Phase relations of phlogopite with magnesite from 4 to 8 GPa

Enggist, Andreas; Chu, Linglin; Luth, Robert W.

Phase relations of phlogopite with magnesite from 4 to 8 GPa

Author

Enggist, Andreas

Chu, Linglin

Luth, Robert W.

Date

2012

Keywords

carbonated peridotite, experimental petrology, mantle, melting

Abstract (summary)

To evaluate the stability of phlogopite in the presence of carbonate in the Earth's mantle, we conducted a series of experiments in the KMAS-HO-CO system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl-mag; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1,150 to 1,550°C. We bracketed the solidus between 1,200 and 1,250°C at pressures of 4, 5 and 6 GPa and between 1,150 and 1,200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus, magnesite is the first phase to react out, and enstatite and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from ~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a cold (40 mWm) subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and can coexist with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite and carbonate at a depth of 180-200 km.

Publication Information

Enggist, A., Chu, L., & Luth, R. (2012). Phase relations of phlogopite with magnesite from 4 to 8 GPa. Contributions To Mineralogy & Petrology, 163(3), 467-481. doi:10.1007/s00410-011-0681-9