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Refining lunar impact chronology through high spatial resolution (40)Ar/(39)Ar dating of impact melts.

Mercer CM, Young KE, Weirich JR, Hodges KV, Jolliff BL, Wartho JA, van Soest MC - Sci Adv (2015)

Bottom Line: However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment.Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga.The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.

ABSTRACT
Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe (40)Ar/(39)Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.

No MeSH data available.


Related in: MedlinePlus

UVLAMP 40Ar/39Ar results for 77115,121.(A) Summed PDP of melt dates (red) with individual dates depicted as small red circles above. The gray region represents the range of dates obtained for clasts in 77115,121. (B) Isotope correlation plot of 40Ar/36Ar versus 39Ar/36Ar and linear regression (dashed line) for melt analyses in 77115,121. The inset figure is an enlarged plot of the boxed region near the origin. The data have been corrected for the presence of nucleogenic and cosmogenic isotopes (see Materials and Methods for details). Error ellipses and uncertainties are all 2σ.
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Figure 2: UVLAMP 40Ar/39Ar results for 77115,121.(A) Summed PDP of melt dates (red) with individual dates depicted as small red circles above. The gray region represents the range of dates obtained for clasts in 77115,121. (B) Isotope correlation plot of 40Ar/36Ar versus 39Ar/36Ar and linear regression (dashed line) for melt analyses in 77115,121. The inset figure is an enlarged plot of the boxed region near the origin. The data have been corrected for the presence of nucleogenic and cosmogenic isotopes (see Materials and Methods for details). Error ellipses and uncertainties are all 2σ.

Mentions: We used the UVLAMP system to target 15 small melt volumes and 11 monomineralic and lithic clasts for 40Ar/39Ar dating in 77115,121 (Fig. 1A, fig. S1, and table S2). Although the analytical uncertainties of individual dates are relatively large as a consequence of the small material volumes ablated, the population of melt dates constitute a single major mode on a probability density plot (PDP; Fig. 2A). When plotted on an isotope correlation diagram (Fig. 2B), these 15 melt analyses form an isochron with a date of 3.834 ± 0.020 Ga [mean squared weighted deviation (MSWD) = 1.01]. Analyses of clasts in 77115,121 yielded considerably older dates, ranging from 4.23 ± 0.25 Ga to 3.892 ± 0.067 Ga (table S2). Note that a few of the melt dates have probability density distributions that overlap those of the clast population, potentially indicating that the ablation volumes may have been contaminated by small amounts of clast material that was undetected during the targeting process (see Supplementary Materials and Methods). However, given the value of the MSWD for the isochron (1.01) and because there are no clear visual indications for clast contamination from an inspection of the laser pits under a microscope, there is no strong statistical or observational basis for identifying and rejecting potential outliers in the regression.


Refining lunar impact chronology through high spatial resolution (40)Ar/(39)Ar dating of impact melts.

Mercer CM, Young KE, Weirich JR, Hodges KV, Jolliff BL, Wartho JA, van Soest MC - Sci Adv (2015)

UVLAMP 40Ar/39Ar results for 77115,121.(A) Summed PDP of melt dates (red) with individual dates depicted as small red circles above. The gray region represents the range of dates obtained for clasts in 77115,121. (B) Isotope correlation plot of 40Ar/36Ar versus 39Ar/36Ar and linear regression (dashed line) for melt analyses in 77115,121. The inset figure is an enlarged plot of the boxed region near the origin. The data have been corrected for the presence of nucleogenic and cosmogenic isotopes (see Materials and Methods for details). Error ellipses and uncertainties are all 2σ.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4644078&req=5

Figure 2: UVLAMP 40Ar/39Ar results for 77115,121.(A) Summed PDP of melt dates (red) with individual dates depicted as small red circles above. The gray region represents the range of dates obtained for clasts in 77115,121. (B) Isotope correlation plot of 40Ar/36Ar versus 39Ar/36Ar and linear regression (dashed line) for melt analyses in 77115,121. The inset figure is an enlarged plot of the boxed region near the origin. The data have been corrected for the presence of nucleogenic and cosmogenic isotopes (see Materials and Methods for details). Error ellipses and uncertainties are all 2σ.
Mentions: We used the UVLAMP system to target 15 small melt volumes and 11 monomineralic and lithic clasts for 40Ar/39Ar dating in 77115,121 (Fig. 1A, fig. S1, and table S2). Although the analytical uncertainties of individual dates are relatively large as a consequence of the small material volumes ablated, the population of melt dates constitute a single major mode on a probability density plot (PDP; Fig. 2A). When plotted on an isotope correlation diagram (Fig. 2B), these 15 melt analyses form an isochron with a date of 3.834 ± 0.020 Ga [mean squared weighted deviation (MSWD) = 1.01]. Analyses of clasts in 77115,121 yielded considerably older dates, ranging from 4.23 ± 0.25 Ga to 3.892 ± 0.067 Ga (table S2). Note that a few of the melt dates have probability density distributions that overlap those of the clast population, potentially indicating that the ablation volumes may have been contaminated by small amounts of clast material that was undetected during the targeting process (see Supplementary Materials and Methods). However, given the value of the MSWD for the isochron (1.01) and because there are no clear visual indications for clast contamination from an inspection of the laser pits under a microscope, there is no strong statistical or observational basis for identifying and rejecting potential outliers in the regression.

Bottom Line: However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment.Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga.The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique.

View Article: PubMed Central - PubMed

Affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA.

ABSTRACT
Quantitative constraints on the ages of melt-forming impact events on the Moon are based primarily on isotope geochronology of returned samples. However, interpreting the results of such studies can often be difficult because the provenance region of any sample returned from the lunar surface may have experienced multiple impact events over the course of billions of years of bombardment. We illustrate this problem with new laser microprobe (40)Ar/(39)Ar data for two Apollo 17 impact melt breccias. Whereas one sample yields a straightforward result, indicating a single melt-forming event at ca. 3.83 Ga, data from the other sample document multiple impact melt-forming events between ca. 3.81 Ga and at least as young as ca. 3.27 Ga. Notably, published zircon U/Pb data indicate the existence of even older melt products in the same sample. The revelation of multiple impact events through (40)Ar/(39)Ar geochronology is likely not to have been possible using standard incremental heating methods alone, demonstrating the complementarity of the laser microprobe technique. Evidence for 3.83 Ga to 3.81 Ga melt components in these samples reinforces emerging interpretations that Apollo 17 impact breccia samples include a significant component of ejecta from the Imbrium basin impact. Collectively, our results underscore the need to quantitatively resolve the ages of different melt generations from multiple samples to improve our current understanding of the lunar impact record, and to establish the absolute ages of important impact structures encountered during future exploration missions in the inner Solar System.

No MeSH data available.


Related in: MedlinePlus