Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535

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 >similar to 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.

Automated summary

The evolution of the lunar interior is examined using samples returned by the Apollo missions, specifically focusing on the magnesian suite of rocks. Results show intra-crystalline compositional heterogeneity in these rocks, contradicting previous assumptions of slow cooling at high temperatures. Diffusion chronometry indicates that these heterogeneities could not have survived magmatic temperatures, providing insights into the thermal history of the lower lunar crust and supporting reactive melt infiltration as the mechanism of formation for the magnesian suite.