Composition of the mantle lithosphere beneath south-central Laurentia: Evidence from peridotite xenoliths, Knippa, Texas

Abstract

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.

Ocean Basalt Simulator version 1 (OBS1): Trace element mass balance in adiabatic melting of a pyroxenite‐bearing peridotite

“…Fractional melting is the most plausible mechanism for melt extraction in the adiabatic melting of a mantle peridotite [Johnson and Kinzler, 1989;McKenzie, 1984;Norman, 1998;Raye et al, 2011;Sano et al, 2012]. However, melts from the pyroxenite can react with the ambient peridotite and freeze in it.…”

Manganese incorporation in synthetic hercynite

“…In air and at atmospheric pressure, magnetite oxidises rapidly to maghemite, γ-Fe2O3, one of the few naturally occurring spinels which contains high concentrations of cation vacancies (e.g., Johnson and Jensen, 1974). The stability of magnetite over a range of conditions found within the deep Earth and its incorporation of mixed-valent Fe means that reactions between olivine, orthopyroxene and spinel can be used to determine prevalent fO2 conditions in spinel-bearing mantle peridotites (for example see Raye et al, 2011, and references therein). There have been numerous studies of other Fe-bearing spinels under varying fO2 conditions, although the majority of these are within the fields of material science and solid state chemistry due to their importance in a wealth of applications from high-density magnetic recording media to reduction catalysts.…”

Structure and origin of the rifted margin of the northern Gulf of Mexico

“…1) (Young and Lee, 2009;Raye et al, 2011), the Mesozoic rift may have localized in thickened crust of the Ouachita orogenic belt (Culotta et al, 1992). To the east, the Ouachita suture lies farther north in Arkansas, ~500 km north of the Gulf coast (Houseknecht and Matthews, 1985;Keller et al, 1989).…”