X RAY IMAGE ANALYSIS: MULTI-SCALE MODAL ABUNDANCE AS APPLIED TO THE ALLENDE METEORITE

This study investigates the use of elemental X ray mapping techniques and computer software as an alternate and efficient method to point counting. Cut and polished slabs and thin sections of the Allende Meteorite were investigated using this new method.

The main instrument in the field of petrography is the polarized light microscope (PLM). The PLM offers a tried and true method of identifying minerals in thin section. X ray maps obtained from an electron microprobe can map more area faster and offer a view of minerals showing definite compositional boundaries, which appear quite different with PLM.

The Allende Meteorite is a carbonaceous chondrite which formed from the accretion of the earliest solids within the cooling proto-planetary disc. The solids within Allende, and other carbonaceous chondrites, (called “inclusions”) are some of the most unaltered primitive minerals known to mankind. The abundance of these inclusions provides a direct correlation to the factors influencing their formation.

Random cut slabs of Allende were selected from the American Museum of Natural History's (AMNH) collection in New York City. In addition, a single thin section was examined using standard point counting methods and compared. The samples were scanned using AMNH's electron microprobe with fixed beam stage motion mapping at a resolution of 13, 5, or 3 microns/pixel. An area of 38.84 cm2 (4.9x107 pixels) was mapped in Si, Fe, Ca, Al, Mg (WDS) and S, Ni, and Ti or Mn (EDS) in three slabs and six sections. This number is in contrast to similar work done with meteorites using PLM.

The maps were then stitched into a mosaic computer image and each individual map was layered into an illustration program. The illustration program allowed for manual outlining of individual inclusions as evident by their elemental boundaries (ie: Mg boundary identifies the outer edges of an olivine rich chondrule). As inclusion boundaries were hand drawn they were checked independently by each author. A program written in IDL was used to count the numbers of pixels of each kind of inclusion. A ratio was taken of the number of pixels in each type of inclusion over the entire number of pixels in the image. The end result is a modal abundance of each individual category of inclusion in a particular slab.