Mineral chemistry and genetic relations among H-group chondrites

Abstract

The chondrites Bremervorde and Tieschitz (both H3), Beaver Creek, Menow, Monroe and Quenggouk (all H4), Allegan and Ambapur Nagla (both H5), and Butsura and Kernouve (both H6) were studied. Ca-poor pyroxenes almost invariably show enrichment in Mg relative to coexisting olivines; this was established in Tieschitz chondrules and in Bremervorde by crystal-liquid equilibria. In each chondrite, Ca-poor pyroxenes typically have less than 2 mol % wollastonite. In petrologic types 3 and 4, chondrule mesostases frequently show enrichment in normative diopside, which occurs as a resolvable modal phase in the type 6 chondrites. Partition of Ca between the two pyroxenes was initiated by crystal-liquid fractionation. Various chemical inhomogeneities may be traced from type 3 meteorites to those of type 6. Polycrystalline taenite occurs in Tieschitz, Bremervörde, Menow, Quenggouk and Butsura; it is rarely present in Kernouve. This textural form of taenite is interpreted as a relict solidification structure produced from a quenched liquid, which must have cooled rapidly to below about 700 °C. We argue that equilibration of the H-group chondrites took place during cooling, mainly from about 700 °C, which is compatible with several lines of evidence. If metamorphism is defined as change induced by an increase in temperature, then in the H-group chondrites studied we recognize it only in transient reheating probably produced by shock.