NEODYMIUM AND STRONTIUM ISOTOPIC CONSTRAINTS ON SOIL SOURCES IN BARBADOS, WEST-INDIES

Neodymium and strontium isotopic compositions and Sm/Nd ratios are use d to constrain the sources of silicate-rich soils developed on uplifte d Pleistocene coral-reef limestones on Barbados, West Indies. The geog raphic and geologic setting of Barbados facilitates the application of these tracers to the evaluation of the following soil sources: (1) Pl eistocene reef Limestone regolith, (2) Tertiary carbonate rocks, sands tones, and mudstones that are exposed in northeastern Barbados, (3) vo lcanic ash erupted from the Lesser Antilles arc, (4) Saharan dust tran sported by trade winds, and (5) fertilizer. The soils have epsilon(Nd) values that range from -6.6 to -1.9, Sr-87/Sr-86 values of 0.70890 to 0.71067, and Sm/Nd ratios of 0.223-0.260. The Pleistocene limestone c omponent is the most significant source of Sr in the soils and a negli gible source of Nd. Comparison of Sm and Nd concentrations and neodymi um isotopic compositions of soil samples that are weathered to varying extents indicates that Sm and Nd are relatively unfractionated and re tained in the soils during weathering. epsilon(Nd) and Sm/Nd Variation s in the soils, therefore, primarily reflect the compositions and prop ortions of the silicate sources. Mass balance calculations based on Sm -Nd systematics require that the silicate soil components contain betw een 30-85% volcanic ash, with the remaining silicate fraction comprise d of old, continentally-derived sediment. In contrast to Sm and Nd, Sr is mobilized and removed from the soils during weathering. Strontium from volcanic and carbonate sources is preferentially removed relative to continental silicate sources. The strontium isotopic compositions of the soils, therefore, reflect the combined effects of the degree of weathering and the compositions and proportions of the soil sources. Mass balance calculations indicate that at least 35-60% of the initial Sr in the soils has been removed by weathering. These results illustr ate (1) the utility of radiogenic isotopes in identifying and quantify ing soil sources and weathering processes, (2) the compositional influ ence of numerous sources on soils, even those developed in a relativel y isolated area such as Barbados, and (3) the domination of Barbados s oil Sm-Nd systematics by nonregolith eolian components.