CO2-Undersaturated Melt Inclusions From the South West Indian Ridge Record Surprisingly Uniform Redox Conditions

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jats:titleAbstract</jats:title>jats:pThe behavior of Fejats:sup3+</jats:sup> during mantle partial melting strongly influences the oxidation state of the resulting magmas, with implications for the evolution of the atmosphere's oxidation state. Here, we challenge a prevailing view that low‐degree partial melts are more oxidized due to the incompatible behavior of Fejats:sup3+</jats:sup>. Our study is based on measurements of Fejats:sup3+</jats:sup>/∑Fe along with major, minor, trace and volatile elements in olivine‐ and plagioclase‐hosted melt inclusions of COjats:sub2</jats:sub> undersaturated mantle melts in South West Indian Ridge lava. These inclusions record minimum entrapment pressures equivalent to depths up to 10 km below the seafloor, record magma ascent rates of 0.03–0.19 m/s, and display exceptionally high COjats:sub2</jats:sub>/Ba, COjats:sub2</jats:sub>/Rb, and COjats:sub2</jats:sub>/Nb ratios, indicative of a COjats:sub2</jats:sub>‐rich mantle source. Accounting for fractional crystallization, we find a uniform melt oxidation state (with an Fejats:sup3+</jats:sup>/ΣFe at 0.140 ± 0.005 at MgO = 10 wt.%) that displays no systematic variation with major, minor, volatile or trace element contents, thus providing no evidence for a relationship between the degree of partial melting and Fejats:sup3+</jats:sup>/ΣFe. This can be explained by efficient buffering of Fejats:sup3+</jats:sup>/∑Fe and jats:italicf</jats:italic>Ojats:sub2</jats:sub> of mid‐ocean ridge basalt melts by their surrounding mantle and/or a decrease in the bulk peridotite‐melt Fejats:sub2</jats:sub>Ojats:sub3</jats:sub> partition coefficient with increasing partial melting. We conclude that changes in the Earth's upper mantle temperature over geological time need not have affected the oxidation state of volcanic products or of the atmosphere.</jats:p>

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Publication status: Published


Funder: Konstantin Ignatyev

Keywords
37 Earth Sciences, 3703 Geochemistry, 3705 Geology, 3706 Geophysics
Journal Title
Geochemistry, Geophysics, Geosystems
Conference Name
Journal ISSN
1525-2027
1525-2027
Volume Title
Publisher
American Geophysical Union (AGU)
Sponsorship
Labex VOLTAIRE (ANR‐10‐LABX‐100‐01)