Abstract

Equilibrium in the system MgO—SiO2—H2O has been determined at temperatures up to 1000°C and at maximum pressures of water vapor varying from 15,000 lbs/in2 at this maximum temperature, to 30,000 lbs/in2 in the range 900°—600°C, and 40,000 lbs/in2 in the range 600°–300°C. Thus were fixed the univariant pressure-temperature curves of the following five reactions: I. serpentine + brucite forsterite + vapor, II. serpentine forsterite + talc + vapor, III. forsterite + talc =; enstatite + vapor, IV. talc enstatite + quartz + vapor, and V. brucite periclase + vapor. Pure magnesian serpentine has a maximum temperature of existence at approximately 500°C, varying only about 10° in the whole range of pressure 2000 to 40,000 lbs/in2. Forsterite is stable in contact with water vapor down to a temperature of about 430°C (at 15,000 lbs/in2). Only below that temperature is it transformed into serpentine and brucite. Iron-bearing olivines are stable in contact with water vapor down to still lower temperatures.

No liquid is formed in any composition of the system throughout the range of temperatures and pressures at which experiments were conducted, a condition which remains unchanged when the mixtures have upwards of 7 per cent FeO.

There is consequently no likelihood that any magma can exist that can be called a serpentine magma and certainly no possibility of its existence below 1000°C. There seems no escape from the conclusion that ultramafics can be intruded only in the solid state.

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