163-6 Frictionites from Langtang, Nepal: Comparison with Impact Glasses and Impact Melts

Session: Impact Cratering Processes Across the Solar System: In Memory of Dr. Bevan M. French

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Abstract:

On Earth, various types of naturally formed glasses exist: (1) volcanic glasses (e.g., obsidians), (2) impact glasses produced by hypervelocity impacts of an extraterrestrial body (including tektites), (3) lightning-produced glasses (fulgurites), which are tube-like structures formed when bolts of lightning strike the ground, (4) combustion-metamorphic glasses produced by burning organic material (e.g., coal, oil, gas) in sediments/rocks, (5) friction-produced glasses (or melts; frictionites) that are produced by massive landslides and faulting. Some human-related processes can also produce glasses, such as nuclear bomb explosions (trinitite etc). In general, glass formed during an impact event, is the result of (short-term) high temperatures (and pressures) during the impact event, and the glass usually forms by melting (whole-rock or mineral melting, such as in the case of the mineral lechatelierite from quartz), but there are also a few cases in which minerals are transformed to an amorphous (glassy) state as a result of just high pressure (diaplectic glasses). However, not every glass found in a geological context, even if suggestive, is indeed of impact origin; for glass to be identified as “impact-derived”, detailed analyses are necessary. This applies also to glasses possibly formed during airburst events. There are reports in the literature in which glasses of unknown origin were designated to be of impact origin and were used to claim some unusual impact events, such as at the Köfels rockslide in Austria, which was alleged to have been caused by an impact event. It is, therefore, of interest to perform detailed mineralogical, petrographic, and geochemical studies of all varieties of natural glasses to see if there are some unique characteristics with which to tell their modes of origin apart. For such a study, we collected fresh samples of frictionites from Tsergo Ri in Nepal, one of the world's largest rockslides in crystalline rocks, and reasonably young, probably only a few tens of thousands of years. These rocks are mainly composed of a glassy matrix with some minerals, including lechatelierite. Besides mineralogical and petrographic work, our studies involve detailed geochemical analyses (e.g., Os isotopic compositions), to establish similarities and differences to impact glass. Petrographic observations indicate some suevite-like features but without shock effects. First isotope data indicate derivation from local crystalline rocks, as is anticipated.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9085

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