Geochemical Perspectives Letters

Journal: Geochemical Perspectives Letters

Abbreviation

Geochem. Persp. Let.

Publisher

European Association of Geochemistry

ISSN

2410-339X
2410-3403

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Publications 1 - 10 of 14

Dominance of benthic flux of REEs on continental shelves: implications for oceanic budgets
Deng, Kai; Yang, Shouye; Du, Jianghui; et al. (2022)
Geochemical Perspectives Letters
Rare earth elements (REEs) are powerful tools to track oceanic biogeochemical processes. However, our understanding of REE sources is incomplete, leading to controversial interpretations regarding their oceanic cycling. Continental margin sediments are often assumed to be a major source, but the sediment pore water data required to understand the processes controlling that potential source are scarce. Here, we measure and compile pore water and estuarine REE data from the Changjiang (Yangtze) estuary–East China Sea shelf. We show that release of REEs, from shallow pore water to overlying seawater, is coupled to Mn reduction. In contrast, REEs are removed in deep pore water, perhaps via formation of an authigenic REE-bearing phase. This sedimentary source can potentially explain REE addition in the estuary at mid-high salinity. Our calculations suggest that the benthic flux is the largest Nd source (∼40 %) on the East China Sea shelf. Globally, however, despite a higher benthic Nd flux on the advection-dominated shelf, the much more extensive deep ocean still dominates the total area-integrated benthic flux. Our results call for a more extensive investigation of the magnitude of the benthic flux of REEs to the oceans.
Molybdenum isotope constraints on the temporal development of sulfidic conditions during Mediterranean sapropel intervals
Sweere, Tim C.; Hennekam, Rick; Vance, Derek; et al. (2021)
Geochemical Perspectives Letters
Mediterranean sapropels represent some of the largest scale deoxygenation events in recent Earth history. Here, we use high resolutionMo isotope data for seven such events (sapropels S3 to S9) to semi-quantitatively constrain past H2S concentrations using a new interpretive framework. Bottom water H2S was present for all studied sapropels, but the extent of redox changes varied considerably between them, the ultimate driver likely being variations in monsoon strength. Nearquantitative removal of Mo (δ98Mo > 2‰) during deposition of sapropels S5 and S7 suggests predominantly highly sulfidic conditions with long deep water residence times, comparable to the modern Black Sea, whereas considerably lower δ98Mo values for sapropels S3, S4, S8, and S9 (-0.4 to +0.9‰) imply mildly euxinic conditions only (0
Moderately volatile element behaviour at high temperature determined from nuclear detonation
Day, J.M.D.; Moynier, Frédéric; Sossi, Paolo A.; et al. (2020)
Geochemical Perspectives Letters
The first nuclear detonation, at the Trinity test site, is an analogue for high temperature volatile loss during planet formation processes. We report Cu isotope and abundance data, comparing them with Zn and K isotopes in trinitite glasses sampled with distance from the detonation centre. Copper concentrations drop, and isotopic compositions become ~0.3 ‰ higher with proximity to the detonation. Relative sensitivity to high temperature evaporative isotopic fractionation processes occur in the order: Zn > Cu >> K. Lower volatility of K relative to Cu and Zn results from its low activity coefficient in silicate melts. Lunar mare basalt and some tektite Cu, Zn and K isotope compositions can be modelled using empirical fractionation factors (α) derived from trinitite. Combined with isotopic variations measured in mare basalts, the results support volatile loss during a magma ocean phase for the Moon.
Rapid onset of ocean anoxia shown by high U and low Mo isotope compositions of sapropel S1
Andersen, Morten B.; Matthews, Alan; Bar-Matthews, Miryam; et al. (2020)
Geochemical Perspectives Letters
Authigenic uranium isotope compositions of Holocene sapropel S1 (delta U-238(auth) = + 0.10 to + 0.52 %; ODP core 967, 2550 mbsl) are significantly higher than the proposed upper boundary (+0.2%) associated with the transport-porewater diffusion model for sediment uranium uptake. It is shown that these high delta U-238(aut) values are compatible with rapid initial slowdown of thermohaline overturning and the development of an anoxic water column. These conditions would favour U uptake in an organic-rich floccule layer overlying the sediment-water interface. The high delta U-238(aut) values correlate with low delta Mo-98(auth) values (+0.02 to -0.88%), interpreted to reflect weakly euxinic conditions controlled by thiomolybdate-molybdate solution equilibria. The S1 data contrast markedly with published data from last interglacial sapropel S5 from the same core, which show delta U-238(aut) and delta Mo-98(auth) characteristics compatible with a restricted euxinic basin due to progressive slowdown in the thermohaline circulation. The U-Mo isotope data for S1 are similar to a range of published palaeo-settings. Sapropels are therefore shown to be useful templates for the unravelling of the interplay between productivity and deep water renewal times in ancient settings.
The primordial He budget of the Earth set by percolative core formation in planetesimals
Roth, Antoine S.G.; Liebske, Christian; Maden, Colin; et al. (2019)
Geochemical Perspectives Letters
The pyroxenite-diamond connection
Kiseeva, Ekaterina S.; Wood, B. J.; Ghosh, Sujoy; et al. (2016)
Geochemical Perspectives Letters
Pieces of the Earth’s mantle occurring either as tectonic fragments or xenoliths in volcanic rocks are dominantly peridotites, assemblages of olivine, ortho- and clinopyroxene with minor garnet and/or spinel. They frequently contain pyroxene-rich inclusions which have compositions intermediate between peridotite and basalt. These pyroxenites typically contain varying amounts of more iron-rich (than peridotite) clinopyroxene, orthopyroxene, garnet and/or spinel and are commonly compositionally layered. Surprisingly, despite their subordinate abundance in mantle fragments, pyroxenitic compositions appear be the dominant sources of majoritic garnet inclusions in diamonds, the principal window into the mineralogy of the deep upper mantle and the transition zone (Kiseeva et al., 2013a). In this study we show that the pyroxenite-diamond association is a consequence of the interaction between basaltic and peridotitic compositions in the presence of carbonate melt and that layering of the pyroxenites is a natural consequence of this interaction. Reduction of carbonate to carbon at high pressures is responsible for the genetic connection between pyroxenite and diamond and the abundance of pyroxenitic inclusions reflects this connection rather than a high abundance of this rock type in the mantle.
Contributions of visible and invisible pores to reactive transport in dolomite
Tutolo, Benjamin M.; Luhmann, Andrew J.; Kong, Xiang-Zhao; et al. (2020)
Geochemical Perspectives Letters
Recent technical advances have demonstrated the importance of pore-scale geochemical processes for governing Earth’s evolution. However, the contribution of pores at different scales to overall geochemical reactions remains poorly understood. Here, we integrate multiscale characterisation and reactive transport modelling to study the contribution of pore-scale geochemical processes to the hydrogeochemical evolution of dolomite rock samples during CO2-driven dissolution experiments. Our results demonstrate that approximately half of the total pore volume is invisible at the scale of commonly used imaging techniques. Comparison of pre- and post-experimental analyses demonstrate that porosity-increasing, CO2-driven dissolution processes preferentially occur in pores 600 nm–5 μm in size, but pores <600 nm in size show no change during experimental alteration. This latter observation, combined with the anomalously high rates of trace element release during the experiments, suggests that nanoscale pores are accessible to through-flowing fluids. A three dimensional simulation performed directly on one of the samples shows that steady state pore-scale trace element reaction rates must be ∼10× faster than that of dolomite in order to match measured effluent concentrations, consistent with the large surface area-to-volume ratio and high reactivity of these pores. Together, these results yield a new conceptual model of pore-scale processes, and urge caution when interpreting the trace element concentrations of ancient carbonate rocks.
Nucleosynthetic s-Process Depletion in Mo from Ryugu samples returned by Hayabusa2
Nakanishi, N.; Yokoyama, T.; Ishikawa, A.; et al. (2023)
Geochemical Perspectives Letters
Initial analyses of samples collected from two locations on the asteroid Ryugu indicated that the mineralogical, chemical, and isotopic characteristics of the Ryugu samples show similarities to carbonaceous chondrites, particularly the Ivuna-type (CI) group. In this study, we analysed a composite sample of four bulk Ryugu samples (A0106, A0106-A0107, C0107, and C0108) collected from both sampling locations that were combined in order to determine its mass independent Mo isotopic composition and reveal contributions from diverse nucleosynthetic sources. The ɛ⁹⁴Mo and ɛ⁹⁵Mo values for the Ryugu sample are characterised by the carbonaceous chondrite (CC)-type, which is consistent with the nucleosynthetic isotope compositions observed for other elements (Cr, Ti, Fe, and Zn). The Ryugu composite sample, however, is characterised by greater s-process depletion of Mo isotopes compared with any known bulk carbonaceous chondrite, even including CI chondrites. The observed Mo isotopic signature in the Ryugu composite was most likely caused by either incomplete digestion of s-process-rich presolar SiC, or biased sampling of materials enriched in aqueously-formed secondary minerals characterised by s-process-poor Mo isotopes, resulting from the physicochemical separation between s-process-rich presolar grains and a complementary s-process-poor aqueous fluid in the Ryugu parent body.
Calcium isotope fractionation during melt immiscibility and carbonatite petrogenesis
Antonelli, Michael A.; Sartori, Gino; Giuliani, Andrea; et al. (2023)
Geochemical Perspectives Letters
Stable calcium isotopes have been used to suggest that subducted marine carbonates are frequently involved in the formation of carbonatites. Significant Ca isotope fractionations during carbonatite petrogenesis, however, could lead to a dramatically different picture. We present Ca isotope data for (i) coexisting (immiscible) carbonatite and silicate melts from high temperature centrifuging piston cylinder experiments, (ii) primary apatite and calcite/dolomite from natural carbonatites, and (iii) ab initio estimates for equilibrium Ca isotope partitioning in calcite, dolomite, and ankerite. Carbonatitic melts have lower δ⁴⁴Ca than their conjugate silicate melts, with an equilibrium fractionation factor [1000lnα(1000K)] of −0.21 ± 0.06 (tSE). We develop a quantitative four stage model for carbonatite petrogenesis (partial melting followed by fractional crystallisation, carbonatite-silicate melt immiscibility, and calcite/apatite accumulation) that fully explains our natural data (average δ⁴⁴CaBSE of −0.30 ± 0.03 ‰) and those from recent studies, without requiring isotopic contributions from recycled marine carbonates. Our results suggest that lighter isotopes of similarly bound cations (e.g., Mg, Fe, Sr, Ba, Zn) should be preferentially incorporated into carbonatitic melts and that calciocarbonatite formation involves melt immiscibility after differentiation of mantle-derived alkaline CO₂-bearing silicate melts.
Direct sensing of total alkalinity profile in a stratified lake
Afshar, M. Ghahraman; Tercier-Waeber, M. L.; Wehrli, B.; et al. (2017)
Geochemical Perspectives Letters

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