Magnesian basalt clasts from the EET 92014 and Kapoeta howardites and a discussion of alleged primary magnesian HED basalts

Abstract

We have performed petrologic and geochemical studies on pyroxene-phyric, magnesian basalt clasts from the howardites EET 92014 and Kapoeta. Pyroxenes in the EET 92014 clast are strongly zoned from Wo 4.5En 68.9Fs 26.6 cores to Wo 22.2En 18.5Fs 59.2 rims. The pyroxene has also suffered later FeO metasomatism along healed fractures that has resulted in compositions with atypically high FeO/MnO ratios. Metasomatized pyroxenes reach compositions as ferroan as WO 3.2En 37.0Fs 59.8. Pyroxenes in the Kapoeta clast have cores that are more magnesian, Wo 1.8En 80.2Fs 18.0, and are less strongly zoned, to Wo 3.6En 51.0Fs 45.4. Atypically high FeO/MnO in the most ferroan pyroxenes in this clast coupled with late ferroan olivine (up to ∼Fa 64), reflect late-stage addition of FeO to the melt rather than igneous fractionation. The EET 92014 clast is enriched in the refractory incompatible lithophile elements and shows a refractory lithophile element pattern similar to Stannem and Bouvante. The clast is enriched in Cr 2O 3 and MgO and depleted in Al 2O 3 and CaO compared to main group or Stannern trend eucrites. Based on bulk major and trace lithophile element composition, this pyroxene-phyric clast cannot be directly related to basaltic eucrites. This clast is either an impact melt of a trace element-rich, polymict eucrite or howardite target, or a primary melt of a more magnesian source region than that which produced basaltic eucrites. We favor the former interpretation. Low siderophile element abundances, within the ranges for monomict eucrites, indicate <0.5% chondritic contamination in the clast, however. The Kapoeta clast has a major element composition within the range of howardites and likely is an impact melt of such a target lithology. Comparison with other reported magnesian basalt clasts from howardites indicates that in all likelihood, all heretofore alleged primary magnesian basalt clasts are impact melts of polymict targets. Two magnesian eucrites, ALHA81001 and Pomozdino, and magnesian eucrite clasts from the polymict eucrite Petersburg may possibly be primary melts of a magnesian source region not directly related to basaltic eucrites.