Oxygen isotope constraints on the petrogenesis of volcanic arc magmas from Martinique, Lesser Antilles

Abstract

Volcanic rocks from the island of Martinique in the Lesser Antilles island arc are highly diverse isotopically. Values of δ 18O (‰ SMOW) in lavas erupted over the last 5 Ma from five stratigraphically discrete and geochemically distinct volcanic centers are: Morne Jacob= +7.5to+7.9‰;Pitons du Carbet= +8.0to+8.9‰;Diamant= +6.3to+7.4‰;Piton Mt. Conil, +7.2to+7.3‰, andMt. Pelée= +6.5to+7.4‰. 13 of the 27 samples have been corrected for variable amounts of secondary alteration and low-temperature hydration. The least 18O-rich andesites on Martinique have δ 18O values similar to those of the northern Lesser Antilles ( δ 18O = +6.0 to +7.3‰ ) and other intra-oceanic arcs like the Marianas ( δ 18O = +5.5 to +6.8‰ ). O-isotope ratios are compositionally dependent; correlations between δ 18O and radiogenic isotope ratios ( 87Sr/ 86Sr, 143Nd/ 144Nd, 206Pb/ 204Pb) and indices of fractionation, show that modification of isotopic compositions occurred during magmatic differentiation rather than at source. A two-stage model is proposed for the origin and differentiation of magmas on Martinique. Primary basaltic melts, which are broadly similar in composition to primitive island-arc magmas elsewhere (e.g. Marianas, Aleutians, South Sandwich Islands), are generated from the subduction-modified asthenospheric mantle wedge. These magmas ascend into the arc crust and differentiate. On Martinique, crystal fractionation is accompanied by contamination with isotopically distinct crust, including a component of terrigenous sediments. Thus, subduction-zone processes at the Lesser Antilles and other island arcs are similar, involving only a small (albeit significant isotopically) volumetric contribution from subducted sediment.