Venus: Basalt Geochemistry and Planet Composition

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Venus Basalt Geochemistry and Planet Composition

• Allan Treiman
• Lunar and Planetary Institute
• For Venus Geochemistry Workshop, Feb. 2009

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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

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Venera/VEGA XRF
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Venera/VEGA Gamma-Ray

• Detector was CsI(Tl)
• Lots of gammas from 137Cs

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Venera/VEGA XRF Data, c.

• Note large uncertainties!

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How Good is Good Enough?

• Depends on purpose
• E.g., Tectonic implications of basalt

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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

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Ca-Ti-Al
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Depletion of Volatile Elements
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U Th - CI ratio!
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K/U - Crucial Ratio
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FeO - like Earth

• Geochemistry - basalt gt 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

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Venera/VEGA XRF Data, c.

• The 8 FeO solution

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FeO/MnO

• Geochemistry
• Fe Mn are both a bit less volatile than K
  during condensation gt Venus should have FeO/MnO
  near CI ratio
• Fe Mn follow closely in igneous processing
• Fe is siderophile, goes to core gt 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

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Mg/Fe - like Earth

• Geochemistry Mg 100Mg/(MgFe) molar
• Igneous fractionation separates Mg and Fe, Mg/Fe
  in basalt might give mantle Mg/Fe
• Fe is siderophile, goes to core gt 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
  7313-21 (2??? Primitive Earth basalts have Mg
  68, gt source mantle of Mg 91.
• Venus and Earth have similar mantle compositions
  (?) gt comparable core sizes.

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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 ???