The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

Abstract

We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Vol- canoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO ( 9 wt%) from Mt. Shasta has olivine d 18 O( d 18 OOl) values of 5.9-6.1%, which are about 1% higher than those observed in olivine from normal mantle-derived magmas. In con- trast, HAOT lavas from Medicine Lake have d 18 OOl values ranging from 4.7 to 5.5%, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate d 18 OOl values. The oxygen isotope composi- tion of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable d 18 OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4% range in d 18 OOl reflects a heter- ogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5% variation. The high-d 18 O values observed in magnesian lavas, and prin- cipally in the HAOT, are difficult to explain by a single- stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18 O. It is here explained by flow of older, pre- enriched portions of the mantle through the slab window beneath the South Cascades.