Abstract
Talc and 10-Å phase are hydrous phases that are implicated in fluid processes and rheological behaviour in subduction zones. Natural samples of talc show limited compositional variation away from the MgO–SiO2–H2O (MSH) endmember, with only substitution of Fe2+ for Mg occurring in significant amounts. In experiments at 2 GPa, talc containing 0.48 apfu Fe2+ begins to break down in the divariant field talc + anthophyllite + quartz at ~ 550 °C, a temperature ~ 270 °C lower than in the MSH system. At 4 GPa, Fe-bearing talc breaks down over a wide temperature interval in the divariant field talc + enstatite + coesite. The large decrease in temperature of the beginning of talc breakdown shows that Fe2+ is partitioned strongly into enstatite and anthophyllite with respect to talc. In phase reversal experiments at 6.5 GPa, the beginning of the dehydration of 10-Å phase containing 0.48 apfu Fe2+ was bracketed between 575 °C and 600 °C, a temperature ~ 100 °C lower than the MSH endmember reaction. The relative positions of the talc and 10-Å phase dehydration reactions indicate that the latter is able to accommodate greater Fe substitution, and is, therefore, more stable in Fe-bearing systems. In experiments at 6.2 GPa, 650 °C in the systems MgO–Al2O3–SiO2–H2O (MASH) and Na2O–MgO–Al2O3–SiO2–H2O (NMASH), 10-Å phase was synthesised that contains up to 0.5 apfu Al in the system MASH (compared to 0.8 in the starting material) and up to 0.4 apfu Al + 0.4 apfu Na in the system NMASH (compared to 0.7 of each of Al and Na in the starting material). Further experiments are required to determine if higher Al and Na contents in 10-Å phase are possible. The much higher Al and Na contents than found in talc indicate that, as with Fe, substitution of these elements enlarges the 10-Å phase stability field with respect to talc. In contrast to the effect of Fe, Al and Na also increase the stability of 10-Å phase relative to its thermal breakdown products enstatite + coesite.
Highlights
Talc is an important carrier of H 2O into the mantle at subduction zones, where it may occur in a wide range of rock types, from metamorphosed sediments to metabasites and metamorphosed ultramafic rocks (Evans and Guggenheim 1988)
Talc is unusual among trioctahedral phyllosilicates in showing limited solid solution away from the endmember composition
From a comparison of the stabilities of Fe-bearing talc and TAP and their ability to incorporate other cations into their structures, we suggest that in typical compositions in subduction zones, the talc stability field is reduced with respect to the simple system MgO–SiO2–H2O (MSH) whereas the TAP stability field is much expanded
Summary
Talc is an important carrier of H 2O into the mantle at subduction zones, where it may occur in a wide range of rock types, from metamorphosed sediments to metabasites and metamorphosed ultramafic rocks (Evans and Guggenheim 1988). Its endmember composition is Mg3Si4O10(OH), making it most likely to form in Mg- and Si-rich bulk compositions. Such rocks include metasomatised peridotite in the descending slab, in which talc may be carried to mantle depths of at least 150 km (Pawley and Wood 1995), and Communicated by Timothy L. Talc is unusual among trioctahedral phyllosilicates in showing limited solid solution away from the endmember composition. Previous studies of talc stability have focussed on this composition.
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