The Volcanology of Venera and VEGA Landing Sites and the Geochemistry of Venus

Magellan images confirm volcanic interpretations of television panoramas and geochemical data obtained by Venera and VEGA landers. Magellan and Venera/VEGA data are consistent with mafic volcanic compositions at all seven landing sites, although such compositions were proven at only three sites. Rock analyses show an unexpectedly high fraction of potassium-rich lavas. If the seven Venera and VEGA samples are approximately representative of the upper crust, just 10 km of the average material distributed globally would contain as much potassium as contained in Earth's continental and oceanic crust. Despite the likelihood that Venus' crust is overwhelmingly mafic, in terms of highly incompatible elements it may represent the differentiated magmatic equivalent of Earth's granitoid continents and of the Moon's anorthositic highlands. Although associated tectonic and petrogenetic aspects of lunar and Venusian crustal genesis were different, crust-forming processes on both objects (and possibly early terrestrial continent formation) may have involved cataclysmic partial melting of large fractions of these planets' mantles. A fairly mafic alkaline character of Venus' crust is attributed to a generally anhydrous, carbonated composition of Venus' mantle, which contrasts with a generally uncarbonated and variably hydrated composition of Earth's mantle. Aside from this important difference, Venus' mantle is geochemically almost identical to Earth's and is distinct from all other planetary objects for which compositional data exist. The compositional similarity of the mantles of Venus and Earth implies that the combined geochemical effects of nebular condensation, planetary accretion, and core formation were not substantially different for these planets.