Iron oxidation state in garnet (Monviso mate-ophilite, western Alps) as monitor of dehydration-redox reactions during subduction: a micro-xanes and electron microprobe ("flank method") comparative study

Although it is accepted that the mantle wedge above subduction zones is oxidized, the exact processes of relative oxidation are still controversial (e.g. Parkinson & Arculus, 1999). It is widely recognized that oxidized components are transferred to the overlying mantle wedge by fluid phases coming from the subducting slab (e.g. Parkinson & Arculus, 1999; Peslier et al., 2002; Malaspina et al., 2009; Rowe et al., 2009; Hirschmann, 2009); however, the study of the redox reactions occurring at the interface between the subducting slab and the mantle wedge, is often hampered by the difficulties in accurately determining the oxidation state of phase assemblages involved in these redox reactions. In this contribution we report the results of a micro-XANES study on strongly-zoned garnet crystals from an eclogitic FeTi-oxide gabbro (Monviso meta-ophiolites, western Alps), aimed to the determination of the Fe3+/ΣFe variation across the crystals. The studied sample extraordinarily well preserves evidence of its prograde evolution during subduction down to a depth of ca. 80 km (Groppo & Castelli, 2010). Because in the studied garnet the Fe3+/ΣFe variations are small and occur on a micrometric scale, the challenge of this study was twofold: (i) the spatial resolution must be of the order of microns, and (ii) the spectral resolution must be as small as possible in order to appreciate the differences in the oxidation state of garnet. The results of the XANES spectra, acquired with a spatial resolution of 1.7 μm × 5.3 μm, show that the Fe3+/ΣFe ratio decreases from a maximum of 0.14 in the garnet core to a minimum of 0.00 in the garnet rim. These results are in excellent agreement with those previously obtained using the “flank method” at the electron microprobe. Comparing the results obtained with the two techniques, it can be concluded that micro-XANES can be successfully used to estimate Fe3+/ΣFe variations in natural garnets at a high spatial resolution (≤ 5 μm), provided that the differences in Fe3+/ΣFe are higher than ca. 5%. The “flank method” is the only method which has demonstrated, so far, to be sensitive to smaller variations (i.e. < 5%) of the Fe3+/ΣFe ratio in garnet, for ΣFe > 8 wt%. The measured Fe3+/ΣFe zoning in garnet, combined with the already estimated P-T conditions of its growth, provides qualitative information about the redox equilibria occurring during subduction of the studied FeTi-oxide metagabbro. Dehydration-redox equilibria involving the breakdown of lawsonite and chlorite (i.e. Qtz+Lws+Chlss+Omp1ss+Grt1ss=Grt2ss+Omp2ss+H2O+O2) are responsible for a significant chemical re-equilibration of garnet (and omphacite), resulting in a progressive decrease of its Fe3+/ΣFe ratio toward the rim and, consequently, in the release of oxygen. Dehydration-redox reactions of this type, occurring at least locally during subduction of the oceanic crust, may be likely involved in the oxidation of the mantle wedge. References Groppo C. & Castelli D. (2010). J. Petrol. 51, 2489-2514. Hirschmann M.M. (2009). Science 325, 545-546. Malaspina N., Poli S. & Fumagalli P. (2009). J. Petrol. 50, 1533-1552. Parkinson I.J. & Arculus R.J. (1999). Chem. Geol. 160, 409-423. Peslier A.H., Luhr J.F. & Post J. (2002). Earth Planet. Sci. Lett. 201, 69-86. Rowe M.C., Kent A.J.R. & Nielsen R.L. (2009). J. Petrol. 50, 61-91.