N. Namiki et Sc. Solomon, VOLCANIC DEGASSING OF ARGON AND HELIUM AND THE HISTORY OF CRUSTAL PRODUCTION ON VENUS, J GEO R-PLA, 103(E2), 1998, pp. 3655-3677
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.