Molybdenum contents of sulfides in ancient glacial diamictites: Implications for molybdenum delivery to the oceans prior to the Great Oxidation Event

Su, Li; Junkin, William; Gaschnig, Richard M.; Ash, R. D.; Piccoli, Philip M.; Candela, Philip A.; Rudnick, Roberta L.

doi:10.1016/j.gca.2019.09.011

Cited by 10 publications

(4 citation statements)

References 113 publications

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“…Consistent with this finding, diamictites from 2.90 to 2.43 Ga show only small degrees of Mo mobilization ( 48 , 49 ). Retention of Mo in diamictites is also expected because Mo 4+ often substitutes for Ti 4+ in titaniferous, weathering-resistant minerals, meaning only a fraction of crustal Mo is hosted in sulfides and mobilized during oxidative weathering ( 28 , 50 ). For instance, Greaney et al ( 28 ) estimated that ~60% of the Mo hosted in the modern upper continental crust could be contained within sulfide minerals.…”

Section: Resultsmentioning

confidence: 99%

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

et al. 2021

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Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O 2 ) at Earth's surface before the Great Oxidation Event. Quantifying this early O 2 has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O 2 partial pressures (PO 2 ) and O 2 production fluxes during the Archean. We consider two end-member scenarios. First, if O 2 was evenly distributed throughout the atmosphere, then PO 2 > 10 -6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O 2 accumulation was instead spatially restricted (e.g., occurring only near the sites of O 2 production), then O 2 production fluxes >0.01 Tmol O 2 /year were required. Archean O 2 levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.

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Section: Resultsmentioning

confidence: 99%

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

et al. 2021

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“…The δ 98 Mo signature of the modern un-weathered UCC is surprisingly difficult to pin down. Upper crustal igneous rocks and molybdenite grains display a wide range of δ 98 Mo values, encompassing almost the entire range of fractionation seen in river and marine environments (Breillat et al, 2014 (Freymuth et al, 2016;Voegelin et al, 2014;Yang et al, 2017;, excluding samples affected by hydrothermal activity, while MORB (average −0.09 ± 0.02 2σ ) and OIB (average −0.14 ± 0.06 2σ ) tend to be relatively lighter than most felsic igneous rocks (Bezard et al, 2016;Liang et al, 2017). While no Mo isotope fractionation was observed by Yang et al (2015) during igneous differentiation in an anhydrous intraplate setting, isotopic fractionation is proposed to occur in subduction zones due to crystallization of Mo-bearing minerals, which leaves arc magmas -therefore the UCC -isotopically heavier than mafic and ultramafic rocks (Voegelin et al, 2014;König et al, 2016;Wille et al, 2018 Because arc magmas can be significantly isotopically heavier than the mantle and MORB (König et al, 2016;Wille et al, 2018), we would expect the evolution from a more mafic to more felsic continental crust to shift the UCC to heavier values with time.…”

Section: Fractionation During Crustal Differentiation and Compositionmentioning

confidence: 99%

Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust

Greaney

Rudnick

Romaniello

et al. 2020

Earth and Planetary Science Letters

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Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43-2.90 Ga), syn-GOE (2.20-2.39 Ga), and post-GOE (0.33-0.75 Ga). Pre-GOE composites have an average δ 98 Mo NIST3134 of +0.03 (± 0.18), syn-GOE composites average −0.29 (± 0.60), and post-GOE composites average −0.45 (± 0.51). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ 98 Mo signature as +0.03 ± 0.18 (2σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems.

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“…17 The role of pyrite in Mo accumulation has, however, been questioned because of its absence or low Mo concentrations in pyrite framboids. 18,19 One alternative mechanism is the formation of a distinct colloidal precipitate through the coprecipitation of MoS 4 2− with iron and sulfide, rather than Mo adsorption to pyrite, yielding FeMo(IV)S 4 . This is a pathway for the enrichment of Mo in euxinic basins, such as the Black Sea.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Modern river studies suggest that pyrite weathering is the main source of Mo to oceans ,− while chemical leaching of marine and marsh sediments using concentrated nitric acid suggests that Mo is associated with the pyritic phase. , Further work shows a possible role of MoO 4 2– in the formation of pyrite when in the presence of elemental sulfur . The role of pyrite in Mo accumulation has, however, been questioned because of its absence or low Mo concentrations in pyrite framboids. , One alternative mechanism is the formation of a distinct colloidal precipitate through the coprecipitation of MoS 4 2– with iron and sulfide, rather than Mo adsorption to pyrite, yielding FeMo(IV)S 4 . This is a pathway for the enrichment of Mo in euxinic basins, such as the Black Sea .…”

Section: Introductionmentioning

confidence: 99%

Adsorption of Tetrathiomolybdate to Iron Sulfides and Its Impact on Iron Sulfide Transformations

Miller

Dougherty

et al. 2020

ACS Earth Space Chem.

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Molybdenum (Mo) in marine sediments has been used as a paleoproxy to provide evidence for past oceanic euxinic and sulfidic conditions through its association with pyrite. Here, we examine the adsorption of Mo to the pyrite precursors mackinawite and greigite and assess the robustness of this association during iron sulfide phase transformations. Tetrathiomolybdate (MoS 4 2− ) adsorption experiments were done using mackinawite and greigite that had been characterized using powder X-ray diffraction and Raman spectroscopy. Adsorption of tetrathiomolybdate to mackinawite and to a primarily greigite mixture was similar. Both showed little change to the mineral phase upon adsorption. Relative to previously published data on pyrite, there was a much greater amount of Mo adsorption and a different mode of adsorption. A mackinawite/greigite mixture was also synthesized through an alternative method that more closely mimicked environmental conditions with a brief in situ aging to form an initial phase of iron sulfide, likely highly disordered mackinawite, and the near-immediate addition of MoS 4 2− . X-ray photoelectron spectroscopy results support the adsorption of tetrathiomolybdate and its concomitant reduction to Mo(IV). The Mo-adsorbed mackinawite/greigite mixture was transformed through heating into a greigite/pyrite mixture while monitoring Mo release to the aqueous phase. Here, the sorption of Mo on the solid phase promoted the transformation of mackinawite into pyrite upon heating without diagenetic loss of Mo to the aqueous phase. These results support the early capture of MoS 4 2− to less-stable forms of iron sulfide with negligible diagenetic loss during subsequent transformation. This work continues to point to Mo(VI) as a plausible oxidant of FeS to FeS 2 within natural euxinic settings.

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Molybdenum contents of sulfides in ancient glacial diamictites: Implications for molybdenum delivery to the oceans prior to the Great Oxidation Event

Cited by 10 publications

References 113 publications

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust

Adsorption of Tetrathiomolybdate to Iron Sulfides and Its Impact on Iron Sulfide Transformations

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