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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS012 Oral Presentation 2192
The fate of morb crust at the core-mantle boundary
Prof. Kei Hirose
Department of Earth and Planetary Sciences Tokyo Institute of Technology
Kenji Ohta, Thorne Lay, Nagayoshi Sata, Yasuo Ohishi
Subduction of mid-oceanic ridge basalt (MORB) gives rise to strong chemical heterogeneities in Earths
mantle, possibly down to the core-mantle boundary. The subducted MORB is denser than the
surrounding mantle by about 3% at the base of the mantle (Hirose et al., 2005), so if slabs penetrate to
the D region, MORB material could be stable there. Geodynamical simulations have demonstrated that
this magnitude of density contrast can induce separation of MORB crust from the balance of the slab
and the dense MORB-enriched materials may have accumulated in dense, chemically distinct piles at the
bottom of the mantle (Christensen and Hofmann, 1994; Tackley, 1998; McNamara and Zhong, 2004).
These piles may be formed underneath upwelling regions. However, direct seismological evidence for
the presence of MORB piles in the lowermost mantle has not been presented, in part because the
velocities properties of MORB are not well characterized for lowermost mantle conditions. We precisely
determined the phase relations in both pyrolite and MORB compositions at high pressures and
temperatures corresponding to lowermost mantle conditions, based on in-situ X-ray diffraction
measurements at BL10XU of SPring-8. Pressures were estimated from the new P-V-T equation of state
of Au (Sata et al., 2007), which is consistent with the MgO pressure scale proposed by Speziale et al.
(2001). Experimental results demonstrate that the post-perovskite phase transition occurs in pyrolite
between 116 and 121 GPa at 2500 K, while post-perovskite and SiO2 phase transitions occur in MORB
at ~4 GPa lower pressure at the same temperature. Theory predicts that these phase changes in
pyrolite and MORB cause shear wave velocity increase and decrease, respectively. Near the northern
margin of the large low shear velocity province in the lowermost mantle beneath the Pacific, reflections
from a negative shear velocity jump near 2520-km depth are followed by reflections from a positive
velocity jump 135 to 155-km deeper. These negative and positive velocity changes are consistent with
the expected phase transitions in a dense pile containing a mixture of MORB and pyrolitic material. This
may be a direct demonstration of the presence of subducted MORB crust in the deep mantle.
Keywords: morb, core mantle boundary, post perovskite