Major-element vapor fractionation on the lunar surface: An unusual lithic fragment from the Luna 20 fines

Abstract

A 250-μm fragment in the Luna 20 fines has a very fine-grained “igneous” texture and has the composition (wt.%): SiO 2, 41.1; TiO 2, 0.35; Al 2O 3, 27.2; Cr 2O 3, 0.14; FeO, 4.2; MnO, 0.06; MgO, 8.5; CaO, 17.8; Na 2O, 0.05; and K 2O < 0.02. It contains ∼ 65% plagioclase An 99–100, ∼ 15% olivine Fo 90, ∼ 2% Mg-Al spinel and the remainder an unusual interstitial phase with composition SiO 2, 34.8; TiO 2, 1.78; Al 2O 3, 18.3; Cr 2O 3, 0.04; FeO, 14.1; MnO, 0.22; MgO, 5.0; CaO, 24.1; Na 2O, 0.34; K 2O < 0.02. This fragment probably represents a portion of a normal highland rock (anorthositic norite) which was heated to a very high temperature by impact, lost volatiles including SiO 2, and then partially crystallized. The observed phases and their inferred crystallization sequence are consistent with experimental results in the system CaO MgO Al 2O 3 SiO 2 (Schairer and Yoder, 1969), assuming the unusual phase to be a residual glass. This type of internal fractionation, leading to silica depletion in the residuum, is different from that normally observed in lunar rocks and is attributed to slightly lower bulk SiO 2 resulting from vapor fractionation due to impact (which also results in lower Na 2O and other volatiles). Because differentiation of the type shown by this fragment is rare in lunar materials, we infer that such major-element vapor fractionation is uncommon on the surface of the moon. The experimental CaO MgO Al 2O 3 SiO 2 phase relations also have a bearing on the lunar model proposed by D.L. Anderson in 1973: his “refractory” original lunar composition would differentiate to produce silica deficient liquids, like the unusual phase in our fragment, rather than the normal lunar crustal rocks.