Density profiles of pyrolite and MORB compositions across the 660 km seismic discontinuity

Abstract

Precise density changes across 660 km seismic discontinuity have been determined for pyrolite and mid-oceanic ridge basalt (MORB) compositions at 1873 K at high pressures using a combination of in situ X-ray diffraction on unit cell measurements and chemical composition analyses on the recovered products, using a multiple sample chamber cell assembly. Density jump for the post-spinel transformation in pyrolite was determined to be ∼9%, which is comparable to that of PREM. The density of MORB is found to be lower than that of pyrolite at the depths of ∼590–680 km (using Anderson's equation of state for Au), however, it becomes denser than the pyrolite below ∼680 km. The density cross-over region at ∼590–680 km may contribute to the stagnation of subducted slabs near the 660 km discontinuity, as suggested by seismological observations as well as earlier experimental studies based on quench experiments and extrapolated in situ X-ray measurements.

Keywords:

• pyrolite
• MORB
• density
• 660 km seismic discontinuity
• in situ X-ray diffraction
• multiple sample chamber

Acknowledgements

We thank N. Nishiyama, Y. Sueda, T. Inoue, D. Yamazaki, Y. Higo, A. Kurio, K. Ochi, D. Watanabe and T. Kunimoto for useful discussions and technical support at SPring-8. We also thank S. Sakamoto and D.-N. Hamane for useful comments on ATEM analyses and T. Minagawa and X. Liu for providing us with the standard ATEM samples. We are grateful to Y. Wang for his careful reading and useful comments on this manuscript. We appreciate Y. Fei for a review of this manuscript. The in situ X-ray diffraction experiment was conducted at BL04B1, SPring-8 (Proposed No. 2002B0331-CD2-np, 2003A0291-ND2-np).