Experimental study of phase relations involving osumilite in the system K2O-FeO-MgO-Al2O3-SiO2-H2O at high pressure and temperature |
| |
| Authors: | N AUDIBERT P BERTRAND B J HENSEN |
| |
| Institution: | Laboratoire de Pétrologie Physique, UniversitéParis-7 and I.P.G.P, CNRS-URA 1093, 75251 Paris Cedex 05, France;Department of Applied Geology, University of New South Wales, Sydney, NSW 2052, Australia |
| |
| Abstract: | Abstract Phase relations and mineral chemistry for garnet (Grt), orthopyroxene (Opx), sapphirine (Spr), water-undersaturated cordierite (Crd), osumilite (Osu), sillimanite (Sil), K-feldspar (Kfs), quartz (Qtz) and a water-undersaturated liquid (Liq) have been determined experimentally in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) under low PH2O and fO2 conditions. Four compositions have been studied with 100 Mg/(Mg + Fe)] ranging from 65.6 to 89.7. Based on our experimental data, a P-T grid is derived for the KFMASH system in the presence of quartz, orthopyroxene and liquid. Osumilite has been found in various mineral assemblages from 950 to 1100°C and 7.5 to 11 kbar. In the temperature range 1000-1100°C, the pair Os-Grt is stable over a pressure range of about 3kbar. The divariant reaction Os + Opx = Grt + Kfs + Qtz runs to the right with increasing pressure. Because osumilite is the most magnesian phase it is restricted to Mg-rich compositions at high pressure. The reaction defining the upper pressure stability limit of Os-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950–100°C. Over the entire temperature range investigated osumilite is not stable beyond 12 kbar. The data imply a restricted pressure range between 11 and 12 kbar for the stability of the assemblage Os-Opx-Sil-Kfs-Qtz. At 1050°C and above, osumilite occurs in various mineral assemblages together with the high-T pair Spr-Qtz. When coexisting with garnet, orthopyroxene or sapphirine, osumilite is always the most magnesian phase. At 1050 and 1100°C, liquid is invariably the most Fe-rich phase in the run product. Our data support a theoretical P-T grid for the KFMAS system in which osumilite is stable outside the field of the high-T assemblage Spr-Qtz. Moreover, our grid indicates that Os-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than Os-Grt, in agreement with natural occurrences. Osumilite is stable over a large pressure range, such that in Mg-rich rocks, and at high temperature, it can occur at any depth in normal thickness continental crust. |
| |
| Keywords: | experimental petrology granulite osumilite |
|
|
