Andreas Enggist

Abstract: 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–H2O–CO2 system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl90-mag10; 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 CO2-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−2) 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.

Abstract: To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg2SiO4–MgSiO3–H2O and Mg2SiO4–MgSiO3–KCl–H2O systems at 5 GPa. In the Mg2SiO4–MgSiO3–H2O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg2SiO4–MgSiO3–KCl–H2O systems with molar Cl/(Cl + H2O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C, and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H2O in the fluid in the Mg2SiO4–MgSiO3–H2O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system.

Abstract:

Abstract: First results in the phlogopite + magnesite (KMASH-CO2) system demonstrate that a potassium-bearing fluid will be the metasomatic agent at sub-continental-lithospheric-mantle conditions with a continental geotherm of 40 mWm-2. In this case, phlogopite can be stable to a depth of 200 km in the presence of carbonate, and will coexist with potassic fluids. Assuming a hotter geotherm of 44 mWm-2, these fluids can be present to a depth of about 180 km. Beyond this depth, at the base of a thick sub-continental lithospheric mantle, a hydrous, potassium- and CO2-rich silicate melt would be the metasomatic agent. In this system, garnet is present above solidus as a residual phase, which implies that a K-CO2-H2O-enriched metasomatic fluid or melt could react with garnet peridotite to form phlogopite.

Abstract: Previously, we reported that in the phlogopite + magnesite system, phlogopite can be stable to a depth of 200 km in the presence of carbonate at a cooler lithospheric mantle geotherm of 40 mWm-2. At the base of a hotter sub-continental lithospheric mantle (SCLM), phlogopite and magnesite will react to form enstatite, olivine, garnet and hydrous, potassiumand CO2-rich melt [1, 2]. Here we present first results of phlogopite + diopside + enstatite ± carbonate. KCMASH In the presence of pyroxenes, phlogopite is stable to 1300°C and 5.5 GPa and starts to break down at 6 GPa; amphibole becomes stable and coexists with remaining phlogopite to higher pressures, which is in agreement with [3]. The solidus is located at 1400 and 1350°C at 5 and 4 GPa, respectively, where phlogopite reacts out over a temperature range of about 50°C and garnet, enstatite, diopside, olivine and melt is in equilibrium. KCMASH-CO2 Adding carbonate to the pyroxene-bearing system lowers the thermal stability of phlogopite considerably: Phlogopite starts to react out at 4 GPa and ~950°C. An experiment at 6 GPa and 1000°C still contains phlogopite, which may reflect the sluggish kinetics of the breakdown reaction. Above the solidus, melt coexists with garnet, enstatite, diopside, and olivine. Melt quenches to amphibole and phlogopite of around 10 &m in size. KCMASH±CO2 vs. KMASH-CO2 No hydrous solution was seen escaping from the capsules upon breach. Amphibole is the new phase occurring, either primary, to higher pressures, or, above solidus, as an additional quench product. First results indicate that in KCMASH-CO2 and at SCLM conditions, phlogopite is stable to about 4.5 and < 4 GPa with a 40 and 44 mWm-2 geotherm, respectively. [1] Enggist et al. (2009) Eos Trans. AGU 90(52) Abstract V51B-1669. [2] Enggist & Luth (2010) GeoCanada2010, expanded Abstract, 4p. [3] Sudo & Tatsumi (1990) Geophys. Res. Lett. 17(1), 29–32.

Abstract: We are investigating the stability of phlogopite in presence of carbonates at the conditions of the Earth’s upper mantle (4 - 8 GPa; 1050 -1550°C) to evaluate the potential of phlogopite as a source for fluids or liquids that will metasomatise mantle rocks. To date, the stability of phlogopite in carbonated peridotite remains unresolved. It was proposed that phlogopite would break down above 5 GPa (≈ 160 km in depth) following the reaction phlogopite + enstatite + magnesite = olivine + garnet + alkali- and water-rich melt [1], whereas another study suggested a reaction already at lower pressures of about 4 GPa (≈ 130 km in depth) to olivine + enstatite + garnet + alkali- and water-rich melt [2]. We present initial results (4 - 6 GPa) in the simple phlogopite + magnesite system. At 4 GPa and sub-solidus temperatures phlogopite breaks down to a non-stoichiometric phlogopite that is lower in K2O, SiO2 and richer in Al2O3 relative to the ideal composition, plus pyrope and highly potassic fluid, coexisting with magnesite. Magnesite remains stable to 1250 and 1200°C at 4 and 6 GPa, respectively, where enstatite and forsterite first appear. A significant change in texture is observed at what we interpret as the solidus at temperatures of 1350 and 1300°C at 4 and 6 GPa, respectively, with the appearance of loose and elongated skeletal crystals of phlogopite (50 - 300 μm in length) in the hot part of the capsule, along with the disappearance of smaller eu- to subhedral primary phlogopite. The skeletal crystals are interpreted to be precipitated upon the quench from the liquid. This quench phlogopite tends to be richer in K2O and SiO2 and poorer in Al2O3 compared to primary but the best criterion to distinguish between them is texture. Above the solidus, forsterite and pyrope (+ spinel at low pressures) are present as residual phases. Our results show that phlogopite can be stable to a depth of 200 km in the presence of carbonate, and will coexist with potassic fluids at sub-continental-lithospheric-mantle conditions. The presence of garnet as a residual phase in these experiments implies that a K-CO2-H2O-enriched metasomatic fluid or liquid infiltrating garnet peridotite could react with garnet to form phlogopite. [1] Wendlandt and Eggler, 1980, American Journal of Science, v. 280, p. 421-458. [2] Ulmer and Sweeney, 2002, Geochimica et Cosmochimica Acta, v. 66, p. 2139-2153. DE: [3612] MINERALOGY AND PETROLOGY / Reactions and phase equilibria

Abstract: Chlorine has been found as a major constituent of fluid inclusions in fibrous diamonds (Navon et al., 1988) and has been interpreted to associate with K to form a KCl-bearing brine (e.g., Klein-BenDavid et al., 2006). To examine the effect of such a KCl-bearing brine on the melting behavior of the Earth’s mantle, we conducted experiments in the Mg2SiO4-MgSiO3-H2O and Mg2SiO4-MgSiO3-KCl-H2O systems at 5 GPa and 1100-1700°C. In the Mg2SiO4-MgSiO3-H2O system, the temperature of the solidus is ~1230°C, and both forsterite and enstatite coexist with the liquid under supersolidus conditions. In the Mg2SiO4-MgSiO3-KCl-H2O systems with molar Cl/(Cl+H2O) ratios of 0.2, 0.4 and 0.6, the temperatures of the solidus are ~1430°C, ~1530°C and ~1580°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. The increase in the temperature of the solidus demonstrates the significant effect of KCl on elevating the solidus of the Mg2SiO4-MgSiO3-H2O system by reducing the activity of H2O in the fluid. If KCl is present in the Earth’s mantle, it will prevent melting at the H2O-saturated solidus, and the KCl-bearing brine will be a robust agent for mantle metasomatism at temperatures greater than that of the H2O-saturated solidus. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich melt) could happen at pressures over 5 GPa in KCl-bearing systems, which needs to be verified by experiments on the MgSiO3-KCl-H2O system in future work. Klein-BenDavid, O., Richard, W. and Navon, O., 2006, TEM imaging and analysis of microinclusions in diamonds; a close look at diamond-growing fluids: American Mineralogist, v. 91, p. 353-365. Navon, O., Hutcheon, I.D., Rossman, G.R. and Wasserburg, G.J., 1988, Mantle-derived fluids in diamond micro-inclusions: Nature, v. 335, p. 784-789.

Abstract: 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.