EPSL ELSEVIER Earth and Planetary Science Letters 142 (1996) 425-437 Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies C. Holmden a, * , R.A. Creaser a, K. Muehlenbachs a, S.M. Bergstrom b, S.A. Leslie ’ a Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alta. T6G 2E3, Canada b Department of Geological Sciences, The Ohio State University, 155 S. Oval Mall, Columbus, OH 43210.139, USA ’ Depurtment of Earth Science, University of Arkansus at Little Rock, Little Rock, AR 72204. USA Received 27 October 1995; revised 7 May 1996; accepted 3 June 1996 Abstract Pristine conodonts (CA1 4 IS), inarticulate brachiopods, and conulariids, all from a single hand sample of Ordovician limestone, define a co-varying trend of *‘Sr/ “Sr and Sr concentration. Most of the apatitic fossils have *‘Sr/ *6Sr ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a general susceptibility of biogenic apatites to post-depositional Sr exchange. ‘Ihe largest isotopic shifts were measured in inarticulate brachiopods and conulariids, and deduced for conodont basal body material. Conodont crown material exhibits the smallest effects. The Sr exchange effects 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 mapping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. In contrast to Sr, all coexisting fossil apatites have identical initial 143Nd/ l”Nd ratios over a wide range of Nd concentration. No relationship between *‘Sr/ 86Sr and ‘43Nd/ l”Nd was observed despite a pronounced antithetic pattern of Sr and Nd distribution both between the fossil types, and within individual conodonts containing preserved basal body material. In agreement with earlier studies, it is concluded that the bulk of the Nd in fossil apatites is from seawater that originally overlay the depositional site. Keywords: Sr-87/Q-86; Nd- 144/Nd- 143; conodonts; paleo-oceanography; bentonite; diagenesis; Ordovician; phosphates 1. Introduction Conodonts are tooth-like microfossils thought to have comprised the feeding apparatus of an extinct eel-like organism which ranged stratigraphically from * Corresponding author’s present address: Division of Geologi- cal and Planetary Sciences, Mail Code 170-25, California Institute of Technology, Pasadena, CA 91125, USA. E-mail: holmden@arms.gps.caltech.edu the Late Cambrian to the Late Triassic. Widely studied for biostratigraphic correlation of sedimen- tary rocks, conodonts and other biogenic phosphates are increasingly being used to elucidate the chem- istry of past seawater. Due to their high rare earth element (REE) and Sr contents, biogenic apatites have become the principal substrate for inferring compositional changes in the ‘43Nd/ ‘44Nd of an- cient seawater [l-S], and provide an alternate sub- strate to biogenic calcite for recording variations in past seawater 87Sr/ 86Sr [8- 121. 0012-821X/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SOOl2-821X(96)001 19-7