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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  • spanning geochemical
  • sciences

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Formation of Archean continental crust constrained by boron isotopes

The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicate differentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga.

M.A. Smit, A. Scherstén, T. Næraa, R.B. Emo, E.E. Scherer, P. Sprung, W. Bleeker, K. Mezger, A. Maltese, Y. Cai, E.T. Rasbury, M.J. Whitehouse

Geochem. Persp. Let. (2019) 12, 23–26 | doi: 10.7185/geochemlet.1930 | Published 14 November 2019

Mobility of chromium in high temperature crustal and upper mantle fluids

Chromium is mobile in ultramafic magmas but its mobility in high temperature fluids has long been unclear. Studies of some chromium-rich ophiolites have suggested chromium mobility in upper mantle fluids. However, the mechanism is poorly understood because Cr(III) is so insoluble in water. We used previous estimates of aqueous Cr species and published experimental and ab initio studies of the solubility of Cr2O3 in HCl and KCl fluids at 500–1200 °C and 0.1–6.0 GPa together with the Deep Earth Water Model to calibrate a thermodynamic equation of state for the Cr(II) complex CrCl(OH)0. Our model predicted low Cr solubilities (0.1 mg kg-1 H2O) in a mid-ocean ridge fluid and very high solubilities (3500 mg kg-1 H2O) in saline, peridotitic diamond-forming fluid consistent with expectations for these environments. In pelitic, mafic eclogitic, and serpentinite fluids, predicted Cr solubilities varied widely depending on the oxygen fugacity and Cl concentration. The predicted predominance of Cr(II) in deep fluids and Cr(III) in minerals suggests that precipitation of Cr mineral components is a redox reaction.

J. Huang, J. Hao, F. Huang, D.A. Sverjensky

Geochem. Persp. Let. (2019) 12, 1–6 | doi: 10.7185/geochemlet.1926 | Published 13 November 2019

Orbitally forced sphalerite growth in the Upper Mississippi Valley District

Groundwater plays an important role in global water cycles and Earth’s climate system. Nevertheless, the geologic history of groundwater activity remains unclear due to limited data. Sphalerite colour banding in the Upper Mississippi Valley District (USA) is apparently caused by variation in oxidation state during precipitation which is controlled by the penetration of deeply circulating groundwater. Here, time series analysis of the grayscale profile of the Permian sphalerite banding in this district shows the banding correlates with Earth’s eccentricity, obliquity, and precession forcing. We have found that the astronomical forcing paced the fluxes of heat and precipitation and regulated the penetration of groundwater into the ore zone and thus the sphalerite banding. The results demonstrate that banding in sphalerite follows the Milankovitch climate frequencies over 104 – 105 years. Therefore, we show that groundwater has a major role in depositing the iron-rich bands of the sphalerite and, as a final corollary, that the banding itself can be used to decipher the effects of climate on groundwater variations in the global water cycle.

M. Li, H.L. Barnes

Geochem. Persp. Let. (2019) 12, 18–22 | doi: 10.7185/geochemlet.1929 | Published 12 November 2019

Hadean protocrust reworking at the origin of the Archean Napier Complex (Antarctica)

The origin of the first continents is still poorly constrained due to the great scarcity of >3.7 Ga rocks. The Napier Complex (East Antarctica) hosts such rocks but the extreme metamorphic conditions it experienced have compromised most isotopic systematics. Here we have studied Mount Sones and Gage Ridge orthogneisses from the Napier complex using microbeam (LA-MC-ICP-MS) U-Pb and Lu-Hf isotope measurements in zircon, together with 146,147Sm-143,142Nd isotope systematics in the corresponding whole rocks to uncover primary information about their origin. Our U-Pb results reveal that these orthogneisses formed at 3794 ± 40 and 3857 ± 39 Ma, respectively, by reworking of 4456-4356 Ma mafic protocrust, as testified to by 176Lu-176Hf and 147,146Sm-143,142Nd systematics. Other Eoarchean terranes in Greenland, Canada, and China also show involvement of Hadean crust(s) in their formation which suggests that protocrusts were massively reworked to form new continents around the Hadean-Eoarchean boundary. Such a mechanism would account for the absence of early-formed protocrust from the geological record despite recent models proposing rapid crustal growth in the Hadean (~25 % of present day volume or surface).

M. Guitreau, M. Boyet, J.-L. Paquette, A. Gannoun, Z. Konc, M. Benbakkar, K. Suchorski, J.-M. Hénot

Geochem. Persp. Let. (2019) 12, 7–11 | doi: 10.7185/geochemlet.1927 | Published 8 November 2019

Experimental evidence for fluid-induced melting in subduction zones

Although subduction zones are the main source of seismic and volcanic hazards on Earth, the causes of melting in volcanic arcs are still not fully understood. Recent models suggested that melting in the mantle wedge is not caused by hydrous fluids, but by sediment melts ascending from the subducted slab. A main argument for these models was that hydrous fluids are “too dilute” to produce the trace element enrichment observed in arc magmas. Here we demonstrate experimentally that even moderate salinities enhance the partitioning of trace elements such as the light rare earths, alkalis, alkaline earths, Pb, and U into the fluid by several orders of magnitude. Our data therefore show that saline hydrous fluids released from the basaltic part of the oceanic crust may produce the enrichment in LILE and light REE elements, and the negative Nb-Ta anomaly observed in typical arc magmas.

G. Rustioni, A. Audétat, H. Keppler

Geochem. Persp. Let. (2019) 11, 49–54 | doi: 10.7185/geochemlet.1925 | Published 22 October 2019

190Pt-186Os geochronometer reveals open system behaviour of 190Pt-4He isotope system

Platinum Group Minerals are typically dated using the 187Re-187Os and 190Pt-186Os isotope systems and more recently using the 190Pt-He geochronometer. The 187Re-187Os and 190Pt-186Os compositions of Pt-alloys from the Kondyor Zoned Ultramafic Complex (ZUC) analysed here reveal overprinting for both geochronometers except in one alloy exhibiting the most unradiogenic 187Os/188Os and most radiogenic 186Os/188Os signatures. These signatures argue for an Early Triassic mineralisation, when silicate melts/fluids derived from the partial melting of an Archean mantle crystallised to form the Kondyor ZUC while the 190Pt-He chronometer supports an Early Cretaceous mineralisation. We propose that Kondyor ZUC represents the root of an alkaline picritic volcano that constitutes the remnants of an Early Triassic island arc formed during the subduction of the Mongol-Okhotsk ocean seafloor under the Siberia craton. After the Early Cretaceous collision of Siberia with the Mongolia-North China continent, the exhumation of deep-seated structures - such as the Kondyor ZUC - allowed these massifs to cool down below the closure temperatures of the Pt-He and K-Ar, Rb-Sr isotope systems, explaining their Early to Late Cretaceous ages for the Kondyor ZUC.

A. Luguet, G.M. Nowell, E. Pushkarev, C. Ballhaus, R. Wirth, A. Schreiber, I. Gottman

Geochem. Persp. Let. (2019) 11, 44–48 | doi: 10.7185/geochemlet.1924 | Published 22 October 2019

Mercury reallocation in thawing subarctic peatlands

Warming Arctic temperatures have led to permafrost thaw that threatens to release previously sequestered mercury (Hg) back into the environment. Mobilisation of Hg in permafrost waters is of concern, as Hg methylation produced under water-saturated conditions results in the neurotoxin, methyl Hg (MeHg). Thawing permafrost may enhance Hg export, but the magnitude and mechanisms of this mobilisation within Arctic ecosystems remain poorly understood. Such uncertainty limits prognostic modelling of Hg mobilisation and impedes a comprehensive assessment of its threat to Arctic ecosystems and peoples. Here, we address this knowledge gap through an assessment of Hg dynamics across a well-studied permafrost thaw sequence at the peak of the growing season in biologically active peat overlying permafrost, quantifying total gaseous mercury (TGM) fluxes, total mercury (HgTot) in the active layer peat, porewater MeHg concentrations, and identifying microbes with the potential to methylate Hg. During the initial thaw, TGM is liberated, likely by photoreduction from permafrost where it was previously stored for decades to centuries. As thawing proceeds, TGM is largely driven by hydrologic changes as evidenced by Hg accumulation in water-logged, organic-rich peat sediments in fen sites. MeHg in porewaters increase across the thaw gradient, a pattern coincident with increases in the relative abundance of microbes possibly containing genes allowing for methylation of ionic Hg. Findings suggest that under changing climate, frozen, well-drained habitats will thaw and collapse into saturated landscapes, increasing the production of MeHg and providing a significant source of the toxic, bioaccumulative contaminant.

M.F. Fahnestock, J.G. Bryce, C.K. McCalley, M. Montesdeoca, S. Bai, Y. Li, C.T. Driscoll, P.M. Crill, V.I. Rich, R.K. Varner

Geochem. Persp. Let. (2019) 11, 33–38 | doi: 10.7185/geochemlet.1922 | Published 14 October 2019

A secretive mechanical exchange between mantle and crustal volatiles revealed by helium isotopes in 13C-depleted diamonds

Fluid inclusions trapped in fast-growing diamonds provide a unique opportunity to examine the origin of diamonds, and the conditions under which they formed. Eclogitic to websteritic diamondites from southern Africa show 13C-depletion and 15N-enrichment relative to mantle values (δ13C = -4.3 to -22.2 ‰ and δ15N = -4.9 to +23.2 ‰). In contrast the 3He/4He of the trapped fluids have a strong mantle signature, one sample has the highest value so far recorded for African diamonds (8.5 ± 0.4 Ra). We find no evidence for deep mantle He in these diamondites, or indeed in any diamonds from southern Africa. A correlation between 3He/4He ratios and 3He concentration suggests that the low 3He/4He are largely the result of ingrowth of radiogenic 4He in the trapped fluids since diamond formation. The He-C-N isotope systematics can be best described by mixing between fluid released from subducted altered oceanic crust and mantle volatiles. The high 3He/4He of low δ13C diamondites reflects the high 3He concentration in the mantle fluids relative to the slab-derived fluids. The presence of post-crystallisation 4He in the fluids means that all 3He/4He are minima, which in turn implies that the slab-derived carbon has a sedimentary organic origin. In short, although carbon and nitrogen stable isotope data show strong evidence for crustal sources for diamond-formation, helium isotopes reveal an unambiguous mantle component hidden within a strongly 13C-depleted system.

S. Mikhail, J.C. Crosby, F.M. Stuart, L. DiNicola, F.A.J. Abernethy

Geochem. Persp. Let. (2019) 11, 39–43 | doi: 10.7185/geochemlet.1923 | Published 10 October 2019

Evidence for anorthositic crust formed on an inner solar system planetesimal

During the first million years of solar system history, planetesimals experienced extensive melting powered by the radioactive decay of 26Al (Lee et al., 1977). To date, the only known anorthositic crust on a solar system body is that of the Moon, formed by plagioclase flotation on top of the magma ocean (Wood et al., 1970). Here we show evidence from the ungrouped achondrite meteorite Northwest Africa (NWA) 8486 that an anorthositic crust formed on a planetesimal very early in solar system history (<1.7 Ma). NWA 8486 displays the highest anomalies in Eu and Sr found in achondrites so far and, for the first time, this characteristic is also identified in clinopyroxene. Elemental modelling, together with calculated timescales for crystal settling, show that only the melting of an anorthosite can produce NWA 8486 within the first 5 million years of solar system history. Our results indicate that such a differentiation scenario was achievable over short timescales within the inner solar system, and must have contributed to the making and elemental budget of the terrestrial planets.

P. Frossard, M. Boyet, A. Bouvier, T. Hammouda, J. Monteux

Geochem. Persp. Let. (2019) 11, 28–32 | doi: 10.7185/geochemlet.1921 | Published 7 October 2019

Loss of immiscible nitrogen from metallic melt explains Earth’s missing nitrogen

Nitrogen and carbon are essential elements for life, and their relative abundances in planetary bodies are important for understanding planetary evolution and habitability. The high C/N ratio in the bulk silicate Earth (BSE) relative to chondrites has been difficult to explain through partitioning during core formation and outgassing from molten silicate. Here we propose a new model that may have released nitrogen from the metallic cores of accreting bodies during impacts with the early Earth. Experimental observations of melting in the Fe-N-C system via synchrotron X-ray radiography of samples in a Paris-Edinburgh press reveal that above the liquidus, iron-rich melt and nitrogen-rich liquid coexist at pressures up to at least 6 GPa. The combined effects of N-rich supercritical fluid lost to Earth’s atmosphere and/or space as well as N-depleted alloy equilibrating with the magma ocean on its way to the core would increase the BSE C/N ratio to match current estimates.

J. Liu, S.M. Dorfman, M. Lv, J. Li, F. Zhu, Y. Kono

Geochem. Persp. Let. (2019) 11, 18–22 | doi: 10.7185/geochemlet.1919 | Published 30 July 2019

Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons

Abiotic formation of n-alkane hydrocarbons has been postulated to occur within Earth's crust. Apparent evidence was primarily based on uncommon carbon and hydrogen isotope distribution patterns that set methane and its higher chain homologues apart from biotic isotopic compositions associated with microbial production and closed system thermal degradation of organic matter. Here, we present the first global investigation of the carbon and hydrogen isotopic compositions of n-alkanes in volcanic-hydrothermal fluids hosted by basaltic, andesitic, trachytic and rhyolitic rocks. We show that the bulk isotopic compositions of these gases follow trends that are characteristic of high temperature, open system degradation of organic matter. In sediment-free systems, organic matter is supplied by surface waters (seawater, meteoric water) circulating through the reservoir rocks. Our data set strongly implies that thermal degradation of organic matter is able to satisfy isotopic criteria previously classified as being indicative of abiogenesis. Further considering the ubiquitous presence of surface waters in Earth’s crust, abiotic hydrocarbon occurrences might have been significantly overestimated.

J. Fiebig, A. Stefánsson, A. Ricci, F. Tassi, F. Viveiros, C. Silva, T.M. Lopez, C. Schreiber, S. Hofmann, B.W. Mountain

Geochem. Persp. Let. (2019) 11, 23–27 | doi: 10.7185/geochemlet.1920 | Published 29 July 2019

Graphite floatation on a magma ocean and the fate of carbon during core formation

Carbon is a strongly siderophile element and current models assume that in a magma ocean, most of the carbon is sequestered into the core. Here we show that (i) for an initially highly reduced magma ocean, most of the carbon will be reduced to graphite, which is less dense than a peridotite melt; (ii) the graphite can be dynamically stable at the surface of a magma ocean; (iii) the equilibrium of the primordial atmosphere with graphite buffers CO and CO2 fugacity to such low values, that only traces of carbon dissolve in the magma ocean. Therefore, under very reducing conditions, most of the carbon may remain near the surface during core formation of a terrestrial planet and the extent of carbon sequestration into the core may be limited. We suggest that the ureilite meteorites may be the remnants of such a graphite-rich surface layer on a partially or completely molten planetesimal. A similar, graphite-enriched surface may also exist on Mercury.

H. Keppler, G. Golabek

Geochem. Persp. Let. (2019) 11, 12–17 | doi: 10.7185/geochemlet.1918 | Published 9 July 2019

182W evidence for core-mantle interaction in the source of mantle plumes

Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth’s core during its segregationaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth’s mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tungsten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle.

H. Rizo, D. Andrault, N.R. Bennett, M. Humayun, A. Brandon, I. Vlastelic, B. Moine, A. Poirier, M.A. Bouhifd, D.T. Murphy

Geochem. Persp. Let. (2019) 11, 6–11 | doi: 10.7185/geochemlet.1917 | Published 20 June 2019