Source: http://info.crpg.cnrs-nancy.fr/spip.php?rubrique40
Timestamp: 2019-04-25 22:36:11+00:00

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Exchanges between the deep envelopes of our planet and the surface are largely associated with the formation and movement of magma or deep fluids. The movement of these liquid phases is closely related to the internal dynamics of the Earth (plate tectonics, convection, and growth and recycling of the crust), to its differentiation, and to the creation of heterogeneities. The programme "Magma and deep fluids" is intended to contribute to the understanding and quantification of the processes involved in these exchanges, focusing on three points : the processes of magmatic evolution and their kinetics ; the modes of magma transfer between the mantle and crust ; and finally the origin, distribution, and role of deep silicate and non-silicate fluids. The approach is multidisciplinary, closely combining experimental petrology, field observation, and elemental and isotopic geochemical measurements at different scales.
These studies are based on the development of innovative isotopic tools that rely on the CRPG analytical facilities (ion probes, noble gas mass spectrometers, TIMS, and next-generation ICP-MS), and on the CRPG experimental petrology lab.
Ayalew, D., R. Pik, N. Bellahsen, L. France, and G. Yirgu. "Differential fractionation of rhyolites during the course of crustal extension, Western Afar (Ethiopian Rift)." Geochemistry, Geophysics, Geosystems G3 20, no. 2 (2019): 571–593.
Résumé : We report field observation, age, chemical (major and trace elements), and isotope (Sr‐Nd‐Pb) data for felsic volcanic rocks from Central Afar and adjacent western margin. Investigated volcanic rocks are dominantly rhyolites with minor trachytes, and they are geochemically similar. Their ages range from ~30 Ma (prerift stage), ~20 Ma (early synrift), ~8–4 Ma (main thinning event) to ~2.5–0.1 Ma (late synrift/continental breakup), representing the entire volcanic‐tectonic events that occurred episodically. Major element variations are consistent with fractionation of gabbroic cumulates. Trace element and isotope data preclude an origin by crustal anatexis; the rhyolites are rather genetically linked to the associated basalts and variously contaminated by the crust during differentiation of magmas. Chemical and isotopic data of the rhyolites support an origin by open system differentiation at deep crustal levels (hot wall rock and high r = rate of assimilation/rate of crystallization) and shallow crustal levels (cold wall rock and low r) with a change in the composition of the assimilated material from lower crustal to upper crustal type. Assimilation appears to decrease in recent times with Quaternary rhyolites, emplaced nearby the active magmatic segments in Afar, which exhibit the isotopic compositions closest to original mantle signature. This is compatible with a crust below the active magmatic segments resulting from important addition of juvenile basic magmas. Such results and interpretations provide actual constrains to suggest that the present‐day stage is probably very close to continental breakup, which will be achieved once the continental crust will be entirely replaced by new magmatic crust.
Belissont, R., M. Munoz, M. C. Boiron, B. Luais, and O. Mathon. "Germanium crystal chemistry in Cu-bearing sulfides from micro-XRF mapping and micro-XANES spectroscopy." Minerals 9, no. 4 (2019): 227.
Résumé : Germanium is considered a critical element, with a demand that has sharply increased due to booming high-technology industries. To understand Ge incorporation mechanisms in natural systems, we investigate Ge speciation in Cu-bearing sulfide minerals using synchrotron X-ray fluorescence (XRF) chemical mapping and Ge K-edge µ-X-ray absorption near-edge structures (µ-XANES) spectroscopy. The samples investigated include (i) a homogeneous chalcopyrite from the Kipushi polymetallic deposit (Central African copperbelt, D.R. Congo) and (ii) a zoned Ge-rich chalcopyrite from the Barrigão Cu deposit (Iberian pyrite belt, Portugal). First, our spectroscopic analysis supports the occurrence of tetrahedrally-coordinated Ge4+ in chalcopyrite, independently from origins or zoning patterns observed for these minerals. Then, based on statistical analyses of XRF chemical maps, we demonstrate that tetravalent germanium most likely incorporates chalcopyrite through the Fe crystallographic site via coupled substitutions with the following form : (2x + 3y)Fe3+<->(x + 2y)(Ge,Sn)4+ + x(Zn,Pb)2+ + y(Cu,Ag)+, although the presence of lattice vacancies cannot be completely excluded.
Couzinié, S., O. Laurent, C. Chelle-Michou, P. Bouilhol, J. L. Paquette, Gannoun A.M., and J. F. Moyen. "Detrital zircon U–Pb–Hf systematics of Ediacaran metasediments from the French Massif Central:Consequence Detrital zircon U–Pb–Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin." Precambrian Research 324 (2019): 269–284.
Résumé : Combining U–Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U–Pb/Lu–Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA–(MC)–ICP–MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 ± 5.5 and 556.8 ± 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55–0.65 Ga zircons with considerably scattered εHf(t) from −19 to +14; (ii) varied amounts of 0.65–1.0 Ga zircons showing dominantly positive εHf(t) with the exception of the c. 1.0 Ga zircons; and (iii) ≥1.8 Ga zircons (mostly between 1.9–2.1 and 2.5–2.8, up to 3.2 Ga) with εHf(t) ranging between +5 and −7. Multidimensional scaling of our U–Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60–75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume.
Delon, R., S. Demouchy, Y. Marrocchi, M. A. Bouhifd, P. Cordier, A. Addad, Burnard, and P.G. "Argon storage and diffusion in Earth’s upper mantle." Geochimica et Cosmochimica Acta 253 (2019): 1–18.
Résumé : In this study, fine-grained polycrystalline olivine was doped with argon at static high pressure (0.30 ± 0.01 GPa) and high temperature (1050 ± 25 °C) conditions during 24 h in a Paterson press, and analysed using a step-heating extraction protocol coupled with noble gas mass spectrometry to investigate argon storage and diffusivity in Earth’s upper mantle. Our results show that a single diffusion mechanism controlled argon diffusion in our samples during the step heating experiments. Effective Ar diffusion in olivine has a low activation energy, implying that argon diffusivity is governed by both grain boundary and lattice diffusion. Mean values of lattice diffusion parameters obtained from our results and by reprocessing literature data are Ea = 166 ± 44 kJ mol−1 and D0 = 10−7.04 ± 1.13 m2·s−1, and grain boundary diffusion parameters determined from our data are Ea = 22 ± 5 kJ·mol−1 and D0 = 10−12.87 ± 0.3 m2·s−1. Isotopic diffusivity ratios were constant and close to the values determined by Graham’s law in the C-regime (i.e., bulk diffusion dominated by grain boundary diffusion) and A-regime (i.e., bulk diffusion controlled by grain boundary and lattice diffusion in proportion to the segregation of Ar between those sites), but varied in the B-regime (i.e., bulk diffusion controlled by both grain boundary and lattice diffusion in a complex manner), implying a higher isotopic fractionation in the kinetic B-regime. Extrapolation to typical mantle grain sizes implies that around 22% of the argon in the upper mantle can be stored at grain boundaries and that effective diffusion is mostly in the A-regime, suggesting a low isotopic fractionation and diffusivities faster than lattice diffusivities alone. The consideration of grain boundaries as a potential Ar storage site can modify equilibrium during partial melting and significantly enrich a liquid in Ar during fluid percolation. The grain size dependence of Ar storage and diffusivity highlights the underestimated role of grain boundaries in the upper mantle, especially in zones of reduced grain size (via dynamic recrystallization) possibly followed by fluid percolation and/or partial melting, such as in subduction zones or below oceanic ridges.
Deng, Z., M. Chaussidon, P. Savage, F. Robert, R. Pik, and F. Moynier. "Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks." Proceedings of the National Academy of Sciences of the United States of America 116, no. 4 (2019): 1132–1135.
Résumé : Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.
Fallick, A. E., G. Giuliani, T. Rigaudier, A. J. Boyce, V. L. Pham, and V. Pardieu. "Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble, with special reference to Vietnamese deposits." Comptes Rendus. Géoscience 351 (2019): 27–36.
Résumé : Oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble from Vietnam and other worldwide deposits have been determined in order to characterize the O-isotope fractionation between calcite and spinel. In Vietnam, the Δ18Occ–sp (= 3.7 ± 0.1‰ for six samples from the An Phu and Cong Troi deposits) is remarkably constant. The combination of these data with those obtained on calcite–spinel pairs of Paigutan (Nepal, n = 2), Ipanko (Tanzania, n = 1), and Mogok (Myanmar, = 2) are also consistent with an overall Δ18Occ–sp of 3.6 ± 0.3‰ for all the spinel samples (n = 11). The straight line correlation δ18Occ = 0.96 δ18Osp + 4.4 is excellent despite their worldwide geographic spread. The increment method of calculating oxygen isotope fractionation gave a geologically unreasonable temperature of formation for both minerals at 1374 °C when compared to temperatures obtained by mineral assemblage equilibrium of these marble type deposits, between 610 and 750 °C. The constant Δ18Occ–sp reflects a constant temperature for this amphibolite facies assemblage, whose current best estimate is calculated at 620 ± 40 °C, but unquantified uncertainties remain.
Giuliani, G., L. A. Groat, D. Marshall, A. E. Fallick, and Y. Branquet. "Emerald deposits : A review and enhanced classification." Minerals 9, no. 105 (2019): 1–63.
Résumé : Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (+-V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic ; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids ; degree of metamorphism ; styles of minerlization, i.e., veins, pods, metasomatites, shear zone ; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types : (Type I) Tectonic magmatic-related with sub-types hosted in : (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others) ; (IB) Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia) ; (IC) Granitic rocks (Nigeria). (Type II) Tectonic metamorphic-related with sub-types hosted in : (IIA) Mafic-ultramafic rocks (Brazil, Austria) ; (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA) ; (IIC) Metamorphic rocks (China, Afghanistan, USA) ; (IID) Metamorphosed and remobilized either type I deposits or hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified deposits.
Giuliani, G., L. A. Groat, D. Marshall, and A. Fallick. "Emerald deposits : a review and enhanced classification." Minerals (2019): 1–63.
Maunder, B., J. van Hunen, P. Bouilhol, and V. Magni. "Modeling slab temperature: A reevaluation of the thermal parameter." Geochemistry, Geophysics, Geosystems G3 20 (2019).
Résumé : We reevaluate the effects of slab age, speed, and dip on slab temperature with numerical models. The thermal parameter Φ = t v sin θ, where t is age, v is speed, and θ is angle, is traditionally used as an indicator of slab temperature. However, we find that an empirically derived quantity, in which slab temperature T ∝ log (t−av−b) , is more accurate at depths <120 km, with the constants a and b depending on position within the slab. Shallower than the decoupling depth (~70–80 km), a~1 and b~0, that is, temperature is dependent on slab age alone. This has important implications for the early devolatilization of slabs in the hottest (youngest) cases and for shallow slab seismicity. At subarc depths (~100 km), within the slab mantle, a~1 and b~0 again. However, for the slab crust, now a~0.5 and b~1, that is, speed has the dominant effect. This is important when considering the generation of arc magmatism, and in particular, slab melting and the generation of slab‐derived melange diapirs. Moving deeper into the Earth, the original thermal parameter performs well as a temperature indicator, initially in the core of the slab (the region of interest for deep water cycling). Finally, varying the decoupling depth between 40 and 100 km has a dominant effect on slab temperatures down to 140‐km depth, but only within the slab crust. Slab mantle temperature remains primarily dependent on age.
Richard, A., C. Morlot, L. Créon, N. Beaudoin, V. S. Balistky, S. Pentelei, V. Dyja-Person, G. Giuliani, I. Pignatelli, H. Legros et al. "Advances in 3D imaging and volumetric reconstruction of fluid and melt inclusions by high resolution X-ray computed tomography." Chemical Geology 508 (2019): 3–14.
Résumé : Fluid and melt inclusions are tiny pockets of fluid and melt trapped in natural and synthetic minerals. Characterizing the 3D distribution of fluid and melt inclusions within minerals, their shape and the volume fraction of their different phases is crucial for determining the conditions of crystal growth and paleostress analysis. However, their relatively small size (typically 5 to 100 μm), complex shape, heterogeneous content, the opaque nature of some host minerals and projection bias frequently hamper accurate imaging and volumetric reconstruction using conventional microscopic techniques. High resolution X-ray computed tomography (HRXCT) is a non-destructive method which uses contrasts of X-ray attenuation in a series of contiguous radiographs with different view angles to reconstruct the 3D distribution of areas of different densities within a large variety of materials. In this work, we show the capabilities of HRXCT for: (i) imaging the 3D distribution of aqueous and hydrocarbon-bearing fluid inclusions and silicate melt inclusions in a crystal; (ii) characterizing the shape of fluid and melt inclusions and (iii) reconstructing the total volume and the volume of the different phases (liquid, glass, crystal, vapor) of fluid and melt inclusions. We have used a variety of hand specimens and chips of transparent and opaque minerals (olivine, quartz, feldspar, garnet, emerald, wolframite), that we analyzed using three different HRXCT setups. When a resolution of ~1 μm3/voxel is achieved, HRXCT allows identifying >5 μm fluid inclusions, and the identification and volumetric reconstruction of the different phases can be carried out with reasonable confidence for relatively large (>25 μm) inclusions. Density contrasts are high enough to properly identify: (i) a silicate melt inclusion, and its different phases (glass, vapor and crystals such as clinopyroxene and spinel) in an olivine crystal; (ii) aqueous monophase (liquid) and two-phase (liquid + vapor) fluid inclusions in transparent and opaque minerals (quartz, garnet, emerald, wolframite). In the case of hydrocarbon-bearing fluid inclusions containing a vapor phase and two liquid phases (oil and aqueous solution), the two liquid phases could not be distinguished from each other. Volumetric reconstruction of liquid and vapor phases of aqueous and hydrocarbon-bearing fluid inclusions show compatible results with independent calculations using known pressure, temperature, molar volume and composition (P-T-V-x) conditions of trapping or imaging using confocal laser scanning microscopy respectively. Collectively, our results show that HRXCT is a promising tool for non-destructive characterization of fluid and melt inclusions.
Voudouris, P., C. Mavrogonatos, I. Graham, G. Giuliani, V. Melfos, S. Karampelas, V. Karantoni, K. Wang, A. Tarantola, K. Zaw et al. "Gem corundum deposits of Greece : Geology, mineralogy and genesis." Minerals 2019, no. 49 (2019): 1–42.
Résumé : Greece contains several gem corundum deposits set within diverse geological settings, mostly within the Rhodope (Xanthi and Drama areas) and Attico-Cycladic (Naxos and Ikaria islands) tectono-metamorphic units. In the Xanthi area, the sapphire (pink, blue to purple) deposits are stratiform, occurring within marble layers alternating with amphibolites. Deep red rubies in the Paranesti-Drama area are restricted to boudinaged lenses of Al-rich metapyroxenites alternating with amphibolites and gneisses. Both occurrences are oriented parallel to the ultra-high pressure/high pressure (UHP/HP) Nestos suture zone. On central Naxos Island, colored sapphires are associated with desilicated granite pegmatites intruding ultramafic lithologies (plumasites), occurring either within the pegmatites themselves or associated metasomatic reaction zones. In contrast, on southern Naxos and Ikaria Islands, blue sapphires occur in extensional fissures within Mesozoic metabauxites hosted in marbles. Mineral inclusions in corundums are in equilibrium and/or postdate corundum crystallization and comprise : spinel and pargasite (Paranesti), spinel, zircon (Xanthi), margarite, zircon, apatite, diaspore, phlogopite and chlorite (Naxos) and chloritoid, ilmenite, hematite, ulvospinel, rutile and zircon (Ikaria). The main chromophore elements within the Greek corundums show a wide range in concentration : the Fe contents vary from (average values) 1099 ppm in the blue sapphires of Xanthi, 424 ppm in the pink sapphires of Xanthi, 2654 ppm for Paranesti rubies, 4326 ppm for the Ikaria sapphires, 3706 for southern Naxos blue sapphires, 4777 for purple and 3301 for pink sapphire from Naxos plumasite, and finally 4677 to 1532 for blue to colorless sapphires from Naxos plumasites, respectively. The Ti concentrations (average values) are very low in rubies from Paranesti (41 ppm), with values of 2871 ppm and 509 in the blue and pink sapphires of Xanthi, respectively, of 1263 ppm for the Ikaria blue sapphires, and 520 ppm, 181 ppm in Naxos purple, pink sapphires, respectively. The blue to colorless sapphires from Naxos plumasites contain 1944 to 264 ppm Ti, respectively. The very high Ti contents of the Xanthi blue sapphires may reflect submicroscopic rutile inclusions. The Cr (average values) ranges from 4 to 691 ppm in the blue, purple and pink colored corundums from Naxos plumasite, is quite fixed (222 ppm) for Ikaria sapphires, ranges from 90 to 297 ppm in the blue and pink sapphires from Xanthi, reaches 9142 ppm in the corundums of Paranesti, with highest values of 15,347 ppm in deep red colored varieties. Each occurrence has both unique mineral assemblage and trace element chemistry (with variable Fe/Mg, Ga/Mg, Ga/Cr and Fe/Ti ratios). Additionally, oxygen isotope compositions confirm their geological typology, i.e., with, respectively d18O of 4.9 +-0.2‰for sapphire in plumasite, 20.5‰for sapphire in marble and 1‰ for ruby in mafics. The fluid inclusions study evidenced water free CO2 dominant fluids with traces of CH4 or N2, and low CO2 densities (0.46 and 0.67 g/cm3), which were probably trapped after the metamorphic peak. The Paranesti, Xanthi and central Naxos corundum deposits can be classified as metamorphic sensu stricto (s.s.) and metasomatic, respectively, those from southern Naxos and Ikaria display atypical magmatic signature indicating a hydrothermal origin. Greek corundums are characterized by wide color variation, homogeneity of the color hues, and transparency, and can be considered as potential gemstones.
Wang, K. K., I. T. Graham, L. Martin, P. Voudouris, G. Giuliani, A. Lay, and A. Fallick S. J. Harris. "Fingerprinting Paranesti rubies through oxygen isotopes." Minerals 9, no. 91 (2019): 1–14.
Résumé : In this study, the oxygen isotope (d18O) composition of pink to red gem-quality rubies from Paranesti, Greece was investigated using in-situ secondary ionization mass spectrometry (SIMS) and laser-fluorination techniques. Paranesti rubies have a narrow range of d18O values between 0 and +1h and represent one of only a few cases worldwide where d18O signatures can be used to distinguish them from other localities. SIMS analyses from this study and previous work by the authors suggests that the rubies formed under metamorphic/metasomatic conditions involving deeply penetrating meteoric waters along major crustal structures associated with the Nestos Shear Zone. SIMS analyses also revealed slight variations in d18O composition for two outcrops located just 500 m apart : PAR-1 with a mean value of 1.0h +- 0.42h and PAR-5 with a mean value of 0.14h +- 0.24h. This work adds to the growing use of in-situ methods to determine the origin of gem-quality corundum and re-confirms its usefulness in geographic “fingerprinting”.
Amalberti, J., P. Burnard, L. Tissandier, and D. Laporte. "The diffusion coefficients of noble gases (He-Ar) in a synthetic basaltic liquid: One-dimensional diffusion experiments." Chemical Geology 480 (2018): 35–43.
16 mol% MgO, 25 mol% CaO) at temperatures of 1673 K and 1823 K: DHe= 2.75 ± 0.25×10−6 cm2·s−1 (T =1673 K); DHe =4.77 ± 0.42 ×10−6 cm2·s−1 (T =1823 K); and DAr =9.3 ± 1.3 ×10−7 cm2·s−1 (T =1673 K). Combining these new high temperature data with diffusion coefficients measured on the same composition just above the glass transition temperature, we determined the activation energy Ea and the preexponential factor D0 for He and Ar diffusion in silicate liquids: D0 = 1.72 ± 0.9×10−2 cm2·s−1 and Ea =136.5 ± 3.2 kJ/mol for Ar; D0 =1.8 ± 0.5×10−4 cm2·s−1 and Ea =57.6 ± 1.5 kJ/mol for He. Because He and Ar have very different activation energies for diffusion in the liquid state, the ratio DHe/DAr is strongly sensitive to temperature, decreasing from 145 at the glass transition temperature (1005 K) to 2 at 1823 K. The implication is that the kinetic fractionation of He relative to Ar in magmas is likely to be more important during the cooling stages than during the earlier, high temperature stages of magmatic history.
Asafov, E. V., A. V. Sobolev, A. A. Gurenko, N. T. Arndt, V. G. Batanova, M. V. Portnyagin, D. Garbe-Schönberg, and S. P. Krasheninnikov. "Belingwe komatiites (2.7 Ga) originate from a plume with moderate water content, as inferred from inclusions in olivine." Chemical Geology 478 (2018): 39–59.
Résumé : Major and trace elements, and volatile components have been measured in melt inclusions in olivine from fresh 2.7 Ga old komatiites from the Reliance Formation of the Belingwe Greenstone Belt, Zimbabwe. Reconstructed compositions of melt inclusions contain 20–23.5 wt% MgO and up to 0.3 wt% H2O ; these compositions probably represent those of the erupted lava. In inclusions in relatively evolved (low Fo) olivines, an excess of Na2O, CaO, Li, La, Cu, Rb as well as volatile components (H2O, F, Cl and S) relative to other highly incompatible elements is attributed to assimilation of seawater altered mafic material. No assimilation signature is observed for the most primitive melt inclusions hosted in the magnesium rich olivines. The primary melt composition, estimated using melt inclusions and the composition of the most magnesian olivine (Fo 93.5), contains up to 27.5 wt% MgO and ca. 0.2 wt% H2O. The presence of H2O slightly depressed the liquidus temperature to ca. 1513 °C. Our results suggest formation of the Belingwe komatiite magma at ca. 7 GPa pressure and ca. 1790 °C temperature in a mantle plume. The plume picked up water and probably chlorine through interaction with a hydrous transition mantle zone in the way similar to that previously proposed by Sobolev et al. (2016) for komatiites in Canada.
Balcone-Boissard, H., G. Boudon, J. D. Blundy, C. Martel, R. A. Brooker, E. Deloule, C. Solaro, and V. Matjuschkin. "Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions." Contributions to Mineralogy and Petrology 173 (2018): 101.
Résumé : Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H2O, CO2, SO2, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤ 8 wt% H2O). CO2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO2. Some low-pressure (< 3 kbars) inclusions have elevated CO2 (up to 1100–1150 ppm), suggestive of fluxing of shallow magmas with CO2-rich fluids. CO2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8–7.5 kbars) indicates that magmas originate from a vertically-extensive (3–27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75–557 ppm), Cl (1525–3137 ppm), Br (6.1–15.4 ppm) and SO2 (< 140 ppm), and for the northern part it’s F (92–798 ppm), Cl (1506–4428 ppm), Br (not determined) and SO2 (< 569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3 ± 2.7), indicating that the magma source at depth is similar. The high H2O content of Dominica magmas has implications for hazard assessment.
Ballouard, C., M. Poujol, J. Mercadier, E. Deloule, P. Boulvais, J. M. Baele, M. Cuney, and M. Cathelineau. "Uranium metallogenesis of the peraluminous leucogranite from the Pontivy-Rostrenen magmatic complex (French Armorican Variscan belt): the result of long-term oxidized hydrothermal alteration during strike-slip deformation." Mineralium Deposita 53, no. 5 (2018): 601–628.
Résumé : In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 < U < 9 ppm; Th/U > 3), the leucogranite samples are characterized by highly variable U contents (~ 3 to 27 ppm) and Th/U ratios (~ 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies.
Baudouin, C., F. Parat, and T. Michel. "CO2-rich phonolitic melt and carbonatite immiscibility in early stage of rifting: Melt inclusions from Hanang volcano (Tanzania)." Journal of Volcanology and Geothermal Research 358 (2018): 261–272.
gas phase indicating that carbonatite:silicate (18:82) liquid immiscibility occurred during nephelinite magmatic evolution. The silicate glasses have trachytic composition (Na+K/Al=1.6–7.2, SiO2=54–65.5 wt%) with high CO2 (0.43 wt% CO2), sulfur (0.21–0.92 wt% S) and halogens (0.28–0.84 wt% Cl; 0.35–2.54 wt% F) contents and very low H2O content (b0.1 wt%). The carbonate phase is an anhydrous Ca-Na-K-S carbonate with 33 wt% CaO, 20 wt% Na2O, 3 wt% K2O, and 3 wt% S. The entrapped melt in nepheline corresponds to evolved interstitial CO2-rich phonolitic composition (Na + K/Al = 6.2–6.9) with 6 ± 1.5 wt% CO2 at pressure of 800 ± 200 MPa after crystallization of cpx (17%), nepheline (40%) garnet (6.5%) and apatite (1.7%) from Mg-rich nephelinitic magma. During ascent, immiscibility in phonolitic melt inclusions leads to Ca-Na carbonate melt with composition within the range of carbonate melt from Oldoinyo Lengai and Kerimasi, in equilibrium with trachytic silicate melt (closed-system, P b 500MPa). The CO2-rich phonolitic melt inclusions from Hanang volcano may represent an early stage of differentiation before Na carbonatitic magmatism observed at Oldoinyo Lengai.
Bonnetti, C., X. Liu, J. Mercadier, M. Cuney, E. Deloule, J. Villeneuve, and W. Liu. "The genesis of granite-related hydrothermal uranium deposits in the Xiazhuang and Zhuguang ore fi elds, North Guangdong Province, SE China: Insights from mineralogical, trace elements and U-Pb isotopes signatures of the U mineralisation." Ore Geology Reviews 92 (2018): 588–612.
Résumé : Granite-related hydrothermal U deposits from the Xiazhuang and Zhuguang ore fields are located within the Nanling Metallogenic Belt in South China. These deposits are hosted in Triassic and Jurassic granites that were emplaced during the Indosinian orogeny and the Yanshanian post-orogenic extension. Four main types of granitic host rocks were identified based on geochemical characteristics as the most favourable sources of U for the mineralisation in the district : (i-ii) Indosinian peraluminous S-type (Maofeng) and L-type leucogranite (Baishuizhai) ; (iii) Indosinian highly fractionated high-K calc-alkaline A2-type granite (Xiazhuang, Maofeng and Youdong) and (iv) early Yanshanian highly fractionated high-K, (Fe) calc-alkaline A2-type granite (Sundong, Siqian and Changjiang). They contain high U concentrations ranging from 15 to 28 ppm indicating the crystallisation of uraninite, which is a source of U easily leachable by oxidised fluids for the formation of hydrothermal mineralisation.
This study characterises the mineralogical, chemical and isotopic signatures of the U mineralisation from five representative deposits and occurrences in the Xiazhuang (Baishuizhai, Shituling, Xiwang and Xianshi) and Zhuguang (Mianhuakeng) ore fields. The 175 ± 16 Ma Baishuizhai occurrence and 162 ± 27 Ma Shituling deposit represent an early stage of U mineralisation (175–145 Ma) that occurred during the early Yanshanian. Disseminated Th-bearing uraninite at Baishuizhai and the association of U oxides with alteration minerals such as epidote and chlorite in both Baishuizhai and Shituling indicate temperatures >250 °C for the hydrothermal system. The early Yanshanian granites (190–150 Ma) provided the heat source and magmatic fluids to the hydrothermal system while U-rich Indosinian granites provided U for the Zr-Th-Ta-bearing U mineralisation. In contrast, the mineralisation from Xiwang, Mianhuakeng and Xianshi deposits (107 ± 16, 93 ± 15 and 79 ± 11 Ma, respectively) represent several pulses of the main mineralisation stage (110–50 Ma) that occurred during the late Yanshanian crustal extension. The mineralisation occurred at relatively low temperatures (<250 °C) as W-Y-Nb-Ti-rich pitchblende and uraninite veins associated with quartz, fluorite, calcite and pyrite. During Cretaceous-early Cenozoic crustal extension, oxidising meteoric and/or basinal waters percolated downward into the granitic basement through deep faults and mixed with CO2-rich magmatic fluids. These thermal solutions circulated along opening fractures and leached U from Indosinian and early Yanshanian U-rich granites and also possibly from early Yanshanian U mineralisation. After heating, the ore-forming fluids ascended along structures leading to the decrease of the hydrostatic fluid pressure, which promoted the deposition of U in veins. Therefore, granite-related hydrothermal U mineralisation in the province, formed over a long time span (>100 Ma) in two major stages related to different genetic conditions.
Bosia, C., F. Chabaux, E. Pelt, A. Cogez, P. Stille, E. Deloule, and C. France-Lanord. "U-series disequilibria in minerals from Gandak River sediments (Himalaya)." Chemical Geology 477, no. 22-34 (2018).
Résumé : The impact of mineralogical sorting on U-Th-Ra variations observed for the Ganga sediments in the Himalayan alluvial plain due to transport and deposition processes highlights the importance to better understand the U-series isotopic system in Himalayan minerals. For this purpose, U, Th, and Ra concentrations, along with 238U–234U–230Th–226Ra radioactive disequilibria, were measured in several pure mineral fractions, i.e. monazite, zircon, titanite, ilmenite, rutile, garnet, magnetite, quartz, biotite and muscovite, separated from one bank sediment of the Gandak River (Ganga Basin). The data, obtained through a double digestion protocol in high pressure and high-temperature vessels, confirm that U and Th concentrations and isotopic ratios of the sediments are mainly influenced by a few minor mineral species that are very enriched in U and Th, namely, monazite, titanite and epidote, as well as major minerals with lower U and Th concentrations, such as quartz. More importantly, the data clearly indicate that, similarly to whole rock samples, the U-Th-Ra systematics of individual minerals have likely recorded complex U-Th-Ra fractionations. In particular, the 226Ra enrichment observed in the biotite and clay fractions likely results from the affinity of Ra for phyllosilicate minerals, while the 230Th and, to a lesser extent the 234U enrichment observed for the other minerals might result from a nuclide adsorption process on the Fe-oxide coatings present on the grains or directly on the mineral surface. The linear trend outlined by the minerals phases in the (226Ra/230Th) vs (230Th/234U) diagram suggests that 230Th is probably mobilized from zircons, the only fraction presenting (230Th/234U) < 1, and from monazites, and redistributed among the all minerals during weathering. Such a scenario suggest that zircon weathering, evidenced in this study, must have been very recent, estimated< 2000 years by a modeling approach, including simulation of nuclides mobility by alpha-recoil ejection and loss and gain processes.
Burg, J. P., and P. Bouilhol. "Timeline of the South-Tibet--Himalayan belt: the geochronological record of subduction, collision, and underthrusting from zircon and monazite U-Pb ages." Canadian Journal of Earth Sciences (2018).
Résumé : The “exact timing” of collision between India and Eurasia is a recurring questions arising in discussions and various publications. Exactitude is a ubiquitous pursuit for all tectonic events. With the example of the Himalaya-South Tibet collision system, a short review of arguments from different approaches suggests that this pursuit is in vain, but that our knowledge is already sufficient to provide an acceptably “precise” timing of the main events. We reviewed U-Pb ages of zircons and monazites considering that major tectonic events can produce thermal effects strong enough to be signalled in high-temperature geochronology. This review equally shows that exactitude in timing is beyond the precision of the methods and the rock record. General consistency between geologic and thermochronologic records strengthens previous interpretations of the collisional orogenic system. At variance with most tectonic interpretations, we argue that the Tsangpo Suture in South-Tibet results from two merged subduction zones, and that island arcs may be part of the root of the Eurasian paleo-active margin. The two main collisional events closely followed each other at ca. 50 and 40 Ma.
Cadoux, A., G. Iacono-Marziano, B. Scaillet, A. Aiuppa, T. A. Mather, D. M. Pyle, E. Deloule, E. Gennaro, and A. Paonita. "The role of melt composition on aqueous fluid vs. silicate melt partitioning of bromine in magmas." Earth and Planetary Science Letters 498 (2018): 450–463.
Résumé : Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their environmental impact. To date, bromine partitioning between silicate melts and the gas phase is very poorly constrained, with the only relevant experimental studies limited to investigation of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments were performed using natural silicate glasses with mafic, intermediate and silicic compositions. For each composition, experiments were run with various Br contents in the initial fluid (H2O–NaBr), at T–P conditions representative of shallow magmatic reservoirs in volcanic arc contexts (100–200 MPa, 900–1200 °C). The resulting fluid/melt partition coefficients (DBrf/m) are : 5.0 ± 0.3 at 1200 °C–100 MPa for the basalt, 9.1 ± 0.6 at 1060 °C–200 MPa for the andesite and 20.2 ± 1.2 at 900 °C–200 MPa for the rhyodacite. Our experiments show that DBrf/m increases with increasing SiO2 content of the melt (as for chlorine) and suggest that it is also sensitive to melt temperature (increase of DBrf/m with decreasing temperature). We develop a simple model to predict the S–Cl–Br degassing behaviour in mafic systems, which accounts for the variability of S–Cl–Br compositions of volcanic gases from Etna and other mafic systems, and shows that coexisting magmatic gas and melt evolve from S-rich to Cl–Br enriched (relative to S) upon increasing degree of degassing. We also report first Br contents for melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and calculate the mass of bromine available in the magma reservoir prior to the eruptions under consideration. The discrepancy that we highlight between the mass of Br in the co-existing melt and fluid prior to the Merapi 2010 eruption (433 and 73 tons, respectively) and the lack of observed BrO (from space) hints at the need to investigate further Br speciation in ‘ash-rich’ volcanic plumes. Overall, our results suggest that the Br yield into the atmosphere of cold and silicic magmas will be much larger than that from hotter and more mafic magmas.
Dalou, C., J. Boulon, K. T. Koga, R. Dalou, and R. L. Dennen. "DOUBLE FIT: Optimization procedure applied to lattice strain model." Computers and Geosciences 117 (2018): 49–56.
parameters while accounting for the elastic response of two different sites to trace element substitution in various minerals.
Debret, B., P. Bouilhol, M. L. Pons, and H. Williams. "Carbonate transfer during the Onset of slab devolatilization: New insights from Fe and Zn stable isotopes." Journal of Petrology 59, no. 6 (2018): 1145–1166.
in the aCO2 of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-T conditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction.
Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas.
Eeken, T., S. Goes, H. A. Pedersen, N. Arndt, and P. Bouilhol. "Seismic evidence for depth-dependent metasomatism in cratons." Earth and Planetary Science Letters 491 (2018): 148–159.
Résumé : The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100–150 km. The metasomatic component can consist of 0.5–1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100–200◦C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.
Fournier, J., E. Régnier, F. Faure, X. Le Goff, H. P. Brau, E. Brackx, and O. Pinet. "Application of the JMAK model for crystal dissolution kinetics in a borosilicate melt." Journal of Non-Crystalline Solids 489 (2018): 77–83.
limiting the dissolution to be known, i.e. the diffusion and the activation energy of the dissolution (475 kJ/mol). These conclusions are the same as those resulting from the application for T > Tliq. This shows the continuity of the dissolution around Tliq which is, in addition, confirmed by the chemical profile study performed by microprobe analysis. Moreover, it validates the application of the JMAK model for crystal dissolution whatever the temperature. Furthermore, the study of the crystal morphologies during dissolution indicates that a mechanism called “crystal attack/splitting” occurs for T > Tliq and leads to a change in value of the Avrami exponent.
Fournier, J., E. Régnier, F. Faure, X. Le Goff, H. P. Brau, E. Brackx, and O. Pinet. "Modeling of dissolution kinetics of rare earth crystals in a borosilicate glass melt." Journal of Non-Crystalline Solids 481 (2018): 248–253.
these results, a second and most common approach is the measure of the chemical profiles at the crystal/melt interfaces by microprobe. The conclusions obtained by this last method are in agreement with the conclusions based on JMAK model. All these results allowed to confirm that the JMAK model is well suited to model crystals dissolution in silicate melts.
Giuliani, G., and A. E. Fallick. "Isotope signatures of gem minerals." Wooshin Gemological Institute of Korea 5 (2018): 2–9.
Résumé : Natural variations in isotopic ratios for gem minerals have been reported for only five light elements namely carbon and nitrogen for diamond, oxygen for oxides, and hydrogen, oxygen and boron for silicates, and heavier elements such as copper for turquoise and sulphur for lapis-lazuli. Stable isotope geochemistry deals with isotopic variations that arise from isotope exchange reactions or mass-dependent fractionations which take place during the physical and chemical processes responsible for the formation of gems. The magnitude and temperature dependence of isotopic fractionation factors between minerals and their parental fluids permit to reconstitute the geological history of gems in terms of source of the element and origin of the fluids. Here, we will focus on some gem minerals viz diamond, ruby, sapphire, spinel, emerald, zoisite, grossular garnet and agate. We shall demonstrate by different examples that the isotopic signatures used for their identification are directly controlled by their geological history.
Giuliani, G., J. Dubessy, D. Ohnenstetter, D. Banks, Y. Branquet, J. Feneyrol, A. E. Fallick, and J. E. Martelat. "The role of evaporites in the formation of gems during metamorphism of carbonate platforms: a review." Mineralium Deposita 53 (2018): 1–20.
metamorphism at moderate to high temperatures of evaporite-rich and organic matter-rich protoliths to form gem minerals.
Giuliani, G., J. Dubessy, I. Pignatelli, and D. Schwarz. "Fluid inclusions study of trapiche and non-trapiche rubies from the Mong Hsu deposit, Myanmar." The Canadian Mineralogist 56 (2018): 1–13.
growth sectors separated by six dendrites running from a central core to the six corners. Fluid inclusions (FI) have been studied in both types of ruby, i.e., trapiche and non-trapiche crystals. Microthermometry combined with Raman spectrometry investigations of primary and secondary FI proved the existence of CO2-H2S-COS-S8-AlO(OH)-bearing fluids with diaspore and native sulfur daughter minerals. The carbonic fluid belongs to the CO2–H2S system with CO2 as a dominant component. Minor COS (1–2 mol.%) and diaspore indicate that H2O (~1 mol.%) was present in the paleofluid. The amount of H2S in FI from trapiche ruby is between 6.9 and 7.4 mol.% for the dendrites and from 7.2 to 15.7 mol.% for the growth sectors. The H2S content is approximately 10 mol.% in FI from non-trapiche rubies.
(0.56 , ds , 0.68) records variation of fluid pressure in the metamorphic system. This variation led to episodically local fluid overpressure and hydraulic fracturing in the marble. During such episodes, changes in driving-force conditions allowed for the formation of the trapiche texture in ruby: the development of dendrites and growth sectors occurred under high and low drivingforce conditions, respectively. Non-trapiche ruby in veinlets formed concurrently under thermodynamic conditions similar to those registered for the growth sectors in trapiche ruby.

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