Chemically Robust Carbon Particles in Peat from the Tunguska Impact Site

Details Chemically Robust Carbon Particles in Peat from the Tunguska Impact Site Chemically Robust Carbon Particles in Peat from the Tunguska Impact Site

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.521h&link_type=abstract

Meteoritics, vol. 30, no. 5, page 521

Computer Science

Carbon, Diamond, Meteorites, Tunguska

Scientific paper

The Tunguska impact event of 1908 caused the destruction of a large forested area but left no impact crater. It was interpreted to have exploded in mid-air and thus would disperse material in a fireball over a wide geographical area. Several attempts have been made to locate particles associated with the impact in materials such as tree resin (1) and even in Antarctic snow (2). One of us (JN) attended an expedition to the Tunguska impact area and collected samples of peat from a depth of 60 cm which is thought to be the level in the peat which holds the impact horizon and indeed contains charred vegetation. The peat was subjected to standard demineralization procedures using mineral and oxidizing acids to destroy all but the most resistant mineral phases. An emphasis was placed on the chromic acid treatment to remove organic carbon as the peat was inevitably of extremely high organic content but perchloric acid was also used to oxidize graphite. The residue after treatment has subsequently been examined using a scanning electron microscope (Jeol JSM-820I at 30kv) with a Kevex system to obtain an energy dispersion scan and subjected to stepped combustion to determine the carbon abundance and isotopic composition. The SEM revealed the predominance in the residue of chromium containing phases possibly as a result of the extensive chromic acid treatments but several very pure carbon-rich particles were also observed. These carbon particles were anhedral and up to 15 micrometers in size. Due to the nature of the acid treatment performed upon this sample only a very robust form of carbon could survive and the most likely mineral is diamond. Further analyses using a transmission electron microscope and selected are electron diffraction will be performed to resolve the true mineralogy of these carbon particles. Stepped heating experiments on the acid resistant residue in conjunction with static mass spectrometry (3) has been used to give both carbon and nitrogen contents as well as carbon and nitrogen isotope compositions. Carbon accounted for 0.6% by weight of the residue and was released between 550-650 degrees C with a delta^(13)C of -25 per mil. The nitrogen yield was 0.02% by weight of nitrogen released between 500-700 degrees C with a delta^(15)N of ca. 0 per mil. These represent terrestrial values and indicate that the carbon particles do not have an extraterrestrial signature. If indeed these carbon particles are diamond and related to the explosion of the Tunguska object then they may well have formed by a plasma process within the fireball (similar to CVD). Further refinements regarding the purification of acid resistant materials are in progress. References: [1] Ceccini S. et al. (1995) in Abstracts Presented to ESF Network on Impacts Meeting, Ancona, Italy, May 12-17, 1995. [2] Roccia R. et al. (1990) GSA Spec. Pap. 247, 189. [3] Wright I. P. and Pillinger C. T. (1989) USGS Bull., 1890, 9.

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