STRONTIUM DIFFUSION KINETICS IN AMPHIBOLES AND SIGNIFICANCE TO THERMAL HISTORY DETERMINATIONS

Strontium tracer-diffusion coefficients have been determined in hornblende and tremolite single crystals using experiments conducted at a confining pressure of 200 MPa at temperatures ranging from 700 to 960 degrees C. A quasi-hydrothermal experimental technique was devised to avoid solution-precipitation effects. Depth profile analyses were made with an ion microprobe. The Arrhenius relation for diffusion parallel to the c-crystallographic direction in the Gore Mountain hornblende over this temperature interval has an activation energy (Q) = 260 (+/-12) kJ/mol and a pre-exponential factor (D-0) = 4.9((+18.)(-1.3)) x 10(-8) m(2)/s. At 800 degrees C, transport rates parallel to the c-axis measured in the two tremolite samples (AMNH#30563 and Barrie Township, Ontario) are 3.2 X 10(-21) m(2)/s and 2.0 x 10(-21) m(2)/s, respectively. These values are factors of 3 and 5 slower than those observed for the hornblende sample at the same temperature. Transport rates parallel and perpendicular to the c-axis are the same within the estimated uncertainty for all three compositions. Experiments that differed in f(O2) by a factor of 6 x 10(7) [MnO-Mn3O4 (MNO) and magnetite-wustite (MW)] for the hornblende gave the same strontium diffusion rates within a factor of two. Crystals with cleavage faces, or polished with alumina or diamond, gave the same D values, provided the crystals were annealed prior to the diffusion experiment. Using a combination of closed-system Rb-Sr systematics and strontium diffusion kinetics it is now possible to determine the thermal histories of hornblende-bearing igneous and metamorphic rocks. Since the diffusion rates for strontium in amphiboles are relatively slow, hornblende will be more retentive of Sr than of other species for which transport rates are known (Ar, O, H) and will be a useful tool to record higher temperature portions of cooling paths.