Experimental study of phase relations involving osumilite in the system K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O at high pressure and temperature

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.