Antigorite: High-pressure stability in the system MgOSiO 2H 2O (MSH)

Abstract

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 experiments. Results on reaction (1) were found to be in good agreement with former experimental determinations by both Johannes [Johannes, W., 1975. Zur Synthese und thermischen Stabilität von Antigorit. Fortschr. Mineral. Beih. 53, 36.] and Evans et al. [Evans, B.W., Johannes, W., Oterdoom, H., Trommsdorff, V., 1976. Stability of crysotile and antigorite in the serpentinite multisystem. Schweiz. Mineral. Petrogr. Mitt. 56, 79–93.]. From our experiments the invariant point ( I 1), interconnecting the two reactions, can be located at about 15 kbar/650°C. This is consistent with the thermodynamic calculations using the dataset of Berman [Berman, R.G., 1988. Internally consistent thermodynamic data for minerals in the system J. Petrol. 29, 445–522.]; however, it is in contrast to recent experimental studies of Ulmer and Trommsdorff [Ulmer, P., Trommsdorff, V., 1995a. Serpentine stability to mantle depths and subduction-related magmatism. Science 268, 858-861.] who determined I 1 at 21 kbar/730°C. Our PT-conditions for I 1 could be confirmed by equilibrium experiments on reaction (10) talc + forsterite ↔ clinoenstatite + water, which is generated at I 1 as well. Up to about 25 kbar the breakdown reaction (2) is nearly pressure-independent. Towards still higher pressures the d P/d T-slope of reaction (2) bends and becomes negative. Schreinemakers analysis as well as thermodynamic calculations of the upper pressure-stability of antigorite show that the possible antigorite breakdown reaction (3) antigorite ↔ clinoenstatite + brucite + water and reaction (4) brucite + clinoenstatite ↔ forsterite + water could originate at a new invariant point I 2, provided that the reactions (2) and (11) brucite + antigorite ↔ forsterite + water intersect. Bracketing equilibrium (4) and combining these results with those on reaction (2), I 2 was located at only about 51 kbar/490°C, compared to 77 kbar/680°C according to Berman's data. However, when taking into account the dense hydrous magnesium silicate (= DHMS)-phase A, Mg 7Si 2O 8(OH) 6, the phase relations of antigorite are changed resulting (i) in the metastability of I 2 and reaction (3) and (ii) in a new invariant point I 7 at about 44 kbar and 580°C generating the new antigorite breakdown-reaction (16) antigorite ↔ phase A + clinoenstatite + water. On the basis of these new data on the stability of antigorite, earlier conclusions about dehydration depths in subducted serpentine-bearing oceanic lithosphere have to be reconsidered. The maximum pressure stability of antigorite according to reaction (16) extends between 44 and 55 kbar, that is between about 130 and 160 km depths, as opposed to about 75 kbar (220 km) following Ulmer and Trommsdorff (see above). Because many different thermal regimes are possible in subduction zones, no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time.