Constraining the formation and transport of lunar impact glasses using the ages and chemical compositions of Chang’e-5 glass beads

Abstract

Impact glasses found in lunar soils provide a possible window into the impact history of the inner solar system. However, their use for precise reconstruction of this history is limited by an incomplete understanding of the physical mechanisms responsible for their origin and distribution and possible relationships to local and regional geology. Here, we report U-Pb isotopic dates and chemical compositions of impact glasses from the Chang’e-5 soil and quantitative models of impact melt formation and ejection that account for the compositions of these glasses. The predominantly local provenance indicated by their compositions, which constrains transport distances to <~150 kilometers, and the age-frequency distribution are consistent with formation mainly in impact craters 1 to 5 kilometers in diameter. Based on geological mapping and impact cratering theory, we tentatively identify specific craters on the basaltic unit sampled by Chang’e-5 that may have produced these glasses and compare their ages with the impact record of the asteroid belt.

A solar wind-derived water reservoir on the Moon hosted by impact glass beads

“…In the other scenario, the CE5 impact glasses could have lost most of the original water inventory of their precursors during their formation at high temperature, following which solar wind-derived water started diffusing inwards into these glass beads on the Moon's surface. This scenario is consistent with the notable lower abundances of volatile elements such as Na 2 O (most <0.05 wt%) and K 2 O (<0.06 wt%) in CE5 impact glasses compared with CE5 mare basalt fragments (Na 2 O ≈ 0.6 wt% and K 2 O ≈ 0.1-0.2 wt%, on average) 39 . This binary mixing model, between a water-poor but relatively high D/H endmember and a water-rich but extremely low D/H endmember, reproduces well the range of water abundances and δD values measured in CE5 impact glasses (Fig.…”

“…To investigate this possibility, we carried out a systematic characterization of the petrography, major element composition, Raman characteristics, water abundance and hydrogen isotope composition on the impact glass beads returned by the CE5 mission, aiming to identify and characterize the missing water reservoir on the Moon's surface. A recent geochronological study of CE5 impact glass beads has shown that they formed more or less continuously for the past 2 Gyr, with prominent peaks in formation ages at ~575 million years ago (Ma), 380 Ma, 68 Ma and 35 Ma, originating predominantly from a few 1-to 5-km-diameter impact craters in the Em4 (Eratosthenian-aged mare) basaltic unit where CE5 landed 39 . shadowed regions within Cabeus crater 11 .…”

“…It is not possible to reconstruct the orientation of the exposed surface of the studied impact glass beads on the lunar surface. Thus, we carried out a simplistic diffusion modelling based on the measured profiles, assuming a constant Cs at the rims for each bead based on their diverse Pb-Pb ages 39 . The diffusion duration is roughly a maximum constraint for qualitatively estimating the efficiency of solar windderived OH/H 2 O recharged in impact glass beads.…”

“…This prediction is consistent with the similar geochemical characteristics between CE5 impact glass beads and CE5 bulk basalts 82 . The Pb-Pb dating of the CE5 impact glass beads indicates that they were formed in the range of 4-2,000 Ma by various local meteoritic impacts 39 .…”

Geological timescales’ aging effects of lunar glasses

“…Lunar glasses, which are a common component in lunar regolith, originate mainly from meteorite impacts ( 13 ). They have experienced extremely long time scale’s weathering and aging (about millions to billions of years) but maintained the glassy nature ( 14 , 15 ). As weathering mainly affects the surface region (~100 to 200 nm) of lunar glasses ( 16 ), the inner part of lunar glasses could be the ideal model to evaluate the long-term aging effects of glasses.…”