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

Abstract

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.