Ultra-High Pressure Metamorphism: Where do we go From Here?

Ultra-High Pressure Metamorphism (UHPM) has its roots in the early 1960s discovery that glaucophane is stable only at high pressure and low temperature - implying that blueschists were carried to 10s of km rapidly and returned to the surface equally rapidly. Controversy erupted. Only with the advent of plate tectonics was the conceptual problem overcome and blueschists recognized as identifying subduction zones. Similar controversies arose upon discovery of coesite (1984) and diamond in metamorphosed crustal lithologies, and were repeated when microstructural indicators identified exhumation from greater depths (1996). Such findings are now commonplace from collision terranes and depths of subduction have been pushed to >350 km and exhumation from ~400 km. In the debates about the reliability of each discovery, the potential information content of UHPM rocks has often been lost in arguments about depth. It is time for us to develop ways to tease out of these rocks information about the exhumation trip up the subduction zone and, perhaps, also the voyage downwards. Solid inclusions in zircon have been used somewhat for this purpose but there must be ways to use isotopes and fluid inclusions to detect fluid flow through these rocks that must have occurred, probably during both downward and upward transport. We have probably only scratched the surface of information contained in the felsic carriers of the mafic and ultramafic rocks that record the greatest depths. Most of this felsic component has partially melted on the way back up or at least reverted to low-pressure assemblages. How do we interrogate them beyond examining their zircons for inclusions of high-pressure minerals? There is also now emerging a dimension of UHPM beyond subduction zones. Microdiamonds have now been identified in tiny melt pools of xenoliths in Ocean Island Basalts (OIBs). Silicates and oxides from the lower mantle, including phase egg (a clear identifier of continental material), have been discovered in diamonds. Most recently, coesite and diamond have been found and stishovite implied in chromitite from unmetamorphosed and unshocked ophiolites. Thus, UHPM of subducted material has struggled for 40+ years against the very old idea that continents and mantle have been very efficiently segregated by isostasy. Now, we are witnessing the dawn of a similar revolution showing that there are also traces of the very deep earth that can be carried to the surface by volcanic processes that can carry messages from much deeper than the depth of generation of the magmas.