The stability of antigorite in the systems MgO-SiO₂-H₂O (MSH) and MgO-Al₂O₃-SiO₂-H₂O (MASH): The effects of Al³⁺substitution on high-pressure stability

Abstract

The high-pressure stability of antigorite in the systems MSH and MASH was investigated using two structurally and chemically well-constrained natural samples. Careful sample selection and characterization ensured that the only significant difference between the samples was Al content, one sample being essentially Al free, and the other containing 3.06(2) wt% Al 2 O 3 . In the system MSH, the reaction antigorite = forsterite + clinoenstatite + water was bracketed, under water-saturated conditions, between 630 and 650 ∞C at 1.6 GPa, between 620 and 660 ∞C at 2.5 GPa, between 620 and 660 ∞C at 3.9 GPa, and between 4.5 and 5.0 GPa at 520 ∞C. In the system MASH, the reaction antigorite = forsterite + clinoenstatite + chlorite + water was bracketed, under water-saturated conditions, between 660 and 700 ∞C at 2.0 GPa, between 660 and 680 ∞C at 2.9 GPa, and between 5.0 and 5.5 GPa at 600 ∞C. At pressures above 5.8 GPa, intersection of this reaction with the reaction chlorite + clinoenstatite = pyrope + forsterite + water leads to an additional reaction whereby the Al component of the antigorite is transferred to pyrope upon antigorite breakdown. The addition of a few weight percent Al 2 O 3 into antigorite is shown to stabilize the antigorite structure to significantly higher temperatures and pressures, possibly by minimizing structural misfit among the component octahedral and tetrahedral sheets. This effect partially explains the considerable discrepancies noted between previous studies on the stability of antigorite at high pressure. In addition, antigorite breakdown in the system MASH transfers a significant volume of water to chlorite-bearing assemblages, thereby greatly increasing the range of temperatures over which water is tied up in hydrous phases relative to the system MSH.suggested by Zussman (1954) and Kunze (1956Kunze ( , 1958, antigorite can be considered to consist of alternating sinusoidal octahedral and tetrahedral layers. The tetrahedral layers reverse polarity every half wavelength. There are two types of reversal, named 8-reversal or 6-reversal after the occurrence of 8-and 6-membered tetrahedral rings. The 8-reversals effectively lead to the loss of 3 brucite units, and thus departure from the generalized serpentine formula. The range of chemical variation within antigorite is, however, small, and compositions depart only slightly from that of the generalized serpentine formula. Kunze (1961) expressed the composition of individual antigorites by the formula M 3m-3 T 2m O 5m (OH) 4m-6 , where m is the number of tetrahedra in a single chain defined by the wavelength a 0 , M = octahedral cations (Mg, Fe 2+ Ni, Mn 2+ , Al), and T = tetrahedral cations (Si, Al). Mellini et al. (1987) determined compositions of natural antigorite in the range M 2.79 T 2 O 5 (OH) 3.57 to M 2.87 T 2 O 5 (OH) 3.74 , corresponding to a variation in m from 14 to 23. TEM studies have shown that antigorite samples with one unique structural state are very rare; more commonly, a considerable range in m is noted. Antigorite with t...

Two types of antigorite serpentinite controlling heterogeneous slow-slip behaviours of slab–mantle interface

“…Incorporation of Al in antigorite extends its thermal stability by up to 80 ºC. However, the effect of Al decreases with decreasing pressure (Bromiley and Pawley, 2003) and it is inferred to be about half the maximum at depths around continental Moho. The effects of Fe on the same reaction have been examined using thermodynamic calculations (Ulmer and Trommsdorff, 1999;Padrón-Navarta et al, 2010).…”

“…However, natural antigorite may also contain significant amounts of Al and Fe. Experimental results show that Al contents in antigorite significantly affect the antigorite-out reaction at high-pressure conditions: antigorite → olivine + Opx + H 2 O (Ulmer and Trommsdorff, 1995;Wunder and Schreyer, 1997;Bromiley and Pawley, 2003;Padrón-Navarta et al, 2010). Incorporation of Al in antigorite extends its thermal stability by up to 80 ºC.…”

“…Phase diagrams for the MSH system (Evans et al, 1976;Ulmer and Trommsdorff, 1999;Bromiley and Pawley, 2003;Evans, 2004) indicate that the stable mineral assemblage in serpentinized harzburgite changes with increasing temperature in the following order: (1) lizardite + brucite (Liz + Brc), (2) antigorite + brucite (Atg + Brc), (3) antigorite + olivine (Atg + Ol). It has been suggested that the Atg + Brc assemblage may be dominant in mantle rocks adjacent to subduction boundaries (Peacock and Hyndman, 1999;Hyndman and Peacock, 2003).…”

“…Reaction curves for the MSH system refer to Ulmer and Trommsdorff (1999), Bromiley and Pawley (2003) and Evans (2004). Stability of mineral assemblages in harzburgite …”

Experimental constraints on formation of hematite in olivine at high pressures and temperatures

“…However, no DHMS phases were observed in the run products, which is significant for understanding the phase relations of the hydrous harzburgite. Experimental studies of the serpentine (antigorite) system (Ulmer and Trommsdorff 1995;Wunder and Schreyer 1997;Irifune et al 1998;Bromiley and Pawley 2003;Komabayashi et al 2005) have shown that antigorite transforms into olivine + enstatite + H 2 O under high P-T conditions (>1073 K at 8 GPa), but transforms into phase A/phase E + enstatite + H 2 O at high-pressure and relatively low-temperature conditions (<1073 K at 8 GPa) (Irifune et al 1998;Komabayashi et al 2005). In Fig.…”