Geochemical constraints on petrogenic processes on Venus

Abstract

Abundances of three incompatible elements (K, U, Th) have been determined by robotic spacecraft in five surface materials on Venus. We present these data normalized to terrestrial normal mid‐ocean ridge basalt (NMORB) derived from the depleted mantle. Relative to NMORB, all of the Venus materials studied are enriched in all of these elements. The moderately enriched (Venera 9 and 10, Vega 1 and 2) basaltic rocks are similar to one another in the sense of their enrichment trends (U N >K N ) but differ from the highly enriched Venera 8 material, where the trend is K N >U N . This difference implies that the enrichment pattern for the basaltic materials is not controlled by crystallization of an NMORB‐like magma or by contamination of such a magma by highly enriched Venera 8 material within the crust, and the Venera 8 material cannot have evolved from the magma of any of the basaltic rocks. Our calculations show the K‐U‐Th pattern for any of the Venus rocks analyzed was not controlled by batch partial melting of primitive mantle. The Venera 8 material could be produced as a partial melt from eclogitic tholeiite, but none of the basaltic materials could be because the sense of their U N >K N trends is opposite to the enrichment trends in calculated models (K N >U N ). The Venus basalts differ from fresh terrestrial rocks (NMORBs and oceanic island‐arc volcanics) in having nearly constant K/U ratios, while terrestrial rocks have nearly constant Th/U ratios. This may suggest an unusual composition of mantle source(s) of the Venus basalts and/or unusual fractionation process(es) on that planet.