Geochemical Aspects of the Geological History of Venus

Introduction: The current knowledge of the geological history of Venus can be interpreted in terms of the history of the geochemical environments on this planet. One can distinguish two parts of the geologic history of Venus: the later part, for which we have morphological records supplemented by geochemical measurements on the surface and in the atmosphere, and the earlier part, for which there is indirect evidence recorded in some compositional characteristics of the atmosphere of the planet. The latter part of the history: For the later part, which occupies the last 0.5-1 b.y. of the history of Venus [1], one can consider the geochemistry of the processes occurring in the crust and in the upper mantle , and the surface geochemistry. Geochemical processes in the crust and upper mantle are obviously the magmatic processes. Considering the known dryness and other parameters of the atmosphere, it is logical to suggest that hydrothermal activity is not appropriate in the crust and probably on Venus magmas are dryer than terrestrial magmas. Some clue to the composition of Venus magmas can be found from the characteristics of the observed volcanic formations. The most widespread (> 80 % of the surface) volcanic formations are represented by plains [e.g., 2-4]. Most of these show either extensive and morphologically very distinct flow-like features, obviously formed by solidified lava flows, or clusters of kms-size gently-sloping shields. Numerous larger volcanic constructs, many of which are more than a hundred kilometers in diameter, are also seen [5]; however the area they occupy is relatively small. These constructs are typically gently-sloped. The gentle slopes of the majority of the Venus volcanic constructs and the large areal sizes of lava flows on the generally horizontal plains, suggest low lava viscosity. Considering that the most widespread volcanic plains on the Earth and the Moon are assemblages of basaltic lavas which, in turn, are expected products of partial melting of the chondrite-like materials composing the silicate parts of all terrestrial planets [6], one can suggest that the majority of the observed products of Venus volcanism are also basaltic. This is in agreement with the deduced low viscosity of the lavas and is supported by geochemical measurements [7] in seven sites where the Venera-Vega probes landed (all on the plains). So the dominant geochemical processes in the Venus crust and the upper mantle are probably partial