Evidence for adiabatic decompression melting in the Southern Mariana arc from high-Mg lavas and melt inclusions

Unusually magnesian (Mg# -76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al2O3 from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17-17.86 wt%, 0.35-0.5% TiO2, 42-50% SiO2 and 1.66-3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182-334 ppm) and H2O contents (0.11-0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log Delta FMQ of + 1.53-1.66) which lie at the low end of the range estimates for arc basalts and picrites (Delta FMQ = + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367 degrees C at 1-1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.