Sodium-rich metasomatism in the upper mantle: Implications of experiments on the pyrolite-Na 2 O-rich fluid system at 950°C, 20 kbar

Details Sodium-rich metasomatism in the upper mantle: Implications of experiments on the pyrolite-Na 2 O-rich fluid system at 950°C, 20 kbar Sodium-rich metasomatism in the upper mantle: Implications of experiments on the pyrolite-Na 2 O-rich fluid system at 950°C, 20 kbar

Sep 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.2285m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 51, Issue 9, pp.2285-2294

Computer Science

2

Scientific paper

Sodium-rich metasomatism in the upper levels of the mantle has been modelled by reacting pyrolite with alkali-bearing H 2 O fluids containing minor CO 2 and concentrations of Na 2 O and Na 2 O + K 2 O (K/K + Na = 0.1 ) up to 4.0 g alkalies/10 g H 2 O at 20 kbar and 950°C. With increasing alkali concentration, the amounts of amphibole (pargasite-edenite) and olivine increase as orthopyroxene and clinopyroxene decrease. Amphiboles show progressive increases in Na (and K) and Si concentrations and decreases in Al and Ca concentrations suggesting the dominant substitution mechanism is (Na, K) + SiAl + Ca. These results and least squares mass balance calculations suggest the reaction of clinopyroxene + orthopyroxene + spinel produces amphibole + olivine. In nature, upper mantle spinel lherzolite is commonly veined by a variety of rock types which may contain Ti-pargasite as a magmatic crystallization product. Pargasite-edenite occurs interstitially in spinel Iherzolite, often spatially related to Ti-pargasite and may be produced by hydrous fluids evolved during late stage crystallization of the veined rocks. This is supported by the close compositional correlation between the natural pargasite-edenite amphiboles and those produced in this study. The present study suggests that up to 43 wt.% amphibole may be accommodated in pyrolite in the presence of Na 2 O-rich H 2 O-CO 2 fluids. This represents 0.8 wt.% H 2 O and 1.7 wt.% Na 2 O in the hydrated pyrolite composition and indicates the importance of sodium in determining the extent of metasomatism. Sodium also lowers the solidus temperature of pyrolite by more than 50°C over the H 2 O-saturated pyrolite system at 20 kbar.

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