What is "normal oceanic mantle"?

Normal oceanic mantle project?

(1) Background of Research Project and Past-related Achievements of Applicants

We propose to conduct a research program toward understanding of the mantle dynamics from an innovative observational approach by answering two fundamental questions in Earth science:

(a) "What is the physical condition for the lithosphere-asthenosphere boundary (LAB)?" and
(b) "Is the mantle transition zone (MTZ) a major water reservoir of the Earth?"

The "normal" ocean floor is the best window to approach these questions as it allows us to see the inside of the Earth through it without the disturbance due to the thick and heterogeneous continental crust. However, any approach, if ever attempted, has not yet been successful because of technological difficulties in obtaining high-quality geophysical data in the ocean.

The present investigators had led the Ocean Hemisphere network Project (OHP) in 1996-2001, in which a network of geophysical observatories was built in the western Pacific region. Data from the OHP network, especially from broadband seismographs on land and under water, precise magnetometers, submarine cables to measure electric field, successfully provided improved global images of the Earth's interior in terms of seismic velocities and electrical conductivity (e.g., Utada et al., GRL, 2003). During the OHP project, we had also developed a set of new portable ocean bottom instruments, broadband ocean bottom seismometers (BBOBSs) and ocean bottom electro-magnetometers (OBEMs).

These new observation technologies have been fully utilized in the 5-year Stagnant Slab Project (SSP) that succeeded the OHP since 2004. In the SSP, we have carried out a long-term (3 years in total) joint observation of BBOBSs and OBEMs in the Philippine sea (Shiobara et al., EOS, 2009) to study the MTZ where the subducted Pacific slab appears to be stagnating. We have made significant contributions to the SSP by obtaining results such as the estimation of water content in the MTZ by joint interpretation of seismic and electromagnetic tomography (Koyama, Utada et al., AGU Monograph, 2006) and the seismic evidence for water transportation deep into the mantle by subducting slab (Kawakastu & Watada, Science, 2007).

Recently, we developed further innovative instruments (BBOBS-nx: broadband ocean bottom seismometer next generation; EFOS: Earth electric field observation system). By improving the mechanical coupling between the sensor housing and seafloor sediments, the BBOBS-nx enables us to record horizontal seismic motions, as well as vertical ones, with a typical noise level comparable to land observations. This gives us a strong advantage over other OBS's because it allows us to apply station-based powerful seismic analysis methods commonly used on land, such as the receiver function and shear-wave splitting analyses, for ocean bottom data. The EFOS, on the other hand, measures the electric voltage difference at the seafloor by using a 10 km long cable. Compared to the OBEM measuring the electric field with a spacing of only 5 m, it successfully reduces the noise level to 1/10 or lower so as to provide reliable estimates of electromagnetic responses in a wide period range (1,000-500,000 s) that have high sensitivity to the electrical conductivity in the upper mantle and in the transition zone. Therefore, we are now capable of providing strong constraints to answer the two fundamental questions (a) and (b) listed above by applying our advanced observational technologies to the "normal oceanic mantle" (as opposed to the mantle beneath subduction zones, hot spots or mid-oceanic ridges).

(2) Importance and Necessity of this Project and its Expected Impact upon the Target Field of Research

Plate tectonics is based on a concept that a rigid lithosphere moves over a weaker asthenosphere, and thus the precise knowledge of the processes at the LAB is fundamental to understand how our planet works. It is especially so for the oceanic plate, because the relatively simple nature of its creation and evolution should allow us easier elucidation of such processes compared to the continental plate. There are several views for the fluidity of the asthenosphere: models ascribing to partial melting (e.g., Anderson & Sammis, 1970), to the effect of grain size (e.g., Jackson, 2002) and to the effect of water (e.g., Karato & Jung, 1998). We expect to be able to distinguish these views from observations by estimating the LAB depth, sharpness, contrast and age-dependence via receiver function method, and by determining the distributions of seismic velocities and electrical conductivity above and below LAB, including their anisotropy, via various imaging techniques. These geophysical data are then given to dynamical and rheological interpretations, which will lead us to a new model of oceanic lithosphere and asthenosphere to solve the long-standing question what defines the oceanic plate. Recently we obtained seismological evidence indicating a sharp boundary corresponding to the LAB and proposed a new model of asthenosphere (Kawakatsu et al., Science, 2009), which is now called a partially molten melt-lubricated asthenosphere model. We will rigorously test all possible hypotheses, including our own, by multi-parametric analyses of high quality geophysical data in the proposed project.

The presence of water in the solid Earth plays essential roles in various significant problems of geodynamics, such as controlling viscosity, solidus condition, chemical partitioning, and so on. The distribution of water in the Earth, thus, is one of the most crucial knowledge, not only in understanding the dynamics and evolution of the Earth, but also in investigating the mechanism of earthquakes and volcanic eruptions. Recent results of mineral physics indicate that the materials in the MTZ can resolve much larger amount of water than those in other parts of the mantle (Ohtani et al., 2004). It is also suggested that the MTZ is a water reservoir in the Earth, as water transported either from the surface to the lower mantle or in the reverse direction will be dissolved in the MTZ. We have clarified how water is transported and distributes in the subduction zone mantle by the observational results of the SSP (Kawakatsu and Watada, Science, 2007; Utada et al., EPSL, 2009). However, the question may never be fully solved without the knowledge for the "normal oceanic mantle" that occupies the largest part of the entire mantle. We aim in the proposed project to give a quantitative estimate on the presence of water in the Earth's interior, especially in the MTZ, by joint interpretation of seismic and electromagnetic data.

The present project is expected to make two significant contributions to solid Earth science. One is made by providing important scientific results to answer the fundamental questions (a) and (b) on normal oceanic mantle as described above, and the other by displaying a new scientific approach to understand the mantle dynamics. In seismology, temporal array observations using leased portable broadband seismometers by the IRIS/PASSCAL program (started in late 1980's) made a significant contribution to the scientific progress. All users of the PASSCAL seismometers are requested to share data open to the community. As a result, a lot of scientific results, unexpected at the proposal of individual project, were obtained. Thus, the PASSCAL project introduced a breakthrough in seismology by providing an infrastructure to the community. If we were able to demonstrate, with this funding, that our technologies enable us to carry out ocean bottom array observations (jointly seismic and electromagnetic) of high quality similar to those on land, it would introduce a new infrastructure (e.g., PASSCAL for ocean) that will induce a new trend in the Earth science. This can be considered as another breakthrough in observational mantle dynamics.

(3) Research Objectives and Targeted Goals of this Project

The present project aims to answer the two fundamental questions (a) and (b) by a long-term deployment of advanced ocean bottom instruments (BBOBS, OBEM, BBOBS-nx and EFOS) on the normal ocean floor in the western Pacific. In the project, we will extensively employ seismological data analyses such as receiver function analysis, shear-wave splitting analysis, surface wave dispersion analysis/tomography, and Vp/Vs tomography, each of which will provide high resolution images. We will also employ electromagnetic imaging methods, which are developed rapidly in recent years largely by the effort of present investigators. Electromagnetic tomography (3-dimensional electrical conductivity inversion) and anisotropic conductivity inversion will provide additional and strong constraints on the presence and configuration of water and/or melt in the upper mantle down to the MTZ depth. Thus obtained variety of geophysical information will be envisaged to construct a new model of the oceanic mantle, by combining with the information on various physical properties and conditions of the mantle that will be provided by co-investigators. In summary, the proposed project aims to make a significant contribution to knowledge on the mantle structure and dynamics from observational approach.