Plutonic Xenoliths Reveal the Influence of Cryptic Melt-Mush Reaction Processes in the Plumbing System Beneath St Vincent, Lesser Antilles Arc

Abstract In subduction zones, it is widely established that magmas are stored as crystal dominated mush within sub-volcanic plumbing systems. In these mush-dominated systems, it is likely that melt-mush reactions between migrating melts and the pre-existing mush influence the chemical evolution of magmas. However, melt-mush reactions and their effect on the chemical evolution of arc magmas can be elusive and cannot be constrained when studying erupted lavas in isolation. In this study, we focus on the island of St. Vincent in the Lesser Antilles volcanic arc because (1) the composition of erupted lavas has been interpreted to reflect simple fractional crystallisation (FC), with minimal influence of other magmatic processes, and (2) an abundance of plutonic xenoliths (erupted fragments of crystal mush) can be found within the eruptive products. Thus, we are able to compare interpretations gleaned from the chemistry of erupted lavas with new observations of the crystal mush in the same magmatic system. To this end, textural analyses were undertaken on seventeen representative plutonic xenoliths from St Vincent, and four of these samples (two olivine [OL] gabbros, two hornblende-olivine gabbros) were studied in detail via element mapping, mineral trace element analyses and geochemical modelling. The chemical, textural and mineralogical characteristics of the olivine gabbros were best explained via FC in the mid-upper crust (~6–18 km depth). However, the hornblende-olivine gabbros (two of seventeen samples studied) contained clear textural evidence for melt-mush reaction in the mid-upper crust. The trace element compositions of minerals such as clinopyroxene in these two samples were best reproduced via assimilation-FC (AFC) modelling, simulating melt-mush reactions, supporting the textural evidence. Our plutonic xenoliths reveal that in addition to FC, cryptic (i.e. not directly recorded in lavas) melt-mush reaction processes also contribute to magma chemical evolution, particularly influencing trace elements, within the sub-volcanic plumbing system. Textural evidence for melt-mush reaction is increasingly reported in plutonic xenoliths from other active arcs and exhumed arc crustal sections, suggesting that this process is ubiquitous in mush-dominated arc plumbing systems. Melt-mush reaction therefore represents an important process contributing to arc magma and arc crust trace element chemical diversity.