HR: 1330h
AN: V52A-0422    [PDF]
TI: Large Iron Isotope Variations in Mantle Minerals: The Influence of Oxidation State and the Implications for the Iron Isotope Heterogeneity of the Upper Mantle
AU: * Williams, H M
EM: williams@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Sonneggstr. 5, Z\"{u}rich, 8092 Switzerland
AU: Peslier, A H
EM: Anne.Peslier@mail.uh.edu
AF: Texas Center for Superconductivity, University of Houston, Houston, TX 77204 United States
AU: McCammon, C
EM: Catherine.McCammon@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universit\"{a}t Bayreuth, Bayreuth, 95440 Germany
AU: Teutsch, N
EM: teutsch@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Sonneggstr. 5, Z\"{u}rich, 8092 Switzerland
AU: Levasseur, S
EM: levasseur@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Sonneggstr. 5, Z\"{u}rich, 8092 Switzerland
AU: Burg, J
EM: jpb@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Sonneggstr. 5, Z\"{u}rich, 8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw,ethz.ch
AF: ETH-Z\"{u}rich, Sonneggstr. 5, Z\"{u}rich, 8092 Switzerland
AB: Controversy exists over whether or not there are significant variations in the iron isotope compositions of terrestrial mantle rocks, and whether stable isotopic fractionation can take place at high temperatures. Recent studies imply that significant iron isotopic fractionations exist between high temperature mantle minerals (Zhu et al., 2002, EPSL 200, 1-2). This suggests that isotope fractionation at the mineral scale has the potential to influence the terrestrial mass balance for iron isotopes. We present MC-ICP-MS iron isotope data for minerals from mantle peridotite to determine if there is isotopic fractionation between minerals, and what the physical and chemical parameters that could cause fractionation are. In order to evaluate the level of mantle iron isotope heterogeneity at the mineral scale, spinel and spinel-garnet facies peridotites, representative of sub-arc mantle, sub-continental cratonic lithospheric mantle and MORB-source mantle, were selected. Systematic isotopic fractionations exist between silicate minerals. The $\delta$ $^{57}$Fe/$^{54}$Fe compositions of the olivines are $\sim$ 0.20 $\permil$ lighter than those of clinopyroxene and orthopyroxene, consistent with the results of Zhu et al. Although the differences are small, they are greater than our long-term reproducibility (0.13 $\permil$ 2 S.D.). The ranges in the $\delta$ $^{57}$Fe/$^{54}$Fe compositions of olivine, clinopyroxene and orthopyroxene are 0.6, 0.7 and 0.9 $\permil$, respectively. Spinels show a range of over 1.70 $\permil$. The lightest isotopic compositions are for minerals from island-arc peridotites, the heaviest are for minerals from xenoliths that sample sub-continental cratonic mantle. The spinel $\delta$ $^{57}$Fe/$^{54}$Fe compositions correlate negatively with spinel ferric/total iron contents, determined by M\"{o}ssbauer spectroscopy, suggesting that this parameter has a strong influence on isotope fractionation. The $\delta$ $^{57}$Fe/$^{54}$Fe compositions of the silicate minerals show some correlation with those of co-existing spinels. One explanation for the variations is that significant differences exist between the iron isotope compositions of sub-arc and sub-continental cratonic lithospheric mantles. These differences could arise as a consequence of melt extraction or melt-rock interaction processes as ferric iron partitions preferentially to ferrous iron into silicate melts. The iron isotope compositions of spinels may be especially sensitive to such processes because spinels are the main carriers of oxidized Fe in the shallow mantle (Wood and Virgo, 1989, GCA, 50, 6).
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting