Metamorphism of the ordinary chondrites: A review

The activity of chromite in multicomponent spinels: Implications for T‐fO2 conditions of equilibrated H chondrites

“…In addition to temperature, oxygen fugacity (fO 2 ) is a key measure of physical conditions during metamorphism: the presence of Fe-rich metal in ordinary chondrites requires reducing conditions, and increases in the ratio of oxidized to metallic iron from H to L to LL chondrites have long been used to infer corresponding increases in fO 2 (e.g., Fredriksson et al 1968;Dodd 1969). In addition, variations in fO 2 within each chondrite group have been inferred based on systematic changes in the compositions and abundances of FeO-bearing phases (e.g., Fredriksson et al 1968;Dodd 1969;Rubin et al 1988;McSween and Labotka 1993).…”

Thermal metamorphism of the Mócs meteorite (L6) revealed by optical microscopy and BSE imaging

“…The higher temperature effects resulting in such textures were described in detail by van Schmus and Wood (1967), Dodd (1969), and Huss et al (2006). In this process, after obscuring the chondrules, the textural integration of matrix, chondrules, chondrule fragments, and inclusions advances with increasing metamorphic grade.…”

“…The clinopyroxene glass occurs in veins and in melt pockets near veins in the Mócs meteorite (Gyollai et al in press), and formed by post-shock annealing. Olivine is equilibrated by Type 4, but pyroxene equilibration is not completed until Type 5 ( van Schmus and Wood 1967;Dodd 1969). In the most metamorphosed members of Type 6 (classified by some workers as Type 7), the CaO content of low-Ca pyroxene may be somewhat higher (> 1 wt %) than in members that have experienced lower peak temperatures (Dodd et al 1975;Mittlefehldt and Lindstrom 2001).…”

Complex Thermal Histories of L Melt Breccias NWA 5964 and NWA 6580

“…Low shock stage chondrites may still maintain textural and compositional indicators of the thermal metamorphism that occurred while a part of the parent body. Dodd (1969) proposed a layered asteroid model in which higher petrologic grades are from deeper within the body than lower grades, allowing higher grades to cool more slowly as they are partially thermally insulated by the overriding layers. This model, the so-called onion-shell model, is generally accepted in some form for ordinary chondrites, although there is evidence for parent body disruption and re-accretion events (e.g., Hutchison, 2004;Taylor et al, 1987).…”

“…Similar to larger bodies and planets, the material was presumably heated more the deeper it was buried within the planetesimal. This model is known as the onion shell model (Dodd, 1969). The degree of thermal metamorphism is recorded as the petrographic type, with type groupings grading from type 3, minimally altered, to type 6, close to melting and original textures nearly destroyed (Dodd, 1981).…”