EMPIRICAL GEOTHERMOBAROMETRY FOR GARNET PERIDOTITES AND IMPLICATIONS FOR THE NATURE OF THE LITHOSPHERE, KIMBERLITES AND DIAMONDS

Abstract

As a result of a study on the influence of Cr on geothermometry and geobarometry empirical garnet-orthopyroxene geobarometry has been refined. The applicability to simple and complex systems is demonstrated and the modelling procedure briefly outlined. The suitability of some geothermometers for application to garnet lherzolite xenoliths and their limits of precision are discussed. Fields of pressure-temperature estimates for garnet lherzolites from various provinces are shown. Low-temperature xenoliths (< 1050°C) give estimates in accord with a conductive geotherm for the continental lithosphere. African xenoliths (both South African and “off-craton” Namibian) record a distinctive pattern, exhibiting a “temperature discontinuity”, where high-temperature xenoliths (> 1050°C) give near-isobaric estimates for a range of temperatures. The depth of this “temperature discontinuity” is shallower for off-craton than for on-craton suites. Similar patterns are indicated for suites from U.S.A. and the U.S.S.R.