bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristic magnetic behavior of subaerial and submarine lava units from the Hawaiian Scientific Drilling Project (HSDP‐2)
Authors: Kontny Agnes , Vahle Carsten , de Wall Helga
Published: May 2003

This study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of Mauna Loa and Mauna Kea volcanoes collected from the 3109 m deep HSDP‐2 drill hole in Hawaii. Three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion. Group 1 shows homogenous titanomagnetite with low Xmt, low Curie temperatures (TC: 100°–200°C) and weak median demagnetizing fields (< 20 mT). Further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility (MS) and hysteresis loops, which indicate a contribution from ferrimagnetic Cr‐Al spinel below ca. −160°C in the 1b‐type samples. Group 2 samples, with exsolution lamellae of ilmenite in the titanomagnetites, have higher TC (480°–580°C) and higher coercive forces (20–40 mT). Group 3, the highest oxidation stage, is characterized by titanohematite‐bearing assemblages with enhanced median demagnetizing fields (35–85 mT) and a significantly different low‐temperature MS behavior. MS core logging shows a systematic variation occurs in the subaerial lava flows, directly related to the degree of high temperature oxidation and their flow morphology. Aa lava flows have higher mean MS than other lava flow types. Besides these factors, MS appears to be also affected by the magma composition of the various shield‐building stages. Mauna Loa subaerial lava flows generally show lower mean susceptibilities (4.6 ± 3 × 10−3 SI) than subaerial Mauna Kea lava flows (9.8 ± 5 × 10−3 SI). As submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks. Some hyaloclastites and pillow breccias show low MS (< 1 × 10−3 SI), small amounts of nearly pure magnetite (TC = 580°C) and high coercive forces up to 110 mT suggesting single domain and/or superparamagnetic behavior. The controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows, which creates large grain size variations in titanomagnetites of varying compositions. Hydrothermal alteration, as described from ocean floor or Icelandic basalts, is not an important process that influences the magnetic properties in the ocean island basalts from the HSDP‐2 drill hole.
Evidence for major‐element heterogeneity in the mantle source of abyssal peridotites from the Southwest Indian Ridge (52° to 68°E)
Authors: Seyler Monique , Cannat Mathilde , Mével Catherine
Published: May 2003

A suite of 53 samples of mantle spinel lherzolites and harzburgites dredged at 13 sites between 52°E and 68°E along the Southwest Indian Ridge has been studied for petrography and mineral major element chemistry. Results show that the residual mantle beneath this very slow‐spreading/cold ridge is strongly heterogeneous in modal and mineral compositions at local and regional scales and underwent greater extents of melting than predicted by melting model and by compositions of the basalts dredged with the peridotites. Along‐axis, the peridotite compositional variability defines a concave pattern with increasing depletion at both ends of the studied section (e.g., approaching Rodrigues Triple Junction to the East and Gallieni fracture zone to the west) that cannot be matched with the basalt compositions. Clinoyroxenes reflect depleted compositions (low modal abundances, high Cr and Mg, low Ti contents) but are paradoxally enriched in jadeite component, a feature that distinguishes these peridotites from common abyssal peridotites. Textural data show that major depletion in basaltic components and pyroxene Na enrichment are early features of the studied peridotites. In most samples, Na is nevertheless correlated with Ti suggesting that initial clinopyroxenes had high Na/Ti contents. Samples at both ends of the studied area have even higher Na/Ti ratios because of higher Na enrichment and higher Ti depletion, indicating metasomatic interaction. We conclude that along‐axis compositional variations characterizing these peridotites are primary controlled by major element heterogeneity in the initial mantle, that have been preserved because of low degrees of melting beneath the Southwest Indian Ridge.
Influence of grain size evolution on convective instability
Authors: Hall Chad E. , Parmentier E. M.
Published: May 2003

Grain size, one of the most important microstructural properties of materials, evolves during creep deformation to minimize the free energy of polycrystalline aggregates. We apply a model of grain size evolution to the study of convective instability of cooling boundary layers. The grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep. The onset of convection is sensitive to grain growth rates and the initial grain size. The formation of convective instabilities is enhanced by stresses induced by plate motions; therefore small‐scale convection is more likely to occur beneath fast‐moving plates. In finite amplitude convection, grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude. Such viscosity contrasts are sufficient to influence the dynamics of convection, often leading to domains which remain isolated from the well‐mixed convecting fluid. A composite viscosity including diffusion creep, which has a lower activation energy than dislocation creep, reduces the effective temperature dependence of viscosity.
Adaptation of a flow‐through leaching procedure for Mg/Ca paleothermometry
Authors: Benway H. M. , Haley B. A. , Klinkhammer G. P. , Mix A. C.
Published: May 2003

Mg/Ca ratios in planktonic foraminifera reflect calcification temperatures and are thus useful for sea surface temperature (SST) reconstructions. Despite the obvious utility of this paleoceanographic tracer, problems of dissolution, gametogenic calcification, and contaminant phases have thus far limited confidence in Mg/Ca‐based reconstructions. Here we show strong evidence of Mg heterogeneity in foraminiferal calcite by sequentially measuring the composition of different forms of calcite (ontogenetic, gametogenic, diagenetic) in the same shells, while monitoring and removing contaminant phases. A new flow‐through method combines chromatographic technology and inductively coupled plasma mass spectrometry (ICP‐MS) in a series of cleaning and dissolution reactions monitored continuously with time‐resolved analysis (TRA). This combination of slow, controlled dissolution and TRA provides a complete elemental description of contaminant phases and sorts the cleaned calcium carbonate based on dissolution sensitivity. Examination of partially dissolved shells with electron microscopy suggests that the flow‐through method simulates the natural dissolution sequence and effectively separates the different calcite domains within a single foraminiferal shell. Heterogeneity of Mg/Ca in foraminiferal calcite is clearly demonstrated in flow‐through analysis. Foraminiferal shells have initially high Mg levels that decrease steadily throughout dissolution. Later dissolution yields lower Mg/Ca, which is likely due to a combination of subsurface calcification and biomineralization effects. Mg/Ca ratios from the most dissolution‐sensitive (high‐Mg) portions of surface‐dwelling species in core tops are used to calculate calcification temperatures. A comparison of late Holocene core top data with World Ocean Atlas SST data indicates that the flow‐through method does yield viable SST estimates. Furthermore, a depth transect in the eastern tropical Pacific suggests that this approach provides the opportunity to extract initial calcification temperatures despite partial dissolution of foraminiferal shells.
Electronic data publication in geochemistry
Authors: Staudigel Hubert , Helly John , Koppers Anthony A. P. , Shaw Henry F. , McDonough William F. , Hofmann Albrecht W. , Langmuir Charles H. , Lehnert Kerstin , Sarbas Baerbel , Derry Louis A. , Zindler Alan
Published: May 2003

Many disciplines of geochemistry have no data reporting standards, and their use of metadata is inadequately developed. This presents problems to the quality of the published science, and it limits the utility of computers in data analysis and the exploitation of Information Technology (IT). We discuss problems of data and metadata publication, in particular for geochemistry, and offer solutions to these problems in the form of consistent data publication formats and a proposal for publication of metadata in geochemistry. Metadata are grouped according to types (location, sampling, characterization), and this grouping allows for the transfer of these formats to other Earth science disciplines. In a companion paper [Helly et al., 2003], we illustrate how these metadata groupings can be used in an IT context. Formats presented here are comprehensive and allow for modification and expansion. It is the hope of the authors that this paper initiates a constructive discussion of data formats and metadata in geochemistry. The most recent contributions to this discussion may be found at http:\\earthref.org\metadata\GERM\.
Scalable models of data sharing in Earth sciences
Authors: Helly John , Staudigel Hubert , Koppers Anthony
Published: May 2003

Many Earth science disciplines are currently experiencing the emergence of new ways of data publication and the establishment of an information technology infrastructure for data archiving and exchange. Building on efforts to standardize data and metadata publication in geochemistry [Staudigel et al., 2002], here we discuss options for data publication, archiving and exchange. All of these options have to be structured to meet some minimum requirements of scholarly publication, in particular reliability of archival, reproducibility and falsifiability. All data publication and archival methods should strive to produce databases that are fully interoperable and this requires an appropriate data and metadata interchange protocol. To accomplish the latter we propose a new Metadata Interchange Format (.mif) that can be used for more effective sharing of data and metadata across digital libraries, data archives, and research projects. This is not a proposal for a particular set of metadata parameters but rather of a methodology that will enable metadata parameter sets to be easily developed and interchanged between research organizations. Examples are provided for geochemical data as well as map images to illustrate the flexibility of the approach.
Resolving atmospheric contaminants in mantle noble gas analyses
Authors: Harrison D. , Burnard P. G. , Trieloff M. , Turner G.
Published: May 2003

As part of an ongoing initiative to constrain mantle noble gas budgets, we evaluate both recently published high‐quality noble gas data and report new data in order to rationalize the use of three‐dimensional (3‐D) modeling techniques. Modeling of these data shows that traditional 2‐D mixing plots are not adequate tools to fully constrain mixing systems and mantle end‐member compositions. We show that these mantle noble gas analyses are compromised to varying degrees by the addition of a fractionated atmospheric contaminant, irrespective of eruption setting. This component can also be present in analyses with mantle‐like neon or argon isotope ratios. Previous estimates for the mantle end‐member 40Ar/36Ar and 129Xe/130Xe compositions of most of these samples are likely to be minimum values. In fact it may well be that there are no analyses in the present literature that have completely unadulterated mantle volatiles present.
Degassing and contamination of noble gases in Mid‐Atlantic Ridge basalts
Authors: Burnard P. , Harrison D. , Turner G. , Nesbitt R.
Published: May 2003

New He, Ne, Ar and CO2 stepped‐crushing data from the Mid‐Atlantic Ridge show that contamination of basalts by atmospheric noble gases involves three or more components: unfractionated air, fractionated air with high 36Ar/22Ne (≥45) and fractionated air with low 36Ar/22Ne (≤5). In addition, the magmatic noble gases trapped in these basaltic glasses are variably fractionated such that 4He/40Ar* (where the asterisk indicates corrected for atmospheric contamination based on all 36Ar being atmospheric in origin) is in the range 3–12. Single samples have a range in 4He/40Ar* with the highest ratios in the final crush steps, consistent with the most fractionated (highest 4He/40Ar*) volatiles trapped in the smallest vesicles. It is not possible to distinguish between batch and Rayleigh degassing mechanisms. The complexities of the contamination and magmatic fractionation processes means that it is not possible to estimate 40Ar/36Ar of the mantle source to these basalts other than it must be higher than the highest ratio measured (26,200 ± 5200). Noble gas/CO2 ratios are also variable. While some CO2 adsorption during crushing exaggerates the variations in He/CO2 and Ar/CO2, we show that it is not possible to account for the entire variation as an analytical artefact: some of the variation is present in the vesicles. Variations in He/CO2 cannot be attributed to solubility controlled degassing because of the broadly similar solubilities of He and CO2 in tholeiitic magmas. The large range in He/CO2 in these glasses (factor of 10) is not accompanied by indications of major changes in melting regime or source region chemistry, therefore is thought to reflect late‐stage (magmatic) fractionation of CO2 from the noble gases. It is not possible to identify an explicit mechanism, although both CO2 reduction (e.g., to hydrocarbons or graphite) and kinetic CO2‐noble gas fractionation could account for the variations.
Incompatible element ratios in oceanic basalts and komatiites: Tracking deep mantle sources and continental growth rates with time
Authors: Condie Kent C.
Published: May 2003

Ratios of elements with similar incompatibilities in the mantle can be used to characterize magma sources through time. Nb/Y and Zr/Y distributions in oceanic basalts support the existence of a long‐lived, deep depleted source in mantle. Zr/Y, Nb/Y, Zr/Nb, and Nb/Th ratios in oceanic basalts and komatiites suggest that depleted and recycled components, together probably with an enriched component, were present in the deep mantle by 3.5 Ga. Low Zr/Nb and Hf/Sm ratios and high La/Yb and Nb/Y ratios in some plume basalts and Al‐depleted komatiites may reflect majorite fractionation. High Zr/Nb ratios and low Nb/Y ratios in Archean Al‐undepleted komatiites may record partial melting of a Mg‐perovskite source in deep mantle plumes in which Mg‐perovskite crystallizes and accumulates in komatiite melts during ascent. Oceanic greenstone basalts show a gradual increase in the Nb/Th ratio with time with a relatively sudden increase at about 2 Ga. This trend is consistent with gradual continental growth and with a major episode of continental growth at 2.7 Ga. Nb/Th ratios in some Early Archean basalts may record extraction of up to 25% of the present volume of continental crust from the early upper mantle. An alternative explanation for the rapid increase in Nb/Th in oceanic basalts at 2 Ga is that a catastrophic 2.7‐Ga event in the mantle changed the composition or/and location of the primary volume of mantle from which continental crust was extracted.
Magmatic subsidence of the East Pacific Rise (EPR) at 18°14′S revealed through fault restoration of ridge crest bathymetry
Authors: Carbotte Suzanne M. , Ryan William B. F. , Jin Wen , Cormier Marie‐Helene , Bergmanis Eric , Sinton John , White Scott
Published: May 2003

The fault restoration technique of De Chabalier and Avouac [1994] is applied to an ultra‐high‐resolution bathymetry data set from the East Pacific Rise (EPR) at 18°14′S. Fault offsets are calculated and subtracted from the original seafloor bathymetry to examine the morphology of the unfaulted seafloor surface within an area encompassing the ridge axis 400 × 1600 m in dimension. The restored topography reveals a gently sloping seafloor ∼200 m wide, which slopes 5° inward toward the spreading axis. We attribute this inward sloping seafloor to subsidence within the axial trough due to subsurface magmatic deflation. The vertical deformation field extracted from the bathymetry is used to characterize the ridge axis fault population present in the area. Median fault throws (9 m for inward‐facing and 8 m for outward‐facing faults) are comparable to values measured for nearby mature abyssal hill fault populations, suggesting a genetic link. However, median fault spacings (70 and 46 m) are an order of magnitude smaller, and estimated total extensional strain is 3×–4× greater than values measured for ridge flank faults. These differences indicate the axial fault population at 18°14′S cannot be rafted onto the ridge flanks to form abyssal hill faults without significant modification, presumably via volcanic burial. We attribute the dense faulting observed in this area to slip on axial fissures during subsidence of the crestal region. The surface subsidence of a volcanic edifice can be modeled in terms of volume change in the magma source reservoir and volume of magma withdrawn from the reservoir. Using the relationship derived for a sill‐like magma body, we estimate that the axial depression at 18°14′S could represent magma withdrawal associated with a small number (4–22) of the frequent dike injection and eruption events required to build the upper crust during the evolution of the trough. The subsidence volumes represented by the axial topography at 18°14′S are a small percentage of the underlying upper crustal sections (3–4%), suggesting that only a minor mismatch between magma recharge and withdrawal from the axial melt lens during ongoing plate separation could account for this pronounced axial depression.
Interaction between small‐scale mantle diapirs and a continental root
Authors: van Thienen P. , van den Berg A. P. , de Smet J. H. , van Hunen J. , Drury M. R.
Published: May 2003

A possible mechanism for adding material to a continental root is by means of upwellings from the convecting mantle subject to pressure release partial melting. We present results of numerical modeling of the interaction of melting diapirs with continental roots in an Archean setting characterized by a mantle potential temperature of 1750°C in a two‐dimensional (2‐D) Cartesian geometry. In an extension of earlier work [de Smet et al., 2000b] we have investigated the influence of mantle rheology on the behavior of diapirs. We have in particular looked at the difference in behavior of diapirs using a composite rheology combining both grain size sensitive diffusion creep and dislocation creep mechanisms. We have used the grain size, here taken to be uniform, as a control parameter to obtain model cases with varying contribution from the two creep mechanisms. The diapirs in the composite rheology model rise much faster than they do in a purely Newtonian model. Observed diapiric ascent times from 230 km depth to the top of the ascent path at about 80 km depth are approximately 1 Myr for a Newtonian model (averaged 14 cm/yr) compared to about 50 thousand years for a composite rheology model (averaged 3 m/yr) with the same parameters for the Newtonian component. This clearly indicates the large impact of the dislocation creep component of the viscous deformation process. We have also investigated the effect of an increase in the viscosity due to dehydration during partial melting. This increase has a strong influence on the development of rising diapirs. The ascent velocity and lateral spreading of the diapirs at the end of their ascent are effectively reduced when a viscosity increase by a factor of 10 is applied, and the effect becomes stronger for larger factors. Average vertical velocities range from 1.4 cm/yr for a factor 10 to 2 mm/yr for a factor 200. The most striking result of the viscosity increase due to dehydration is the reduction of the ascent velocity, thereby stretching the characteristic timescale of the diapiric intrusion process to a value between 5 and 50 Myr for dehydration viscosity prefactor values of 10 and 200, respectively. In contrast with the strong difference between the Newtonian and the composite rheology models, small differences are found in the overall dynamics between the composite rheological models, characterized by different values of the uniform grain size. The composite rheological models exhibit self‐regulating behavior where substantial differences between the relative contributions of the two creep components result in very similar effective viscosities, due to a local dominance of dislocation creep at high stresses and corresponding similar flow dynamics. Stress levels and P,T paths in the modeling results are consistent with estimates obtained from Precambrian peridotite bodies which are interpreted to have originated from asthenospheric diapirism.
The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP‐2 drill core
Authors: Eisele Jürgen , Abouchami Wafa , Galer Stephen J. G. , Hofmann Albrecht W.
Published: May 2003

We analyzed Pb isotopic compositions of 50 samples from the HSDP‐2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple‐spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/204Pb from 37.97 to 38.22. In 207Pb/204Pb‐206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/204Pb‐206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as “Kea‐lo8,” “Kea‐mid8,” and “Kea‐hi8.” The 206Pb/204Pb isotope ratios exhibit rapid shifts by ∼0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ∼100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP‐2 in terms of mixing of end‐members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP‐2 arrays converge to a common end‐member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/204Pb ratios. This topology suggests that the HSDP‐2 arrays were produced by mixing of at least four end‐members. The origin of these end‐members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end‐member of the three Pb isotope arrays contains material with elevated μ values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP‐2 Kea‐lo8, Kea‐mid8, and Kea‐hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea‐lo8 array is present between ∼180 and 370 ka and samples more peripheral parts of the plume, while the Kea‐mid8 and Kea‐hi8 arrays occur in the deeper parts of the core (∼370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding “lengths” of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small‐scale heterogeneities derived from the D″ layer in the lower mantle.
Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano
Authors: Blichert‐Toft Janne , Weis Dominique , Maerschalk Claude , Agranier Arnaud , Albarède Francis
Published: May 2003

The present work reports multiple collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) measurements of the isotopic compositions of Hf and Pb in the first 3 km of the deep core retrieved by the Hawaii Scientific Drilling Project. The measurements cover all the samples from the standard geochemical reference set, glasses from the deep hole, and replicates from the pilot hole. Both Hf and Pb are less radiogenic in Mauna Loa compared to Mauna Kea. The transition between Mauna Kea and Mauna Loa lavas in the deep core is progressive for εHf and 208Pb/204Pb, but a sharp discontinuity is observed for 208Pb*/206Pb*. There is no correlation between the alkalinity of the samples and isotopic composition. In detail, the Hf isotope compositions of samples from the pilot hole are not all identical to those of the HSDP‐2 core for samples retrieved from a similar depth, suggesting that steep topography existed at the time of emplacement or that a different eruptive sequence was recorded. The strong correlation between 208Pb*/206Pb* and 3He/4He (He data from M. D. Kurz et al. (Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project, submitted to Geochemistry Geophysics Geosystems, 2002)) requires the episodic incorporation of a component that resembles the basalts erupted by either Kilauea or the Loihi eruptive centers (this component is referred to as K/L). The data suggest that some 500 kyr ago, Mauna Kea was tapping a mantle source similar to that tapped by Kilauea today. Isotopic variability of Pb and He cannot be accounted for by radiogenic ingrowth in a closed system, but requires the mixing of mantle source components with distinct outgassing histories. The time series of isotopic and concentration data in Mauna Kea samples spanning about 350,000 years of age indicate the recurrence of geochemical patterns in the melting column. Ignoring the most recent alkalic samples, we find that the dominant fluctuations of εHf and 207Pb/204Pb correspond to a period of 50,000 years. For La/Yb, Zr/Nb, 87Sr/86Sr, 206Pb/204Pb, 207Pb/206Pb, and 208Pb/206Pb, a dominant period of ca. 18,000 years is obtained. Once provision is made for the existence of harmonics, the consistency between the isotopic spectrum of the pilot hole and the HDSP‐2 core is very good. The input of the K/L component does not seem to be periodic. We use these recurrence intervals in conjunction with the upwelling rate deduced from buoyancy flux and seismic evidence of the maximum dimension of scatterers to constrain the radius of the Hawaiian plume conduit to be in the range of 10–50 km and the upwelling velocity to be in the range of 0.13–3 m/yr. Plausible vertical length scales of heterogeneities in the conduit are 6.5–160 km.
Geochemistry of Kauai shield‐stage lavas: Implications for the chemical evolution of the Hawaiian plume
Authors: Mukhopadhyay S. , Lassiter J. C. , Farley K. A. , Bogue S. W.
Published: May 2003

We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield‐stage lavas from Kauai, Hawaii. The range of 3He/4He ratios (17–28 RA) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3He/4He ratios are restricted to the preshield stage of Hawaiian magmatism. 3He/4He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3He/4He ratios (up to 8 RA in ∼102 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three‐component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted‐mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3He/4He end‐member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206Pb/204Pb, anomalously low 207Pb/204Pb relative to 206Pb/204Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3He/4He reservoir in the Hawaiian plume and suggest that the proportion of the different mantle components in the plume have changed significantly in the past 5 Myr. The long‐term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas require more complex geochemical heterogeneities than suggested by radially zoned plume models. These complexities may arise from heterogeneities in the thermal boundary layer and through variable entrainment of ambient mantle by the upwelling plume.
Paleomagnetism of the southwestern U.S.A. recorded by 0–5 Ma igneous rocks
Authors: Tauxe Lisa , Constable Catherine , Johnson Catherine L. , Koppers Anthony A. P. , Miller Winter R. , Staudigel Hubert
Published: May 2003

The issue of permanent nondipole contributions to the time‐averaged field lies at the very heart of paleomagnetism and the study of the ancient geomagnetic field. In this paper we focus on paleomagnetic directional results from igneous rocks of the southwestern U.S.A. in the age range 0–5 Ma and investigate both the time‐averaged field and its variability about the mean value. Several decades of work in the southwestern United States have resulted in the publication of paleomagnetic data from over 800 individual paleomagnetic sites. As part of a new investigation of the San Francisco Volcanics, we collected paleomagnetic samples from 47 lava flows, many of which have been previously dated. The new data combined with published data are highly scattered. Contributions to the scatter were considered, and we find that removal of data sets from tectonically active areas and judicious selection according to Fisher's [1953] precision parameter results in an axially symmetric data distribution with normal and reverse modes that are indistinguishable from antipodal. Monte Carlo simulations suggest that a minimum of 5 samples per site are needed to estimate the precision parameter sufficiently accurately to allow its use as a determinant of data quality. Numerical simulations from statistical paleosecular variation models indicate the need for several hundred paleomagnetic sites to get an accurate determination of the average field direction and are also used to investigate the directional bias that results from averaging unit vectors rather than using the full field vector. Average directions for the southwestern U.S.A. show small deviations from a geocentric axial dipole field, but these cannot be considered statistically significant. Virtual geomagnetic pole (VGP) dispersions are consistent with those from globally distributed observations analyzed by McElhinny and McFadden [1997]. However, a systematic investigation of the effect of imposing a cutoff on VGPs with large deviations from the geographic axis indicates that while it may reduce bias in calculating the average direction, such a procedure can result in severe underestimates of the variance in the geomagnetic field. A more satisfactory solution would be to use an unbiased technique for joint estimation of the mean direction and variance of the field distribution.
The evolution of the Izu Bonin ‐ Mariana volcanic arcs (NW Pacific) in terms of major element chemistry
Authors: Straub Susanne M.
Published: May 2003

New and published analyses of major element oxides (SiO2, TiO2, Al2O3, FeO*, MnO, MgO, CaO, K2O, Na2O and P2O5) from the central Izu Bonin and Mariana arcs (IBM) were compiled in order to investigate the evolution of the IBM in terms of major elements since arc inception at ∼49 million years ago. The database comprises ∼3500 volcanic glasses of distal tephra fallout and ∼500 lava samples, ranging from the Quaternary to mid‐Eocene in age. The data were corrected to 4 wt% MgO in order to display the highly resolved temporal trends. These trends show that the IBM major elements have always been “arc‐like” and clearly distinct from N‐MORB. Significant temporal variations of some major element oxides are apparent. The largest variations are displayed by K4.0. The data support a model wherein the K2O variability is caused by the addition of slab component with strongly differing K2O contents to a fairly depleted subarc mantle; variable extents of melting, or mantle heterogeneity, appear to play a negligible role. The other major element oxides are controlled by the composition and processes of the subarc mantle wedge. The transition from the boninitic and tholeiitic magmatism of the Eocene and Oligocene to the exclusively tholeiitic magmatism of the Neogene IBM is proposed to reflect a change in the composition of the subarc mantle wedge. The early boninitic magmas originate from an ultra‐depleted subarc mantle, that is residual to either the melting of E‐MORB mantle, or of subcontinental lithospheric mantle. During the Eocene and Oligocene, this residual mantle is gradually replaced by Indian MORB mantle advected from the backarc regions. The Indian MORB mantle is more radiogenic in Nd isotope ratios but also more fertile with respect to major and trace elements. Therefore the Neogene tholeiites have higher Al2O3 and TiO2 contents and lower mg# numbers at given SiO2 content. After the subarc mantle replacement was complete in the late Oligocene or early Miocene, the Neogene IBM entered a “steady state” that is characterized by the continuous advection of Indian MORB mantle from the reararc, which is fluxed by fluids and melt components from slab. The thickness of the IBM crust must have grown with time, but any effects of crustal thickening on the major element chemistry of the IBM magmas appear to be minor relative to the compositional changes that are related to source composition. Therefore next to the processes of melting, the composition of the mantle sources must play a major role in creating substantiative heterogeneities in the major element chemistry of the arc crust.
Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components
Authors: Snyder Glen , Poreda Robert , Fehn Udo , Hunt Andrew
Published: May 2003

Gases and fluids from four geothermal fields of Central America were analyzed for nitrogen, methane, and helium concentrations, isotopic composition, and 129I/I ratios in order to determine the sources of volatiles in these systems. Results for gas ratios and isotopic compositions for three of the fields are consistent with observations from other subduction zones. Ratios of N2/3He are only slightly higher than average arc values of 1 × 108 and the volcanic flux of N2 for the Central American systems is estimated to be between 1.6 × 108 and 3.2 × 108 mol/yr. Analysis of 129I/I ratios indicates the presence of a subducted organic component (25–30 Ma) as well as of a much older crustal component (40–65 Ma) throughout the study area. The magmatic flux of nitrogen and noble gases in Central America was then extrapolated to determine the degree of nitrogen recycling in island arc systems. Global N2 flux is estimated at 2.7 × 109 to 5.4 × 109 mol/yr, which is comparable to the global mid‐ocean ridge flux, and represents 29–58% of the subducted sediment flux. This flux estimate is consistent across the N2–CO2–He systems and suggests that nearly all of the nitrogen supplied to the mantle wedge is devolatilized beneath the volcanic front. The Momotombo geothermal field of Nicaragua is characterized by exceptionally high excess nitrogen and methane values, and the close correlation of these two gases indicates a common source. While it is not uncommon for sedimentary basins with high heat flow to have excess nitrogen, the Momotombo geothermal field is unique in that the high N2/3He gases have essentially magmatic 3He/4He ratios. The high excess nitrogen component of Nicaragua is related to the older iodine end‐member, pointing to a crustal origin. The crustal nitrogen contributions along the Nicaraguan portion of the arc are on the order of 2.2 × 108 mol N2/yr or roughly equal to the magmatic contribution along the entire Central American volcanic arc. The results for Momotombo indicate that the release of nitrogen during reorganization of island arc systems may have a significant impact on the global flux of volcanic nitrogen.
Lithium isotope geochemistry of the Hawaiian plume: Results from the Hawaii Scientific Drilling Project and Koolau Volcano
Authors: Chan Lui‐Heung , Frey Frederick A.
Published: May 2003

We determined lithium isotopic compositions of Mauna Loa and Mauna Kea basalts from the 3.1 km drill hole of the Hawaiian Scientific Drilling Project (HSDP); for comparison Li isotopic ratios were also determined for basalts from Koolau volcano. These two suites of samples define geochemical extremes in the range of Hawaiian shield lavas. The 400 Ka record of Mauna Kea in the HSDP core shows temporal fluctuations between low δ7Li (∼4‰ relative to the L‐SVEC standard) and high δ7Li (5–6‰), suggesting that the source components in the Hawaiian plume are heterogeneous in Li isotopic composition. Based on SiO2 content and isotopic ratios of He, Li, Nd, Hf and Pb, three geochemical groups are identified in Mauna Kea lavas. Mauna Kea basalts between 1900 and 2500 mbsl have relatively low δ7Li of about 4‰. They are low SiO2 lavas distinguished by the highest 3He/4He and 208Pb/204Pb, and low 176Hf/177Hf and 143Nd/144Nd. Like basalt from Loihi seamount, this Mauna Kea group is considered to originate from the core of the plume. Above 1900 mbsl, high δ7Li lavas with high SiO2 contents appear in both the submarine and subaerial sections. They are marked by low 3He/4He and high 176Hf/177Hf. The 7Li‐rich signature of some samples (δ7Li up to 5.7) is indicative of recycled oceanic crust in the plume. This magma group defines the Kea component. The low SiO2 lavas in the subaerial section have low δ7Li (∼4‰), 3He/4He and 208Pb/204Pb. Their δ7Li values overlap the range of δ7Li in unaltered mid‐ocean ridge basalt (MORB) and are consistent with upper mantle material entrained by the plume or contamination of plume‐derived magmas by the Pacific lithosphere. The δ7Li of Koolau lavas mostly fall within the range of 4.5 ± 0.3‰. Exceptions are two samples that have δ7Li of 2–3‰. The lightest isotopic values may indicate subducted Li that was isotopically fractionated during slab dehydration. In contrast to other isotopic systems, most Koolau samples, however, resemble Mauna Kea samples in Li isotopic composition. Mauna Loa samples have δ7Li values of 3.5 to 4.9‰, within the range of the Koolau and Mauna Kea lavas. Based on these data, the Loa trend volcanoes and Kea trend volcanoes have largely overlapping Li isotopic compositions. In summary, the Hawaiian plume is not highly variable in Li isotopic composition; δ7Li is typically ∼4‰ with perturbations by subducted components to lower and higher ratios (2.5 to 5.7‰). The overlap of most Hawaiian basalt and MORB in their range of Li isotopic ratios suggests minor influence of recycled oceanic crust in the plume and perhaps similar Li isotopic ratios in the upper and lower mantle.
Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean
Authors: Hodell David A. , Venz Kathryn A. , Charles Christopher D. , Ninnemann Ulysses S.
Published: May 2003

We demonstrate that the carbon isotopic signal of mid‐depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial‐to‐interglacial variations in benthic δ13C, whereas the amplitude of the δ13C signal at Site 1088 (∼2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic δ13C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate‐to‐deep δ13C gradients (Δ13CI‐D) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well‐ventilated water above ∼2500 m from poorly ventilated water below. The Δ13CI‐D signal parallels the Vostok atmospheric pCO2 record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100‐kyr cycle in Δ13CI‐D during the mid‐Pleistocene supports a change in vertical fractionation and deep‐water ventilation rates in the Southern Ocean, and is consistent with possible CO2‐forcing of this climate transition.
Kinetics of crystal‐melt reaction in partially molten silicates: 1. Grain scale processes
Authors: Liang Yan
Published: May 2003

When major elements in a partial melt and its host crystalline matrix are out of chemical equilibrium and at a constant temperature and pressure, dissolution or precipitation takes place. The kinetics of crystal‐melt reequilibration in a mono‐mineralic and partially molten silicate was examined using a combined numerical, theoretical, and experimental approach. The salient feature of crystal‐melt reaction in a partially molten system is dissolution and reprecipitation. There are at least two regimes of crystal‐melt reequilibration in a two‐phase aggregate: diffusion‐in‐melt‐limited dissolution (referred to as Regime I dissolution) and diffusion‐in‐solid‐limited reprecipitation (designated as Regime II dissolution). Regime I dissolution follows the series rule and is rate‐limited by the slowest diffusing component in the melt. It always over dissolves when extensive solid solution exists in the crystal. This over‐dissolution is compensated by diffusion‐in‐solid‐limited reprecipitation at a later time when the magnitude of the diffusive flux in the melt is smaller than that in the crystal. The rate of Regime II dissolution can be described by a parallel rule and is dominated by the component that has the largest diffusive flux in the solid. The dissolved mineral composition is practically identical to the initial crystal composition during Regime I dissolution when the diffusion rate in the melt is more than three orders of magnitude faster than the diffusion rate in the crystal. Local averaged melt composition is on and moves along the liquidus of the dissolving crystal during Regime II dissolution. The final equilibrium crystal and melt compositions and crystal‐melt proportion are identical to the values predicted by the lever rule. Another important feature of the kinetics of crystal‐melt reaction in a multicomponent system is the variations of crystal and melt compositions at their interface as a function of dissolution time even under the assumption of local thermodynamic equilibrium at their interface. Variations of interface melt composition during crystal melting or dissolution have sometimes been attributed to interface disequilibrium. The validity of such conclusions needs to be reexamined. Diffusive fractionation in a multicomponent crystal‐melt mixture gives rise to nonlinear concentration paths in composition space. Such nonlinear mixing process cannot be fully resolved by simple mass balance calculations. Given that solid solutions are common among many rock‐forming minerals, and diffusion rates of major and minor constituents in these minerals can differ by orders of magnitude, dissolution and reprecipitation are likely to play an important role in crystal‐melt interaction under upper mantle and lower crust conditions.
Hydrogen concentration analyses using SIMS and FTIR: Comparison and calibration for nominally anhydrous minerals
Authors: Koga Kenneth , Hauri Erik , Hirschmann Marc , Bell David
Published: May 2003

We report analyses of hydrogen abundance in experimentally annealed and natural mantle minerals using FTIR and use these data to establish calibration lines for measurement of H2O concentrations in olivine, pyroxenes, garnet, amphibole and mica by secondary ion mass spectrometry (SIMS). We have reduced the detection limit for H2O analysis by SIMS to 2–4 ppm H2O (by weight) through careful attention to sample preparation and vacuum quality. The accuracy of the SIMS calibrations depends on the choice of FTIR extinction coefficients; however, all of the calibrations reported here are shown to be consistent with measurements on standards whose H2O abundance has been determined independently via manometry or nuclear reaction analysis. The resulting calibrations are accurate to 10–30% at the 95% confidence limit, with improvements possible through the use of higher‐H2O standards. Using our SIMS calibration, we determined hydrogen concentrations in coexisting olivine, orthopyroxene, and glass from a single melting experiment at 2 GPa and 1380°C. Olivine/melt and orthopyroxene/melt partition coefficients are equal to 0.0020 ± 0.0002 and 0.0245 ± 0.0015, respectively, and the orthopyroxene/olivine coefficient is 12 ± 4 (2σ uncertainties).
Blueschist preservation in a retrograded, high‐pressure, low‐temperature metamorphic terrane, Tinos, Greece: Implications for fluid flow paths in subduction zones
Authors: Breeding Christopher M. , Ague Jay J. , Bröcker Michael , Bolton Edward W.
Published: May 2003

The preservation of high‐pressure, low‐temperature (HP‐LT) mineral assemblages adjacent to marble unit contacts on the Cycladic island of Tinos in Greece was investigated using a new type of digital outcrop mapping and numerical modeling of metamorphic fluid infiltration. Mineral assemblage distributions in a large blueschist outcrop, adjacent to the basal contact of a 150‐meter thick marble horizon, were mapped at centimeter‐scale resolution onto digital photographs using a belt‐worn computer and graphics editing software. Digital mapping reveals that while most HP‐LT rocks in the outcrop were pervasively retrograded to greenschist facies, the marble‐blueschist contact zone underwent an even more intense retrogression. Preservation of HP‐LT mineral assemblages was mainly restricted to a 10–15 meter zone (or enclave) adjacent to the intensely retrograded lithologic contact. The degree and distribution of the retrograde overprint suggests that pervasively infiltrating fluids were channelized into the marble‐blueschist contact and associated veins and flowed around the preserved HP‐LT enclave. Numerical modeling of Darcian flow, based on the field observations, suggests that near the marble horizon, deflections in fluid flow paths caused by flow channelization along the high‐permeability marble‐blueschist contact zone likely resulted in very large fluid fluxes along the lithologic contact and significantly smaller fluxes (as much as 8 times smaller than the input flux) within the narrow, low‐flux regions where HP‐LT minerals were preserved adjacent to the contact. Our results indicate that lithologic contacts are important conduits for metamorphic fluid flow in subduction zones. Channelization of retrograde fluids into these discrete flow conduits played a critical role in the preservation of HP‐LT assemblages.