COMPARISON OF MANTLE SAMPLES FROM A CV CHONDRITE PARENT BODY AND FROM EARTH

A small number of meteorite samples have been found that appear to be mantle samples from relatively large differentiated parent bodies. NWA 3133, first described by Irving et al. (2004), is a lherzolitic metachondrite from northwest Africa with chemistry nearly identical to Allende and other CV3 carbonaceous chondrites. Many mantle xenolith samples from the southwestern United States have mineralogy similar to NWA 3133. Typically they are olivine websterites, containing <30% olivine. Some contain spinel and a very small number contain garnet. Sample KB59 was collected on the eastern side of Kilbourne Hole, New Mexico. Texturally, NWA 3133 and KB59 are quite similar, although grains in KB59 are much larger. NWA 3133 contains 40% olivine, 28% orthopyroxene, 12% clinopyroxene, 12% plagioclase and 8% oxidized Fe-Ni alloy as major phases. KB59 contains 20% olivine, 40% orthopyroxene, 35% clinopyroxene, and 5% spinel. Fe/Mg values for olivine, orthopyroxene and clinopyroxene in NWA 3133 are 0.30, 0.24, and 0.16, respectively. Fe/Mg values in KB59 are about 0.11 for all three minerals. NWA 3133 and KB59 clinopyroxenes contain 0.71 and 1.30 wt% Cr₂O₃, respectively. KB59 clinopyroxene and orthopyroxene contain significantly more Al₂O₃ (6.0 wt% vs 1.5 wt%) than NWA 3133 pyroxenes).

Plagioclase-bearing terrestrial mantle xenoliths are rare due to Eath's crustal thickness. Spinel-bearing meteoritic lherzolites are unknown due to parent body size. The different aluminous minerals (plagioclase An₅₂Ab₄₆Or₂ in NWA 3133, and spinel in KB59) result from different depths of formation. Geobarometry suggests that KB59 equilibrated at depths of about 55 Km (18 Kb). Garnet-bearing pyroxenites from the same region equilibrated at greater depth. NWA 3133 formed at considerably shallower depths than KB59 but deriving a value is complicated because of unknown effects of solid solutions on mineral equilibria such as olivine + plagioclase = clinopyroxene + orthoproxene + spinel. (Fe/Mg)_opx/(Fe/Mg)_cpx and (Fe/Mg)_ol/(Fe/Mg)_cpx values are 1.47 and 1.84 for NWA 3133, and 1.01 and 0.99 for KB59, respectively. The different K_d values reflect different temperatures of equilibration. Geothermometry yields 1150°C for KB59 and 825°C for NWA 3133. If more samples of metamorphosed chondrites are found and analyzed, it may be possible to reconstruct geotherms of CV parent bodies.