EVAPORATION OF OLIVINE - LOW-PRESSURE PHASE-RELATIONS OF THE OLIVINE SYSTEM AND ITS IMPLICATION FOR THE ORIGIN OF CHONDRITIC COMPONENTS IN THE SOLAR NEBULA

Low-pressure phase relations and vapor pressures of forsterite and fay alite were studied with the Knudsen method in the temperature range of 1400-1860-degrees-C for forsterite and 1100-1160-degrees-C for fayali te. The triple points are 5.2 X 10(-5) bar (1890-degrees-C) and 6.3 X 10(-8) bar (1217-degrees-C) for forsterite and fayalite, respectively. The enthalpy and entropy of evaporation of forsterite are DELTAH(v) = 543 +/- 33 kJ /mol and DELTAS(v) = 169 +/- 21 J/(mol . K), and those for fayalite are 502 +/- 9 kJ/mol and 199 +/- 6 J/(mol . K), respectiv ely. By assuming that gas and olivine are ideal solutions, vaporous an d solidus curves of the olivine solid solution system in the pressure range from 10(10) to 10(-3) bar were calculated from the thermochemica l data. At pressures above the triple point of forsterite, the phase d iagram is the same as that at 1 bar, because the vapor field is at hig her temperatures than those of liquid-bearing fields. At pressures bet ween the triple points of forsterite and fayalite, liquid-bearing stab ility fields appear below the gas-solid stability field in the Fe-rich portion of the system. The compositional range of liquid-bearing fiel ds expand with increasing pressure; at 10(-4) bar, the liquid-bearing fields cover most of the olivine system (X(Mg) < 0.9). Liquid is absen t at pressures below the triple point of fayalite, regardless of the c omposition. Large Mg/Fe fractionation between gas and solid is more ex tensive, compared to the solid and liquid relationship. When heated in equilibrium, olivine melts before evaporation in the pressure range o f 10(-7)-10(-3) bar. In a solar nebula with total preSSure of 10(-3) b ar, olivine melts when the nebula is enriched in dust by more than an order of magnitude over the solar nebular value and when the (Mg + Si + Fe)/H ratio increased. The origin of type IA and II chondrules and m atrix olivine in ordinary and carbonaceous chondrites can be explained in terms of the pressure of olivine gas. Type IA chondrules were form ed in the gas-solid phase field where volatiles (FeO, Na2O, and others ) were lost during heating events responsible for chondrule formation. The residual materials became enriched in refractory elements. Type I I chondrules were formed in the temperature range of the liquid-solid phase field. Under these conditions, the original compositions were re tained after heating of chondrule formation. Provided that type IA and II chondrules were formed at a similar temperature, type IA chondrule s were formed at an olivine pressure one order of magnitude lower than that of type II chondrules. In this model, matrix olivine is a conden sate from gas which formed by partial evaporation of chondritic materi al during type IA chondrule formation.