Upper mantle electrical resistivity structure beneath the Southwest Indian Ridge 37ºE

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 国立極地研究所 ３階ラウン

Electromagnetic constraints on a melt region beneath the central Mariana back-arc spreading ridge

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 13 (2012): Q10017, doi:10.1029/2012GC004326.An electrical resistivity profile across the central Mariana subduction system shows high resistivity in the upper mantle beneath the back-arc spreading ridge where melt might be expected to exist. Although seismic data are equivocal on the extent of a possible melt region, the question arises as to why a 2-D magnetotelluric (MT) survey apparently failed to image any melt. We have run forward models and inversions that test possible 3-D melt geometries that are consistent with the MT data and results of other studies from the region, and that we use to place upper bounds on the possible extent of 3-D melt region beneath the spreading center. Our study suggests that the largest melt region that was not directly imaged by the 2-D MT data, but that is compatible with the observations as well as the likely effects of melt focusing, has a 3-D shape on a ridge-segment scale focused toward the spreading center and a resistivity of 100 Ω-m that corresponds to ∼0.1–∼1% interconnected silicate melt embedded in a background resistivity of ∼500 Ω-m. In contrast to the superfast spreading southern East Pacific Rise, the 3-D melt region suggests that buoyant mantle upwelling on a ridge-segment scale is the dominant process beneath the slow-spreading central Mariana back-arc. A final test considers whether the inability to image a 3-D melt region was a result of the 2-D survey geometry. The result reveals that the 2-D transect completed is useful to elucidate a broad range of 3-D melt bodies.TM and NS are supported by the scientific program of “TAIGA” (Trans-crustal Advection and In situ reaction of Global sub-seafloor Aquifer)” sponsored by the MEXT of Japan, and are also supported by the JSPS for Grant-In-Aid for Scientific Research (21244070). Participation in the Marianas experiment by RLE and ADC was supported by NSF grant OCE0405641.2013-04-2

Workshop report: Exploring deep oceanic crust off Hawai‘i

For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (> 110 Ma) and young (< 20 Ma) ocean crust, our sampling remains relatively shallow (< 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the long-term history of seawater–basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop “Exploring Deep Oceanic Crust off Hawai`i” brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai`i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a−1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai`i would provide an essential “pilot hole” to inform the design of future mantle drilling

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 11 (2010): Q09003, doi:10.1029/2010GC003101.This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.Japanese participation in the Marianas experiment was supported by Japan Society for the Promotion of Science for Grant-In-Aid for Scientific Research (15340149 and 12440116), Japan-U.S. Integrated Action Program and the 21st Century COE Program of Origin and Evolution of Planetary Systems, and by the Ministry of Education, Culture, Sports, Science, and Technology for the Stagnant Slab Project, Grant-in Aid for Scientific Research on Priority Areas (17037003 and 16075204). U.S. participation was supported by NSF grant OCE0405641. Australian support came from Flinders University. T. M. is supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Deep Ocean Exploration Institute

Measurement of geomagnetic field at sea during JARE-30, 1988-1989

Three components and total intensity of the geomagnetic field were measured by STCM (Shipboard Three Components Magnetometer) and a proton magentometer at the same time at sea during the 30th Japanese Antarctic Research Expedition. The measurements of total intensity of the geomagnetic field by a proton magnetometer showed a variation of noises in harmony with the variation of ship's velocity. This variation of noises may be caused by instability of the sensor due to large velocity of the ship. The measurements of three components of the geomagnetic field by STCM revealed short wavelength noiscs which were caused by small yawing of the ship. Reliable absolute values of three components of the geomagnetic field were obtained by adapting the total intensity of the geomagnetic field measured by a proton magnetometer to the data of three components of the geomagnetic field measured by STCM. In the present state of STCM, it is necessary to measure three components and total intensity of geomagnetic field by STCM and a proton magnetometer simultaneously, in case of obtaining the absolute values of three components of the geomagnetic field. These results suggest that improvement of measurement of the geomagnetic field at sea will be required

A new miniaturized magnetometer system for long-term distributed observation on the seafloor

Abstract We have developed a new magnetometer system specialized to multipoint and long-term observations on the seafloor to promote marine or ocean-bottom experiments mainly for the electromagnetic sounding of the Earth’s interior. In situ magnetic field observation on the seafloor is an essential geophysical technique to investigate structures of the oceanic crust and the upper mantle, many of which are still frontier as to observational evidences. The in situ and long-term observations require long-term-operable and small-size magnetometer systems, which are placed on the seafloor over a year in pressure-resistant cases without external power supply and communication. We have designed and developed a new electric circuit board of small size and lower power consumption for the magnetometer system. Our new magnetometer system, what we call “DOKODEMO MAG,” is suitable to be installed in a pressure-resistant cylinder of 36 mm diameter and can operate independently over 2 years with a smaller amount of batteries than the conventional system because its power consumption was saved to ~ 33 mW. This magnetometer system is capable of observing orthogonal three-axis magnetic fields continuously with sampling frequency of 5 Hz at maximum and an accuracy of ~ 0.1 nT. The system also records tilt and temperature of the system and voltage of the batteries. Prototype models of this magnetometer system were tested for in situ operation for 5 months on the seafloor around the Kikai caldera in the south of the Kyusyu Island, SW Japan. The results of the test showed sufficient performance of our new magnetometer system and its potential of future usage for every type of marine or ocean-bottom operations

PRELIMINARY REPORT OF THREE COMPONENTS OF GEOMAGNETIC FIELD MEASURED ON BOARD THE ICEBREAKER SHIRASE DURING JARE-30, 1988-1989

Measurement of three components of geomagnetic field was carried out on board the icebreaker SHIRASE during the 30th Japanese Antarctic Research Expedition (JARE-30). Vector anomalies of geomagnetic field were obtained and the directions of magnetic lineations were determined from the vector anomalies. They are in good agreement with the results previously reported along the ship\u27s tracks (e. g. ROYER et al. : Tectonophysics, 155,235,1988; ROYER and SANDWELL : J. Geophys. Res., 94,13755,1989), except for the Antarctic continental margin and the Enderby Basin. In the Antarctic continental margin, N-S trending magnetic structure that coincides with the Australian-Antarctic depression is detected between Australia and Antarctica and the local magnetic anomaly that seems to be caused by the Napier Complex appears between 50°E and 60°E along 63°S. Further, in the Enderby Basin around 60°S, N-S and NNE-SSW trending magnetic lineations which have never been reported before are detected. These results may suggest new constraints on the evolution of the Indian Ocean

GEOMAGNETIC ANOMALY FIELD VECTOR OFF WESTERN AUSTRALIA

Vector data of the geomagnetic anomaly field were obtained during the 32nd Japanese Antarctic Research Expedition (JARE-32) off Western Australia. The strikes of the magnetic boundaries at their position were derived from vector data of the geomagnetic anomaly field. These strikes were interpreted as the directions of magnetic anomaly lineations originated either by seafloor spreading (seafloor spreading anomaly) or by morphological structures (structural magnetic anomaly). Some strikes of structural magnetic anomaly are inferred to be of fracture zone origin. The strikes of the seafloor spreading anomaly and fracture zone in the Argo Abyssal Plain are concordant with those previously found. Structural magnetic anomaly strikes in the continental rise off Western Australia coincide with the fracture zone trend in the neighboring basin, and locate in the extension of the fracture zone trends in the basin. On the Naturalist Plateau, couples of NW-SE and NNW-SSE structural magnetic anomaly strikes with almost constant spacing are observed. These strikes are inferred to be from the fracture zone trend with an offset structure. NW-SE fracture zone trends have been reported in the Perth Basin neighboring the Naturalist Plateau, suggesting that these NW-SE fracture zone trends extended over to the Naturalist Plateau. The couples of NW-SE and NNW-SSE trend in the Naturalist Plateau probably indicate a change of direction of the fracture zone in time, namely NNW-SSE direction may show the initial breakup trend between India and Australia-Antarctica