AN EXPERIMENTAL INVESTIGATION ON THE P-T STABILITY OF MG-STAUROLITE IN THE SYSTEM MGO-AL2O3-SIO2-H2O

The pressure-temperature stability field of Mg-staurolite, ideally Mg4 Al18Si8O46(OH)(2), was bracketed for six possible breakdown reactions in the system MgO-Al2O3-SiO2-H2O (MASH). Mg-staurolite is stable at wa ter pressures between 12 and 66 kbar and temperatures of 608-918 degre es C, requiring linear geotherms between 3 and 18 degrees C/km. This p hase occurs in rocks that were metamorphosed at high-pressure, low-tem perature conditions, e.g. in subducted crustal material, provided they are of appropriate chemical composition. Mg-staurolite is formed from the assemblage chlorite + kyanite + corundum at pressures <24 kbar, w hereas at pressures up to 27 kbar staurolite becomes stable by the bre akdown of the assemblage Mg-chloritoid + kyanite + corundum. Beyond 27 kbar the reaction Mg-chloritoid + kyanite + diaspore = Mg-staurolite + vapour limits the staurolite field on its low-temperature side. The upper pressure limit of Mg-staurolite is marked by alternative assembl ages containing pyrope + topaz-OH with either corundum or diaspore. At higher temperatures Mg-staurolite breaks down by complete dehydration to pyrope + kyanite + corundum and at pressures below 14 kbar to enst atite + kyanite + corundum. The reaction curve Mg-staurolite = talc kyanite + corundum marks the low-pressure stability of staurolite at 1 2 kbar. Mg-staurolite does not coexist with quartz because alternative assemblages such as chlorite-kyanite, enstatite-kyanite, talc-kyanite , pyrope-kyanite, and MgMgAl-pumpellyite-kyanite are stable over the e ntire field of Mg-staurolite.