Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.430s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 430
Computer Science
Comets, Fireballs, Impact Craters, Impacts, Microtektites, Neutron Activation Analysis, Strewn Field, Tektites, Trace Elements
Scientific paper
New estimates of the total masses of tektites and Ir fallout in the Australasian strewnfield offer information about the crater diameter of the large source crater or craters that launched Australasian tektites and Australasian microtektites into extra-atmospheric trajectories 0.77 Ma ago. Australasian microtektites (<1 mm diameter) have been found in many deep-sea sediments, though generally at moderate fluences (only one value >200 cm^-2) [1]. Recently, cores having exceptionally high microtektite fluences (>1000 cm^-2) have been discovered: ODP hole 758B [2] and ODP holes 767B and 769A [3]. Analyses of sediments from ODP 758B (Eastern Indian Ocean) and from ODP 769A (Sulu Sea) near SE Asia show small Ir enhancements associated with the microtektites. We determined a net Ir fluence of 2.0 +- 0.5 ng cm^-2 in 758B and a less well-defined value of 1.3 +- 0.6 ng cm^-2 in 769A [4]. Addition of the new cores to the set studied by Burns [4] yields a microtektite distribution that exponentially decreases outward from southeast Asia, the fluence dropping a factor of 2 in ~400 km. In southeast Asia the trend merges with a roughly estimated mass fluence of ~0.9 g cm^-2 inferred from evidence of a melt sheet in NE Thailand. Integration of the inferred distribution yields a total mass of Australasian tektites of 2.7 x 10^16 g, 270X greater than the Glass et al. value of 10^14 g commonly cited in the literature. We can use the Ir fluence of 2.0 ng cm^2 in the main peak of core 758B and the assumption that the fallout pattern was the same as that of the tektites to estimate the mass of the impacting body. The microtektite fluence in the main peak in 758B is 3970 cm^2 corresponding to a mass fluence of 7.94 x 10^-2 g cm^2. Multiplying the Ir/microtektite mass ratio times the integrated tektite mass of 2.7 x 10^16 g yields an Ir mass of 6.8 x 10^17 ng. Assuming that the non-icy part of the projectile consisted of chondritic matter having an Ir concentration of 600 ng/g, we calculate the mass of the impacting body to be 1.1 x 10^15 g. A typical density of 3.6 g cm^-3 yields a volume of 3.1 x 10^14 cm^3 and thus a sphere with a radius of 420 m. A mean Ir content of KT boundary sediments of 80 ng cm^2 corresponds to a mass of the KT impactor of 6.8 x 10^17 g of CM-chondrite-like material, 620X greater than our inferred mass of the Australasian impactor. The inferred mass of the body responsible for the Eltanin impact is 2.5 x 10^14 g, 4X smaller than that calculated for the Australasian impactor. If, as numerous people have suggested, the projectile that produced the tektites was a comet, the chondritic matter would have been accompanied by a roughly equal mass of H2O ice and the total volume was 1.4 x 10^15 cm^3, corresponding to a sphere of radius 700 m. The collision of such bodies with the Earth at an asteroidal (as opposed to cometary velocity) of 20 km s^-1 would release of 2.2 x 10^20 J (chondrite) or 4.4 x 10^20 J (comet), sufficient to excavate craters having diameters 14 and 17 km, respectively, assuming a simple scaling relationship. The approximate mass required to produce a 10-km crater is one-half and one- quarter, respectively, of our estimated masses for the chondritic and cometary projectiles. Thus the formation of a crater large enough to launch tektites through the atmosphere is consistent with the formation of a multitude of smaller craters required to account for the layered tektites [5]. The most probable location of the missing large crater is in the Mekong Valley, in mountainous areas experiencing high erosion rates, or buried beneath Mekong sediments. References: [1] Burns (1990) Ph.D. thesis, Univ. of Delaware. [2] Smit et al. (1991) Proc. ODP, Sci. Res., 121, 489-503. [3] Schneider et al. (1992) EPSL, 111, 395-405; [4] Schmidt et al. (1993) GCA, submitted; [5] Wasson (1991) EPSL, 102, 95-109.
Schmidt Georg
Wasson John T.
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