WICHITA ANORTHOSITES: CHEMISTRY, GEOCHRONOLOGY, TECTONIC AND PLANETARY SIGNIFICANCE

The Electron Microprobe Laboratory was put into operation at Oklahoma State University (OSU) in 2002 after ConocoPhillips made a gracious contribution of a JEOL 733. A course in Electron Microprobe Analysis was developed that allowed graduate and undergraduates the opportunity to work hands on with the machine. This course is the only one offered in the Geology Department at OSU that provides undergraduates with a chance to work with state-of-the-art technical tools. In the Electron Microprobe Analysis course, each student is given the opportunity to choose his or her own research project or to work with a partner on a shared project. This project received funding from the NASA-Oklahoma Space Grant Consortium.

The lunar highlands are composed of anorthosite, a crystalline rock containing mainly plagioclase feldspar thought to have formed in a magma ocean where the lunar crust experienced fractional crystallization. Six NASA Apollo missions returned 382 kg of lunar material and Soviet Luna missions returned 321 g, but only a small portion of these were anorthosites. This lack in availability makes it necessary to identify an anorthositic terrestrial analog for further understanding of lunar formation. Three samples of anorthosites from the Wichita Mountains in SW Oklahoma were collected during the course field trip. These samples were chemically analyzed with the OSU Electron Microprobe, and were found to be primarily calcic plagioclase feldspar with inclusions of zircon, thorite, Fe-oxides, and rutile. The zircons are typically >250 microns, and backscatttered electron images and X-ray element maps of the grains show that they are compositionally zoned. Four zircons in a sample collected at N34 43 336 and W98 41 593 were dated in thin section using the UCLA Ion Microprobe and range in age from 367±12 Ma to 508±9 Ma.