PETROGENETIC HISTORY OF NWA 6963 AS REVEALED BY ELEMENT ZONING AND TEXTURAL ANALYSES OF AUGITE AND PIGEONITE

Northwest Africa (NWA) 6963 is a coarse-grained Martian meteorite that is thought to represent an intrusive gabbro with cumulate pyroxene, which possibly represents a feeder dike to the plumbing system for the basaltic shergottites. To further constrain the crystallization history and the depth of emplacement, we investigate major and trace element zoning in augite and pigeonite (and one maskelynite grain), and use electron-backscattered diffraction (EBSD) to examine the cumulate pyroxene grains.

A thin and thick section of NWA 6963 were analyzed for major elements and X-Ray element maps using a Cameca SX100 electron microprobe at the Open University, UK. The results of these analyses and maps were used to avoid alteration-filled fractures during trace element analyses by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Open University, UK. Finally, the same sample was analyzed using an Oxford Instruments Symmetry detector at the Plymouth Electron Microscopy Centre at the University of Plymouth, UK for EBSD.

NWA 6963 has complexly zoned composite pyroxenes, with augite and pigeonite cores with similar Mg#’s and relatively flat zoning profiles. The chemistry and textures of the cores suggest that these cores are in equilibrium, and crystallized at the same temperature (~1250 °C). Both augite and pigeonite cores are rimmed by Fe-rich pigeonite. The chemistry and textures of the pyroxene cores and rims are likely due to accumulation during magmatic crystallization. Previously we suggested this was in a feeder dike system, but here our results are more consistent with accumulation in a large intrusive environment (such as a sill or magma chamber); however, without geologic context or companion samples it is currently impossible to rule out accumulation at the base of a very large (>>100 m) differentiated flow.