Extension of the High Temperature Limit of Cooling Geospeedometry.

Abstract

Geospeedometer proposed so far to estimate cooling history of rocks has a limit in terms of the maximum temperature above which we cannot deduce the history. The limit is governed by kinetic processes for geospeedometry and cooling rates; for example the olivine-spinel geospeedometry based on kinetics of Mg-Fe2+ exchange between olivine and spine! with the maximum grain size of a few millimeters cannot record any cooling history above 800°C if the cooling rate is <∼10-4°C/year. In order to extend the limit, a larger distance scale for geospeedometry is needed. By using mineralogical variations forming rock structures, whose distance scale is up to a meter order or more, we can in principle estimate thermal history of rocks up to their so!idus temperatures, if effects of polycrystalline and multi-phase nature and the initial conditions for a large scale rock system are correctly assessed. The olivine-spinel geospeedometry was extended to deduce high temperature thermal history of a dunite-chromitite layer sampled from the Iwanai-lake peridotite mass, Hokkaido. The Fe-Mg variation over 6 cm in dunite from the contact is analyzed by a 1-D diffusion model with a composition-dependent diffusivity and a changeable cooling rate. The dunite-chromitite geospeedometry indicates that the Iwanai-lake mass was cooled from ∼1000°C to 900°C with increasing cooling rate from <0.01°C/year to -0.3°C/year. This suggests that the slower cooling after the melting event was followed by the ascent of the mass that resulted in the inferred rapid cooling at ∼900°C.