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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 73217,83.(A) Summed PDP of the dates for the three melt domains with individual dates depicted as small colored circles above (yellow, domain 1; blue, domain 2; red, domain 3). Vertical gray bars indicate ranges of two populations of clast dates from 73217,83. The vertical dashed lines indicate U/Pb dates for zircons and phosphates from another section of 73217 interpreted by Grange et al. (2) to represent impact melting events. (B to D) Isotope correlation plots of 40Ar/36Ar versus 39Ar/36Ar with linear regressions shown as dashed lines. Inset figures are enlarged plots of the boxed regions in their parent figures. 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. (B) Isotope correlation plot for analyses in domain 1. (C) Isotope correlation plot of domain 2 analyses. Note: The one analysis that was rejected as an outlier is not shown (see the text for details). It would plot below the regressed line, up and to the right outside of the present bounds. (D) Isotope correlation plot of domain 3 data. Points shown as dark red ellipses were selected from mode 3A of the PDP (see the text for details) and regressed (upper line) to constrain the upper bound of the range of apparent ages measured for domain 3. Points shown as light red ellipses were selected from mode 3B of the PDP and regressed (lower line) to constrain the lower bound of the range of apparent ages for domain 3. The locations of the analyses that constitute modes 3A and 3B are shown in fig. S2. All other data are shown as unfilled ellipses.
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Figure 3: UVLAMP 40Ar/39Ar results for 73217,83.(A) Summed PDP of the dates for the three melt domains with individual dates depicted as small colored circles above (yellow, domain 1; blue, domain 2; red, domain 3). Vertical gray bars indicate ranges of two populations of clast dates from 73217,83. The vertical dashed lines indicate U/Pb dates for zircons and phosphates from another section of 73217 interpreted by Grange et al. (2) to represent impact melting events. (B to D) Isotope correlation plots of 40Ar/36Ar versus 39Ar/36Ar with linear regressions shown as dashed lines. Inset figures are enlarged plots of the boxed regions in their parent figures. 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. (B) Isotope correlation plot for analyses in domain 1. (C) Isotope correlation plot of domain 2 analyses. Note: The one analysis that was rejected as an outlier is not shown (see the text for details). It would plot below the regressed line, up and to the right outside of the present bounds. (D) Isotope correlation plot of domain 3 data. Points shown as dark red ellipses were selected from mode 3A of the PDP (see the text for details) and regressed (upper line) to constrain the upper bound of the range of apparent ages measured for domain 3. Points shown as light red ellipses were selected from mode 3B of the PDP and regressed (lower line) to constrain the lower bound of the range of apparent ages for domain 3. The locations of the analyses that constitute modes 3A and 3B are shown in fig. S2. All other data are shown as unfilled ellipses.

Mentions: During UVLAMP analyses of 73217,83, we targeted 13 melt volumes in domain 1, 10 melt volumes in domain 2, 40 melt volumes in domain 3, and 8 clasts within domains 2 and 3 (Fig. 1B, fig. S2, and table S3). Represented on a PDP, the populations of 40Ar/39Ar dates from the three domains and clasts are distinct and span a range from ca. 4.0 to 3.2 Ga (Fig. 3A). One population of clast dates range from 3.788 ± 0.088 Ga to 3.721 ± 0.052 Ga, whereas a second group of clast dates range from 3.973 ± 0.019 Ga to 3.902 ± 0.030 Ga. Both sets of clasts are older than the melt matrixes of their host breccia domains (fig. S2 and table S3). All 13 melt analyses from domain 1 form an isochron with a date of 3.808 ± 0.013 Ga (MSWD = 0.86; Fig. 3B). Note that the PDP for domain 1 has a minor mode that overlaps the younger population of clast dates and a minor mode in the PDP of domain 3 dates (Fig. 3A). Although this minor mode in domain 1 may represent some analyses that incorporated mixed materials, we found no visual evidence for this during our inspection of the ablation pits (see Supplementary Materials and Methods). Also, because the MSWD is near unity, we find no statistical 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 73217,83.(A) Summed PDP of the dates for the three melt domains with individual dates depicted as small colored circles above (yellow, domain 1; blue, domain 2; red, domain 3). Vertical gray bars indicate ranges of two populations of clast dates from 73217,83. The vertical dashed lines indicate U/Pb dates for zircons and phosphates from another section of 73217 interpreted by Grange et al. (2) to represent impact melting events. (B to D) Isotope correlation plots of 40Ar/36Ar versus 39Ar/36Ar with linear regressions shown as dashed lines. Inset figures are enlarged plots of the boxed regions in their parent figures. 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. (B) Isotope correlation plot for analyses in domain 1. (C) Isotope correlation plot of domain 2 analyses. Note: The one analysis that was rejected as an outlier is not shown (see the text for details). It would plot below the regressed line, up and to the right outside of the present bounds. (D) Isotope correlation plot of domain 3 data. Points shown as dark red ellipses were selected from mode 3A of the PDP (see the text for details) and regressed (upper line) to constrain the upper bound of the range of apparent ages measured for domain 3. Points shown as light red ellipses were selected from mode 3B of the PDP and regressed (lower line) to constrain the lower bound of the range of apparent ages for domain 3. The locations of the analyses that constitute modes 3A and 3B are shown in fig. S2. All other data are shown as unfilled ellipses.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: UVLAMP 40Ar/39Ar results for 73217,83.(A) Summed PDP of the dates for the three melt domains with individual dates depicted as small colored circles above (yellow, domain 1; blue, domain 2; red, domain 3). Vertical gray bars indicate ranges of two populations of clast dates from 73217,83. The vertical dashed lines indicate U/Pb dates for zircons and phosphates from another section of 73217 interpreted by Grange et al. (2) to represent impact melting events. (B to D) Isotope correlation plots of 40Ar/36Ar versus 39Ar/36Ar with linear regressions shown as dashed lines. Inset figures are enlarged plots of the boxed regions in their parent figures. 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. (B) Isotope correlation plot for analyses in domain 1. (C) Isotope correlation plot of domain 2 analyses. Note: The one analysis that was rejected as an outlier is not shown (see the text for details). It would plot below the regressed line, up and to the right outside of the present bounds. (D) Isotope correlation plot of domain 3 data. Points shown as dark red ellipses were selected from mode 3A of the PDP (see the text for details) and regressed (upper line) to constrain the upper bound of the range of apparent ages measured for domain 3. Points shown as light red ellipses were selected from mode 3B of the PDP and regressed (lower line) to constrain the lower bound of the range of apparent ages for domain 3. The locations of the analyses that constitute modes 3A and 3B are shown in fig. S2. All other data are shown as unfilled ellipses.
Mentions: During UVLAMP analyses of 73217,83, we targeted 13 melt volumes in domain 1, 10 melt volumes in domain 2, 40 melt volumes in domain 3, and 8 clasts within domains 2 and 3 (Fig. 1B, fig. S2, and table S3). Represented on a PDP, the populations of 40Ar/39Ar dates from the three domains and clasts are distinct and span a range from ca. 4.0 to 3.2 Ga (Fig. 3A). One population of clast dates range from 3.788 ± 0.088 Ga to 3.721 ± 0.052 Ga, whereas a second group of clast dates range from 3.973 ± 0.019 Ga to 3.902 ± 0.030 Ga. Both sets of clasts are older than the melt matrixes of their host breccia domains (fig. S2 and table S3). All 13 melt analyses from domain 1 form an isochron with a date of 3.808 ± 0.013 Ga (MSWD = 0.86; Fig. 3B). Note that the PDP for domain 1 has a minor mode that overlaps the younger population of clast dates and a minor mode in the PDP of domain 3 dates (Fig. 3A). Although this minor mode in domain 1 may represent some analyses that incorporated mixed materials, we found no visual evidence for this during our inspection of the ablation pits (see Supplementary Materials and Methods). Also, because the MSWD is near unity, we find no statistical 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