Tektite glasses from Belize, Central America: Petrography, geochemistry, and search for a possible meteoritic component

Abstract

The presence of tektite-like glasses from a geographically restricted area in Belize (Central America) has been known for several decades. We comprehensively studied 18 such Belize glasses by a variety of petrographic and geochemical methods, including major and trace element analysis, radiogenic isotopic composition (Rb-Sr, Sm-Nd, and Re-Os), water content, oxidation state, and cosmogenic radionuclides. The aims were to determine their compositional variation, their mode of formation and possible source rocks, and their relation to known tektites, and to search for evidence of an extraterrestrial component.In terms of petrography, the samples are similar to tektites from the four “classical” strewn fields, with the presence of lechatelierites, schlieren, and vesicles; these are also widely accepted indicators of an impact origin. No close similarities to volcanic glasses are evident. Water contents are very low, and iron oxidation states are mostly reduced, in both cases similar to observations for other tektites. The geochemical and isotopic data presented, such as Cr, Co and Ni elemental abundances and interelement ratios, as well as trace element patterns are typical for local and regional volcanics from the active Central American Arc. Particular similarities to material comparable to volcanic rocks from Honduras or Guatemala are noted. This is confirmed by Sr-Nd isotope signatures of the Belize glasses, showing close similarities to Central American volcanics in general, and Honduran and Guatemalan volcanic, in particular. Osmium concentrations and 187Os/188Os ratios are comparable to arc volcanics from world-wide locations, but - in a few of the samples - elevated Ir concentrations, near-chondritic Pt/Ir and 187Os/188Os ratios can also be interpreted with the admixture of a minor meteoritic component to some of the Belize samples. 10Be concentrations are consistent with values typical of both, young or deeply buried soils and with values for Central American volcanics, which carry subducted 10Be.Geochemical data clearly indicate a source different from that of the Australasian tektites. Both isotope data sets for the Belize glasses indicate a close relationship to local arc lavas, especially those from Guatemala and Honduras, suggesting that the glasses were not deposited very far from their source. The main evidence that the Belize glasses are of impact origin are their petrographic characteristics and low water content. The evidence from 10Be is consistent with, but does not require, a model of formation for the Belize glasses by an impact on loosely consolidated surface sediments exposed to rain. A probable meteoritic component is low and heterogeneously distributed.