Abstract:
The primordial He budget of the Earth’s interior is commonly thought to have been set by full liquid metal-silicate equilibration in a terrestrial magma ocean. However, incomplete metal-silicate equilibration during accretion will have a substantial effect on this budget. Here we present liquid-solid partitioning experiments indicating that He behaves as a moderately siderophile element during percolative core formation in planetesimals. Mass balance considerations show that even minor disequilibrium will allow the Earth’s early core to incorporate sufficient primordial He—and possibly other noble gases—to supply the lower mantle throughout Earth’s history. We conclude that the high ³He/⁴He ratios in basalts may well represent primarily the last vestiges of metal-silicate disequilibrium in a terrestrial magma ocean preserved from the time of Earth’s formation.

Geochem. Persp. Let. (2018) 9, 26–31 | doi: 10.7185/geochemlet.1901 | Published 21 January 2018

Abstract:
Noble gases serve as unique tracers of the origin and evolution of Earth’s volatile reservoirs owing to their inert nature and contribution from extinct and extant radioactivities. However, noble gases are low in abundance relative to many other elements, particularly in the Earth’s mantle. Additionally, mantle-derived samples show large post-eruptive atmospheric contamination, rendering the determination of the primary mantle composition challenging. The sources of mantle krypton and xenon remain debated due to their partially resolvable excess, if any, relative to the atmosphere. Atmospheric noble gases also appear to be recycled into the mantle via subduction, progressively overprinting the initial mantle signature. Here we develop a new protocol to accumulate non-contaminated mantle-derived xenon, in particular the low abundant ^124-126-128Xe. The results show the highest excesses in ^124-126-128Xe ever measured in the mantle relative to the atmosphere and point toward a chondritic origin for mantle xenon. The fissiogenic isotopes ^{131-132-134-136}Xe allow the onset of efficient xenon recycling in the mantle to be constrained at around 3 Gyr ago, implying that volatile recycling before 3 Ga would have been negligible.

Geochem. Persp. Let. (2018) 9, 21–25 | doi: 10.7185/geochemlet.1833 | Published 20 December 2018

Abstract:
Scandium is the subject of increasing interest and is even becoming critical for a number of institutions in Europe and the U.S.A. among others because of its economic value and supply risk. Despite this growing awareness, little is known about its geochemical behaviour. Recent studies have shown that Sc can be strongly associated with iron oxides including goethite but the mode of interaction is not yet fully understood. We investigated the incorporation of Sc in goethite structure in controlled batch systems. For the first time, we show the formation of a solid αFeOOH-αScOOH solution consistent with an ideal solution despite significant differences in atomic radii between Sc³⁺ and Fe³⁺. We believe that beyond this fundamental knowledge concerning the affinity of Sc for goethite, these results provide important insights into Sc speciation in primary or secondary sources, including Sc-rich lateritic deposits and bauxite residues after alumina extraction, for the future design of extraction processes with a low environmental footprint, as an alternative to current energy-intensive technologies.

Geochem. Persp. Let. (2018) 9, 16–20 | doi: 10.7185/geochemlet.1832 | Published 19 December 2018

Abstract:
The mechanisms driving the chemical complementarity between depleted MORB mantle (DMM) and continental crust (with an average 'andesitic' composition) remain unclear. By investigating Archean komatiites, and modern enriched (E) and normal (N) MORB samples, we demonstrate that partial melting of the mantle does not fractionate Ti isotopes, whereas intracrustal differentiation causes significant Ti isotopic fractionation between melts and minerals, specifically Fe-Ti oxides. Thus, Ti isotope ratios are tracers of these two magmatic regimes. N-MORB and late Archean (2.9-2.7 Ga) komatiites are depleted in the heavier Ti isotopes compared to E-MORB and middle Archean (3.5-3.3 Ga) komatiites. We show that the depletion in the heavier Ti isotopes of the DMM is due to mantle recycling of the isotopically light residues from the generation of felsic continental crust over 3.5-2.7 Ga. This process must have reached a steady state by ≈ 2.5 Ga, based on the uniform Ti isotopic composition of contemporary N-MORBs and late Archean komatiites. This change is likely due to a decrease in the mantle potential temperature related to the emergence of plate tectonics.

Geochem. Persp. Let. (2018) 9, 11–15 | doi: 10.7185/geochemlet.1831 | Published 13 December 2018

Abstract:
The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform δ¹⁸O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform δ¹⁸O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.

Geochem. Persp. Let. (2018) 9, 6–10 | doi: 10.7185/geochemlet.1830 | Published 7 December 2018

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

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

Abstract:
The radiogenic neodymium isotope ratio ¹⁴³Nd/¹⁴⁴Nd (expressed as εNd) has been applied to examine seawater elemental budgets, sedimentary provenance, oceanic water mass source and circulation, large scale geochemical cycling, and continental crust growth rates. These applications are underpinned by the assumption that during sedimentary processing the parent/daughter (samarium/neodymium) ratio is conservative during low temperature fluid related processes. In this study, we report εNd data from two streams draining sedimentary formations in the Arctic archipelago of Svalbard. The εNd value of the dissolved load is offset from stream suspended sediment samples by up to 5.5 epsilon units. We demonstrate that dissolved load εNd values are controlled by the dissolution of labile phases present in the catchment rocks which are isotopically distinct from the silicate residue and account for up to 12 % Nd in the bulk sediment. This study highlights; 1) the potential for incongruent release of Nd isotopes to seawater from rocks and sediments, with implications for the isotopic composition of seawater, and 2) the large scale decoupling between a rapidly exchanging labile reservoir and a silicate-bound reservoir during sediment recycling.

Geochem. Persp. Let. (2018) 8, 43–46 | doi: 10.7185/geochemlet.1828 | Published 8 November 2018

Abstract:
The scale and magnitude of compositional heterogeneity in the mantle has important implications for the understanding of the evolution of Earth. Heterogeneity of the upper mantle is often evaluated based on mid-ocean ridge basalt compositions, despite their homogenisation prior to eruption. In this study we present Nd and Sr isotope data obtained by micro-drilling single plagioclase and clinopyroxene crystals in gabbroic cumulates of the Semail Ophiolite (Oman) and show that mantle source variability is better preserved in the lower crust than in the extrusive suite. Analysis of sub-nanogram quantities of Nd in plagioclase revealed a range in ¹⁴³Nd/¹⁴⁴Nd_i in the Wadi Abyad crustal section that is three times greater than recorded in the extrusive suite. The isotopic variability is preserved in plagioclase, whereas clinopyroxene is isotopically homogeneous. These data imply that the mantle is heterogeneous on the scale of melt extraction, and that a significant proportion of homogenisation of erupted melts occurs in the oceanic crust, not the mantle.

Geochem. Persp. Let. (2018) 8, 37–42 | doi: 10.7185/geochemlet.1827 | Published 5 November 2018

Abstract:
The accessory mineral zircon is widely used to constrain the timing of igneous processes such as magma crystallisation or eruption. However, zircon U-Pb ages record zircon crystallisation, which is not an instantaneous process. Zircon saturation calculations link zircon crystallisation, temperature, and melt fraction, allowing for the estimation of zircon crystallisation distributions as a function of time or temperature. Such distributions provide valuable prior information, enabling Bayesian estimates of magma eruption time and allowing for comparison of the relative accuracy of common weighted-mean and youngest-zircon age interpretations with synthetic datasets. We find that both traditional interpretations carry a risk of underestimating the uncertainty in eruption age; a low mean square of weighted deviates (MSWD) does not guarantee the accuracy of weighted-mean interpretations. In the absence of independent confirmation that crystallisation timescale is short relative to analytical uncertainties, a Bayesian approach frequently provides the most accurate results and is least likely to underestimate uncertainty. Since U-Pb zircon studies now routinely resolve geological age dispersion due to increasing analytical precision, such considerations are increasingly critical to future progress in resolving rates and dates of Earth processes.

Geochem. Persp. Let. (2018) 8, 31–35 | doi: 10.7185/geochemlet.1826 | Published 2 October 2018

Abstract:
Fluid inclusions in diamond provide otherwise inaccessible information on the origin and nature of carbonaceous fluid(s) in the mantle. Here we evaluate the role of subducted volatiles in diamond formation within the Siberian cratonic lithosphere. Specifically, we focus on the halogen (Cl, Br and I) and noble gas (He, Ne and Ar) geochemistry of fluids trapped within cubic, coated and cloudy fibrous diamonds from the Nyurbinskaya kimberlite, Siberia. Our data show Br/Cl and I/Cl ratios consistent with involvement of altered oceanic crust, suggesting subduction-derived fluids have infiltrated the Siberian lithosphere. ³He/⁴He ranging from 2 to 11 R_A, indicates the addition of a primordial mantle component to the SCLM. Mantle plumes may therefore act as a trigger to re-mobilise subducted carbon-rich fluids from the sub-continental lithospheric mantle, and we argue this may be an essential process in the formation of fluid-rich diamonds, and kimberlitic magmatism.

Geochem. Persp. Let. (2018) 8, 26–30 | doi: 10.7185/geochemlet.1825 | Published 1 October 2018