Report for the Joint Use/Research of the Institute for Planetary Materials,Okayama University
2022 fiscal year second call
01/19/2023
Category:International Joint Research

Name of the research project: The relative strength of MORB and pyrolite at the upper/lower
mantle boundary
Principal applicant: Fang Xu
Affiliated institution and department: Department of Earth Sciences, University College London
Collaborator Name: David Dobson
Affiliated institution and department: Department of Earth Sciences, University College London
Collaborator Name: Daisuke Yamazaki
Affiliated institution and department: Institute for Planetary Materials, Okayama University
Research report:
Subducted oceanic lithosphere is composed of two chemically distinct layers, namely
mid-ocean-ridge basalt (MORB) and depleted peridotite layers (harzburgite, Hbg). The fate ofMORB and Hbg layer has important implications for: (1) complex slab behavior and mantle
convection around 660-1000 km depth; (2) the production of seismic scatterers in the lower mantle;(3) the source of enriched OIB at the core-mantle boundary and the base of the transition zone and(4) the composition and origin of enriched components in OIB magmas. The strength (viscosity)
contrast of the MORB layer with the slab core and surrounding mantle plays an important role incontrolling the behavior of MORB layer, e.g., a moderately stronger (0-2 orders) MORB layer isfavored for the delamination of MORB atop 660 km (Karato, PEPI 99, 103-111, 1997). However,
little is known about the viscosity of deep subducted slab and its components. Therefore, we
propose to measure the relative strength of MORB and Hbg under the range of pressure andtemperature conditions of the subducting slab across the mantle transition zone to ~800 km depth.
We planned to conduct deformation experiments on MORB and Hbg two layered samples, i.e.,
pre-synthesized ringwoodite (Rw)/post-spinel (PS) sample against MORB composition, which isdominated by majorite garnet and stishovite synthesized at the same pressure with Rw/PS, with
D111 apparatus. The strain contrast enables a direct measurement of the relative strength of MORBand Hbg hence to imply the behavior of slab at lower mantle transition zone and top lower mantle.
Before and after the deformation experiments, the samples are analyzed by XRD, SEM and EDS

carefully for phase identification, structure, and composition.
During the visit, firstly, three testing experiments were conducted to calibrate 8/3 cell assemblyin D111 apparatus. In total four deformation experiments on Rw and MORB two layered samples
were conducted at 1200-1600 ℃. We found that the Rw and MORB shows very small strain
contrast, i.e., smaller than a factor of two, which indicates a comparable viscosity of Rw andMORB. Before the good load for deformation of Rw was found, three Rw samples partiallytransformed to post-spinel mixture (PS) in deformation experiment, thus the viscosity contrast of
PS+Rw and MORB were obtained at 1400 ℃. It turns out that the PS+Rw samples is much softer
than MORB sample, up to one order of difference. It strongly suggests that the PS just after thephase transition, which shows eutectoid texture of ferropericlase and bridgmanite with grain size of~200 nm, might be much softer than majorite garnet. We therefore pre-synthesized PS sample witheutectoid texture and conducted the deformation experiment at 1400 ℃; a large strain in PS andlarge strain contrast with MORB sample was observed.
The results of the joint use this time, plus our previous result on wadsleyite and PS indicate thathighly likely that MORB and Hbg layer have comparable strength through transition zone to lower
mantle except extreme softening of PS at the depth just below PS phase transition, which is ~660km. It will give a significant implication for slab delamination and MORB stagnation at top lower
mantle. More experiments are needed to further study the details. We will propose the next term
joint use with IPM on the future study on this project.