Research ArticlePLANETARY SCIENCE

The end of the lunar dynamo

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Science Advances  01 Jan 2020:
Vol. 6, no. 1, eaax0883
DOI: 10.1126/sciadv.aax0883
  • Fig. 1 NRM demagnetization of matrix glass subsamples from breccias 15465 and 15015.

    Shown are endpoints of the NRM vectors during progressive alternating field (AF) and thermal demagnetization. Closed and open symbols represent projections of the NRM vectors onto the horizontal (N-E) and vertical (U-E) planes, respectively. (A) 15465 subsample 4-2. (B) 15465 subsample 5-3. (C) 15015 subsample 229a1m. (D) 15015 subsample 229b8. Inset: Magnified view of HT demagnetization steps for 229b8. The legend in (A) shows the sample holder magnetic moment (denoted by the size of the large black box) and the MIT SRM moment resolution (denoted by the size of the small black box) (section S3). The initial NRM, AF levels, and temperatures for selected demagnetization steps are labeled. For both breccias, after removal of low coercivity (LC) and low temperature (LT) components (blue arrows), there is no discernible origin-trending magnetization in the high coercivity (HC)/high temperature (HT) range (as indicated by scattered vector endpoints).

  • Fig. 2 Paleointensity estimates for subsamples of breccias 15465 and 15015.

    (A and C) ARM paleointensity experiments on 15465 subsample 6-3 and 15015 subsample 229a1m, respectively. Paleointensities are estimated from NRM lost during AF demagnetization as a function of ARM gained in a 50-μT DC bias field and a 260-mT AF. AF steps used to calculate the LC and HC paleointensities are colored blue and red, respectively. Paleointensities and their uncertainties (95% confidence intervals) are shown for the HC range. Insets in (A) and (C) show the decay of NRM and ARM during progressive AF demagnetization. (B and D) ARM paleointensity fidelity tests on 15465 subsample 6-3 and 15015 subsample 229a1l, respectively. Legends list TRM-equivalent fields for ARMs acquired in a range of DC bias fields in an AF of 260 mT and assuming ARM/TRM = 1.34 (section S4) (49). Horizontal dashed lines indicate the noise level due to acquisition of spurious ARM during AF demagnetization. Inset in (D) shows a magnified view of the moment decay.

  • Fig. 3 Thermal paleointensity estimate for 15015 matrix glass.

    Shown is the NRM lost during progressive thermal demagnetization versus pTRM gained by heating in a laboratory field of 3 μT for subsample 229b8. Inset: Magnified view of 300° to 680°C temperature steps. NRM lost and pTRM gained steps are denoted with squares, with blue and red symbols denoting data in the LT and HT ranges, respectively. pTRM checks for alteration are denoted with triangles. The HT range has a paleointensity value of 0.24 ± 0.24 μT.

  • Fig. 4 Paleointensities for breccias 15465 and 15015 and those of other modern measurements of lunar rocks.

    Points labeled 15015 and 15465 are new paleointensity estimates reported by this study, while the remaining points are previously measured values (6, 9). Red points denote upper limits on the field (i.e., values indistinguishable from zero), while blue points denote nonzero values (i.e., detections of the paleofield). Vertical and horizontal arrows and error bars indicate the paleointensity and age limits and 1-SD uncertainties, respectively. The blue and red shaded regions indicate the epochs when the dynamo is inferred to be active and have ceased, respectively. Vertical and horizontal arrows and error bars indicate the paleointensity and age limits and uncertainties, respectively. The vertical dashed lines are the lifetime for proposed dynamo mechanisms: thermal convection in a dry mantle (11), impact-driven changes in mantle rotation (50), thermal convection in a dry mantle covered by a thermal blanket (12), thermal convection in a wet mantle (13), lunar precession (16), and core crystallization (15). The upper and lower horizontal dashed lines denote the maximum field predicted by energy flux scaling (36) and the predicted field exceeded by all published dynamo models for >90% of the dynamo lifetime, respectively. Data are tabulated in table S22.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/1/eaax0883/DC1

    Section S1. Overview of samples

    Section S2. Breccia thermal history

    Section S3. NRM

    Section S4. Paleointensities

    Section S5. VRM experiments

    Section S6. Magnetization carriers

    Section S7. 40Ar/39Ar, 38Ar/37Ar, and 40Ar/36Ar chronometry

    Fig. S1. Photomicrographs of 15465.

    Fig. S2. Photomicrographs of 15015.

    Fig. S3. Schematic time-temperature transformation curve for a generic cooling melt.

    Fig. S4. Estimated melt cooling rate for breccias 15465 and 15015.

    Fig. S5. Temperature distribution inside a clast with a one-dimensional contact with a cooling melt.

    Fig. S6. Temperature distribution inside a spherical clast surrounded by a linearly cooling melt.

    Fig. S7. Location of our 15465 subsamples relative to the parent sample 15465, 44, 115.

    Fig. S8. Magnetization directions in 15465 inferred from PCA.

    Fig. S9. AF demagnetization of 15465.

    Fig. S10. Thermal demagnetization of 15465.

    Fig. S11. Location of our 15015 subsamples and cuts relative to the parent samples 229a1, 229a3, and 229b.

    Fig. S12. Magnetization directions in 15015 inferred from PCA.

    Fig. S13. AF demagnetization of 15015 glass subsamples.

    Fig. S14. Thermal demagnetization of 15015 subsamples.

    Fig. S15. Magnetic overprints in 15015 subsamples from bandsaw cutting at JSC.

    Fig. S16. Paleointensity estimates for breccia 15465 matrix glass and clast samples.

    Fig. S17. Paleointensity estimates for breccia 15015 subsamples.

    Fig. S18. Thermal paleointensity experiments for breccia 15465 subsamples.

    Fig. S19. Thermal paleointensity experiments for breccia 15015 glass subsamples.

    Fig. S20. Paleointensity fidelity tests for breccia 15465.

    Fig. S21. Paleointensity fidelity tests for breccia 15015.

    Fig. S22. VRM acquisition by 15465 glass.

    Fig. S23. VRM acquisition by 15015 glass.

    Fig. S24. Electron microprobe analysis of magnetization carriers in 15465.

    Fig. S25. Hysteresis and IRM acquisition/demagnetization curves for 15465 glass and clast subsamples.

    Fig. S26. Dunlop-Day plot showing the domain state of breccias compared to other lunar rocks analyzed in the MIT Paleomagnetism Laboratory.

    Fig. S27. FORC analysis of 15465 breccia subsamples.

    Fig. S28. Electron microprobe analysis of magnetization carriers in 15015.

    Fig. S29. Hysteresis and IRM acquisition/demagnetization curves for 15015 glass subsamples.

    Fig. S30. FORC analysis for 15015 glass.

    Fig. S31. Ar release spectra for 15465 glass subsample 6-4-1.

    Fig. S32. Ar release spectra for 15465 clast subsample 6-2.

    Fig. S33. Ar release spectra for 15015 glass subsample 229b1.

    Fig. S34. Ar release spectra for 15015 clast subsample 229a1a.

    Fig. S35. Ar three-isotope plot for 15465 glass subsample 6-4-1.

    Fig. S36. Ar three-isotope plot for 15465 clast subsample 6-2.

    Fig. S37. Ar three-isotope for 15015 glass subsample 229b1.

    Fig. S38. Geologic and magnetization history of breccia 15465.

    Fig. S39. Geologic and magnetization history of breccia 15015.

    Table S1. Comparison between the glass compositions of breccias 15465, 15015, and 15498.

    Table S2. Estimated cooling rate for 15465 and 15015 lunar rock compositions.

    Table S3. Distances between 15465 matrix glass melt interface and our clast subsamples.

    Table S4. NRM components during AF or thermal demagnetization for 15465 subsamples.

    Table S5. NRM components during AF or thermal demagnetization for 15015 subsamples.

    Table S6. Paleointensity estimates for 15465.

    Table S7. Paleointensity upper limits for 15465 and 15015 based on the AREMc method.

    Table S8. Paleointensity estimates for 15015.

    Table S9. Statistics for thermal paleointensity experiments for breccias 15465 and 15015.

    Table S10. pTRM check parameters for double-heating experiments.

    Table S11. Paleointensity fidelity tests for breccias 15465 and 15015.

    Table S12. Upper paleointensity limits on breccias using different paleointensity methods.

    Table S13. WDS of 15465 metal grains.

    Table S14. Rock magnetic parameters for 15465 and 15015 subsamples.

    Table S15. WDS of 15015 metal grains.

    Table S16. 40Ar/39Ar degassing data for 15465 glass 6-4-1.

    Table S17. 40Ar/39Ar degassing data for 15465 clast 6-2.

    Table S18. 40Ar/39Ar degassing data for 15015 glass 229b1.

    Table S19. 40Ar/39Ar degassing data for 15015 clast 229a1a.

    Table S20. 40Ar/39Ar, 40Ar/36Ar, and 38Ar/36Ar analyses of breccia 15465.

    Table S21. 40Ar/39Ar, 40Ar/36Ar, and 38Ar/36Ar analyses of breccia 15015.

    Table S22. Modern paleointensity analyses of Apollo samples.

    Database S1. Demagnetization data on 15465 and 15015.

    References (51103)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Overview of samples
    • Section S2. Breccia thermal history
    • Section S3. NRM
    • Section S4. Paleointensities
    • Section S5. VRM experiments
    • Section S6. Magnetization carriers
    • Section S7. 40Ar/39Ar, 38Ar/37Ar, and 40Ar/36Ar chronometry
    • Fig. S1. Photomicrographs of 15465.
    • Fig. S2. Photomicrographs of 15015.
    • Fig. S3. Schematic time-temperature transformation curve for a generic cooling melt.
    • Fig. S4. Estimated melt cooling rate for breccias 15465 and 15015.
    • Fig. S5. Temperature distribution inside a clast with a one-dimensional contact with a cooling melt.
    • Fig. S6. Temperature distribution inside a spherical clast surrounded by a linearly cooling melt.
    • Fig. S7. Location of our 15465 subsamples relative to the parent sample 15465, 44, 115.
    • Fig. S8. Magnetization directions in 15465 inferred from PCA.
    • Fig. S9. AF demagnetization of 15465.
    • Fig. S10. Thermal demagnetization of 15465.
    • Fig. S11. Location of our 15015 subsamples and cuts relative to the parent samples 229a1, 229a3, and 229b.
    • Fig. S12. Magnetization directions in 15015 inferred from PCA.
    • Fig. S13. AF demagnetization of 15015 glass subsamples.
    • Fig. S14. Thermal demagnetization of 15015 subsamples.
    • Fig. S15. Magnetic overprints in 15015 subsamples from bandsaw cutting at JSC.
    • Fig. S16. Paleointensity estimates for breccia 15465 matrix glass and clast samples.
    • Fig. S17. Paleointensity estimates for breccia 15015 subsamples.
    • Fig. S18. Thermal paleointensity experiments for breccia 15465 subsamples.
    • Fig. S19. Thermal paleointensity experiments for breccia 15015 glass subsamples.
    • Fig. S20. Paleointensity fidelity tests for breccia 15465.
    • Fig. S21. Paleointensity fidelity tests for breccia 15015.
    • Fig. S22. VRM acquisition by 15465 glass.
    • Fig. S23. VRM acquisition by 15015 glass.
    • Fig. S24. Electron microprobe analysis of magnetization carriers in 15465.
    • Fig. S25. Hysteresis and IRM acquisition/demagnetization curves for 15465 glass and clast subsamples.
    • Fig. S26. Dunlop-Day plot showing the domain state of breccias compared to other lunar rocks analyzed in the MIT Paleomagnetism Laboratory.
    • Fig. S27. FORC analysis of 15465 breccia subsamples.
    • Fig. S28. Electron microprobe analysis of magnetization carriers in 15015.
    • Fig. S29. Hysteresis and IRM acquisition/demagnetization curves for 15015 glass subsamples.
    • Fig. S30. FORC analysis for 15015 glass.
    • Fig. S31. Ar release spectra for 15465 glass subsample 6-4-1.
    • Fig. S32. Ar release spectra for 15465 clast subsample 6-2.
    • Fig. S33. Ar release spectra for 15015 glass subsample 229b1.
    • Fig. S34. Ar release spectra for 15015 clast subsample 229a1a.
    • Fig. S35. Ar three-isotope plot for 15465 glass subsample 6-4-1.
    • Fig. S36. Ar three-isotope plot for 15465 clast subsample 6-2.
    • Fig. S37. Ar three-isotope for 15015 glass subsample 229b1.
    • Fig. S38. Geologic and magnetization history of breccia 15465.
    • Fig. S39. Geologic and magnetization history of breccia 15015.
    • Table S1. Comparison between the glass compositions of breccias 15465, 15015, and 15498.
    • Table S2. Estimated cooling rate for 15465 and 15015 lunar rock compositions.
    • Table S3. Distances between 15465 matrix glass melt interface and our clast subsamples.
    • Table S4. NRM components during AF or thermal demagnetization for 15465 subsamples.
    • Table S5. NRM components during AF or thermal demagnetization for 15015 subsamples.
    • Table S6. Paleointensity estimates for 15465.
    • Table S7. Paleointensity upper limits for 15465 and 15015 based on the AREMc method.
    • Table S8. Paleointensity estimates for 15015.
    • Table S9. Statistics for thermal paleointensity experiments for breccias 15465 and 15015.
    • Table S10. pTRM check parameters for double-heating experiments.
    • Table S11. Paleointensity fidelity tests for breccias 15465 and 15015.
    • Table S12. Upper paleointensity limits on breccias using different paleointensity methods.
    • Table S13. WDS of 15465 metal grains.
    • Table S14. Rock magnetic parameters for 15465 and 15015 subsamples.
    • Table S15. WDS of 15015 metal grains.
    • Table S16. 40Ar/39Ar degassing data for 15465 glass 6-4-1.
    • Table S17. 40Ar/39Ar degassing data for 15465 clast 6-2.
    • Table S18. 40Ar/39Ar degassing data for 15015 glass 229b1.
    • Table S19. 40Ar/39Ar degassing data for 15015 clast 229a1a.
    • Table S20. 40Ar/39Ar, 40Ar/36Ar, and 38Ar/36Ar analyses of breccia 15465.
    • Table S21. 40Ar/39Ar, 40Ar/36Ar, and 38Ar/36Ar analyses of breccia 15015.
    • Table S22. Modern paleointensity analyses of Apollo samples.
    • References (51103)

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    Other Supplementary Material for this manuscript includes the following:

    • Database S1 (.zip format). Demagnetization data on 15465 and 15015.

    Files in this Data Supplement:

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