The Trials and Tribulations of Noritic Lunar Meteorite Arguin 002

THE TRIALS AND TRIBULATIONS OF NORITIC LUNAR METEORITE ARGUIN 002. J. F. Snape1, R. Tartèse1, K. H. Joy1, M. Boruah2, B. G. Rider-Stokes2, M. Anand2 and M. J. Whitehouse2, 1Department of Earth and Environmental Sciences, The University of Manchester, UK. ([email protected]), 2School of Physical Sciences, The Open University, UK, 3Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden. Introduction: Arguin 002 was discovered in Mauritania and officially approved as a lunar norite meteorite on 19th March 2023 [1]. The sample has been interpreted as representing a plutonic magma derived from a KREEP-free mantle source, and analyses of several zircon and apatite grains yielded an average 207Pb/206Pb date of 4342±9 Ma, which was interpreted as a crystallisation age [2]. In this study, we present Pb and Rb-Sr isotope analyses for a range of phases in a section of Arguin 002 to better constrain the chronology of the rock’s formation and modification, as well as the nature of the mantle source from which it was derived. This information is combined with a comparison of the sample’s bulk composition and remote sensing data to investigate the location on the lunar surface from which the rock may have originated. Sample description: A detailed petrography of Arguin 002 is presented in [2], which describes the sample as comprising coarse-grained (1-5 mm) orthopyroxene (47% by mode) with exsolved patches and lamellae of clinopyroxene (5%), and plagioclase (36%) that has been completely converted to maskelynite. Additionally, the rock contains melt veins (10%) and patches of anhedral Si-rich glass (2%). The maskelynite, melt veins, Si-rich glass and irregular nature of the clinopyroxene exsolution lamellae indicate the rock experienced significant metamorphic modification. Methods: Backscatter electron images and energy dispersive spectrometry (EDS) elemental maps were acquired with a Hitachi TM4000 tabletop scanning electron microscope at the University of Manchester (UoM), these were used to identify locations for Secondary Ion Mass Spectrometry (SIMS) and laser ablation inductively-coupled plasma mass spectrometry analyses (LA-ICP-MS). The Pb isotope analyses were performed using a Cameca IMS 1280 ion microprobe at the NordSIMS facility, in the Swedish Museum of Natural History, using a similar methodology to that outlined in previous studies [34]. The Rb–Sr isotope analyses were performed at the UoM with a Teledyne Analyte Excite+193 nm ArF Excimer laser ablation system and an Agilent 8900 ICP-MS, following a similar procedure to that described by [5]. Results: A total of 30 Pb isotope analyses were made in plagioclase/maskelynite, pyroxene, Si-rich glass, Ca-phosphates and the melt veins common in the sample. A regression is formed from 21 of these analyses, excluding the pyroxene, 1 plagioclase and 1 Ca-phosphate analysis for evidence of terrestrial contamination (Fig. 1a). This regression is interpreted as an isochron, equating to a date of 4328±6 Ma (95% conf.; MSWD = 0.95; P = 0.52). The least radiogenic Pb isotope composition is measured in the Si-rich glass and has a composition of 204Pb/206Pb = 0.024±0.003 and 207Pb/206Pb = 1.54±0.06, providing the best estimate for an initial Pb composition in the sample. The Rb-Sr isotope compositions were measured in plagioclase, pyroxene, Si-rich glass and the melt veins. An isochron is formed from 69 such analyses, equating to a date of 4469±160 Ma (95% conf.; Figure 1: (a) Pb and (b) Rb–Sr isotope compositions measured in Arguin 002 by SIMS and LA-ICP-MS. Error ellipses represent 2 sigma analytical uncertainties. MSWD = 1.2; P = 0.1), and an initial 87Sr/86Sr isotope ratio of 0.6997±0.0012 (Fig. 1b). Discussion: These isochron dates are similar to the 207Pb/206Pb apatite and zircon date reported previously [2]. Contrary to the previous study, and given that these new isochron dates comprise analyses of maskelynite and melt vein material, we interpret the dates as most likely representing resetting of the isotope systems, potentially as a result of an impact event. While the resetting of isotope systematics within different mineral phases will not occur uniformly, if this occurs relatively close in time to the original formation of the rock, the disturbance to the isochron relationship may be unresolvable within the uncertainties of the measurements. With this in mind, we propose that the Arguin 002 crystallisation age could not be significantly older than ~4.35 Ga. Following the approach of previous studies for modelling multiple stage Pb isotope evolution of the Moon [6,7], the initial Pb composition obtained for Arguin 002 indicates that the rock was derived from a mantle source with a 238U/204Pb ratio (µ-value) of 474±44 (2σ). While this is significantly lower than µ-values predicted for the sources of KREEP basalts (e.g. 15386 source µ-value = 3400-3950; [3]), it is similar to those predicted for many Apollo mare basalts where assimilation of between 1-15% KREEP has been proposed [6,8,9]. The Pb–Pb isochron resetting age of Arguin 002 is identical to the age recently proposed for the South Pole-Aitken (SPA) impact basin [10]. Notably, the previous study of the sample identified SPA as a likely location on the lunar surface from which the meteorite was sourced [2]. Following a similar logic, we have compared the bulk composition of Arguin 002 [2] with the Lunar Prospector gamma-ray spectrometer global orbital chemistry dataset [11], using the same chi-squared approach demonstrated in previous studies [10,12] to identify regions on the lunar surface closest in composition to Arguin 002 (Fig. 2). By contrast with the previous study, our results indicate a more ambiguous connection with SPA. The highest probabilities are less than 0.5 and clustered around Mare Crisium and Mare Smythii, the latter of which was also identified in the previous study as a potential source locality [2]. The ambiguity in this result may be due to an inappropriate comparison of a crystalline rock with the composition of lunar regolith. Conslusions: Analyses of the Pb and Rb–Sr isotope analyses of lunar meteorite Arguin 002 provide a date of 4328±6 Ma, which is interpreted as likely representing impact resetting. The initial Pb composition of the sample indicates derivation from a mantle source with a minor Apollo-like KREEP contribution. Finally, while this resetting age is consistent with recent estimates for the SPA basin, comparison of the meteorite’s bulk chemistry with orbital measurements of lunar regolith compositions are unable to confidently confirm the SPA region as the meteorite’s source locality. References: [1] Gattacceca et al. (2024) Met. Bull. no. 112. [2] Wang. Z. et al. (2025) Comm. Earth & Env., 6, 170. [3] Snape J. F. et al. (2016) EPSL, 451, 149–158. [4] Merle R. E. et al. (2020) Meteoritics & Planet. Sci., 55, 1808–1832. [5] Oliveira B. H. et al. (2025) Meteoritics & Planet. Sci., 60, 392–421. [6] Merle et al. (2024). [7] Che et al. (2025). [8] Snyder et al. (1994). [9] Jerde et al. (1994). [10] Joy K. H. et al. (2024) Nat. Astron., 9, 55–65. [11] Prettyman et al. (2006) JGR Planets, 111, E12007. [12] Calzada-Diaz et al. (2015) Meteoritics & Planet. Sci., 50, 214–228. Figure 2: Probability map of Arguin 002 bulk chemistry being a match to all nine element abundances measured by the Lunar Prospector gamma-ray spectrometer. White diamonds indicate the locations of the Apollo (A), Luna (L) and Chang’e landing sites.