Boron Isotopic Compositions of High-Pressure Hydrous Phases from the Slab-Mantle Wedge Interface

Abstract:

The breakdown or formation of high-pressure hydrous minerals are known to play an important role in the transfer of volatiles and fluid mobile elements from the subducted lithosphere to the mantle wedge in subduction zones, as well as in arc magma genesis. Hydrous mineral formation (e.g. chlorite and serpentine) at P, T conditions corresponding to the slab-mantle wedge interface is observed in experimental studies but has not yet been verified in natural high pressure orogenic peridotites, as there are few definitive tools to distinguish prograde hydrous mineral formation from those formed during exhumation, aside from petrographic textural studies. Here we investigate the boron isotopic composition of hydrous minerals in mafic-ultramafic lithologies both found in an exhumed slab-wedge interface, the Higashi-Akaishi peridotite body of the Sanbagawa metamorphic belt in southwest Japan. The quartz eclogite records peak metamorphic conditions of 2.3-2.4 GPa, 675-740 ºC (Miyamoto et al., 2007), whereas the garnet clinopyroxenite records higher peak conditions of 2.9-3.8 GPa and 700-810ºC, prior to rapid exhumation (Enami et al., 2004). Secondary ion mass spectrometry of paragonite inclusions in garnet porphyroblasts identified in the quartz eclogite yielded δ¹¹B values of ~+5‰ with B concentrations of ~35 ppm. Chlorite is found in textural equilibrium with the anhydrous phases in the garnet clinopyroxenite and records very light B isotopic values (-21.5 ± 4.1‰) and relatively low B concentrations (~3 ppm). Our petrographic and boron isotopic observations in chlorite are consistent with its formation at relatively higher pressures than paragonite. Differences in protolith compositions are likely responsible for differences in B concentrations.

Enami et al., J. Metamorphic Geol., 22, 1-15 (2004).

Miyamoto et al., J. of Min. and Petro. Sci., 102, 352-367 (2007)