GeoScienceWorld
GeoRef Record 1998-067502

A petrologic and isotopic study of winonaites; evidence for early partial melting, brecciation, and metamorphism

A petrologic and isotopic study of winonaites; evidence for early partial melting, brecciation, and metamorphism

Abstract

Winonaites have reduced mineral compositions and mineralogy and O isotopic compositions distinct from primitive achondrite groups, other than silicate inclusions in IAB and IIICD iron meteorites. These factors, with the abundance and distribution of metal and troilite, are used to classify winonaites which are described in detail with many photomicrographs, modes, mineralogy (low-Ca pyroxene, olivine, Ca pyroxene, Fe, Fe-Ni-metal, with minor chromite, daubreelite, alabandite, schreibersite, graphite, K-feldspar and apatite), bulk major and trace element compositions, olivine and pyroxene analyses, and trapped and cosmogenic noble gases and their isotopes. The results show that winonaites are primitive achondrites that suffered limited partial melting, with brecciation and metamorphism of the precursor body. The precursor was probably chondritic unlike any known chondrite group. (super 39) Ar/ (super 40) Ar age determinations of samples of Pontlyfni, Winona and Mt Morris range 4400-4540 m.y.; cosmic ray exposure ages are approximately 20-80 m.y. Brecciation and metamorphism of the precursor body occurred early in the solar system.