A melt inclusion record of volatiles, trace elements and Li-B isotope variations in a single magma system from the Plat Pays Volcanic Complex, Dominica, lesser antilles

Abstract

Glass inclusions in plagioclase and orthopyroxene from dacitic pumice of the Cabrits Dome, Plat Pays Volcanic Complex in southern Dominica reveal a complexity of element behavior and Li-B isotope variations in a single volcanic center that would go unnoticed in a whole-rock study. Inclusions and matrix glasses are high-silica rhyolite with compositions consistent with about 50% fractional crystallization of the observed phenocrysts. Estimated crystallization conditions are 760-880°C, 200MPa and oxygen fugacity of FMQ + 1 to +2 log units (where FMQ is the fayalite-magnetite-quartz buffer). Many inclusion glasses are volatile-rich (up to 6 wt % H2O and 2900ppm Cl), but contents range down to 1 wt % H2O and 2000ppm Cl as a result of shallow-level degassing. Sulfur contents are low throughout, with <350ppm S. The trace element composition of inclusion glasses shows enrichment in light rare earth elements (LREE; (La/Sm)n = 2·5-6·6) and elevated Ba, Th and K contents compared with whole rocks and similar or lower Nb and heavy REE (HREE; (Gd/Yb)n = 0·5-1·0). Lithium and boron concentrations and isotope ratios in melt inclusions are highly variable (20-60ppm Li with δ7Li = +4 to +15 ± 2‰ 60-100ppm B with δ11B = +6 to +13 ± 2‰) and imply trapping of isotopically heterogeneous, hybrid melts. Multiple sources and processes are required to explain these features. The mid-ocean ridge basalt (MORB)-like HREE, Nb and Y signature reflects the parental magma(s) derived from the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlations in inclusion glasses indicate coupled enrichment in strongly fluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements, which can be explained by fractional crystallization of plagioclase, orthopyroxene and Fe-Ti oxides. The δ7Li and δ11B values are at the high end of known ranges for other island arc magmas. We attribute the high values to a 11B and 7Li-enriched slab component derived from sea-floor-altered oceanic crust and possibly further enriched in heavy isotopes by dehydration fractionation. The heterogeneity of isotope ratios in the evolved, trapped melts is attributed to shallow-level assimilation of older volcanic rocks of the Plat Pays Volcanic Complex. © The Author 2005. Published by Oxford University Press. All rights reserved.