Are there 100s of ppm water in nominally anhydrous minerals of non-carbonaceous asteroids?
Dennis Harries1
European Space Resources Innovation Centre (ESRIC), Luxembourg Institute of Science and Technology, 41 rue du Brill, L4422 Belvaux, Luxembourg1

Non-carbonaceous (NC) asteroids of mainly the S type comprise a large fraction of objects in the main asteroid belt and among
the near-Earth asteroid population. Materials represented by them have played significant roles in the accretion of theterrestrial planets, but generally NC materials have been considered essentially dry relative to carbonaceous chondrites (CC).
In consequence, NC planetesimals were usually not considered as carriers of H2O delivered to the early Earth. Recently, SIMS
studies of regolith particles returned by JAXA’s Hayabusa mission from S-type/LL-group asteroid 25143 Itokawa havereported 240 to 990 ppm H2O in the nominally anhydrous minerals (NAMs) orthopyroxene, olivine, and albite [1,2].
Additional data has indicated similar contents in NAMs of equilibrated chondrites of the L and LL groups [1,3] and in NAMsof unequilibrated chondrites [4].
H2O contents on the order of 100s of ppm in NAMs of ordinary chondrites would have substantial implications for the earlyaccretion of H2O to the proto-Earth and would also imply high lithostatic pressures during thermometamorphism withinchondritic planetesimals not consistent with current size estimates. Moreover, high water-to-rock ratios during metamorphism
could indicate much higher bulk H2O contents of pristine equilibrated chondrites beyond the defect-bound hydroxyl
component, e.g., in the form of fluid inclusions. This would have considerable implications for the targeting of S-type nearEarth asteroids for in-situ space resources utilization (ISRU).
In order to shed more light on the H2O contents in asteroidal NAMs, we have conducted a survey of H2O in equilibratedchondrites and achondrites using the NanoSIMS 50L at the Open University, UK [5]. This contribution serves to place theseresults in perspective with other recent findings. The NAMs studied comprise olivine and orthopyroxene from a set of
equilibrated ordinary chondrites of the L and LL groups (Baszkówka, Bensour, Kheneg Ljouâd, and Tuxtuac) and several
ultramafic achondrites (Zakłodzie, Dhofar 125, Northwest Africa [NWA] 4969, NWA 6693, and NWA 7317). For calibration
we used terrestrial olivine and orthopyroxene with H2O contents determined by Fourier transform infrared spectroscopy. Our
99.7% (~3SD) detection limits achieved were 3.6 to 5.4 ppm H2O for olivine and 7.7 to 10.9 ppm H2O for orthopyroxene.
Our survey did not identify any meteorite sample that consistently showed H2O contents in NAMs above our detection limits.
Other recent SIMS studies of H2O in NAMs of ungrouped achondrites and acapulcoites/lodranites, including NWA 6704
(paired with our sample of NWA 6693) and Dhofar 125, did not detect H2O above 2.1 ppm [6] and 6.6 ppm [7], respectively.
SIMS analyses of NAMs in unequilibrated chondrites by have also indicated very low H2O contents of 8 to 14 ppm in thesematerials [8].
Finding H2O contents one to two orders of magnitude lower than previously reported suggests that the parent planetesimals of
the highly metamorphic chondrites, primitive achondrites and ultramafic achondrites studied were efficiently degassed duringmetamorphism and melting, if they initially had held substantial amounts of volatiles [5,6]. The discrepancies between SIMS
analyses of Itokawa samples/equilibrated chondrites showing 100s of ppm H2O on the one hand and equilibrated chondrites
showing hardly any detectable H2O on the other hand may stem from two contributions: Systematic errors of the analyses
and/or heterogeneity of the samples. Sample contamination leading to systematically biased H2O contents is a major suspect. Itappears more likely to occur than H2O loss from the strongly bound hydroxyl defects in NAMs, which requires substantial orprolonged heating to diffusively remove H2O. The studies available rarely analysed the same samples or meteorites, rendering
it difficult to rule out sample heterogeneity. However, there is currently no other indication to support the hypothesis that
common equilibrated chondrites underwent metamorphism under vastly different peak pressures and H2O fugacities, leading toheterogeneous H2O contents in NAMs among different meteorites.
Hence, the presence of 100s of ppm H2O in samples of asteroid Itokawa and equilibrated non-carbonaceous chondrites isdoubtful and the role of their parent bodies in delivering water to the terrestrial planets needs to be questioned.
References[1] Jin Z. and Bose M. 2019. Science Advances 5:eaav8106. [2] Chan Q.H.S. et al. 2021. Scientific Reports 11:5125. [3] Jin Z.
et al. 2021. Planetary Science Journal 2:244. [4] Stephant A. et al. 2017. Geochimica et Cosmochimica Acta 199:75–90. [5]Harries D. et al. 2023. Meteoritics & Planetary Science 58:705–721. [6] Newcombe M.E. et al. 2023. Nature 615:854–857. [7]Stephant A. et al. 2023. Earth and Planetary Science Letters 615:118202. [8] Shimizu K. et al. 2021. Geochimica et
Cosmochimica Acta 301:230-258.