Moon and Earth : compositional differences inferred from siderophiles, volatiles, and alkalis in basalts

Abstract We have compared RNAA analyses of 18 trace elements in 25 low-Ti lunar and 10 terrestrial oceanic basalts. According to Ringwood and Kesson, the abundance ratio in basalts for most of these elements approximates the ratio in the two planets. Volatiles (Ag, Bi, Br, Cd, In, Sb, Sn, Tl, Zn) are depleted in lunar basalts by a nearly constant factor of 0.026 ± 0.013, relative to terrestrial basalts. Given the differences in volatility among these elements, this constancy is not consistent with models that derive the Moon's volatiles from partial recondensation of the Earth's mantle or from partial degassing of a captured body. It is consistent with models that derive planetary volatiles from a thin veneer (or a residuum) of C-chondrite material; apparently the Moon received only 2.6% of the Earth's endowment of such material per unit mass. Chalcogens (Se and Te) have virtually constant and identical abundances in lunar and terrestrial basalts, probably reflecting saturation with Fe(S, Se, Te) in the source regions. Siderophiles show diverse trends. Ni is relatively abundant in lunar basalts (4 × 10 −3 × Cl -chondrites), whereas Ir, Re, Ge, Au are depleted to 10 −4 −10 −5 × Cl . Except for Ir, these elements are consistently enriched in terrestrial basalts: Ni 3 × , Re 370 ×, Ge 330 × , Au 9 × . This difference apparently reflects the presence of nickel-iron phase in the lunar mantle, which sequesters these metals. On Earth, where such metal is absent, these elements partition into the crust to a greater degree. Though no lunar mantle rock is known, an analogue is provided by the siderophile-rich dunite 72417 (~0.1% metal) and the complementary, siderophile-poor troctolite 76535. The implied metal-siderophile distribution coefficients range from 10 4 to 10 6 , and are consistent with available laboratory data. The evidence does not support the alternative explanation advanced by Ringwood—that Re was volatilized during the Moon's formation, and is an incompatible element (like La or W 4+ ) in igneous processes. Re is much more depleted than elements of far greater volatility: (Re/U) Cl ∼- 4 × 10 −6 vs ( T 1/ U ) Cl = 1.3 × 10 −4 , and Re does not correlate with La or other incompatibles. Heavy alkalis (K, Rb, Cs) show increasing depletion with atomic number. Cs/Rb ratios in lunar basalts, eucrites, and shergottites are 0.44, 0.36, and 0.65 × Cl, whereas the value for the bulk Earth is 0.15–0.26. These ratios fall within the range observed in LL and E6 chondrites. supporting the suggestion that the alkali depletion in planets, as in chondrites, was caused by localized remelting of nebular dust (= chondrule formation). Indeed, the small fractionation of K, Rb and Cs, despite their great differences in volatility, suggests that the planets, like the chondrites, formed from a mixture of depleted and undepleted material, not from a single, partially devolatilized material.