Lunar Phosphates Record Impact Cratering Events at Micro to Nano Scales
Bibliographic record
Abstract
The field of planetary mineralogy has greatly benefited from recent studies of accessory minerals that utilise µm- and nm-scale analytical techniques such as EBSD, APT, TEM and SIMS. Apatite and merrillite have been of particular interest, as they record vital information on the volatile content, U-Pb ages and trace-element composition of various planetary materials. However, the extent to which shock-deformation, pervasive among all planetary materials, affects the distribution of these valuable geochemical tracers is still poorly understood. Here we focus on exploring the U-Pb and Pb-Pb ages of apatite and merrillite in a set of variably shocked lunar rocks, building on previous nanostructural analyses of the phosphates. We carried out U-Pb and Pb-Pb analyses of phosphates in Apollo 17 samples of the Mg-suite rocks (76535, 76335, 76255, 72255, 78235 and 78236) using the CAMECA 1280 ion microprobe at the NordSIMS facility (Stockholm). In addition, we applied a recently developed approach of conducting high-precision U-Pb and Pb-Pb analyses by ID-TIMS of extracted phosphate grains (Jack Satterly Lab, University of Toronto). For this purpose, individual ~50x50x30 µm crystals of apatite and merrillite were extracted directly from thin sections using a Xe+ plasma FIB. As determined by SIMS, 207Pb/206Pb systematics of the unshocked or weakly shocked apatite in 76535 and 76335 is undisturbed, implying cooling of the rock below the closure temperature of Pb diffusion in apatite (~450°C) at ~4.2 Ga, ~100 Ma younger than what is interpreted as the rock’s crystallization age. Phosphates that experienced similar levels of deformation but were in proximity or in direct contact with the impact melt in samples 76255 and 72255 show almost complete age resetting (~3.92 Ga). The SIMS determined age of 16 phosphates in sample 76255 is 3922.2 ± 6.7 Ma (2σ) and agrees with the previously published 207Pb/206Pb phosphate ages of impact melt breccias found within the same boulder and was interpreted as the timing of the Imbrium impact. These recrystallized phosphates yield comparable TIMS Pb-Pb ages (3917.8 ± 1.8 Ma and 3921.0 ± 1.3 Ma, 2σ) with significantly lower internal uncertainties than that of the individual SIMS measurements and may represent multiple impact-events close to the Imbrium event. SIMS U-Pb analyses of highly shocked phosphates (78235 and 78236) reveal a discordia line with an upper intercept of ~4.2 Ga and a lower intercept of ~0.5 Ga. We interpret this new, younger age as a minor thermal event that reactivated existing shock-induced nm-scale grain boundaries, as visualised by APT, within the apatite population to allow for Pb-loss at ~0.5 Ga. We propose a small crater located near the Apollo 17 landing site as a possible source of this sample. By correlating micro- to nanostructural characterization with in-situ age systematics we show that apatite and merrillite are powerful thermochronometers that provide a new approach to dating which has the potential to discriminate between temporally similar events. This can greatly aid in unravelling the bombardment record of solar system and be helpful when dealing with samples of limited availability (e.g. space return missions).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".