Moon rock collected by Apollo 17 astronauts reveals new details about lunar evolution

rod

Oct 22, 2019
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This report has some interesting comments. "Until now, it was thought that at some point after the moon's formation it experienced a 100-million-year period of cooling. But in looking at this sample, "we found that it could have taken no longer than 20 [million years] for this sample to cool to the point of complete solidification," Nelson said. "If the sample had cooled as slowly as previously suggested, then it should have no variations in P[phosphorus] content," Nelson said, adding that any variations would have long ago been "ironed out." This finding changes our understanding of the moon's history and evolution by a whopping 80 million years."

Some other reports are out now on this lunar sample. Advanced analysis of Apollo sample illuminates Moon's evolution, https://phys.org/news/2021-12-advanced-analysis-apollo-sample-illuminates.html, "Sophisticated analysis of a rock sample taken from the Moon during the Apollo 17 mission revealed new information about the complex cooling and evolutionary history of the Moon. Apollo 17 astronauts collected the rock sample troctolite 76535 from the Moon's surface in 1972..."

Here is the referenced paper: Chemical heterogeneities reveal early rapid cooling of Apollo Troctolite 76535, https://www.nature.com/articles/s41467-021-26841-4, 14-Dec-2021. "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."

My note. Newer models for Theia hitting the proto-earth and making the Moon, feature a lunar magma ocean that lasted 200 million years. Here is an earlier report on 76535 sample. Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago, https://ui.adsabs.harvard.edu/abs/2020NatAs...4..974W/abstract, May 2020. "Accurately constraining the formation and evolution of the lunar magnesian suite is key to understanding the earliest periods of magmatic crustal building that followed accretion and primordial differentiation of the Moon. However, the origin and evolution of these unique rocks is highly debated. Here, we report on the microstructural characterization of a large (~250-μm) baddeleyite (monoclinic-ZrO2) grain in Apollo troctolite 76535 that preserves quantifiable crystallographic relationships indicative of reversion from a precursor cubic-ZrO2 phase. This observation places important constraints on the formation temperature of the grain (>2,300 °C), which endogenic processes alone fail to reconcile. We conclude that the troctolite crystallized directly from a large, differentiated impact melt sheet 4,328 ± 8 Myr ago. These results suggest that impact bombardment would have played a critical role in the evolution of the earliest planetary crusts."

My note. The age of 4.328 billion years old is getting closer to the newer dates for when Theia impacted the proto-earth to create the Moon. Also the new Chinese lunar rocks dated 2 billion years old, will be interpreted with a very different geologic age and past on the Moon from the Apollo sample 76535. Some juggling solutions will be needed :)
 

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