Lunar thin section 12002
This low-Ti basalt is typical of the younger mare lavas. High-Ti lavas (9 to 13% TiO2) are generally confined to the period 3.85 to 3.55 Ga, whereas the low-Ti lavas (1 to 5% TiO2) are most common in the period 3.45-3.15 Ga (Meyer, 1987). This basalt is thought to have been derived from an olivine-pyroxene source rock at a depth of ~300 km. The sample was collected from the southeastern end of Mare Procellarum by the Apollo 12 mission.
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This rock contains mostly brownish elongate pyroxene, colorless lath-shaped plagioclase, colorless blocky olivine, and opaques. The abundance of elongate pyroxene and plagioclase suggests rapid cooling. In cross-polarized light the elongate, colorful minerals are mostly pyroxene, and the blocky ones mostly olivine. They can actually be distinguished better by color in plane, where the olivine is colorless and pyroxene is brownish. Field width is 8 mm. Views in plane and cross-polarized light. |
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Close-up of a some skeletal olivine crystals encased in masses of pyroxene and plagioclase. In the cross-polarized light view the skeletal olivines include the one with pink-and-orange birefringence in the center, and the one with bluish birefringence in the upper right, in which four masses of melt were trapped. Notice how the ends of the skeletal arms are rounded, suggesting partial resorption following skeletal growth. Field width is 4 mm. Views in plane and cross-polarized light. |
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Close-up of an elongate pyroxene, just left of center, and a skeletal olivine crystal to the right of center. Notice how the brown color of the pyroxenes in plane-polarized light becomes darker toward the margins. This change follows increasing iron and titanium contents near the pyroxene rims. The chemical zoning in the pyroxenes and olivines changes their birefringence (both minerals) and optic orientation (pyroxene only). This causes rims of these minerals to be a different birefringent color than the cores. Field width is 4 mm. Views in plane and cross-polarized light. |
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Close-up of radiating bundles of pyroxene in the matrix, suggestive of high cooling rates. Pyroxene interference color zoning is clearly seen here. Field width is 1.6 mm. Views in plane and cross-polarized light. |
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Chromite occurs in this sample, suggesting it is one of the more primitive lavas. In olivine, chromite is pristine. In the matrix, chromite is overgrown (partially replaced by?) ulvöspinel. In this image, the dark gray shades are silicates. The medium pinkish-gray laths are ilmenite, the light gray in the center of grains is chromite, the pinkish-brown overgrowths on chromite are ulvöspinel, the creamy brown bits are troilite, and the bright white grains are iron. Notice how the chromite grain embedded in olivine (right) is euhedral and not overgrown with ulvöspinel. Field width is 0.8 mm. |
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Close-up of a chromite overgrown (replaced?) by ulvöspinel. Colors are as above. Notice the large bright white native iron grain, and the small specks of iron in the triangular troilite grain toward the lower right. Field width is 0.32 mm. |
