Eclogites, Pyroxene Geothermn, and Layered Mantle Conve!tion

consumption of '2C-CH4. Such fractionation may occur to dissolve methane transported more slowly through the 7-to 25-cm-thick sedinent cover over-lying the methane production zone via molecular diffusion or to mthane bubbles that become lodged in overyling sulfate-reducing sediments between low-tide bubbling episodes. Gas bubble methane transport in our site accounts for approximately 84% of the total annual methane flux of 5.7 ± 2.6 mol m2 (13). The potential importance of carbon isotopic fractionation of methane during its transport up through an oxidizing sediment column has been discussed by Stevens and Rust (8). Temperatures of equilibration for the majority (81 percent) ofthe eclogite xenoliths of the Roberts Victor kimberlite pipe in South Africa range between 10000 and 12501C, falling essentially on the gap of the lower limb of the subcontinental inflected geotherm derived from garnet peridotite xenoliths. In view of the Archean age (>2.6 x 109 years) ofthese elogites and their stratigraphic position on the geotherm, it is proposed that the inflected part of the geotherm represents the convective boundary layer beneath the conductive lid of the 'lithospheric plate. The gradient of 8 Celsius degrees per kilometer for the inflection is characteristic of a double thermal boundary layer and suggests layered convection rather than whole mantle conve,ction for the earth. IN A STUDY OF UPPER MANTLE PETROL-ogy, Boyd (1) estimated the equilibra-tion conditions of ultramafic xenoliths ,containing the assemblage garnet, orthopy-roxene, clinopyroxene, and olivine from kimberlites in northern Lesotho. Temperature and pressure estimates of the Lesotho xenoliths defined a curvilinear trend ranging in temperature from 9000 to 1400°C corre-spo,nding to depths of 100 to 200 km. Boyd interpreted this trend as the segment of a fossil geotherm at the time of kimberlite eruption. The lower temperature-pressure end of this xenolith trend coincided essentially with the continental shield geotherm of Clark and Ringwood (2), although the trend showed an inflection, beginning at 1100°C, and extending to 1400°C and a depth of 200 km (Fig. 1). Another characteristic of this geotherm is that the xenoliths plotting on the shallow limb of the geotherm displayed a coarsely granular texture whereas those that defined the inflected limb were intensely sheared. Boyd also interpreted the point of inflection on this geotherm as marking the top of the low-velocity zone beneath Lesotho at the time of kimberlite eruption around 90 million years ago. In addition to this Lesotho geotherm, inflected geotherms based on the pyroxene geotherm,ometry-barometry approach …