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

Mare volcanism on the Moon is closely tied to the distribution of heat-producing elements, unlike Earth where additional processes such as crustal recycling influence volcanism. Existing returned samples (Apollo, Luna) and meteorites indicated mare volcanism was most active ~3.8–3.3 Ga and waned by ~2.9–2.8 Ga, broadly consistent with thermal evolution models. However, Chang’e-5 returned basalts directly dated to be 800–900 Myr younger than previously sampled, extending mare volcanism to ~2.0 Ga and challenging prevailing models of lunar thermal evolution. Constraining eruptive fluxes (thickness, volume) for these young basalts can illuminate late-stage lunar magmatism. Remote-sensing based estimates for <2.8 Ga units face major uncertainties (crater chronology between 3–1 Ga, degraded flow-front recognition, and ambiguities in converting crater morphology to thickness/volume). This study uses mineral chemical zoning and diffusion chronometry on Chang’e-5 olivine and clinopyroxene to reconstruct post-eruption cooling timescales, infer lava flow thickness/volume, and place these results in the context of mare eruption flux through time.

Previous work linked lunar mare volcanism to internal heat production and modeled its duration with thermal evolution frameworks. Age distributions from Apollo/Luna samples and lunar meteorites indicated peak volcanism at 3.8–3.3 Ga with decline by ~2.9–2.8 Ga. Remote sensing studies estimated thicknesses and eruption rates of young mare units, but suffer from: (i) large uncertainties in crater-counting chronologies between 3 and 1 Ga; (ii) difficulty identifying flow fronts due to impact gardening and erosion; and (iii) limited constraints converting crater morphology to unit thickness/volume. Remote datasets nonetheless suggested enhanced activity at ~1.8–2.2 Ga and a peak eruption rate near ~1.7 Ga in Oceanus Procellarum. The Chang’e-5 samples provide direct ground-truth to refine these inferences by coupling petrochronology, diffusion chronometry, and conductive cooling models to estimate flow thickness and eruptive volumes for ~2 Ga lavas.

• Sample selection and petrography: Analyzed 42 crystals (21 olivine, 17 clinopyroxene; plus 4 clinopyroxene from literature) from 26 basalt clasts spanning porphyritic, subophitic, poikilitic, and equigranular textures across three CE-5 soil samples. High-resolution BSE imaging and X-ray maps characterized simple, largely concentric zoning patterns.
• EMPA/SEM: Major/minor elements in olivine (Mg, Fe, Si, Mn, Al, Ca) measured by Cameca SXFive EMPA (25 kV, 400 nA, ~5 µm beam); detection limits down to tens of ppm; mapping at 20 kV. Clinopyroxene analyzed on JEOL JXA-8100 (15 kV, 20 nA). Textures documented by Thermo Apreo SEM. Standards and ZAF matrix corrections applied.
• EBSD crystallography: Determined olivine crystallographic orientations (Oxford/HKL EBSD on Nova NanoSEM 450) to account for diffusion anisotropy when modeling along measured traverses.
• Zoning classification: Distinguished growth- vs diffusion-dominated olivine zoning using Al–Fo behavior (Al sluggish vs Fe–Mg mobile). Linear Fo–Al correlations indicate growth-controlled zoning; invariant Al with varying Fo indicates diffusion-dominated profiles.
• Diffusion chronometry: Modeled Fe–Mg and Mn concentration profiles in olivine using DIPRA (finite-difference multi-element diffusion). Parameters: diffusion anisotropy (D[001] ≈ 6× D[100]/D[010]); oxygen fugacity at NNO–5.5; temperatures estimated per crystal from Fo using PETROLOG and KD(Fe–Mg)Ol–melt = 0.33 with ±30 °C uncertainty; accounted for orientation from EBSD. For diffusion-dominated crystals, used core Fo as initial interior value and rim Fo at boundary; modeled single-stage timescales; Mn co-modeled as a check. For growth-dominated crystals, times estimated from grain size and experimental olivine growth rates (10^−10 to 10^−11 m/s).
• Thermal modeling for flow thickness: 1D conductive cooling modeled in COMSOL for a hot basaltic flow (initial 1200 °C; density ~2940 kg/m³; literature heat capacity and thermal conductivity) overlying colder basalt (Em4/P58 over Em3; properties from prior studies). Top boundary radiates to space; base conducts into substrate. For each crystal, matched diffusion timescale to modeled cooling from core to rim temperatures at depth to infer flow thickness; propagated uncertainties from diffusion times and temperature estimates.
• Geochemical context: Compared olivine core–melt equilibrium (exchange coefficients) and pyroxene Al/Ti to constrain crystallization sequence and degree of fractional crystallization; integrated with Sr–Nd–Pb isotope uniformity and U–Pb ages indicating a single eruptive event.

• Zoning and compositions: Nearly all olivine (21/22 profiles) and all clinopyroxene are normally zoned, indicating limited magma recharge or crustal assimilation. One olivine is compositionally homogeneous with a thin Fe-rich rim; one olivine shows reverse zoning (rim Fo 36.8 to core Fo 31.1). Olivine spans Fo ~61 down to Fo ~10. Clinopyroxene Mg# ranges 12.4–61.0, matching evolved compositions; thin Fe-rich rims in porphyritic samples reflect rapid quenching.
• Growth vs diffusion classification: Of 21 olivines with Al measured, 6 show linear Fo–Al trends (growth-dominated), 15 show invariant Al with varying Fo (diffusion-dominated).
• Cooling timescales: Growth-dominated olivines indicate 2.9 to 8.9 days. Diffusion-dominated olivines indicate 25.6 to 602.3 days (~50% >3 months). A reverse-zoned olivine yields a diffusion time of ~188.7 days. Times are lower bounds due to fixed-temperature assumptions.
• Flow thickness: Inferred individual-crystal thicknesses from ~4.5 to 55 m, mostly 8–30 m; average minimum thickness ~18.5 m assuming clasts derive from the slowest-cooling location. Consistent with remote-sensing thickness estimates for unit Em4/P58 (39.1–62.7 m).
• Eruptive volume and flux: Assuming uniform thickness over the relatively flat Em4/P58 unit (average slope ~3.3°), estimated flow volume ~473–894 km³, evidencing a large-volume late-stage eruption. Compilation with historical mare data indicates an enhancement in eruption flux around ~2.0 Ga, concentrated in Oceanus Procellarum, contradicting a simple monotonic decline of lunar volcanism.
• Magmatic history: Sr–Nd–Pb isotopic uniformity and U–Pb ages support a single eruptive event from a common source. Olivine cores are near equilibrium with bulk rock, suggesting closed-system crystallization from parental melt. Crystallization sequence includes early spinel, concurrent augite and plagioclase (pyroxene Al/Ti ~2), >40% pre-eruptive fractional crystallization with further fractionation during flow cooling, explaining textural stratigraphy across the flow.

By coupling diffusion chronometry of mineral zoning with conductive cooling models, the study constrains the cooling timescales, thickness, and volume of the ~2 Ga Chang’e-5 basalt flow. These results directly quantify a substantial eruptive flux at ~2 Ga, demonstrating that lunar volcanism did not diminish monotonically but included episodic, high-flux events late in lunar history. The mostly normal zoning and isotopic homogeneity indicate a simple magmatic evolution without significant recharge or assimilation, reinforcing interpretation of a single, large eruptive event. The inferred thickness and volume align with remote-sensing estimates and support a surge in volcanic output in Oceanus Procellarum at ~2.0 Ga. Potential drivers for this late-stage activity likely relate to fusible mantle components, Earth–Moon tidal heating, and/or mantle convection cycles, aided by favorable crustal conditions (thin crust and deep fractures associated with the Imbrium basin) that facilitated magma ascent.

This work provides direct, sample-based constraints on late-stage lunar volcanism by deriving cooling timescales, lava flow thickness, and eruptive volumes for Chang’e-5 basalts. The findings reveal an episodic surge in eruption flux around 2.0 Ga and revise models of lunar thermal evolution that predict a simple decline in activity. The integration of mineral diffusion chronometry with thermal modeling offers a robust framework for reconstructing eruption dynamics of lunar mare basalts. Future work could extend these approaches to additional CE-5 clasts and other young mare units, refine thermal models with variable substrate and atmospheric boundary conditions, and further probe mantle source characteristics and potential external forcings (e.g., tidal heating) through integrated geochemical–geophysical studies.

• Diffusion timescales assume fixed or average temperatures per crystal; this likely yields lower-bound times. Temperature estimates from Fo content and KD carry ±30 °C uncertainty.
• Diffusion anisotropy and orientation are accounted for via EBSD, but residual uncertainties remain.
• Growth times for growth-dominated olivines rely on experimental growth rates (10^−10–10^−11 m/s) not calibrated for all lunar conditions.
• Thermal model is 1D and assumes uniform properties (initial lava at 1200 °C, constant thermal parameters), radiative top boundary, and conductive base into cold basalt; potential presence of regolith and property variations are not fully constrained.
• Flow thickness estimates assume clasts sampled the slowest-cooling location and that analyzed clasts derive from the same flow; true stratigraphic positions are inferred from textures and may introduce bias.
• Some mineral profiles lack core plateaus, implying partial overprinting of original core compositions.
• Remote comparisons (thickness, flux) inherit uncertainties from crater chronology and geomorphic interpretations.