This Special Session was organized and chaired by Orson L. Anderson for the Fifty-Fifth Annual Meeting of the American Geophysical Union, held in April 1974. Five invited papers were presented, four dealing with the mantle of the earth and the fifth with samples of the core.
The first paper, 'The Mantle Sample From the Kimberlites and an Estimated Geotherm,' was by F.R. Boyd, Jr., of the Geophysical Laboratory. Some of the results have been published previously [Boyd, 1973a, b]. Basically, Boyd compared the compositions of pyroxenes in ultramafic nodules in kimberlites with compositions of pyroxenes synthesized under known pressures and temperatures to establish what he interprets as a fossil (Cretaceous) geotherm beneath Lesotho (Africa) and elsewhere, for example, beneath Montana [Hearn and Boyd, 1973] (see Figure 1). In these instances, the high-pressure (deep) leg of the geotherm is inflected to higher temperatures. The rocks that plot on the lowtemperature, shallower segment of the geotherm have a ‘normal’ granular texture, but those on the deep (high-temperature) segment are sheared. Boyd suggests that movement of the Cretaceous lithospheric plates sheared the underlying deepseated mantle rocks, concomitantly heating them above their predrift temperatures. Eruption of the kimberlites at about this time quenched in the high-temperature, high-pressure compositions of the pyroxenes, thus preserving a record of the perturbed geotherm.