Abstract:
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_2­-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.

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

Abstract:
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.

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

Abstract:
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.

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

Abstract:
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.

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

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).

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

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.

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

Abstract:
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.

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

Abstract:
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 (δ¹⁵N_µm) 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 ‰ < δ¹⁵N_µm < +30.7 ‰). Rayleigh fractionation is required to yield such large δ¹⁵N variations. Based on these data, we propose a model in which negative δ¹⁵N_µm 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 δ¹⁵N_µm 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 δ¹⁵N_µm values reflect an ocean redox stratification tightly related to O₂ production by oxygenic photosynthesisers in a mid-Archean ocean 3.0 Gyr ago.

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

Abstract:
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 λ_RL= 0.528 (λ_RL = 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.

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