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The Martha's Vineyard Tektite Revisited: A Distinct Subcategory of North American Tektite

Introduction: Tektites, a specific type of impact glass, are formed by quenching impact melt from terrestrial impact structures, and occur as part of the distal ejecta deposits [1-3]. Today, tektites occur in four strewn fields (Ivory Coast, Central European, North American, and Australasian,) associated with the Bosumtwi, Ries, Chesapeake Bay, and an unidentified impact structure, respectively. Compositionally, tektites are unlike other non-impact glasses as they typically have higher SiO2 and FeO contents and relatively lower alkali and volatile element concentrations [2-3]. The North American strewn field, the only strewn field in the Western Hemisphere, is dominated by two subcategories: georgiaites (found in East-Central GA) and bediasites (found in east Texas near College Station). Microtektites and tektite fragments have been found in sediments in Barbados [4-5] as well as in sediments from the DSDP Site 612 off the coast of New Jersey, all of which have chemistries consistent with a coastal plain sedimentary target, but subtle measurable differences from the georgiaties and bediasites [5]. There have been reports of tektites recovered in Cuba, which are also likely part of the North American strewn field but little is known about these tektites. Lastly, one tektite has been found at Grey Head, Martha’s Vineyard [6-7]. The Martha’s Vineyard tektite shares many similar characteristics with georgiaites. Major elemental geochemistry is nearly identical to georgiatites and it also appears translucent olive green, suggesting similar iron content and oxidation states. Likewise it has a K-Ar age of 34 Ma identical to all of the N. American tektites [7-8]. However, only one was recovered (and it was not in place), and the Martha’s Vineyard tektite has widely been considered to be a humanlytransported georgiaite. Albin [9, 11] noted however, slight variations in major element chemistry (particularly enrichment in Ca concentration) and suggested that it may in fact be a distinct tektite. Samples and Methods: Here we present new trace element geochemistry for the Martha’s Vineyard and georgia tektites. We analyzed two georgiaites (GA21 and GA30) from the Fernbank Science Center collection in Atlanta, and the Martha’s Vineyard tektite currently in the Smithsonian National Museum of Natural History’s collection (catalog #2082). Analyses were conducted in the Facility for Isotope Research and Student Training (FIRST Lab) at Stony Brook University. Concentrations were measured using laserablation mass spectrometry on an Agilent 7500 quadrupole ICP-MS with a New Wave 213 laser. The values reported for each sample is based on 15 spots each having a diameter of 120 μm and a 40 sec dwell time. All analyses were run using He as the carrier gas, and data reduction was conducted in the Iolite software [10]. Results: The Martha’s Vineyard tektite has nearly identical trace element concentrations as the georgiaites (Figure 1-2). All are consistent with melting of eastern US coastal plain sediments and a Chesapeake Bay Impact Structure source. However, the Martha’s Vineyard tektite shows significant differences from georgiaties in four elements: Zn, Sb, Pb, and U (Figure 3).