AR-39-AR-40 DATING OF MESOSIDERITES - EVIDENCE FOR MAJOR PARENT BODY DISRUPTION LESS-THAN-4 GA AGO

39Ar-40Ar ages were measured on 17 samples of 14 different mesosiderites and indicate major degassing of Ar by one or more thermal events less than 4.0 Ga ago. Three samples gave release patterns of constant age, whereas the other samples typically showed 200-400 Ma increases in 39Ar-40Ar age with increasing gas extraction temperature. These stepwise releases indicate ages of ~3.4-3.8 Ga (1 Ga = 109 a); averaged ages for individual meteorites are 3.45-3.82 Ga. No substantial evidence exists in the temperature releases for 39Ar-40Ar ages older than 4.0 Ga and the Ar ages appear to be in contradiction to 244Pu fission track ages previously reported for a few mesosiderites. The Ar ages probably date the event that caused disturbance in previously reported Rb-Sr, Sm-Nd, and Pb-Pb ages of Estherville and other mesosiderites. Data on release of Ar as a function of extraction temperature were used in Arrhenius plots to estimate activation energies for Ar diffusion of 18-52 kcal/mol. These values were then used to calculate closure temperatures against Ar loss of 2-340°C and 180-600°C for assumed cooling rates of 1°C/Ma and 1°C/a, respectively. The Ar data suggest that, for certain assumed models of mesosiderite formation, the 39Ar- 40Ar ages may date the time of formation of those metal textures in mesosiderites which have led others to calculate cooling rates of ~1°C/ Ma. Models which call for metal-silicate mixing ~4.5 Ga ago, followed by cooling at 1°C/Ma from temperatures >1150°C, seem unlikely. Single-event models which produce the silicate-metal mixture at high temperatures, rapidly cool the objects to temperatures of ~500°C, followed by cooling of ~1°C/Ma and are not consistent with the Ar data unless the thermal event occurred <4 Ga ago. Collisional break-up of the mesosiderite parent object < 3.9 Ga ago, followed by gravitational reassembly, is consistent with the Ar ages and metal textures. We suggest that metal-silicate mixing occurred ~4.4 Ga ago, but that a major collisional event disrupted the mesosiderite parent planet < 3.9 Ga ago, heated mesosiderites to >500°C, and left them deeply buried to undergo slow cooling. © 1990.