2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 14
Presentation Time: 11:30 AM

The Brangane Asteroid Family Disruption: Source of the Late Eocene 2.5 Million Years of Dust Deposition, Impacts, and H Chondrite Meteoroids


HILDEBRAND, Alan R., Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada, PARKER, Alex, Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada, KYTE, Frank T., Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, CA 90095-1567 and SHUKOLYUKOV, Alex, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212, ahildebr@ucalgary.ca

Extraterrestrial material fell on the Earth at an enhanced rate during the Late Eocene (~35 Ma): anomalous 3He abundances in marine sediments indicate that the flux of <50 μm dust was enhanced by up to ~8 times for ~2.5 My; during this interval at least two multi-km bodies impacted to form the Popigai and Chesapeake Bay craters.

Siderophile element ratios from impactites of the Popigai crater and the associated, globally-distributed, spinel-bearing spherules have been interpreted as indicating an L or LL chondrite impactor. Cr isotope data from the spinel-bearing spherules unequivocally indicate that this impactor was an ordinary chondrite, but don't distinguish between H, L and LL. However, disruption of an H chondrite parent is indicated by a peak in H chondrite cosmic ray exposure ages of 33 - 36 My matching the interval of enhanced dust production. The meteorites of this peak have types of 3 to 6 indicating that a sizeable body was disrupted.

Increasing numbers of asteroid orbits allow identification of additional asteroid families whose cogenesis may be further tested with SDSS spectral data. Only one young S-type family named after asteroid 606 Brangäne has an age (50±40 My) encompassing the indicated Late Eocene disruption. Analysis of proper orbital elements in conjunction with SDSS colours now yields ~40 members of the Brangäne family. The amount of spectral reddening of the family members is reasonable for this age. SDSS family colours are unusual compared to those of other S-type families, but this could be consistent with an H chondrite lithology as some of the other S-type families are indicated as of L chondrite affinity. The Brangäne family is relatively well positioned in the inner part of the Middle Belt near the 3:1 resonance to allow large bodies to be perturbed into Earth-crossing orbits within the necessary <1 My.