Multiple fluid-rock interactions in the subduction channel evidenced by magnesium isotopes

Abstract

Fluids from the subducted oceanic slab are closely associated with mass transfer in subduction zones, which significantly affect the composition of arc magmas and deep mantle. Previous studies have shown that magnesium isotopes can be used to identify the source of fluids and to trace carbonate cycling. However, the Mg isotope variation as a function of fluids in the subduction channel remains obscure. As large-scale fluid flows and strong fluid-rock interactions can significantly enhance elemental mobility, Mg isotope variations may be recorded in metabasaltic rocks with large-scale veins (which represent original fluid pathways). Here we present Mg isotope data for different parts of a metabasaltic block from Tianshan (ultra)high pressure metamorphic belt, southwest of China to reveal the Mg isotope composition of fluids and their effects in the subduction channel. All the samples have been divided into three groups depending on their position in each “pillow” of the metamorphic block, i.e., those from the omphacite-rich interior, glaucophane-rich vein and transitional part. The omphacite-rich interior is mainly composed of omphacite and phengite, with minor garnet, while the glaucophane-rich vein mainly consists of garnet, glaucophane and epidote. The transitional part between the omphacite-rich interior and glaucophane-rich vein is dominated by epidote and carbonate, and aragonite has been widely replaced by dolomite. Together with our previous study, we think that the metabasaltic block has experienced two stages of rehydration overprinting after eclogitization: the first stage rehydration led to the common enrichment of LILEs in the whole block compared to its protolith E-MORB; the second stage rehydration resulted in the carbonate reprecipitation at the transitional part and the redistribution of garnet component.

The δ²⁶Mg values of the omphacite-rich interior are lower (−0.42 ‰ to −0.34 ‰) than those of MORBs (−0.25 ± 0.06 ‰), which probably indicates that subduction dehydration can lead to lighter Mg isotope composition of the residual eclogite and heavier Mg isotope composition of the released fluids. Furthermore, the lighter Mg isotope composition of the transitional part (δ²⁶Mg = −0.67 ‰ ∼ −0.63 ‰) and glaucophane-rich vein (δ²⁶Mg = −0.71 ‰ ∼ −0.54 ‰) may be caused by the reprecipitation of dolomite and garnet through Ca-Mg exchange as the result of interaction with externally derived carbonate rich fluids. The residual fluid after the reprecipitation of carbonate minerals is also expected to show a heavier Mg isotope composition. In addition, the recrystallization of dolomite in the subduction channel could not only hamper the influence of subducted carbonate-derived fluids on the heavier Mg isotope composition of arc magmas compared to MORBs, but also facilitate the transfer of carbonate with lighter Mg isotope compositions into the deep mantle.