Phase Relations of Harzburgite and MORB up to the Uppermost Lower Mantle Conditions: Precise Comparison With Pyrolite by Multisample Cell High‐Pressure Experiments With Implication to Dynamics of Subducted Slabs

Abstract

We determined phase relations of harzburgite and basalt (mid-ocean ridge basalt, MORB) at 12-28 GPa and 1600-2200°C with a large number of experiments using a multianvil high-pressure apparatus. These phase relations were precisely compared with those of pyrolite simultaneously determined by multisample cell technique. The post-spinel (pSp) transition of harzburgite occurs at 23 GPa and 1600°C with the boundary slope of −3 ± 1 MPa/°C. The post-garnet (pGt) transition boundary of MORB, defined as the beginning of the majorite garnet-bridgmanite transition, is located at 25 GPa and 1600°C, with a slope of 1.5 ± 1 MPa/°C. The pSp transition of harzburgite occurs at higher pressure by 0.5 GPa at 1600°C and has the more negative slope than that of pyrolite (−1 ± 1 MPa/°C). The pGt transition of MORB occurs at higher pressure by 3 GPa than the pSp transition of harzburgite. At 1600°C, the density of pyrolite is smaller than those of harzburgite and MORB before the pSp transition of pyrolite (pyrolite-harzburgite/MORB = −0.17 g/cm 3 ). After the pSp transition of pyrolite, harzburgite has lower densities than that of pyrolite at 22-27 GPa (pyrolite-harzburgite = 0.31 and 0.08 g/cm 3 before/after the pSp transition in harzburgite, respectively). On the other hand, the density of MORB becomes the highest among the three rocks above 25 GPa after the pGt transition of MORB (pyrolite-MORB = −0.18 g/cm 3 ). The present density contrasts suggest that harzburgite and MORB may stagnate and accumulate in the transition zone by slab subduction, resulting in that the pyrolitic part of slabs is subducted into the lower mantle.

Effect of Water on Lattice Thermal Conductivity of Ringwoodite and Its Implications for the Thermal Evolution of Descending Slabs

“…The water‐bearing phase in a slab located in the MTZ are expected to consist of nominally anhydrous minerals (e.g., wadsleyite and ringwoodite) and dense hydrous magnesium silicates (DHMS, such as superhydrous Phase B or Phase D) (Ohtani et al, 2004). Once ringwoodite reaches the base of the MTZ, it decomposes into LM assemblage (e.g., Ishii et al, 2018; Ishii, Huang, et al, 2019; Ishii, Kojitani, & Akaogi, 2019; Litasov et al, 2005), which is expected to host only ≈1,000 ppm of water (Fu et al, 2019; Litasov et al, 2003). Due to the release of water around 660‐km depth, this decomposition most likely causes major slab dehydration (Schmandt et al, 2014).…”

Delayed Growth of Ferropericlase and Bridgmanite Controls Slab Residence at the 660‐km Discontinuity

“…The high‐ P displacement (∼27 GPa) of the majorite breakdown is consistent with the MAS univariant majorite = bridgmanite + corundum reaction that stabilizes corundum as a suitable host for excess alumina (Hirose et al, 1999; Figures 1 and 3). Most experimental results favor the T ‐sensitive growth of hexagonal new‐Al‐silicate phases (Mookherjee et al., 2012), a Ca‐ferrite‐type or a layered Ca‐Al‐Si oxide mineral (zagamiite [CaAl 2 Si 3.5 O 11 ]; Ma, 2018) instead of corundum, though the P–T conditions of the modeled reaction involving corundum are consistent with those involving other aluminous experimental phases (Ishii et al., 2019; Hirose et al., 1999; Litasov & Ohtani, 2005; Litasov et al., 2005; Ono et al., 2001). The boundaries of the new‐Al‐silicate and Ca‐ferrite‐type phases correspond well with the experimental results of Ishii et al.…”

“…Higher‐variance changes that control the mineral transformation of basaltic crust will result in a reduction of its density during isobaric heating. This is mostly related to thermal expansion and less so to the growth of corundum and Ca‐ferrite‐type phases, which retain large uncertainties on their stability in association to difficulties tracking the distribution of Al‐rich phases in experimentation (Figures 3 and 5; Hirose et al, 1999; Ishii et al., 2019). Volume expansion is more pronounced at depths of 680 km relative to 660–670 km on account of the growth of majorite (Figure 5).…”

“…This variability influences the rate of change of density as well as a depth-dependency: cool subduction is predicted to induce pronounced densification at deeper conditions (∼700 km) than warmer subduction. Collectively these transformations induce volume reductions (0.2-0.3 cm 3 ) less significant than the breakdown of ringwoodite and are consist-CHAPMAN AND CLARKE 10.1029/2020JB021487 ent with the uncertainties of experimental relations (Figures 4-6;Ishii et al, 2019).…”

Oceanic plateau of the Hawaiian mantle plume head subducted to the uppermost lower mantle

“…In thermodynamic equilibrium, we find that the transition from a garnet-dominated assemblage to a bridgmanite-dominated assemblage in MORB occurs at 30.2 GPa (810 km depth) along the 1200 ˚C adiabat. This is considerably deeper than what was found in a recent experiment (25 GPa, or 700 km depth) (58), that was designed to produce conditions of thermodynamic equilibrium, and that was not considered in our determination of the species parameters used in our HeFESTo calculations. However, the subducted Hawaiian plume head may not be in thermodynamic equilibrium.…”