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

Abstract

A 6 STRACT 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 (K20-Fe0-Mg0-A1203-Si0,-H,0) under low PHz0 and fs 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 1OOO-1100" C, the pair 0s-Grt is stable over a pressure range of about 3 kbar. The divariant reaction 0 s + 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 0s-Grt is located around 11 kbar with a nearly flat dP/dT slope over the temperature range 950-1100" 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 0s-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 0s-Opx-Sil-Kfs-Qtz has a more restricted pressure and compositional stability domain than 0s-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.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…Sapphirine is typically indicative of high to extreme crustal temperatures (e.g. Hensen, 1971Hensen, , 1972bHensen, , 1986Hensen, , 1987Green, 1971, 1973;Chatterjee and Schreyer, 1972;Newton 1972;Newton et al, 1974;Ellis et al, 1980;Grew, 1980;Harley, 1985Harley, , 1986Sandiford, 1985a;Sandiford et al, 1987;Powell and Sandiford, 1988;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Podlesskii et al, 2008;Podlesskii, 2010), with sapphirine stability in silica-undersaturated rock compositions expanded to lower temperatures than for sapphirine þ quartz bearing assemblages (e.g. Schreyer and Seifert, 1969a;Seifert, 1974;Ackermand et al, 1975;Hensen, 1987;Kelsey et al, 2005).…”

“…Ultrahigh temperature (UHT) metamorphism is the most thermally extreme type of regional crustal metamorphism, defined by Harley (1998b) as non-igneous crustal temperatures above 900 C. UHT metamorphism of continental crust is now widely accepted as a relatively commondrather than anomalousdcharacteristic of deep crustal processes, greatly assisted by the large (and growing) number of localities worldwide that contain mineral assemblages whose stability has been constrained experimentally (Schreyer and Seifert, 1969a,b;Hensen and Green, 1970;Hensen, 1971Hensen, , 1972aHensen, ,b, 1977Hensen and Essene, 1971;Hensen and Green, 1971, 1972, 1973Chatterjee and Schreyer, 1972;Newton 1972;Doroshev and Malinovskiy, 1974;Newton et al, 1974;Seifert, 1974;Ackermand et al, 1975;Kiseleva, 1976;Arima and Onuma, 1977;Annersten and Seifert, 1981;Bertrand et al, 1991;Motoyoshi et al, 1993;Audibert et al, 1995;Carrington, 1995;Carrington andHarley, 1995a,b, 1996;Das et al, 2001Das et al, , 2003Hollis and Harley, 2003;Brigida et al, 2007;Fockenberg, 2008;Podlesskii et al, 2008;Podlesskii, 2010) and reinforced by calculated phase equilibria (Kelsey et al, 2004;Kelsey et al, 2005;Podlesskii et al, 2008;Podlesskii, 2010;Taylor-Jones and Powell, 2010;Holland and Powell, 2011;…”

Extending our understanding of Ultrahigh temperature crustal metamorphism

“…Experiments on biotite dehydration melting (Carrington and Harley, 1995a) and related studies of osumilite stability relations at low f o2 (Audibert et al, 1995;Carrington and Harley, 1995b) indicate that at pressures of greater than 6 kbar the assemblage osumilite + garnet (Os + Grt) is restricted to temperatures of >900°C in KFMASH (see Harley, 1998a for discussion) and that this assemblage is replaced by the assemblage Opx + Sil + Kfs + Qz at pressures higher than 8 9 kbar. This result is consistent with the distribution of mineral assemblages the Napier Complex, where a NW to SE baric gradient indicated by geobarometry 1987;Harley, 1985; is manifested in the occurrence of Os + Grt in the Tula Mountains compared with the more common occurrence of Opx + Sil + Kfs + Qz southwards (Audibert et al, 1995;Carrington and Harley, 1995b).…”

“…Mineral assemblages in sapphirine , spinel and orthopyroxene sillimanite metapelitic granulites that are constrained by experimentally derived and calculated FeO MgO Al 2 O 3 SiO 2 (FMAS) (Hensen, 1971;Hensen and Green, 1973;Grew, 1980;Bertrand et al, 1991) and KFMASH (Audibert et al, 1995;Carrington and Harley, 1995a;1995b;Holland et al, 1996;White et al, 2001) petrogenetic grids have up until now provided the main evidence for UHT metamorphism. However, such rocks may be scarce or non existent in some high grade terrains that are candidates for UHT status (e.g.…”

“…The a H 2 O conditions calculated for specific P T conditions in the KFMASH system following the methods of Harley et al (2002) (Carrington and Harley, 1995b). Osumilite bearing assemblages that have equilibrated at 7 9 kbar in the Napier Complex of East Antarctica Grew, 1982;Audibert et al, 1995) and other UHT terrains imply a H 2 O of <0.08.…”