Barringer Crater Stishovite: Oxygen-18 Rich Relative to Bulk Target Rock; J.F

756 LPSC XIX

BARRINGER CRATER STISHOVITE: OXYGEN-18 RICH RELATIVE TO BULK TARGET ROCK; J.F. McHone and L.P. Knauth, Dept. of Geology, Arizona State University, Tempe AZ 85287-1604

We have measured 6180 for samples of stishovite, coesite, and whole-rock shocked and unshocked Coconino Sandstone from Barringer (Meteor) Crater, Arizona. In analyses from this study and from published whole-rock data (I), stishovite is distinctly enriched in 018 by about 2 0100 relative to all other samples. SUMMARY OF 6180 (doo SMOW) DATA COCONINO SANDSTONE, METEOR CRATER ARlZONA Previous Study' 6'80 Whole-rock, Normal, Unshocked 1 2.8 Whole-rock, Normal, Unshocked 1 2.9 Whole-rock, Lightly Shocked 13.0 Whole-rock, Lightly Shocked 13.0 Whole-rock, Moderately Shocked 1 3.8 Whole-rock, Strongly Shocked 12.6 Whole-rock, Strongly Shocked 12.5 Whole-rock, Frothy Glass 12.6 Whole-rock, Frothy Glass 12.5 This Study Whole-rock, Normal, Unshocked 13.6 Whole-rock, Moderately Shocked 1 2.8 Whole-rock, Frothy Glass 12.8 Coesite 13.2 Coesite 13.2 Stishovite + Coesite 13.7 Stishovite + Coesite 13.7 Stishovite 15.4 Stishovite 15.0 Stishovite 15.1 *After Taylor & Epstein, (ref.1 )

Two possible mechanisms might produce anomolous 6180 values in stishovite: 1) a previously unrecognised form of isotope shock fractionation or 2) formation of stishovite by selective shock metamorphism of a quartz component having an original 0181dhtio different from that of bulk Coconino Sandstone. Kieffer (2) and Kieffer et al. (3) reported individual grains of Coconino Sandstone commonly exhibit pre-impact quartz cement overgrowths. They observed that stishovite occured only at grain boundaries and suggested that it formed by direct nucleation from host quartz rather than crystallization from hot silica melts as did the more abundant coesite. Overgrowths on detrital quartz form at sedimentary temperatures and are therefore enriched in 6180 relative to the igneous-metamorphic quartz originally deposited as clastic sediment. If stishovite forms preferentially in quartz overgrowths, it should be richer in 6180 than samples of whole-rock, as observed.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LPSC XIX 757

STISHOVITE: OXYGEN-18 RICH McHone, JF. and LP. Knauth

Porous, sedimentary rocks respond somewhat differently to an impact event than do solid, crystalline targets (4,5,6). Peak pressure pulses due to passage of a shock wave through a porous medium, such as a sandstone, are thought to be concentrated near edges of colliding grain boundaries. In contrast, pressure pulses passing through solid crystalline rocks, such as a granite, are more uniformly distributed; coesite and stishovite form as minute aggregates in crystallographicaly controled lamellae within individual quartz grains. We suggest stishovite at Meteor Crater was derived selectively from quartz overgrowth cements and that the observed stishovite 6180 enrichment relative to bulk Coconino is a result of shock-wave pressures concentrated near grain boundaries in a porous, sedimentary target rock. Acknowledgements: The writers express gratitude to the following Arizona State University staff : Sarah L. Kealy provided isotope analyses, Charles F. Lewis prepared coesite and stishovite separates, Dr. A. Yates performed X-ray diffraction powder analyses, and Dr. R. A. Nieman performed nuclear magnetic resonance analyses. A grant from the Barringer Crater Company, Princeton, NJ, provided partial support for J.F.M. References: 1) Taylor, H.P.,Jr. and Epstein, S., 1969. J. Geophys. Res., vol. 74, no. 23, p.6834- 6844. 2) Kieffer, S.W., 1971. J. Geophys. Res., vol. 76, no.23, p.5449-5473. 3) Kieffer, S.W., P. P. Phakey, and J.M. Christie, 1976. Cont. Min. Petrol., vol. 59, p.41- 93. 4)Stoffler, D., 197 1. J. Geophys. Res., vo1.76, no.23, p.5474-5488. 5) Robertson, B.P., 1980. Lunar and Planet. Sci. Conf.11 th, p.929-934. 6) Schaal, R.B., F. Horz, T.D. Thompson, and J.F. Baur, 1979. Proc. Lunar Sci. Conf., loth, p.2547-2571.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System