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On the Origin of Porphyritic Chondrules
Milton Blander, Laura Unger2, Arthur Pelton3 and Gunnar Ericksson3
'Argonne National Laboratory, Argonne, IL 60439-4837
2Purdue University-North Central, Westville, IN 46391
3Ecole Polytechnique, Montreal, Quebec, Canada H3C 3A7
Abstract
A unique computer program for the calculation of the complex equilibria in a
cooling nebular gas has been used to explore a possible origin of porphyritic chondrules,
(the major class of chondrules in chondritic meteorites) as well as other classes of
chondrules. A keystone of the calculation is a method of accurately calculating the
thermodynamic properties of molten multicomponent aluminosilicates which allows one
to deduce the compositions of silicate condensates as functions of temperature and
pressure of a nebular gas. This computer program is coupled with a chemical equilibrium
algorithm for the calculation of complex equilibria in multicomponent-multiphase systems
with at least 1000 chemical species and with a data base of over 5000 solid, liquid and
gaseous species.
In addition, we have considered the effect of two major predicted nucleation
constraints on the formation of porphyritic chondrules; these are (a) a large barrier to the
formation of condensed iron alloys which leads to the supersaturation of iron in the vapor.
This results in the incorporation of much more iron oxide in the silicates than one has at
equilibrium and (b) a barrier to the condensation of solid silicates which leads to the
condensation of metastable subcooled liquid silicates which ultimately form chondrules.
Constrained equilibrium calculations for a gas containing H, C, O, Si, S, Fe, Ca,
Na, Al and Mg in solar proportions indicate that types IA and 11 porphyritic chondrules
