Discussion
Recent studies of Ultra-High-Pressure Metamorphism have replaced xenoliths from kimberlites as the source of the deepest rocks brought to Earth’s surface. Here we have briefly described both a mantle peridotite that apparently has experienced a one-way trip to the surface from about 400 km and a shale that was subducted to more than 350km and returned. Depths at least as great have been suggested in the form of pyroxene exsolution rods in previously majoritic (super-silicic) garnets from the Western Gneiss region of Norway (Spengler et al., 2005). To that is now added potentially equal depths sampled in the upwelling of mantle beneath ocean spreading centers. The only material that has come to the surface with evidence of deeper origin than these examples of the mantle convection system are inclusions in diamonds from the mantle transition zone and lower mantle (Hayman et al., 2005; Wirth et al., 2007; Kaminsky, 2012 and reference therein).
What can we infer from this new direct evidence of mantle convection? The first is that at least some of the harzburgites in ophiolites do not represent the residue of melting left over from generation of the basalts that overlie them. Chromite is a very dense material. Thus, the only way the massive chromites carrying a very deep signal can have risen from great depth is if they were carried by a larger volume of less dense but strong rocks. The only possibility is the harzburgite in which the chromite is embedded (exactly analogous to the UHPM eclogites and peridotites being carried by the quartzofeldspathic rocks that surround them). Indeed, diamonds have recently been found in the harzburgites of Luobusa (J. Yang, personal communication, 2010). This change in understanding of the mantle portions of ophiolites opens a window for further prospecting for other deep mantle mineralogy.
The second implication we can draw is that if continental material, even sediments, can reach depths in excess of 350 km and return, there must have been other cases in which the “point of no return” was reached when upper continental crust becomes more dense than ambient mantle and never returns (Irifune et al., 1994; Dobrzhinetskaya and Green, 2007; Liu et al., 2007b; Wu et al., 2009). In particular, Irifune et al. (1994) and Wu et al. (2009) showed that if the point of no return is reached by such lithologies, they would fall to the base of the mantle transition zone (MTZ) but not into the lower mantle. Evidence of chemical components suggestive of continental sediments and crust is recorded in ocean island basalts (e.g. Rapp et al., 2008). The materials from Luobusa may be evidence of return of “partially digested” upper mantle material. The dominant chemical components of the high-pressure material from Luobusa discussed here are SiO2 and Al2O3 and surprisingly include boron. These major components strongly suggest an upper crustal origin and the presence of boron implicates sediments. However, the Nitrogen isotopes of this rock are incompatible with a crustal origin (Dobrzhinetskaya et al., 2009) and therefore most likely represent a component added from the lower mantle. Thus, the simplest explanation of all of these observations would appear to be that some continental material gets subducted to the base of the MTZ where it can be contaminated by at least volatiles from the deeper mantle. Furthermore, ocean island basalts probably are either generated at that depth or they interact with such contaminants on their way to the surface.
Lastly, the question arises as to whether chromitites, which are only found in opholites, are generated at great depth rather than at shallow depth as is currently understood. Or could chromitites like those in Luobusa be, like some of their included constitutents, relics of a previous Wilson Cycle? In either case, they provide immediately a point for prospecting for deep materials. The role of ophiolites in providing information about the deep mantle has only begun to be investigated. Modeling studies are proving productive in creating viable scenarios of subduction and exhumation guided by these mineralogical discoveries in continental collision terranes (see for example Gerya et al., 2002; Roda et al., 2010) and very likely will provide additional insight into the dynamics of mantle upwelling as well.