Derivation of primary magmas and melting of crustal materials on Venus: Some preliminary petrogenetic considerations

Abstract

Crustal formation and evolution processes are of critical importance in the geochemical and thermal evolution of planets. As an aid to understanding these processes on Venus, we develop a general paradigm for: (1) the derivation of primary magmas, and (2) the range of possible conditions for remelting of crustal materials and the evolution of the products of remelting. We use as a basis for this paradigm the present knowledge of the bulk and surface composition, thermal structure, and surface geological and geochemical processes. For the range of conditions of derivation of primary magmas and crustal remelting, a wide range of magma types is possible, and no magma type can be arbitrarily excluded from consideration on Venus. We conclude that magmatic and volcanic activity on Venus, in its broadest sense, could be very similar to that on the Earth, although eruption styles are expected to vary due to environmental conditions (Head and Wilson, 1986). Major differences in magmatic and volcanic activity are likely to occur in two environments on Venus: (1) those analogous to terrestrial island arcs, where due to the absence of water, melts should be SiO+ndersaturated, and the more fluid melt products may produce widespread deposits of SiOz-poor ferrobasalts rather than more viscous SiOz-rich magmas and composite volcanoes, and (2) those in plains regions influenced by mantle plumes and hot spots, where highly picritic melts may periodically flood vast regions of the surface.

Processes of crustal formation and evolution on Venus: An analysis of topography, hypsometry, and crustal thickness variations

“…Whether melting or density inversion occurs is dependent on the conductive geotherm (Anderson, 1980;Hess and Head, 1989,199Ob). For a conductive geotherm of 25"C/km, the basalt liquidus is crossed at about 40 km depth (Figure lc) (Hess and Head, 1990b). If a thickness of crust equivalent to this depth was produced over the history of the planet, then the crustal production rate would be 4.1 km3/yr and the thickness production rate would be about 8.9 km/b.y.…”

Geochemical constraints on petrogenic processes on Venus

“…GRS-measured K, U, and Th Figure 1 Calculations of partitioning during partial melting were made using the mass balance The K-U-Th pattern for partial melts could be influenced by volatile components such as H20 and CO2. However, an attempt to allow for the role of volatiles in Venus petrogenesis would raise more questions than it answered because the abundances of volatile species in the Venus interior are poorly constrained (see Hess and Head [1990] for a summary), and necessary data on (low-density fluid)/melt, fluid/mineral partition coefficients, particularly for accessory minerals and for a range of fluid compositions, are among the suggestions for future work made by Green [1994] in his recent review. …”

Lakshmi Planum, Venus: Characteristics and models of origin

“…Hess and Head (1989) demonstrated on a theoretical basis that no magma type may be arbitrarily excluded from consideration for Venus. The greatest difference between magma types on Venus and Earth may occur at convergent zones, such as proposed for Ishtar Terra, due to the suspected lack of water in the Venusian crust.…”