An investigation of closure temperature of the biotite Rb-Sr system: The importance of cation exchange

Factors controlling closure in the biotite Rb-Sr system were investigated i n a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed sy stem. Calcite-dolomite thermometry temperatures of similar to 450 degreesC to 500 degreesC and feldspar thermometry temperatures of similar to 300 deg reesC to 400 degreesC provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol.% phlogopite and has high Rb (similar to 900 ppm) compared to Sr (similar to0.3 ppm), giving precise Rb-Sr ages. Carefully separated and sized phlogopite shows a range of Rb-Sr ages that do not simply decrease with grain size as predicted by current m odels of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma w ith a decrease in mean grain diameter from 1.16 mm to 0.74 mm, but grains w ith mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contra sts with Ar-Ar data for single phlogopites, which do show a decrease in age with decreasing grain size. The Rb-Sr age pattern is due to Rb-loss during cooling, which is most pronounced in the finest fraction. The phlogopites are restricted to a a-cm-thick layer in calcite marble; Sr-87/Sr-86 of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopite s over this distance and that the phlogopite was not equilibrating with an "infinite reservoir." Ion microprobe traverses across grains of different m inerals reveal systematic core-rim variations in major and trace element co ncentrations. In particular, Sr decreases from calcite core to rim, but inc reases from core to rim in K-feldspar, whereas Rb decreases from core to ri m in phlogopite but also increases from core to rim in K-feldspar. These gr adients are interpreted as indicating the direction of transport of element s during cooling as a result of cation exchange reactions; calcite and phlo gopite were sources for Sr and Rb, respectively, whereas K-feldspar acted a s a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally im portant in controlling the apparent ages recorded by the mica grains. In co ntrast, closure temperature calculations for geochronological systems based on classic Dodsontype models assume parent and daughter element concentrat ions are homogeneous across grains and do not change with time, only isotop ic exchange is modeled. Closure in mica Rb-Sr systems will depend both on t he factors that control isotopic exchange (grain size, mode, Sr-87 diffusio n coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their p artition coefficients). Copyright (C) 2001 Elsevier Science Ltd.