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Title: The effect of solid solution on the stabilities of selected hydrous phases during subduction
Author: Howe, Harriet
ISNI:       0000 0004 6499 0277
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2017
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Previous studies on complex chemical systems, approximating enriched ultramafic compositions, have shown that the stability fields of certain phyllosilicate minerals may be shifted through solid solution. This project focuses on three hydrous phases predicted to play an important role in water transfer and storage during subduction. Talc, and at higher pressures the 10 A phase, are expected form in enriched abyssal peridotite within the cold interior of a lithospheric slab, whilst the sodic amphibole eckermannite is expected to be present in the overlying hydrated basalt. Multi-anvil and piston cylinder press experiments in the FeO-MgO-SiO2-H2O (FMSH), NaO-MgO-Al2O3-SiO2-H2O (NMASH), and MgO-Al2O3-SiO2-H2O (MASH) systems have sought to determine the effect of solid solution on the stability on talc and the 10 A phase, with comparison to the end-member MgO-SiO2-H2O (MSH) system. The reaction talc + H2O = 10 A phase has been bracketed in the MSH system at 4.8 GPa/560 ˚C and 5 GPa/640 ˚C, confirming the estimated reaction position from Pawley et al. (2011). Previously unknown values for the entropy and enthalpy of formation of the 10 A phase have been calculated as DeltaHf = -6172.02 kJ and DeltaSf = 320.075 JK-1. At 2 GPa talc containing 0.48 apfu Fe2+ breaks down in the divariant field talc + anthophyllite + quartz + H2O from ~550 ˚C, initiating talc dehydration at temperatures ~270 ˚C lower than in the MSH system. At 4 GPa Fe-bearing talc breaks down in the divariant field talc + enstatite + coesite. A run at 5.2 GPa and 555 ˚C produced 10 A phase containing 0.48 apfu Fe2+. Between 575 ˚C and 600 ˚C at 6.5 GPa phase reversal experiments bracketed the initiation of Fe-bearing 10 A phase dehydration in the divariant field 10 A phase + enstatite + coesite + H2O, corresponding to a reduction in thermal stability of around ~100 ˚C compared to the end-member. The relative positions of the talc and 10 A phase dehydration reactions suggest the latter is able to accommodate greater Fe substitution, and is therefore more stable in the FMSH system. The assemblages 10 A phase + enstatite + coesite + jadeite and 10 A phase + enstatite + pyrope + coesite, were synthesised in the NMASH and MASH systems, respectively. Compositional analysis indicates that the 10 Å phase in these samples contains < 1 weight % Al2O3, with negligible Na. This suggests that Al3+ substitution in talc and the 10 Å phase is unlikely to exert the same stabilising effect observed in a number of other phyllosilicates. Eckermannite was produced in further NMASH experiments at 6.2 GPa. Compositional and structural analysis indicates near-full A-site occupancy and a composition close to that of the end-member, deviating through a minor binary exchange towards Mg-katophorite. This exchange is proposed to stabilise eckermannite to high pressures, beyond previously published limits for sodic amphibole stability. Updated stability fields for talc, the 10 Å phase, and eckermannite were applied to a thermal model for subduction. This predicts that 10 Å phase containing 0.48 apfu Fe2+ may be stable to depths of ~260 km, compared to ~280 km for the end-member. With increasing pressure and temperature Fe-bearing 10 Å phase will dehydrate across a depth range, resulting in either total de-volatilisation, or transfer to other stable high pressure hydrous phases enabling the transport of water to the deeper regions of the mantle.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Talc ; Ten Angstrom Phase ; Subduction ; DHMS ; Experimental Petrology