Sodium-rich metasomatism in the upper mantle: implications of experiments on the pyrolite-Na/sub 2/O-rich fluid system at 950/sup 0/C, 20 kbar

Journal Article · Tue Sep 01 04:00:00 EDT 1987 · Geochim. Cosmochim. Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(87)90281-X· OSTI ID:5100184

McNeil, A M; Edgar, A D

Sodium-rich metasomatism in the upper levels of the mantle has been modeled by reacting pyrolite with alkali-bearing H/sub 2/O fluids containing minor CO/sub 2/ and concentrations of Na/sub 2/O and Na/sub 2/O + K/sub 2/O (K/K + Na = 0.1) up to 4.0 g alkalies/10 g H/sub 2/O at 20 kbar and 950/sup 0/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) + Si in equilibrium Al + Ca. These results and least squares mass balance calculations suggest the reaction of clinopyroxene + orthopyroxene + spinel produces amphibole + olivine. The present study suggest that up to 43 wt.% amphibole may be accommodated in pyrolite in the presence of Na/sub 2/O-rich H/sub 2/O-CO/sub 2/ fluids. This represents 0.8 wt.% H/sub 2/O and 1.7 wt.% Na/sub 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/sup 0/C over the H/sub 2/O-saturated pyrolite system at 20 kbar.

• Research Organization: Univ. of Western Ontario, London (Canada)
• OSTI ID: 5100184
• Journal Information: Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:9; ISSN GCACA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
ALKALI METALS
AMPHIBOLE
CHEMICAL COMPOSITION
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISTRY
EARTH MANTLE
ELEMENTS
FLUIDS
GEOCHEMISTRY
GEOTHERMAL FLUIDS
HYDROTHERMAL ALTERATION
KINETICS
MASS BALANCE
METALS
MINERALOGY
MINERALS
OXYGEN COMPOUNDS
PARAGENESIS
PYROXENES
REACTION KINETICS
SILICATES
SILICON COMPOUNDS
SODIUM