Upper mantle origin of plagioclase megacrysts from plagioclase-ultraphyric mid-oceanic ridge basalt

摘要

Rigorous petrogenetic interpretation of data from melt inclusions (MI) and their host phenocrysts commonly requires knowledge of the crystallization pressure. Pressures of trapping of MI are commonly estimated based on the H2O and CO2 concentrations of the inclusions, but their original concentrations may be modified as a result of various post-entrapment processes. Most studies of mafic magmas employ olivine- or pyroxene-hosted MI, largely based on the assumption that plagioclase phenocrysts do not retain volatiles and/or are the product of late-stage crystallization. We have investigated the effects of post-entrapment processes on plagioclase-hosted MI in one plagioclase-ultraphyric basalt from the Blanco transform fault (offshore Oregon, USA). Our results demonstrate that plagioclase-hosted MI do not lose CO2 even after being heated to the entrapment temperature for 4 d. The reconstructed CO2 (glass + vapor) concentration in the MI indicates that the plagioclase megacrysts crystallized at depths equal to or below the local Moho, at minimum pressures of 230-910 MPa. Our results confirm that the plagioclase crystallized in the mantle, and that reliable trapping pressures can be obtained from MI in plagioclase following rehomogenization experiments.