Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.u33a0004d&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #U33A-0004
Other
1099 General Or Miscellaneous, 2129 Interplanetary Dust, 6015 Dust
Scientific paper
Thirty-five deep ocean sediment samples from the Central Pacific sediment core LL-44 GPC-3 were examined for their noble gas composition and mineralogy. The samples spanned from 30 to 71 Ma in age, including the Cretaceous/Tertiary (K/T), Paleocene/Eocene (P/E), and Eocene/Oligocene (E/O) boundaries. From each bulk sediment sample, magnetic grains (5-200 μg/g; 1-20 μm diameter) were isolated and analyzed. Noble gas measurements determined the helium and neon isotopic compositions and the abundance of extra-terrestrial (ET) noble gases for the bulk and magnetic fractions. 3He/4He ratios of 3.1x10-4 and 20Ne/22Ne ratios of 9.96-12.62 are consistent with the SEP (solar energetic particle) signature seen in both zero-age magnetic grains (Z-MAG) from the central Pacific sediments and stratospheric interplanetary dust particles (IDPs). The isolated magnetic fraction typically consisted of less than 1% of the bulk sample while accounting for 3 to 10% of the bulk sediment 3He with a maximum of 40% at K/T boundary. The magnetic isolates revealed no significant differences of 3He/4He, 20Ne/22Ne, or (3He/20Ne)solar from the bulk GPC-3 sediments or Z-MAG grains. No temporal variation in He or Ne or anomalous gas signatures associated with the boundaries present within this time interval was observed. Scanning electron microscope analysis was utilized to determine the mineralogy of the magnetic isolates in an effort to distinguish between the continuous flux of interplanetary dust particles and the flux associated with major impact events. This information may also assist in determining the carrier phase of ET noble gases. Anomalously high Ni, Mg, Al, and Cr compositions combined with low or nonexistent abundances of titanium can distinguish extraterrestrial spinel grains from terrestrial spinel. The SEM analysis revealed stark contrasts in composition and morphology in the samples associated with the boundaries included in this study. A survey of ``background'' samples revealed an abundance of titanium rich Fe-Ti oxides, magnetite, and spinel proper with weathered appearance or typical cubic spinel structure. The terrestrial types of spinels also constituted a significant portion of the spinel grains in boundary samples. Only the boundary samples contained an estimated 20-60% extra-terrestrial spinel grains. The K/T and E/O boundaries revealed an abundance of spinel grains with ET compositions. Many ET spinel grains were distinguished by their peculiar ``honeycomb'' or ``skeletal'' morphology, which are indicative of rapid cooling from a vapor cloud after impact. Other grains with ET composition retained the traditional spinel structure and have a more uncertain mode of formation. The Paleocene/Eocene bulk sample did not provide an adequate magnetic separate for a complete SEM analysis, although two grains revealed ET composition. The noble gas analysis reveals an ET component of He and Ne throughout the entire sampling interval. The carrier component of the noble gases is clearly concentrated in the magnetic fraction, but is not readily distinguished by the SEM analysis. However this method provides an accurate and resilient method of locating impact boundaries within deep ocean sediment cores and may eventually lead to an understanding of the carrier phase for ET noble gas accretion in interplanetary dust particles and meteorites.
Darrah Thomas H.
Poreda Robert J.
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