High pressure breakdown of antigorite to spinifex-textured olivine and orthopyroxene, SE Spain

The prograde, high pressure, transition from antigorite serpentinite to enstatite-olivine rock occurs along a tectonically undisturbed pro®le at Cerro del Almirez, SE Spain. The reactant assemblage is antigorite + olivine with tremolite rimming precursor diopside. The product assemblage of tremolite + chlorite + enstatite + olivine has a spinifex-like texture with arborescent or radiating olivine elongated parallel to [001] and with radially grown enstatite. Product enstatite is very poor in Al2O3. Due to numerous oriented submicroscopic inclusions of chromian magnetite, product olivine has a brownish pleochroism and a bulk chromium content similar to precursor antigorite. Titanian clinohumite with a ̄uorine content of 0.45±0.50 wt% persisted beyond the breakdown of antigorite. The partitioning of iron and magnesium amongst the silicate phases is almost identical to that at lower pressures. Average Kd values Mn/Mg and Ni/Mg are 0.17 and 0.70 for antigorite-olivine pairs and 1.83 and 0.22 for orthopyroxene-olivine pairs, respectively. These data are useful in discriminating generations of olivine grown on each other. From the ®eld data a phase diagram topology for a portion of the system CaO-MgO-SiO2-H2O is derived. This topology forms the basis for extrapolations into inaccessible P-T regions. Introduction Subduction of serpentinized lithospheric mantle is considered an important process, because its dehydration provides a source of H2O that may trigger partial melting and a€ect rheological and seismic properties of mantle rocks. The knowledge of phase relations in subducting serpentinite is important for monitoring these processes. Therefore, in recent years, a great number of experimental studies have been carried out in order to determine high pressure phase relations of hydrous ultrama®c systems (Yamamoto and Akimoto 1977; Pawley and Wood 1995, 1996; Wunder and Schreyer 1997; Bose and Ganguly 1995; Ulmer and Trommsdor€ 1995). Attempts to model these systems thermodynamically for pressures and temperatures corresponding to various subduction zone environments (Evans and Guggenheim 1988; Pawley and Wood 1995) have been only partly successful and are faced with increasing diculties, particularly at high pressures, because of the lack of relevant thermodynamic data. In contrast to experimental work and thermodynamic modelling the ®eld evidence for high pressure metamorphism of serpentinite has received relatively little attention. For example, the major sources of H2O in subducting hydrous mantle, the breakdown reactions of antigorite and talc have been studied repeatedly in experiments and theory but only a part of them have been recovered in the ®eld. The relative stability ®elds of talc and antigorite parageneses, which are crucial in evaluating the phase relations of subducting serpentinites, are only incompletely known for parts of the lithosphere. Trommsdor€ and Evans (1972, 1974) demonstrated that at moderate pressures of up to 10 kbar antigorite breaks down to forsterite + talc at temperatures below 600 °C. Depending on pressure this product assemblage is then consumed by various dehydration steps until the ultimate product, enstatite + olivine, is formed. According to experimental studies (Ulmer and Trommsdor€ 1995; Wunder and Schreyer 1997) and internally Contrib Mineral Petrol (1998) 132: 139±148 Ó Springer-Verlag 1998 V. Trommsdor€ (&) á O. MuÈ ntener Institut fuÈ r Mineralogie und Petrographie, ETH Zentrum, CH-8092 ZuÈ rich, Switzerland V. Lo pez Sa nchez-Vizcaõ no Depto. Geologia, Escuala Universitaria Polite cnica, Universidad de Jaen, E-23700 Linares, Spain M.T. Go mez-Pugnaire Depto. Mineralogia y Petrologia, Facultad de Ciencias, Universitad de Granada, E-18002 Granada, Spain Editorial responsibility: J. Hoefs consistent thermodynamic data (Berman et al. 1986; Berman 1988) the intermediate dehydration products are absent at high pressures and antigorite breaks down directly to enstatite + olivine. Although antigorite is known to exist to conditions of eclogite facies (Bearth 1967; Evans 1977; Scambelluri et al. 1995), the direct breakdown of antigorite to enstatite + olivine remains to be demonstrated in the ®eld (Evans and Guggenheim 1988, p. 267). This breakdown reaction, however, is considered to be a major source of H2O for calc-alkaline magmatism (Ulmer and Trommsdor€ 1995). It is the purpose of this study to present a case where this reaction can be demonstrated in the ®eld and to derive from the relations of phases coexisting in the ®eld a phase diagram topology relevant for ultrama®c rocks.