Spinel-garnet lherzolite transition in the system CaO-MgO-Al2O3-SiO2 revisited:: An in situ X-ray study

Large discrepancies are reported for the near-solidus, pressure-temperature location of the spinel to garnet lherzolite univariant curve in the system CaO-MgO-Al2O3-SiO2 (CMAS). Experimental data obtained previously from the piston-cylinder apparatus indicate interlaboratory pressure differences of up to 30% relative. To investigate this disparity-and because this reaction is pivotal for understanding upper mantle Petrology-the phase boundary was located by means of an independent method. The reaction was studied via in situ X-ray diffraction techniques in a 6-8 type multianvil press. Pressure is determined by using MgO as an internal standard and is calculated from measured unit cell volume by using a newly developed high-temperature equation of state for MgO. Combinations of real-time and quenched-sample observations are used to bracket the phase transition. The transition between 1350 and 1500degreesC was reversed, and the reaction was further constrained from 1207 to 1545degreesC. Within this temperature range, the transition has an average dT/dP slope of similar to40 10degreesC/kbar, consistent with several previous piston-cylinder studies. Extrapolation of our curve to 1575degreesC, an established temperature of the P-T invariant point, yields a pressure of 25.1 +/- 1.2 kbar. We also obtained a real-time reversal of the quartz-coesite transition at 30.5 +/- 2.3 kbar at 1357degreesC, which is about 2 to 4 kbar lower in pressure than previously determined in the piston-cylinder apparatus. Copyright (C) 2002 Elsevier Science Ltd.