Paleosecular Variation, Geochronology, And Magnetic Mineralogy Of Pleistocene Igneous Rocks Of Ascension Island, South Atlantic Ocean

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Eos, Transactions, American Geophysical Union




Paleomagnetic samples were collected from 68 sites on Ascension Island between May 14 and June 18, 2001. Paleomagnetic analyses of samples were performed at the University of Puget Sound paleomagnetic laboratory. Preliminary results indicate that 32 sites yield normal directions, six sites give reversed directions, and three sites exhibit intermediate directions (VGP latitude less than 60°). Eight sites exhibit too much scatter in paleomagnetic directions to be useful and nineteen sites have yet to be analyzed as of 9/1/02. Mean directions for both normally and reversely magnetized units tend to exhibit inclinations steeper than expected for Ascension Island. Ar/Ar dating of 20 samples was completed at the University of Wisconsin, Madison. Ages range from 50.9+/-7.9 ka to 1086.2+/-15.2 ka. Ages and polarities indicate directions represent the Matuyama Chron, Brunhes Chron, and Jaramillo Subchron. Sixteen samples from twelve sites were examined using reflected light microscopy at Tarleton State University. Eight sites appear to be relatively unaltered. Three sites show moderate alteration, but appear to retain substantial primary magnetic mineralogy and record primary magnetic directions. One site is extensively altered and probably records secondary magnetism. Optical examination confirms inferences from thermal demagnetization that magnetic mineralogy is largely dominated by titanomagnetite with some units containing significant hematite. A substantial paleosecular variation (PSV) database has been compiled for the past 5 Ma, but numerous areas, including the South Atlantic are underrepresented. This study has produced enough magnetic directions from the last million years or more to fill a gap in the PSV database, which can be used to test and refine emerging geodynamo models.