Stratigraphic processes in the lunar regolith - additional insight from neutron fluence measurements on bulk soils and lithic fragments from the deep drill cores

Abstract

Three of the Apollo missions sampled 2-3 m sections of the regolith with deep drill cores. Measurements of the integrated neutron exposure, the fluence, of bulk samples from these cores provided considerable insight into the processes of sedimentation, erosion and mixing in the lunar regolith. We have attempted to gain further knowledge about the nature of the lunar gardening processes by measuring the fluences in lithic fragments and selected bulk samples (<300 μm soils) from the Apollo 15 and 17 deep drill cores. Additional measurements on bulk samples from the Apollo 15 core have increased the spatial resolution of the depth dependence of fluence in this section. The new data more fully define the very smooth profile that was originally measured by Russ et al. (1972). The exact correspondence between this well-defined fluence profile and the depth dependence of the lunar neutron flux leaves virtually no doubt that all but the top few decimeters of this vertical section of the regolith had lain undisturbed for about half a billion years prior to its retrieval by the Apollo 15 astronauts. Fluences in soil breccias appear to reflect the average fluence of materials which comprise them rather than the residence time of the lithic fragments in the regolith. By contrast, fluences in millimeter sized pebbles of basalt are, on the average, less than in bulk samples from the same depths in the cores. The difference manifests an average residence time in the regolith that is ~300 m.y. less for igneous pebbles than for fine-grained soils (<300 μm). The difference in the regolith residence times between the igneous pebbles and the bulk samples in the Apollo 15 core has established an upper limit of 750 m.y. on the stratigraphic age of the cored materials. The lower limit of 420 m.y. was previously determined assuming the section to have been emplaced by very rapid sedimentation. One basaltic fragment from the Apollo 17 drill stem has a relative regolith residence time which conflicts with a very long stratigraphic history that had been proposed to describe the fluence profile in the core. An alternative model describing rapid sedimentation in the last 200 m.y. is believed to be a more accurate description of the processes that formed this stratigraphic section. Both the Apollo 15 and 17 cores appear to contain thick slabs of material that were laid down rather rapidly. Materials throughout the length of each slab have had nearly identical average residence times in the regolith. There must be lunar gardening processes that can deposit at least 2-3 m of well-mixed material in a relatively short time.

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