O¹⁸/O¹⁶, Si³⁰/Si²⁸, D/H, and C¹³/C¹2 ratios in lunar samples

Abstract

The total range in O¹⁸/O¹⁶ in all analyzed whole-rock Apollo 11 and Apollo 12 samples is from δO¹⁸ = 5.53 to 6.60; the Si³⁰/Si²⁸ ratio varies from δSi³⁰ = -0.49 to + 0.31. The lunar igneous rocks (ilmenite basalts and microgabbros) show a smaller variation, δO¹⁸ = 5.53 to 5.90 and δSi³⁰ = - 0.28 to 0.0, with lunar micro breccias and fines being enriched in both δO¹⁸ and δSi³⁰ relative to igneous rocks. The whole-rock samples of the moon thus have an exceedingly homogeneous oxygen and silicon isotopic composition. The surfaces of grains of the lunar microbreccias and soils are, however, highly enriched in Si³⁰ and O¹⁸. The maximum measured δ-values of the surface oxygen and silicon are about +50 and +25 per mil, respectively, heavier than the bulk soils and rocks from the moon. These grain surfaces also appear to be depleted in total oxygen relative to total silicon. These O¹⁸ and Si³⁰ enrichments fit a simple square-root-of-the-mass fractional vaporization model, and they also correlate very well with the concentrations of solar wind hydrogen in the lunar samples. The isotopic enrichments are therefore probably (1) the result of particle bombardment of the lunar grain surfaces by micrometeorites, heavy ions, and/or solar protons, or (2) the result of condensation of a Si³⁰ and O¹⁸ enriched residue onto the grain surfaces. During vaporization due to meteorite bombardment the lighter isotopes may have preferentially escaped from the moon. A maximum value for the deuterium concentration in lunar hydrogen gas (almost wholly of solar wind origin) has been estimated to be about 5 ppm. Taking into account the contribution of deuterium formed by cosmic-ray spallation processes the D/H ratio of the solar wind therefore probably is no larger than 3 x 10⁻⁶. The δC¹³ values in the Apollo 12 breccia and soil samples are not as high as in the Apollo 11 samples, but are still higher than in terrestrial reduced carbon. This C¹³-rich carbon is strongly concentrated in the lunar fines and breccias. Thus, the heavy isotopes of carbon, silicon, and oxygen are all enriched in the lunar fines, apparently most strongly in the surface coatings of the grains and/or the finest size-fraction.

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