26 Planetary Geology (Posters)

THE SPATIAL DISTRIBUTION AND CHEMICAL DIFFERENCES OF TEKTITES FROM A CRATER IN THE TASMAN SEA

Paper number: 26-10
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Geophysical evidence suggests that there is a structure, named the Mahuika crater with a 20 km diameter that lies on the southern edge of the New Zealand continental shelf. If an impacting body created this structure, the surrounding area should contain evidence of such an impact. The most reliable and widespread evidence of an impact is that of tektites. Tektites form when an impactor liquefies its target and sends the melt into the atmosphere. The Mahuika tektite field contains glassy tektites that appear orange, light green, and clear in visible light. The tektite field does not extend radially outward from the crater; instead, it appears to extend roughly thirty degrees to either side of a line drawn southwest from the center. This shape suggests an impact that occurred at a very low angle relative to the earth’s crust. This oblique angle also agrees with Aboriginal legends that tell of a great fireball passing through the sky in southeastern Australia. A great circle path that uses the northwestern most sighting of the fireball and the location of the structure in the Tasman Sea, almost exactly bisects the field where tektites have been found.

Tektites have been found over 220 km from the crater. Despite these large distances the chemistry of the tektites is surprisingly similar. The tektites were analyzed using energy dispersive x-ray analysis. Despite the inherent error in this technique, the analysis showed average weight percents of 48% O, 27% Si, 10% Fe, 3% Mg, 3% Al, 3% K and trace Ni. More precise chemical data should be available in September 2003 when electron microprobe analysis is complete. In addition to similar compositions the tektites also have a very uniform size. All of the tektites range from 40-51 microns in diameter. This size range fits with the scaling predicted for a crater the size of the Mahuika structure. It is also important to note that the tektite field was mapped using dredges. If you assume a normal sedimentation rate of 1cm/thousand yrs, then the event happened within the last thousand years. The date of the event will become more constrained after a C-14 date is obtained from the fossils in surface samples.