Kamacite and olivine in ordinary chondrites: Intergroup and intragroup relationships

Abstract

A suite of 134 ordinary chondrites (OCs) was analyzed by electron microprobe to determine olivine and kamacite compositions. Equilibrated members of the three main OC groups have the following ranges of olivine Fa and kamacite Co: H (17.3–20.2 mol% Fa; 4.4–5.1 mg/g Co); L (23.0–25.8 mol% Fa; 7.0–9.5 mg/g Co); LL (26.6–32.4 mol% Fa; 14.2–370 mg/g Co). However, the high-Co (200–370 mg/g Co) metal phase in highly oxidized LL chondrites is probably not kamacite. Group ranges of kamacite Ni content overlap, but mean Ni values of equilibrated OCs decrease from H (69.2 mg/g) to L (65.4 mg/ g) to LL (49.8 mg/g). The concomitant increase in kamacite Co and decrease in kamacite Ni in the H-L-LL sequence is probably due to a combination of two effects: 1. (1) the decrease in kamacite grain size from H through LL (the smaller LL grains reach equilibrium faster and thereby acquire lower Ni contents), and 2. (2) the greater oxidizability of Fe relative to Co (kamacite grains thus become richer in Co as the oxidation state increases from H through LL). There are significant intragroup differences in olivine and kamacite composition: type-3 and type-4 OCs tend to have lower olivine Fa and kamacite Co and Ni values than type-6 OCs. The lower Fa in H-L-LL3 chondrites may be due either to the lack of equilibration between fine FeO-rich and coarse FeOpoor silicates in type-3 OCs, or to the acquisition of less FeO-rich material (possibly type-II chondrules or fine-grained matrix material) by type-3 OCs during agglomeration. The lower kamacite Co and Ni contents of H-L-LL3 chondrites may reflect the presence in type-3 OCs of a relict nebular metal component with positively correlated Co and Ni; such a component has previously been inferred for the primitive, ungrouped chondrites, Al Rais, Renazzo, and ALH85085. Aberrant olivine and/or kamacite grains with compositions significantly different than the majority in the whole-rock occur in at least half and probably virtually all equilibrated OCs. Aberrant kamacite grains are not uniformly distributed among the OCs; they are more common in L and LL than in H chondrites. Chondrites containing aberrant grains are fragmental breccias that were brecciated after cooling from high metamorphic temperatures. The different OC parent bodies accreted at different heliocentric distances and acquired distinct bulk and mineralogical compositions. It is probable that more than three OC parent bodies were formed: Netschaevo is from an OC body more reduced than H chondrites; Tieschitz and Bremervörde may be from an OC body intermediate in its properties between H and L chondrites; and ten other OCs may be from a body intermediate between L and LL. Within each individual OC group, there are many chondrites with non-overlapping olivine compositional distributions. These meteorites clearly have not equilibrated with each other; they may have agglomerated at opposite extremes of the parent body's accretion zone. Their existence is consistent with maximum metamorphic temperatures having occurred in small planetesimals prior to the accretion of large asteroids.