FROM METAMORPHISM IN NEW ENGLAND TO RED-OX REACTIONS IN THE SOLAR NEBULA: A LONG AND WINDING ROAD THROUGH REACTION SPACE

Introduction: The reaction space approach was developed to understand changes in abundances and compositions of minerals in rocks undergoing metamorphism and was initially applied to mafic rocks in New England [1]. In this approach, (1) phase compositions are re-written using a set of components chosen so that reactions causing changes in mineral abundances can be considered separately from changes in mineral compositions, and (2) the bulk rock reaction is expressed in terms of progress along axes in an algebraic space. This more precise understanding of the effects on a reacting system can be used in turn to lead to a better understanding of the causes of reactions.

Model system: Oxidation, reduction and sulfidation during formation of chondritic meteorites in the solar nebula were evaluated using reaction space. The main minerals of ordinary (OCs), enstatite (ECs) and R-chondrites (RCs) include olivine, low-Ca and high-Ca pyroxenes, feldspar, troilite and Fe,Ni-metal. Additional sulfides occur in RCs and ECs. O- and S-bearing gases were available to react with solids in the solar nebula.

All transfers of mass between the silicate, sulfide and metal subsystems in ECs, OCs and RCs can be described as progress along two reactions: (R_m) Mg₂SiO₄ + 2 FeMg_-1 = 2 Fe + SiO₂ + O₂; and (R_s) Mg₂SiO₄ + 2 FeMg_-1 + S₂ = 2 FeS + SiO₂ +O₂. Increasing reduction is indicated by progress on R_m, increasing sulfidation by progress on R_s, whereas oxidizing conditions are indicated by minor progress on both reactions.

Results: Reaction progress on R_m and R_s in chondrites were determined by speciation of Fe between silicates, sulfides and metals in three data sets: (1) wet chemical analyses reported from the Smithsonian Museum [2], and (2) from the National Institute of Polar Research-Japan [3]; (3) modes and electron microprobe data collected at Waseda University. All three data sets show wide ranges in progress on R_m and only minor variations on R_s, suggesting rather constant abundances of S and wide variations in redox, possibly due to variations in H₂O ice in the solar nebula.

References: [1] Thompson J.B. et al (1982) JPetrol 23:1-27. [2] Jarosewich E. (2006) MaPS 41:1381-1382. [3] Yanai K. and Kojima H. (1995) Catalog of the Antarctic Meteorites, NIPR, 230 p.