Ar-ar Dating of Thermal Events on Meteorite Parent Bodies

Parent bodies of meteorites have experienced a variety of thermal events caused by early internal metamorphism and impact heating. Among radiometric chronometers, the 39 Ar-40 Ar technique is particularly sensitive to such thermal events. For over two decades the JSC laboratory has acquired 39 Ar-40 Ar data on a wide variety of meteorite types. The purpose here is to summarize some JSC data (and some literature data) and to address issues of the thermal histories of meteorite parent bodies. Impact Reset Ages: Among meteorites, the eucrites/howardites and ordinary chondrites show abundant evidence of impact disturbance or resetting of chronometers (1). Most eucrites/howardites dated give Ar-Ar ages (and some Rb-Sr and Pb-Pb ages) in the range of ~3.4-4.2 Gyr and suggest that the impact cataclysm which reset most ages of lunar highland rocks also occurred on the HED parent body. This supports the suggestion that the HED parent is relatively large, and may be Vesta. Among ordinary chondrites, impact resetting of Ar-Ar ages is usually associated with shock characteristics, probably produced by large impacts on the parent bodies, and not by smaller, later impacts that initiate cosmic ray exposure of meteorites. These chondrite shock ages are mostly <1 Gyr, and Ar-Ar ages of many shocked L-chondrites suggest a major impact event 0.5 Gyr ago. Presumably chondrites registering earlier (>1 Gyr) impact ages have been destroyed by gravitational interactions or erosion of parent body surfaces. A few clasts from chondritic breccias give Ar-Ar ages consistent with the cataclysmic period of ~3.5-4.0 Gyr. Presumably these were reset by processes similar to those that reset Ar-Ar ages of eucrites (1). Surprisingly, several other classes of meteorites (see below) give little evidence of impact resetting of Ar-Ar ages more recently than 4.4 Gyr ago, an observation that may reflect their parent body sizes and orbits. Ar-Ar dating of 13 mesosiderites show age " plateaus " with varying degrees of slope. These give average ages in the range of 3.77-4.13 Gyr and an overall average age of 3.94 ±0.10 Gyr (2). Earlier we interpreted these ages to represent breakup and reassembly of the mesosiderite parent body. However, other recent investigations indicate that mesosiderite breccias formed earlier than this. Mesosiderite cooling rates derived from Ni diffusion in the metal and closure temperatures for Ar diffusion indicate that these meteorites cooled exceptionally slowly (~0.1-0.5 O C/Myr). Thus, it now seems equally probable that the younger Ar-Ar ages were …