Water in lunar anorthosites and evidence for a wet early Moon

Journal name:
Nature Geoscience
Volume:
6,
Pages:
177–180
Year published:
DOI:
doi:10.1038/ngeo1735
Received
Accepted
Published online

The Moon was thought to be anhydrous since the Apollo era1, but this view has been challenged by detections of water on the lunar surface2, 3, 4 and in volcanic rocks5, 6, 7, 8, 9 and regolith10. Part of this water is thought to have been brought through solar-wind implantation2, 3, 4, 7, 10 and meteorite impacts2, 3, 7, 11, long after the primary lunar crust formed from the cooling magma ocean12, 13. Here we show that this primary crust of the Moon contains significant amounts of water. We analysed plagioclase grains in lunar anorthosites thought to sample the primary crust, obtained in the Apollo missions, using Fourier-transform infrared spectroscopy, and detected approximately 6ppm water. We also detected up to 2.7ppm water in plagioclase grains in troctolites also from the lunar highland upper crust. From these measurements, we estimate that the initial water content of the lunar magma ocean was approximately 320ppm; water accumulating in the final residuum of the lunar magma ocean could have reached 1.4wt%, an amount sufficient to explain water contents measured in lunar volcanic rocks. The presence of water in the primary crust implies a more prolonged crystallization of the lunar magma ocean than a dry moon scenario and suggests that water may have played a key role in the genesis of lunar basalts.

At a glance

Figures

  1. Representative polarized FTIR spectra of plagioclase from FANs.
    Figure 1: Representative polarized FTIR spectra of plagioclase from FANs.

    a,b, Spectra for 15415,238 (a) and 60015,787 (b) at mutually perpendicular orientations (Ext1, Ext2 and Ext3: optical extinction directions 1, 2 and 3) are normalized to 1mm and shifted vertically for comparison. The dashed line indicates the baseline position used for water content estimations. The narrow peaks (3,000–2,800cm−1) most probably come from organic contamination on the mineral surface during sample preparation22, 23. The spectra with the same label (for example, Ext1) for different crystals were not taken at the same crystal orientation relative to the mineral crystallographic axis.

  2. FTIR spectra of plagioclase from 15415,238 before and after heating at 1,000[thinsp][deg]C for 24[thinsp]h.
    Figure 2: FTIR spectra of plagioclase from 15415,238 before and after heating at 1,000°C for 24h.

    FTIR analyses were performed on the sample before (upper thick curve with the dashed baseline used for water content estimation) and after the heating experiment (lower thin curve) at the same orientations of the sample relative to the polarizer (Ext1 and Ext2, respectively). The diminished band (~ 3,700 to ~3,100cm−1) in the spectrum of the heated sample demonstrates that dehydration occurred. The band in the untreated sample is due to absorption of O–H bond vibration, and not an artefact in the baseline.

  3. Water contents in LMO products and mantle sources of basalts through time.
    Figure 3: Water contents in LMO products and mantle sources of basalts through time.

    Model ages are used for primary magma ocean products12, 13 and isochron ages for the basalts (Supplementary Table S2). Water contents of the initial magma ocean (LMO), the first crystallized olivine cumulate (first Ol), co-crystallized pyroxene cumulate (cxt Px) and urKREEP were estimated from the water content measured in FAN plagioclases. The black dashed line from LMO to urKREEP shows the water content evolution in magma ocean residua. The water contents of mantle sources with isochron ages <4.0 Gyr were calculated assuming 20% (green) or 3% partial melting (purple; Supplementary Table S2).

  4. Representative polarized FTIR spectra of troctolite 76535,164.
    Figure 4: Representative polarized FTIR spectra of troctolite 76535,164.

    a,b, Spectra of plagioclase (a) and olivine (b) at two mutually perpendicular orientations (Ext1 and Ext2) are normalized to 1mm and shifted vertically for comparison. The dashed line indicates the baseline position used to calculate water concentrations (see Supplementary Information S2). No obvious OH bands are observed in the spectra taken with orientation Ext1 of plagioclase or in any of the olivine spectra. The narrow peaks between 3,000 and 2,800cm−1 most probably come from organic contamination on the mineral surface during sample preparation22, 23.

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Author information

Affiliations

  1. Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA

    • Hejiu Hui &
    • Clive R. Neal
  2. Jacobs Technology, ESCG, Mail Code JE23, Houston, Texas 77058, USA

    • Anne H. Peslier
  3. ARES, NASA-Johnson Space Center, Mail Code KR, Houston, Texas 77058, USA

    • Anne H. Peslier
  4. Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

    • Youxue Zhang

Contributions

H.H. conceived this study and performed the analyses and experiments. Y.Z. provided the terrestrial plagioclase grains. A.H.P and Y.Z. assisted in experiments and FTIR analyses. H.H., A.H.P., Y.Z. and C.R.N. discussed the data and wrote the paper.

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The authors declare no competing financial interests.

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