A new pre-treatment pTRM Method for Paleointensity Determination: Application to Cretaceous Volcanic Rock Samples from China and Pacific Ocean

HR: 0830h
AN: GP31B-0755 [PDF]
TI: A new pre-treatment pTRM Method for Paleointensity Determination: Application to Cretaceous Volcanic Rock Samples from China and Pacific Ocean
AU: * Zhao, X
EM: xzhao@es.ucsc.edu
AF: Institute of Geophysics and Planetary Geophysics, University of California, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Zheng, Z
EM: tei@sogo-geo.co.jp
AF: Geoscience Division, Sogokaihatsu Co, Ltd., Nishi-Koiwa 1-30-16, Tokyo, 133-0057 Japan
AU: Coe, R S
EM: rcoe@es.ucsc.edu
AF: Institute of Geophysics and Planetary Geophysics, University of California, 1156 High Street, Santa Cruz, CA 95064 United States
AB: We present a pre-treatment Thellier method which involves partial alternating field (AF) demagnetization prior to regular Thellier heating steps. Our method can help improve the experimental scheme in the Thellier paleointensity method and extract paleointensity information from many samples that contain pseudo-single domain (PSD) and multidomain (MD) grains. Although NＦl's theory satisfactorily explains the magnetic properties of single-domain (SD) grains, which are ideal recorders of both direction and intensity of the past geomagnetic field, numerous subsequent paleomagnetic research, coming from both continents and oceans, showed that only rarely the magnetic carriers in rocks are pure SD grains. In most cases, samples contain PSD and MD particles as remanence carriers. Theoretical and experimental studies have shown that the law of additivity of partial TRM (pTRM) required in the Thellier paleointensity method is no longer valid for MD particles, as well as for PSD particles that contain more MD-like grains. We have developed a technique that combines BEI photo observation and AF pre-treatments to make sample remanence more SD-like. We found that mild AF pre-cleaning is effective to erase the MD spectrum, leaving the SD-like spectrum untouched. We estimate the paleointensity of samples based on comparison of their pTRM blocking temperature spectra and NRM unblocking temperature spectra. We have applied our technique to several Cretaceous aged rock samples from China and the Pacific Ocean. Preliminary results show that these samples can be pre-treated by AF demagnetization to render them more useful for paleointensity studies.
DE: 1519 Magnetic mineralogy and petrology
DE: 1521 Paleointensity
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting