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Volume 18

About the cover: Reflected-light image of pyrite grains (etched with sodium hypochlorite
solution) associated with barite from the 3.49 Ga Dresser Formation, Pilbara Craton, Western Australia,
studied by Liu et al. in Letter 2113. One pyrite grain shows a core-rim texture. SHRIMP-SI analyses reveal negative Δ33S values similar to those of associated barite in the rim, but distinctively positive Δ33S values in the core. This indicates two generations of pyrites in the Dresser system, with Δ33S-positive pyrites preceding Δ33S-negative ones. The two groups of pyrites with positive and negative Δ33S values reported in previous studies may thus be connected.
Image by Li Liu. Download high-resolution cover.

Rise of major subaerial landmasses about 3.0 to 2.7 billion years ago

Abstract:
The emergence of subaerial landmasses should have inevitably modulated the chemical composition of the atmosphere-ocean and long term climate. However, it remains controversial when major subaerial landmasses first emerged in Earth’s history. Here we show that the mean K/La of globally continental mafic volcanic rocks declined from a fairly high value (∼2125) to a magmatic background (∼582, indicated by continental mafic plutonic rocks) during 3.0–2.7 Ga. It can only be explained by a progressive reduction in the proportion of submarine hydrothermally altered mafic volcanic samples and thus records a gradual rise of major subaerial landmasses from 3.0 to 2.7 Ga, likely to a present day level since 2.7 Ga. The rise and maintaining of major subaerial landmasses were intrinsically controlled by a dynamic balance of mountain building processes dominantly driven by plate tectonics and subsidence due to weathering erosion and thermal relaxation.

C.-T. Liu, Y.-S. He

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Geochem. Persp. Let. (2021) 18, 1–5 | doi: 10.7185/geochemlet.2115 | Published 25 May 2021

The pressure-induced local structural change around tungsten in silicate glass

Abstract:
Tungsten is one of the key elements to understand the conditions and timing of planetary core formation. While the metal-silicate partitioning of tungsten has been extensively studied, the effects of pressure and silicate melt composition have been controversial. Here we have investigated the local environment of tungsten in a basaltic glass up to 67 GPa based on EXAFS spectroscopy and found that the W-O bond length increases in a pressure range from 10 to 32 GPa, indicating that W6+ ion increases the coordination number from four to six. It is known that the coordination of silicon also changes at similar pressure range, suggesting that the coordination structure of trace element tungsten may be controlled by the Si-O coordination. The coordination change of tungsten will affect its metal-silicate partitioning and may explain the previously observed change in the pressure effect around 5 GPa, when considering the difference between melt and glass. This also suggests further change in the pressure dependence above 32 GPa where tungsten is predominantly sixfold coordinated. In addition, the effect of silicate melt composition may diminish at such pressure range.

K. Ozawa, K. Hirose, Y. Kuwayama, Y. Takahashi

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Geochem. Persp. Let. (2021) 18, 6–10 | doi: 10.7185/geochemlet.2116 | Published 1 June 2021

207Pb-excess in carbonatitic baddeleyite as the result of Pa scavenging from the melt

Abstract:
For the last two decades, the end of the voluminous phase of eruptions of the Siberian Traps large igneous province has been constrained by a U-Pb date of discordant baddeleyite collected from the Guli carbonatite intrusion with the assumption that the discordance resulted from unsupported 207Pb. In this study we have re-analysed baddeleyite from the same intrusion and found two types of discordance: (1) due to 207Pb-excess, and (2) radiogenic lead loss from high U mineral inclusions. The former implies that baddeleyite is an efficient scavenger of protactinium during crystallisation, leaving the magma depleted in this element. Together with a published high precision U-Pb date of 252.24 ± 0.08 Ma for the Arydzhansky Formation, our new date of 250.33 ± 0.38 Ma for the Guli carbonatite constrains the total duration of the voluminous eruptions of the Siberian Traps LIP at 1.91 ± 0.38 million years. The lower intercept of the (231Pa)/(235U) corrected discordance line yields a date of 129.2 ± 65.0 Ma, which points to the widespread Early Cretaceous rifting in East and Central Asia.

A.V. Ivanov, F. Corfu, V.S. Kamenetsky, A.E. Marfin, N.V. Vladykin

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Geochem. Persp. Let. (2021) 18, 11–15 | doi: 10.7185/geochemlet.2117 | Published 15 June 2021

Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era

Abstract:
Eukaryotes reached ecological importance in the late Neoproterozoic Era, some one billion years after their emergence. Their slow rise to prominence has been ascribed to prolonged environmental stagnation, but testing this idea requires an appraisal of the evolution of atmospheric and ocean chemistry. Establishing a nuanced geochemical history is, however, challenging due to the paucity of well preserved sedimentary rocks. Here, we present new Mo isotope ratios from black shale units spanning ∼1560 to ∼1170 Ma. These results, combined with literature data, reveal potential episodic expansions of oxygenated and/or mildly reducing conditions during the Mesoproterozoic Era, suggesting fluctuating oxygen availability that could have exerted a crucial control on the evolution of eukaryotes.

Y. Ye, S. Zhang, H. Wang, X. Wang, C. Tan, M. Li, C. Wu, D.E. Canfield

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Geochem. Persp. Let. (2021) 18, 16–21 | doi: 10.7185/geochemlet.2118 | Published 15 June 2021