VOLCANIC DEGASSING OF ARGON AND HELIUM AND THE HISTORY OF CRUSTAL PRODUCTION ON VENUS

We develop a new methodology linking Ar-40 and He-4 degassing to crust al production on Venus in order to examine two different scenarios for the history of magmatism on that planet, both consistent with recent analyses of the cratering record of the Venus surface. The first scena rio includes episodic global resurfacing events and modest levels of m agmatism between such events, while the second scenario invokes differ ent rates of steady magmatism before and after a given transition time . Our degassing models include distinct mantle, crustal, and atmospher ic reservoirs. Diffusive transfer of Ar-40 and He-4 from the crust to the atmosphere is also taken into account. The Ar-40 abundance in the present atmosphere reflects the integrated degassing and magmatism ove r most of planetary history. In contrast, helium escapes from the plan etary atmosphere within a characteristic residence time of 200 Myr to 1.8 Gyr, so the present atmospheric He-4 abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first sce nario and to the two steady rates pf magmatism and the transition time under the second. Unfortunately, large uncertainties in mineral parti tion coefficients and in the Ar and He mixing ratios in the lower atmo sphere of Venus presently prevent the use of our degassing models to d istinguish among crustal formation history models. We therefore explor e the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements th at will most strongly constrain crustal production history. As an impo rtant step in the development of the Ar degassing model, the K budget in the bulk silicate fraction of Venus is re-examined on the basis of Venera and Vega gamma ray measurements of K, U, and Th concentrations in surface materials. For U and Th concentrations in the bulk silicate planet of 18-29 ppb and 64-94 ppb, respectively, the degree of mantle melting that formed the surface materials at the Venera 9 and 10 and Vega 1 and 2 landing sites is calculated to lie between 0.02 and 0.16. The degree of melting calculated for materials at each landing site i s combined with measurements of surface K concentration to estimate th e K concentration in the bulk silicate portion of the planet at betwee n 100 and 300 ppm. These results suggest that Venus and Earth have sim ilar heat production.