SCANNING ELECTRON MICROSCOPY OF KIMBERLITES FROM WYOMING AND COLORADO

This study deals with samples collected from kimberlite intrusions in southern Wyoming and northeast Colorado and applies techniques of backscattered electron (BSE) imaging, digital imaging processing, and x-ray microanalysis supplemented by light microscope petrography and whole rock (XRF) chemistry. The rock bodies (pipes) are intruded into Precambrian crystalline basement rocks and are associated with feeder dikes and sills. The intrusions in Wyoming have been assigned a Lower Paleozoic age and those in Colorado are believed to be Devonian. The rocks range from coarse to fine-grained and contain varying amounts of chlorite, phlogopite, a variety of oxides, calcite, pyroxene, high Mg olivine, plagioclase, and serpentine. Some samples are holocrystalline but others are definitely fragmental consisting of angular mineral grains and pieces of both igneous and sedimentary rock (preexisting wall rock). All samples show evidence of post-emplacement alteration (diagenesis). Most of these rocks contain less than 50% SiO and have the approximate composition of an ultrabasic igneous rock such as peridotite . Those which exceed 50% SiO show evidence of considerable alteration which may indicate that some of the silica is secondary. Image analysis techniques show that individual samples have a wide range of textures but are made up of a small number of components, but that there are considerable differences in compositions within the whole collection. Examination of the literature on kimberlites shows considerable diversity in the taxonomy and nomenclature of such rocks, especially in regards to textures and mineral compositions. This is apparently due to the variety of models proposed for the origin and emplacement of kimberlites. We have compared our material to specimens obtained from other localities (Northwest Territories, South Africa, Arkansas, etc.) which have been identified as being Kimberlites and have found them to be comparable. We conclude that, although all the rocks examined show considerable variety of textures and compositions, they all qualify to be called kimberlites. Furthermore, the processes by which these rock bodies became emplaced involved both crystallization and fragmentation. Indeed some of the rock bodies (pipes) may be considered to be diatremes.