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Melting of Phlogopite-bearing Assemblages in the Earth’s Mantle Open Access


Other title
Experimental Petrology
Phase relations
Type of item
Degree grantor
University of Alberta
Author or creator
Enggist, Andreas
Supervisor and department
Luth, Robert (Earth and Atmospheric Sciences)
Examining committee member and department
Mar, Arthur (Chemistry)
Chacko, Tom (Earth and Atmospheric Sciences)
Foley, Stephen (Johannes Gutenberg-Universität Mainz)
Stachel, Thomas (Earth and Atmospheric Sciences)
Herd, Christopher (Earth and Atmospheric Sciences)
Department of Earth and Atmospheric Sciences

Date accepted
Graduation date
Doctor of Philosophy
Degree level
Phlogopite, an alkali-rich and water-bearing mineral, is a common phase in the Earth’s upper mantle. Its breakdown could generate melts or stabilize fluids that will metasomatize mantle rocks. To date, the effect of CO2 on phlogopite stability remains unconstrained. To evaluate the stability of phlogopite in the presence of carbonate, experiments were conducted in the KMAS-H2O-CO2, KCMAS-H2O and KCMAS-H2O-CO2 systems at pressures from 4 to 8 GPa and temperatures from 1100 to 1600°C. The solidus of KMAS-H2O-CO2 was bracketed between 1200 and 1250°C at pressures of 4, 5 and 6 GPa, and between 1150 and 1200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus magnesite reacts out, and enstatite and olivine appear. The solidus of KCMAS-H2O was bracketed between 1250-1300C at 4 and 5 GPa, and between 1300-1350C at 6, 7 and 8 GPa. The solidus of KCMAS-H2O-CO2 was bracketed between 1150-1200C at 4, 5 and 6 GPa, and between 1100-1150C at 7 and 8 GPa. Below the solidus in both systems, phlogopite is in equilibrium with enstatite, diopside, garnet, ±magnesite and a fluid. At 7 GPa phlogopite coexists with potassic richterite, enstatite, diopside, garnet, ±magnesite and a fluid. Potassic richterite is the stable K-bearing phase at 8 GPa and is in equilibrium with enstatite, diopside, garnet, ±magnesite and a fluid. Olivine forms at the solidus and coexists with enstatite, diopside, garnet and melt. The solidus of CO2-bearing systems is lowered such, that, in a very hot subduction environment, alkali-rich, CO2-bearing melts can originate at a depth of ~240 km (~7.5 GPa). In a 40-mWm-2 subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and to 190 km in the presence of pyroxene with carbonate. Coexisting fluids become Si-rich with increasing pressure. Ascending alkali- and CO2-rich melts from greater depths could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite, carbonate and stabilizing a fluid at a depth of 170 to 200 km. Fluid and melt in KCMAS-H2O-CO2 remain immiscible phases to pressures >8 GPa.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Enggist A, Chu L, Luth RW (2012) Phase relations of phlogopite with magnesite from 4 to 8 GPa. Contrib Mineral Petrol 163:467-481. doi:10.1007/s00410-011-0681-9

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