ISOTOPIC AND ELEMENTAL SYSTEMATICS OF SR AND ND IN 454-MA BIOGENIC APATITES - IMPLICATIONS FOR PALEOSEAWATER STUDIES

Pristine conodonts (CAI less than or equal to 1.5), inarticulate brach iopods, and conulariids, all from a single hand sample of Ordovician l imestone, define a co-varying trend of Sr-87/Sr-86 and Sr concentratio n. Most of the apatitic fossils have Sr-87/Sr-86 ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a gener al susceptibility of biogenic apatites to post-depositional Sr exchang e. The largest isotopic shifts were measured in inarticulate brachiopo ds and conulariids, and deduced for conodont basal body material. Cono dont crown material exhibits the smallest effects. The Sr exchange eff ects are strongly dependent on differences in apatite composition, as revealed by contrasting Ca/P ratios. Although conodont crown material (with low Ca/P ratios) is less prone to isotopic disturbance relative to other types of coexisting apatite fossils, high resolution X-ray ma pping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. In con trast to Sr, all coexisting fossil apatites have identical initial Nd- 143/Nd-144 ratios over a wide range of Nd concentration. No relationsh ip between Sr-87/Sr-86 and Nd-143/Nd-144 was observed despite a pronou nced antithetic pattern of Sr and Nd distribution both between the fos sil types, and within individual conodonts containing preserved basal. body material. In agreement with earlier studies, it is concluded tha t the bulk of the Nd in fossil apatites is from seawater that original ly overlay the depositional site.