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

• Author / Creator

  Enggist, Andreas

• 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.

• Subjects / Keywords

  • Magnesite
  • Experimental Petrology
  • Metasomatism
  • Phlogopite
  • Phase relations

• Graduation date

  Fall 2012

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R32D91

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.