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Volume 3, Number 2

Articles in this issue

Scandium speciation in a world-class lateritic deposit

Abstract:
Scandium (Sc) has unique properties, highly valued for many applications. Future supply is expected to rely on unusually high-grade (up to 1000 ppm) lateritic Sc ores discovered in Eastern Australia. To understand the origin of such exceptional concentrations, we investigated Sc speciation in one of these deposits. The major factors are unusually high concentrations in the parent rock together with lateritic weathering over long time scales in a stable tectonic context. At microscopic and atomic scales, by combining X-ray absorption near-edge structure spectroscopy, X-ray diffraction and microscopic and chemical analyses, we show that Sc-rich volumes are associated with iron oxides. In particular, Sc adsorbed on goethite accounts for ca. 80 % of the Sc budget in our samples. The remaining Sc is incorporated in the crystal structure of haematite, substituting for Fe3+. Scandium grades reflect the high capacity of goethite to adsorb this element. In contrast, the influence of haematite is limited by the low levels of Sc that its structure can incorporate. These crystal-chemical controls play a major role in lateritic Sc deposits developed over ultramafic–mafic rocks.


M. Chassé, W.L. Griffin, S.Y. O'Reilly, G. Calas
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Geochem. Persp. Let. (2017) 3, 105-114 | doi: 10.7185/geochemlet.1711 | Published 23 November 2016

Water in alkali feldspar: The effect of rhyolite generation on the lunar hydrogen budget

Abstract:
Recent detection of indigenous hydrogen in a diversity of lunar materials, including volcanic glass (Saal et al., 2008), melt inclusions (Hauri et al., 2011), apatite (Boyce et al., 2010; McCubbin et al., 2010), and plagioclase (Hui et al., 2013) suggests water played a role in the chemical differentiation of the Moon. Water contents measured in plagioclase feldspar, a dominant mineral in the ancient crustal lunar highlands have been used to predict that 320 ppm water initially existed in the lunar magma ocean (Hui et al., 2013) whereas measurements in apatite, found as a minor mineral in lunar rocks, representing younger potassium-enriched melt predict a bulk Moon with <100 ppm water. Here we show that water in alkali feldspar, a common mineral in potassium-enriched rocks, can have ~20 ppm water, which implies magmatic water contents of ~1 wt. % in chemically evolved rhyolitic magmas. The source for these wet, potassium-rich magmas probably contained ~1000 ppm H2O. Thus, lunar granites with ages from 4.3–3.9 Ga (Meyer et al., 1996) likely crystallised from relatively wet melts that degassed upon crystallisation. Geochemical surveys by the Lunar Prospector (Jolliff et al., 2011) and Diviner Lunar Radiometer Experiment (Glotch et al., 2010; Jolliff et al., 2011) indicating the global significance of evolved igneous rocks suggest that the formation of these granites removed water from some mantle source regions, helping to explain the existence of mare basalts with <10 ppm water, but must have left regions of the interior relatively wet as seen by the water content in volcanic glass and melt inclusions. Although these early-formed evolved melts were water-rich, their petrogenesis supports the conclusion that the Moon’s mantle had <100 ppm water for most of its history.


R.D. Mills, J.I. Simon, C.M.O’D. Alexander, J. Wang, E.H. Hauri
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Geochem. Persp. Let. (2017) 3, 115-123 | doi: 10.7185/geochemlet.1712 | Published 28 November 2016

Evidence of sub-arc mantle oxidation by sulphur and carbon

Abstract:
The oxygen fugacity (ƒO2) of the Earth’s mantle at subduction zones exerts a primary control on the genesis of mineral deposits in the overlying magmatic arcs and on speciation of volcanic gases emitted into the atmosphere. However, the processes governing mantle ƒO2 such as the introduction of oxidised material by subduction are still unresolved. Here, we present evidence for the reduction of oxidised fluid-borne sulphur and carbon during alteration of depleted mantle by slab fluids at ultra-high pressure in the Bardane peridotite (Western Gneiss Region, Norway). Elevated ferric iron in metasomatic garnet, determined using synchrotron X-ray absorption near edge structure (XANES) spectroscopy, indicates that this process drove oxidation of the silicate assemblage. Our finding indicates that subduction oxidises the Earth’s mantle by cycling of sulphur and carbon.


A. Rielli, A.G. Tomkins, O. Nebel, J. Brugger, B. Etschmann, R. Zhong, G.M. Yaxley, D. Paterson
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Geochem. Persp. Let. (2017) 3, 124-132 | doi: 10.7185/geochemlet.1713 | Published 27 January 2017

Repulsion between calcite crystals and grain detachment during water–rock interaction

Abstract:
Weathering in carbonate rocks is often thought to be governed by chemical dissolution. However, recent studies have shown that mechanical detachment of tiny grains contributes significantly to the overall surface retreat. Whether this detachment is caused by shear forces acting at the surface, or repulsive forces acting between grains, was not known. In this study, we used atomic force microscopy to examine the mechanism of grain detachment and we demonstrate that it occurs even in the absence of shearing fluid flow. This suggests that the removal of grains from rock surfaces can be caused by repulsive forces between calcite grains. Although these repulsive forces are expected to be sensitive to the ionic strength of the solution, we did not find enough evidence to demonstrate a correlation between salinity and the frequency of grain detachment. Importantly, our findings suggest that grain detachment occurs during water-rock interaction under low flow conditions over a range of salinities, with potential impacts on geological carbon sequestration and enhanced oil recovery in carbonate formations.


Y. Levenson, S. Emmanuel
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Geochem. Persp. Let. (2017) 3, 133-141 | doi: 10.7185/geochemlet.1714 | Published 31 January 2017

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016

Comment on “A cometary origin for atmospheric martian methane” by Fries et al., 2016:
Reports of transient plumes of martian atmospheric methane (Mumma et al., 2009; Webster et al., 2015) have led to suggestions of biologic or abiotic surface sources. Schuerger et al. (2012) examined the production of methane near the surface from interplanetary dust particles. They found this mechanism was capable of yielding the background value of methane, but could not reproduce plume densities by bolide, airburst or other meteor impact process. Fries et al. (2016) draw on the work of Schuerger et al. (2012) and propose that the methane plumes are sourced instead from intense meteor showers with conversion at high altitudes.


M.M.J. Crismani, N.M. Schneider, J.M.C. Plane
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Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1715 | Published 18 February 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017

Reply to Comment on “A cometary origin for martian atmospheric methane”
by Crismani et al., 2017:

First, I would like to extend thanks to Crismani et al. for their commentary, which highlights an important uncertainty acknowledged in Fries et al. (2016), namely whether the total mass of infall material required to produce the observed methane is on par with that available from meteor showers. I would like to disagree on two points presented in the letter, and then discuss the implications of their findings.


M. Fries,
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Geochem. Persp. Let. (2017) 3 | doi: 10.7185/geochemlet.1716 | Published 18 February 2017


Volume 3, Number 1

About the cover: Immiscible C-H-O fluid inclusions in quartz formed at 2.5 GPa and 600 °C as discussed in Letter 1702 by Li et al. Credit: Yuan Li.
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The print version of this issue will be mailed to EAG members early 2017.

Articles in this issue

The statistical mechanical basis of the triple isotope fractionation relationship

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

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

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

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

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

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

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

The 176Lu-176Hf systematics of ALM-A: A sample of the recent Almahata Sitta meteorite fall

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

Warm Archean oceans reconstructed from oxygen isotope composition of early-life remnants

Abstract:
Deciphering the surface conditions on the Earth during Archean times (> 2.5 billion years ago – Ga) is crucial to constrain the conditions that promoted the development of life. The progressive shift through time of the oxygen isotopic compositions of Precambrian siliceous sediments – the so-called cherts – has been interpreted as indicating a secular decrease of seawater temperature by 50-80 °C from the early Archean to the present-day. However, this interpretation has been questioned, notably because it assumes that the seawater oxygen isotopic composition has remained globally constant since 3.5 Ga, though this has never been tested by direct isotopic measurements on Archean samples. Here we report measurements of the oxygen isotopic composition of carbonaceous matter indigenous to Precambrian cherts up to ca. 3.5 Ga. These new results demonstrate that the oxygen isotope composition of seawater during most of the Precambrian remained around 0 ± 5 ‰, which is consistent with the composition of present day seawater. Combined with the chert oxygen isotope composition record, this indicates that ca. 3.5 Ga ago ocean bottom-water temperatures were ~50-60 °C higher than today.  


R. Tartèse, M. Chaussidon, A. Gurenko, F. Delarue, F. Robert
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Geochem. Persp. Let. (2017) 3, 55-65 | doi: 10.7185/geochemlet.1706 | Published 18 October 2016

Pressure-induced ion pairing in MgSO4 solutions: Implications for the oceans of icy worlds

Abstract:
At ambient temperature, liquid water transforms from a low-density to a high-density dynamic structure at ~0.2 GPa. The transition persists in electrolyte solutions; however, its effects on solute properties are unknown. We obtained Raman spectra of 0.5–2.0 molal MgSO4 solutions at 21 °C and 10-4 to ~1.6 GPa. Above about 0.4 GPa, we observed an increase in the MgSO4 contact ion pair abundance with pressure, regardless of concentration. This phenomenon contravenes the general rule that dissolved salts dissociate upon compression, and is likely caused by the structural collapse in the solvent with pressure due to increased hydrogen-bond breaking. Increasing ion association in high-pressure aqueous solutions implies that, at a given salinity, high-density water in deep, cold planetary oceans and pore waters will possess lower ionic strength and electric conductivity than previously thought. This behaviour will also lead to higher ocean salinity in the interiors of Pluto and the largest icy moons of Jupiter and Saturn, Ganymede, Callisto, and Titan, or in exoplanet water-worlds, through enhancement of submarine silicate weathering.  


C. Schmidt, C.E. Manning
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Geochem. Persp. Let. (2017) 3, 66-74 | doi: 10.7185/geochemlet.1707 | Published 18 October 2016

Stable vanadium isotopes as a redox proxy in magmatic systems?

Abstract:
Recycling pathways of multivalent elements, that impact our understanding of diverse geological processes from ore formation to the rise of atmospheric oxygen, depend critically on the spatial and temporal variation of oxygen fugacity (fO2) in the Earth’s interior. Despite its importance, there is currently no consensus on the relative fO2 of the mantle source of mid-ocean ridge basalts compared to the sub-arc mantle, regions central to the mediation of crust-mantle mass balances. Here we present the first stable vanadium isotope measurements of arc lavas, complemented by non-arc lavas and two co-genetic suites of fractionating magmas, to explore the potential of V isotopes as a redox proxy. Vanadium isotopic compositions of arc and non-arc magmas with similar MgO overlap with one another. However, V isotopes display strikingly large, systematic variations of ~2 ‰ during magmatic differentiation in both arc and non-arc settings. Calculated bulk V Rayleigh fractionation factors (1000 lnαmin-melt of -0.4 to -0.5 ‰) are similar regardless of the oxidation state of the evolving magmatic system, which implies that V isotope fractionation is most influenced by differences in bonding environment between minerals and melt rather than changes in redox conditions. Thus, although subtle fO2 effects may be present, V isotopes are not a direct proxy for oxygen fugacity in magmatic systems.  


J. Prytulak, P.A. Sossi, A.N. Halliday, T. Plank, P.S. Savage, J.D. Woodhead
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Geochem. Persp. Let. (2017) 3, 75-84 | doi: 10.7185/geochemlet.1708 | Published 26 October 2016

Direct sensing of total alkalinity profile in a stratified lake

Abstract:
We demonstrate the direct detection of a total alkalinity depth profile through the use of an integrated thin layer electrochemical modulation instrument which acts as an alkalinity sensor. The technique uses a chemically selective proton pump that alters the concentration of hydrogen ions in the thin layer sample. As the proton pump releases hydrogen ions the resulting pH is recorded at the pH probe placed directly opposite the thin sample gap. This results in an acid-base titration that takes place directly in the thin layer sample and therefore obviates the need for traditional sample manipulation. Collected samples from a stratified lake were assessed with this alkalinity probe to record a total alkalinity profile, indicating a substantial increase from 2.59 to 4.11 mM with depth. Results of the new method were in excellent agreement with titration alkalinity data, and offer the potential for autonomous on site measurements of this key parameter.  


M. Ghahraman Afshar, M. Tercier-Waeber, B. Wehrli, E. Bakker
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Geochem. Persp. Let. (2017) 3, 85-93 | doi: 10.7185/geochemlet.1709 | Published 10 November 2016

Late accretion history of the terrestrial planets inferred from platinum stable isotopes

Abstract:
Late accretion of chondritic material to differentiated planetary bodies is thought to have been common in the early solar system. However, the timing and scale of admixing this material to terrestrial planets are poorly constrained. Using platinum (Pt) stable isotope data in a range of solar system bodies, we show that Earth’s post-Archean mantle has chondritic 198Pt/194Pt, consistent with addition of a chondritic late-veneer after core formation. Conversely, terrestrial Archean samples record non-chondritic, heavy, 198Pt/194Pt, indicating preservation of early mantle components that escaped complete mixing with the late-veneer. These data suggest admixing of ≤50 % of the eventual full late-veneer inventory. Such effective mixing within Earth’s mantle by 3.85 Ga is most consistent with modern-style plate tectonics.  


J.B. Creech, J.A. Baker, M.R. Handler, J.-P. Lorand, M. Storey, A.N. Wainwright, A. Luguet, F. Moynier, M. Bizzarro
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Geochem. Persp. Let. (2017) 3, 94-104 | doi: 10.7185/geochemlet.1710 | Published 8 November 2016


Volume 2, Number 2

About the cover: A recovered large orthopyroxene crystal from experimental annealing at 2 GPa and 1100 °C as discussed in Letter 1616 by Yang et al. Credit: Xiaozhi Yang. Download high-resolution cover.

Articles in this issue

Experimental evidence that microbial activity lowers the albedo of glaciers

Abstract:
Darkening of glacier and ice sheet surfaces is an important positive feedback to increasing global temperatures. Deposition of impurities on glaciers is primarily believed to reduce surface albedo, resulting in greater melt and mass loss. However, no study has yet included the effects of biological activity in albedo reduction models. Here, we provide the first experimental evidence that microbial activity can significantly decrease glacier surface albedo. Indeed, the addition of nutrients at ice meltwater concentrations to microbe-impurity mixtures resulted in extensive microbial organic carbon fixation and accumulation in Greenland Ice Sheet surface debris. Accumulated organic carbon, over the period of a melt season, darkened the glacial debris in our experiments from 31.1 % to 15.6 % surface reflectivity (used as an analogue for albedo in our calculations), generating a strongly absorbing surface. Our experiments are the first to quantify the microbially-induced potential melt increase for the Greenland Ice Sheet (up to an average of 17.3±2.5 Gt yr-1 at present and up to ~85 Gt yr-1 by 2100, based on our first order calculations). Mass loss from glaciers will conceivably intensify through enhanced microbial activity, resulting from longer melt seasons and fertilisation from anthropogenic sources.


M. Musilova, M. Tranter, J.L. Bamber, N. Takeuchi, A.M. Anesio
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Geochem. Persp. Let. (2016) 2, 106-116 | doi: 10.7185/geochemlet.1611 | Published 11 March 2016

Compositional symmetry between Earth’s crustal building blocks

Abstract:
Arc magmatism drives the production of modern continental crust. However, the mode of crustal differentiation in the geologic past, particularly in the Archean, remains controversial. Herein I adopt a compositional approach to interrogate a global, igneous geochemical database (EarthChem Library) and document the evolving compositional history of basalt, andesite, and rhyolite, which represent the three main crustal building blocks. Basalt and andesite share synced geochemical histories and progressively incompatible-element-rich and compatible-element-poor compositions that are consistent with extensive partial mantle-melting during the Archean. Post-Archean basaltic to andesitic rocks also tend to be more alkaline in character, which coupled with their high field-strength- and large-ion lithophile-element signature, points to the increased influence of Phanerozoic-style intra-plate magmatism on the global, rock record. Coeval rhyolitic rocks are depleted in these same elements, suggesting that post-Archean felsic magmas track the evolving compositions of their basaltic to andesitic source, which are, in turn, controlled by the partial melting trend. These complementary, or symmetric, geochemical trends between rock types shifted during the Proterozoic and heralded the onset of modern compositional relationships between crustal building blocks.


C.J.M. Lawley
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Geochem. Persp. Let. (2016) 2, 117-126 | doi: 10.7185/geochemlet.1612 | Published 11 March 2016

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

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

Nitrogen isotope fractionation during terrestrial core-mantle separation

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

Release of subducted sedimentary nitrogen throughout Earth’s mantle

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

Molecular hydrogen in mantle minerals

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

CO2-fluxing collapses metal mobility in magmatic vapour

Abstract:
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 significa1705nt 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
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Geochem. Persp. Let. (2016) 2, 169-177 | doi: 10.7185/geochemlet.1617 | Published 18 May 2016

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

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


Volume 2, Number 1

About the cover: Typical glacial cryoconite hole system as discussed in Letter 1605 by Bagshaw et al. Credit: Liane G. Benning.
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Articles in this issue

The pyroxenite-diamond connection

Abstract:
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.


E.S. Kiseeva, B.J. Wood, S. Ghosh, T. Stachel
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Geochem. Persp. Let. (2016) 2, 1-9 | doi: 10.7185/geochemlet.1601 | Published 15 October 2015

A cometary origin for martian atmospheric methane

Abstract:
Methane has been reported repeatedly in the martian atmosphere but its origin remains an obstinate mystery. Possible sources include aqueous alteration of igneous rocks, release from ancient deposits of methane/water ice clathrates, infall from exogenous sources such as background interplanetary dust, or biological activity. All of these sources are problematic, however. We hypothesise that delivery of cometary material includes meteor outbursts, commonly known as “meteor showers”, may explain martian methane plumes. Correlations exist between the appearance of methane and near-approaches between Mars and cometary orbits. Additional correlations are seen between these interactions and the appearance of high-altitude dust clouds on Mars, showing that large amounts of material may be deposited on Mars during these encounters. Methane is released by UV breakdown of delivered cometary material. This hypothesis is testable in future Mars/cometary encounters. A cometary origin for methane would reveal formation of methane through processes that are separate from any geological or biological processes on Mars.


M. Fries, A. Christou, D. Archer, P. Conrad, W. Cooke, J. Eigenbrode, I.L. ten Kate, M. Matney, P. Niles, M. Sykes, A. Steele, A. Treiman
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Geochem. Persp. Let. (2016) 2, 10-23 | doi: 10.7185/geochemlet.1602 | Published 02 December 2015

Tracing Earth’s O2 evolution using Zn/Fe ratios in marine carbonates

Abstract:
Through Earth history, atmospheric oxygen has increased from initial values near zero to its present day level of about 21 % by volume; concomitantly, changes in ocean redox conditions have fundamentally altered global biogeochemical cycles. While there is a reasonable understanding of where oxygen history begins and ends, the quantitative timetable of oxygenation that links the endpoints has proven contentious. Equilibrium between marine surface environments and the overlying atmosphere suggests that carbonate-based redox proxies could refine palaeoredox records in time and space. Here we explore the use of Zn/Fe ratios to infer the evolution of atmospheric O2 through time, based on marine carbonate rocks that are well characterised in terms of depositional age, environmental setting, and diagenetic history. While Fe and Zn in the shallow ocean are mainly sourced from hydrothermal inputs, their redox sensitivities differ significantly, so that geological intervals with higher O2 would be characterised by stepped increases in Zn/Fe as preserved in shallow marine carbonates. Therefore, Zn/Fe analyses of ancient carbonates allow us to constrain past atmospheric pO2 levels, providing a secular record of atmospheric O2 over the past 3.5 billion years. In particular, we corroborate an earlier proposal that for much of the Proterozoic Eon, O2 levels were as low as 0.1-1 % of present atmospheric level. We conclude that Zn/Fe in shallow marine carbonate rocks has potential to provide a quantitative tracer for the long-term redox evolution of the oceans and the rise of atmospheric O2.


X.-M. Liu, L.C. Kah, A.H. Knoll, H. Cui, A.J. Kaufman, A. Shahar, R.M. Hazen,
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Geochem. Persp. Let. (2016) 2, 24-34 | doi: 10.7185/geochemlet.1603 | Published 02 December 2015

Carbon isotope discrimination in C3 land plants is independent of natural variations in pCO2

Abstract:
The δ13C of terrestrial C3 plant tissues and soil organic matter is important for understanding the carbon cycle, inferring past climatic and ecological conditions, and predicting responses of vegetation to future climate change. Plant δ13C depends on the δ13C of atmospheric CO2 and mean annual precipitation (MAP), but an unresolved decades-long debate centres on whether terrestrial C3 plant δ13C responds to pCO2. In this study, the pCO2-dependence of C3 land plant δ13C was tested using isotopic records from low- and high-pCO2 times spanning historical through Eocene data. Historical data do not resolve a clear pCO2-effect (-1.2 ± 1.0 to 0.6 ± 1.0 ‰/100 ppmv). Organic carbon records across the Pleistocene-Holocene transition are too affected by changes in MAP, carbon sources, and potential differential degradation to quantify pCO2-effects directly, but limits of ≤1.0 ‰/100 ppmv or ~0 ‰/100 ppmv are permissible. Fossil collagen and tooth enamel data constrain pCO2-effects most tightly to -0.03 ± 0.13 and -0.03 ± 0.24 ‰/100 ppmv between 200 and 700 ppmv. Combining all constraints yields a preferred value of 0.0 ± 0.3 ‰/100 ppmv (2 s.e.). Recent models of pCO2-dependence imply unrealistic MAP for Cenozoic records.


M.J. Kohn
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Geochem. Persp. Let. (2016) 2, 35-43 | doi: 10.7185/geochemlet.1604 | Published 8 January 2016

Processes controlling carbon cycling in Antarctic glacier surface ecosystems

Abstract:
Glacier surface ecosystems, including cryoconite holes and cryolakes, are significant contributors to regional carbon cycles. Incubation experiments to determine the net production (NEP) of organic matter in cryoconite typically have durations of 6-24 hours, and produce a wide range of results, many of which indicate that the system is net heterotrophic. We employ longer term incubations to examine the temporal variation of NEP in cryoconite from the McMurdo Dry Valleys, Antarctica to examine the effect of sediment disturbance on system production, and to understand processes controlling production over the lifetimes of glacier surface ecosystems. The shorter-term incubations have durations of one week and show net heterotrophy. The longer term incubations of approximately one year show net autotrophy, but only after a period of about 40 days (~1000 hours). The control on net organic carbon production is a combination of the rate of diffusion of dissolved inorganic carbon from heterotrophic activity within cryoconite into the water, the rate of carbonate dissolution, and the saturation of carbonate in the water (which is a result of photosynthesis in a closed system). We demonstrate that sediment on glacier surfaces has the potential to accumulate carbon over timescales of months to years.


E.A. Bagshaw, M. Tranter, J.L. Wadham, A.G. Fountain, A. Dubnick, S. Fitzsimmons
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Geochem. Persp. Let. (2016) 2, 44-54 | doi: 10.7185/geochemlet.1605 | Published 21 January 2016

North Atlantic hotspot-ridge interaction near Jan Mayen Island

Abstract:
At slow to ultraslow spreading rates along mid-ocean ridges, thicker lithosphere typically impedes magma generation and tectonic extension can play a more significant role in crustal production (Dick et al., 2003). The source of anomalously high magma supply thus remains unclear along ridges with ultraslow-spreading rates adjacent to Jan Mayen Island in the North Atlantic (Neumann and Schilling, 1984; Mertz et al., 1991; Haase et al., 1996; Schilling et al., 1999; Trønnes et al., 1999; Haase et al., 2003; Mertz et al., 2004; Blichert-Toft et al., 2005; Debaille et al., 2009). Here we show that Jan Mayen volcanism is likely the surface expression of a small mantle plume, which exerts significant influence on nearby mid-ocean ridge tectonics and volcanism. Progressive dilution of Jan Mayen geochemical signatures with distance from the hotspot is observed in lava samples from the immediately adjacent Mohns Ridge, and morphological indicators of enhanced magma supply are observed on both the Mohns Ridge and the nearby Kolbeinsey Ridge, which additionally locally overlies a highly heterogeneous, eclogite-bearing mantle source. These morphological and geochemical influences underscore the importance of heterogeneous mantle sources in modifying melt supply and thus the local expression of tectonic boundaries.


L.J. Elkins, C. Hamelin, J. Blichert-Toft, S.R. Scott, K.W.W. Sims, I.A. Yeo, C.W. Devey, R.B. Pedersen
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Geochem. Persp. Let. (2016) 2, 55-67 | doi: 10.7185/geochemlet.1606 | Published 22 January 2016

Rapid cooling of planetesimal core-mantle reaction zones from Mn-Cr isotopes in pallasites

Abstract:
Pallasite meteorites, which consist of olivine-metal mixtures and accessory phosphates crystallised from silico-phosphate melts, are thought to represent core-mantle reaction zones of early differentiating planetesimals. Pallasite meteorites can be linked to five distinct planetesimals, indicating that they are default products of differentiation. However, their formation modes (deep, shallow, and impact environments) and age are still elusive. We have investigated the trace element and Mn-Cr isotopic signatures of Main-Group pallasite olivine, finding enhanced Mn, P and 53Cr/52Cr near crystal rims which indicates early ingrowth of radiogenic 53Cr* in silico-phosphate melts. Mn-Cr isotopic data corroborate previous Hf-W isotopic data, indicating an early metal-silicate separation event but additionally that rapid cooling generated silico-phosphate eutectic melts with high Mn/Cr within ~2.5 to 4 Myr of Solar System formation. These melts formed before most known samples of planetesimal crusts (eucrite and angrite meteorites) and are among the earliest evolved planetary silicates. Additionally, Mn-rich phosphates in other, non-Main-Group pallasite meteorites suggest that core-mantle reaction zones are generic, datable features of differentiation.


S.J. McKibbin, T.R. Ireland, P. Holden, H.St.C. O’Neill, G. Mallmann
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Geochem. Persp. Let. (2016) 2, 68-77 | doi: 10.7185/geochemlet.1607 | Published 28 January 2016

A stable isotope doping method to test the range of applicability of detailed balance

Abstract:
The principle of detailed balance (PDB) has been a cornerstone for irreversible thermodynamics and chemical kinetics for a long time (Wegscheider, 1901; Lewis, 1925; Onsager, 1931), and its wide application in geochemistry has mostly been implicit and without experimental testing of its applicability. Nevertheless, many extrapolations based on PDB without experimental validation have far reaching impacts on society’s mega environmental enterprises. Here we report an isotope doping method that independently measures simultaneous dissolution and precipitation rates and can test this principle. The technique reacts a solution enriched in a rare isotope of an element with a solid having natural isotopic abundances (Beck et al., 1992; Gaillardet, 2008; Gruber et al., 2013). Dissolution and precipitation rates are found from the changing isotopic ratios. Our quartz experiment doped with 29Si showed that the equilibrium dissolution rate remains unchanged at all degrees of undersaturation. We recommend this approach to test the validity of using the detailed balance relationship in rate equations for other substances.


Z. Liu, J.D. Rimstidt, Y. Zhang, H. Yuan, C. Zhu
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Geochem. Persp. Let. (2016) 2, 78-86 | doi: 10.7185/geochemlet.1608 | Published 15 February 2016

Intramolecular fractionation of hydrogen isotopes in silicate quenched melts

Abstract:
The interplay between the chemical composition and the molecular structure of silicate melts was central to the evolution of the Earth’s crust, mantle and core. This interplay also affects geochemical records such as the partitioning of isotopes between minerals, melts and fluids in the Earth’s interior. For instance, large 2H/1H fractionations between silicate melts and aqueous fluids have been observed at high temperature and pressure. Such behaviour may be promoted by the occurrence of 2H/1H intramolecular fractionation within the molecular structure of silicate melts. New Raman spectroscopy and 1H and 2H Nuclear Magnetic Resonance (NMR) spectroscopy data reveal the source of such 2H/1H intramolecular isotopic fractionation, showing that 1H and 2H fractionate between the silicate tetrahedral units. Such a process might affect other isotopic systems (e.g., N, C, or S) where the isotopes interact with the melt silicate network.


C. Le Losq, B.O. Mysen, G.D. Cody
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Geochem. Persp. Let. (2016) 2, 87-94 | doi: 10.7185/geochemlet.1609 | Published 16 February 2016

Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments

Abstract:
Natural attenuation of uranium in subsurface environments is generally assigned to immobilisation processes due to microbial reduction of U(VI). Recent laboratory studies have established that the end products of such a process include both low solubility biogenic uraninite and more labile non-crystalline U(IV) species. Indeed, biogenic uraninite formation may be inhibited in the presence of organic or inorganic phosphoryl ligands, leading to the formation of non-crystalline U(IV)-phosphate complexes or nanoscale U(IV)-phosphate solids. Such species have been observed in shallow contaminated alluvial aquifers and can thus be suspected to form in other important environments, among which lacustrine sediments have a global environmental significance since they may represent major uranium accumulation reservoirs in riverine watersheds. Here, on the basis of microscopic, spectroscopic and chemical extraction analyses, we report the occurrence of mononuclear U(IV)-phosphate/silicate complexes, accompanied by nano-crystalline ningyoite-like U(IV)-phosphate minerals, as major scavengers for uranium in lacustrine sediments downstream from a former uranium mine in France. This observation reveals that uranium trapping mechanisms during early diagenesis of lacustrine sediments can virtually exclude uraninite formation, which has important implications for better modelling uranium cycling in natural and contaminated freshwaters. Moreover, our results raise issues concerning the long term fate of mononuclear U(IV) complexes and U(IV) phosphate nano-minerals, especially with respect to re-oxidation events.


G. Morin, A. Mangeret, G. Othmane, L. Stetten, M. Seder-Colomina, J. Brest, G. Ona-Nguema, S. Bassot, C. Courbet, J. Guillevic, A. Thouvenot, O. Mathon, O. Proux, J.R. Bargar
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Geochem. Persp. Let. (2016) 2, 95-105 | doi: 10.7185/geochemlet.1610 | Published 16 February 2016


Volume 1, Number 1

About the cover: The battle between open and closed lava pathways: Holuhraun, Iceland, 10 January 2015. The mosaic of rafted lava plates displays the dynamics of a lava channel. The plates constrict the flow of lava downstream, which on the other hand push and tear the plates apart. The degassing of the lava can be seen as a blue haze in the central and left part of the image. Credit: Gro Birkefeldt Møller Pedersen gro@hi.is.
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Articles in this issue

Preserved macroscopic polymeric sheets of shell-binding protein in the Middle Miocene (8 to 18 Ma) gastropod Ecphora

Abstract:
The genus Ecphora of Muricid gastropods from the mid-Miocene Calvert Cliffs, Maryland is characterised by distinct reddish-brown colouration that results from shell-binding proteins associated with pigments within the outer calcite (CaCO3) portion of the shell. The mineral composition and robustness of the shell structure make Ecphora unique among the Neogene gastropods. Acid-dissolved shells produce a polymeric sheet-like organic residue of the same colour as the initial shell. NMR analysis indicates the presence of peptide bonds, while hydrolysis of the polymeric material yields 11 different amino acid residues, including aspartate and glutamate, which are typical of shell-binding proteins. Carbon and nitrogen elemental and isotopic analyses of the organic residue reveals that total organic carbon ranges from 4 to 40 weight %, with 11 < C/Nat < 18. Isotope values for carbon (-17 < δ13C < -15‰) are consistent with a shallow marine environment, while values for nitrogen (4 < δ15N < 12.2‰) point to Ecphora's position in the trophic structure with higher values indicating predator status. The preservation of the pigmentation and shell-binding proteinaceous material presents a unique opportunity to study the ecology of this important and iconic Chesapeake Bay organism from 8 to 18 million years ago.


J.R. Nance, J.T. Armstrong, G.D. Cody, M.L. Fogel, R.M. Hazen

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Associated EAG Press Release

Geochem. Persp. Let. (2015) 1, 1-9 | doi: 10.7185/geochemlet.1501 | Published 20 January 2015

Rapid response of silicate weathering rates to climate change in the Himalaya

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

Growth of upper plate lithosphere controls tempo of arc magmatism: Constraints from Al-diffusion kinetics and coupled Lu-Hf and Sm-Nd chronology

Abstract:
Most magmatism occurs at mid-ocean ridges, where plate divergence leads to decompression melting of the mantle, and at volcanic arcs, where subduction leads to volatile-assisted decompression melting in the hot mantle wedge. While plate spreading and subduction are continuous, arc magmatism, particularly in continental arcs, is characterised by >10-50 Myr intervals of enhanced magmatic activity followed by rapid decline (DeCelles et al., 2009). In some cases, such as the Andes, this pattern has recurred several times (Haschke et al., 2002). Abrupt changes in plate convergence rates and direction (Pilger, 1984) or repeated steepening and shallowing of subducting slabs (Kay and Coira, 2009) have been suggested as triggering flare-ups or terminating magmatism, but such scenarios may not be sufficiently general. Here, we examine the thermal history of deep crustal and lithospheric xenoliths from the Cretaceous Sierra Nevada batholith, California (USA). The deepest samples (~90 km), garnet-bearing spinel peridotites, show cooling-related exsolution of garnet from high-Al pyroxenes originally formed at >1275 °C. Modelling of pyroxene Al diffusion profiles requires rapid cooling from 1275 to 750 °C within ~10 Myr. Also suggesting deep-seated, rapid cooling is a garnet websterite from ~90 km depth with nearly identical Lu-Hf (92.6 ± 1.6 Ma) and Sm-Nd (88.8 ± 3.1 Ma) isochron ages to within error. Thermal modelling shows that this cooling history can be explained by impingement of the base of the Sierran lithosphere against a cold subducting slab at ~90 km depth, precluding cooling by shallowing subduction. Rather, the coincidence of the radiometric ages with the magmatic flare-up (120-80 Ma) suggests that the hot mantle wedge above the subducting slab may have been pinched out by magmatic (± tectonic) thickening of the upper plate, eventually terminating mantle melting. Magmatic flare-ups in continental arcs are thus self-limiting, which explains why continental arc magmatism occurs in narrow time intervals. Convective removal of the deep arc lithosphere can initiate another magmatic cycle.


E.J. Chin, C.-T.A. Lee, J. Blichert-Toft
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Geochem. Persp. Let. (2015) 1, 20-32 | doi: 10.7185/geochemlet.1503 | Published 8 April 2015

Beyond the cellulose: Oxygen isotope composition of plant lipids as a proxy for terrestrial water balance

Abstract:
There is growing interest in using stable isotopes to measure the impact of shifting water regimes on terrestrial ecosystems. The analysis of oxygen isotopes (δ18O) of plant cellulose has been widely used for that purpose, but its application is limited by cellulose’s short life in most soils and sediments. Here we compare δ18O values of cellulose and plant lipids (hexane-extractable compounds) to assess the value of bulk lipids as a proxy for water balance. Using a set of field experiments with three C3 and three C4 species, we found significant differences in 18O enrichment in response to irrigation regime, with a strong linear relationship observed between cellulose and lipid signals. Imposed drought increased lipid δ18O values of all species relative to controls and also affected the carbon isotope composition (δ13C) of cellulose, reflecting increased water-use efficiency in C3 plants. Lipid extracts did not differ with respect to δ13C values, but δ18O signals consistently recorded drought effects in C3 and C4 species, regardless of variation in productivity and abundance of oxygen-containing groups. These results show that oxygen isotope composition of plant lipids can be used as a proxy for changing water regimes.


L.C.R. Silva, G. Pedroso, T.A. Doane, F.N.D. Mukome, W.R. Horwath
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Geochem. Persp. Let. (2015) 1, 33-42 | doi: 10.7185/geochemlet.1504 | Published 22 April 2015

Time-lapse zirconography: Imaging punctuated continental evolution

Abstract:
The continental crust (CC) contains a 4.4 billion year record of Earth evolution (Armstrong, 1991; Bowring and Housh, 1995; Wilde et al., 2001; Harrison et al., 2005; Hawkesworth et al., 2010; Condie and Kroener, 2013). Understanding its growth is central to a broad range of research, including the geodynamics of the planet's interior (O'Neill et al., 2007; Korenaga, 2013), the evolution of the atmosphere (Barley et al., 2005) and even the evolution of life itself (Lowe and Tice, 2007). The growth of the continents is a competition between rates of crustal production and destruction (Armstrong, 1991; Hawkesworth et al., 2010), but which of these dominates the CC record is debated. Here I examine a large database of detrital zircon U-Pb ages (n > 200,000; Voice et al., 2011) as a function of the age of the sedimentary rock in which they are found. This provides snapshots of the Earth's CC age distribution through time; in essence, producing a time-lapse movie of CC evolution. The analysis strongly indicates that the growth of the CC was episodic.


S.W. Parman
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Geochem. Persp. Let. (2015) 1, 43-52 | doi: 10.7185/geochemlet.1505 | Published 9 May 2015

Copper isotope evidence for large-scale sulphide fractionation during Earth’s differentiation

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
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Associated EAG Press Release

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

Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds

Abstract:
The fate of subducted oceanic lithosphere and its role in the planet-scale geochemical cycle is a key problem in solid Earth studies. Asthenospheric and transition zone minerals included in diamond have been interpreted as representing subducted oceanic crust based on inclusion REE patterns and strong 13C depletion of their host diamond (d13C as low as -23 ‰). This view/explanation, however, has been challenged by alternative interpretations that variable carbon isotopic compositions either result from high temperature fractionation involving carbides, or reflect primordial, unhomogenised mantle reservoirs. Here, we present the first oxygen isotope analyses of inclusions in such ultradeep diamonds – majoritic garnets in diamond from Jagersfontein (South Africa). The oxygen isotope compositions provide unambiguous evidence for derivation of the inclusions from subducted crustal materials. The d18OVSMOW values of the majorites range from +8.6 ‰ to +10.0 ‰, well outside that of ambient mantle (+5.5 ±0.4 ‰) and indicate that the protoliths were very heavily weathered at relatively low temperatures. When this information is combined with the broadly eclogitic composition of the majoritic garnets, a derivation from subducted sea-floor basalts is implied. Based on the association between the heavy oxygen and light carbon, the light carbon isotope composition cannot relate to deep mantle processes and is also ultimately derived from the crust.


R.B. Ickert, T. Stachel, R.A. Stern, J.W. Harris
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Geochem. Persp. Let. (2015) 1, 65-74 | doi: 10.7185/geochemlet.1507 | Published 9 June 2015

Magnesium isotopic systematics of metapelite in the deep crust and implications for granite petrogenesis

Abstract:
Fluid-absent reactions that involve the breakdown of hydrous minerals produce granites in the continental crust. The minerals in the deep continental crust thus play a significant control on the chemical compositions of granites, but how and to what extent it may influence their isotopic signatures is poorly understood. Here we present Mg isotope data for a suite of amphibolite- to granulite-facies metapelites as well as the biotite and garnet minerals therein from the Ivrea Zone, NW Italy. The mineralogy of metapelites changes from biotite-dominated in the amphibolite-facies to garnet-dominated in the granulite-facies. The bulk Mg isotopic compositions (δ26Mg = -0.23 to +0.20 ‰) do not correlate with metamorphic grade, indicating negligible Mg isotopic variation caused by metamorphism of metapelites. By contrast, the δ26Mg values of biotite vary widely from -0.08 to +1.10 ‰, and increase with increasing metamorphic grade. Correspondingly, coexisting garnets become isotopically heavy (-1.22 to +0.10 ‰) as metamorphism proceeds, in order to equilibrate with the biotite with a fractionation equation of 103 lnαbiotite-garnet = 0.96 × 106/T2, which can be used as a novel geothermometer. Our results indicate a nearly closed system for Mg isotopes of metapelites during metamorphism, and the bulk Mg isotopic compositions are therefore reconciled by the shifting garnet and biotite modes accompanied by increasing mineral δ26Mg values. The systematic Mg isotopic variation in the biotite implies a possible Mg isotope fractionation between melts and residues during biotite dehydration melting, which makes Mg isotopes a potential monitor of crustal melting and a tracer of granite petrogenesis.


S.-J. Wang, F.-Z. Teng, F. Bea
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Geochem. Persp. Let. (2015) 1, 75-83 | doi: 10.7185/geochemlet.1508 | Published 22 June 2015

Environmental pressure from the 2014–15 eruption of Bárðarbunga volcano, Iceland

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 10.7‒11.8 Mt, more than the amount emitted from Europe in 2011. The ground level concentration of SO2 exceeded the 350 µg m3 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
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Associated EAG Press Release

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

The effect of warming climate on nutrient and solute export from the Greenland Ice Sheet

Abstract:
Glacial meltwater runoff is likely an important source of limiting nutrients for downstream primary producers. This has particular significance for regions surrounding the Greenland Ice Sheet, which discharges >400 km of meltwater annually. The Arctic is warming rapidly but the impact of higher discharge on nutrient export is unknown. We present four years of hydrological and geochemical data from a large Greenland Ice Sheet catchment that includes the two highest melt years on record (2010, 2012). Measurements reveal significant variation in dissolved solute (major ion) and estimated dissolved macronutrient (nitrogen, phosphorus and silica) fluxes, with increases in higher melt years. Labile particulate macronutrients dominate nutrient export, accounting for ~50 % of nitrogen and >80 % of both phosphorus and silica. The response of ice sheet nutrient export to enhanced melting is largely controlled by particle bound nutrients, the future supply of which is uncertain. We propose that the Greenland Ice Sheet provides an underappreciated and annually dynamic source of nutrients for the polar oceans, with changes in meltwater discharge likely to impact marine primary productivity in future decades.


J.R. Hawkings, J.L. Wadham, M. Tranter, E. Lawson, A. Sole, T. Cowton, A.J. Tedstone, I. Bartholomew, P. Nienow, D. Chandler, J. Telling
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Geochem. Persp. Let. (2015) 1, 94-104 | doi: 10.7185/geochemlet.1510 | Published 23 June 2015