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The chlorine isotope fingerprint of the lunar magma ocean.

Boyce JW, Treiman AH, Guan Y, Ma C, Eiler JM, Gross J, Greenwood JP, Stolper EM - Sci Adv (2015)

Bottom Line: The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon.Instead, (37)Cl/(35)Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high (37)Cl/(35)Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean.These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon's history.

View Article: PubMed Central - PubMed

Affiliation: Division of Geological and Planetary Sciences, Caltech, 1200 East California Boulevard, Pasadena, CA 91125, USA. ; Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095-1567, USA.

ABSTRACT
The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because (37)Cl/(35)Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, (37)Cl/(35)Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high (37)Cl/(35)Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon's history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

No MeSH data available.


Related in: MedlinePlus

Bulk Th and La/Lu versus δ37Cl.(A and B) Measures of KREEP contribution, bulk rock Th abundance (A), and bulk rock La/Lu (B), as a function of δ37Cl, with trace element data from (43, 54, 59). Symbols are the same as those in previous figures. For all basalts, δ37Cl is strongly correlated with Th and La/Lu, suggesting that pure urKREEP would have δ37Cl ≥+30‰.
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Figure 5: Bulk Th and La/Lu versus δ37Cl.(A and B) Measures of KREEP contribution, bulk rock Th abundance (A), and bulk rock La/Lu (B), as a function of δ37Cl, with trace element data from (43, 54, 59). Symbols are the same as those in previous figures. For all basalts, δ37Cl is strongly correlated with Th and La/Lu, suggesting that pure urKREEP would have δ37Cl ≥+30‰.

Mentions: We conclude that none of the three tests of the lava-outgassing hypothesis are consistent with this new data set, and therefore, we develop other explanations, focusing on processes that are implied by correlations between d37Cl and other geochemical properties: The δ37Cl values of apatite from lunar basalts are correlated with Cl content, which is known to be enriched in KREEP. KREEP is the name given to a chemical component—possibly derived from the urKREEP, the last vestige of the lunar magma ocean—rich in potassium (K), the rare earths (REE), and phosphorus (P) and other elements that are incompatible during the crystallization of magmas (29–31). Incompatible elements are those that are preferentially excluded from crystals growing from a melt and therefore concentrated in that residual melt—in this case, the lunar magma ocean. The lunar magma ocean first crystallizes from below, then from both above and below, resulting in a “sandwich horizon” of melt with decreasing volume and increasing abundances of incompatible elements (31). KREEP (and the proposed urKREEP source) have long been implicated in the trace element variations of lunar basalts [for example, (32)], with different units having different contributions of KREEP (sometimes referred to as “KREEPiness”). More KREEP-rich (or “KREEPy”) samples are observed to have higher abundances of trace elements such as Th, Cl, as well as the major and trace elements that make up the acronym KREEP. Model compositions of the urKREEP (29) also indicate that the urKREEP may have elevated La/Lu ratios, relative to the mare basalt source. Thus, trace element abundances and ratios can be used to determine the magnitude of KREEP contamination/addition. The δ37Cl values of mare basalt apatites are also positively correlated with abundances and ratios in their host rocks of the elements that are enriched in the lunar KREEP component, notably bulk Th abundance and La/Lu abundance ratios derived from the literature (Fig. 5) (29, 31).


The chlorine isotope fingerprint of the lunar magma ocean.

Boyce JW, Treiman AH, Guan Y, Ma C, Eiler JM, Gross J, Greenwood JP, Stolper EM - Sci Adv (2015)

Bulk Th and La/Lu versus δ37Cl.(A and B) Measures of KREEP contribution, bulk rock Th abundance (A), and bulk rock La/Lu (B), as a function of δ37Cl, with trace element data from (43, 54, 59). Symbols are the same as those in previous figures. For all basalts, δ37Cl is strongly correlated with Th and La/Lu, suggesting that pure urKREEP would have δ37Cl ≥+30‰.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4643783&req=5

Figure 5: Bulk Th and La/Lu versus δ37Cl.(A and B) Measures of KREEP contribution, bulk rock Th abundance (A), and bulk rock La/Lu (B), as a function of δ37Cl, with trace element data from (43, 54, 59). Symbols are the same as those in previous figures. For all basalts, δ37Cl is strongly correlated with Th and La/Lu, suggesting that pure urKREEP would have δ37Cl ≥+30‰.
Mentions: We conclude that none of the three tests of the lava-outgassing hypothesis are consistent with this new data set, and therefore, we develop other explanations, focusing on processes that are implied by correlations between d37Cl and other geochemical properties: The δ37Cl values of apatite from lunar basalts are correlated with Cl content, which is known to be enriched in KREEP. KREEP is the name given to a chemical component—possibly derived from the urKREEP, the last vestige of the lunar magma ocean—rich in potassium (K), the rare earths (REE), and phosphorus (P) and other elements that are incompatible during the crystallization of magmas (29–31). Incompatible elements are those that are preferentially excluded from crystals growing from a melt and therefore concentrated in that residual melt—in this case, the lunar magma ocean. The lunar magma ocean first crystallizes from below, then from both above and below, resulting in a “sandwich horizon” of melt with decreasing volume and increasing abundances of incompatible elements (31). KREEP (and the proposed urKREEP source) have long been implicated in the trace element variations of lunar basalts [for example, (32)], with different units having different contributions of KREEP (sometimes referred to as “KREEPiness”). More KREEP-rich (or “KREEPy”) samples are observed to have higher abundances of trace elements such as Th, Cl, as well as the major and trace elements that make up the acronym KREEP. Model compositions of the urKREEP (29) also indicate that the urKREEP may have elevated La/Lu ratios, relative to the mare basalt source. Thus, trace element abundances and ratios can be used to determine the magnitude of KREEP contamination/addition. The δ37Cl values of mare basalt apatites are also positively correlated with abundances and ratios in their host rocks of the elements that are enriched in the lunar KREEP component, notably bulk Th abundance and La/Lu abundance ratios derived from the literature (Fig. 5) (29, 31).

Bottom Line: The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon.Instead, (37)Cl/(35)Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high (37)Cl/(35)Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean.These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon's history.

View Article: PubMed Central - PubMed

Affiliation: Division of Geological and Planetary Sciences, Caltech, 1200 East California Boulevard, Pasadena, CA 91125, USA. ; Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095-1567, USA.

ABSTRACT
The Moon contains chlorine that is isotopically unlike that of any other body yet studied in the Solar System, an observation that has been interpreted to support traditional models of the formation of a nominally hydrogen-free ("dry") Moon. We have analyzed abundances and isotopic compositions of Cl and H in lunar mare basalts, and find little evidence that anhydrous lava outgassing was important in generating chlorine isotope anomalies, because (37)Cl/(35)Cl ratios are not related to Cl abundance, H abundance, or D/H ratios in a manner consistent with the lava-outgassing hypothesis. Instead, (37)Cl/(35)Cl correlates positively with Cl abundance in apatite, as well as with whole-rock Th abundances and La/Lu ratios, suggesting that the high (37)Cl/(35)Cl in lunar basalts is inherited from urKREEP, the last dregs of the lunar magma ocean. These new data suggest that the high chlorine isotope ratios of lunar basalts result not from the degassing of their lavas but from degassing of the lunar magma ocean early in the Moon's history. Chlorine isotope variability is therefore an indicator of planetary magma ocean degassing, an important stage in the formation of terrestrial planets.

No MeSH data available.


Related in: MedlinePlus