Gabbro‐eclogite reaction rate and its geophysical significance

Abstract

The gabbro‐garnet granulite‐eclogite transformation may play a significant role in driving the motions of terrestrial lithospheric plates. Whether or not this transformation is in fact important as a driving mechanism for plate tectonics depends on the relationship of the reaction time to geologic time. Solid state diffusion under completely dry conditions is investigated as a possible model for the gabbro‐eclogite reaction, with the result that it could not produce the transition in geologically meaningful times at temperatures less than circa 600°–800°C in the earth's upper mantle. Other reaction mechanisms must exist for the geologically rapid occurrence of the phase change at lower temperatures. It is found that one of these mechanisms can be grain interstitial diffusion in a mantle with minute amounts of water. In this model, dissolved ions migrate through water films surrounding mineral grains to sites of reaction. A water‐undersaturated mantle contains a small quantity of hydrous phases, such as chlorite, amphibole, or talc, the presence of which implies that interstices within the rock can contain water in equilibrium with these minerals and at a pressure PH 2O which is less than the pressure in the rock. Implicit then is the presence of other gases and/or structural rock integrity. This PH 2O is calculated for serpentine, tremolite, and talc as a function of temperature and rock pressure. Various pertinent cations are sufficiently mobile in aqueous solution that at high temperature and high pressure, diffusion through water will not significantly slow the reaction. Rather, pressure‐induced solubility of ions in this water vapor is the important rate‐limiting process in the model. Rock pressure and temperature must be such as to generate at least ∼0.5‐1 kbar of PH 2O in the presence of the hydrous phases for geologically short reaction times. Under ambient conditions PH 2O is quite small, the cations are relatively insoluble, and the reaction time is geologically long. Upon subduction of a basaltic upper crust or lithosphere, for example, an increase in PH 2O occurs, and with increasing pressure the mineral solubility in this supercritical water increases dramatically, yielding geologically short reaction times; for example, ∼20 m.y. for chlorite‐containing rocks with ∼10−5‐cm film thickness for ion diffusion at depths of ∼15‐30 km and at temperatures of ∼150°–300°C for different heating models of the descending slab. For gabbros in which amphibole (tremolite)‐pyroxene equilibria buffer the partial pressure of water, depths of ∼55–70 km and temperatures of 400°–550°C are required for rapid eclogitization, again for different slab heating models. Thus contrary to previous suggestions, the gabbro‐eclogite transformation, as it probably occurs in the descending or spreading lithosphere, is not simply rate‐controlled by temperature but depends heavily on pressure and on the nature of the minor hydrous minerals present.