Experimental Calibration of a Geobarometer for Eclogites Based on Al Substitution in Clinopyroxene

We present an experimentally calibrated geobarometer for a wide range of eclogites based on the solubility of Al-Tschermaks in omphacitic clinopyroxene coexisting with garnet. Experimental investigations of the substitution of Al(IV) on the tetrahedral site in clinopyroxene reveal strong negative pressure dependence of 4-fold coordinated Al, whereas temperature has only a minor effect on Al solubility. Experiments were carried out in a multi-anvil apparatus and in a piston cylinder apparatus at pressures of 3 to 7 GPa and temperatures from 1350 to 1490 °C. The synthetic bulk composition represents hydrous mid-ocean ridge basalt (MORB). Experiments yielded typical eclogitic two phase assemblages consisting of garnet and omphacite solid solutions. All experiments were buffered in terms of Al with garnet as Al-bearing phase. To test our geobarometer we reviewed the available literature covering a wide range of experimental conditions and compositions from anhydrous pyroxenites to carbonated eclogites and found that the observed correlation between Al(IV) and pressure is also applicable to the already existing data. Although the systems show major variations in chemical composition the ratio of Al(IV) over Al(total) plus Ca negates these differences in most instances. Therefore, the presented geobarometer opens up the possibility to determine pressures of eclogites/ultramafic igneous rocks without invoking accessory phases.