TY - JOUR KW - diffusion KW - isotope KW - metasomatism KW - monomineralic ID - discovery1462110 N2 - Isotopic fractionation of cationic species during diffusive transport provides a novel means of constraining the style and timing of metamorphic transformations. Here we document a major (?1?) decrease in the Mg isotopic composition of the reaction front of an exhumed contact between rocks of subducted crust and serpentinite in the Syros mélange zone. This isotopic perturbation extends over a notable length scale (?1 m), implicating diffusion of Mg through an intergranular fluid network over a period of ?100 kyr. These novel observations confirm models of diffusion-controlled growth of reaction zones formed between rocks of contrasting compositions, such as found at the slab?mantle interface in subduction zones. The results also demonstrate that diffusive processes can result in exotic stable isotope compositions of major elements with implications for mantle xenoliths and complex intrusions. TI - Extreme Magnesium Isotope Fractionation at Outcrop Scale Records the Mechanism and Rate at which Reaction Fronts Advance EP - 58 Y1 - 2015/01// AV - public JF - Journal of Petrology A1 - von Strandmann, PAEP A1 - Dohmen, R A1 - Marschall, HR A1 - Schumacher, JC A1 - Elliott, T SN - 0022-3530 UR - http://dx.doi.org/10.1093/petrology/egu070 N1 - This is a pre-copyedited, author-produced PDF of an article accepted for publication in von Strandmann, PAEP; Dohmen, R; Marschall, HR; Schumacher, JC; Elliott, T; (2015) Extreme Magnesium Isotope Fractionation at Outcrop Scale Records the Mechanism and Rate at which Reaction Fronts Advance. Journal of Petrology, 56 (1) 33 - 58. 10.1093/petrology/egu070, following peer review. The version of record [insert complete citation information here] is available online at: http://dx.doi.org/10.1093/petrology/egu070 IS - 1 VL - 56 SP - 33 ER -