Abstract:
The abundance of dolomite in ancient carbonate sediments, and its apparent rarity today, has important implications for the coupled Ca-Mg-C-cycles in seawater and global climate. Despite its importance, there are large differences between published records of dolomite abundance vs. geologic age, mainly due to complexities in adequately sampling heterogeneous bedrock. We overcome this issue by using dissolved Mg²⁺ and Ca²⁺ measurements in rivers draining carbonate-bearing bedrock. Because rivers weather broad areas, this approach integrates the geochemical composition of much larger volumes of carbonate compared to sample based methods. The average Mg/(Ca + Mg) molar ratio in rivers declines with decreasing bedrock age, from 0.44 at ∼485 million year old (Ma) to 0.14 at ∼5 Ma, suggesting a decreasing percentage of dolomite in carbonate sequences across the Phanerozoic Eon. These data are hard to reconcile with any model that relies only upon oscillatory drivers to explain the dolomite abundance record, such as sea level or episodic expansions of ocean anoxia, and have important implications for the oceanic Mg cycle.

J.M. Husson, L.A. Coogan

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2023) 26, 1–6 | doi: 10.7185/geochemlet.2316 | Published 26 May 2023

Article views: 12461

Abstract:
The crystallisation ages of lunar samples provide critical constraints on the minimum formation age of the Moon and its early evolution. Zircon crystals from Apollo 17 lunar impact melt breccia 72255 preserve ancient domains with a concordant average uranium-lead radiometric date of 4460 ± 31 Ma (Zhang et al., 2021), the oldest lunar zircon yet reported. To assess the possible mobility of radiogenic lead in zircon, which may lead to redistribution and clustering of Pb atoms that may cause a U-Pb age bias (Valley et al., 2014), we investigated a zircon grain from Zhang et al. (2021) by atom probe tomography (APT). The atomic spatial resolution analysis of individual mineral grains demonstrates the absence of nanoscale clustering of lead, which supports a 4.46 Ga ancient formation age for lunar zircon in sample 72255. This age pushes back the age of the first preserved lunar crust by ∼40 Myr and provides a minimum formation age for the Moon within 110 Myr after the formation of the solar system.

J. Greer, B. Zhang, D. Isheim, D.N. Seidman, A. Bouvier, P.R. Heck

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2023) 27, 49–53 | doi: 10.7185/geochemlet.2334 | Published 23 October 2023

Article views: 10234

Abstract:
Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018

Article views: 7493

Abstract:
The concept of the ‘magma series’ and the distinction between alkaline, calc-alkaline and tholeiitic trends has been a cornerstone in igneous petrology since the early 20^th century, and encodes fundamental information about the redox state of divergent and convergent plate tectonic settings. We show that the ‘Bowen and Fenner trends’ that characterise the calc-alkaline and tholeiitic types of magmatic environments can be approximated by a simple log ratio model based on three coupled exponential decay functions, for A = Na₂O + K₂O, F = FeO_T and M = MgO, respectively. We use this simple natural law to define a ‘Bowen-Fenner Index’ to quantify the degree to which an igneous rock belongs to either magma series. Applying our model to a data compilation of igneous rocks from Iceland and the Cascade Mountains effectively separates these into tholeiitic and calc-alkaline trends. However the simple model fails to capture the distinct dog-leg that characterises the tholeiitic log ratio evolution, which can be attributed to the switch from ferrous to ferric iron-bearing minerals. Parameterising this switch in a two stage magma evolution model results in a more accurate fit to the Icelandic data. The same two stage model can also be fitted in A–T–M space, where ‘T’ stands for TiO₂. This produces a new way to identify calc-alkaline and tholeiitic rocks that does not require the conversion of FeO and Fe₂O₃ to FeO_T. Our results demonstrate that log ratio analysis provides a natural way to parameterise physical processes that give rise to these magma series.

P. Vermeesch, V. Pease

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2021) 19, 1–6 | doi: 10.7185/geochemlet.2125 | Published 30 September 2021

Article views: 4159

Abstract:
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, silicate-carbonatite melt immiscibility, and calcite/apatite accumulation) that fully explains our natural data (average δ⁴⁴Ca_BSE 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.

M.A. Antonelli, G. Sartori, A. Giuliani, E.A. Schauble, J. Hoffmann, M.W. Schmidt

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2023) 28, 13–19 | doi: 10.7185/geochemlet.2338 | Published 1 December 2023

Article views: 3894

Abstract:
The composition of continental crust records the balance between construction by tectonics and destruction by physical and chemical erosion. Quantitative constraints on how igneous addition and chemical weathering have modified the continents’ bulk composition are essential for understanding the evolution of geodynamics and climate. Using novel data analytic techniques we have extracted temporal trends in sediments’ protolith composition and weathering intensity from the largest available compilation of sedimentary major element compositions: ∼15,000 samples from 4.0 Ga to the present. We find that the average Archean upper continental crust was silica-rich and had a similar compositional diversity to modern continents. This is consistent with an early Archean, or earlier, onset of plate tectonics. In the Archean, chemical weathering sequestered ∼25 % more CO₂ per mass eroded for the same weathering intensity than in subsequent time periods, consistent with carbon mass balance despite higher Archean outgassing rates and more limited continental exposure. Since 2.0 Ga, over long (>0.5 Gyr) timescales, crustal weathering intensity has remained relatively constant. On shorter timescales over the Phanerozoic, weathering intensity is correlated to global climate state, consistent with a weathering feedback acting in response to changes in CO₂ sources or sinks.

A.G. Lipp, O. Shorttle, E.A. Sperling, J.J. Brocks, D.B. Cole, P.W. Crockford, L. Del Mouro, K. Dewing, S.Q. Dornbos, J.F. Emmings, U.C. Farrell, A. Jarrett, B.W. Johnson, P. Kabanov, C.B. Keller, M. Kunzmann, A.J. Miller, N.T. Mills, B. O’Connell, S.E. Peters, N.J. Planavsky, S.R. Ritzer, S.D. Schoepfer, P.R. Wilby, J. Yang

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2021) 17, 21–26 | doi: 10.7185/geochemlet.2109 | Published 2 March 2021

Article views: 3655

Abstract:
Ferruginous conditions prevailed in the oceans through much of Earth’s history. However, minerals recording these conditions remain difficult to interpret in terms of biogeochemical processes prior to lithification. In Lake Towuti, Indonesia, ferruginous sediments are deposited under anoxic sulfate-poor conditions similar to the Proterozoic oceans, allowing the study of mineralogical (trans)formations during microbial diagenesis.
Comprehensive pore water geochemistry, high resolution geochemical core profiles, and electron microscopy of authigenic minerals revealed in situ formation of magnetite, millerite, and abundant siderite and vivianite along a 100 m long sequence. Framboidal magnetites represent primary pelagic precipitates, whereas millerite, a sulfide mineral often overlooked under sulfate-poor conditions, shows acicular aggregates entangled with siderite and vivianite resulting from saturated pore waters and continuous growth during burial. These phases act as biosignatures of microbial iron and sulfate reduction, fermentation and methanogenesis, processes clearly traceable in pore water profiles.
Variability in metal and organic substrates attests to environment driven processes, differentially sustaining microbial processes along the stratigraphy. Geochemical profiles resulting from microbial activity over 200 kyr after deposition provide constraints on the depth and age of mineral formation within ferruginous records.

A. Vuillemin, M. Morlock, A. Paskin, L.G. Benning, C. Henny, J. Kallmeyer, J.M. Russell, H. Vogel

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2023) 28, 20–26 | doi: 10.7185/geochemlet.2339 | Published 6 December 2023

Article views: 3567

Abstract:
Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (δ⁷Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive δ⁷Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO₂ degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO₂ drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.

P.A.E. Pogge von Strandmann, A. Desrochers, M.J. Murphy, A.J. Finlay, D. Selby, T.M. Lenton

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2017) 3, 230–237 | doi: 10.7185/geochemlet.1726 | Published 15 June 2017

Article views: 3529

Abstract:
Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope ¹⁸²Hf decayed entirely to ¹⁸²W in the mantle after metal-silicate segregation. Therefore, the ¹⁸²W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the ¹⁸²W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019

Article views: 3369

Abstract:
The effusive six months long 2014‒2015 Bárðarbunga eruption (31 August‒27 February) was the largest in Iceland for more than 200 years, producing 1.6 ± 0.3 km³ of lava. The total SO₂ emission was 11.8 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO₂ exceeded the 350 µg m⁻³ hourly average health limit over much of Iceland for days to weeks. Anomalously high SO₂ concentrations were also measured at several locations in Europe in September. The lowest pH of fresh snowmelt at the eruption site was 3.3, and 3.2 in precipitation 105 km away from the source. Elevated dissolved H₂SO₄, HCl, HF, and metal concentrations were measured in snow and precipitation. Environmental pressures from the eruption and impacts on populated areas were reduced by its remoteness, timing, and the weather. The anticipated primary environmental pressure is on the surface waters, soils, and vegetation of Iceland.

S.R. Gíslason, G. Stefánsdóttir, M.A. Pfeffer, S. Barsotti, Th. Jóhannsson, I. Galeczka, E. Bali, O. Sigmarsson, A. Stefánsson, N.S. Keller, Á. Sigurdsson, B. Bergsson, B. Galle, V.C. Jacobo, S. Arellano, A. Aiuppa, E.B. Jónasdóttir, E.S. Eiríksdóttir, S. Jakobsson, G.H. Guðfinnsson, S.A. Halldórsson, H. Gunnarsson, B. Haddadi, I. Jónsdóttir, Th. Thordarson, M. Riishuus, Th. Högnadóttir, T. Dürig, G.B.M. Pedersen, Á. Höskuldsson, M.T. Gudmundsson

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2015) 1, 84–93 | doi: 10.7185/geochemlet.1509 | Published 29 June 2015

Article views: 3300

Abstract:
Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem.

X. Peng, M. Chen, S. Chen, S. Dasgupta, H. Xu, K. Ta, M. Du, J. Li, Z. Guo, S. Bai

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2018) 9, 1–5 | doi: 10.7185/geochemlet.1829 | Published 27 November 2018

Article views: 63816

Abstract:
The crystallisation ages of lunar samples provide critical constraints on the minimum formation age of the Moon and its early evolution. Zircon crystals from Apollo 17 lunar impact melt breccia 72255 preserve ancient domains with a concordant average uranium-lead radiometric date of 4460 ± 31 Ma (Zhang et al., 2021), the oldest lunar zircon yet reported. To assess the possible mobility of radiogenic lead in zircon, which may lead to redistribution and clustering of Pb atoms that may cause a U-Pb age bias (Valley et al., 2014), we investigated a zircon grain from Zhang et al. (2021) by atom probe tomography (APT). The atomic spatial resolution analysis of individual mineral grains demonstrates the absence of nanoscale clustering of lead, which supports a 4.46 Ga ancient formation age for lunar zircon in sample 72255. This age pushes back the age of the first preserved lunar crust by ∼40 Myr and provides a minimum formation age for the Moon within 110 Myr after the formation of the solar system.

J. Greer, B. Zhang, D. Isheim, D.N. Seidman, A. Bouvier, P.R. Heck

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2023) 27, 49–53 | doi: 10.7185/geochemlet.2334 | Published 23 October 2023

Article views: 44244

Abstract:
Chemical weathering of silicate rocks is a primary drawdown mechanism of atmospheric carbon dioxide. The processes that affect weathering are therefore central in controlling global climate. A temperature-controlled “weathering thermostat” has long been proposed in stabilising long-term climate, but without definitive evidence from the geologic record. Here we use lithium isotopes (δ⁷Li) to assess the impact of silicate weathering across a significant climate-cooling period, the end-Ordovician Hirnantian glaciation (~445 Ma). We find a positive δ⁷Li excursion, suggestive of a silicate weathering decline. Using a coupled lithium-carbon model, we show that initiation of the glaciation was likely caused by declining CO₂ degassing, which triggered abrupt global cooling, and much lower weathering rates. This lower CO₂ drawdown during the glaciation allowed climatic recovery and deglaciation. Combined, the data and model provide support from the geological record for the operation of the weathering thermostat.

P.A.E. Pogge von Strandmann, A. Desrochers, M.J. Murphy, A.J. Finlay, D. Selby, T.M. Lenton

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2017) 3, 230–237 | doi: 10.7185/geochemlet.1726 | Published 15 June 2017

Article views: 40171

Abstract:
The differentiation of Earth into a metallic core and silicate mantle left its signature on the chemical and isotopic composition of the bulk silicate Earth (BSE). This is seen in the depletion of siderophile (metal-loving) relative to lithophile (rock-loving) elements in Earth’s mantle as well as the silicon isotope offset between primitive meteorites (i.e. bulk Earth) and BSE, which is generally interpreted as a proof that Si is present in Earth’s core. Another putative light element in Earth’s core is sulphur; however, estimates of core S abundance vary significantly and, due to its volatile nature, no unequivocal S isotopic signature for core fractionation has thus far been detected. Here we present new high precision isotopic data for Cu, a chalcophile (sulphur-loving) element, which shows that Earth’s mantle is isotopically fractionated relative to bulk Earth. Results from high pressure equilibration experiments suggest that the sense of Cu isotopic fractionation between BSE and bulk Earth requires that a sulphide-rich liquid segregated from Earth’s mantle during differentiation, which likely entered the core. Such an early-stage removal of a sulphide-rich phase from the mantle presents a possible solution to the long-standing 1st terrestrial lead paradox.

P.S. Savage, F. Moynier, H. Chen, J. Siebert, J. Badro, I.S. Puchtel, G. Shofner

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2015) 1, 53–64 | doi: 10.7185/geochemlet.1506 | Published 4 June 2015

Article views: 32671

Abstract:
Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope ¹⁸²Hf decayed entirely to ¹⁸²W in the mantle after metal-silicate segregation. Therefore, the ¹⁸²W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the ¹⁸²W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019

Article views: 30730

Abstract:
The effusive six months long 2014‒2015 Bárðarbunga eruption (31 August‒27 February) was the largest in Iceland for more than 200 years, producing 1.6 ± 0.3 km³ of lava. The total SO₂ emission was 11.8 ± 5 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO₂ exceeded the 350 µg m⁻³ hourly average health limit over much of Iceland for days to weeks. Anomalously high SO₂ concentrations were also measured at several locations in Europe in September. The lowest pH of fresh snowmelt at the eruption site was 3.3, and 3.2 in precipitation 105 km away from the source. Elevated dissolved H₂SO₄, HCl, HF, and metal concentrations were measured in snow and precipitation. Environmental pressures from the eruption and impacts on populated areas were reduced by its remoteness, timing, and the weather. The anticipated primary environmental pressure is on the surface waters, soils, and vegetation of Iceland.

S.R. Gíslason, G. Stefánsdóttir, M.A. Pfeffer, S. Barsotti, Th. Jóhannsson, I. Galeczka, E. Bali, O. Sigmarsson, A. Stefánsson, N.S. Keller, Á. Sigurdsson, B. Bergsson, B. Galle, V.C. Jacobo, S. Arellano, A. Aiuppa, E.B. Jónasdóttir, E.S. Eiríksdóttir, S. Jakobsson, G.H. Guðfinnsson, S.A. Halldórsson, H. Gunnarsson, B. Haddadi, I. Jónsdóttir, Th. Thordarson, M. Riishuus, Th. Högnadóttir, T. Dürig, G.B.M. Pedersen, Á. Höskuldsson, M.T. Gudmundsson

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2015) 1, 84–93 | doi: 10.7185/geochemlet.1509 | Published 29 June 2015

Article views: 29566

Abstract:
Chromium (Cr) isotopes in marine sedimentary rocks can be used as a sensitive proxy for ancient atmospheric oxygen because Cr-isotope fractionation during terrestrial weathering only occurs when pO₂ exceeds a threshold value. This is a useful system when applied to rocks of mid-Proterozoic age, where fundamental questions persist about atmospheric pO₂ and its relationship to biological innovation. Whereas previous studies have focused on temporally limited iron-rich sedimentary rocks, we present new Cr-isotope data from a suite of mid-Proterozoic marine carbonate rocks. Application of the Cr-isotope proxy to carbonate rocks has the potential to greatly enhance the temporal resolution of Proterozoic palaeo-redox data. Here we report positive δ⁵³Cr values in four carbonate successions, extending the mid-Proterozoic record of Cr-isotope fractionation – and thus pO₂ above threshold values – back to ~1.1 Ga. These data suggest that pO₂ sufficient for the origin of animals was transiently in place well before their Neoproterozoic appearance, although uncertainty in the pO₂ threshold required for Cr-isotope fractionation precludes definitive biological interpretation. This study provides a proof of concept that the Cr-isotopic composition of carbonate rocks can provide important new constraints on the oxygen content of the ancient atmosphere.

G.J. Gilleaudeau, R. Frei, A.J. Kaufman, L.C. Kah, K. Azmy, J.K. Bartley, P. Chernyavskiy, A.H. Knoll

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2016) 2, 178–187 | doi: 10.7185/geochemlet.1618 | Published 24 May 2016

Article views: 28389

Abstract:
The dynamic process of subduction represents the principal means to introduce chemical heterogeneities into Earth's interior. In the case of nitrogen (N) - atmosphere's most abundant gas - biological-activity converts N₂ into ammonium ions (NH₄⁺), which are chemically-bound within seafloor sediments and altered oceanic crust that comprise the subducting slab. Although some subducted N re-emerges via arc-related volcanism (Sano et al., 1998), the majority likely bypasses sub-arc depths (150-200 km) and supplies the deeper mantle (Li et al., 2007; Mitchell et al., 2010; Johnson and Goldblatt, 2015; Bebout et al., 2016). However, the fate of subducted N remains enigmatic: is it incorporated by the shallow convecting mantle - the source of ridge volcanism, or is the deeper mantle - nominally associated with mantle plumes - its ultimate repository? Here, we present N-He-Ne-Ar isotope data for oceanic basalts from the Central Indian Ridge (CIR)-Réunion plume region to address this issue. All on-axis samples with depleted MORB mantle (DMM) affinities (³He/⁴He = 8 ± 1 R_A; Graham, 2002) have low N-isotopes (mean δ¹⁵N = -2.1 ‰), whereas those with plume-like ³He/⁴He display higher values (mean δ¹⁵N = 1.3 ‰). We explain these data within the framework of a new mantle reference model to predict a time-integrated net N regassing flux to the mantle of ~3.4 × 10¹⁰ mol/yr, with the plume-source mantle representing the preferential destination by a factor of 2-3. The model has implications for the present-day imbalance between N subducted at trenches and N emitted via arc-related volcanism, the N-content of Earth's early atmosphere, as well as relationships between N₂ and the noble gases in mantle reservoirs, including ³He/⁴He-δ¹⁵N relationships in plume-derived lavas.

P.H. Barry, D.R. Hilton

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2016) 2, 148–159 | doi: 10.7185/geochemlet.1615 | Published 3 May 2016

Article views: 28251

Abstract:
Current models assume that hydrogen was delivered to Earth already in oxidised form as water or OH groups in minerals; similarly, it is generally believed that hydrogen is stored in the present mantle mostly as OH. Here we show by experiments at 2-7 GPa and 1100-1300 °C that, under reducing conditions, molecular hydrogen (H₂) has an appreciable solubility in various upper mantle minerals. This observation suggests that during the accretion of the Earth, nebular H₂ could have been delivered to the growing solid planet by direct dissolution in a magma ocean and subsequent incorporation in silicates. Moreover, the presence of dissolved molecular H₂ in the minerals of the lower mantle could explain why magmas sourced in this region are rich in hydrogen, despite the fact that lower mantle minerals contain almost no OH groups.

X. Yang, H. Keppler, Y. Li

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2016) 2, 160–168 | doi: 10.7185/geochemlet.1616 | Published 18 March 2016

Article views: 26837

Abstract:
Chemical weathering of continental rocks plays a central role in regulating the carbon cycle and the Earth's climate (Walker et al., 1981; Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon dioxide globally (Beaulieu et al., 2012). However, the role of climate variability on chemical weathering is still strongly debated. Here we focus on the Himalayan range and use the lithium isotopic composition of clays in fluvial terraces to show a tight coupling between climate change and chemical weathering over the past 40 ka. Between 25 and 10 ka ago, weathering rates decrease despite temperature increase and monsoon intensification. This suggests that at this timescale, temperature plays a secondary role compared to runoff and physical erosion, which inhibit chemical weathering by accelerating sediment transport and act as fundamental controls in determining the feedback between chemical weathering and atmospheric carbon dioxide.

A. Dosseto, N. Vigier, R. Joannes-Boyau, I. Moffat, T. Singh, P. Srivastava

HTML | PDF | PDF + SI

Geochem. Persp. Let. (2015) 1, 10–19 | doi: 10.7185/geochemlet.1502 | Published 20 February 2015

Article views: 26205