SAMARIUM NEODYMIUM ELEMENTAL AND ISOTOPIC SYSTEMATICS IN SEDIMENTARY-ROCKS

Trace element (Sm, Nd, Th, Sc) and Nd-isotopic data for sedimentary rocks are examined in order to evaluate (1) possible secular trends in the composition of the sedimentary mass and upper continental crust and (2) the role of tectonic setting in controlling the chemical and Nd-isotopic composition of the sedimentary mass. When plotted against either sedimentation age or average provenance age (determined mainly from Nd-model ages), Sm/Nd in sedimentary rocks decreases abruptly from about 0.21 to 0.19 at the Archean-Proterozoic transition, consistent with a change towards a more felsic upper crustal composition. The change is only somewhat less pronounced for turbidite sediments than for those of continental affinity. A trend toward increasing Sm/Nd, approaching or exceeding Archean values, is observed for Phanerozoic turbidites. This latter change is interpreted as indicating undifferentiated young crust as a component in the sediment, which has been lost from the older post-Archean record due to sedimentary and igneous intracrustal recycling processes. When plotted against sedimentation age, Th/Sc ratios exhibit a trend directly comparable to that of Sm/Nd. In this case, Th/Sc increases abruptly at the Archean/Proterozoic boundary from about 0.5 to 1.0 and then decreases to about 0.7 during the Phanerozoic. Nd-isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher epsilon(Nd) reflecting younger provenance, and higher and more scattered Sm/Nd reflecting less homogenized and differentiated provenance. It is likely that these distinctions can be traced back for about 1-2 Ga but, with the current data base, are not readily resolved in older sedimentary rocks. The degree to which the preserved sedimentary mass is open to new additions from the mantle, either through replacement during sedimentary recycling or sediment mass growth, is highly variable but averages about 15-25% during the Phanerozoic.