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Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Greenwood, James P., et al. Presented by Patrick Harner. Deuterium/Hydrogen. δ D = {[(D/H) sample /(D/H) VSMOW ] – 1} x 1,000 Terrestrial water δ D range from ~-500‰ to ~+100‰; mean ocean value of O‰

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Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

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  1. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon Greenwood, James P., et al. Presented by Patrick Harner

  2. Deuterium/Hydrogen • δD = {[(D/H)sample/(D/H)VSMOW] – 1} x 1,000 • Terrestrial water δD range from ~-500‰ to ~+100‰; mean ocean value of O‰ • Terrestrial mantle δD of ~-100‰ • Solar system δD range -1,000‰ to >+100,000‰ www.nasa.gov

  3. Lunar Samples • Analyzed Apatite (Ca5(PO4)3(F, Cl, OH)) • Mare basalt apatite associated with late-stage crystallization features • Apatite grain fragments from late-stage matrix of two breccia • One apatite grain from a high phosphate intrusive alkali anorthositeclast apod.nasa.gov

  4. Methods • Lunar samples, fresh thin sections • Light microscopy and electron imaging • Ion microscope analysis: SIMS, SCAPS • Quantitative electron microprobe analysis

  5. Mare Basalts • Apatite enriched in H compared to surrounding minerals • Water content in mare basalt apatite grains range from 0 to 6,050ppm • Similar water content from basalts from separate landing sites with different ages

  6. Water Content • Plagioclase represent non-hydrous background • One low-water mare basalt sample; slowly cooled, extensive degassing • Highland alkali anorthositeclast apatite grain show 80 ± 10 ppm; predate mare basalt volcanism

  7. Isotopic Variability • δD range from +391‰ to +1,010‰ • Variability of δD within individual grains • Homogenous H2O% • No strong correlation with cosmic-ray exposure • Zonation of minor elements

  8. Possible Explanations • Earth-Moon system was able to diffusively equilibrate its volatiles after Moon-forming impact; fractionation caused by hydrodynamic escape of hydrogen during formation • Addition of cometary water to an early Moon that inherited its δD from Earth’s mantle • Assimilation of lunar regolith enriched in water from impact

  9. References • Boyce, J. W., et al., Nature 466, 466-469 (2010) • Greenwood, James P., et al., Geophys. Res. Lett. 35, L05203 (2008) • Greenwood, James P., et al., Nature 1050, 79-82 (2011) • Sakamoto, N., et al., 72 Annual Meteoritical Society Meeting (2009) • Taylor, L. A., et al., Am. Mineral 89, 1617-1624 (2004)

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