Geochimica a Cosmochimica Acta Vol. 55, pp. 2695-2698 Copyright 0 1991 Pergamon Press plc. Printed in U.S.A. 0016-7037/91/$3.00 + .@I LETTER Age and geomorphic history of Meteor Crater, Arizona, from cosmogenic 36C1 and 14Cin rock varnish FRED M. PHILLIPS,’MAREK G. ZREDA,’ STEWART S. SMITH,’ DAVID ELMORE,“* PETER W. KUBIK,“+ RONALDI. DORN,~and DAVID J. RODD~ ‘Geoscience Department, New Mexico Tech, Socorro, NM 87801, USA 2Nuclear Structure Research Laboratory, University of Rochester, Rochester, NY 14627, USA ‘Geography Department, Arizona State University, Tempe, AZ 85287, USA %.S. Geological Survey, Flagstaff,AZ 86001, USA zyxwvutsrqponmlkjihgfedcbaZYXWVUT (Received March 7, 199 1; accepted in revised form August 6, 199 1) Abstract-Using cosmogenic 36C1 buildup and rock varnish radiocarbon, we have measured the exposure age of rock surfaces at Meteor Crater, Arizona. Our 36C1 measurements on four dolomite boulders ejected from the crater by the impact yield a mean age of 49.7 + 0.85 ka, which is in excellent agreement with an average age of 49 + 3 ka obtained from thermoluminescence studies on shock-metamorphosed dolomite and quartz. These ages are supported by undetectably low 14Cin the oldest rock varnish sample. THE AGES OF TERRESTRIAL impact structures yield important information on meteorite fluxes. Young meteorite craters are difficult to date by conventional means, but may be amenable to recently developed surface exposure dating techniques such as rock varnish dating or cosmogenic nuclide accumulation methods. For an initial application of surface exposure dating to meteorite craters we selected Barringer Meteor Crater in northern Arizona, USA. Barringer Crater is the largest ter- restrial crater (1.2 km diameter and 170 m depth) whose impact origin is proven by meteorite fragments (comparison with other craters in GRIEVE, 1979). The crater was originally estimated to be 25,000 t- 5,000 y old (SHOEMAKER, 1983; SHOEMAKER and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA KIEFFER, 1979), based on soil development within the crater and on the ejecta blanket and on the stra- tigraphy of lacustrine sediments in the crater. However, a recent thermoluminescence study on impact-shocked dolo- mite and sandstone yielded an average age of 49,000 + 3,000 y (SUTTON 1985a,b). The considerable discrepancy between these dates motivated further research into the crater chro- nology, using cosmogenic 36C1 (half-life 30 1,000 y) and var- nish 14C(half-life 5,730 y). Chlorine-36 is produced in rocks at the surface of the earth by cosmic-ray spallation, mainly of K and Ca, and by acti- vation of 35C1 by cosmic-ray neutrons (PHILLIPS et al., 1986; FABRYKA-MARTIN, 1988). Cosmogenic 36C1 significantly above subsurface concentrations is produced only to depths of a few meters below the earth’s surface (FABRYKA-MARTIN, 1988; LAL, 1987), and its buildup has been shown to be a regular function of time (PHILLIPSet al., 1986). Zreda et al. (1990, 199 1) have determined 36C1production rates (nor- * Present address: Physics Department, Purdue University, West Lafayette, IN 47907, USA. + Present address: Institut Fiir Mittelenergiephysik, ETH-H&g- gerberg, CH-8093 Ziirich, Switzerland. malized to sea level and 90” N latitude) of 4,160 f 3 10 atoms ‘%l (mol K)-’ yr-’ and 3,050 + 210 atoms 36C1 (mol Ca)-’ yr-‘, and a thermal neutron capture rate of (3.07 _t 0.24)* lo5 neutrons (kg rock)-’ yr-‘. Meteor Crater is an excellent subject for cosmogenic nuclide accumulation dating because we can identify and sample one geological unit (the Kaibab For- mation) that was virtually completely shielded from cosmic rays by 10 m of Moenkopi Sandstone prior to the impact (RODDY, 1978). Boulders of Kaibab Formation were nearly instantaneously exposed to cosmic radiation when they were ejected from the crater by the impact. The date of the impact can be determined by measuring the amount of cosmogenic 36C1 that has accumulated, provided that the boulder surfaces are not strongly eroded. Erosion rates in the range of milli- meters per thousand years will have little effect on cosmogenic 36C1 dates but loss of slabs of decimeter or greater thickness would re&ce the apparent age. For 36C1 dating we sampled five large boulders of siliceous dolomite of the Kaibab Formation, ranging in height from 1 to 7 m above the land surface. We attempted to select boulders that would have stood above the surface of the post- impact ejecta blanket. Boulder surfaces were examined for visual evidence of weathering and erosion and only surfaces that appeared stable were sampled. Three of the boulders were from the crater rim and two from the ejecta blanket surrounding the crater (Fig. 1). Samples were obtained by chiseling pieces from the top 2 cm of the centers of the boulder tops. The samples were ground and leached in deionized water to remove any meteoric chlorine. Chlorine was extracted from approximately 100 g samples by dissolution in nitric and hydrofluoric acid and precipitated as AgCl. Details of the extraction and analytical methods can be found in ZREDA et al. (1990, 199 1). The 36Cl/C1ratio of the AgCl precipitate was measured by accelerator mass spectrometry (ELMORE et al., 1979) at the University of Rochester. Major elements and chlorine were measured by x-ray fluorescence and rare earth 2695