The ordinary chondrites (OCs) comprise three petrographic series, each ranging from type 3 (the UOCs), with distinct chondrules, little crystalline intergrowth and unequilibrated minerals, to type 6 or 7, in which chondrules have largely disappeared into a granular textured fabric with little mineralogical disequilibrium. There are conflicting interpretations of the textural progression. Many believe that the OCs formed by accretion of 'cold' silicate chondrules, chondrule fragments, metal, sulphide and fine-grained material, followed by thermal metamorphism. A minority argues that the constituents accreted 'hot' and that the textural progression resulted from different cooling rates; type 3 cooled rapidly, types 6 and 7 cooled slowly. Hot accretion has been attacked by Haack et al., from the premise that chondrules are nebular, and by Rubin, who suggests that OCs are 'brecciated on millimeter-sized scales' to explain the presence of grains that do not fit the metamorphic model. Some OCs are not post-crystallisation breccias and contain mineralogical evidence that is incompatible with cold accretion and prograde metamorphism.
Lunar and Planetary Science Conference
- Pub Date:
- March 1996
- ACCRETION: HOT;