EVAPORATION OF OLIVINE - LOW-PRESSURE PHASE-RELATIONS OF THE OLIVINE SYSTEM AND ITS IMPLICATION FOR THE ORIGIN OF CHONDRITIC COMPONENTS IN THE SOLAR NEBULA
H. Nagahara et al., EVAPORATION OF OLIVINE - LOW-PRESSURE PHASE-RELATIONS OF THE OLIVINE SYSTEM AND ITS IMPLICATION FOR THE ORIGIN OF CHONDRITIC COMPONENTS IN THE SOLAR NEBULA, Geochimica et cosmochimica acta, 58(8), 1994, pp. 1951-1963
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.