Geology of the Venera 8 landing site region from Magellan data: Morphological and geochemical considerations

A photogeologic analysis of high-resolution Magellan radar images covering the Venera 8 landing site area has shown that: (1) the majority of the region is characterized by mottled plain; (2) a younger plains complex exists in the western half of the landing circle; and (3) there are steep-sided domes and a possible collapsed caldera inside the landing circle. A preliminary photogeological analysis of all the geochemically studied Venera/Vega sites has shown that for the Venera 8 and 13 sites, where nontholeiitic compositions were measured for the surface material, steep-sided domes were found. Meanwhile, for the five sites where geochemical signatures of tholeiitic basalts were identified (Venera 9, 10, and 14 and Vega 1 and 2), steep-sided domes were not found. Emissivity, reflectivity, altimetry, and rms slope values inside the landing circle are typical for the Venusian plains. A continuation of searching through the literature for terrestrial igneous rocks with similar K2O, U, and Th contents as the Venera 8 material resulted in the finding of another group of rocks which are K2O-U-Th analogs of the material (in addition to a previously identified group of evolved intermediate subalkaline rocks). These potential analogs are dike rocks (lamprophyres) with mostly mafic silica contents. Among these analogs is a sample having the same K2O and Th content as the Venera 8 material (no uranium content was measured) and a primitive bulk chemistry close to the Venera 13 material. Both evolved and mafic terrestrial igneous rocks identified as analogs or as potential analogs to the Venera 8 material showed a distinct enrichment of their magma and/or magma source in incompatible elements. This indicates the involvement of continental crust material in the terrestrial analogs, and it suggests that crustal material enriched in incompatible elements may also be present on Venus.

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