40Ar39Ar dating,Ar diffusion properties,and cooling rate determinations of severely shocked chondrites

Determinations of ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex ${}^{40}\text{Ar}{}^{39}\text{Ar}$ release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of ⁴⁰Ar by the event, but retained from ~ 2 to ~45% of their ⁴⁰Ar. When calculated values of the diffusion parameter, $\text{D}\text{a}{}^2$, for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct $\text{K}\text{Ca}$ ratios and different average temperatures for Ar release. The experimentally determined values of $\text{D}\text{a}{}^2$ for the high temperature phase of several severely shocked chondrites are ~10^?7 to 10^?5sec^?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, $\text{D}\text{a}{}^2$ values, and fraction of ⁴⁰Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10^?2 10^?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from ⁴⁰Ar and Ni disagree by a factor of ~10⁵. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time.