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 in a detailed study of an amphibolite-facies metacarbonate from the central Swiss Alps. Oxygen isotope data suggest that the rock cooled as a closed system. Calcite-dolomite thermometry temperatures of ;450°C to 500°C and feldspar thermometry temperatures of ;300°C to 400°C provide evidence of extensive Ca-Mg and Na-K exchange during cooling. Biotite in the sample is 90 mol.% phlogopite and has high Rb (;900 ppm) compared to Sr (;0.3 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 models of closure temperature. Rb-Sr ages decrease from 18.1 Ma to 16.6 Ma with a decrease in mean grain diameter from 1.16 mm to 0.74 mm, but grains with mean diameter of 0.54 mm show an increase again to 17.6 Ma. This contrasts 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 2-cm-thick layer in calcite marble; Sr/Sr of the calcite decreases away from the phlogopite band over 4 cm, indicating that the calcite was moving towards Sr-isotope equilibration with the phlogopites over this distance and that the phlogopite was not equilibrating with an “infinite reservoir.” Ion microprobe traverses across grains of different minerals reveal systematic core-rim variations in major and trace element concentrations. In particular, Sr decreases from calcite core to rim, but increases from core to rim in K-feldspar, whereas Rb decreases from core to rim in phlogopite but also increases from core to rim in K-feldspar. These gradients are interpreted as indicating the direction of transport of elements during cooling as a result of cation exchange reactions; calcite and phlogopite were sources for Sr and Rb, respectively, whereas K-feldspar acted as a sink for both elements. This chemical equilibration was taking place at the same time as isotopic equilibration during cooling, and was equally important in controlling the apparent ages recorded by the mica grains. In contrast, closure temperature calculations for geochronological systems based on classic Dodsontype models assume parent and daughter element concentrations are homogeneous across grains and do not change with time, only isotopic exchange is modeled. Closure in mica Rb-Sr systems will depend both on the factors that control isotopic exchange (grain size, mode, Sr diffusion coefficients) and those that control chemical exchange (grain size, mode, Rb and Sr diffusion coefficients, Rb and Sr contents of phases and their partition coefficients). Copyright © 2001 Elsevier Science Ltd