Intellectual Merit: IODP Expedition 330 to the Louisville Seamounts starts 12 December 2010 from Auckland, New Zealand. A primary objective of drilling at the Louisville volcanic chain is to document the possible motion of the Louisville hotspot between 80 and 50 Ma. High quality paleomagnetic data are required to document this motion and to compare with the 15° shift observed for the Hawaiian hotspot over this time interval. These data, together with detailed radiometric ages, will provide the basis for calibrating and testing various geodynamic models.The most robust paleolatitude information will be derived from detailed demagnetization studies of the aziumuthally-unoriented core samples. In addition, we will deploy the Göttingen Borehole Magnetometer (GBM) to make 3-component magnetic borehole measurements. The main benefit of the GBM is that fiber-optic gryos should allow fully oriented component magnetic anomalies to be determined and magnetization declination to be estimated, facilitating identification of individual flows and a more robust estimate of hotspot and Pacific plate motions. Tests reveal that the sinker bar and centralizer to be used with the GBM both generate substantial magnetic anomalies (up to 10,000 nT at a distance of 1m). These large signals, and importantly the likely changes in these signals on different deployments, compromise the benefits of the magnetic logging.We therefore propose fabrication of a nonmagnetic sinker bar that will allow determination of accurate component magnetic anomalies (and remanent declinations) on Expedition 330 and subsequent expeditions. The large magnetic signature of the existing sinker bar and centralizer were documented in August; the short time before Expedition 330 begins requires rapid requires rapid fabrication of this tool. Broader Impacts: High quality paleomagnetic and age data are required to document the motion of the Louisville hotspot, a primary goal of Expedition 330. The proposed GBM magnetic logging has the potential to provide fully oriented vector magnetic anomalies in the borehole, significantly enhancing the constraints on the past motion of the Louisville hotspot and Pacific plate motion. A number of Expedition participants will be involved in interpretation of the borehole magnetometer results and integrating these results with data from the recovered core material.

Project Report

This project funded fabrication of a nonmagnetic bar that allowed more accurate measurement of the magnetic field within boreholes drilled during Integrated Ocean Drilling Program Expedition 330 to the Louisville seamount trail, a chain of submarine volcanoes in the southwest Pacific. Why is measuring the magnetic field within these boreholes useful? The submarine volcanoes sampled form a linear chain that is the southern hemisphere equivalent of the Hawaiian volcanic chain. Both chains of volcanoes are thought to have formed as the Pacific plate moved over deep and relatively stationary melting anomalies in the underlying mantle. To the extent that these melting anomalies are stationary, the resulting volcanic chains provide a record of absolute motion of the Pacific plate. The direction of Earth's magnetic field varies in a simple way as a function of latitude and therefore provides the most direct test of whether the melting regions have been stationary. If the melting anomaly was stationary, then the magnetic field direction should be the same in all the volcanoes sampled from a chain and this field direction can be determined by accurately measuring the field in the borehole. Although a magnetometer capable of measuring all components of the borehole magnetic field was available, the additional components needed to deploy this magnetometer were found to generate large magnetic fields and a nomagnetic replacement bar was needed to allow accurate measurements in the borehole. We fabricated a nonmagnetic bar and subsequent testing confirmed that it had negligible magnetic signal, as expected from the aluminum material used. The bar and borehole magnetometer was successfully deployed at two of the drill sites during Expedition 330. Comparison with the results of a second magnetometer also used during borehole logging operations illustrated the improvement with the nonmagnetic bar. The resulting borehole magnetometer data are presently being analyzed and should provide a valuable additional data set for evaluating whether the melting anomaly responsible for the Louisville chain has remained stationary. The new nonmagnetic bar is available for use with other logging deployments where the magnetic signal for the logging string should be minimized.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Thomas Janecek
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University of California-San Diego Scripps Inst of Oceanography
La Jolla
United States
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