Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.504f&link_type=abstract
Meteoritics, vol. 30, no. 5, page 504
Physics
Impact Melting, Impacts, Shock, Tektites
Scientific paper
The Australasian strewn field is the youngest and best preserved deposit of tektites and thus holds important information about the physics of large impact-induced explosions on Earth. In particular, the scale of compositional variation in and among tektites yields information about the scale of mixing during impact and ejection and thus the energy of the process. Splash-form tektites are widely distributed, internally homogeneous, depleted in volatiles, and show variations in composition on the regional scale, implying relatively efficient mixing and thus high energies of formation [1]. In contrast, layered tektites are distributed over a smaller region (principally in Vietnam, Laos, Cambodia and NE Thailand), less depleted in volatiles, internally heterogeneous at the microscopic scale, implying lower energies of formation. While systematic regional variations in the composition of layered tektites have been reported [2], this interpretation is complicated by the uncertainty in the original provenance of some samples and by current strategies of sampling and analysis, which may not give representative results. In order to evaluate compositional variability of layered tektites on the hand-sample to regional scale, we have analyzed the major and trace-element composition of 9 fragments of layered tektites from 6 known localities in a 50x40 km region in NE Thailand. Together with multiple analyses from a single large deposit of tektite fragments [3], we have quantified the scale of compositional heterogeneity in layered tektites. Experimental Method and Results: Each fragment weighed between 20 to 50 g, was cleaned of surface material before processing, and was analyzed in duplicate by wavelength-dispersive X-ray fluorescence spectroscopy at Stanford University. Major elements show the following relative variation: CaO (50%), MgO (34%), TiO2 (24%), Al2O3 (23%), Fe2O3* (19%), Na2O (19%), K2O (10%). In general, the SiO2 content varies inversely with CaO, MgO, TiO2, and Al2O3. Of the 18 trace elements measured, Zn, V, Ce, Ni, Cr, Cu, Ga, and Sr show relative variation larger than 20%. In contrast to the heterogeneity exhibited by the sample set as a whole the four samples analyzed from a single deposit near Ban Huai Sai are compositionally identical within experimental uncertainty. The composition of the tektites analyzed does not correlate in any way to locality. The three samples from the locality of Ban Non Hung show the greatest variation of the entire sample set, and span a compositional range comparable to that reported for all Australasian layered tektites [2]. Discussion: Our results show that the composition of layered tektites from a single deposit may be very homogeneous on the hand-sample scale but highly variable from deposit to deposit within a km2-sized area. In contrast, splash-form tektites show a smaller magnitude of compositional variation over much larger distances (100's of km) [2]. This difference implies more efficient mixing during the formation of splash-form tektites than during the formation of layered tektites. The relative efficiency of mixing in the blast cloud reflects the conditions of formation, including temperature and the Reynolds number (turbulence) of flow.. On the regional scale, these results call into question the use of regional trends in the composition of layered tektites to infer the location of the source crater [e.g., 2]. The high degree of local (km-scale) compositional variability requires that an unrealistic number of layered tektites must be analyzed in order to obtain a sufficiently accurate average for the composition of layered tektites in a particular area. Reported trends [2], based on only a few analyses for most localities, are likely to be artifactual. Acknowledgments: Work at Lawrence Livermore National Laboratory was performed under the auspices of the U.S. Department of Energy under contract number W-7405-Eng-48. _ References: [1] Koeberl C. (1994) GSA Spec. Pap. 293, 133-151. [2] Schnetzler C. (1992) Meteoritics, 27, 154-165. [3] Fiske P. S. et al. (1995) Meteoritics, in press.
Fiske Peter S.
Lowe T. K.
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