Paleomagnetic and rock magnetic studies of basement basalts recovered during IODP Expeditions 320/321
Abstract
During IODP Expeditions 320 and 321, basement basalts with estimated ages of about 53, 50, 46, 38, 26, 24 and 18 Ma were recovered from eastern equatorial Pacific. We took cylindrical mini cores with 2.5 cm diameter from split core-catcher samples or working halves. From Site U1337 (estimated crust age of 24 Ma), four and two mini cores were taken from azimuthally un-oriented core segments with about 10-20 cm long from Holes C and D, respectively. Secondary components were erased by 2-28 mT AFD and characteristic components could be obtained except one core from Hole C (SEM observation clarified that magnetic minerals of the core were eroded by secondary calcite veins). Median destructive fields (MDFs) of the cores range between 10 and 25 mT, and thus the characteristic components are not considered to be affected by drilling remanence. Mean paleomagnetic inclinations (I) are estimated as I=-23.1 (N=3) for Hole C and I=17.2 (N=2) for Hole D, based on the inclination-only statistics by Arason and Levi (2010). Corresponding paleolatitudes are calculated as 8.80 deg for Hole C and 12.1 deg for Hole D, but these are estimations only from spot-readings of the ancient geomagnetic field. One mini core was taken from a relatively vertically long cobble (about 6 cm long) from Hole B of Site U1333. It gives a reference paleomagnetic inclination of I=-11.4 (N=1) after AF cleaning, apparent paleolatitude of which is calculated as 5.76 deg. Other basement basalts were recovered as fragments in the size up to about 10 cm. Unfortunately no orientation information is available for these samples and it is impossible to convert paleomagnetic directions in to geographically meaningful ones. For most samples, thermomagnetic experiments showed Curie temperatures of 300-400 C for main phase and irreversible increase in induced magnetizations above 400-500C on heating. This indicates that main magnetic carriers of the samples are Ti-rich titanomagnetite and their low-temperature oxidation products of titanomaghemite. However, some samples did not show `titanomaghemite’-like thermomagnetic curves. They might be able to use for absolute geomagnetic paleointensity determinations and we will apply the LTD-DHT Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003) to them.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMGP11A0736Y
- Keywords:
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- 1500 GEOMAGNETISM AND PALEOMAGNETISM