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

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

Aug 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..64..305o&link_type=abstract

Earth and Planetary Science Letters, Volume 64, Issue 2, p. 305-315.

Computer Science

108

Scientific paper

The subsolidus mineralogy and the isobarically-invariant solidus of peridotite in the presence of small amounts of H2O and CO2 have been determined from 13.5 to 26 kbar pressure. The composition of fluid buffered by amphibole peridotite, below 17.5 kbar, is CO2-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 H2O-rich. Quenched liquids could not be analyzed, but they are inferred to be carbonate-rich by analogy with synthetic systems. Buffering of fO2 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.

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