Impact of crustal assimilation on the Lesser Antilles
arc lava geochemistry

Abstract

Characterization of the mantle source of arc magmas using the composition of erupted lavas
is only possible after the assessment of the effects of crustal assimilation. While the impact
of crustal assimilation on continental arc lavas is commonly investigated, it is often ignored
or debated in oceanic arcs. Nowhere has the debate been more acute than in the Lesser Antilles
arc which is characterised by extreme ranges in geochemistry from typical oceanic arc to
continental crust-like compositions. Given the oceanic provenance of the arc, these
geochemical features can be inherited either from subducted sediments, or from crustal
contamination by continental sediments intercalated in the arc crust. In this thesis, the problem
is reassessed using a whole rock-to sub-grain scale trace element and isotopic (Sr-Nd-Hf-Pb-
O isotopes) investigation of volcanic rocks from St Lucia Island, thought to encompass most
of the arc’s isotopic heterogeneities. In addition, an Os isotopic investigation of some of the
most mafic and well constrained lavas from along the arc was performed.
New isotopic composition of St Lucia lavas confirm that the island covers most of the arc’s
compositional variations. Intra-crystal 87Sr/86Sr heterogeneities as well as the co-variation of
lava radiogenic isotopes and trace elements with SiO2 and mineral d18O suggest that crustal
assimilation affected lavas as mafic as basaltic andesites and is responsible for the isotopic
and trace element heterogeneities observed in the Island. The assimilant is thought to be a mix
of detrital and organic sediments, likely to have been part of the sedimentary basin intruded
by the arc. Assimilation of sediment is thought to occur in the middle crust of the arc and to
be tightly linked with the production of silicic magmas.
Sr, Nd, Hf and Pb isotopic compositions of the lavas that avoided significant sediment
assimilation suggest that no more than 2% of slab-derived sediment was added to the source
of St Lucia magmas. However, an along arc 187Os/188Os investigation suggests that even the
most mafic lavas, having escaped sediment assimilation, were modified on their way to the
surface by assimilation of the igneous arc roots, possibly a plagioclase-rich cumulate. Care
must therefore be applied before characterising the source using mafic lava compositions
since this process is thought to modify the Os and Sr isotopes, as well as La/Sm and Sr/Th
ratios. This suggests that, in the Lesser Antilles arc, very few lavas preserved their original
mantle source characteristics.