The Size of Ultrahigh-Pressure Terranes

Coesite is the ultrahigh-pressure (UHP) polymorph of SiO2 (commonly quartz) and its occurrence indicates deep subduction to depth >90 km. Figuring out the size (rock volume) of UHP terranes as well as the lithologies affected are of high importance to understand the fundamental geodynamic processes leading to their formation and exhumation. For many UHP terranes worldwide this is a matter of debate, particularly because coesite is difficult to find and only preserved in the form of tiny mineral inclusions in rigid host minerals like garnet. Although the analysis of mineral inclusions in detrital garnet represents an effective tool to trace the extent of UHP rocks, this technique is still time consuming. Preliminary work indicates that coesite is also present as single mineral grains in sediments shed from UHP rocks (in very low amounts). The aim of this project is to develop a technique to enrich coesite by gravimetric and magnetic separation, to detect the coesite grains by semi-automated Raman spectroscopy, and to apply this new concept to the Saxonian Erzgebirge (E Germany) or Western Gneiss Region (SW Norway) to shed new light on the size of these UHP terranes.

• Tasks: Experimental Sample Preparation (sieving, density separation, magnetic separation, embedding, grounding and polishing), Data Acquisition and Evaluation (Raman)
• Methods: Raman Spectroscopy
• Field work: Required (Saxonian Erzgebirge or Western Gneiss Region)
• Project Start: Anytime
• Project Type: Master Thesis