RECORD OF DEEP UPWELLING IN OPHIOLITES: HIGHLY REDUCED ENVIRONMENT CONTAINING VERY HIGH PRESSURE PHASES AND NITRIDES FROM AT LEAST 300 KM DEPTH

It is well known that ophiolites are rocks recording fossil oceanic spreading centers; the specific oceanic environment may be a point of controversy but since the birth of plate tectonics, they have been recognized as the locus of formation of oceanic lithosphere. My colleagues and I have recently established that at least some ophiolites contain vestiges of a highly-reduced deep mantle environment characterized by the presence of native metals, diamonds, silicon carbide and very high-pressure minerals. This environment is not associated with subduction of the ophiolite but is a primary feature of the ophiolite mantle. Specifically, the environment is encased in massive chromite where it has been protected from the low-pressure, oxidized, upper mantle environment. Here, I will discuss in some detail this environment from the Luobusa ophiolite from the Inda/Asia suture in Tibet.

The Luobusa sample described here, although small, establishes beyond a doubt that this environment exists at depths of more than 300 km, perhaps much more (Yang et al., Geology, 2007; Dobrzhinetskaya et al., PNAS, 2009). The rock consists almost completely of coesite, kyanite, and an unknown amorphous phase, with interstitial and included TiN and the cubic form of BN. The coesite is pseudomorphic after stishovite, the high-pressure polymorph of SiO₂ that is not stable at pressures less then 10 GPa at mantle temperatures (~1300¢ªC). TiN, osbornite, has only one previously described terrestrial occurrence but is abundant in meteorites, especially iron meteorites; this is the first recorded natural occurrence of boron nitride. The material also contains many native metals, including native Fe, and also SiC and the high-pressure polymorph of TiO₂ that has been described twice previously from UHPM rocks but does not yet have a mineralogical name. The presence of this phase confirms a minimum pressure of ~10 GPa at 1300¢ªC implied by stishovite pseudomorphs.

Despite the abundant evidence of highly-reducing conditions, the massive chromites show high ratios of Fe^3+/Fe_total. (Ruskov et al., J. Met. Geol., 2010). Thus, these rocks appear to be the first natural example of pressure-induced disproportion of Fe²⁺ into Fe³⁺ plus Fe⁰ observed in very-high-pressure experiments. The origin of this material is problematic. The high concentration of SiO₂ and Al₂O₃ suggests that this material is perhaps recycled crustal material from the surface. However, nitrogen isotopes are incompatible with such an origin. At minimum, these observations require that the massive chromites carrying the high-pressure signal have their origin at very high pressure, perhaps in the lower mantle. In addition, they require that the surrounding harzburgites must have been solid during upwelling in order to transport the much more dense chromites. Thus, the interpretation of the mantle section of ophiolites needs to be revisited. These rocks are much more than the partial-melting residue of the overlying oceanic crust. It is also critical to realize that the Luobusa chromites are not unique. My colleague, J. Yang, has determined that another ophiolite, from the polar Urals, also contains diamonds, native metals, and SiC. Thus, rather than being a unique curiosity, this mineral assemblage characterizes a subset of ophiolites, the extent of which is not yet known. Thus, this rock and others yet to be describe provide a new window into the deep mantle; only future work will be able to discern how deep; what we see requires a minimum of 300 km but there is not hard upper bound on the depth.