H₂O - δD Systematics from the Unbrecciated Lunar Norite Meteorite, Arguin 002

H2O - δD SYSTEMATICS FROM THE UNBRECCIATED LUNAR NORITE METEORITE, ARGUIN 002. M. Boruah1, B. G. Rider-Stokes1, M. Anand1, L. F. White1, X. Zhao1, A. Stephant2, J. F. Snape3, R. Tartèse3 , I. Franchi1, S. L. Jackson1. 1School of Physical Sciences, The Open University, UK, 2Istituto Nazionale di Astrofisica, Rome, Italy, 3Department of Earth and Environment Sciences, University of Manchester, UK. Corresponding Author: m[email protected] Introduction: Water, present even in trace amounts, plays a crucial role in understanding the thermal and magmatic evolution of a planetary body. With the discovery of water in pyroclastic glasses [1], several previous studies have well established the presence of water (with the term ‘water’ hereafter referring to OH, H, and H2O) on the Moon [2]. However, the sources and processes responsible for the retention and distribution of H2O on the Moon remain unconstrained. This uncertainty is due to several reasons: (1) Our current understanding is largely based on lunar samples collected during the Apollo, Luna, and Chang’e missions, which are only representative of specific regions on the Moon, resulting in a sampling bias [3]. (2) Most of these measurements have been largely focused on mineral-hosted melt inclusions [4] and apatite [5, 6]. Apatite is a late-stage mineral formed after the crystallization of 90-95% of the parent melt. In contrast, studies on nominally anhydrous minerals (NAMs) such as olivine, pyroxene, and plagioclase, which are likely to retain the indigenous signatures of the magmatic water, have been limited [7]. This is because measuring water in NAMs, especially in a lunar meteorite, is extremely challenging as they store water in their lattice defects, making them highly susceptible to alteration, especially isotopic compositions [8]. (3) Precise measurement of indigenous lunar water is further complicated by the need to account for several processes, such as magmatic degassing, crustal assimilation, solar wind implantation, spallation reactions, and terrestrial contamination, which are all known to alter the isotopic compositions [9]. In this work, we report the petrogenetic history, water abundance, and isotopic compositions measured in the nominally anhydrous mineral, orthopyroxene (OPX), from the lunar norite, Arguin 002. This study aims to provide insights into the fundamentals responsible for water retention in differentiated planetary bodies like the Moon, along with providing constraints on the sources responsible for volatile delivery to the Earth-Moon system. Sample: Arguin 002 (AG 002) is the only unbrecciated lunar norite, found in Mauritania in 2021 [10]. A recent study confirms it is a KREEP-free Mg suite rock with a crystallization age of 4341.5 ± 9.3 Ma, potentially sampled from/near the South Pole Aitken (SPA) basin [11]. Analytical Methods: Preliminary analysis was performed using a TESCAN Clara Scanning Electron Microscope (SEM) at OU. Before analysis, four small chips of AG 002 were embedded in a 1-inch round resin block and carbon-coated (5 µm thick). Mineral chemistry was acquired using a JEOL Field Emission Gun JXA-IHP200F electron probe micro-analyzer (EPMA) at the University of Cambridge. Figure 1: Energy dispersive X-ray spectroscopy (EDS) map of AG 002 with plagioclase in red, pyroxene in green, chromite in yellow, and silica in blue. The chips were then removed from the resin, mounted in indium, and gold-coated for water measurements using the Cameca NanoSIMS 50L at OU, following established protocols [9, 12]. A primary Cs+ beam of 1 nA was rastered over a 12 µm × 12 µm area to pre-sputter for 3 minutes to remove any surface contamination. Two terrestrial standards of OPX - NMNH 116,610 –10 and NMNH 116,610 –29 were used for H−/18O− versus H2O calibration to determine H2O abundances in OPX. The background in the H2O content was corrected using the H−/18O− measured on the anhydrous San Carlos olivine. The measured hydrogen (D/H) isotopic composition is expressed in δD (‰) format, where: δD = {(Rsample/RVSMOW) – 1}× 1000 where R is the D/H ratio, and the reference used is VSMOW (Vienna Standard Mean Ocean Water). The measured values were then corrected for instrumental mass fractionation (IMF), background, and spallation reaction. A cosmic ray exposure age (~0.11 Ma) of the sample was calculated statistically following the methodology mentioned in [9, 12]. Results: AG 002 appears as an unbrecciated with a coarse-grained (~2 mm average grain size) texture (Fig. 1). It is dominated by two minerals, orthopyroxene (En60.9Fs32.8Wo6.2, n=31) and plagioclase (An92.3, n=18). No olivine was found in this section. Clinopyroxene (En40.4Fs18.5Wo41.3, n=2) occurs as irregular exsolution lamellae and small blebs. Other minerals, such as chromite, troilite, amorphous silica, monazite, zircon, and Ca-phosphates, were also found in the section. Evidence of terrestrial weathering is limited to the presence of minor calcite along the fractures. The presence of melt veins, amorphous silica, and the transformation of plagioclase feldspar to maskelynite indicates that the rock has experienced a significant level of shock. Figure 2: Plot of Mg# (pyroxene) vs. An# (plagioclase) suggests a Mg-suite origin for the sample. The data for norites, gabbronorites, spinel troctolites, and troctolites are from [13]. Seven OPX grains were successfully targeted for NanoSIMS analyses. After spallation correction, the H2O abundances measured in these grains ranges from 1.7 to 12.4 ± 0.2 ppm, with a weighted average of 4.1 ± 0.1 ppm, and the δD values ranges from - 819 ± 59 ‰ to 29 ± 138 ‰ with a weighted average of - 351 ± 90 ‰, (Fig. 3). The data was collected in two sessions, and the background measured based on the H2O content of amorphous San Carlos olivine improved from 4.6 to 4.0 ppm. The background corrected detection limit (BLOD) of H2O, defined by three times the uncertainty (1SD) of the analytical background, was 0.37 and 0.8 ppm. Figure 3: Plot of δD values (‰) vs. H2O contents (ppm) in OPX in AG 002 in comparison with the apatite from Norite 77215 and 78235 [15]. The grey shaded region represents the average δD = -350 ± 90 ‰. Discussion: The mineral chemistry data of the major silicates, plotted on an Mg# vs. An# diagram (Fig. 2), confirm the Mg-suite origin of the sample. However, it falls within the uncertainty of the existing geochemical field of the Apollo Mg-suite rocks [13], and this is because the OPX present in the sample exhibits low Mg# (63-73). This lower Mg# could be linked to the evolved nature of the parental melt. A recent study has also shown that AG 002 is a KREEPfree Mg-suite rock with evolved REE and deeper Eu anomalies formed from a primary melt generated by low-degree partial melting of KREEP-free mantle source, followed by a higher degree (~95-97%) of fractional crystallisation of this primary melt [11]. The H2O content measured in plagioclase from FAN 60015 (3.4 ppm [14]; 5 ppm [7]) falls within the range of H2O content measured in this study. In contrast, H2O content in apatites from Apollo norites (77215 and 78235; [15]) is remarkably higher. Furthermore, while the measured range of δD in AG 002 largely overlaps with the Apollo norites [15] except for two points with extremely lower δD (Fig. 3), it is significantly lower than the average δD in FAN 60015 = +310 ‰ [7]. The higher δD measured in FAN 60015 could be due to enhanced degassing in shallow crustal rocks, as suggested in [7]. While the extremely low δD measured in this study could be a result of solar wind implantation (δD = -1000‰ [16]). However, due to the lack of sufficient data points, there is no visible correlation between H2O content and δD measured in this study. Notably, given the shocked nature of the sample, the lack of correlation could also be attributed to the effects of shock on the hydrogen isotopic compositions. Therefore, to better constrain the H₂O–δD systematics, additional data shall be collected with microand nano-structural investigation of OPX, which shall be presented at the conference. Acknowledgment: This work is funded by UKRI-STFC grant. We thank Ms. Kay Knight for her assistance with sample preparation. Dr Iris Buisman is greatly thanked for assisting with the EPMA work. Jared Collins is greatly thanked for their donation of material. References: [1] Saal A.E. et al. (2008) Nature 454, 192-195. [2] Anand M et al (2014) Phil.Trans.R.Soc.A372: 20130254. [3] McCubbin F.M. et al. (2023) Reviews in Min. & Geochem. Vol. 89 pp. 729-786. [4] Saal A.E. et al. (2013) Science, 340, 1317. [5] Barnes J.J. et al. (2013) Chemical Geology. 337–338:48-55. [6] Stephant A. et al. (2019) Geochim et Cosmochim Acta 266, 163–183. [7] Hui H. et al. (2017) Earth Planet. Sci. Lett., 473, 14–23. [8] Distel A. et al. (2023), LPI Contrib. No. 3036, 6205. [9] Stephant A. et al. (2021) Geochim et Cosmochim Acta 297, 203–219. [10] Gattacceca et al. (2024) Met. Bull. no. 112. [11] Wang. Z. et al. (2025) Nature Communication, 6,170. [12] RiderStokes. B.G. et al. (2024) Earth Planet. Sci. Lett. 642,, 118860. [13] Shearer. C.K. et al. (2015) American Mineralogist, Vol. 100, 294–325. [14] Hui H. et al. (2013) Nature Geosci. 6, 177-180. [15] Barnes J.J. et al. (2014) Earth Planet. Sci. Lett. 390, 244–252. [16] Wiens R.C. et al. (2004), Earth Planet. Sci. Lett. 266, 549-565.