SOLID-SOLID REACTIONS MEDIATED BY A GAS-PHASE - AN EXPERIMENTAL-STUDYOF REACTION PROGRESS AND THE ROLE OF SURFACES IN THE SYSTEM OLIVINE+IRON METAL

The intergranular fluid involved in solid-solid reactions is tacitly a ssumed to be a melt or a (C-O-H-S-Cl-F)-bearing phase. We have studied the system olivine+metal using diffusion couple experiments, in situ reaction progress monitoring using Knudsen-cell mass spectrometry, and thermodynamic-kinetic analysis to show that a dry vapor phase coexist ing with solids (silicates, oxides, metals) has all the characteristic s of a classical petrologic ''intergranular fluid,'' and it is a viabl e transport agent for major rock-forming elements such as Mg, Fe, or S i in many petrologic situations. Some of the major conclusions of the work are: (1) ignoring the vapor phase leads to incorrect estimation o f degrees of freedom and consequently, incorrect interpretations of mi neral assemblages and zonation; (2) normally refractory elements such as Mg may in some cases be more volatile than O-2; and (3) reaction mo deling using free-energy minimization allows the main parameters contr olling reaction progress, pathway, and products (assemblage, abundance of phases, and composition) to be identified. These parameters includ e: available reactive surface area; volume of the reaction system; dif fusion rates in the product solid; temperature; and relative rates of reaction to transport (in/out of the system). Components other than th ose appearing explicitly in the mass-balance equations (e.g., f(O2) in the olivine+metal system) may play an important role. Transport of Mg in the vapor phase away from local reaction sites explains the compos itional zoning of olivine around FeNi-metal inclusions and simultaneou sly provides a mechanism for the growth of at least some of the fayali te-rich rims in Allende and other meteorites of the CV3-class. Similar considerations may play a role in terrestrial problems where metal an d silicate coexist, e.g., the primitive terrestrial magma ocean and th e ''D'' layer.