Agglutinates as recorders of regolith evolution: Application to the Apollo 17 drill core

Abstract

We report chemical data (27 major, minor, and trace elements by INAA) for agglutinates from 26 depth intervals of the Apollo 17 deep drill core (sections 70009–70001) and compare their compositions with those of the soils in which they occur. The results show that seven stratigraphic units contain agglutinate populations in which the “average agglutinate” is in equilibrium (E) with the soil in which it resides and thus has a chemical composition that falls within the compositional range for the various size fractions. Four units contain average agglutinate populations with compositions out of equilibrium (O) with the soils in which they reside, and two units contain agglutinate populations that are close to the compositional limits of the soil size fractions and are designated (M) for mixed. The sequence is: (0–22 cm) E; (28–47 cm) O; (58–70 cm) E; (81 cm) O; (90–98 cm) E; (108–118 cm) M; (113–178 cm) E; (189 cm) O; (198–210 cm) E; (220–242 cm) O; (253 cm) E; (258–269 cm) M; (278–286 cm) E. This agglutinate sequence suggests a scenario in which several closely‐spaced depositional events were involved in the formation of the A‐17 drill core rather than a continuous accumulation process. Chemical mixing models indicate that agglutinates from section 70008 (22–48 cm) are enriched in mare components (81%), containing 55% orange glass and 26% ilmenite basalt. Agglutinates from core section 70003 (220–242 cm) did not form from the KREEPy soils that occur in this section but were probably mixed in from stratigraphic horizons above and below this unit. The Apollo 17 drill core soils have complex histories and agglutinates occurring in this stratigraphic sequence are powerful recorders of soil maturation and mixing events.