The origin of tektites: A geochemical discussion

摘要

Tektites are a group of natural glasses occurring in four different strewn fields on earth. They are generally small, brownish to black, partly transparent, spherically symmetric, and sometimes aerodynamically ablated. Strewn fields are geographically restricted areas on earth where tektites are found, usually in association with microtektites. Microtektites are spherules of up to about 1mm in diameter and are retrieved from deep sea sediments. Recently microtektites have been found to co-occur with tektite fragments. Besides chemical, isotopical, and age arguments this gives final proof for a genetic relationship between tektites and microtektites. Chemically tektites are Si-rich glasses (SiO_2 between 65 and 85wt%), not unlike some well known terrestrial impact glasses. The major element chemistry allows the distinction of tektites of different strewn fields, and the use of some geochemical diagrams allows the further distinction between different sub-strewn fields. Within one strewn field, we can distinguish between splash-form tektites (normal tektites) and so called Muong Nong tektites, which differ from normal tektites in respect to a higher volatile content and greater inhomogeneity, besides being of generally larger size. The trace element chemistry reveals a close similarity of tektites to terrestrial rocks, especially to surface sediments. No similarity with lunar rocks or terrestrial mantle derived rocks can be observed. Trace element ratios and rare earth element patterns are especially useful in further delineating the type of precursor rocks for tektite production. The study of isotopes, like Rb/Sr, Sm/Nd, or the lead isotopes, adds further credibility to the connection between tektites and upper crustal sediments. For two of the four strewn fields a clear connection, based on chemical, isotopical, and age considerations, between tektites and impact craters, has been established. This is the Ries crater and moldavite and the Bosumtwi crater and Ivory Coast tektite connection. The consideration of various theories of tektite origin in view of the known facts leads to the conclusion that only the production of tektites during an impact on earth is consistent with the data. Analogous studies of impact glasses associated with terrestrial impact craters (like the Zhamanshin crater) give a picture which is in accordance with the impact model for tektites. Some of these glasses show an enrichment of the platinum group and other cosmic fingerprint elements compared to the terrestrial sedimentary background, these elements being the only ones from the impacting body which may survive the impact. Tektite analyses point in the same direction, and it may be possible to get further clues on the nature of the projectile by studying these elements.