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Subcalcic diopsides from kimberlites: Chemistry,exsolution microstructures,and thermal history
Authors:Robert H. McCallister  Gordon L. Nord Jr.
Affiliation:(1) Department of Geosciences, Purdue University, 47907 West Lafayette, IN, USA;(2) U.S. Geological Survey, 959 National Center, 22092 Reston, VA, USA
Abstract:Twenty-six subcalcic diopside megacrysts (Ca/(Ca+ Mg)) = 0.280–0.349, containing approximately 10 mol% jadeite, from 15 kimberlite bodies in South Africa, Botswana, Tanzania, and Lesotho, have been characterized by electron microprobe analysis, X-ray-precession photography, and transmission electron microscopy. Significant exsolution of pigeonite was observed only in those samples for which Ca/(Ca+Mg)lE0.320. The exsolution microstructure consists of coherent (001) lamellae with wavelengths ranging from 20 to 31 nm and compositional differences between the hosts and lamellae ranging from 10 to 30 mol% wollastonite. These observations suggest that the exsolution reaction mechanism was spinodal decomposition and that the megacrysts have been quenched at various stages of completion of the decomposition process.Annealing experiments in evacuated SiO2 glass tubes at 1,150° C for 128 hours failed to homogenize microstructure, whereas, at 5 kbar and 1,150° C for only 7.25 hours, the two lattices were homogenized. This ldquopressure effectrdquo suggests that spinodal decomposition in the kimberlitic subcalcic diopside megacrysts can only occur at depths less than sim15 km; the cause of the effect may be the jadeite component in the pyroxene. ldquoApparent quenchrdquo temperatures for the exsolution process in the megacrysts range from 1,250° C to 990° C, suggesting that decomposition must have commenced at temperatures of more than sim1,000° C.These P–T limits lead to the conclusion that, in those kimberlites where spinodal decomposition has occurred in subcalcic diopside megacrysts, such decomposition occurred at shallow levels (<15 km) and, at the present erosion level, temperatures must have been greater than 1,000° C.
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