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Published July 15, 2014 | Supplemental Material
Journal Article Open

Magma redox and structural controls on iron isotope variations in Earth's mantle and crust

Abstract

The heavy iron isotopic composition of Earth's crust relative to chondrites has been explained by vaporization during the Moon-forming impact, equilibrium partitioning between metal and silicate at core–mantle-boundary conditions, or partial melting and magma differentiation. The latter view is supported by the observed difference in the iron isotopic compositions of MORBS and peridotites. However, the precise controls on iron isotope variations in igneous rocks remain unknown. Here, we show that equilibrium iron isotope fractionation is mainly controlled by redox (Fe^(3+)/Fe_(tot) ratio) and structural (e.g., polymerization) conditions in magmas. We measured, for the first time, the mean force constants of iron bonds in silicate glasses by synchrotron Nuclear Resonant Inelastic X-ray Scattering (NRIXS, also known as Nuclear Resonance Vibrational Spectroscopy – NRVS, or Nuclear Inelastic Scattering – NIS). The same samples were studied by conventional Mössbauer and X-ray Absorption Near Edge Structure (XANES) spectroscopy. The NRIXS results reveal a +0.2 to +0.4‰ equilibrium fractionation on ^(56)Fe/^(54)Fe ratio between Fe^(2+) and Fe^(3+) end-members in basalt, andesite, and dacite glasses at magmatic temperatures. These first measurements can already explain ∼1/3 of the iron isotopic shift measured in MORBs relative to their source. Further work will be required to investigate how pressure, temperature, and structural differences between melts and glasses affect equilibrium fractionation factors. In addition, large fractionation is also found between rhyolitic glass and commonly occurring oxide and silicate minerals. This fractionation reflects mainly changes in the coordination environment of Fe2+ in rhyolites relative to less silicic magmas and mantle minerals, as also seen by XANES. We provide a new calibration of XANES features vs. Fe^(3+)/Fe_(tot) ratio determinations by Mössbauer to estimate Fe^(3+)/Fe_(tot) ratio in situ in glasses of basaltic, andesitic, dacitic, and rhyolitic compositions. Modeling of magma differentiation using rhyolite-MELTS shows that iron structural changes in silicic magmas can explain the heavy iron isotopic compositions of granitoids and rhyolites. This study demonstrates that iron stable isotopes can help reveal planetary redox conditions and igneous processes. Other heterovalent elements such as Ti, V, Eu, Cr, Ce, or U may show similar isotopic variations in bulk rocks and individual minerals, which could be used to establish past and present redox condition in the mantles of Earth and other planets.

Additional Information

© 2014 Elsevier B.V. Published by Elsevier B.V. Received 13 July 2013, Revised 15 April 2014, Accepted 21 April 2014, Available online 23 May 2014. N.D. thanks the NSF Petrology and Geochemistry (EAR1144429) and NASA Cosmochemistry (NNX12AH60G) programs for support. M.R. thanks the support of the French ANR program (2011JS56 004 01, FrIHIDDA). The EPMA facility in Lille is supported by the European Regional Development Fund (ERDF). Constructive criticisms by Elizabeth Cottrell, Sune Nielsen, an anonymous reviewer, and editor Bernard Marty helped improve the manuscript. Discussions with Andrew J. Campbell, Jennifer M. Jackson and Wenli Bi were greatly appreciated.

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