Micro Visions 3.00

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.

One centimeter square polished surface. Packed chondrules and dark matrix. Incident light. NWA 8276 L3.00

Metal in and around a chondrule. Field of view is 3 mm wide. Incident light. NWA 8276 L3.00

A small metal-layered chondrule. Field of view is 3 mm wide. Incident light. NWA 8276 L3.00

Radial pyroxene chondrule with bleached rim indicative of parent body aqueous alteration. FOV = 3 mm wide. Incident light. NWA 8276 L3.00

Same RP chondrule in partially crossed-polarized light. FOV = 3 mm wide. NWA 8276 L3.00

Edge of the same altered RP chondrule in XPL. FOV = 0.3 mm wide. NWA 8276 L3.00

Overview in XPL showing that many chondrules are porphyritic olivine pyroxene chondrules. FOV = 8.6 mm wide. NWA 8276 L3.00

POP chondrule in XPL. FOV = 3 mm wide. NWA 8276 L3.00

POP chondrule in XPL. FOV = 3 mm wide. NWA 8276 L3.00

Polysomatic barred olivine chondrule in XPL. FOV = 3 mm wide. NWA 8276 L3.00

Barred olivine chondrule in XPL. FOV = 3 mm wide. NWA 8276 L3.00

Same barred olivine chondrule in plane-polarized light. No devitrification of the glass between bars is apparent. If the glass had started to crystallize we might have seen fine needles extending from bars into the glass between them. FOV = 0.3 mm wide. NWA 8276 L3.00

Same barred olivine chondrule in XPL. The space between the bars is dark attesting to its glassy state. The softly defined violet zones are places where the bars do not occupy the entire thickness of the thin section sample. The FOV = 0.3 mm wide. NWA 8276 L3.00

About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
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