Impact of crustal assimilation on the Lesser Antilles arc lava geochemistry
thesisposted on 28.03.2022, 17:41 by Rachel Christine Bezard
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 18O 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 sourceof 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.