Garnet lherzolite xenoliths in the kimberlites of northern Lesotho: revised P-T equilibration conditions and upper mantle Palaeogeotherm

Evidence is presented that the inflected palaeogeotherm for northern Lesotho, previously highlighted by Boyd (1973), Boyd and Nixon (1973, 1975), Finnerty and Boyd (1984, 1987), is essentially an artifact of the unsatisfactory, over-simplified barometer formulation (based on MacGregor 1974) employed. The absence of an inflection in the palaeogeotherm for Udachnaya, Siberia based on P-T estimates for garnet lherzolite xenoliths calculated with the same barometer, does not prove the reality of an inflected palaeogeotherm for northern Lesotho. Rather, it reflects, at least in part, chemical differences between the equivalent deformed, high-T xenoliths in these two areas — most importantly expressed in the respective contents of Jadeite relative to ureyite in the constituent orthopyroxenes. Accurate estimation of P-T equilibration conditions for garnet lherzolite xenoliths requires both complete and precise mineral analyses and adequate consideration of the influence of minor elements, such as Cr and Na, on the element exchange reaction thermometers and barometers employed. The barometer formulation of Nickel and Green (1985) is judged to be the best currently available. As no single thermometer is entirely satisfactory and dependable throughout the P-T range of interest, equilibration temperatures are currently best assessed as a mean value obtained from application of the most accurate formulations for both the two-pyroxene solvus thermometer (Bertrand and Mercier 1985) and Fe²⁺-Mg²⁺ exchange reactions between garnet-clinopyroxene (Powell 1985), garnet-orthopyroxene (Harley 1984a) and garnet-olivine (O'Neill and Wood 1979) mineral pairs. Such best P-T estimates for xenoliths in the kimberlites of northern Lesotho indicate a somewhat elevated, non-inflected, upper mantle palaeogeotherm, compatible with a 120–145 km thick thermally conductive lithosphere above a convecting asthenosphere. The common coarse textured, chemically depleted, garnet lherzolite xenoliths appear mostly to have originated from close to the base of the lithosphere whilst the contrasting deformed, higher T, more chemically fertile xenoliths have come from the underlying asthenosphere. There is evidence for slight variations in the heat flux within the mantle beneath northern Lesotho at the time of emplacement of the Thaba Putsoa and Mothae kimberlites, only some 16 km apart, and also possibly for a regional variation in the thickness of the lithosphere.