Congratulations to PhD student Nicole McGowan for publishing her work in the prestigious journal Geology. Here is the abstract of her article:
Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (Tibet) contain diamonds and a highly reduced trace-mineral association that suggests that the chromitites formed at ultra-high pressure (UHP) corresponding to the Transition Zone (>400 km; >12.5 GPa). However, trace-element signatures of the chromites are indistinguishable from those of typical ophiolitic chromitites (e.g. Antalya Complex, Turkey), implying primary crystallisation from typical arc-type melts at shallow depths. New data on geochronology and Fe oxidation state may explain this conundrum. We propose that a lithospheric mantle slab containing the crystallised chromitite was subducted to the Transition Zone, where chromite inverted to a high-pressure polymorph and reacted locally with reducing fluids to form the highly reduced trace-mineral assemblage. Thermo-mechanical modelling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.
Nicole M. McGowan, William L. Griffin, José M. González-Jiménez, Elena Belousova, Juan Carlos Afonso, Rendeng Shi, Catherine A. McCammon, Norman J. Pearson, and Suzanne Y. O’Reilly (2015) Tibetan chromitites: Excavating the slab graveyard
Geology, February 2015, v. 43, p. 179-182, doi:10.1130/G36245.1