Melt Production Beneath Mt. Shasta from Boron Data in Primitive Melt Inclusions
Résumé
Most arc magmas are thought to be generated by partial melting of the mantle
wedge induced by inÞltration of slab-derived ßuids. However, partial melting
of subducting oceanic crust has also been proposed to contribute to the melt
generation process, especially when young and hot lithosphere is being subducted.
The isotopic composition of boron measured in situ in olivine-hosted
primitive melt inclusions in a basaltic andesite from Mt. Shasta, California, is
characterized by large negative values that are also highly variable (d11B 5
Ð21.3 to Ð0.9 per mil). The boron concentrations, from 0.7 to 1.6 parts per
million, are lower than in most other arc lavas. The relation between concentration
and isotopic composition of boron observed here supports a hypothesis
that materials left after dehydration of the subducting slab may have contributed
to the generation of basaltic andesite lavas at Mt. Shasta.