P-bearing Olivines from the “Luna-20” Soil Samples, Their Sources and Possible Phosphorus Substitution Mechanisms in Lunar Olivine

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Rocks with P-bearing olivine were found in soil samples delivered by the “Luna-20” automated station. They are ascribed to the highland anorthosite–norite (more rarely, gabbro-norite)–troctolite rock series enriched in phosphorus and other incompatible elements, but are not related to typical KREEP rocks enriched in incompatible elements. Their source is presumably of hybrid origin and related to primary high- Mg suite (HMS) rocks. The occurrence of high- and low-Cr populations of P-bearing olivine in different structural rock types can be attributed to the annealing-related more rapid chromium diffusion (relative to that of phosphorus) in olivine from metamorphosed rocks. This assumption is supported by stoichiometric formula calculations of these olivines. An alternative explanation for these olivine populations is their derivation from at least two different sources. Disequilibrium crystallization of the P-bearing olivines, which is confirmed by an intricate phosphorus zoning, excludes the existence of P-rich melts, which is consistent with previous observations. At the same time, olivine fractionation can be responsible for the phosphorus content in lunar melts. The incorporation of phosphorus in olivine of the “Luna-20” anorthosite troctolites is presumably controlled by a coupled substitution mechanism of divalent cations and silicon for phosphorus and chromium in the tetrahedral and octahedral sites (Milman-Barris et al., 2008). Another possible mechanism is the substitution of divalent cations in octahedral sites by phosphorus and chromium, which provides the possible presence of P³⁺.