The Cl isotope composition and halogen contents of Apollo-return samples
Anthony Gargano1,2, Zachary Sharp3,2, Charles Shearer4
1Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-0001; agargano@unm.edu.
Summary
Lunar mare basalts show significant depletion in fluorine and chlorine, suggesting early degassing during the Moon-forming Giant Impact. Distinct isotopic signatures point to a light vapor source, not equilibrium fractionation.
Area of Science:
- Geochemistry
- Planetary Science
- Isotope Geochemistry
Background:
- Lunar mare basalts exhibit substantial depletion in fluorine (F) and chlorine (Cl) compared to Earth's mid-ocean ridge basalts.
- Two distinct Cl-bearing components with elevated isotopic compositions (δ³⁷Cl) were identified in lunar samples.
Purpose of the Study:
- To investigate the origin of halogen depletion and isotopic signatures in lunar mare basalts.
- To determine the processes responsible for the observed high δ³⁷Cl values and their implications for lunar evolution.
Main Methods:
- Analysis of water-soluble and structurally bound chloride in lunar mare basalts.
- Measurement of chlorine isotope ratios (δ³⁷Cl) in different mineral phases.
- Correlation analysis of F/Cl ratios and δ³⁷Cl values with lithological components.
Main Results:
- Water-soluble Cl averages δ³⁷Cl of 3.0 ± 4.3‰, while structurally bound Cl averages 7.3 ± 3.5‰.
- High and divergent δ³⁷Cl values suggest early, extensive degassing of an isotopically light vapor, inconsistent with equilibrium fractionation.
- Ferroan anorthosites (FANs) display the highest δ³⁷Cl values, linked to incorporation of HCl vapor from an evolved lunar magma ocean (LMO).
Conclusions:
- Lunar halogen depletion primarily resulted from the Moon-forming Giant Impact.
- Localized degassing during late-stage lunar magma ocean (LMO) crystallization or later melt crystallization explains high δ³⁷Cl in apatite.
- The KREEP component is unlikely to be the sole source of high lunar Cl isotope values; other mechanisms involving evolved LMO vapor are indicated.
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