Mr Joshua Brown
PhD project:The volcanic roots of the Lesser Antilles island arc – insights from mineral isotope stratigraphy
The Lesser Antilles island arc, formed by the subduction of the American plate below the Caribbean plate, erupts magmas with a wide range of geochemical compositions. Some magmas from the central-southern islands of the arc (e.g. Martinique, St Lucia) display extreme enrichment in radiogenic isotope ratios (e.g. high 87Sr/86Sr), with values nearing those of crustal sediments. Many magmas from other islands along the arc also display a geochemical signature consistent with addition of crustal/sedimentary material to the sub-arc mantle from which these magmas are initially derived.
My PhD aims to answer the debate as to whether these “crustal” signatures are derived from sediments entering the subduction zone, or from crustal material assimilated by ascending magma in the roots of the arc volcanoes. To do this, I am studying cumulate xenoliths, derived from different depths within the Antilles crust, to investigate intra-crustal processes. Specifically, I will focus on Sr isotope compositions in cumulate plagioclase, using the principle that if the deepest, earliest formed plagioclase show enriched/”crustal” Sr isotope ratios, then the dominant contributor to the “crustal” signature is the subduction component. However, a progressive increase in Sr isotope ratios from deep to shallow samples would provide strong evidence for the role of crustal assimilation. This has implications for understanding the origin of the volatiles in Lesser Antilles magmas that produce explosive volcanism and associated hazards on the islands.
My initial studies will focus on Martinique and St Lucia, to determine the origin of the unusual geochemical characteristics of these magmas. Cumulates from St Vincent will also be studied, to represent an island with a contrasting, more mantle like magma composition.
The project is funded by the NERC IAPETUS DTP and is being run in collaboration with the VOILA project (http://www.voila.ac.uk/).
- Volcanology, Isotope geochemistry, Igneous petrology