Abstract
The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. Reconciling the paradoxical geochemical features of this suite constitutes a feasibility test of lunar differentiation models. Here we present the results of a microanalytical examination of the archetypal specimen, troctolite 76535, previously thought to have cooled slowly from a large magma body. We report a degree of intra-crystalline compositional heterogeneity (phosphorus in olivine and sodium in plagioclase) fundamentally inconsistent with prolonged residence at high temperature. Diffusion chronometry shows these heterogeneities could not have survived magmatic temperatures for >~20 My, i.e., far less than the previous estimated cooling duration of >100 My. Quantitative modeling provides a constraint on the thermal history of the lower lunar crust, and the textural evidence of dissolution and reprecipitation in olivine grains supports reactive melt infiltration as the mechanism by which the magnesian suite formed.
Highlights
The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions
Lunar evolution has been defined by the samples returned by the Apollo missions, culminating in the generally accepted lunar magma ocean (LMO) model
This model postulates that the Moon was once largely molten, and the lithologies present on the Moon are produced from geochemical reservoirs defined by crystallization products of this melt
Summary
The evolution of the lunar interior is constrained by samples of the magnesian suite of rocks returned by the Apollo missions. The magnesian suite (Mg-suite) was found to possess characteristics that prompted amendments to the basic LMO model This suite contains the combination of some of the most anorthitic plagioclase (An98–84) and the most Mg-rich mafic silicates (Mg# 95–60) ever found in lunar samples[1], suggesting crystallization from a parent melt that is highly primitive (unmodified by fractional crystallization). Elardo et al.[12,13] demonstrated that mixing these cumulates with ur-KREEP would lower the liquidus of the system, and radiogenic heat from the decay of potassium and thorium would have helped produce the Mgsuite parent melt Another possibility is that cooling and crystallization of an impact melt sheet generated during an impact event[14,15] caused direct mixing of anorthite-rich crust, ur-. 76535 has been used to quantify the amount of water in the primitive Moon[19], the evolution and cessation of the lunar magnetic dynamo[20], and the effect of shock on radiometric ages[21]
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