Graduate Thesis Or Dissertation
 

A chemical and petrological study of igneous lithic clasts from the kapoeta howardite

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  • Instrumental and radiochemical neutron activation analyses supported by electron microprobe mineral analyses (Paul Warren, University of New Mexico) and petrographic observations are presented for approximately forty individual clasts separated from the Kapoeta howardite. These clasts are classified into eucritic, cumulate-like, mafic (plagioclase ± olivine-bearing pyroxenites), pyroxenite, polymict and/or metal-rich and chondritic components. Geochemical inferences are presented which suggest significant compositional variations in the eucritic clasts as compared to normal 'main' group eucrites. Clasts of mafic lithologies have chemistries which suggest primary magmas. These primitive primary compositions can be derived from sources which are essentially CH or CL ordinary chondrite-like compositions with the exception of ~̲13-fold depletions in the volatile alkali elements (Na, K, Rb and Cs). An extensive partial melt origin if preferred for the genesis of these mafic magmas. Source Fe' (Fe/Fe-I-Mg molar) values suggested by the most Mg-rich eucritic and Antarctic eucrites (-0.28) vs. the ordinary chondrite source values (-0.23) are reasonably close; however, normal eucrites suggest source Fe' values of -0.35. Although eucritic clasts and known Antarctic eucrites can reasonably be derived from sources which produced the mafic magmas by partial melting followed by minor (<20%) orthopyroxene fractionation, normal eucrites having greater and nearly constant Fe' values and nearly unfractionated trivalent REE patterns cannot be produced by similar mechanisms and are therefore concluded to have significantly different source compositions. In such cases a secondary melt origin from enriched non-chondritic sources is inferred for normal eucrites. Eucritic and pyroxenitic cumulates have compositions which infer the presence of trapped liquid components. The trapped liquid probably resulted from inefficient solid phase accumulation due to small parent body gravitational fields. Final conclusions are presented which attempt to account for the compositional variations observed in normal eucrites, diogenites, pallasites and the howardite lithic clasts observed in this work. The evolutionary sequence includes (1) initial production of extensive core melts leaving pallasitic-like residuals, (2) generation of near surface low-degree partial melts (Mg-rich eucritic clasts), (3) crystallization of extruded primary melts (diogenites and cumulate eucrites) and (4) cumulate remelting (normal eucrites) episodes. This sequence of igneous events requires that the KPB was heated by a meltmobile heat source such as primordial ²⁶A1.
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