ULTRAHIGH-PRESSURE INDICATOR MINERALS: WHERE TO LOOK AND HOW TO LOOK FOR THEM

Since their first descriptions nearly 40 years ago, ultrahigh-pressure (UHP) rocks have challenged conventional tectonic paradigms and stimulated interdisciplinary work bridging mineralogy, petrology, tectonics, and geodynamics. Because UHP rocks are now a key part of the study of subduction zones and orogenic belts, the methods used to identify these rocks should be not only robust, but also accessible to researchers of all specialties working on convergent margins.

We present an overview of the well-studied UHP indicator minerals coesite, diamond, and (decomposed) majoritic garnet. These phases are central to the petrologic literature on UHP rocks and are likely to be relevant to collaborative studies of convergent margins. We enumerate the methods commonly used to recognize these phases and discuss the relevance of each indicator mineral to different rock types. We detail the important cautionary measures for identifying and interpreting these phases that can help avoid spurious UHP identification. For coesite, identification using Raman spectroscopy alone should be accompanied by analysis ruling out contaminant Si-based polishing grit and/or xenotime. For diamond, thin sections should be prepared without diamond-based polishes and diamonds should be far from sample surfaces. Ultrasonic cleaning helps to dislodge retained polishing grit but does not quantitatively remove contaminant. Decomposed majoritic garnet can be demonstrated from exsolved silicates (± oxides, phosphates) if precursor (pre-exsolution) garnet compositions are carefully reconstructed and the crystallography of the exsolution system is characterized, particularly if energetically favorable edge-to-edge matching crystallographic relationships are present. Our aim is that discussion of these best practices will strengthen the interdisciplinary discussion between researchers interested in UHP rocks.