Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.281r&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 281
Computer Science
Scientific paper
Two occurrences of vitreous rocks (fulgurites) that have resulted from the fusion of Etnean lavas, have been ascribed to the result of lightning striking the basalts and melting fresh volcanic rocks [1]. Rapidly quenched melts appear as tubular cavities that preserve the path of the discharge. Glass droplets (D <= 500 micrometers) are always dispersed around the fused lava tube and show several petrographic similarities with chondrules found in ordinary chondrites (presence of melilite, radiating skeletal fassaite, etc). In this process, high temperatures (T>1800 K) have probably been reached during timescales <=10 sec. Because it has been suggested that lightning discharges may have played an important role in the formation of chondrules [2], we have analyzed the oxygen isotope compositions of these fulgurites (our experimental protocol is described elsewhere [3]). The glass (free from any contamination from the unmelted basalt) is 1.5o/oo depleted in ^18O relative to its measured initial isotopic composition (delta^18O = +5.6o/oo); most of the data define a mass-dependent fractionation relationship (i.e. delta^17O = 0.52 x delta^18O). Therefore the data clearly do not reproduce the oxygen isotope anomaly defined for meteorites, which has a slope of 1 in the diagram delta^17O versus delta^18O (i.e. delta^17O = 1.0 x delta^18O). Nevertheless, it should be noted that some glass samples scatter around this canonical value of 0.52 with minor departures from a purely mass-dependent fractionation. If these results are confirmed by additional determinations (now in progress) on the separated glassy droplets, the following conclusions can be proposed: 1) lightning discharges do not yield oxygen isotope anomalies similar to those measured in chondrules and 2) an isotope exchange between hot chondrules and their parent nebular gas--presumably "anomalous" in its oxygen isotopes-- seems difficult to achieve within the duration of the rapid cooling of the melt. This last point is illustrated by the fact that no marked isotope exchange with atmospheric molecular oxygen is detectable. REFERENCES: 1] CLOCCHIATTI R. Eur. J. Mineral., 2, p. 479-494 (1990). 2] WHIPPLE F.L. Science 153, 54-56 (1966). 3] ROBERT F., REJOU-MICHEL A. & JAVOY M. Earth Planet. Sci. Lett. 108, 1-9 (1992).
Javoy Marc
Robert Frédéric
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