitation. This is supported by the spatial distribution of compositional zoning in plagioclase, which demonstrates that pressure-solution or related diffusion-controlled processes were insufficient to obscure primary magmatic fabrics. The Skaergaard adcumulates did not form by viscous compaction. Instead we suggest that they formed by primary processes involving mass transport in a thin mushy layer. Compaction is most likely to occur in slowly cooled intrusions in which the bulk magma crystallizes abundant dense minerals. We present preliminary observations of microstructures in norites from the lower Main Zone of the Bushveld Intrusion, South Africa, and in plagioclase-rich cumulates from the Fe–Ti oxide-rich Baima Intrusion, SW China. The evidence for dislocation creep in both intrusions is unambiguous, although deformation was insufficient to obliterate all traces of the primary magmatic fabrics and unlikely to have been sufficient to significantly reduce the volume of interstitial liquid.

title: Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion
abstract: The intercumulus liquid of a crystal mush fills pore spaces, and typically solidifies to form overgrowths on cumulus grains and poikilitic post-cumulus minerals. If the liquid is immobile, solidification produces zoned intercumulus minerals, as a result of progressive fractionation of the residual liquid. Convection within the mush results in buffering of the liquid composition, and thus limits mineral zonation. For fully solidified cumulates, fossil' changes in liquid composition or porosity are difficult to identify. However, detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material. Ti contents of plagioclase overgrowths, in samples from the lowermost parts of the Skaergaard Intrusion, show strong variations with anorthite content. With decreasing XAn, Ti concentrations first rise and then fall, consistent with changing TiO2 contents of the intercumulus liquid during solidification. TiO2 in plagioclase decreases sharply at An55, reflecting local saturation of Fe-Ti oxides. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is limited. Other than slight reverse zones that may occur on the plagioclase margins, XAn falls continuously during crystallization. The reverse zoning is interpreted as the result of compaction-driven dissolution and reprecipitation of plagioclase. The continual decrease in XAn is exploited, together with back-scattered electron images of the cumulates, to produce calibrated images showing regions of progressive crystallization. This allows the regions crystallizing at each stage of solidification to be visualized. These images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain-boundary melt films. This approach can provide information about the progressive reduction of porosity during cumulate solidification.

title: Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry
abstract: Olivine, orthopyroxene, and spinel compositions within seafloor peridotites yield important information about the nature of Earth’s mantle. Major element compositions of these minerals can be used to calculate oxygen fugacity, a thermodynamic property critical to understanding phase equilibria in the upper mantle. This study examines how hydrothermal alteration at the seafloor influences peridotite chemistry. The Tonga Trench (South Pacific Ocean) exposes lithospheric forearc peridotites that range from highly altered to completely unaltered and provides an ideal sample suite for investigating the effect of alteration on spinel peridotite major element chemistry and calculated oxygen fugacity. Using the Tonga peridotites, we develop a qualitative alteration scale rooted in traditional point-counting methodology. We show that high degrees of serpentinization do not affect mineral parameters such as forsterite number in olivine, iron site occupancy in orthopyroxene, and Fe 3+ /ΣFe ratio in spinel. Additionally, while serpentinization is a redox reaction that leaves behind an oxidized residue, the oxygen fugacity recorded by mantle minerals is unaffected by nearby low-temperature serpentinization. As a result, oxygen fugacity measured by spinel oxybarometry in seafloor peridotites is representative of mantle processes, rather than an artifact of late-stage seafloor alteration.

title: Oxide minerals in the Turnagain ultramafic complex, northwestern British Columbia
abstract: The oxide minerals in the Turnagain ultramafic complex, an Alaskan-type intrusion of possible Upper Triassic age in northwestern British Columbia, offer some important clues concerning the composition of magmas from which they separated and the chemical changes that they underwent during the cooling process. Chromite occurs as disseminations and vein-like concentrations in dunite, and as disseminations in wehrlite and olivine clinopyroxenite. Ilmenite is found in hornblendite. A small amount of primary magnetite is associated with intercumulus sulfide. Chrome spinel exhibits a very wide range of compositions. The ratio 100 Mg/(Mg + Fe2+) ranges from 69.7–3.6, and this is paralleled by a decrease in Cr2O3 (60.1–9.3%), and by increases in Fe2O3, TiO2, and MnO. Chromite rims are poorer in Cr and richer in Fe3+ than chromite cores. Magmatic differentiation was responsible for most of these variations. Subsolidus re-equilibration caused Mg/Fe2+ zoning in coexisting chromite and olivine, the ratio decreasing in...

title: Redox states of lithospheric and asthenospheric upper mantle
abstract: AbstractThe oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities (
$$f_{O_2 } $$
) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative
$$f_{O_2 } $$
 is predominantly controlled by sliding Fe3+-Fe2+ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing
$$f_{O_2 } $$
 with increasing refractoriness, to a relative
$$f_{O_2 } $$
 well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for
$$f_{O_2 } $$
 control. The solubility of carbon in H2O-rich fluid (and presumably in H2O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum
$$f_{O_2 } $$
 in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower
$$f_{O_2 } $$
 limit may be given by the H2O +C=CH4+O2 (C-H2O) equilibrium. Below C −H2O a fluid becomes CH4 rather than CO2-bearing and carbonatitic melt presumably unstable. The actual
$$f_{O_2 } $$
 in deep Archaean lithosphere is then a function of the activities of CO2 and MgCO3. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate
$$f_{O_2 } $$
 probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt
$$f_{O_2 } $$
 relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe3+/Fe2+ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform
$$f_{O_2 } $$
 shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along
$$f_{O_2 } $$
-pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative
$$f_{O_2 } $$
 will be constrained to below the CCO equilibrium and will be a function of
$$a_{CO_2 } $$
. Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along
$$f_{O_2 } $$
-decompression paths controlled by continuous Fe3+-Fe2+ solid-melt equilibria. These equilibria will involve increases in
$$f_{O_2 } $$
 relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.

title: Metasomatic interactions in the lithospheric mantle: petrologic evidence from the Lherz massif, French Pyrenees
abstract: To better understand the processes of metasomatism in the lithospheric mantle, we have undertaken a detailed petrologic study of wallrock adjacent to late stage mafic dikes at the Lherz massif. Traverse I contains a garnet-pyroxenite dike, along with hornblendite dikelets which cut harzburgite. Traverses II and III contain an anhydrous garnet-pyroxenite dike and a hornblendite dike in lherzolite, respectively. There is evidence for three successive metasomatic events in these samples: (1) crystallisation of Al-rich grain-boundary spinels ± associated phases from a silicate melt, (2) formation of apatite related to carbonatite metasomatism, (3) FeTi metasomatism related to the intrusion of garnet-pyroxenite dikes and hornblendite dikes. The first two events were diffuse in character, and although they penetrated a relatively large volume of peridotite, interaction with the peridotite was not pervasive. The result was the development of localised chemical gradients at the mm scale. The carbonatite metasomatism caused a net oxidation and chemical enrichment of the peridotite. Metasomatic effects related to the intrusion of the garnet-pyroxenite and hornblendite dikes are spatially restricted. Primary and secondary spinel and clinopyroxene compositions indicate that melt migrating from the hornblendite dikes was mostly responsible for pervasive metasomatism at the scale of Successive overprinting of one metasomatic event by another has led to the production of enriched peridotites with complex geochemical signatures. The variable extent of melt-peridotite interaction can lead to geochemical heterogeneities at a variety of length scales. Textural relations reveal little, if any, difference in the mechanism of infiltration of a silicate or carbonatite melt through harzburgite or lherzolite.

title: Peridotites and basalts reveal broad congruence between two independent records of mantle fO2 despite local redox heterogeneity
abstract: The oxygen fugacity (fO2) of the oceanic upper mantle has fundamental implications for the production of magmas and evolution of the Earth's interior and exterior. Mid-ocean ridge basalts and peridotites sample the oceanic upper mantle, and retain a record of oxygen fugacity. While fO2 has been calculated for mid-ocean ridge basalts worldwide (>200 locations), ridge peridotites have been comparatively less well studied (33 samples from 11 locations), and never in the same geographic location as basalts. In order to determine whether peridotites and basalts from mid-ocean ridges record congruent information about the fO2 of the Earth's interior, we analyzed 31 basalts and 41 peridotites from the Oblique Segment of the Southwest Indian Ridge. By measuring basalts and peridotites from the same ridge segment, we can compare samples with maximally similar petrogenetic histories. We project the composition and oxygen fugacity of each lithology back to source conditions, and evaluate the effects of factors such as subsolidus diffusion in peridotites and fractional crystallization in basalts. We find that, on average, basalts and peridotites from the Oblique Segment both reflect a source mantle very near the quartz–fayalite–magnetite (QFM) buffer. However, peridotites record a significantly wider range of values (nearly 3 orders of magnitude in fO2), with a single dredge recording a range in fO2 greater than that previously reported for mid-ocean ridge peridotites worldwide. This suggests that mantle fO2 may be heterogeneous on relatively short length scales, and that this heterogeneity may be obscured within aggregated basalt melts. We further suggest that the global peridotite fO2 dataset may not provide a representative sample of average basalt-source mantle. Our study motivates further investigation of the fO2 recorded by ridge peridotites, as peridotites record information about the fO2 of the Earth's interior that cannot be gleaned from analysis of basalts alone.

title: Closure temperature in cooling geochronological and petrological systems
abstract: AbstractClosure temperature (Tc) of a geochronological system may be defined as its temperature at the time corresponding to its apparent age. For thermally activated diffusion (D=Doe−E/RT it is given by 
$$T_c = R/[E ln (A \tau D_0 /a^2 )]$$
(i) in which R is the gas constant, E the activation energy, τ the time constant with which the diffusion coefficient D diminishes, a is a characteristic diffusion size, and A a numerical constant depending on geometry and decay constant of parent. The time constant τ is related to cooling rate by 
$$\tau = R/(Ed T^{ - 1} /dt) = - RT^2 /(Ed T/dt).$$
 (ii) Eq. (i) is exact only if T−1 increases linearly with time, but in practice a good approximation is obtained by relating τ to the slope of the cooling curve at Tc.If the decay of parent is very slow, compared with the cooling time constant, A is 55, 27, or 8.7 for volume diffusion from a sphere, cylinder or plane sheet respectively. Where the decay of parent is relatively fast, A takes lower values. Closure temperatures of 280–300° C are calculated for Rb-Sr dates on Alpine biotites from measured diffusion parameters, assuming a grain size of the order 0.5 mm.The temperature recorded by a “frozen” chemical system, in which a solid phase in contact with a large reservoir has cooled slowly from high temperatures, is formally identical with geochronological closure temperature.

title: Cations in olivine, Part 2: Diffusion in olivine xenocrysts, with applications to petrology and mineral physics
abstract: Diffusivities for calcium, iron, magnesium, manganese and aluminum have been measured for St. John's olivine undergoing cation exchange with synthetic basaltic melts. The variety of temperature, pressure and fO2 conditions under which the diffusivities were measured complement the equilibrium-partitioning study of calcium in olivine-bearing basalts by Jurewicz and Watson, 1988. Olivine was found to be anisotropic with respect to the diffusion of calcium, iron, magnesium and manganese. This anisotropy is a weak function of temperature, but strongly dependent upon oxygen fugacity.Because diffusion is independent of olivine composition over the small range of compositions used in this study, it could be shown that the absolute values of the diffusion coefficients were also functions of temperature and fO2. At near-atmospheric total pressure and an oxygen fugacity of 10−8atm, DFe>DMn>DCa and DMg≥DMn for a range of geologically reasonable temperatures. These relative diffusivities were shown to change with oxygen fugacity. The power-law dependence of diffusion on oxygen partial-pressure was determined for each cation and the results are consistent with the range of values given by Stocker (1978) and by other workers.For Ca and Fe, the effect of hydrostatic pressure on diffusion appears to be weak, at least for transport parallel to the c crystallographic direction. Unfortunately, no true activation volumes (or other pressure-related parameters) could be computed because the oxygen fugacity was not held constant over changes in pressure, and because accurate post-experiment reconstruction of sample orientation was not possible. Al was found to enter high-pressure olivines at concentrations of up to 0.14 weight percent, thus allowing aluminum diffusion to be characterized. The diffusivity of aluminum is, within error, the same as iron at 20 kb at 1430° C at the ambient fO2 of our piston-cylinder cells. This correspondence suggests that diffusion of Al may depend on transport of either Fe or of Fe +3 defects. While the results of these experiments are generally consistent with results published elsewhere, there are important inconsistencies. Tracer diffusion and interdiffusion in pure, ordered, olivine endmembers (e.g., tephroite and forsterite) showed significantly higher activation energies. This discrepancy could reflect the role of Fe+3 defects in diffusion; however, it may also suggest that order-disorder phenomena may be significant factors influencing diffusion in analog systems.The results of this study are applied to four petrologic problems: (1) calculation of rates of equilibration for olivine xenocrysts; (2) calculation of closure temperatures for the CaO/MgO olivine/basalt geothermometer (Jurewicz and Watson 1988); (3) delineation of an intrinsic-/O2 geobarometer; and (4) investigation of the dependence of olivine dissolution upon crystallographic orientation. In addition, it is demonstrated that diffusion-exchange experiments are useful for studying the dominant point-defect mechanisms for cation diffusion.

title: Diffusion in Oxides
abstract: Non-silicate oxide minerals are minor but important constituents of many igneous and metamorphic rocks, a major component of Earth’s lower mantle, and are well represented in planetary and meteoritic materials. Oxide minerals also have important roles in technology, for example as semiconductors, thermal and electrical insulators, fuel cell components, substrates for thin films, photovoltaic materials, and as products of metal oxidation. Because of their technological applications, and their fundamental interest to geosciences, materials science, physics and chemistry, the diffusion properties of many oxide minerals have been studied intensively, using a wide range of experimental, analytical and computational approaches. In many cases, particular attention has been devoted to deciphering the atomic-level mechanisms involved in diffusion. With the possible exceptions of metals and halides, oxides have probably been studied in more detail with regard to their point defect and diffusion properties than any other group of minerals.

The oxide minerals considered in this chapter are relatively simple in terms of their structure and chemistry, but nonetheless exhibit quite complicated diffusion behavior in many cases. Due to the simplicity of the minerals, and to the amount and quality of data available, the origin of many of these complicated behaviors is fairly well understood. In magnetite, for example, cation diffusion rates have a complex dependence on oxygen fugacity. This dependence is due to internal redox reactions, and to a transition from an interstitial diffusion mechanism at low oxygen fugacities to a vacancy mechanism under more oxidizing conditions. Oxygen and titanium diffusion rates in rutile also vary strongly with f O2, due to internal reduction of titanium and the associated production of oxygen vacancies and titanium interstitials. Some cations in rutile exhibit strong diffusional anisotropy, which is thought to result from rapid diffusion along interstitial “channels” that extend along the c direction in …

title: Distribution of Ferric Iron in some Upper-Mantle Assemblages
abstract: The distribution of ferric iron among the phases of uppermantle rocks, as a function of pressure (¥), temperature (T) and bulk composition, has been studied using Fe Mb'ssbauer spectroscopy to determine the Fe3*/lLFe ratios of mineral separates from 35 peridotite and pyroxenite samples. The whole-rock Ft complement of a peridotite is typically shared approximately evenly among the major anhydrous phases (spinel and/ or garnet, orthopyroxene and clinopyroxene), with the important exception ofolivine, which contains negligible Fe3*. Whole-rock Fe3* contents are independent oftheTand P of equilibration of the rock, but show a well-defined simple inverse correlation with the degree of depletion in a basaltic component. Fe in spinel and in both pyroxenes from the spinel lherzolite fades shows a positive correlation with temperature, presumably owing to the decrease in the modal abundance of spinel In garnet peridotites, the Fe3* in garnet increases markedly with increasing T and P, whereas that in clinopyroxene remains approximately constant. The complex nature of the partitioning of Fe between mantle phases results in complicated patterns of the activities of the Fe3*-bearing components, and thus in calculated equilibrium

title: SILICA ACTIVITY IN IGNEOUS ROCKS
abstract: In igneous rocks the occurrence of such mineral pairs as leucite and sanidine, or nepheline and albite indicates that silica activity may be buffered over a range of temperatures. Using published thermody'namic data, the variation of silica activity with temperature for these and other reactions has been calculated. Two bounding reactions are Mg2SiO{+SiO2 :2MgSiO: and CaTiOr*SiOz:CaTiSiOs; the former separates the tholeiitic magma series from alkali olivine-basalt and all its derivatives, whereas the latter separates the feldspar-present from the feldspar-free assemblages, and thus the nephelinites (sensulato) from the alkali olivine-basalt series' The lower limit of siiica activitf in silicate rocks is probably defined by the reaction KAlSiO4+SiOr: KAlSirO6. Throughout the whole range (7, X) of igneous rocks, silica activity varies by slightly more than two orders of magnitude; surprisingly, in the tholeiitic series (olivine tholeiite to quartz-rhyolite) silica activity may only increase from 10-0 15 to 10-0 10 over the range 1 150 to 850'C. The calculated melilite reactions fali in the feldspar-free region, and suggest that the feldspar-melilite antipathy in volcanic rocks is influenced by silica activity. In calciumrich pyroxenes AlzOa increases with decreasing as;s, until plagioclase ceases to precipitate, and in association with melilite, AlzOr in pyroxene declines' The sequence of o510, curves correspond above 1050oC with the desilication steps of the CIPW norml however two alternatives to these steps are proposed. The desilication of orttroclase should precede that of albite and the desilication of leucite should precede that of diopside.

title: Oxygen fugacity of basaltic magmas and the role of gas-forming elements
abstract: It is suggested that major variations in the relative oxygen fugacity of a basaltic magma are caused primarily by gas-forming elements, especially carbon and hydrogen. According to this theory, carbon, present in the source region of a basaltic magma, reduces the host magma during ascent, as isothermally carbon becomes more reducing with decreasing pressure. For an anhydrous magma such as lunar basalts, this reduction continues through the extrusive phase and the relative oxygen fugacity decreases rapidly until buffered by the precipitation of a metallic phase. For hydrous magmas such as terrestrial basalts, reduction by carbon is eventually superceded by oxidation due to loss of H2 generated by the reaction of C with H2O and by thermal dissociation of H2O. The relative oxygen fugacity of a hydrous magma initially decreases as a magma ascends from the source region and then increases until magnetite crystallization curbs the rising trend of the relative oxygen fugacity.

title: The effect of oxygen fugacity on the redox state of natural liquids and their crystallizing phases
abstract: For magmas cooling under conditions such that their oxygen contents are held fixed, or are subject to only minor loss or gain, there are several generalizations that are evident. If the early crystallizing phase has a lower redox state than that of the liquid, for example an olivine phenocryst in a tholeiitic liquid in an oceanic environment, then as temperature decreases, the progressive increase of ferric iron and sulphur in the liquid will induce the formation of a spinel phase, and an immiscible sulfide liquid. The latter Fe-S-O liquid may contain more O than S at FMQ (Doyle and Naldrett, 1987), providing an adequate reservoir for the oxygen needed to sustain a tholeiitic liquid line of descent on the FMQ buffer (Ghiorso and Carmichael, 1985). In combination, the continued formation of olivine, spinel and sulfide liquid, will regulate the iron redox state of the residual liquid, so that it may resemble a buffered path. Addition or subtraction of oxygen to the system (liquid plus solids) at constant temperature will initially change the proportions of the solids. But should the oxygen fugacity change, then the iron redox ratio in the crystalline phases (and the liquid) will also change. In those highly oxidized natural liquids whose iron redox ratio is greater than that of the precipitating spinel and rhombohedral oxide assemblage, then ultimately a representative of the pseudobrookite series will form, and thereafter the liquid line of descent involving a progressive increase in Fe 2 O 3 may cease. The assemblage of a rhombohedral (FeTiO 3 -Fe 2 O 3 ) solid solution and a pseudobrookite solid solution will have the effect of constraining the iron redox ratio in the liquid, and at constant temperature will act as a buffer to small changes in the oxygen content of the system. Again if the oxygen fugacity changes, at constant temperature, then the solid phases must either change in composition by changing their individual redox ratios, or one will disappear. Thus one problem of modelling a general liquid line of descent essentially hinges on the relationship between oxygen fugacity and the iron redox ratio of the phases that enter above the solidus. For natural silicate liquids this relationship is well known, but it has yet to be established for any ferromagnesian silicate solid solution series. Today the modelled crystallization path has to constrain the composition of the ferromagnesian minerals to their idealized composition, and for tholeiitic magmas this seems reasonably successful. For more alkaline-rich magmas, such as olivine-basalt or more silica-poor varieties, augite can show a large range in composition but experimental calibration of this is sadly lacking. Of additional concern is the fact that in these magmas a sulfide phase can even occur in combination with a sulphate-bearing solid phase (Luhr et al., 1984). The "bottom line" is that a vast experimental effort is required to establish the connection between composition, iron redox state, temperature and oxygen fugacity for the ferromagnesian minerals, particularly augite, biotite, the igneous amphiboles, in equilibrium with liquids.

title: Experimentally Derived Intermediate to Silica-rich Arc Magmas by Fractional and Equilibrium Crystallization at 1·0 GPa: an Evaluation of Phase Relationships, Compositions, Liquid Lines of Descent and Oxygen Fugacity
abstract: Differentiation of hydrous primary, mantle-derived magmas is a fundamental process to generate evolved intermediate to SiO2-rich compositions forming the bulk of the continental and island arc crust. This study focuses on the results of equilibrium and fractional crystallization experiments at 1 0 GPa using two different primary magmas representing deep (90 km) and shallow (35 km) mantle extraction depths. Experiments on a hydrous high-Mg basalt were conducted at graphite-saturated and more oxidized conditions (NNO to NNOþ 2, where NNO is nickel–nickel oxide buffer) to exploit the influence of fO2 on phase assemblages and the evolution of derivative liquids. The liquid line of descent (LLD) was simulated from liquidus to near-solidus conditions ranging from 1330 C to 720 C. H2O contents varied from about 2 0 to more than 10 wt %. The LLD covers the entire compositional range from high-Mg basalt to high-silica rhyolite and evolves from metaluminous to peraluminous compositions at 56–60 wt % SiO2 under oxidizing conditions. The observed crystallization sequences and the LLD reveal contrasting behavior depending on oxidation state, H2O content and equilibrium versus fractional crystallization. Equilibrium crystallization of high-Mg basalt under reducing conditions is initially dominated by olivine fractionation followed by Cr-rich spinel, clinopyroxene (cpx), and orthopyroxene (opx). Finally, between 1060 and 1000 C, amphibole formed by a peritectic reaction consuming cpxþolivine and forming amphiboleþopx, resulting in 16% silica-undersaturated trachy-basaltic liquid. Equilibrium crystallization of the same composition under oxidizing conditions is characterized by strongly enhanced olivine and cpx fractionation and suppression of opx only occurring as a result of the peritectic amphibole-forming reaction at and below 1040 C. The liquid at 980 C is a peraluminous, alkali-poor, high-Al andesite representing 15% residual liquid. Fractional crystallization of the high-Mg basalt under oxidizing conditions evolves through fractionation of early olivine joined by cpx at 1200 C, followed by opx and hercynitic spinel (1140–1080 C) and amphibole at 1050 C coexisting with cpx (and spinel) to 980 C. At 950 C both garnet and plagioclase (plag) join amphibole as liquidus phases. This paragenesis (plus ilmenite and apatite) persists to 750 C with 16% residual liquid relative to the initial basaltic composition. Liquids evolve continuously from basalt to rhyolite, crossing the metaluminous–peraluminous boundary at about 60 wt % SiO2. Fractional crystallization of the basaltic andesite starting material differs at high temperature, where opx and cpx are the liquidus phases (1200–1080 C), followed by amphibole at the expense of opx. Below VC The Author(s) 2018.  2018, Vol. 59, No. 1, 11–58  February 2018

title: On the conditions for lower crustal convective instability
abstract: Continental crust is thought to be formed as a result of arc magmatism, but many of the lavas produced in these settings are basaltic, while those that are silicic are commonly evolved, with lower Mg #s than the continental crust. The bulk composition of continental crust can be produced by mixing of end-member basaltic and silicic compositions, via magma mixing or in mechanical, tectonic juxtaposition, but some process is required to remove the cumulates and residues formed during generation of the silicic, “granitic” end-member. We consider convective instability of dense mafic and ultramafic lower crust as a means to remove mafic residues of basalt differentiation in order to produce end-member compositions that can mix to form the bulk composition of the continental crust. Using a range of lower crustal and mantle bulk compositions, ranging from mafic and ultramafic cumulates to primary liquid compositions, we calculated the subsolidus phase assemblage and resulting density. The results show that densities of likely lower crustal lithologies can exceed those of the mantle (by ∼50–250 kg m−3), but the density contrast is a strong function of composition, temperature, and pressure. For a “cold” geotherm with a Moho temperature of 300°C, relevant to cratonic settings, densities of all of the lower crustal compositions that we considered, except granulite, exceed the density of the underlying mantle at pressures as low as 0.8 GPa. For a “hot” geotherm with a Moho temperature in the range of 800–1000°C, the density of the lower crust is much more variable, with gabbroic and granulite compositions having lower densities than the mantle, while “arc gabbronorite” and ultramafic cumulate compositions having higher densities than the mantle at pressures similar to that for the cold geotherm. Instability times calculated for a two-dimensional Rayleigh-Taylor convective instability, where a dense lower crustal layer sinks into a lower-density mantle, show that high temperatures (>700°C, or >500°C with a background strain rate) are required for this process to occur on a timescale of 10 Myr with rheological parameters expected for the crust and mantle. The high temperature required for dense lower crustal mafic-ultramafic cumulates to sink into the mantle suggests that this process is restricted to arcs, volcanic rifted margins, and continental regions that are undergoing extension, are underlain by a mantle plume, or have had part of the conductive upper mantle removed.

title: The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states
abstract: Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.

title: Construction of the granitoid crust of an island arc. Part II: a quantitative petrogenetic model
abstract: Results of simple model calculations that integrate cumulate compositions from the Kohistan arc terrain are presented in order to develop a consistent petrogenetic model to explain the Kohistan island arc granitoids. The model allows a quantitative approximation of the possible relative roles of fractional crystallization and assimilation to explain the silica-rich upper crust composition of oceanic arcs. Depending in detail on the parental magma composition hydrous moderate-to-high pressure fractional crystallization in the lower crust/upper mantle is an adequate upper continental crust forming mechanism in terms of volume and compositions. Accordingly, assimilation and partial melting in the lower crust is not per se a necessary process to explain island arc granitoids. However, deriving few percent of melts using low degree of dehydration melting is a crucial process to produce volumetrically important amounts of upper continental crust from silica-poorer parental magmas. Even though the model can explain the silica-rich upper crustal composition of the Kohistan, the fractionation model does not predict the accepted composition of the bulk continental crust. This finding supports the idea that additional crustal refining mechanism (e.g., delamination of lower crustal rocks) and/or non-cogenetic magmatic process were critical to create the bulk continental crust composition.

title: Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle
abstract: Quaternary hornblende andesites have erupted along a fault zone near El Penon, central Mexico. One of these flows contains 1‐2 cm xenoliths of amphibole-rich spinel lherzolite and chromite websterite. These xenoliths are rare samples of subarc upper mantle from a region of continuing subduction, and they are the most oxidized mantle peridotites yet described (fayalitemagnetite-quartz +1.5 to 2.4). The abundant amphibole and high oxygen fugacities that characterize these xenoliths are significant because they provide direct evidence for metasomatism of the mantle wedge by slab-derived fluids. Phenocrysts of hornblende and lack of plagioclase phenocrysts in the host andesite indicate that it equilibrated with high water content (>8 wt%), and the presence of xenoliths implies rapid (26 km/day) ascent.

title: Melt‐mantle interactions beneath the Kamchatka arc: Evidence from ultramafic xenoliths from Shiveluch volcano
abstract: Ultramafic xenoliths of spinel dunite, harzburgite, lherzolite, amphibole/phlogopite-bearing pyroxenite, and clinopyroxenite occur in andesitic pyroclastic debris from the 1964 eruption of Shiveluch volcano, Kamchatka. Peridotites have coarse/protogranular, porphyroclastic, and granuloblastic textures, abundant kink-banded olivine, and refractory mineral compositions with forsteritic olivine (Fo88–94) and Cr-rich spinel (100*Cr/Cr + Al = 47–83). Orthopyroxene (opx) is also Mg-rich but occurs only as a fibrous mineral present along olivine grain boundaries, in monomineralic veins that crosscut coarse olivine, and in veins with amphibole and phlogopite that crosscut coarse-grained peridotites. Textural evidence and mineral compositions indicate that the peridotites and hydrous pyroxenites were replacive dunites that formed by melt-rock reactions involving the dissolution of pyroxene and precipitation of olivine. The fibrous opx and millimeter-scale veins of phlogopite, amphibole, and opx are interpreted as the autometasomatic products of hydrous magmas that were trapped in the uppermost mantle (<45 km). In this interpretation, opx was produced by reaction between late-stage, silica-rich, hydrous fluids/melts and olivine in the dunite protolith, and the millimeter-scale veins of phlogopite, amphibole, and opx are the volatile-enriched, deuteric products that were liberated during the final stages of magma crystallization. The absence of textural equilibrium suggests that the late-stage replacement process which produced the fibrous opx occurred shortly prior to the eruption that carried the xenoliths to the surface. On the basis of two-pyroxene thermometry and Ca-in-olivine barometry, the xenoliths equilibrated between 800 and 1000°C and 1.03 and 2.21 GPa. This implies that the xenoliths were carried from sub-Moho depths, a result consistent with geophysical estimates of crustal thickness. Olivine-opx-spinel equilibria indicate that the xenoliths are strongly oxidized with fO2 from +1.4–2.6 log units above the fayalite-magnetite-quartz (ΔFMQ) buffer in peridotites, +1.7–2.3 ΔFMQ in hydrous pyroxenites, and +2.4–3.3 ΔFMQ in cumulate clinopyroxenites. High fO2 in the peridotites is attributed to the melt-rock reactions that formed the dunite protolith. These results therefore suggest that interaction between oxidized melts and peridotite wall rock at shallow mantle depths plays a significant role in creating and modifying the uppermost mantle and deepest crust in some subduction settings.

title: Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review
abstract: The petrological characteristics of peridotite xenoliths exhumed from the lithospheric mantle below the Western Pacific arcs (Kamchatka, NE Japan, SW Japan, Luzon-Taiwan, New Ireland and Vanuatu) are reviewed to obtain an overview of the supra-subduction zone mantle in mature subduction systems. These data are then compared with those for peridotite xenoliths from recent or older arcs described in the literature (e.g. New Britain, Western Canada to USA, Central Mexico, Patagonia, Lesser Antilles and Pannonian Basin) to establish a petrological model of the lithospheric mantle beneath the arc. In currently active volcanic arcs, the degree of partial melting recorded in the peridotites appears to decrease away from the fore-arc towards the back-arc region. Highly depleted harzburgites, more depleted than abyssal harzburgites, occur only in the frontal arc to fore-arc region. The degree of depletion increases again to a degree similar to that of the most depleted abyssal harzburgites within the back-arc extensional region, whether or not a back-arc basin is developed. Metasomatism is most prominent beneath the volcanic front, where the magma production rate is highest; silica enrichment, involving the metasomatic formation of secondary orthopyroxene at the expense of olivine, is important in this region because of the addition of slab-derived siliceous fluids. Some apparently primary orthopyroxenes, such as those in harzburgites from the Lesser Antilles arc, could possibly be of this secondary paragenesis but have been recrystallized such that the replacement texture is lost. The Ti content of hydrous minerals is relatively low in the sub-arc lithospheric mantle peridotites. The K/Na ratio of the metasomatic hydrous minerals decreases rearward from the fore-arc mantle as well as downward within the lithospheric mantle. The lithospheric mantle wedge peridotites, especially metasomatized ones from below the volcanic front, are highly oxidized. Shearing of the mantle wedge is expected beneath the volcanic front, and is represented by fine-grained peridotite xenoliths.

title: Iron Isotope Fractionation and the Oxygen Fugacity of the Mantle
abstract: The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (δ57/54Fe) of mantle spinels. Spinel δ57/54Fe values correlate with relative oxygen fugacity, Fe3+/ΣFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling.

title: Voluminous granitic magmas from common basaltic sources
abstract: Granitic—rhyolitic liquids were produced experimentally from moderately hydrous (1.7–2.3 wt% H2O) medium-to-high K basaltic compositions at 700 MPa and fO2 controlled from Ni-NiO −1.3 to +4. Amount and composition of evolved liquids and coexisting mineral assemblages vary with fO2 and temperature, with melt being more evolved at higher fO2s, where coexisting mineral assemblages are more plagioclase- and Fe–Ti oxide-rich and amphibole-poor. At fO2 of Ni–NiO +1, typical for many silicic magmas, the samples produce 12–25 wt% granitic–rhyolitic liquid, amounts varying with bulk composition. Medium-to-high K basalts are common in subduction-related magmatic arcs, and near-solidus true granite or rhyolite liquids can form widely, and in geologically significant quantities, by advanced crystallization–differentiation or by low-degree partial remelting of mantle-derived basaltic sources. Previously differentiated or weathered materials may be involved in generating specific felsic magmas, but are not required for such magmas to be voluminous or to have the K-rich granitic compositions typical of the upper continental crust.

title: Ti-V plots and the petrogenesis of modern and ophiolitic lavas
abstract: Plots of Ti vs. V for many modern volcanic rock associations are diagnostic of tectonic setting and can be used to determine possible tectonic settings of ophiolites as well. The basis of this plot is the variation in the crystal/liquid partition coefficients for vanadium, which range with increasing oxygen fugacity from > 1 to 50. Back-arc basin basalts may have either arc-like or MORB-like Ti/V ratios, and sample suites from single back-arc basins may have Ti/V ratios ranging from 10 to 50. This range in Ti/V ratios in samples from a restricted geographical area may be diagnostic of the back-arc setting. The Ti/V plot is applied here to published data on ophiolites from a variety of postulated settings and in general supports the conclusions of previous investigators. Ophiolites from the western Mediterranean (Corsica, northern Apennines) and the "lower" Karmoy volcanics have Ti/V trends similar to MORB; the "upper" Karmo'y volcanics have alkaline Ti/V ratios. Lavas and tonalites in the Papuan ultramafic belt, the high-Mg andesites of Cape Vogel, and the upper pillow lavas at Troodos all have Ti/V ratios < 20, consistent with formation in an island arc setting. More specific evaluation of the tectonic setting of these and other ophiolites requires application of detailed geologic and petrologic data as well as geochemistry. The Ti/V discrimination diagram, however, is a potentially powerful adjunct to these techniques.

title: Vanadium partitioning and the oxidation state of Archaean komatiite magmas
abstract: The MgO content of komatiite lavas is an important measure of their formation temperature deep in the Archaean mantle, and forms the basis for models of the early Earth's thermal and chemical evolution. Estimates of the primary MgO content of komatiites are sensitive to the oxidation state—characterized by the oxygen fugacity (fO2)—assumed for the magmas during their crystallization. Despite two decades of study, however, fO2 is still poorly constrained for these lavas. Here I present an estimate of the fO2 for komatiite flows, based on the systematics of vanadium partitioning between komatiitic liquid and olivine in six well-characterized komatiite flows of varying ages. This approach shows that the oxidation state of several of these Archaean lava flows was the same as, or possibly more oxidizing than, that of present-day oceanic basalts. These results may require a downward revision of the mantle melting temperature estimated for many komatiites by about 50 °C, and suggest that the mantle was unlikely to be much less oxidized during the Archaean era than at present.

title: Clinopyroxene-liquid partitioning for vanadium and the oxygen fugacity during formation of cratonic and oceanic mantle lithosphere
abstract: The distribution coefficient for vanadium Dvcpx/liq between clinopyroxene and silicate liquid in the system Di-Ab-An has been measured at trace levels as a function of oxygen fugacity (ƒO2) at 100 kPa and 2 GPa. With increasing ƒO2 vanadium changes from compatible to incompatible in clinopyroxene at about one log ƒO2 unit below the nickel-nickel oxide (NNO) buffer. The trend in the Di-Ab-An system represents a reasonable upper limit for Dvcpx/liq versus ƒO2 in natural systems. The Dvcpx/liq versus fo2 systematics from this study are combined into a melting model to evaluate the V budget during partial melting in the mantle. It is shown that fo2 has a very measurable effect on bulk Dvsource/liq along the spinel peridotite solidus, and at 7 GPa along the garnet peridotite solidus. Isobaric melting models are formulated and used to examine the distinctly different covariation of V with degree of depletion in mantle lithosphere from various tectonic settings. The spectrum of oceanic lithosphere can be explained by partial melting in the spinel peridotite stability field at an fo2 of ∼NNO-3, identical to that recorded by midocean ridge basalts. Cratonic peridotites are not well explained by melting in the garnet peridotite stability field at any fo2 but this may be a shortcoming of both the isobaric melting model and the poor constraints for Dvgt/liq with fo2. If cratonic peridotites formed by depletion in the spinel stability field, then fo2 during melting was above ∼NNO-2.5, requiring that mantle lithosphere in the Archean formed under more oxidizing conditions than hitherto realized. The V abundances of cratonic peridotites are collinear with subduction zone peridotites, possibly linking the mantle roots beneath Archean cratons to a more oxidized subduction zone setting.

title: Vanadium in peridotites, mantle redox and tectonic environments: Archean to present
abstract: New measurements of partition of vanadium (V) between spinel, garnet and a pigeonite-like high pressure (P) pyroxene and magnesian liquids on the mantle solidus as a function of oxygen fugacity (fO2) are presented. The spinel-liquid experiments show an effect of Cr/Al in spinel on partitioning, and further suggest that V exists as V3+, V4+ and V5+ in melts at terrestrial fO2. Vanadium is mildly incompatible in both ‘pigeonitic’ pyroxene and garnet between 4.5 and 6.5 GPa on the mantle solidus. Analysis of the fO2-sensitive partitioning for V between mantle minerals and melts are combined with compositional data from peridotite melting experiments to model the covariation of V and Al in peridotite residues produced by non-modal, fractional melting under different fO2 from 1.0 to 7.0 GPa. The partial melting models at 1.0 to 3.0 GPa fit the covariation of V and Al in abyssal peridotites quite well at fO2s similar to those of mid-ocean ridge basalt. Many orogenic massifs and spinel lherzolite xenoliths represent mantle that formed at fO2 higher than that produced at mid-ocean ridges in a range of tectonic environments. A large proportion of spinel-facies Archean cratonic lithosphere formed at fO2s significantly higher than those of abyssal peridotites possibly linking its formation to a convergent margin (arc) tectonic setting. The case for garnet-facies cratonic mantle is equivocal; it may have formed at significantly higher pressures (7.0 GPa), or within the spinel-facies at lower pressures but at significantly higher fO2 than is observed for abyssal peridotites. The imbrication of both oceanic garnet-facies mantle with spinel-facies arc mantle may explain the datasets for some Archean cratons. Overall, the data for Archean mantle melts and residues make clear that models cannot look to reduced, mantle-derived volcanic gases containing H2 and CO to engender early life synthesis, or to promote hydrogen escape and gradual oxygenation of the Archean earth system.

title: Calibration of an Empirical Thermometer and Oxybarometer based on the Partitioning of Sc, Y and V between Olivine and Silicate Melt
abstract: The evolution of basaltic magmas depends on their redox state, hence oxygen fugacity, but there is increasing evidence that this intensive thermodynamic variable may be less well understood in basalts than commonly supposed. The redox state of terrestrial basalts has to a large extent been inferred from the Fe3+/Fe2+ ratios of their quenched glasses. However, this quantity appears to be significantly affected during late and post-eruptive processes in magmatic systems (e.g. by degassing, charge-transfer reactions of redox-variable species, and alteration), so that the degree to which the Fe3+/Fe2+ ratios preserved in basaltic glasses reflect the oxidation state of the magma at high temperature is unclear. Because olivine is the first silicate mineral to crystallize from primitive basaltic liquids on cooling following decompression, the equilibrium partitioning relations preserved in olivine phenocrysts in basalts are, in principle, less disturbed by these late and post-eruptive processes and, therefore, may better reflect the high-temperature (pre-eruptive) conditions of the magma. Here we calibrate an oxybarometer based on the strong sensitivity of the partitioning of vanadium between olivine and silicate melt to oxygen fugacity. Our empirical parameterization, calibrated over a range of redox conditions between four log(10) units above and below the quartz-fayalite-magnetite (QFM) oxygen buffer, takes into account the effects of temperature, olivine composition (i.e. Mg/Fe ratios) and melt composition (namely the activities of CaO, SiO2, AlO1 center dot 5, NaO0 center dot 5 and KO0 center dot 5), and allows oxygen fugacity determinations to within similar to 0 center dot 25 log(10) units. We also explore the sensitivity of the exchange partitioning of Sc and Y between olivine and melt to temperature as a geothermometer. Our calibration indicates that this geothermometer allows temperature to be estimated to within 15 degrees C, but precision is strongly dependent on the Sc and Y measurements in olivine and melt.

title: Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels
abstract: Igneous island arc rocks are more oxidized than midocean-ridge basalts. Whether this originates from an oxidized mantle source or during differentiation or eruption is debated. Newly compiled Fe3+/ΣFe and V/Sc ratios presented here indicate that island arc rocks became more oxidized 800–400 Ma. We attribute this increase in the redox state of island arc rocks to the oxidation of their mantle source from subduction of oxidized oceanic crust following deep-ocean oxygenation and increased marine sulfate concentrations over the same time interval. This provides evidence that the oxidized nature of modern island arc magmas is due to an oxidized mantle source and represents a rare example of a change in the surface biogeochemistry influencing the igneous rock record. A rise in atmospheric O2 levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2 and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth’s surface to the subarc mantle. It also indicates that the rise of atmospheric O2 and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

title: Stable vanadium isotopes as a redox proxy in magmatic systems
abstract: JP was funded by NERC postdoctoral fellowship NE/H01313X/2, with support of the Oxford laboratories from an Advanced ERC grant (NEWISOTOPEGEOSCIENCE) to ANH. PAS by an APA PhD scholarship and ANU Vice-Chancellor’s Scholarship.

title: Melting of mantle peridotite at pressures approaching the spinel to garnet transition: Application to mid‐ocean ridge basalt petrogenesis
abstract: This paper reports experiments carried out between 1.5 and 2.3 GPa using synthetic analogs of the molten peridotite system to investigate the effects of pressure (P), temperature (T), and variable bulk chemistry on the composition of melts multiply saturated with the minerals present in the upper oceanic mantle: olivine, orthopyroxene, augite, and spinel ( garnet). The multidimensional surface defined by melts coexisting with the lherzolite mineral assemblage produced in this study as well as from the literature is fit as a function of the variables: P, melt Mg number, and melt mole percent NaO 0.5 KO 0.5 , Cr 2 O 3 , and TiO 2 , using multiple regression techniques. Forward models of polybaric, near-fractional melting using the parameterization presented here demonstrate that small extent, pooled magmas produced at greater initial pressures of melting differ in composition from small extent, pooled magmas produced at lower initial pressures of melting. Furthermore, the small extent, pooled magmas generated at shallower initial pressures of melting fractionate at low P to magmas that are similar to the high-Na 2 O, low-FeO mid-ocean ridge basalts of the global array of Klein and Langmuir [1987], while the small extent, pooled magmas generated at greater initial pressures of melting fractionate at low P to magmas that do not resemble any mid-ocean ridge basalts present in the global data set. Preliminary data relevant for melting garnet-lherzolite indicate that the systematics of the key major element indicators of depth of melting (FeO and SiO 2 in the melt) observed in melts generated in the spinel stability field persist in melts generated in the lower P portion of the garnet stability field.

title: Bulk-rock Major and Trace Element Compositions of Abyssal Peridotites: Implications for Mantle Melting, Melt Extraction and Post-melting Processes Beneath Mid-Ocean Ridges
abstract: This paper presents the first comprehensive major and trace element data for � 130 abyssal peridotite samples from the Pacific and Indian ocean ridge–transform systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. Although abyssal peridotites are serpentinized, and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REE) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from previous melt extraction events from the fertile mantle source. Light REE (LREE) in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyroxenes (cpx) of the same sample suites. If the cpx LREE record sub-ridge mantle melting processes, then the bulk-rock LREE must reflect post-melting refertilization. The significant correlations of LREE (e.g. La, Ce, Pr, Nd) with immobile high field strength elements (HFSE, e.g. Nb and Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process. The refertilization takes place in the ‘cold’ thermal boundary layer (TBL) beneath ridges through which the ascending melts migrate and interact with the advanced residues. The refertilization apparently did not affect the cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally ‘reactive’, and thus may have affected only cpx rims, which, together with precipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are observed, which are unexpected. The correlation between the two ratios is consistent with observations on basalts that DZr/DHf < 1 and DNb/DTa < 1.

title: The role of water in the petrogenesis of Mariana trough magmas
abstract: Most variations in composition among primitive basalts from the Mariana back-arc trough can be explained by melting mixtures of an NMORB-type mantle source and an H_2O-rich component, provided the degree of melting is positively and approximately linearly correlated with the proportion of the H_2O-rich component in the mixture. We conclude that the degrees of melting by which Mariana trough magmas are generated increase from magmas similar to NMORB, through more H_2O-enriched basalts, to ‘arc-like’ basalts, and that this increase is due to the lowering of the solidus of mantle peridotite that accompanies addition of the H_2O-rich component is likely to be ultimately derived from fluid from a subducting slab, but we propose that by the time fluids reach the source regions of Mariana trough basalts, they have interacted with sufficient mantle material that for all but the most incompatible of elements (with respect to fluid-mantle interaction), they are in equilibrium with the mantle. In contrast, fluids added to the source regions of Mariana island-arc magmas have typically interacted with less mantle and thus retain the signature of slab-derived fluids to varying degrees for all but the most compatible elements. Primitive Mariana arc basalts can be generated by melting mixtures of such incompletely exchanged slab-derived fluids and sources similar to NMORB-type mantle sources, but the degrees of melting are typically higher than those of Mariana trough NMORB and the sources have been variably depleted relative to the back-arc sources by previous melt extraction. This depletion may be related to earlier extraction of back-arc basin magmas or may evolve by repeated fluxing of the sources as fluid is continually added to them in the regions of arc magma generation. If fluid with partitioning behavior relative to the solid mantle similar to that deduced for the H_2O-rich component involved in the generation of Mariana trough basalts were extracted from primitive mantle, the residual mantle would have many of the minor and trace element characteristics of typical oceanic upper mantle; primitive mantle enriched in such fluid would be a satisfactory source for the continental crust in terms of its trace and minor element chemical composition.

title: Mantle melting as a function of water content beneath back-arc basins
abstract: Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings.

title: High field strength and transition element systematics in island arc and back-arc basin basalts: Evidence for multi-phase melt extraction and a depleted mantle wedge
abstract: The geochemical character of the mantle wedge in convergent margin settings is investigated using the high field strength and transition elements Ti, Zr, V, Sc and Y in basaltic rocks from island arcs and associated back-arc basins. Systematic differences are observed in the composition of each arc relative to the adjacent back-arc basin. The arcs have lower average abundances of incompatible high field strength elements and Y, and extend to higher ratios of Ti/Zr, V/Ti and Sc/Y. Partial melting of a normal mid-ocean ridge basalt source accounts for the back-arc basin basalt compositions, but cannot explain the higher element ratios observed in many arc basalts. These arc magmas are consistent with derivation from sources which are more depleted in incompatible elements those that of the back-arc basin basalts. Source depletion by melt extraction, prior to arc magma genesis, explains the similar degree of Y depletion and, by analogy, the heavy REE depletion, to that of the high field strength elements in island arc basalts. The observations are not compatible with the presence of residual titanate phases (e.g. rutile). However, amphibole may be required in the source of a subgroup of arc basalts which have relatively high Zr abundances and anomalously low Ti/Zr and high V/Ti values. These results have implications for subduction zone geochemical budgets, as the influence of any slab-derived component may be enhanced in regions of extensive mantle wedge depletion.

title: The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California
abstract: This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from <1 to >8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and εNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and εNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).

title: Redox-variability and controls in subduction zones from an iron-isotope perspective
abstract: An ongoing controversy in mantle geochemistry concerns the oxidation state of the sources of island arc lavas (IAL). Three key factors control oxidation–reduction (redox) of IAL sources: (i) metasomatism of the mantle wedge by fluids and/or melts, liberated from the underlying subducted slab; (ii) the oxidation state of the wedge prior to melting and metasomatism; and (iii) the loss of melt from IAL sources. Subsequently, magmatic differentiation by fractional crystallisation, possible crustal contamination and degassing of melts en route to and at the surface can further modify the redox states of IAL. The remote nature of sub-arc processes and the complex interplay between them render direct investigations difficult. However, a possible gauge for redox-controlled, high-temperature pre-eruptive differentiation conditions is variations in stable Fe isotope compositions (expressed here as δ 57 Fe) in erupting IAL because Fe isotopes can preserve a record of sub-surface mass transfer reactions involving the major element Fe. Here we report Fe isotope compositions of bulk IAL along the active Banda arc, Indonesia, which is well known for a prominent subducted sediment input. In conjunction with other arc rocks, δ 57 Fe in erupted Banda IAL indicates that fractional crystallisation and possibly crustal contamination primarily control their Fe isotope signatures. When corrected for fractional crystallisation and filtered for contamination, arc magmas that had variable sediment melt contributions in their sources show no resolvable co-variation of δ 57 Fe with radiogenic isotope tracers. This indicates that crustal recycling in the form of subducted sediment does not alter the Fe isotope character of arc lavas, in agreement with mass balance estimates. Primitive sources of IAL, however, are clearly isotopically lighter than those sourced beneath mid-ocean ridges, indicating either preferential Fe 3+ -depletion in the mantle wedge by prior, δ 57 Fe-heavy melt extraction, and/or addition of an isotopically-light slab-derived agent. Based on our findings and previous models of arc redox conditions, we propose a three-stage process to explain the Fe isotope composition of IAL: (i) prior melt depletion lowers Fe 3+ /ΣFe (Fe 3+ over total Fe) in the residues, leaving refractory, δ 57 Fe-light and possibly reduced mantle wedge protoliths beneath arcs. The oxygen fugacity ( f O 2 ) of these refractory protoliths may be up to −2 log 10 units reduced relative to the fayalite–magnetite–quartz synthetic oxygen buffer (ΔFMQ); (ii) oxidised, slab-derived fluids, Fe-poor but possibly rich in sulphate (i.e., S 6+ ), trigger melting of depleted protoliths with minimal effect on δ 57 Fe. Melts derived from this fluid-modified wedge source have high Fe 3+ /ΣFe, oxidised by the reduction of S 6+ , but importantly retain the light δ 57 Fe from their mantle wedge source; (iii) after melt liberation from the mantle wedge, arc magmas initially become progressively oxidised and isotopically heavier in Fe through fractional crystallisation of ferromagnesian silicates. In summary, reduction consequent to Fe 3+ -rich melt extraction and subsequent oxidation, likely by S 6+ -rich fluids, results in a “redox yo-yo” in IAL sources. Fractional crystallisation will further oxidise and elevate δ 57 Fe in erupting IAL. Iron isotope signatures in IAL record a complex magmatic history with no simple relation between δ 57 Fe and calculated f O 2 in erupted lavas. Records of higher f O 2 in subduction zones compared to MORB sources are inherited from the subduction component.