Limited magnesium isotope fractionation during metamorphic dehydration in metapelites from the Onawa contact aureole, Maine

Abstract

Knowledge on the behavior of Mg isotopes during metamorphic dehydration is the prerequisite for applying Mg isotopes as tracers for crustal recycling. Here we report Mg isotopic compositions of metapelites from the Onawa contact aureole, Maine. Except one sample, all metapelites across the aureole, from the wall-rock regional metamorphic rocks to the partially melted rocks adjacent to the pluton, have similar Mg isotopic compositions (d 26 Mg 5 20.09 to 10.12&). This observation indicates limited Mg isotope fractionation during metamorphic dehydration and fluid-rock interaction, due to the low Mg concentration in fluids relative to rocks. Our results suggest that Mg isotopic compositions of metapelites can record those of their protoliths and, hence, recycled clastic sedimentary materials may preserve their low-temperature Mg isotopic signatures through subduction zones. Therefore, Mg isotopes may serve as new tracers for crustal recycling, for example, tracing components experienced weathering cycles within granite sources.

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

“…S3A). These observations agree with previous studies that found limited Mg isotope fractionations in siliciclastic sedimentary rocks during diagenesis and low-grade metamorphism (17,29). A possible influence of modern outcrop weathering can also be excluded because both Bahuang outcrop and the 14ZK core show consistent δ 26 Mg profiles.…”

Magnesium isotope geochemistry in arc volcanism

“…Equivalent sediments in arc crust through which the Martinique lavas erupted could provide such a source, as well, if they were assimilated into the lavas. Furthermore, due to the lack of Mg isotope fractionation during prograde metamorphism (31,33,34), the metamorphic counterparts of the subducting sediments should preserve their original Mg isotopic signature. Previous isotopic studies of Martinique lavas show that the sedimentary input increases with age from old to intermediate lavas whereas it is much more variable in the recent lavas (4).…”

Magnesium isotopic compositions of altered oceanic basalts and gabbros from IODP site 1256 at the East Pacific Rise

“…Therefore, Mg isotopes have also been used in attempts to trace recycled crustal materials in the mantle Shen et al, 2009;Xiao et al, 2013;Yang et al, 2012). Since metamorphic dehydration and high-temperature partial melting only result in limited Mg isotope fractionation (e.g., Li et al, , 2014Liu et al, 2010;Telus et al, 2012;Teng et al, 2007;2010b;Wang et al, 2014a), it is expected that the recycled materials will preserve their original Mg isotopic signatures through subduction zone processes. Thus, the similarity in Mg isotopic compositions of the carbonate-barren AOC and that of the mantle that we demonstrate in this study implies that recycling of such AOC is unlikely to result in heterogeneous Mg isotopic compositions of the mantle at a global scale.…”