VENERA-8 LANDING SITE GEOLOGY REVISITED

Photogeologic mapping of the Magellan images of the Venera 8 site show ed that practically all geologic units of this area correlate with the geologic units distinguished by Basilevsky and Head [1995a, b] in man y other regions of Venus; however, areal abundance of the units in dif ferent areas may significantly differ. In particular, an anomalously h igh abundance of small volcanic shields was found in the landing site area and especially in the landing circle thus suggesting that the mat erial sampled by the Venera 8 could represent these shields. The forma tion of shield fields, instead of vast basaltic floods more typical fo r Venus, is believed to be due to low magma replenishment rates [Crump ler et al., 1997]. These conditions might favor intra-chamber magma di fferentiation and/or contamination in the crustal material, thus being a cause of the enrichment of the material sampled by the Venera 8 in K, U, and Th. This conclusion is very tentative, and a possible petrol ogic link between the presence of shield fields in the landing site an d the nontholeiitic character of the sampled material demands further studies. The observed correlation between the high abundance of small volcanic shields in the landing area and the nontholeiitic character o f the sampled material implies that other small shield field localitie s on Venus could be possible sites of high-K nontholeiitic materials t oo. The Venera 8 landing site should be considered as one of the poten tial targets for future missions to Venus designed to study the surfac e geochemistry of different geologic units.