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 From that data, evaluate similarity of Earth and Venus (‘our sister planet’) –Bulk composition –Mantle composition –Core formation

Venera/VEGA XRF

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

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

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

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

Ca-Ti-Al

Depletion of Volatile Elements

U & Th - CI ratio!

K/U - Crucial Ratio

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

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

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

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* = (2  Primitive Earth basalts have Mg* = 68, => source mantle of Mg* = 91. Venus and Earth have similar mantle compositions (?) => comparable core sizes.

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 40 Ar to release to atmosphere –How much K does Venus really contain ???