Press Releases
Jan. 26, 2009

Subduction of Basaltic Crust into the Lowermost Mantle

— Evidence from Inversion of Seismic Waveforms —
Presenters
  • Kenji Kawai (Researcher, Interactive Research Center of Science, Graduate School of Science and Engineering, Tokyo Institute of Technology; and Visiting Researcher, Dept. of Earth and Planetary Science, School of Science, The University of Tokyo)
  • Robert Geller (Professor, Dept. of Earth and Planetary Science, School of Science, The University of Tokyo)

Summary

Figure 1

Figure 1. The earthquakes (red points) and observatories (blue points) used in our study. The curves connecting these points are the raypaths, and the red portions show the portions of the raypaths sampling the D″region beneath the western Pacific. In this research we mainly used data from the F-net seismic network in Japan.

Figure 2

Figure 2. Our inversions yield a model (MODEL) for the variation of seismic velocity with depth in the D″region beneath the western Pacific. The marked low in velocity at depths around 2700km is interpreted as due to phase transitions in basaltic rocks from perovskite structure (pv) to post-perovskite structure (ppv) and from CaCl2-type structure to α-PbO2-type SiO2, both of which result in a decrease in S-velocities. We also find an increase in the S-velocity at a depth of about 2800km, which is consistent with the phase transition from pv structure to ppv structure expected for average mantle composition models. As the velocity first decreases and then increases with increasing depth we call the model in Fig. 2 an "S-shaped model." This figure also shows, for comparison, a global standard model (PREM) and a model of the average velocity in this region (PREM').

Beneath the Earth's crust lies the mantle, composed of rocks, which occupies over 80% of the Earth's volume, and beneath the mantle is the metallic core (the outer core is liquid and the inner core is solid). The mantle has been convecting for billions of years, and the lowermost several hundred kilometers of the mantle, referred to as the D″(read "D double prime") region is of particular importance for understanding the physical and chemical evolution of the Earth.

Studies of seismological data to determine the Earth's seismological structure in general, and that of D″in particular, have provided information on the physical state (e.g., chemical composition and temperature) of the Earth's interior. Previous research has shown the existence of a low (relative to the global average for that depth range) S-wave velocity zone (which we refer to hereafter as "LVZ") beneath the western Pacific, but the reason for its existence is still a topic for discussion and research. In our research we use methods developed by our research group, called "waveform inversion," to determine Earth structure models objectively and quantitatively directly from observed seismic waveforms. We use waveform inversion to study the LVZ in the D″region beneath the western Pacific. We find that the variation of S-velocity with depth in this region is given by an "S-shaped" model (see below), which suggests the presence of basaltic rocks that have been subducted and transported by convection from shallow depths to the lowermost mantle.

Details

The rocks in the Earth's mantle have been convecting for billions of years. This convection causes generation of fresh basaltic rocks at mid-ocean ridges and subduction of oceanic plates at trenches. The rocks in the upper mantle and lower mantle have different mineralogical phases, and scientific discussion continues on the effect of the transition zone (the boundary zone between the upper and lower mantle) on convection. Geochemical arguments based on isotopic analyses have suggested the existence of separate convection systems in the upper and lower mantles, while arguments based on convection dynamics suggest whole-mantle convection, and intermediate possibilities have also been suggested. Knowledge of the fate of subducted basaltic rocks can thus provide important information on mantle convection and the Earth's evolution. Our analyses of seismic waveforms provide new information that can contribute to resolving the above issues.

Because the D″region is the lower boundary region of convection in the mantle, the possibility of marked heterogeneity of temperature and chemical composition has been suggested. Previous seismological studies of the D″region beneath the western Pacific have suggested the existence of a large-scale S-wave LVZ in this region. However, whether this is due to temperature differences, or compositional differences, or both remains a subject for debate. Previous studies of the seismological structure of this region lacked the spatial resolution (vertical resolution in particular) to resolve this issue. In this study we use methods for waveform inversion developed by our group, and analyze broadband seismic data for the earthquakes and observatories shown in Fig. 1. We obtain the model shown in Fig. 2, which shows the variation of seismic velocity with depth in the D″region beneath the western Pacific. The marked low in velocity at depths around 2700km is interpreted as due to phase transitions in basaltic rocks from perovskite structure (pv) to post-perovskite structure (ppv) and from CaCl2-type structure to α-PbO2-type SiO2, both of which result in a decrease in S-velocities. We also find an increase in the S-velocity at a depth of about 2800km, which is consistent with the phase transition from pv structure to ppv structure expected for average mantle composition models. As the velocity first decreases and then increases with increasing depth we call the model in Fig. 2 an "S-shaped model." This model suggests that basaltic rocks are carried by convection to the base of the mantle.

Publication

Journal
Earth and Planetary Science Letters (Published by Elsevier, Publication of on-line version of paper scheduled for January 20, 2009)
Citation of paper
Konishi, K., K. Kawai,, R. J. Geller, and N. Fuji (2009), MORB in the lowermost mantle beneath the western Pacific: Evidence from waveform inversion, Earth and Planetary Science Letters, in press.