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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Forty-year pollution history of microplastics in the largest marginal sea of the western Pacific

Marine sediments have been considered to be a major sink for microplastics, yet the pollution history of microplastics recorded in these sediments remains poorly understood. Using a combination of 210Pb chronology and quantification of microplastics in undisturbed sediment cores, here we established the forty-year pollution history of microplastics in the northern South China Sea (SCS), the largest marginal sea of the western Pacific. We found that the pollution of microplastics in the northern SCS commenced in the 1980s. A dramatic increase of microplastic abundance in about 1998 marked an important breakpoint for microplastic contamination. Since then, microplastic abundances in the sediments have continued to increase and reached the highest level in 2018. This was well in line with the increasing trend of plastic output in the local industries. Reconstructing regional pollution history further revealed the shift of microplastic depocentres in the northern SCS over the past forty years. We estimated that the microplastic abundances in the sediments at nearshore stations will double by 2028. Our results provide the first example of the reconstruction of microplastic pollution history in marine sediments and new insights into how microplastics contaminated the marginal sea.

M. Chen, M. Du, A. Jin, S. Chen, S. Dasgupta, J. Li, H. Xu, K. Ta, X. Peng

Geochem. Persp. Let. (2020) 13, 42–47 | doi: 10.7185/geochemlet.2012 | Published 3 April 2020

Sulfur loss from subducted altered oceanic crust and implications for mantle oxidation

Oxygen fugacity (fO2) is a controlling factor of the physics of Earth’s mantle; however, the mechanisms driving spatial and secular changes in fO2 associated with convergent margins are highly debated. We present new thermodynamic models and petrographic observations to predict that oxidised sulfur species are produced during the subduction of altered oceanic crust. Sulfur loss from the subducting slab is a function of the protolith Fe3+/ΣFe ratio and subduction zone thermal structure, with elevated sulfur fluxes predicted for oxidised slabs in cold subduction zones. We also predict bi-modal release of sulfur-bearing fluids, with a low volume shallow flux of reduced sulfur followed by an enhanced deep flux of sulfate and sulfite species, consistent with oxidised arc magmas and associated copper porphyry deposits. The variable SOx release predicted by our models both across and among active margins may introduce fO2 heterogeneity to the upper mantle.

J.B. Walters, A.M. Cruz-Uribe, H.R. Marschall

Geochem. Persp. Let. (2020) 13, 36–41 | doi: 10.7185/geochemlet.2011 | Published 2 April 2020

Concentration of meteoritic free organic matter by fluid transport and adsorption

Carbonaceous chondrites contain many abiotic organic compounds, some of which are found in life on Earth. Both the mineral and organic matter phases, of these meteorites, have been affected by aqueous alteration processes. Whilst organic matter is known to be associated with phyllosilicate phases, no such relationship has yet been identified for specific organic compound classes. Furthermore, ongoing sample return missions, Hyabusa 2 and OSIRIS-Rex, are set to return potentially organic rich C-type asteroid samples to the Earth. Consequently, strategies to investigate organic-mineral relationships are required. Here we report spatial data for free/soluble organic matter (FOM/SOM) components (akylimidazole and alkylpyridine homologues) and mineral phases. Low and intermediate molecular weight alkylimidazole homologues are more widely distributed than higher molecular weight members, likely due to their affinity for the aqueous phase. On aqueous alteration of anhydrous mineral phases, transported FOM is adsorbed onto the surface or into the interlayers of the resulting phyllosilicates and thus concentrated and protected from oxidising fluids. Therefore, aiding the delivery of biologically relevant molecules to earth, shortly preceding the origin of life.

C. Potiszil, R. Tanaka, T. Ota, T. Kunihiro, K. Kobayashi, E. Nakamura

Geochem. Persp. Let. (2020) 13, 30–35 | doi: 10.7185/geochemlet.2010 | Published 17 March 2020

Mantle cooling causes more reducing volcanic gases and gradual reduction of the atmosphere

The early atmosphere contained negligible O2 until the Great Oxidation Event (GOE) around 2.4 Ga, but evidence suggests that production of photosynthetic O2 began hundreds of millions of years earlier. Thus, an ongoing debate concerns the trigger of the GOE. One possibility is that volcanic gases became more oxidising over time. Secular cooling of the mantle affects thermodynamic equilibria and also changes the proportions of reduced and oxidised volcanic gases. Here, we examine the consequences of mantle cooling for the evolution of Earth’s atmospheric redox state. Contrary to some previous hypotheses, we show that as the mantle cools, volcanic emissions contain a greater proportion of reducing gases, which produces a more reducing atmosphere. However, the atmosphere became more oxic. Therefore, the redox consequences of other processes, such as secular oxidation of the mantle and/or hydrogen escape to space, must have dominated over that of mantle cooling in shaping the redox evolution of Earth’s atmosphere.

S. Kadoya, D.C. Catling, R.W. Nicklas, I.S. Puchtel, A.D. Anbar

Geochem. Persp. Let. (2020) 13, 25–29 | doi: 10.7185/geochemlet.2009 | Published 16 March 2020

Massive sulfide Zn deposits in the Proterozoic did not require euxinia

Our most important Zn resources occur within clastic-dominated (CD-type) deposits, which are located in a small number of Proterozoic and Phanerozoic sedimentary basins. The most common model for CD-type mineralisation involves sedimentary exhalative (SEDEX) processes, i.e. the venting of metal bearing fluids into a restricted, anoxic H2S-bearing (euxinic) water column. In the Carpentaria Zn Province (Australia), multiple world class deposits are hosted in Proterozoic (1.6 Ga) stratigraphy, where models of the ancient sulfur cycle have also been developed. Focusing on the most recent discovery – the Teena deposit – we report bulk rock and isotopic data (δ34Spyrite values) that provide information on the sulfur cycle during the diagenetic and hydrothermal evolution of the Teena sub-basin. In contrast to the SEDEX model, intervals containing abundant pyrite with highly positive δ34S values (>25 ‰) correspond with euxinic conditions that developed due to high organic loading (i.e. productivity) and not basin restriction. This basin wide feature, which can also be mistaken as a hydrothermal pyrite halo, is genetically unrelated to the subsequent hydrothermal mineralisation that formed beneath the palaeo-seafloor. The formation of CD-type deposits in the Proterozoic does not, therefore, require euxinic conditions.

J.M. Magnall, S.A. Gleeson, N. Hayward, A. Rocholl

Geochem. Persp. Let. (2020) 13, 19–24 | doi: 10.7185/geochemlet.2008 | Published 6 March 2020

Unravelling lunar mantle source processes via the Ti isotope composition of lunar basalts

Formation and crystallisation of the Lunar Magma Ocean (LMO) was one of the most incisive events during the early evolution of the Moon. Lunar Magma Ocean solidification concluded with the coeval formation of K-, REE- and P-rich components (KREEP) and an ilmenite-bearing cumulate (IBC) layer. Gravitational overturn of the lunar mantle generated eruptions of basaltic rocks with variable Ti contents, of which their δ49Ti variations may now reflect variable mixtures of ambient lunar mantle and the IBC. To better understand the processes generating the spectrum of lunar low-Ti and high-Ti basalts and the role of Ti-rich phases such as ilmenite, we determined the mass dependent Ti isotope composition of four KREEP-rich samples, 12 low-Ti, and eight high-Ti mare basalts by using a 47Ti-49Ti double spike. Our data reveal significant variations in δ49Ti for KREEP-rich samples (+0.117 to +0.296 ‰) and intra-group variations in the mare basalts (-0.030 to +0.055 ‰ for low-Ti and +0.009 to +0.115 ‰ for high-Ti basalts). We modelled the δ49Ti of KREEP using previously published HFSE data as well as the δ49Ti evolution during fractional crystallisation of the LMO. Both approaches yield δ49TiKREEP similar to measured values and are in excellent agreement with previous studies. The involvement of ilmenite in the petrogenesis of the lunar mare basalts is further evaluated by combining our results with element ratios of HFSE, U and Th, revealing that partial melting in an overturned lunar mantle and fractional crystallisation of ilmenite must be the main processes accounting for mass dependent Ti isotope variations in lunar basalts. Based on our results we can also exclude formation of high-Ti basalts by simple assimilation of ilmenite by ascending melts from the depleted lunar mantle. Rather, our data are in accord with melting of these basalts from a hybrid mantle source formed in the aftermath of gravitational lunar mantle overturn, which is in good agreement with previous Fe isotope data.

S. Kommescher, R.O.C. Fonseca, F. Kurzweil, M.M. Thiemens, C. Münker, P. Sprung

Geochem. Persp. Let. (2020) 13, 13–18 | doi: 10.7185/geochemlet.2007 | Published 28 February 2020

Accessory mineral constraints on crustal evolution: elemental fingerprints for magma discrimination

Underexplored accessory minerals such as titanite and apatite have the potential to give insights into the nature and the petrogenesis of their host rock. Their trace element and REE-rich compositions carry a record of crystallisation history and chemical characteristics of their source. Moreover, titanite and, to a certain extent, apatite are resistant to erosion during sedimentary cycles which makes them ideal to reconstruct the history of long-eroded continental landmasses. Here we report new trace element data on apatite and titanite from granitoids of different Archean cratons and comparative granitoids from the Phanerozoic. Trace element signatures of both minerals reveal systematic chemical trends in Y, LREE and Sr contents related to the nature of their host magma, which are used to construct discrimination diagrams delineating Archean TTGs from sanukitoids, and modern adakites from S/I-type granites. By comparing Archean granitoids (TTG and sanukitoids) and their Phanerozoic counterparts (adakite and high Ba-Sr granites), we show that the robust nature of these phases makes them reliable recorders of petrogenetic information from Archean rocks, that usually have been affected by secondary processes (metamorphism, deformation, hydrothermal activity). Applied to the rock record, both phases potentially provide detailed archives of magmatic evolution through time.

E. Bruand, M. Fowler, C. Storey O. Laurent, C. Antoine, M. Guitreau, E. Heilimo, O. Nebel

Geochem. Persp. Let. (2020) 13, 7–12 | doi: 10.7185/geochemlet.2006 | Published 26 February 2020

Unravelling the controls on the molybdenum isotope ratios of river waters

The molybdenum (Mo) isotope ratios (δ98/95Mo) of river waters control the δ98/95Mo values of seawater and impact on the use of Mo isotope ratios as a proxy of past redox conditions. The δ98/95Mo values of river waters vary by more than 2 ‰, yet the relative roles of lithology versus fractionation during weathering remain contested. Here, we combine measurements from river waters (δ98/95Modiss), river bed materials (δ98/95MoBM) and soils from locations with contrasting lithology. The δ98/95Mo values of river bed materials (δ98/95MoBM), set by rock type, vary by ~1 ‰ between rivers in New Zealand, the Mackenzie Basin, and Iceland. However, the difference between dissolved and solid phase Mo isotopes (Δ98/95Modiss-BM) varies from +0.3 ‰ to +1.0 ‰. We estimate Mo removal from solution using the mobile trace element rhenium and find that it correlates with Δ98/95Modiss-BM across the sample set. The adsorption of Mo to Fe-Mn-(oxyhydr)oxides can explain the observed fractionation. Together, the amount of Mo released through dissolution and taken up by (oxyhydr)oxide formation on land may cause changes in the δ98/95Mo values of rivers, driving long term changes in the Mo isotope ratios of seawater.

K. Horan, R.G. Hilton, A.J. McCoy-West, D. Selby, E.T. Tipper, S. Hawley, K.W. Burton

Geochem. Persp. Let. (2020) 13, 1–6 | doi: 10.7185/geochemlet.2005 | Published 19 February 2020

Oxidation of green rust by anoxygenic phototrophic Fe(II)-oxidising bacteria

Green rust (GR) may have been a primary mineral phase during the deposition of Fe(III) (oxyhydr)oxides in Precambrian iron formations (IFs). However, the transformation pathways of GR into secondary mineral phases in IFs remain unclear. One potentially relevant mechanism on early Earth is anoxygenic phototrophic microbial oxidation of either dissolved Fe(II) or Fe(II)-bearing minerals that leads to the formation of Fe(III) (oxyhydr)oxides. It is currently unknown whether phototrophic Fe(II)-oxidisers can access lattice Fe(II) in GR. Here, we studied microbial Fe(II) oxidation of carbonate green rust by two anoxygenic phototrophic Fe(II)-oxidising bacteria, Rhodobacter ferrooxidans SW2 and Rhodopseudomonas palustris TIE-1. We found that these two species could oxidise GR to a short range ordered Fe(III) oxyhydroxide, likely ferrihydrite, with faster GR oxidation rates by SW2 than by TIE-1. These results suggest that anoxygenic phototropic Fe(II) oxidation of GR can contribute to the formation of Fe(III) (oxyhydr)oxides and thus, this process could have been an important mechanism for Precambrian IFs deposition in ancient oceans.

X. Han, E.J. Tomaszewski, J. Sorwat, Y. Pan, A. Kappler, J.M. Byrne

Geochem. Persp. Let. (2020) 12, 52–57 | doi: 10.7185/geochemlet.2004 | Published 31 January 2020

Redox transfer at subduction zones: insights from Fe isotopes in the Mariana forearc

Subduction zones are active sites of chemical exchange between the Earth’s surface and deep interior and play a fundamental role in regulating planet habitability. However, the mechanisms by which redox sensitive elements (e.g., iron, carbon and sulfur) are cycled during subduction remains unclear. Here we use Fe stable isotopes (δ56Fe), which are sensitive to redox-related processes, to examine forearc serpentinite clasts recovered from deep sea drilling of mud volcanoes formed above the Mariana subduction zone in the Western Pacific. We show that serpentinisation of the forearc by slab-derived fluids produces dramatic δ56Fe variation. Unexpected negative correlations between serpentinite bulk δ56Fe, fluid-mobile element concentrations (e.g., B, As) and Fe3+/ƩFe suggest a concomitant oxidation of the mantle wedge through the transfer of isotopically light iron by slab-derived fluids. This process must reflect the transfer of either sulfate- or carbonate-bearing fluids that preferentially complex isotopically light Fe.

B. Debret, C.D.J. Reekie, N. Mattielli, H. Beunon, B. Ménez, I.P. Savov, H.M. Williams

Geochem. Persp. Let. (2020) 12, 46–51 | doi: 10.7185/geochemlet.2003 | Published 31 January 2020

Metamorphic evolution of carbonate-hosted microbial biosignatures

Microbial biosignature assemblages captured within mineral substrates experience extreme pressures (P) and temperatures (T) during rock burial and metamorphism. We subjected natural microbial biofilms hosted within thermal spring carbonate to six high pressure, high temperature (HPHT) conditions spanning 500 and 800 MPa and 200 to 550 °C, to investigate the initial petrographic transformation of organic and inorganic phases. We find biogenic and amorphous silica mineralises increasingly mature organic matter (OM) as temperature and pressure increase, with OM expelled from recrystallised calcite at the highest HPHT, captured within a quartz phase. Sulfur globules associated with microbial filaments persist across all HPHT conditions in association with microbially-derived kerogen. These data demonstrate how microbial material captured within chemically-precipitated sediments petrographically evolves in high grade rocks during their first stages of transformation.

C.R. Cousins, S. Mikhail, F. Foucher, A. Steele, F. Westall

Geochem. Persp. Let. (2020) 12, 40–45 | doi: 10.7185/geochemlet.2002 | Published 16 January 2020

Hafnium isotopic disequilibrium during sediment melting and assimilation

Identification of juvenile and mature crustal sources in granite formation relies on radiogenic isotopic systems such as Sm-Nd and Lu-Hf and assumes isotope systems reach equilibrium between the melt and residual phases prior to melt extraction. However, we hypothesise disequilibrium melting and residual zircon result in preferential retention of 177Hf in residues, generating partial melts with higher 176Hf/177Hf ratios. To test this hypothesis, we evaluate radiogenic isotopic signatures of strongly-peraluminous granites from the Chinese Altai. These granites show Nd-Hf isotopic decoupling and inherited zircons with negative ɛHf(t) values providing evidence for incomplete Hf release. This is consistent with the significant depletions in Zr and Hf. The Chinese data compilation shows that strongly-peraluminous and calcic to calc-alkalic, magnesian metaluminous or ferroan peraluminous (often respectively referred to as S- and I-type) granites show elevated ɛHf(t) relative to the terrestrial Hf-Nd isotopic array. Hf isotope disequilibrium marked by the preferential release of radiogenic Hf is likely ubiquitous during anatexis of zircon-rich protoliths.

C. Zhang, D. Liu, X. Zhang, C. Spencer, M. Tang, J. Zeng, S. Jiang, M. Jolivet, X. Kong

Geochem. Persp. Let. (2020) 12, 34–39 | doi: 10.7185/geochemlet.2001 | Published 15 January 2020

Experimental clues for detecting biosignatures on Mars

Forthcoming exploration of Mars aims at identifying fossil biosignatures within ancient clay-rich formations. The subsurface of Mars has mostly acted as a giant freezer for the last 4 Gyr, thereby preserving potential remains of martian life. Yet, volcanism and impactors have periodically triggered the circulation of hydrothermal fluids, inevitably causing alteration of potentially fossilised biogenic organic materials. It thus appears crucial to quantify the impact of hydrothermal processes on organic biogeochemical signals in the presence of clay minerals. Here, we submitted RNA to hydrothermal conditions in the presence of Mg-smectites. Results show heterogeneous organo-mineral residues, with sub-micrometric phosphates, carbonates and amorphous silica particles together with Mg-smectites with interlayer spaces saturated by N-rich organic compounds. Although the chemical structure of RNA did not withstand hydrothermal conditions, clay minerals efficiently trapped organic carbon, confirming the relevance of drilling for organic carbon in ancient martian sediments. In addition, the degradation of RNA in the presence of Mg-smectites led to the precipitation of a quite uncommon mineral assemblage that could be seen as a biosignature per se. Martian targets exhibiting this mineral assemblage will thus constitute high priority and highly relevant candidates for sample return.

J.-C. Viennet, S. Bernard, C. Le Guillou, P. Jacquemot, E. Balan, L. Delbes, B. Rigaud, T. Georgelin, M. Jaber

Geochem. Persp. Let. (2019) 12, 28–33 | doi: 10.7185/geochemlet.1931 | Published 27 November 2019