Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmpp13c1472f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #PP13C-1472
Other
1028 Composition Of Meteorites (3662, 6240), 1065 Major And Trace Element Geochemistry, 6022 Impact Phenomena (5420, 8136), 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
We have analyzed the elemental content of sediments, magnetic grains and microspherules found in the 12.9 ka Younger Dryas boundary layer (YDB) at eight North American Clovis-age sites, a site in Belgium, and 16 Carolina Bays using Prompt Gamma-ray Activation Analysis (PGAA), Instrumental Neutron Activation Analysis (INAA), X-ray fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and low- background gamma-ray counting techniques. Magnetic grains, which sharply peak in the YDB, have typical terrestrial composition at the Gainey, MI site but are highly enriched in titanium averaging 22 wt.% TiO2 at other sites with an average ratio TiO2/FeO=0.73. Magnetic grains, which were distributed throughout the rims of the Carolina Bays, averaged 15 wt.% TiO2 with an average ratio TiO2/FeO=0.79. Magnetic microspherules, found only in the YDB at all sites including Gainey, averaged 34 wt.% TiO2 with an average ratio TiO2/FeO=0.77. The nearly identical TiO2/FeO ratios for magnetic grains and microspherules suggest that they have a common origin. TiO2/FeO=0.12 in Earth's crust, and no terrestrial or extraterrestrial sources have such high ratios except for lunar KREEP with TiO2/FeO=0.63 in Apollo 17 basalt. REE concentrations in magnetic grains have negative Eu signatures characteristic of KREEP and the relative concentrations of Sc, V, Mn, Co, Ni, As, Zr, Sb, Ce, REE, Hf, W, Ta, Th, U, and other trace elements in magnetic grains are consistent with KREEP ratios at all sites except Gainey. Potassium is significantly enriched in 40K in YDB sediments and Clovis chert which is consistent with known Fe-rich meteorites and 40K/K ratios inferred from a comparison of Lunar Prospector Gamma-ray Spectrometer (GRS) and ground truth data. The high concentration of magnetic grains with terrestrial composition at Gainey suggests that they are local ejecta from a nearby impact site. High water content was found in the magnetic grains at all sites, e.g. 18 at.% H at Gainey, which is consistent with their formation in a steam explosion following an impact into the nearby Laurentian Ice Sheet. Although no craters have been identified with the YDB impact, four deep holes extending to 193-723 feet below sea level in Lakes Superior, Michigan, Huron, and Ontario are candidates. Also in Lake Ontario, a proposed Pleistocene-aged 1-km impact crater has been identified at Charity Shoal where a negative magnetic anomaly also exists. Similar titanium-rich compositions of magnetic grains and microspherules at numerous sites far from Gainey suggest that they are ejecta emitted directly from the airburst of the impacting object. The major axes of the Carolina Bays are oriented towards the Great Lakes and the KREEP-like composition of magnetic grains found mixed throughout the Bays establishes their origin in the YDB impact, but the process that formed the Bays is not yet known. The similarity between YDB magnetic grains and microspherules to lunar KREEP is a mystery but it is interestingly that SAU-169, a meteorite ascribed to lunar Procellarum KREEP Terrane origin, landed in Oman near the time of the YDB impact.
Firestone Richard B.
Hagstrum Jonathan T.
Que Hee S. S.
Revay Zs.
Smith Aaron
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