Effects of vaporization and condensation on Apollo 11 glass spherules: implications for cooling rates

Abstract

Fourteen of 40 glass spherules present in a section of the Apollo 11 microbreccia 10019,22 were found to exhibit steep concentration gradients at their rims: the oxides of the relatively volatile elements Na, K, and P increase by, respectively, factors of up to 67, 16, and > 54, at the spherule rims in comparison to the homogeneous centers. These gradients usually extend over 25–30 μ. Furthermore, the natural surfaces of 11 unpolished glass spherules separated from the fines 10084,97, in 60–80% of all cases, show higher concentrations in Na 2O, K 2O, and P 2O 5 than the averages for the central portions of 45 independently analyzed glass spherules. It is suggested that the concentration gradients observed are diffusion gradients and are the result of the impact event which produced the glasses: in the impact melting process, splash drops of melt formed that lost volatiles by vaporization. If the impact-produced vapor cloud surrounding the freely-floating spherules was large enough, then partial condensation of the volatile elements in the vapor cloud should take place upon cooling. These volatile elements condensed on the relatively hot spherules, and thermal diffusion of Na, K and P from the spherule surfaces towards their interiors occurred. Depending upon the temperature, cooling rate, and diffusion coefficient for the particular spherule composition, concentration gradients of different steepness and width should originate. A procedure is described to determine the cooling rate of a spherule from measured concentration gradients, provided the diffusion coefficients are known for the particular glass compositions in question.