The Study of Microstructures in UHP Rocks

Minerals with stability fields restricted to very high pressures (e.g. coesite and diamond) are now known from many continental collision zones of many ages. In addition, mineral associations and microstructures suggestive of extremely high pressures, perhaps representing the mantle transition zone or lower mantle, have now been documented from a wide variety of Ultra-High-Pressure metamorphic rocks, both from xenoliths in kimberlites and related volcanic rocks, and from continental collision zones. In this talk, I will discuss many of these microstructures in terms of how reliable may be the inferences of very great depth and some alternative interpretations that might be placed upon them. As additional new and perplexing observations emerge from UHPM terranes, the need will grow for experimental investigations of a wide variety of bulk chemistries representing continental rocks and sediments. We have begun such experiments on decompression of peridotite from conditions of the mantle transition zone; our successes and failures to date in reproducing natural UHP microstructures can be instrumental in designing similar experiments in other systems.