A NEW SILICATE DISSOLUTION PROCEDURE FOR ISOTOPE STUDIES ON GARNET AND OTHER ROCK-FORMING MINERALS

A new silicate digestion technique, based on sequential acid treatment (HBr --> HF-HNO3 --> HNO3), is presented. The main features are: (i) only ''off the shelf'' equipment is needed, (ii) it avoids HClO4 and H 2SO4, (iii) precipitates which may coat residual grains and inhibit to tal dissolution are removed by ultrasonic agitation, (iv) cations whic h are likely to form fluorides in the course of dissolution are largel y extracted from the silicates before HF is applied, thus minimising t he danger of laboratory fractionation (of e.g. Sm/Nd or U/Pb) and inco mplete spike-sample equilibration, (v) the procedure can be optimised for specific silicates (or mineral chemistries), (vi) thanks to the us e of transparent beakers the state of dissolution can be traced withou t interrupting the process and residual fluorides are easily detected and (vii) blank contribution is easily traced. This technique is an al ternative to steel jacketed Teflon(R) bomb digestion and an improvemen t over traditional closed vial approaches.