Abstract
Clinopyroxenes in Apollo 15 quartz-normative basalts (QNBs) are chemically complex recorders of thermal history. Texturally distinctive augite rims (Wo30En42) mantle the clinopyroxene phenocrysts and contain submicroscopic exsolution lamellae of pigeonite and augite near the (001) and (100) orientations. The size and morphologic variations of exsolution textures were characterized using the transmission electron microscope (TEM) in cooling rate experiments (cooled over the range 0.21° to 150°C/hr) on a synthetic QNB composition and in lunar samples 15597, 15499 and 15058. The size variation of (001) exsolution lamellae is used to estimate the sample’s thermal history. The lamellae coarsen over the temperature interval 1100°–800°C, record cooling rate over this temperature range and may be used as cooling rate speedometers. Augite–pigeonite intergrowths form by spinodal decomposition, homogeneous nucleation, and heterogeneous nucleation. Coarsening of the lamellae produced in the cooling rate experiments may be described by the rate law:
where λ° = 70Å, k = 107.3 and t is time in days. Thermal histories of lunar basalts 15597, 15499 and 15058 estimated with the lamellae coarsening speedometer are comparable to those estimated from grain size.
In the Apollo 15 QNBs three lamellar orientations dominate; (100) and two sets of (001). Heterogeneously nucleated (100) and (001) lamellae grow with C2/c symmetry, but the homogeneously nucleated (001) lamellae grow as coexisting C2/c and P21/c. All orientations form over a restricted temperature (1100–1066°C) but have different orientations as a consequence of the change in cell dimensions that occurs during the C2/c to P21/c transition.
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