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Journal of Petrology Advance Access published online on August 31, 2005
Journal of Petrology, doi:10.1093/petrology/egi063
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received January 30, 2004
Accepted June 16, 2005

Article

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

ANDREY A. GURENKO 1*, ROBERT B. TRUMBULL 2, RAINER THOMAS 2, and JAN M. LINDSAY 3

1 GEOFORSCHUNGSZENTRUM POTSDAM, SECTION 4.2, TELEGRAFENBERG, 14473 POTSDAM, GERMANY; MAX-PLANCK-INSTITUT FÜR CHEMIE, ABTEILUNG GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY; Present address: Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany
2 GEOFORSCHUNGSZENTRUM POTSDAM, SECTION 4.2, TELEGRAFENBERG, 14473 POTSDAM, GERMANY
3 SEISMIC RESEARCH UNIT, UNIVERSITY OF THE WEST INDIES, ST. AUGUSTINE, TRINIDAD

* To whom correspondence should be addressed.
ANDREY A. GURENKO, E-mail: agurenko{at}mpch-mainz.mpg.de


  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, 200 MPa 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 2900 ppm Cl), but contents range down to 1 wt % H2O and 2000 ppm Cl as a result of shallow-level degassing. Sulfur contents are low throughout, with <350 ppm 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 (20-60 ppm Li with {delta}7Li = +4 to +15 ± 2{per thousand}; 60-100 ppm B with {delta}11B = +6 to +13 ± 2{per thousand}) 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 {delta}7Li and {delta}11B 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.

Keywords: subduction; volcanic arcs; igneous processes; melt inclusions; SIMS; trace elements; lithium and boron isotopes; diffusion.
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