Glass From The Cretaceous Tertiary Boundary In Haiti

Tektite-like glasses preserved at the Cretaceous/Tertiary boundary at Beloc in Haiti provide clear evidence of an impact event. The glass composition suggests that the impact occurred on a continental shelf region, generating a silica-rich glass with chemical composition that reflects the melting of continental crustal rocks, and a calcium-rich glass produced by the fusion of marl sediments. These findings indicate that catastrophic release to the atmosphere of 10(15) moles of CO2 from vaporized marl occurred during the impact.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62562/1/349482a0.pd

Methods for determination of the age of Pleistocene tephra, derived from eruption of Toba, in central India

Tephra, emplaced as a result of Pleistocene eruption of the Indonesian ‘supervolcano’ Toba, occurs at many localities in India. However, the ages of these deposits have hitherto been contentious; some workers have argued that these deposits mark the most recent eruption (eruption A, ca 75 ka), although at some sites they are stratigraphically associated with Acheulian (Lower Palaeolithic) artefacts. Careful examination of the geochemical composition of the tephras, which are composed predominantly of shards of rhyolitic glass, indicates that discrimination between the products of eruption A and eruption D (ca 790 ka) of Toba is difficult. Nonetheless, this comparison favours eruption D as the source of the tephra deposits at some sites in India, supporting the long-held view that the Lower Palaeolithic of India spans the late Early Pleistocene. In principle, these tephra deposits should be dateable using the K–Ar system; however, previous experience indicates contamination by a small proportion of ancient material, resulting in apparent ages that exceed the true ages of the tephras. We have established the optimum size-fraction in which the material from Toba is concentrated, 53–61 μm, and have considered possible origins for the observed contamination. We also demonstrate that Ar–Ar analysis of four out of five of our samples has yielded material with an apparent age similar to that expected for eruption D. These numerical ages, of 809 ± 51, 714 ± 62, 797 ± 45 and 827 ± 39 ka for the tephras at Morgaon, Bori, Gandhigram and Simbhora, provide a weighted mean age for this eruption of 799 ± 24 ka (plus-or-minus two standard deviations). However, these numerical ages are each derived from no more than 10–20% of the argon release in each sample, which is not ideal. Nonetheless, our results demonstrate that it is feasible, in principle, to date this difficult material using the Ar–Ar technique; future follow-up studies will therefore be able to refine our preparation and analysis procedures to better optimize the dating

Natural Glasses

International audienceNatural glasses have been used since prehistoric times and are strongly linked to human evolution. On Earth, glasses are typically produced by rapid cooling of melts, and as in the case of minerals and rocks, natural glasses can provide key information on the evolution of the Earth. However, we are aware that natural glasses are products that are not solely terrestrial and that the formation mechanisms give rise to a variety of natural amorphous materials. On the Earth's surface, glasses are scarce compared to other terrestrial bodies (i. e., the Moon), since the conditions on the surface give rise to devitrification or weathering. In order to provide an exhaustive overview, we shall classify natural glasses based on the mechanisms by which they were formed: temperature related, temperaturepressure related, temperature-pressure-volatile related, and others. In this chapter, we will review the most common natural glasses and their technological applications and also the scientific and technological advancements achieved from the study of these natural amorphous materials. Finally, we will provide some insights into the structure and properties of natural glasses and melts