Other
Scientific paper
Oct 1972
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1972gecoa..36.1101p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 36, Issue 10, pp.1101-1113
Other
4
Scientific paper
The quench pH of MgCl 2 solutions equilibrated at 500°C and 1 kbar pressure with talc + quartz is between 0.9 and 1.6, that of solutions equilibrated with forsterite + talc 4.2 to 7.7, and that of solutions equilibrated with brucite + forsterite between 6 and 9. The addition of NaCl and CaCl 2 does not affect the quench pH measurably. The low quench pH of solutions equilibrated with talc + quartz is due to the ionization of HCl O during the quench. The quench pH of solutions equilibrated with forsterite + talc is apparently controlled by the reaction of Mg-OH complexes with HCl O during cooling, that of brucite + forsterite solutions by the precipitation of magnesium hydroxychlorides and/or brucite during cooling. At 500°C and 1 kbar the solutions are neutral to slightly acid. The low quench pH of solutions equilibrated with talc + quartz shows that these can give rise to extensive proton metasomatism with wall rocks during cooling. This is not the case for solutions equilibrated at 500°C and 1 kbar with forsterite + talc or brucite + forsterite, and explains the lack of proton metasomatism around ultrabasic bodies. The SiO 2 concentration in solutions equilibrated with forsterite + talc is sufficient to lead to the precipitation of quartz on cooling. This may explain the occurrence of quartz with some ultramafic bodies. Mg 2 (OH) 3 Cl appears to be a stable phase in equilibrium with brucite + forsterite in runs containing more than 0.1 mole MgCl 2 /kg of solution. Much of the chlorine in altered ultramafic bodies may be present as a component of this and/or other magnesium hydroxychlorides.
Borcsik Maria
Holland Heineich D.
Poty Bernard
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