Multiphase solid inclusions in ultrahigh-pressure metamorphic rocks: a petrographic approach

The study of fluid inclusions in ultra high-pressure (UHP) rocks does not represent an easy task, since most peak inclusions are destroyed during post- metamorphic uplift. Peak metamorphic conditions are in fact largely outside the isochore fields even for the densest fluid solutions, and the exhumation P-T paths of the host rocks strongly favour chemical interaction with the host mineral. Despite these difficulties, a number of documented examples show that inclusions in these metamorphic rocks might be preserved, providing valuable information on the composition of fluids at UHP, which are precursor to subduction fluids. Most peak inclusions in UHP rocks are multiphase solid (MS), containing different silicates salts, and no or very subordinate amount of water. A petrographic approach to MS inclusion studies in UHP rocks is proposed, consisting of a number of successive investigation steps (i.e. identification, chronology, selection, and interpretation). Since MS inclusions represent an integral part of UHP metamorphic phase assemblages, they should be systematically investigated during any petrographic study of such rocks