3.00 is a rarely given petrologic grade assigned to meteorites which experienced the lowest levels of thermal alteration on the parent body. (Aqueous alteration is another matter.) NWA 8276 L3.00 was assigned this grade based on laboratory tests and a study by Grossman and Brearley published in 2005.
Among many findings the study’s authors showed that at the onset of thermal metamorphism the average chromium content of iron rich olivine grains in chondrules was relatively high. As metamorphism proceeded those levels receded. And, at the same time, the variance among the values making up those averages changed – starting with a narrow variance, widening then narrowing again with advancing metamorphism.
Combining these two characteristics, Grossman and Brearley presented a scheme for classification of very low petrologic grade chondrites. See this graphically on Grossman and Brearley (2005) page 113, Fig. 15. These characteristics and hence the method has the advantage of being resistant to the effects of parent body aqueous alteration and terrestrial weathering.
The paper presents numerous other thermal metamorphism correlated phenomena including changes in core to rim Cr zoning in ferroan olivine grains, the development of distinct chromite inclusions, the migration of troilite and the expulsion of sulfur from fine chondrite matrix.
Most of this is invisible to our optical microscope but we are still able to enjoy this near-pristine meteorite in thin section.
