COMPOSITION OF THE MANTLE LITHOSPHERE BENEATH SOUTH-CENTRAL LAURENTIA: EVIDENCE FROM PERIDOTITE XENOLITHS, KNIPPA, TEXAS

Mantle xenoliths in ~83Ma basanites from south-central Texas provide a rare opportunity to examine the lithospheric mantle beneath southern Laurentia. These peridotites represent lithosphere at the boundary between Mesoproterozoic continental lithosphere and transitional Gulf of Mexico passive margin. Here we report petrographic, mineral and major element data for 29 spinel peridotite xenoliths from Knippa and use these to characterize the lithospheric mantle beneath south central Texas. The xenoliths comprise spinel-bearing lherzolites and harzburgites with coarse, equigranular textures. Some peridotites contain veins of lizardite, apatite, pentlandite, and pyrrhotite. There are no pyroxenites or eclogites. The peridotites contain olivine (Fo_89-92), orthopyroxene (En_89-92), clinopyroxene (Wo_{40- 45}En_45-49Fs_3-5) and spinel. Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzolites (Cr# = 0.14-0.21) and harzburgites (Cr# = 0.25-0.36). Mineral and major element compositions indicate that the lherzolites are residues after <10% melt extraction from primitive upper mantle and the harzburgites formed by <15% melt extraction. Calculated oxygen fugacities indicate equilibration of the harzburgites at -1 to + 0.61 and lherzolites at 0 to -2.6 log units with respect to fayalite-magnetite-quartz (FMQ) buffer, similar to lightly metasomatized spinel peridotites elsewhere. The degree of melt depletion and oxidation of the Knippa peridotites are consistent with present data sets for slightly metasomatized lithospheric mantle and/or back-arc samples rather than fore-arc settings. Equilibration temperatures range from 824 to 1068°C (mean= 936°C), calculated at reference pressure of 2.0 GPa. Calculated mean seismic velocities Vs = 4.44 km/sec and Vp =7.87 km/sec show no systematic difference between lherzolites and harzburgites, and agree with present geophysical measurements of upper mantle velocity beneath Texas. The seismic velocities calculated for these samples will provide important constraints for interpretation of EarthScope and other geophysical data sets.