SECULAR BORON ISOTOPE VARIATIONS IN THE CONTINENTAL-CRUST - AN ION MICROPROBE STUDY

Tourmalines of various chemical compositions, hosted in magmatic rocks with ages ranging from 3.8 Ga to the present were analysed for delta-B-11 B by ion microprobe. A large range of delta-B-11 values, from -2 parts per thousand to -30 +/- 1.5 parts per thousand, was observed. The delta-B-11 values are correlated with the chemical composition of the tourmalines, the Li-rich tourmalines being enriched in B-11 compared to Fe and/or Mg-rich tourmalines. This may reflect mixing in the continental crust between a marine reservoir, characterized by high Li contents and delta-B-11 values and a continental reservoir characterized by low Li contents and delta-B-11 values. No relationship between the age and the delta-B-11 values is observed for the Fe- or for the Mg-rich tourmalines. In contrast the delta-B-11 values of the Li-rich tourmalines show a tendency to regularly decrease with increasing age. This pattern is consistent with a simple model of constant boron isotopic fractionation between seawater and the continental crust during the progressive extraction of the continents. A variation in the mean delta-B-11 value of the continental crust, from almost-equal-to - 20 parts per thousand 4 Ga ago to almost-equal-to -7 parts per thousand at present, is predicted by the model. The largest variation occurred during the first 2.5 Ga.