Basalts as probes of planetary interiors: Constraints on the chemistry and mineralogy of their source regions

Abstract

Basalt magmas, derived by the partial melting of planetary interiors, have compositions that reflect the pre-accretionary history of the material from which the planet formed, the planets, subsequent evolutionary history, the chemistry and mineralogy of the source regions, and the intensive thermodynamic parameters operating at the source and emplacement sites. Studies of basalt suites from the Earth, its Moon, and the eucrite parent body reveal compositional differences intrinsic to their source regions which are, in turn, a characteristic of the planet and its formational and evolutionary history. Major interplanetary differences are observed in iron, Mg (Mg + Fe 2+) , TiO 2, Al 2O 3, Na 2O, Cr, Ni, and in volatile element abundances. The most primitive mare basalts have Mg#s ∼ 0.6, on the Earth they are 0.70–0.72 for mid-ocean ridge basalts (MORBs) and up to 0.9 for Archean peridotitic komatiites. Eucrites have Mg#s approaching 0.5 (excepting Binda). These differences reflect inherent differences in Mg (Mg + Fe 2+) of their sources. Striking differences in the TiO 2 abundances of the planetary basalts reflect both inter- and intra-planetary variations in source chemistry. Primitive MORBs and primitive oceanic intraplate tholeiites have a factor of 2–3 difference in TiO 2 at comparable Mg# (0.7–1.2 vs 2–3 wt.% respectively). Three major titania groups are recognized in the mare suite; high TiO 2 (8–13 wt.%), low TiO 2 (2–5 wt.%) and very low TiO 2 (<1 wt.%). The eucrites have TiO 2 contents <1 wt.%. The mare basalts and eucrites have pronounced Na 2O depletion relative to all terrestrial basalts. This is a consequence of the preplanetary accretion loss of volatiles from the material that formed the Moon and the eucrite parent bodies. Mare basalts have consistently lower Al 2O 3 contents than the terrestrial basalts. This may be due either to an inherently lower content of Al 2O 3 in the mare sources or it may reflect Al 2O 3 retention in an aluminous phase. The transition metals are fractionated in all three basalt suites. For terrestrial basalts this may reflect core-separation; however, in the case of the Moon and eucrite parent bodies pre-accretionary separation of metal and silicates is a more reasonable explanation. A pronounced Cr anomaly is observed in terrestrial MORBs but not in the mare basalts. This appears to be related to f O 2 differences in the respective mantles. Overall rare earth element abundances are comparable between all three objects. Mare basalts have a pronounced negative Eu anomaly which is inherited from their source region and is record of plagioclase removal from crystallizing magma ocean early in lunar history (4.4–4.6 Ga). Early separation of plagioclase on the Earth appears to have been a relatively unimportant process.