Abstract
The intracrystalline equilibria for Fe2+ and Mg between the M1 and М2 sites in Ca-rich clinopyroxenes have been determined at a number of temperatures. The required site-occupancy data have been obtained by complete crystal-structure refinement of single-crystal X-ray intensities constrained by the bulk chemistry determined from electron microprobe analysis. The samples used in the study were (1) a discrete clinopyroxene nodule from the Thaba Putsoa kimberlite pipe, Lesotho; (2) a clinopyroxene megacryst found in a nephelinite breccia from Kakanui, New Zealand; and (3) a clinopyroxene from a kaersutite-bearing eclogite, also from Kakanui.
The temperature corresponding to the cation distribution for the discrete nodule is 530° ± 50°C, in agreement with the results of McFadden (1973), who carried out thermal demagnetization experiments on samples of the Ventersdorp lava that was intruded by the kimberlite at the DeBeers mine. On the basis of his experimental work and heat-flow calculations, he obtained an emplacement temperature of 550°-620°C.
The megacryst from Kakanui yields a temperature of 1367° ± 120°C. Although the uncertainty is large, the lower temperature limit is consistent with crystallization temperatures inferred from garnet-clinopyroxene distribution coefficients (Akella and Boyd, 1974; Råheim and Green, 1974), in addition to the experimental study of Merrill and Wyllie (1975). The clinopyroxene from the kaersutite-bearing eclogite, which was referred to the megacryst calibration curve, gives a temperature of 710° ± 50°C, in marked contrast to the temperature of approximately 1250°C calculated from the garnet-clinopyroxene distribution coefficients. The most probable cause of the discrepancy is cooling of the eclogite in the subsolidus to 700°–800°C prior to eruption, as suggested by Merrill and Wyllie (1975).
