HR: 08:45h
AN: P31A-04 INVITED     [Abstracts]
TI: Compositions of Igneous Rocks on Venus
AU: * Treiman, A H
EM: treiman@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, United States
AB: The limited available data on Venus' igneous rocks (from landed geochemistry and radar-based geomorphology and elevation data) are all consistent with basalt of one variety or another. Venus' shield volcanos have shallow slopes, comparable to terrestrial basalt shields; Venus' lava plains are consistent with extensive flows of fluid lava, i.e. basalt; the margins of distinct lava flows are consistent also with fluid like basalt. A few small volcanic constructs (e.g., pancake domes) could represent more silicic lavas, but could also represent basalt extrusives, either crystal-rich or with very slow effusion rates. The Venera and VEGA landers, technical and scientific triumphs that they were, provided limited and imprecise constraints on the chemistry of Venus' basalts. Their landing sites were all in the lowland plains, and so did not sample rocks from any highland: shield volcanos, tesserae, nor the unique plateau and high mountains of Ishtar Terra. The V/V analyses for Mg, Al, and Mn are little more than 2σ detections, and V/V returned no data on Na, Cr, Ni, P and other minor/trace elements. The V/V analyses for K, U, and Th (by γ rays) are imprecise, except for one (Venera 8) with extremely high K (~4% K₂O) and one (Venera 9) with a super- chondritic Th/U abundance ratio (at the 2σ level). Even with this imprecise limited data, a few inferences are fairly sound. [1] The FeO content of Venus basalts is similar to those of Earth basalts, suggesting a comparable mantle composition and thus a similar-sized core. [2] The range of K abundances suggests significant processes of depletion and enrichment in incompatible elements, consistent with repeated or extensive igneous processing. [3] The super-chondritic Th/U value measured by V9 is difficult to generate in low- pressure silicate melt/crystal fractionations. This Th/U value could possibly represent garnet fractionation in the mantle source, action of an ionic fluid (like carbonate melt) or even action of liquid water. [4] The Venus basalts, as a whole, have sub-chondritic Ca/Al. This relative deficit in Ca could arise from weathering at Venus's surface, or could be a primary characteristic possibly reflecting an origin as melted eclogite.
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3640 Igneous petrology
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 6295 Venus
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly