IRON SPECIATION IN E, O, AND R CHONDRITES AS AN INDICATOR OF OXIDATION, REDUCTION AND SULFIDATION IN THE SOLAR NEBULA

Introduction: Different crystallization conditions in the solar nebula and in primitive parent bodies of chondritic meteorites are reflected in part by speciation of iron between silicates and opaque phases. McSween [1] recognized that some CV chondrites had relatively high Fe# (Fe/[Fe+Mg]) in mafic silicates and low abundances of Fe,Ni-rich metal, indicating formation at high f(O₂), whereas other CV chondrites formed under reducing conditions. Likewise, silicate Fe#’s increase from E to H to L to LL to R chondrites, suggesting a shift from low to high f(O₂) in the same sequence [2,3]. Yet, Fe occurs in sulfide minerals as well as silicates and metals, indicating that chondrites might have undergone variable sulfidation. The goal of this study is to examine variations in sulfidation as well as redox in enstatite, ordinary and Rumuruti-like chondrites.

Model system: A reaction space approach [4] was used to identify the main reactions possible between minerals in E, O and R chondrites. All transfers of mass between the silicate, sulfide and metal subsystems 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 [5], and (2) from the National Institute of Polar Research-Japan [6]; (3) modal recombination of electron microprobe data collected at Waseda University for seven chondrites. 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 (due to variations in H₂O?) in the part of the solar nebula where E, O and R chondrites formed. References: [1] McSween H.Y. (1977) GCA 41:1777-1790. [2] Van Schmus W.R. and Wood J.A. (1967) GCA 31:747-765. [3] Weisberg M.K. et al (2006) in MESS II, p. 19-52. [4] Thompson J.B. et al (1982) JPetrol 23:1-27. [5] Jarosewich E. (2006) MaPS 41:1381-1382. [6] Yanai K. and Kojima H. (1995) Catalog of the Antarctic Meteorites, NIPR, 230 p.