Layered Lunar Interior for Hydrogen Isotopes and A Wet Moon during Formation

Hejiu Hui^1,2, Yunbin Guan³, Yang Chen⁴, Anne H. Peslier⁵, Youxue Zhang⁶, Yang Liu⁴, Roberta L. Flemming⁷, 

George R. Rossman³, John M. Eiler³, Clive R. Neal², Gordon R. Osinski⁷

¹State Key Laboratory for Ore Deposit Research & Lunar and Planetary Science Institute
School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China
²Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, USA.
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
⁵Jacobs, NASA-Johnson Space Center, Mail Code X13, Houston, TX 77058, USA.
⁶Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
⁷Department of Earth Sciences & Centre for Planetary Science and Exploration
University of Western Ontario, London, Ontario N6A 5B7, Canada.

Abstract

Knowing how much and when water was trapped within our Moon has fundamental implications on our understanding of how the Earth-Moon system formed. Water has been detected in lunar samples but its abundance, distribution and origin are debated. To address these issues, we report water concentrations and hydrogen isotope ratios of plagioclase from ferroan anorthosites, the only available lithology thought to have crystallized directly from the lunar magma ocean (LMO). Combined with literature data, δ²H values in least-degassed lunar igneous materials range from -280 to +310‰. We interpret the results by hydrogen isotope fractionation by degassing of molecular H₂ in the LMO until the formation of the primary lunar crust, with magmatic δ²H value of primordial water at the beginning of LMO being about -280‰, evolving to about +310‰ at the time of ferroan anorthite crust formation. Intermediate δ²H values observed in igneous rocks could be due to either mixing of the different end members, or from mantle sources that was degassed to different degree during magma evolution. We thereby explain the wide range of hydrogen isotope ratios of lunar lithologies. In the context of this model, H₂O concentration in the moon at the beginning of LMO may be as high as 2600 ppm.