Planetary Materials Research Group

We present a 4.25-billion-year-old volcanic rock petrogenetically linked to the Mg suite, together with a related glass bead from the same Apollo 17 core 73002, which was unsealed as part of NASA’s ANGSA program. This basalt, with porphyritic texture and magnesian-suite composition, has a crystallization age of 4246 ± 4 million years. Phase equilibrium modeling indicates that this picritic basalt is related to glass in 73002 and formed when a mantle melt interacted with anorthositic crust before eruption. The presence of extrusive, olivine-rich Mg-suite samples indicates that high-temperature lunar magmas had sufficient energy or buoyancy to breach the early crust. Subsequent events, namely the late heavy bombardment and mare volcanism, possibly obscured evidence of ancient volcanism, or the early Moon was dominated by intrusive rather than extrusive magmatism. These findings also offer insight into magmatic processes on early Earth before the onset of plate tectonics, where such samples have been destroyed by geologic processes and where most ancient igneous ages have come from individual zircon grains rather than whole rocks. Our results support a dynamically evolving early Moon, expanding our understanding of primordial crust formation and thermal evolution on early Earth and other rocky planets.

The “light mantle” deposit at the base of South Massif in the Moon’s Taurus-Littrow Valley was a primary science target for the Apollo 17 exploration. The possibility that it was a landslide triggered by ejecta from Tycho Crater is critical for establishing the age of Tycho and constraining recent lunar impact chronology; however, the mechanism of emplacement of the deposit has recently been questioned. The newly opened 73001/73002 double drive tube from Station 3 sampled 70.6 cm deep into the regolith and represents the first stratigraphic section of an extraterrestrial landslide deposit returned to Earth. Here we provide an overview of the stratigraphy of the 73001/73002 core based on top to bottom variations revealed by coordinated laboratory analyses and explore constraints on the emplacement of the light mantle deposit. Briefly, the upper ∼10 cm of 73002 contains a disturbed zone from space weathering and emplacement of ejecta from a nearby crater that excavated and ejected basaltic material. Below 10 cm is a nearly uniform unit of immature regolith. These data support a single event for the emplacement of the deposit at this location, followed by weathering and mixing of materials from nearby crater ejecta in the upper 10 cm. Slight variations in chemistry and clast components may reflect the relative stratigraphy of the South Massif slope, with material toward the bottom of 73001 originating from lower slopes and material from higher up in the core representing regolith from higher up the South Massif slopes.

Space weathering alters the surface materials of airless planetary bodies; however, the effects on moderately volatile elements in the lunar regolith are not well constrained. For the first time, we provide depth profiles for stable K and Fe isotopes in a continuous lunar regolith core, Apollo 17 double drive tube 73001/2. The top of the core is enriched in heavy K isotopes (δ⁴¹K = 3.48 ± 0.05‰) with a significant trend toward lighter K isotopes to a depth of 7 cm; while the lower 44 cm has only slight variation with an average δ⁴¹K value of 0.15 ± 0.05‰. Iron, which is more refractory, shows only minor variation; the δ⁵⁶Fe value at the top of the core is 0.16 ± 0.02‰ while the average bottom 44 cm is 0.11 ± 0.03‰. The isotopic fractionation in the top 7 cm of the core, especially the K isotopes, correlates with soil maturity as measured by ferromagnetic resonance. Kinetic fractionation from volatilization by micrometeoroid impacts is modeled in the double drive tube 73001/2 using Rayleigh fractionation and can explain the observed K and Fe isotopic fractionation. Effects from cosmogenic ⁴¹K (from decay of ⁴¹Ca) were calculated and found to be negligible in 73001/2. In future sample return missions, researchers can use heavy K isotope signatures as tracers of space weathering effects.

Samples 73001 and 73002, which make up the lower and upper portions, respectively, of the double drive tube containing regolith (“soil”) collected on the “light mantle” at Station 3 during Apollo 17. Using a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) and fused-bead electron-probe microanalysis (FB-EPMA), we determined the chemical composition of every 0.5 cm dissection interval of the entire 56.9 cm length of the double drive tube, which penetrated to a depth of 70.6 cm below the regolith surface. We used the chemical compositions to model the proportions of different lithologic components found at the Apollo 17 site. Elemental variations with depth were linked to different proportions of these components. Higher amounts of high-Ti mare basalt near the 73002 surface (uniformly dark-toned regolith from 0 to 1.5 cm) indicate mixing of local mare materials by small impact cratering. Decreasing proportions of high-Ti mare basalt below 1.5 cm result from the mixing of dark and light regolith components during the dissection process on Earth. Below about 7.5 cm, compositions indicate consistent amounts of primarily highlands material (<5% high-Ti mare basalt), which can be described as a mixture of noritic impact-melt and anorthositic-norite components. In detail, the modeled anorthositic-norite component, which may represent the pre-basin upper crust in this part of the Moon, ranges from 50 to 60 wt.%. The modeled noritic impact-melt breccia component remains relatively uniform at 35–40 wt.% throughout the length of 73002 and increases to 45 wt.% at the bottom of 73001.