Research Training Program

Smithsonian Institution
National Museum of Natural History

PROJECT SUMMARY
1997

Klint A. Cowan
Antioch College
Yellow Springs, Ohio
Tim McCoy, Ph.D.
Glenn MacPherson, Ph.D.
Supervising Scientists
Department of Mineral Sciences
"Working with two of the world's leading meteoriticists introduced me to a new field of scientific research, and allowed me to intelligently consider a career in planetary geology. "

A macroscopic view: Mass equivalence and brecciation of Mt. Padbury

ABSTRACT
Mesosiderites are a group of stony-iron meteorites comprised of basaltic and metal-rich clasts. These clasts formed by melting and differentiation on an asteroid, and were mixed by one or more impact events. They provide a means to test the hypothesis, proposed by previous researchers, that metal-silicate mass equivalency is indicative of sorting by mass in the nebula from which the solar system formed, because metal-silicate mass equivalence cannot be expected in meteorites with such a complex formation. Their history can be examined to determine the number of geologic events required for formation. Large polished slabs of the Mt. Padbury mesosiderite were chosen for examination. Mt. Padbury experienced impacts, but only minor melting as a result, and contains more pristine material than other mesosiderites. The Smithsonian Institution contains the two largest slabs outside of Australia, where the meteorite was found in 1964. A total of 1311 cm2 of Mt. Padbury were examined, by outlining metal, silicate, and breccia clasts, these outlines were then digitized to determine the area. Metal and silicate clasts are not equivalent in either size or mass, showing that mass equivalency might indicate size sorting in the nebula. Silicate clasts within Mt. Padbury record impact brecciation events, which occurred prior to metal-silicate mixing by the impact of a metal body into the mesosiderite asteroid. Mt. Padbury also contains 1-30 cm2 size mesosiderite blocks, from a pre-existing generation of mesosiderite material on the parent asteroid. At least three impact events occurred in the formation history of Mt. Padbury, one impact which brecciated the silicate material, one which mixed the metal and silicate in the breccia blocks, and one which mixed the metal and silicate of the matrix.

This work was made possible by a grant from the Office of the Director, National Museum of Natural History, Smithsonian Institution.

Letter of Gratitude