Surges in volcanic activity on the Moon about two billion years ago

Understanding the history of lunar mare volcanism is crucial for comprehending the Moon's thermal evolution. While previous studies using remote observations and limited samples have suggested a decline in volcanic activity after 3.8–3.3 billion years ago (Ga), recent findings from the Chang'e-5 mission challenge this view by dating lunar volcanism to as young as 2 billion years ago. This discrepancy highlights the need for more precise constraints on the eruptive fluxes over time, which are currently hampered by uncertainties in crater counting chronology, difficulties in recognizing flow fronts, and limitations in estimating mare unit volumes. The Chang'e-5 basalts, being the youngest lunar basalts currently available, offer a unique opportunity to directly investigate the late-stage volcanic activity and refine our understanding of lunar thermal evolution by studying the volume and timing of these eruptions.

Existing literature points to a correlation between lunar volcanic activity and the distribution of heat-producing elements. Mare volcanism was most active around 3.8–3.3 Ga and then decreased or ceased by 2.9–2.8 Ga based on Apollo, Luna, and meteorite samples. This timeframe is generally consistent with thermal evolution models. However, the Chang'e-5 samples provide direct evidence of volcanism much younger than previously observed, causing significant changes in the accepted model of lunar thermal evolution. Previous attempts to determine volcanic fluxes using remote sensing data have been hampered by substantial uncertainties in crater counting chronology, difficulties in identifying flow fronts due to impact bombardment and erosion, and limitations in estimating mare unit volumes accurately.

This study analyzes the mineralogy and geochemistry of 42 olivine and pyroxene crystals from Chang'e-5 basalts using high-resolution backscattered electron (BSE) imaging, electron microprobe analysis (EMPA), and electron backscatter diffraction (EBSD). The researchers identified four typical textures (porphyritic, subophitic, poikilitic, and equigranular) in the basaltic rock clasts. They focused on olivine crystals, categorizing them into two groups based on their forsterite (Fo) zoning patterns: Group 1 with normal/no chemical zonation, and Group 2 with reverse chemical zonation. Diffusion chronometry was applied to the olivine crystals to estimate post-eruption lava flow cooling timescales using the DIPRA software. This software models the diffusive relaxation of compositional boundaries within zoned minerals, enabling the calculation of cooling times from the measured Fe–Mg and Mn concentrations. The cooling timescales were then used in conjunction with a COMSOL Multiphysics model of heat transfer in lunar lava flows to estimate the thickness of the basalt lava flows. These thickness estimates, combined with previous estimations of eruptive flux throughout lunar history, allowed the researchers to reconstruct the volume and timing of volcanism.

The study found that almost all olivine and clinopyroxene crystals in the Chang'e-5 basalts exhibit normal zoning, indicating minimal magma recharge or shallow-level assimilation during their formation. The cooling timescales derived from olivine crystals range from a few days to several months. Based on these cooling timescales and thermal modeling, the thickness of the Chang'e-5 basalt lava flow was estimated to be between 4.5 and 55 meters, with an average of 18.5 meters. This thickness is consistent with remote observation estimates. The identical Sr–Nd–Pb isotopic compositions and the high-precision U–Pb dating of basalt clasts of different textures suggest that the Chang'e-5 basalts originated from the same source and formed in a single eruptive event. The estimated volume of the Chang'e-5 basalt flow is significant (~473–894 km³), providing direct evidence for a high-volume late-stage volcanic eruption. By compiling previous data with the results from the Chang'e-5 samples, the study revealed an enhanced volcanic eruption flux approximately 2 billion years ago, concentrated mainly in the Oceanus Procellarum basin. This finding indicates that lunar volcanic activity did not decrease monotonically but experienced episodic eruptions even during the Moon's late stage.

The findings of this study directly address the research question of reconstructing the eruptive flux history of the Moon and resolving the discrepancy between previous models and the younger ages observed in Chang'e-5 samples. The significant volume of the 2-billion-year-old Chang'e-5 basalt flow provides compelling evidence for episodic surges in lunar volcanism. The results challenge the notion of a monotonic decline in volcanic activity throughout lunar history and highlight the potential for significant eruptive events even during the late stage of the Moon's thermal evolution. The absence of complex zoning patterns in the Chang'e-5 samples suggests a relatively simple magmatic history without substantial magma replenishment or crustal assimilation. This adds to our understanding of the late-stage magmatic processes on the Moon and the factors that may have triggered these final episodes of volcanic activity.

This study, using data from the Chang'e-5 mission, provides strong evidence for episodic surges in lunar volcanic activity, particularly an increase around 2 billion years ago. This finding revises existing models of lunar thermal evolution, highlighting the importance of considering the potential for significant eruptive events even during the Moon's late stage. Future research could focus on expanding the analysis to a wider range of lunar samples and further investigating the specific mechanisms driving these episodic eruptions. This may involve exploring the role of fusible components in the lunar mantle, tidal heating, and lunar mantle convection cycles.

The study primarily focuses on the Chang'e-5 samples from the Oceanus Procellarum basin and might not be entirely representative of global lunar volcanic activity during this period. The thermal modeling relies on several assumptions regarding the initial temperature, density, heat capacity, and thermal conductivity of the lava flow and underlying materials. Uncertainties in these parameters could affect the accuracy of the estimated flow thickness. Furthermore, the diffusion modeling assumes a constant temperature during cooling, which might not be entirely accurate in reality.