HR: 0800h
AN: V51E-0836    [Abstracts]
TI: Redox-controlled iron isotope fractionation during basalt differentiation?
AU: * Teng, F
EM: teng@geosci.uchicago.edu
AF: Origins Laboratory, Department of the Geophysical Sciences & Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, United States
AU: Dauphas, N
EM: dauphas@uchicago.edu
AF: Origins Laboratory, Department of the Geophysical Sciences & Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, United States
AU: Helz, R
EM: rhelz@usgs.org
AF: U.S. Geological Survey, U.S. Geological Survey, Reston, VA 20192, United States
AB: Theoretical studies suggest that the scale of equilibrium isotope fractionation decreases as the temperature increases and is likely to be negligible during high-temperature processes such as mantle melting and basaltic differentiation. This is confirmed by studies of Li and Mg isotopes, which show no measurable Li and Mg isotopic fractionation during basaltic differentiation at temperatures greater than 1050^oC. However, iron chemistry is unique in that two oxidation states are present in magmas. During crystallization, they partition differently between melts and solids, which can potentially produce measurable isotopic fractionation. We present high-precision Fe isotopic data measured by MC-ICP-MS for a set of well-characterized samples from Kilauea Iki lava lake, Hawaii. The Kilauea Iki lava lake has a relatively simple geological setting and experienced extensive closed-system differentiation that makes it an ideal field laboratory for studying the effect of basaltic differentiation on isotopes. The samples range from olivine-rich cumulates to andesitic segregation veins. The δ⁵⁶Fe values correlate negatively with MgO contents, and correlate positively with Fe³⁺/Fe²⁺ ratios. Olivine cumulates have higher MgO contents (up to 26.87 wt%) and lower δ⁵⁶Fe (down to -0.03‰) and Fe³⁺/Fe²⁺ ratios while late-stage veins have lower MgO contents (down to 2.37 wt%) and higher δ⁵⁶Fe (up to +0.22‰) and Fe³⁺/Fe²⁺ ratios. The Fe isotopic variations (> 0.2‰) in Kilauea Iki lava lake were most likely produced by fractional crystallization, with an estimated isotopic fractionation factor between minerals and melts of ~ - 0.1‰. The oxidation state of Fe affects key chemical properties such as the coordination number. This can explain why Fe isotopes are fractionated during basalt differentiation while Li and Mg are not. Our study suggests that the Fe isotopic fractionation during magmatic differentiation is largely controlled by the oxidation state of iron. Therefore, Fe isotopes can potentially be used to constrain the redox history of terrestrial and exterrestrial magmas
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting