SCINTILLATING SCENARIOS OF SUBDUCTING SLABS: TALES TOLD BY TRACE TI (AND ZR)

The prograde parts of P-T paths for subduction zone complexes have typically been drawn to be consistent with peak metamorphic conditions and steady-state thermal models. That is, they display smoothly increasing temperature and pressure with none of the small-scale variations that have been deduced from the study of continental collision zones.

Application of trace element thermometry to subduction complexes from the islands of Syros and Sifnos, Greece, coupled with phase equilibrium constraints, reveals a different and more dynamic view of subduction tectonics. We are developing analysis protocols for application of the Zr-in-rutile, Zr-in-sphene, Ti-in-quartz, and Ti-in-zircon thermometers for the EMP and SIMS that permit temperature precisions of as little as ±2 °C at ca 500 °C. The temperatures obtained from sphene, rutile, and zircon in this way have the potential for providing a unique P-T-t point of crystallization along the subduction path when considered in the context of the specific mineral-forming reactions, many of which have relatively flat slopes in the blueschists.

Zr-in-rutile thermometry from the northern blueschist unit of Sifnos, Greece, yields temperatures that range from 440-510 °C indicating progressive growth of rutile with increasing temperature. No differences in temperature between blueschist and eclogite units were discerned. Rutile growth from the breakdown of ilmenite is estimated to have occurred at ca 1 GPa, giving an instantaneous geothermal gradient of ca 13 degrees/km. Peak conditions from phase equilibria are estimated at 510-520 and 1.5-1.8 GPa, yielding an inferred P-T trajectory of 2-3 degrees/km. This change in the slope of the P-T path implies either an increase in the rate of subduction or an increase of the angle of subduction. Preliminary data from Syros indicate similar results from the mica schist packages (440-510 °C), but significantly higher temperatures (550-600 °C) from the eclogite units, which suggests the juxtaposition of different tectonic blocks.

These results indicate that subduction occurs by episodes of rapid under-thrusting followed by a period of thermal relaxation, rather than by a steady burial along a perturbed, steady state geotherm.