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Redox Heterogeneity in Mid-Ocean Ridge Basalts as a Function of Mantle Source

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Science  14 Jun 2013:
Vol. 340, Issue 6138, pp. 1314-1317
DOI: 10.1126/science.1233299

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Redox Recycling

Plate tectonics drive the continuous exchange of material between Earth's crust and mantle. Subduction adds crustal materials to the mantle, which influence the composition of erupted lavas at mid-ocean ridges. Because chemical and physical processes in the mantle change over time as a response to the availability of oxygen, the redox state of mid-ocean ridge basalts may trace the history of recycling between crust and mantle. Cottrell and Kelley (p. 1314, published online 2 May) analyzed the relation between the oxidation state of iron in a global suite of mid-ocean ridge basalts and tracers for mantle source composition. Over tectonic time scales, the recycling of reduced carbon in ancient crustal sediments may result in the preservation of more reduced zones in the mantle.

Abstract

The oxidation state of Earth’s upper mantle both influences and records mantle evolution, but systematic fine-scale variations in upper mantle oxidation state have not previously been recognized in mantle-derived lavas from mid-ocean ridges. Through a global survey of mid-ocean ridge basalt glasses, we show that mantle oxidation state varies systematically as a function of mantle source composition. Negative correlations between Fe3+/ΣFe ratios and indices of mantle enrichment—such as 87Sr/86Sr, 208Pb/204Pb, Ba/La, and Nb/Zr ratios—reveal that enriched mantle is more reduced than depleted mantle. Because carbon may act to simultaneously reduce iron and generate melts that share geochemical traits with our reduced samples, we propose that carbon creates magmas at ridges that are reduced and enriched.

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