Browsing by Author "Klein, EM"

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    4.13 - Composition of the Oceanic Crust
    (Treatise on Geochemistry (Second Edition), 2014) White, WM; Klein, EM
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    Isotope evidence of a mantle convection boundary at the Australian-Antarctic Discordance
    (Nature, 1988-12-01) Klein, EM; Langmuir, CH; Zindler, A; Staudigel, H; Hamelin, B
    Basalts from the Southeast Indian Ridge south of Australia form two geographically and isotopically distinct groups that show affinities with either Indian Ocean or Pacific/Atlantic Ocean isotope compositions. The data raise the possibility that there is a sharp boundary between the Indian and Pacific Ocean isotope provinces. The isotope boundary occurs over less than 200km within the Australian-Antarctic Discordance, a technically complex region believed to overlie a zone of downwelling mantle flow. © 1988 Nature Publishing Group.
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    Mixing of basalt magmas
    (Nature, 1989-12-01) Klein, EM
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    Ocean-ridge basalt compositions correlated with palaeobathymetry
    (Nature, 1990-12-01) Keen, MJ; Klein, EM; Melson, WG
    KLEIN and Langmuir1 investigated the relationships between the chemistry and water depth of eruption of zero-age mid-ocean-ridge basalts around the world. They showed that the regionally averaged values of the major-element oxides Na2O and FeO, corrected for low-pressure fractionation, and the ratio CaO/Al2O3, correlate with their axial depths. Klein and Langmuir pointed out that the same principles should also apply to older ocean crust, where the relationships that they had established could be used as 'calibration curves', and that there was a "possibility of petrologically constraining the origins of bathymetrie anomalies throughout the ocean basins"1. Keen2 showed that this can be achieved if the initial depths of eruption of older basalts are found by correcting their present depths below sea level for sediment loading and thermal subsidence. Here we show that the chemistry of basalts from older oceanic crust in the Atlantic and Indian Oceans correlates with their depth restored in this way to the original water depth at the time of eruption. © 1990 Nature Publishing Group.
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    Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges
    (Mantle Flow and Melt Generation at Mid-Ocean Ridges, 1992) Langmuir, CH; Klein, EM; Plank, T
    Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots (“normal” ridges) can be used to define the global correlations among normalized Na2O, FeO, TiO2 and SiO2 contents, CaO/Al2O3 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na2O and depth over distances of 1000 km.Local variations in basalt chemistry from slow- and fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts.Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical.Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements.We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched basalts. Melting processes associated with slow-spreading ridges vary substantially over short distances along strike and lead to the local trends discussed above, irrespective of hot spot influence. In the Pacific, enriched components appear to have been more thoroughly mixed into the mantle, leading to ubiquitous small scale heterogeneities. Melting processes do not vary appreciably along strike, so local chemical variations are dominated by the relative contribution of enriched component on short time and length scales. Thus the extent of mixing and distribution of enriched components influences strongly the contrasting local major element trends. Despite the difference in the distribution of enriched components, the mean compositions of each data set are equivalent. This suggests that the hot spot influence is similar in the two ocean basins, but its distribution in the upper mantle is different. These contrasting relationships between hot spots and ridges may result from differences in both spreading rate and tectonic history. Unrecognized hot spots may play an important role in diverse aspects of EPR volcanism, and in the chemical systematics of the erupted basalts.The observations and successful models have consequences for melt formation and segregation. (1) The melting process must be closer to fractional melting than equilibrium melting. This result is in accord with inferences from abyssal peridotites [Johnson et al., 1990]. (2) Small melt fractions generated over a range of pressures must be extracted rapidly and efficiently from high pressures within the mantle without experiencing low pressure equilibration during ascent. This requires movement in large channels, and possibly more efficient extraction mechanisms than nonnally envisaged in porous flow models with small residual porosity. (3) Diverse melts from the melting regime produce variations in basalts that are observable at the surface. (4) Basalt data can be used to constrain the melting process (e.g. active vs. passive upwelling) and its relationship to segmentation. The data cannot be used to constrain the shape of the melting regime, however, for many shapes lead to similar chemical results. (5) Highly incompatible elements and U-series disequilibria results appear not yet to be explained by melting models, and may require additional processes not yet clearly envisaged.
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    Reconciling geochemical and geophysical observations of magma supply and melt distribution at the 9N overlapping spreading center, East Pacific Rise
    (Geochemistry, Geophysics, Geosystems, 2012-11-01) Wanless, VD; Perfit, MR; Klein, EM; White, S; Ridley, WI
    Early studies of mid-ocean ridge discontinuities, such as transform faults and overlapping spreading centers, suggested a lower magma supply compared to ridge segment centers. This is reflected in bathymetrically deeper ridge axes, decreased hydrothermal activity, and the eruption of more evolved lava compositions. While many signatures of lower magma supply are observed at the 9N overlapping spreading center on the East Pacific Rise, geophysical studies indicate extensive sub-surface melt in the region, suggesting that the present magmatic system is not diminished. Here major and trace element concentrations of erupted lavas are used to better understand magma supply at a large second-order ridge discontinuity. We show that the wide range of lava compositions erupted at the 9N overlapping spreading center is generally consistent with early petrologic models of ridge propagation and require variable degrees of fractional crystallization, extensive magma mixing, and in some instances crustal assimilation. Moderately evolved ferrobasalts and FeTi basalts erupted at the OSC indicate that crustal residence times are long enough for significant crystallization of all magmas within the region, but the presence of dacitic lavas reflects periods of even lower magma supply, where melt replenishment is subordinate to cooling and crystallization. The geophysical observations of extensive melt within the shallow crust are reconciled with the geochemistry of the lavas, if melts are supplied intermittently to the propagating ridge over relatively short timescales. © 2012. American Geophysical Union. All Rights Reserved.
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    Tectonic and magmatic segmentation of the Global Ocean Ridge System: A synthesis of observations
    (2016-01-01) Carbotte, SM; Smith, DK; Cannat, M; Klein, EM
    © 2016 The Author(s).Mid-ocean ridges display tectonic segmentation defined by discontinuities of the axial zone, and geophysical and geochemical observations suggest segmentation of the underlying magmatic plumbing system. Here, observations of tectonic and magmatic segmentation at ridges spreading from fast to ultraslow rates are reviewed in light of influential concepts of ridge segmentation, including the notion of hierarchical segmentation, spreading cells and centralized v. multiple supply of mantle melts. The observations support the concept of quasi-regularly spaced principal magmatic segments, which are 30-50 km long on average at fast- to slow-spreading ridges and fed by melt accumulations in the shallow asthenosphere. Changes in ridge properties approaching or crossing transform faults are often comparable with those observed at smaller offsets, and even very small discontinuities can be major boundaries in ridge properties. Thus, hierarchical segmentation models that suggest large-scale transform fault-bounded segmentation arises from deeper level processes in the asthenosphere than the finer-scale segmentation are not generally supported. The boundaries between some but not all principal magmatic segments defined by ridge axis geophysical properties coincide with geochemical boundaries reflecting changes in source composition or melting processes. Where geochemical boundaries occur, they can coincide with discontinuities of a wide range of scales.
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    The Global Biogeochemical Cycle of Arsenic
    (Global Biogeochemical Cycles, 2022-11-01) Schlesinger, WH; Klein, EM; Vengosh, A
    Direct exploitation and use of arsenic resources has diminished in recent years, but inadvertent mobilizations of As from mineral extractions (metal ores, coal, and phosphate rock) are now as much as ten-fold greater (1,500–5,600 × 109 g/yr) than the As released by the natural rate of rock weathering at the Earth's surface (60–544 × 109 g/yr). Although some As from mining activities enters global cycling through leaching and spills, the amount of dissolved As in rivers (23 × 109 g/yr) is similar to the theoretical mobilization of As from chemical weathering. Anthropogenic emissions to the atmosphere (17–38 × 109 g As/yr) are double the natural background sources (10–25 × 109 g As/yr), largely as a result of the smelting of Cu and other non-ferrous ores. This results in increased atmospheric deposition near regions with high mining and industrial activities, with potential consequences to human health, natural ecosystems and agriculture. Using median values for As, the ratio of anthropogenic to natural emissions to the atmosphere (1.57) suggests a human impact on the global As cycle that rivals those for V, Hg and Pb.
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    Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on mid-ocean ridges: The role of assimilation at spreading centers
    (Chemical Geology, 2011-08-07) Wanless, VD; Perfit, MR; Ridley, WI; Wallace, PJ; Grimes, CB; Klein, EM
    Most geochemical variability in MOR basalts is consistent with low- to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR high-silica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H2O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of ~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ18O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ18O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites. © 2011 Elsevier B.V.