Geochemical Perspectives Letters is an internationally peer-reviewed journal of the European Association of Geochemistry, produced by and for the geochemical community:
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Redox state of the convective mantle from CO2-trace element systematics of oceanic basalts

The redox state of mantle lithologies, based on xenoliths from continental lithospheric mantle, has been shown to decrease with depth and reach oxygen fugacities (fO2) at which graphite/diamond will be the stable form of carbon at pressures greater than about 3-4 GPa (e.g., Frost and McCammon, 2008). On the other hand, the depth-fO2 profile of the convecting mantle remains poorly known. We compare the CO-Ba and CO2-Nb systematics of natural oceanic basalts to the CO2-trace element concentrations that can be generated via contributions from depleted peridotite partial melts and graphite-saturated partial melts of subducted lithologies. Results suggest that to produce the CO2 enrichments relative to the depleted end member observed in natural oceanic basalts, subducted lithologies cannot be graphite-saturated at the onset of melting or must undergo oxidative transformation below the respective volatile-free solidi. Therefore, the oxygen fugacity profile of the continental lithospheric mantle may not be applicable to the deep convecting upper mantle, with the convecting upper mantle to at least 150 km depth being more oxidised than the carbonate vs. graphite/diamond buffer.

J. Eguchi, R. Dasgupta

Geochem. Persp. Let. (2018) 8, 17–21 | doi: 10.7185/geochemlet.1823 | Published 18 September 2018

Influence of metasomatism on vanadium-based redox proxies for mantle peridotite

The multi-valence nature of vanadium means that its geochemical behaviour will be ƒO2-dependent, so that its concentration or V/Sc (or V/Ga), can serve as proxies for oxidation state in mantle peridotites. Compared to Fe3+/Fe2+-based equilibria, such trace elements may be less sensitive to metasomatic processes. To investigate these systematics, we have measured V, Sc, Ga and Fe3+ contents in clinopyroxene from well-characterised spinel peridotite xenoliths from the Massif Central, France. These samples were metasomatised by a variety of agents with different oxidation states.V contents can be modified by metasomatic interactions, and other geochemically similar elements including Sc and Ga can also be added, removed or remain constant. A link between V/Sc and Fe3+-Fe2+ equilibria is apparent. Partial removal of V is caused by different metasomatic agents; the common factor is that all agents were significantly more oxidised than the initial ambient mantle peridotite. This extraction can be understood by a decreasing partition coefficient for V for ΔlogƒO2 > ~FMQ-2. Considering that mineral/melt partitioning of V decreases similarly for all peridotite minerals, the bulk-rock V/Sc will also change during relatively oxidising metasomatic interactions and mirror the results obtained for clinopyroxene.

A.B. Woodland, L. Uenver-Thiele, H.-M. Seitz

Geochem. Persp. Let. (2018) 8, 11–16 | doi: 10.7185/geochemlet.1822 | Published 7 September 2018

Apollo 15 green glass He-Ne-Ar signatures – In search for indigenous lunar noble gases

Identifying indigenous lunar noble gases in samples returned by the Apollo and Luna missions is highly challenging because contributions from the solar wind (SW) and/or cosmogenic nuclides have modified the noble gas signature of the regolith and rocks exposed to space at the lunar surface. Here we re-investigate the possible presence of indigenous noble gases in pyroclastic Apollo 15426 green glasses based on precise measurements of He-Ne-Ar isotopic compositions and abundances. The noble gas content of single glass beads varies by two orders of magnitude, indicating that they experienced highly variable irradiation histories as a result of intense regolith stirring by impact gardening. Four out of the twelve spherules stand out by having the highest He-Ne-Ar abundances and by releasing an isotopically 'solar-like' noble gas component at high temperatures. While a contribution from indigenous noble gases cannot be ruled out, the data are best accounted for by inward diffusion of, and equilibration with, SW-derived volatiles during prolonged space exposure.

E. Füri, L. Zimmermann, A.E. Saal

Geochem. Persp. Let. (2018) 8, 1–5 | doi: 10.7185/geochemlet.1819 | Published 7 September 2018

Hadean geodynamics inferred from time-varying 142Nd/144Nd in the early Earth rock record

Tracking the secular evolution of 142Nd/144Nd anomalies is important towards understanding the crust-mantle dynamics in the early Earth. Excessive scatter in the published data, however, precludes identifying the fine structure of 142Nd/144Nd evolution as the expected variability is on the order of few parts per million. We report ultra-high precision 142Nd/144Nd data for Eoarchean and Palaeoarchean rocks from the Isua Supracrustal Belt (SW Greenland) that show a well-resolved 142Nd/144Nd temporal variability suggesting progressive convective homogenisation of the Hadean Isua depleted mantle. This temporally decreasing 142Nd/144Nd signal provides a direct measure of early mantle dynamics, defining a stirring timescale of <250 Myr consistent with vigorous convective stirring in the early mantle. The 142Nd/144Nd evolution suggests protracted crustal residence times of ~1000-2000 Myr, inconsistent with modern-style plate tectonics in the Archean. In contrast, a stagnant-lid regime punctuated by episodes of mantle overturns accounts for the long life-time estimated here for the Hadean proto-crust.

N.S. Saji, K. Larsen, D. Wielandt, M. Schiller, M.M. Costa, M.J. Whitehouse, M.T. Rosing M. Bizzarro

Geochem. Persp. Let. (2018) 7, 43–48 | doi: 10.7185/geochemlet.1818 | Published 5 September 2018

Comment 2 on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes”

We read with great interest the paper by C. Ballhaus and coauthors (2017) reporting on electrical discharge experiments that showed how SiC and other phases found in mantle-derived rocks can potentially form by lightning strikes (Ballhaus et al., 2017). The experiments are technically innovative and challenging and the results make fascinating reading. In a comment paper, Griffin et al. (2018) noted several lines of evidence that ultra-high pressure (UHP) and super reduced (SuR) minerals in ophiolites do not form by lightning strikes. Here, we add additional comments relating to the geological and mineralogical data from ophiolites that are not compatible with the model of Ballhaus et al. (2017).

J.S. Yang, R.B. Trumbull, P.T. Robinson, F.H. Xiong, D.Y. Lian

Geochem. Persp. Let. (2018) 8, 6-7 | doi: 10.7185/geochemlet.1820 | Published 3 September 2018

Reply 2 to Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes”

We appreciate the comments by Yang et al. (2018) to our recent proposal (Ballhaus et al. 2017) that high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes. We have carried out additional experiments to address the issues raised by Yang et al. (2018). We maintain that the ultra-reduced phases in ophiolites are best explained as plasma precipitates generated by lightning strikes.

C. Ballhaus, H. Blanchard, R.O.C. Fonseca, A. Bragagni

Geochem. Persp. Let. (2018) 8, 8-10 | doi: 10.7185/geochemlet.1821 | Published 3 September 2018

Chemical nature of the 3.4 Ga Strelley Pool microfossils

The biogenicity of putative traces of life found in early-Archean rocks is strongly debated. To date, only equivocal lines of evidence have been reported, which has prevented a full consensus from emerging. Here we report elemental and molecular data from individual organic microfossils preserved within the 3.4 billion-year-old cherts of the Strelley Pool Formation, Western Australia. The present results support the growing body of evidence advocating their biogenicity, promoting them as the oldest known authentic organic microfossils. These microfossils consist of nitrogen- and oxygen- rich organic molecules that have been only slightly degraded despite experiencing temperatures of ~300 °C. Such molecular preservation emphasises the palaeobiological potential of the Earth’s oldest geological record, whilst providing a promising window into the early biosphere.

J. Alleon, S. Bernard, C. Le Guillou, O. Beyssac, K. Sugitani, F. Robert

Geochem. Persp. Let. (2018) 7, 37–42 | doi: 10.7185/geochemlet.1817 | Published 16 August 2018

Nitrogen isotope signatures of microfossils suggest aerobic metabolism 3.0 Gyr ago

There is compelling evidence for early oxygenation of mid-Archean oceans. However, the biological use of molecular oxygen is still not ascertained. Here we report the nitrogen isotope composition measured in isolated microfossils (δ15Nµm) from the 3.0 billion years old Farrel Quartzite metasediments. We show that the quasi-null bulk δ15N values of Farrel Quartzite organic matter encompass a large 15N isotopic heterogeneity at the scale of isolated microfossils (-21.6 ‰ < δ15Nµm < +30.7 ‰). Rayleigh fractionation is required to yield such large δ15N variations. Based on these data, we propose a model in which negative δ15Nµm values determined on film-like and on spheroidal microfossils are explained by ammonia assimilation in the anoxic deeper levels of the water column, whereas positive δ15Nµm values determined on lenticular microfossils were driven by both ammonia assimilation and aerobic oxidation close to the sea surface. Since ammonium aerobic oxidation requires the presence of free molecular O2 within the water column, we further suggest that positive δ15Nµm values reflect an ocean redox stratification tightly related to O2 production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago.

F. Delarue, F. Robert, K. Sugitani, R. Tartèse, R. Duhamel, S. Derenne

Geochem. Persp. Let. (2018) 7, 32–36 | doi: 10.7185/geochemlet.1816 | Published 24 July 2018

Mass-dependent triple oxygen isotope variations in terrestrial materials

High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δ'17O - δ'18O space that can be explained by well understood fractionation processes. The Δ'17O - δ'18O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a λ value of ~0.528, although a better fit to waters with δ18O values >-20 ‰ is δ'17O = 0.52654 (±0.00036) δ'18O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δ'17O - δ'18O space with high δ'18O and low Δ'17O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ'18O values at low fluid/rock ratios, and finally higher Δ'17O when F/R ratios are greater than 1. In order to make the triple isotope data tractable to the entire geological community, consensus on a reporting scheme for Δ'17O is desirable. Adoption of λRL= 0.528 (λRL = slope of δ'17O - δ'18O reference line, the ‘Terrestrial Fractionation Line’ or TFL) would bring the ‘rock’ community in line with well established hydrological reporting conventions.

Z.D. Sharp, J.A.G. Wostbrock, A. Pack

Geochem. Persp. Let. (2018) 7, 27–31 | doi: 10.7185/geochemlet.1815 | Published 1 June 2018