ANTIGORITE - HIGH-PRESSURE STABILITY IN THE SYSTEM MGO-SIO2-H2O (MSH)

The breakdown reactions of antigorite: (1) forming talc + forsterite water at low pressures and (2) forming forsterite + clinoenstatite water at high pressures were determined in reversed equilibrium experi ments. Results on reaction (1) were found to be in good agreement with former experimental determinations by both Johannes [Johannes, W., 19 75. Zur Synthese und thermischen Stabilitat von Antigorit. Fortschr. M ineral. Beih. 53, 36.] and Evans et al. [Evans, B.W., Johannes, W., Ot erdoom, H., Trommsdorff, V., 1976. Stability of crysotile and antigori te in the serpentinite multisystem. Schweiz. Mineral. Petrogr. Mitt. 5 6, 79-93.]. From our experiments the invariant point (I-1), interconne cting the two reactions, can be located at about 15 kbar/650 degrees C . This is consistent with the thermodynamic calculations using the dat aset of Berman [Berman, R.G., 1988. Internally consistent thermodynami c data for minerals in the system a2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2 -TiO2-H2O-CO2. J. Petrol. 29, 445-522.]; however, it is in contrast to recent experimental studies of Ulmer and Trommsdorff [Ulmer, P., Trom msdorff, V., 1995a. Serpentine stability to mantle depths and subducti on-related magmatism. Science 268, 858-861.] who determined I-1 at 21 kbar/730 degrees C. Our PT-conditions for I-1 could be confirmed by eq uilibrium experiments on reaction (10) talc + forsterite <-> clinoenst atite + water, which is generated at I-1 as well. Up to about 25 kbar the breakdown reaction (2) is nearly pressure-independent. Towards sti ll higher pressures the dP/dT-slope of reaction (2) bends and becomes negative. Schreinemakers analysis as well as thermodynamic calculation s of the upper pressure-stability of antigorite show that the possible antigorite breakdown reaction (3) antigorite <-> clinoenstatite + bru cite + water and reaction (4) brucite + clinoenstatite <-> forsterite + water could originate at a new invariant point I-2, provided that th e reactions (2) and (11) brucite + antigorite <-> forsterite + water i ntersect. Bracketing equilibrium (4) and combining these results with those on reaction (2), I-2 was located at only about 51 kbar/490 degre es C, compared to 77 kbar/680 degrees C according to Berman's data. Ho wever, when taking into account the dense hydrous magnesium silicate ( =DHMS)-phase A, Mg7Si2O8(OH)(6), the phase relations of antigorite are changed resulting (i) in the metastability of I-2 and reaction (3) an d (ii) in a new invariant point I-7 at about 44 kbar and 580 degrees C generating the new antigorite breakdown-reaction (16) antigorite <-> phase A + clinoenstatite + water. On the basis of these new data on th e stability of antigorite, earlier conclusions about dehydration depth s in subducted serpentine-bearing oceanic lithosphere have to be recon sidered. The maximum pressure stability of antigorite according to rea ction (16) extends between 44 and 55 kbar, that is between about 130 a nd 160 km depths, as opposed to about 75 kbar (220 km) following Ulmer and Trommsdorff (see above). Because many different thermal regimes a re possible in subduction zones, no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and ti me. (C) 1997 Elsevier Science B.V.