Phase transitions in pyrolitic mantle around 670‐km depth: Implications for upwelling of plumes from the lower mantle

Phase relations in the natural pyrolitic mantle composition (KLB‐1) around 670‐km depth have been determined at 1600–2200°C by high‐pressure experiments using multianvil apparatus. A phase transition between majorite garnet and Al‐bearing Mg‐rich perovskite occurs at depths similar to the postspinel phase transition. The seismic discontinuity observed at this depth could be caused by a combination of both transitions. The majorite‐perovskite transition boundary has a positive Clausius‐Clapeyron slope (+0.0013 GPa/°C for majorite‐out curve), in contrast to the negative slope of the postspinel phase boundary (−0.0028 GPa/°C). Both transition boundaries cross each other at 1700–1800°C. Below this temperature, formation of Mg‐perovskite starts at the majorite‐perovskite or akimotoite‐perovskite transition coexisting with ringwoodite but is predominantly formed by the postspinel phase transition at higher pressures. On the other hand, the stability of majorite significantly expands relative to Mg‐perovskite at higher temperatures. Majorite becomes a dominant postspinel phase, and a majority of Mg‐perovskite is formed by the majorite‐perovskite transition with a positive Clapeyron slope. Phase transition within high‐temperature plumes (>1800°C at 670‐km depth) assists their upwelling from the lower mantle through the 670‐km boundary. The depth of the 670‐km seismic discontinuity becomes much less temperature‐sensitive and is greater in such a higher temperature region.