Geochemical Perspectives Letters is a new internationally peer-reviewed journal of the EAG:
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The 176Lu-176Hf systematics of ALM-A: A sample of the recent Almahata Sitta meteorite fall

The application of Lu-Hf chronometry to meteorites has been compromised by arbitrary results such as dates up to 300 Myr older than the Pb-Pb age of the Solar System, unsubstantiated isochron scatter among different meteorite fractions, and varying initial Hf isotope ratios (176Hf/177Hfi). To determine the cause of the discrepancies and presence of unsupported radiogenic 176Hf, we collected Lu-Hf data for the ureilitic trachyandesite ALM-A, a fragment of the recent Almahata Sitta meteorite fall. The purest feldspar and pyroxene fractions and all 2 M HNO3 washes (i.e. selectively dissolved phosphate minerals) yield a 13-point isochron with a reasonable age of 4569 ± 24 Ma and 176Hf/177Hfi of 0.279796 ± 0.000011. Most impure mineral fractions, in contrast, scatter above this regression. Terrestrial contamination causes the 176Hf excesses, but is effectively removed by handpicking the purest mineral grains. Our study demonstrates 1) the successful application of the Lu-Hf chronometer to ALM-A, and 2) an internal consistency among the Pb-Pb age of the Solar System, the 176Lu decay constant, the Lu-Hf CHUR parameters, and robust estimates of the 176Hf/177Hfi of the Solar System from meteorites.  

R. Bast, E.E. Scherer, A. Bischoff

Geochem. Persp. Let. (2017) 3, 45-54 | doi: 10.7185/geochemlet.1705 | Published 16 September 2016

Cadmium isotope variations in Neoproterozoic carbonates - A tracer of biologic production?

Cadmium concentrations and stable isotopic compositions in seawater are important tools for studying the biogeochemical cycling of Cd in the modern oceans and as a proxy for micronutrient utilisation by phytoplankton. It is now well established that Cd isotopes become “heavier” as the primary production in the surface ocean increases, even though the mechanism driving the isotopic fractionation is still debated. Here, we use this property of Cd isotopes to examine changes that took place in the oceans during the emergence of multicellular life in the Neoproterozoic. Isotopic compositions and concentrations of Cd, N and C are reported in shallow-water carbonates of Ediacaran age from the Xiaofenghe section on the Yangtze Platform, South China. The Cd isotope data - reported as ε112/110Cd - show positive excursions in the cap dolomites, while significantly lighter Cd is found in the overlying strata. After correction for salinity-controlled fractionation into inorganic calcite, calculated palaeo-seawater ε112/110Cdsw range from -2 to +1.5, overlapping values of modern surface seawater. Importantly, ε112/110Cdsw and δ13C show a general positive correlation, as would be expected in bio-productive environments. However, the trend to lighter ε112/110Cd up-section is not that explicitly expected for an “explosion of life” at the end of the Ediacaran. The upper Doushantuo also displays substantial fluctuations in REE abundances, δ15N and δ13C, which may be due to estuarine mixing. Our data suggest that the variations in ε112/110Cd are a result of biologically-induced fractionation in at least some of the Ediacaran carbonates at Xiaofenghe. Further Cd isotope fractionation processes are clearly playing a role as well, such as precipitation of sulphides under anoxic pore-water conditions and fractionation into inorganic carbonates under variable salinity conditions. These effects have to be evaluated carefully when using Cd isotope systematics in ancient marine carbonates to look for palaeo-productivity signals.  

S.V. Hohl, S.J.G. Galer, A. Gamper, H. Becker

Geochem. Persp. Let. (2017) 3, 32-44 | doi: 10.7185/geochemlet.1704 | Published 9 September 2016

Tracking the formation of magma oceans in the Solar System using stable magnesium isotopes

The processes associated with magma ocean formation and solidification can control the earliest compositional differentiation and volatile inventory of planetary bodies. Thus, elucidating the scale and extent to which magma oceans existed in the Solar System is critical for a full understanding of planet formation. Here we show that the magnesium isotope compositions of the co-magmatic diogenite and eucrite meteorites originating from the protoplanet Vesta are distinct and this is a predictable consequence of extensive crystallisation in a shallow magma ocean. The enrichment in the heavy magnesium isotopes observed in eucrites relative to diogenites is consistent with the isotopic differences measured between minerals and whole-rock basalts on Earth and other asteroids. This isotope effect is not readily observed on Earth due to the lower primary melt magnesium contents produced at smaller degrees of melting and less extensive amounts of mafic mineral crystallisation. However, it is discernible on other planetary bodies where magma oceans formed and crystallised and, thus, Mg isotopes provide a tracer of their previous existence.  

M. Schiller, J.A. Dallas, J. Creech, M. Bizzarro, J.A. Baker

Geochem. Persp. Let. (2017) 3, 22-31 | doi: 10.7185/geochemlet.1703 | Published 1 September 2016

Immiscible C-H-O fluids formed at subduction zone conditions

Earth’s long-term carbon cycle, which is regulated by subduction and volcanism, is critical for understanding Earth’s structure, dynamics, and climate change. However, the mechanisms for carbon mobility in subduction zones remain largely unresolved. Aqueous fluids produced by slab devolatilisation may dissolve a considerable amount of carbon, but it is usually assumed that aqueous C-H-O fluids in subduction zones are fully miscible. In order to constrain the nature of aqueous C-H-O fluids in subduction zones, experiments were performed at 0.2 to 2.5 GPa and 600 to 700 oC to study the phase relations of C-H-O fluids in the presence of 3 wt. % NaCl, using the synthetic fluid inclusion technique. The results show that at 0.2 GPa and 700 °C, one single C-H-O fluid phase was present; however, at 1.5 to 2.5 GPa and 600 to 700 oC, H2O and gases of CH4+H2, CH4+CO2, or CO2 coexisted as immiscible fluid phases. These results demonstrate that pressure can significantly expand the miscibility gap of C-H-O fluids and immiscible C-H-O fluids may occur in subduction zones. The likely occurrence of immiscible C-H-O fluids in subduction zones may cause extensive decarbonation and the formation of immiscible CO2-rich fluids, providing an important mechanism for the transfer of slab carbon to the mantle wedge.  

Y. Li

Geochem. Persp. Let. (2017) 3, 12-21 | doi: 10.7185/geochemlet.1702 | Published 18 August 2016

The statistical mechanical basis of the triple isotope fractionation relationship

Multiple stable isotope relationships have found a growing variety of uses in geochemistry and cosmochemistry. Approximations to the statistical-mechanical models for predicting isotope effects have led to the notion that mass fractionation laws are constrained to a “canonical” range of possible values. Despite previous work indicating that these mass fractionation exponents can be highly variable, the concept of a constant relationship remains common. In this study, we demonstrate generically that the mass fractionation exponent, θ, can take any value for small fractionations and that these deviations are measurable. In addition, we characterise and advocate the use of the change/difference in cap-delta as a necessary and more reliable descriptor of multiple isotope fractionation relationships. Deviations from the “canonical” range are demonstrated by experimental data in the geochemically relevant haematite-water system.  

J.A. Hayles, X. Cao, H. Bao

Geochem. Persp. Let. (2017) 3, 1-11 | doi: 10.7185/geochemlet.1701 | Published 17 August 2016

Oxygenation of the mid-Proterozoic atmosphere: clues from chromium isotopes in carbonates

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

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

CO2-fluxing collapses metal mobility in magmatic vapour

Magmatic systems host many types of ore deposits, including world-class deposits of copper and gold. Magmas are commonly an important source of metals and ore-forming fluids in these systems. In many magmatic-hydrothermal systems, low-density aqueous fluids, or vapours, are significant metal carriers. Such vapours are water-dominated shallowly, but fluxing of CO2-rich vapour exsolved from deeper magma is now recognised as ubiquitous during open-system magma degassing. Here, we show that such CO2-fluxing leads to a sharp drop in element solubility, up to a factor of 10,000 for Cu, and thereby provides a highly efficient, but as yet unrecognised mechanism for metal deposition.  

V.J. van Hinsberg, K. Berlo, A.A. Migdisov, A.E. Williams-Jones

Geochem. Persp. Let. (2016) 2, 169-177 | doi: 10.7185/geochemlet.1617 | Published 18 May 2016

Molecular hydrogen in mantle minerals

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

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

Release of subducted sedimentary nitrogen throughout Earth’s mantle

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 RA; 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

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

Nitrogen isotope fractionation during terrestrial core-mantle separation

The origin and evolution of the terrestrial nitrogen remains largely unresolved. In order to understand the potential influence of core-mantle separation on terrestrial nitrogen evolution, experiments were performed at 1.5 to 7.0 GPa and 1600 to 1800 °C to study nitrogen isotope fractionation between coexisting liquid Fe-rich metal and silicate melt. The results show that the metal/silicate partition coefficient of nitrogen FORMULA ranges from 1 to 150 and the nitrogen isotope fractionation Δ15Nmetal-silicate is −3.5 ± 1.7 ‰. Calculations show that the bulk Earth is more depleted in δ15N than the present-day mantle, and that the present-day mantle δ15N of −5 ‰ could be derived from an enstatite chondrite composition via terrestrial core-mantle separation, with or without the addition of carbonaceous chondrites. These results strongly support the notion that enstatite chondrites may be a main component from which the Earth formed and a main source of the terrestrial nitrogen. Moreover, in the deep reduced mantle, the Fe-rich metal phase may store most of the nitrogen, and partial melting of the coexisting silicates may generate oceanic island basalts (OIBs) with slightly positive δ15N values.  

Y. Li, B. Marty, S. Shcheka, L. Zimmermann, H. Keppler

Geochem. Persp. Let. (2016) 2, 138-147 | doi: 10.7185/geochemlet.1614 | Published 21 April 2016

A glimpse into the Roman finances of the Second Punic War through silver isotopes

The defeat of Hannibal’s armies at the culmination of the Second Punic War (218 BC–201 BC) was a defining moment in Western world history. One of the underappreciated consequences of the conflict was the Roman monetary reform of 211 BC, which ushered in a monetary system that would sustain Roman power for the next many centuries. This system would encapsulate many of the issues plaguing finances of governments until today, such as inflation, debasement, and the size of monetary mass. Here we approach the issue of financial fluxes using a newly developed powerful tracer, that of silver isotopic compositions, in conjunction with Pb isotopes, both of which we measured in Roman coinage minted before and after the 211 BC monetary reform. The results indicate that pre-reform silver was minted from Spanish metal supplied by Carthage as war penalty after the First Punic War, whereas post-reform silver was isotopically distinct and dominated by plunder, most likely from Syracuse and Capua. The 211 BC monetary reform and the end of debasement, therefore, were aimed at accommodating new sources of silver rather than being the response to financial duress. The drastic weight reduction of silver coins implemented by the Roman mint was not motivated by metal shortage but by the need to block inflation after a major surge of war booty.

F. Albarède, J. Blichert-Toft, M. Rivoal, P. Telouk

Geochem. Persp. Let. (2016) 2, 127-137 | doi: 10.7185/geochemlet.1613 | Published 15 April 2016