Wed 1st May @ noon, Rm 312.222

Abstract:

We have compiled ample evidence showing that Lesser Antilles magmas are very wet (≥ 4.5 wt% of initial H₂O). Elevated H₂O content of Lesser Antilles magmas and its variation from island to island is reflected in crustal structure of the arc. We used an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions with additional information from experimental petrology, chemistry of lavas and melt inclusions to reconstruct crustal structure of the Lesser Antilles island arc. Lesser Antilles xenoliths show considerable island-to-island variation in xenolith petrology from plagioclase-free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole, indicative of changing magma differentiation depths. Xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km. We use xenolith mineral modes and compositions to calculate seismic velocities (v_P, v_S) and density at the estimated equilibration depths. We create a five-layer model of crustal structure for testing against receiver functions (RF) from island seismic stations along the arc. Lowermost layer (5) comprises peridotite with physical characteristics of mantle xenoliths from Grenada. Uppermost layer (1) consists of 5 km of volcaniclastics and sediments, whose physical properties are determined via a grid inversion routine. The three middle layers (2) to (4) comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island. By inversion we obtain a petrological best-fit for the RF on each island to establish the nature and thicknesses of layers (2) to (4).

Along the arc we see variations in the depth and strength of both Moho and mid-crustal discontinuity (MCD) on length-scales of tens of km. Moho depths vary from 25 to 37 km; MCD from 11 and 25 km. Both discontinuities are located at greater depths in the northern arc. The Moho is the dominant discontinuity beneath some islands (St. Kitts, Guadeloupe, Martinique, Grenada), whereas the MCD dominates beneath others (Saba, St. Eustatius). Along-arc variability in MCD depth and strength is consistent with variation in the inferred magmatic H₂O contents and differentiations depths that, in turn, influence xenolith lithologies. A striking feature is steep, along-arc gradients in v_P similar to those observed at other oceanic arcs. These gradients are reflected in abrupt changes in rates and processes of magma generation in the underlying crust and mantle. We find no evidence for large, interconnected bodies of partial melt beneath the Lesser Antilles. Instead, the crustal velocity structure is consistent with magma differentiation in vertically-extensive, crystal mush-dominated reservoirs. Along-arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge, itself driven by variation in slab-derived H₂O fluxes.

Short bio:

 Elena (Lena) Melekhova is experimental petrologist, with interest in evolution of magmas, specifically: the chemistry of melts, minerals and fluids; role of water in subduction zone environment; and formation of new continental crust. Lena’s main tool is high-pressure, high-temperature experiments, which she combines with field observations and microanalyses of natural rocks and experimental run products. She obtained her undergraduate and M.Sc degree in Geology from Irkustk State University, Russia, and her PhD in experimental petrology from ETH-Zurich, Switzerland. She is has been based at University of Bristol, UK,  since 2007. For the past 12 years Lena’s main focus was the Lesser Antilles volcanic arc and its magmatic evolution. She has also been involved in projects on primitive magmatism in Kamchatka and Vanuatu volcanic arcs, and recently in a project looking at heterogeneity of mantle wedge under Colima volcano Mexico. Currently she is part of a large, strategic, collaborative research project FAMOS (From Arc Magmas to Ore Systems).