Magnesium isotope fractionation processes during seafloor serpentinization and implications for serpentinite subduction

Abstract

Studies of magnesium (Mg) isotope ratios in subduction zone lavas have revealed small but significant offsets from the mantle value with enrichments in the heavy isotopes. However, the very high concentration of Mg in the mantle contrasts with much lower concentrations in the subducted igneous crust and oceanic sediments, making these subduction components unlikely vehicles of the Mg isotope anomalies in arc lavas. Only serpentinites, which in various proportions form part of oceanic plates, have high Mg contents comparable to fresh mantle rocks, and they have thus been regarded as a potential source of exotic Mg in the source of arc magmas. In this study we analyzed serpentinite samples from different oceanic settings for their Mg isotopic compositions. The majority of samples are indistinguishable from the depleted mantle (δ26Mg=-0.24 ‰ ± 0.04 ‰) irrespective of their origin. Only a small number of seafloor-weathered serpentinites are slightly enriched in the heavy isotopes (up to δ26Mg=-0.14 ‰ ± 0.03 ‰), implying that bulk serpentinites are unlikely sources of isotopically anomalous Mg in subduction zones. We also developed a partial dissolution method in which 5 % acetic acid for 180 min was shown to fully dissolve the minerals brucite and iowaite while leaving the serpentine mineral chrysotile essentially undissolved. Partial dissolution of 11 bulk serpentinite samples revealed Mg isotopic composition of brucite (± iowaite) that is systematically ∼0.25 ‰ heavier than that of coexisting serpentine. Thus, preferential breakdown of brucite and/or iowaite in a subducted slab prior to serpentine could preferentially release isotopically heavy Mg, which could subsequently be transported into the source region of arc magmas. Such a scenario would require brucite/iowaite breakdown to occur at pressures in excess of 3 GPa and produce fluids with very high concentrations of Mg that could be transported to arc magma source regions. Whether these conditions are met in nature has yet to be experimentally investigated.