Sediment subduction and magma genesis in the Lesser Antilles: Isotopic and trace element constraints

Abstract

We present analyses of Pb, Sr, and Nd isotope ratios and of K, Rb, Cs, Sr, Ba, U, Th, Pb, and rare earth elements (REE) of a suite of volcanic rocks from the Lesser Antilles island arc, including representatives from all the major islands. An unusually wide range of compositions is present. 87 Sr/86 Sr and 143 Nd/144 Nd overlap extensively with those of oceanic islands and indicate slightly more “enriched” source signatures than most island arcs. Pb isotope ratios range to extremely radiogenic values (206 pb/204 pb = 19.75, 207 pb/204 pb = 15.83) and extend significantly beyond the oceanic basalt field. REE patterns range from slightly light‐rare‐earth depleted to strongly light‐rare‐earth enriched; several exhibit negative Ce anomalies. The Pb isotopic compositions are more radiogenic than those of most marine sediments but are similar to isotope compositions of sediments in front of the arc. This, and other evidence, such as high Cs/Rb ratios and a southward increase in Pb isotope ratios in both arc volcanics and sediments in front of the arc, is regarded as strong evidence for a sediment component in arc magmas. Metasedimentary xenoliths in arc magmas and other evidence indicate that sediment can be a high‐level contaminant of ascending magmas. However, the ubiquitous nature of the “sedimentary” isotopic and trace element signature in arc magmas, including primitive and peridotite‐xenolith‐bearing ones, indicates subducted sediment must also be a component of arc magma sources. Mixing calculations show the isotope geochemistry of most arc magmas is readily explained if their sources are mixtures of depleted mantle and 3% or less subducted sediment. On average, subduction of 90 m of sediment would be required to explain the isotopic compositions of arc magmas. Simple binary mixing models also explain the REE patterns of low‐K basalts but fail to explain all the excess enrichment of the alkalis and alkaline earths relative to the REE. The excess abundance of these elements is probably due to their enrichment (relative to the REE) in an aqueous fluid generated by dehydration of the subducting sediment and basalt. This fluid mixes with (metasomatizes) overlying depleted mantle, which melts to produce the arc magmas.