Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels

Daniel A. Stolper and Claire E. Bucholz
PNAS April 30, 2019 116 (18) 8746-8755; first published April 11, 2019 https://doi.org/10.1073/pnas.1821847116

  1. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved March 13, 2019 (received for review December 21, 2018)

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Significance

Igneous island arc rocks are more oxidized than midocean-ridge basalts. Whether this originates from an oxidized mantle source or during differentiation or eruption is debated. Newly compiled Fe3+/ΣFe and V/Sc ratios presented here indicate that island arc rocks became more oxidized 800–400 Ma. We attribute this increase in the redox state of island arc rocks to the oxidation of their mantle source from subduction of oxidized oceanic crust following deep-ocean oxygenation and increased marine sulfate concentrations over the same time interval. This provides evidence that the oxidized nature of modern island arc magmas is due to an oxidized mantle source and represents a rare example of a change in the surface biogeochemistry influencing the igneous rock record.

Abstract

A rise in atmospheric O2 levels between 800 and 400 Ma is thought to have oxygenated the deep oceans, ushered in modern biogeochemical cycles, and led to the diversification of animals. Over the same time interval, marine sulfate concentrations are also thought to have increased to near-modern levels. We present compiled data that indicate Phanerozoic island arc igneous rocks are more oxidized (Fe3+/ΣFe ratios are elevated by 0.12) vs. Precambrian equivalents. We propose this elevation is due to increases in deep-ocean O2 and marine sulfate concentrations between 800 and 400 Ma, which oxidized oceanic crust on the seafloor. Once subducted, this material oxidized the subarc mantle, increasing the redox state of island arc parental melts, and thus igneous island arc rocks. We test this using independently compiled V/Sc ratios, which are also an igneous oxybarometer. Average V/Sc ratios of Phanerozoic island arc rocks are elevated (by +1.1) compared with Precambrian equivalents, consistent with our proposal for an increase in the redox state of the subarc mantle between 800 and 400 Ma based on Fe3+/ΣFe ratios. This work provides evidence that the more oxidized nature of island arc vs. midocean-ridge basalts is related to the subduction of material oxidized at the Earth’s surface to the subarc mantle. It also indicates that the rise of atmospheric O2 and marine sulfate to near-modern levels by the late Paleozoic influenced not only surface biogeochemical cycles and animal diversification but also influenced the redox state of island arc rocks, which are building blocks of continental crust.

Footnotes

  • ↵1To whom correspondence may be addressed. Email: dstolper{at}berkeley.edu or cbucholz{at}caltech.edu.

Published under the PNAS license.

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Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels
Daniel A. Stolper, Claire E. Bucholz
Proceedings of the National Academy of Sciences Apr 2019, 116 (18) 8746-8755; DOI: 10.1073/pnas.1821847116
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