Processes in Thick Lava Flows: Nakhlites (Mars) and Theo's Flow (Ontario, Earth)

Nakhlites are medium-grained pyroxenites that may have erupted as lava flows on Mars. To elucidate the crystallization environment of the nakhlites, we have begun to examine a terrestrial analog, Theo's flow. This 120-m thick, Archean lava flow contains a 54-m thick pyroxenite zone which bears a striking resemblance to the nakhlite meteorites in texture (Fig. I), with large euhedral augite grains and a fine-grained mesostasis. To quantify these textural similarities, we used crystal size distribution (CSD) analysis on three nakhlite sections, on a sample of the pyroxenitic zone of Theo's flow, and on two thinner pyroxenitic flows associated with Theo's. The similar numeric results support using Theo's flow as an analog for the nakhlites. Calculations made from CSD data of a 5-m thick pyroxenitic flow gave a pyroxene growth rate of 6.1 x 10-'0 c d s , in agreement with estimates for other silicates. Applying this growth rate to the thicker flows and nakhlites, we found that all had experienced steady-state growth periods of about 0.3 yr. Subsequent accumulation and liquid phase sintering, or Ostwald ripening, must then have occurred to produce the final textures observed.