Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body

Abstract

Detailed mineralogic and petrographic data are presented for four isotopically-dated basaltic rock fragments separated from the howardite Kapoeta. Clasts C and ρ have been dated at ~4.55 AE and ~ 4.60 AE respectively, and Clast ρ contains ^(244)Pu and ^(129)I decay products. These are both igneous rocks that preserve all the features of their original crystallization from a melt. They thus provide good evidence that the Kapoeta parent body produced basaltic magmas shortly after its formation (< 100 m.y.). Clast A has yielded a Rb-Sr age of ~ 3.89 AE and a similar ^(40)Ar/^(39)Ar age. This sample is extensively recrystallized, and we interpret the ages as a time of recrystallization, and not the time of original crystallization from a melt. Clast B has yielded a Rb-Sr age of ~ 3.63 AE, and an ^(40)Ar/^(39)Ar age of ⪆ 4.50 AE. This sample is moderately recrystallized, and the Rb-Sr age probably indicates a time of recrystallization, whereas the ^(40)Ar/^(39)Ar age more closely approaches the time of crystallization from a melt. Thus, there is no clearcut evidence for ‘young’ magmatism on the Kapoeta parent body. Kapoeta is a ‘regolith’ meteorite, and mineral-chemical and petrographic data were obtained for numerous other rock and mineral fragments in order to characterize the surface and near-surface materials on its parent body. Rock clasts can be grouped into two broad lithologic types on the basis of modal mineralogy—basaltic (pyroxene- and plagioclase-bearing) and pyroxenitic (pyroxenebearing). Variations in the compositions of pyroxenes in rock and mineral clasts are similar to those in terrestrial mafic plutons such as the Skaergaard, and indicate the existence of a continuous range in rock compositions from Mg-rich orthopyroxenites to very iron-rich basalts. The FeO and MnO contents of all pyroxenes in Kapoeta fall near a line with FeO/MnO ~ 35, suggesting that the source rocks are fundamentally related. We interpret these observations to indicate that the Kapoeta meteorite represents the comminuted remains of differentiated igneous complexes together with ‘primary’ undifferentiated basaltic rocks. The presently available isotopic data are compatible with the interpretation that this magmatism is related to primary differentiation of the Kapoeta parent body. In addition, our observations preclude the interpretation that the Kapoeta meteorite is a simple mixture of eucrites and diogenites. The FeO/MnO value in lunar pyroxenes (~60) is distinct from that of the pyroxenes in Kapoeta. Anorthositic rocks were not observed in Kapoeta, suggesting that plagioclase was not important in the evolution of the Kapoeta parent body, in contrast to the Moon. Both objects appear to have originated in chemically-distinct portions of the solar system, and to have undergone differentiation on different time scales involving differing materials.