DETERMINATION OF SR AND BA PARTITION COEFFICIENTS BETWEEN APATITE AND WATER FROM 5°C TO 60°C: A POTENTIAL NEW THERMOMETER FOR AQUATIC PALEOENVIRONMENTS

Abstract

Apparent partition coefficients of Sr and Ba between calcium phosphate and water were measured experimentally for temperature ranging from 5°C to 60°C. Calcium phosphates were precipitated from an aqueous mixture of Na2HPO4 . 2H2O (10-2 M) and CaCl2 . 2H2O (10-2 M). Spiked solutions of Sr or Ba were introduced into the CaCl2 . 2H2O solution at Sr/Ca and Ba/Ca ratios of 0.1. The experiment consisted in sampling the liquid and solid phases after 1, 6, 48, and 96 h of interaction. The amorphous calcium phosphate (ACP) precipitated early in the experiment was progressively replaced by hydroxylapatite (HAP), except at 5°C where brushite (di-calcium phosphate di-hydrate or DCPD) was formed. We observed that the crystallinity of the solid phase increased with time for a given temperature and increased with temperature for a given time of reaction. With the exception of the experiment at 5°C, yield (R%) and apparent partition coefficients (Ka-wSr/Ca and Ka-wBa/Ca) both decreased with increasing reaction time. After 96 h, R%, Ka-wSr/Ca and Ka-wBa/Ca were observed to be constant, suggesting that the solid phases were at steady-state with respect to the aqueous solutions. The thermodependence of Sr and Ba partitioning between apatite and water at low temperature could therefore be calculated: Log(Ka-wSr/Ca)=0.42+/-0.04(103T-1)-1.87+/-0.12(r2=0.94) Log(Ka-wBa/Ca)=1.96+/-0.06(103T-1)-7.19+/-0.20(r2=0.99) We also performed competition experiments between Sr and Ba. The thermodependence of the Sr/Ba partitioning between apatite and water was calculated after 96 h of reaction: Log(Ka-wSr/Ba)=-0.75+/-0.04(103T-1)+2.39+/-0.14(r2=0.97) This relationship reveals a discrimination of Ba in favor of Sr during their incorporation into HAP. Temperature trends deduced from the Ba/Ca of fish teeth recovered from the K/T boundary mimic those estimated from δ18O(PO4) measurements carried out on the same sample. Unless Sr, Ba and Ca contents of biogenic apatites are modified during diagenesis, their elemental ratios could be used as new and attractive thermometers of aquatic paleoenvironments.