THE MANTLE TRANSITION ZONE: A DEEP EARTH TARGET FOR DECADES OF INTERDISCIPLINARY GEOSCIENCE

From Birch [1952] onward solid Earth scientists have recognized that rapid physical and mineralogical changes from ~300 – 800 km depth warrant scrutiny because of their potential to answer fundamental questions about mantle composition and dynamics. This depth range is referred to as the mantle transition zone based the occurrence of multiple mineral phase transition in abundant mantle minerals, particularly olivine polymorphs. Globally visible seismic discontinuities mark the depths of the olivine-to-wadsleyite (~410 km) and ringwoodite-to-bridgmanite (~660 km) phase transitions. Physical expressions of the latter boundary are complicated by potential overlap with the more gradual majorite garnet-to-bridgmanite transition depending on local mantle temperature and composition. Another more gradual olivine polymorph transition from wadsleyite-to-ringwoodite occurs within the transition zone (~520 km). The potential for seismological imaging of depth variations of these discontinuities in conjunction with controlled laboratory mineral physics constraints and quantitative modeling of mantle flow has long motivated investigations of mantle bulk composition and temperature. There is a large body of evidence that the thermodynamics of these phase transitions act in concert with transitions in density and rheology such that the transition zone acts as a mechanical filter altering the organization of mantle flow. An exciting and controversial direction for more recent and ongoing research is investigation of the mantle transition zone’s role as a marker of volatile fluxes and perhaps a geochemical filter as a result of dehydration melting in volumes of upwelling or downwelling mantle. Continually improving observational, experimental, and integrative modeling capabilities suggest that the mantle transition zone will continue to be a vivid window into deep interior processes and a source of vibrant geoscience debates for years to come.