David H. Eggler

Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridotite: petrologic constraints on the asthenosphere

Abstract The subsolidus mineralogy and the isobarically-invariant solidus of peridotite in the presence of small amounts of H 2 O and CO 2 have been determined from 13.5 to 26 kbar pressure. The composition of fluid buffered by amphibole peridotite, below 17.5 kbar, is CO 2 -rich; amphibole melts completely, although incongruently, near the solidus, contributing materially to the nephelinitic character of the liquids. Fluid-absent amphibole peridotite also melts to nephelinitic liquid. Between 17.5 kbar and 22 kbar, amphibole-carbonate peridotite, without fluid, melts to alkalic, very silica-undersaturated liquid. Above 22 kbar, fluid buffered by carbonate peridotite, with accessory phlogopite, is H 2 O-rich. Quenched liquids could not be analyzed, but they are inferred to be carbonate-rich by analogy with synthetic systems. Buffering of f O 2 by graphite-carbonate-silicates maintains these phase relations if graphite is present. The subsolidus change of peridotite carbonate mineralogy from low-pressure dolomite to high-pressure magnesite has been bracketed at 975°C, 28 kbar. Suboceanic thermal models are consistent with the existence of a relatively deep, partially-molten asthenosphere even beneath old (180 m.y.) lithosphere, but melts should be exclusively carbonate-rich. Beneath younger lithosphere, a shallower zone should change, with increasing depth, from nephelinitic to carbonate-rich. Amounts of liquid are proportional to amounts of volatiles present, because amphibole, phlogopite, and carbonates melt quantitatively at or close to the solidus; in particular, 0.1% water in amphibole peridotite represents about 5% nephelinitic melt.

Mantle Redox Evolution and the Oxidation State of the Archean Atmosphere

Current models predict that the early atmosphere consisted mostly of

Fluids in equilibrium with peridotite minerals: Implications for mantle metasomatism

CO_{2}, N_{2}

Fractionation paths of Atka (Aleutians) high-alumina basalts: Constraints from phase relations

, and

Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana

H_{2}O

Partitioning of elements between silicate melt and H2ONaCl fluids at 1.5 and 2.0 GPa pressure: Implications for mantle metasomatism

, along with traces of