Resolution of the effects of crustal assimilation, sediment subduction, and fluid transport in island arc magmas: Pb[sbnd]Sr[sbnd]Nd[sbnd]O isotope geochemistry of Grenada, Lesser Antilles

Abstract

New lead, strontium, neodymium, and oxygen isotope and chemical data are reported for forty-three samples from the picrite to low-CaO andesite differentiation series (the M-series) of Grenada, Lesser Antilles island arc. Revised isotopic data for the low-Mg, high-Ca C-series basalts show tight correlations with MgO that confirm published interpretations of C-series evolution by coupled assimilation-fractional crystallisation (AFC). δ18O determined by laser fluorination on phenocryst augite increases from mantle values (+5.6) to the highest values observed in Grenada (+6.2) over the small observed range in 143Nd/144Nd, consistent with AFC. The most magnesian C-series basalts can be generated by about 16% olivine fractionation from picrites similar in major elements to M-series picrites, but with greater large ion lithophile element (LILE) enrichment and more MORB-like isotopic ratios. The strong enrichment in LILE relative to rare earth elements (REE), and in REEs relative to Nb and Zr, implies fluid transport of these elements from subducted altered ocean crust. This is consistent with constant 87Sr/86Sr around 0.7045, but lower 143Nd/144Nd and higher Δ207Pband Th/U than MORB require a small (ca. 0.2%) additional subducted sediment contribution. Some LILE (Pb, Rb) are not strongly enriched and Ce/Pb ratios are close to those of MORB. This may be due to release of a substantial part of the slab Pb at shallower depth than the zone of magma generation. Grenada high-silica andesites (>58% SiO2) lie on extensions of C-series AFC correlations, reflecting assimilation with amphibole-dominated fractionation from C-series parents. AFC does not lead to strongly radiogenic Pb in the andesites despite an inferred contaminant With 206Pb/204Pb > 20. This is because Pb is much more incompatible than Nd due to apatite and hornblende fractionation, resulting in progressively slower 206Pb/204Pb increase as magmatic Pb contents rise.M-series picrites and basic andesites display evidence for three-component mixing on SrNd isotope, NdPb isotope, and Pb/Nd ratio— 143Nd/144Nd diagrams. One endmember has isotopic compositions similar to the uncontaminated C-series lavas, although it has less LILE enrichment from subduction fluid. Two groups of M-series picrites are recognized with lower and higher La/Y; these have respectively higher (ca. 0.51290) and lower (ca. 0.51280) 143Nd/144Nd, but similar ranges in 87Sr/86Srand206Pb/204Pb. The sample with the most extreme low 143Nd/144Nd (0.51256) has the lowest 206Pb/204Pb in the M-series: all aspects of the chemistry and isotope systematics of this sample are consistent with generation from a mantle source with a relatively high contribution (ca. 2%) from subducted local seafloor sediment (206Pb/204Pb ≈ 19.2). A progressively greater contribution from this sediment can successfully explain the neodymium isotopic shift from the C-series through the low-La/Y M-series picrites to the high-La/Y M-series picrites. The extent of fluid modification of the mantle source, as monitored by LILE enrichment relative to LREE, broadly decreases as the subducted sediment component increases.Basic andesites of the M-series also display low- and high-La/Y groups, and can be generated from the picrites by amphibole-dominated fractional crystallisation. The highest 206Pb/204Pb,87Sr/86Sr, and Pb/Nd in Grenada are found in low-La/Y basic andesites and suggest that fractionation of low-La/Y picrite was accompanied by some 6% high-level assimilation of crust with high Sr/Ndand206Pb/204Pb > 20. This hypothesis is strongly supported by the presence of 18O-enriched quartz xenocrysts of crustal origin in two of the three basic andesites. Smaller amounts (2–5%) of the same crustal component can satisfactorily model the variation of Pb/Nd,206Pb/204Pb,and87Sr/86Sr at constant 143Nd/144Nd in the picrites, an interpretation supported by correlations between isotope ratios and MgO in the high-La/Y picrites. AFC relationships in the C-series can be modelled using a similar crustal assimilant, but it must have much lower Pb/Nd ratios. The C-series assimilant cannot be M-series magma products, since M-series Pb/Nd ratios are too high, and M-series magmas themselves only achieve high 206Pb/204Pb through crustal assimilation.Lead isotope compositions of Grenada magmas, even picrites, are largely controlled by high-level crustal assimilation, mostly through AFC processes. Since these magmas include some of the most magnesian lavas found in arcs worldwide, we recommend caution in interpreting the lead isotope chemistry of arc magmas elsewhere to be the result of mantle and subduction processes.