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Venus: Basalt Geochemistry and Planet Composition

Venus: Basalt Geochemistry and Planet Composition . Allan Treiman Lunar and Planetary Institute For Venus Geochemistry Workshop, Feb. 2009. Purpose and Plan. Review Geochemistry of Venus’ basalts Analyses from Venera/VEGA Broad conclusions Data quality as significant limitation

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Venus: Basalt Geochemistry and Planet Composition

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  1. Venus: Basalt Geochemistry and Planet Composition Allan Treiman Lunar and Planetary Institute For Venus Geochemistry Workshop, Feb. 2009

  2. Purpose and Plan • Review Geochemistry of Venus’ basalts • Analyses from Venera/VEGA • Broad conclusions • Data quality as significant limitation • From that data, evaluate similarity of Earth and Venus (‘our sister planet’) • Bulk composition • Mantle composition • Core formation

  3. Venera/VEGA XRF

  4. Venera/VEGA Gamma-Ray • Detector was CsI(Tl) • Lots of gammas from 137Cs

  5. Venera/VEGA XRF Data, &c. • Note large uncertainties!

  6. How Good is Good Enough? • Depends on purpose • E.g., Tectonic implications of basalt

  7. Venus Bulk & Mantle Geochemistry • Refractories (U-Th-Ca-Al-Ti) • Close to CI ratio • K-U-Th. Heat-creating elements • Only clue to volatile-depletion • Fe-Mg-Mn • Mantle composition • Core size

  8. Ca-Ti-Al

  9. Depletion of Volatile Elements

  10. U & Th - CI ratio!

  11. K/U - Crucial Ratio

  12. FeO - like Earth • Geochemistry - basalt => mantle • When mantle peridotite melts to form basalt, and in early crystallization of basalt, most elements prefer either melt or crystals • Fe is ‘indifferent’ - FeO content of a basalt remains nearly constant through fractionation, and is nearly that of its source mantle. • FeO of Venus basalts ~ 8%, like primitive (magnesian) Earth mid-ocean basalt • Implies that Venus mantle has ~ 8% FeO, like Earth

  13. Venera/VEGA XRF Data, &c. • The 8% FeO solution

  14. FeO/MnO • Geochemistry • Fe & Mn are both a bit less volatile than K during condensation => Venus should have FeO/MnO near CI ratio • Fe & Mn follow closely in igneous processing • Fe is siderophile, goes to core => FeO/MnO is a measure of core size • Venera/VEGA MnO are 2 upper limits • Uncertainties in Fe & Mn probably correlated • Taking nominal values gives average FeO/MnO = ~50, like Earth at ~60. • Venus and Earth have comparable core sizes

  15. Mg/Fe - like Earth • Geochemistry [Mg* = 100*Mg/(Mg+Fe) molar] • Igneous fractionation separates Mg and Fe, Mg/Fe in basalt might give mantle Mg/Fe • Fe is siderophile, goes to core => mantle Mg/Fe is an indirect measure of core size • Venera/VEGA MgO are mostly 2 upper limits • Most precise data are VEGA 2, which give Mg* = 73+13-21 (2 Primitive Earth basalts have Mg* = 68, => source mantle of Mg* = 91. • Venus and Earth have similar mantle compositions (?) => comparable core sizes.

  16. Implications • Earth and Venus are geochemical ‘twins’ in many respects • [Size, uncompressed density] • Refractory element ratios • Mantle compositions (FeO, Mg*) • Core size (FeO/MnO) • Biggest difference, as constrained by Venera/VEGA, is lower K than Earth • Lower heat production • Less 40Ar to release to atmosphere • How much K does Venus really contain ???

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