AR-39-AR-40 AGE AND PETROLOGY OF CHICO - LARGE-SCALE IMPACT MELTING ON THE L-CHONDRITE PARENT BODY

Our studies of the 105 kg Chico L chondrite show that it contains simi lar to 60% impact melt and the largest volume of impact melt recognize d in stony meteorites. We suggest that it is part of a much larger dik e complex that formed when chondritic impact melt was intruded into ho st chondrite during a large, if not catastrophic, impact on the L chon drite parent body at about 0.5 Ga. Petrologic and Ar-39-Ar-40 dating s tudies were made on several lithologies, including the massive melt zo ne, host chondrite, and melt-chondrite boundaries, for the purpose of studying the melting and thermal histories associated with impacts on small bodies and their effects on the K-Ar chronometer. The chondritic host is shocked to stage S6 and contains pockets and veins of melt. T here are no unmelted clasts in the interior of the melt; coalesced met al-troilite nodules reach up to 2 cm in size. Melt near the contact wi th the host chondrite contains numerous clasts and quenched more rapid ly. Metal-troilite textures suggest cooling rates of similar to 0.1 de grees C/s in the interior of the melt dike during crystallization. Sec ondary kamacite rims indicate cooling at 0.01-1 degrees C/y over the r ange of 700-500 degrees C, consistent with an impact-heated volume of up to a kilometer in thickness. Compositions of olivines and pyroxenes are generally similar in melt and chondritic host, reflecting rapid c rystallization, not metamorphic equilibration. The interior melt shows an overall depletion in K, whereas the melt near the boundary is enri ched in K. The Ar-39-Ar-40 release spectra during stepwise heating of both melt and chondrite samples can be divided into two parts, based o n Ar diffusion properties and K/Ca ratios. The low-temperature, high K /Ca phase of both melt and host chondrite show ages of 0.54-0.78 Ga. A ges of the high-temperature, low K/Ca phase of the melt are comparable or higher, 0.61-1.35 Ga, whereas those of the host chondrite are even higher, 0.87-1.86 Ga, due to lesser degrees of degassing. Isochron pl ots for several melt samples suggest an age of similar to 0.53 Ga and the presence of variable amounts of excess Ar-40 not completely degass ed by the impact. Even this age, however, is significantly higher than the previously reported Rb-Sr isochron age of 0.467 +/- .015 Ga. The apparent retention of radiogenic Ar-40 in the Chico impact melt, in sp ite of its relatively large size, absence of clasts, and moderately sl ow cooling rate below 700 degrees C, raises questions as to the reliab ility of using melts for Ar-39-Ar-40 dating of meteoritic impact event s.