Mantle xenoliths in ~83 Ma 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 harz burgites with coarse, equigranular textures. Some perido tites contain veins of lizardite . There are no pyroxenites or eclogites . The peridotites contain olivine (Fo 89-92 ), orthopyroxene (En 89-92 ), clinopyroxene (Wo 40-45 En 45-49 Fs 3-5 ), and spinel . Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzolites (Cr# = 0.14-0.21) and harz burgites (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, simi lar to lightly metasomatized spinel perido tites 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 backarc samples rather than forearc settings. Equilibration temperatures range from 824 to 1058 °C (mean= 916 °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 harz burgites, and agree with present geophysical measure ments 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.
Peridotite xenoliths from southern Texas consist of spinel lherzolite, harzburgite and minor dunite. Based on phase relations and temperature of equilibration, Knippa xenoliths come from the uppermost mantle, 40–70 km deep. Knippa xenoliths provide rare snapshots of upper mantle processes and compositions beneath south‐central Laurentia. They preserve olivine a‐axis fiber fabrics with a strong concentration of [100] and girdles of [010] and [001]. Assuming a lithospheric mantle having a horizontal flow direction parallel to fast directions, the mantle lithospheric fabric revealed by the xenoliths mostly explains the magnitude of observed shear‐wave splitting observed along the southern margin of the Laurentian craton.
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