1 / 18

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

issac
Télécharger la présentation

Venus: Basalt Geochemistry and Planet Composition

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 ???

More Related