Oxygen‐isotope and trace element constraints on the origins of silica‐rich melts in the subarc mantle

Abstract

Peridotitic xenoliths in basaltic andesites from Batan island in the Luzon arc contain silica‐rich (broadly dacitic) hydrous melt inclusions that were likely trapped when these rocks were within the upper mantle wedge underlying the arc. These melt inclusions have been previously interpreted to be slab‐derived melts. We tested this hypothesis by analyzing the oxygen isotope compositions of these inclusions with an ion microprobe. The melt inclusions from Batan xenoliths have δ18OVSMOW values of 6.45 ± 0.51‰. These values are consistent with the melts having been in oxygen isotope exchange equilibrium with average mantle peridotite at temperatures of ≥875°C. We suggest the δ18O values of Batan inclusions, as well as their major and trace element compositions, can be explained if they are low‐degree melts (or differentiation products of such melts) of peridotites in the mantle wedge that had previously undergone extensive melt extraction followed by metasomatism by small amounts (several percent or less) of slab‐derived components. A model based on the trace element contents of Batan inclusions suggests that this metasomatic agent was an aqueous fluid extracted from subducted basalts and had many characteristics similar to slab‐derived components of the sources of arc‐related basalts at Batan and elsewhere. Batan inclusions bear similarities to “adakites,” a class of arc‐related lava widely considered to be slab‐derived melts. Our results suggest the alternative interpretation that at least some adakite‐like liquids might be generated from low‐degree melting of metasomatized peridotites.