Diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks

Recently obtained data on oxygen diffusion in feldspars, quartz, and hornblende permit the prediction of the apparent 18O/16O temperatures that would be measured in a rock that consisted only of those three minerals, and cooled slowly from high temperature. The computed temperatures would be based on the differences in the 18O/16O ratios between coexisting pairs of minerals. The present calculation takes into account the diffusion rates for oxygen as a function of temperature, the cooling rate of the rock, the mineral grain sizes, and the mode of the rock. For mineral grains 1 mm in radius, and a cooling rate of 100C/m.y., the minimum difference in apparent temperature between quartz-feldspar and feldspar-hornblende pairs will be 1150C, despite the assumption of a normal, uneventful, slow cooling history to room temperature. Further, the apparent quartz-hornblende temperature will range over 300C (590-6200C) depending on the mode of the rock. For a cooling rate of 10000C/m.y., the apparent difference in temperature can be as much as 4000C. Consequently, consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high-grade metamorphic rocks or igneous rocks which are cooled slowly. Departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature, or a complex history for the rock. For some minerals, including hornblende, the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens. (orig.)