A PETROLOGIC AND ISOTOPIC STUDY OF WINONAITES - EVIDENCE FOR EARLY PARTIAL MELTING, BRECCIATION, AND METAMORPHISM

We have conducted detailed petrologic, chemical, and isotopic studies of winonaites to ascertain the genesis of this group of meteorites. Wi nonaites have reduced mineral compositions and mineralogy, and oxygen isotopic compositions distinct from primitive achondrite groups other than silicate inclusions in IAB and IIICD irons. However, winonaites d iffer from IAB and IIICD irons in that they lack the metallic matrices of the latter and consist mostly of silicates. On the basis of these criteria, Winona, Mount Morris (Wisconsin), Tierra Blanca, Pontlyfni, Y-74025, Y-75300, Y-8005, and QUE 94535 are winonaites and Y-75305 and Y-75261 may be winonaites. Winonaites are fine- to medium-grained, mo stly equigranular rocks. Pondyfni and Mount Morris (Wisconsin) contain what appear to be relict chondrules. Several winonaites contain mm-si zed areas that differ substantially in grain size and/or silicate mine ralogy from the surrounding matrix. Fe,Ni-FeS veins are common in many winonaites. Mineral compositions are intermediate between E and H cho ndrites, and reduced sulfides are observed in low-FeO winonaites. Bulk major element compositions are roughly chondritic, although REE eleme nts are fractionated. The Ar-39-Ar-40 ages of three winonaites range f rom greater than or equal to 4.40 Ga (Mount Morris, Wisconsin) to 4.54 Ga (Pontlyfni). Cosmic-ray exposure ages are similar to 20-80 Ma. Tra pped noble gases in these winonaites resemble those in enstatite chond rites. We suggest that the winonaites formed from a chondritic precurs or material unlike that of known chondrites in mineral and oxygen isot opic compositions, and this material may have been heterogeneous in co mposition. Extensive heating caused metamorphism and partial melting o f both Fe,Ni-FeS and silicate material. Impact brecciation during cool ing mixed lithologies with different thermal histories, and subsequent metamorphism produced recrystallization, grain growth, and reduction of mafic silicates. The Ar-39-Ar-40 ages indicate that cooling may hav e been more rapid than observed in IAB irons, although later resetting may have occurred. Copyright (C) 1998 Elsevier Science Ltd.