Ultrahigh-Pressure Minerals from Both Downwelling and Upwelling Environments: Snapshots of Mantle Convection on a Grand Scale

Harry W. Green, II, Larissa Dobrzhinetskaya , and Krassimir N. Bozhilov

The concept of Ultra-High-Pressure Metamorphism (UHPM) grew out of the discovery that blueschist minerals in metasediments required simultaneous high pressure and low temperatures that, in turn, required rapid travel to several 10s of km to grow the minerals in the first place, and similarly rapid return to the surface to avoid their reaction to greenschist facies. Both the trip down and the trip back had to occur faster than any known mechanism could provide. The birth of plate tectonics resolved this conundrum. Subsequent discovery of coesite and diamond in shallow protoliths extended the depth of implied subduction to > 150 km but the context remained transport of surface rocks on a scale small compared to mantle convection. Indeed, the presence of coesite and/or diamond or implication of their former presence became the working definition of UHPM. However, mantle rocks caught up in UHPM terranes also can carry evidence of very deep origin. Prior to that discovery, metamorphic petrology implicitly assumed that travels of rocks begin at or near the surface; solid-state transport upward of dense rock from significant depth was implicitly thought to be impossible. Over geological time, peridotite behaves as a fluid and is the medium by which the mantle of Earth convects, with the unavoidable implication that large volumes of such rock have “seen” the deep upper mantle and some must have circulated deep into the lower mantle. We now understand that these rocks can carry records of their travels in their microstructures. A simple example of this is the ubiquitous presence of pyroxene-spinel symplectites in spinel peridotites of ophiolites and mantle xenoliths, recording the former presence of garnet. A more complex example is exsolution of high-pressure pigeonite as lamellae at high angles to [001] in diopside, recording depths of ~400 km in the Alpe Arami peridotite in the Swiss Alps. UHPM terranes now have yielded “memory” in continental lithologies of almost that deep. Even more surprising, minerals recording comparable depths and a very reducing environment are now known from inside massive chromite of an ophiolite and the presence of diamonds in another demonstrates that this deep environment is not unique. The evidence that ophiolites form at Earth’s surface is overwhelming. Nevertheless, chromites within them show unambiguous evidence of a highly-reduced environment of great depth and high temperature. One of them contains coesite-after-stishovite and TiO2 (II), as well as titanium and boron nitrides, with no evidence of down-pressure reaction. Lastly, we now know that some diamonds contain inclusions from the mantle transition zone and the lower mantle. What more can these rocks tell us? We discuss three things that point to much-needed new work in a variety of directions: (i) the harzburgites of ophiolites must be at least in part solid material that rose from great depth (carrying chromite) rather than the residue of melting from the basalts above them; (ii) continental material has been subducted to the base of the mantle transition zone and we suggest that ocean island basalts are generated there; (iii) Massive chromite carrying a mineralogical record of very high pressure either has a very deep origin or represents material from a previous Wilson Cycle that has been recycled.