Magma Chemical Evolution in the Lesser Antilles Arc Crust: Insights from Plutonic Xenoliths

Magmas generated beneath volcanic arcs at subduction zones must traverse the crust of the overriding plate, via “magma plumbing systems” prior to eruption. Magma plumbing systems are predominantly composed of crystal mush, where low volumes of melt are distributed throughout higher volumes of crystals. To understand processes controlling the chemical compositions of arc lavas, it is essential to constrain how magmas are chemically modified within mushy plumbing systems during storage and transit through the upper plate crust. Arc lavas often carry fragments of crystal mush, known as plutonic xenoliths, to the surface, which can be used to study the influence of crustal processes on arc magma chemical compositions. This thesis investigates plutonic xenoliths from the islands of Martinique and St Vincent in the active intra-oceanic Lesser Antilles arc. These samples are used to determine whether the majority of the trace element and isotopic variation in Lesser Antilles arc lavas can be produced by crustal processes. Strontium isotopic compositions of plagioclase, in plutonic xenoliths derived from different crustal depths, are used to demonstrate that isotopic variation and highly radiogenic isotopic compositions observed in Martinique lavas are acquired via assimilation of upper crustal sediments. Textural features and mineral trace element compositions in St Vincent plutonic xenoliths record polybaric fractional crystallization and melt-mush reaction processes. These two processes control magma chemical evolution in mushes beneath the island and consequently influence the trace element compositions of St Vincent lavas. These findings suggest that majority of trace element and radiogenic isotopic variation in Lesser Antilles arc lavas is generated within mushy magma plumbing systems in the arc crust. An important wider implication is that contributions from crustal processes must be well constrained before using trace element and isotopic compositions of arc lavas to assess subduction recycling processes and mantle source heterogeneity.