The amount of water reserved in a “normal” oceanic mantle transition zone beneath the northwestern Pacific, inferred from data of coincident ocean bottom electromagnetic and seismic observations

Abstract:

"Is the mantle transition zone (MTZ) a major water reservoir of the Earth?" To answer this question, we have inferred the amount of water reserved in a “normal” oceanic MTZ beneath the northwestern Pacific from electrical and seismic structures. The field experiment was conducted with arrays of ocean bottom electromagnetometers and broadband ocean bottom seismometers in 2010-2014. Our innovative ocean bottom instruments, Earth’s electric field observation system was used to measure time-variations of electric field with high S/N ratio at periods of >10⁵s where the electromagnetic field is sensitive to the MTZ, and broadband ocean bottom seismometer of the next generation was used to record the earth motion with as low noise level as land stations.

Electromagnetic data were first analyzed to derive MT and GDS response functions at periods of >10⁵s. Then, model responses were numerically obtained from electrical conductivity models overlain by earth-surface land-ocean heterogeneity. Comparison between the observation and the prediction suggested that a MTZ electrical conductivity structure beneath the northwestern Pacific is similar to an averaged 1-D structure of the north Pacific [Shimizu et al., 2010].

Seismic data were analyzed through a P-wave receiver function method. Depths of 410 and 660 km discontinuities and thickness of the MTZ were variable by areas. The MTZ thickness beneath the southern part of Area A (northwest to the Shatsky Rise), 270 km, is larger a global average of 243 km [e.g., Flanagan & Shearer, 1989], while those beneath Area B (southeast to the Shatsky Rise), 241 and 248 km, are comparable to the average.

The water content and a thermal structure in the MTZ beneath Area A were inferred from the data. First, a temperature perturbation was determined to -70~-150 K from the thickness perturbation of the MTZ. Then, a thermal structure for Area A is assumed to a structure [Katsura et al., 2010] with the temperature perturbation. Electrical conductivity models with variable water contents for the MTZ were constructed based on the thermal structure and conductivity measurements of hydrous wadsleyite and rigwoodite in laboratory [e.g., Yoshino & Katsura, 2010]. Comparisons of predictions from these models with the observation suggest that the maximum amount of water in the MTZ beneath Area A is 0.5 wt%.