Archean Crustal Components in Georgia Tektites? Trace Element Compositions of Georgiaites by Laser Ablation ICPMS

Archean Crustal Components in Georgia Tektites? Trace Element Compositions of Georgiaites by Laser Ablation ICPMS

1Department of Space Sciences, Fernbank Science Center, Atlanta, Georgia 30307 and Department of Geology, University of Georgia, Athens GA 30602-2501 USA

2GEMOC, School of Earth Sciences, Macquarie University, N. Ryde NSW 2109 Australia

Abstract - 23 georgiaites have been analyzed for trace element abundances by laser ablation ICPMS. These data show that georgiaites are compositionally distinct from other tektite groups. Compared to indochinites, bediasites, and moldavites (the IBM group), georgiaites are less enriched in LREE/HREE, have smaller Eu anomalies, lower La/Sc, Th/Sc, and La/Th, and higher Rb/Cs ratios. Trace element characteristics of the IBM group are consistent with targets composed of post-Archean upper crustal sediments [1, 2]. In contrast, the geochemical signature of georgiaites suggests a significant component of Archean-type crust in their targets. Good correlations of refractory trace element ratios (e.g., Ba/Y) with major element compositions indicate mixing rather than loss of Si by volatilization as the cause of compositional variations within the georgiaite group.

Introduction - Eocene age tektites are common in southeastern North America, but their origin, including the location of their parent crater, is poorly understood. A recently discovered crater beneath Chesapeake Bay has been proposed as a possible source for the N.A. tektite strewn field [2,3,4,5]. Georgiaites are characterized by high-silica (Fig. 1) compositions which are distinguished from other tektite groups by low Al2O3 contents and high Na/K ratios [2]. We have analyzed 23 individual georgiaites, 2 indochinites, and a moldavite for their trace element compositions by laser ablation ICPMS to define the geochemical characteristics of georgiaites, and to determine the nature of their source. These are the same georgiaites for which major element data were reported by [5].

Results - Analytical procedures followed those described by [6]. 3-6 analyses were performed on each sample using 44Ca as an internal standard and the NIST 612 glass for external calibration. 24 determinations of the 612 glass as an unknown during the course of this study demonstrate a precision of 2-4% for all elements and an accuracy of ²2% for the average of these analyses relative to the calibration values.

Like other tektite groups [1,2], georgiaites have a range of compositions with good correlations between major and trace element abundances (Fig. 1) [5]. Both mixing and loss of Si by volatilization have been suggested as mechanisms to account for the compositional variations within tektite groups [2]. For the georgiaites, the good correlations among refractory trace element ratios and major element compositions (Fig. 2) suggest that these trends are more likely to result from mixing of different components in the target rather than loss of Si.

Trace element abundances show that georgiaites are distinct from other tektite groups (Tables 1, 2; Fig. 2). The REE patterns of IBM tektites (Fig. 2) clearly show that these impact glasses formed by melting of the Earth's upper crust [1, 2]. La/Th, La/Sc, and Th/Sc ratios, which are diagnostic indicators of post-Archean upper crust [7], are also consistent with this interpretation (Table 2). Rb/Cs ratios of indochinites and moldavites are ~10-20 (Table 2), which are typical values for upper crustal sediments [8]. In contrast, georgiaites have REE patterns more like Archean crust (Fig. 2), which is characterized by less enrichment of LREE/HREE and a smaller Eu anomaly compared to post-Archean crust (Table 2, [7]). La-Th-Sc abundances of georgiaites also suggest a target with compositional characteristics similar to Archean crust (Table 2). Rb/Cs ratios of georgiaites range from 38-52 (Table 2), higher than most sediments, either Archean or post-Archean. The Rb/Cs of Archean crust is poorly known, but appears to be around 35-85 [9]. In general, crystalline crustal rocks tend to have higher Rb/Cs than sediments [9], so the relatively high Rb/Cs of the georgiaites may be indicating crystalline rocks in their target stratigraphy, and would be consistent with a component of Archean-type crust in these tektites. The Chesapeake Bay crater penetrated crystalline Precambrian basement [3], perhaps providing another link with georgiaites, although this may also imply production of these tektites from relatively deep in the target stratigraphy.

[8] Taylor and McLennan (1985) The Continental Crust, Blackwell Sci. Publ., 312 pp

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