Research ArticleGEOPHYSICS

A two-billion-year history for the lunar dynamo

See allHide authors and affiliations

Science Advances  09 Aug 2017:
Vol. 3, no. 8, e1700207
DOI: 10.1126/sciadv.1700207
  • Fig. 1 Mutually oriented 15498 parent chips.

    (A) Chips 15498,274, 15498,282, and 15498,287. (B) Chip 15498,313. (C) Chip 15498,314. The sample contains abundant mare basalt fragments (blue arrows and labels) within a glassy matrix (purple arrows and labels). Surficial melt glass spatter locations are denoted with red arrows and outlines. The scale cubes have widths of 1 cm. The subsamples and scale cube are oriented following the Johnson Space Center (JSC) system for 15498.

  • Fig. 2 Hysteresis curves for 15498.

    The red curve shows the measured data. The blue curve shows the data after application of a paramagnetic slope correction.

  • Fig. 3 40Ar/39Ar thermochronometry constraints on the formation age of breccia 15498.

    (A) Multi-phase, multi-domain diffusion (MP-MDD) model predictions for diffusion of radiogenic 40Ar* experienced by a 1-cm-diameter basalt clast within 15498 resulting from breccia formation between 650 and 3300 Ma (that is, from heating to temperatures ranging between 450° and 675°C), followed by daytime heating to effective mean temperatures ranging between 25° and 56°C after 600 Ma. Observed step heating ages ±1 SD (dark gray boxes) are plotted against the cumulative release fraction of 39Ar released. The 3310 ± 24–Ma age inferred from the HT release steps represents the minimum crystallization age of the basalt clast. The colored steps are model release spectra calculated using MP-MDD model parameters corresponding to breccia formation at varying times (different formation ages are indicated with different colors). The inset displays the number of model degassing steps that are within error of the sample degassing path (individual steps connected by dashed red line) in the LT release fraction (heating steps 2 to 6) for different breccia formation ages (a value of n = 5 indicates all steps fit within error of the model). (B) Reduced χ2 misfit values for model release spectra shown in (A). Misfits are shown both including (black circles) and excluding (gray circles) the first degassing step of the heating experiments. Red shaded box indicates formation ages precluded by the cosmogenic exposure age (≤600 Ma).

  • Fig. 4 Vector endpoint diagrams showing demagnetization of 15498 subsamples.

    (A) AF demagnetization of subsample 282c. (B) Thermal demagnetization of subsample 282t. Open and closed circles represent projections of the NRM vector onto the vertical (Up-E) and horizontal planes (N-E), respectively. Blue, red, and green arrows denote LC/LT, MC/MT, and HC/HT components, respectively. Subsample masses as well as selected AF levels and temperature steps are labeled.

  • Fig. 5 Equal-area stereographic projections of LC/LT and MC/MT magnetization components observed for interior subsamples of 15498.

    (A) LC (circles) and LT (squares) component directions. (B) MC (circles) and MT (squares) component directions. Lines encircling component directions represent the maximum angular deviations associated with each direction. Open symbols (dashed lines) represent directions in the upper hemisphere, whereas filled symbols (solid lines) represent directions in the lower hemisphere. Subsamples from parent chips 274, 282, 287, 313, and 314 are denoted with light blue, medium blue, dark blue, dark green, and light green symbols, respectively.

  • Fig. 6 Equal-area stereographic projection of HC and HT magnetization component directions.

    Shown directions are observed for mutually oriented matrix glass subsamples from the interior of 15498. Symbols and surrounding ellipses represent directions and associated maximum angular deviation values obtained from principal component analysis. AF and thermally demagnetized subsamples are displayed using circles and squares, respectively. Subsamples from parent chips 274, 282, 287, 313, and 314 are shown by light blue, medium blue, dark blue, dark green, and light green symbols, respectively. Open symbols (dashed lines) represent directions in the upper hemisphere, and filled symbols (solid lines) represent directions in the lower hemisphere. The Fisher mean direction and α95 confidence interval (star and surrounding ellipse, respectively) are shown.

  • Fig. 7 Thellier-Thellier paleointensity experiment for subsample 15498,313e.

    (A) Arai plot displaying NRM lost during progressive thermal demagnetization (ordinate) versus laboratory pTRM gained (abscissa). Peak temperatures for selected steps are shown. pTRM checks for alteration are shown as triangles. Paleointensities for unblocking temperature ranges of 250° to 540°C and 560° to 680°C are denoted with dark gray and green symbols, respectively. Gray segments link consecutive thermal steps. (B) Vector endpoint diagram showing zero-field thermal demagnetization steps for subsample 313e. LT and HT components are denoted using blue and green symbols, respectively. Paleointensity experiments were conducted following the IZZI protocol (alternating zero-field and in-field measurements).

  • Fig. 8 Magnetization versus distance from the peripheral impact glass spatter.

    Shown are residual magnetization values for thermally demagnetized 15498 matrix glass subsamples after heating to 300°C. Individual subsample names are labeled. The gray shaded box denotes the zone likely to have been remagnetized by emplacement of the impact glass spatter (approximately three half-widths of the local glass spatter thickness).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/8/e1700207/DC1

    section S1. Sample 15498

    section S2. NRM behavior

    section S3. Paleointensity

    section S4. Rock magnetic properties

    section S5. 40Ar/39Ar and 38Ar/37Ar thermochronology

    fig. S1. Apollo 15 landing site and 15498 sampling context.

    fig. S2. Sample 15498.

    fig. S3. Backscattered scanning electron microscopy images of 15498 matrix showing absence of post-lithification microfracturing.

    fig. S4. BSEM images of FeNi grains in 15498.

    fig. S5. Equal-area stereographic projections of LC/LT and MC/MT magnetization components observed for peripheral subsamples of 15498.

    fig. S6. AF demagnetization of sample 15498,282a over the range of the HC component.

    fig. S7. Thellier-Thellier paleointensity experiments for subsamples 15498,313k1 and 15498,313k2 following the IZZI variant.

    fig. S8. Paleointensity fidelity limit tests for 15498.

    fig. S9. FORC distribution for sample 15498,287b1.

    fig. S10. Rock magnetic experiments on 15498,282a.

    fig. S11. PRM acquisition by 15498 subsample 15498,282a.

    fig. S12. VRM decay experiment on sample 15498,282c.

    fig. S13. The predicted effects of 600 Ma of solar heating at the lunar surface, calculated using the 15498 MP-MDD model.

    fig. S14. Arrhenius plots with calculated diffusion coefficients for 39Ar and 37Ar released during the first 20 release steps.

    fig. S15. Schematic depicting time-temperature conditions underlying our thermochronological models.

    fig. S16. 15498 MP-MDD model predictions for diffusion of 40Ar* resulting from impact heating at 2000 Ma (to temperatures ranging between 450° and 675°C), followed by daytime heating to an effective mean temperature of 69°C after 600 Ma.

    fig. S17. 15498 MP-MDD model predictions for diffusion of 40Ar* resulting from impact heating at various times in lunar history (to temperatures ranging between 450° and 675°C), followed by daytime heating to effective mean temperatures ranging between 35° and 56°C after 600 Ma.

    fig. S18. 15498 MP-MDD model age spectra incorporating diffusion of 40Ar* resulting from impact heating at 650 Ma (to temperatures ranging between 450° and 675°C), followed by daytime heating to an effective mean temperature of 25°C after 600 Ma.

    table S1. WDS measurements of metal grains in 15498 thin sections 298 and 299.

    table S2A. NRM components identified for interior matrix glass subsamples of 15498.

    table S2B. NRM components identified for peripheral matrix glass subsamples of 15498.

    table S2C. Fisher mean component directions derived from 15498 data in table S2A.

    table S3A. Thellier-Thellier paleointensity determinations for 15498 subsamples.

    table S3B. Comparison of pTRM and pTRM check values for 15498 subsamples.

    table S3C. ARM paleointensity determinations for 15498 subsamples.

    table S3D. IRM paleointensity determinations for 15498 subsamples.

    table S4. Rock magnetic and hysteresis parameters.

    table S5. Anisotropy of ARM (85-mT ac field with 0.01-mT dc field).

    table S6. Complete 40Ar/39Ar incremental heating results.

    table S7. Oxide weight percent compositions of K-bearing phases in basalt clast 15498-282-1.

    table S8. Summary of MP-MDD model parameters with cosmogenic 38Ar production rates for 15498.

    table S9. Summary of 40Ar/39Ar chronology for 15498.

    table S10. Reduced χ2 misfit statistics for best-fit thermochronometry models for a variety of breccia formation ages.

    data file S1. 15498 demagnetization data sets.

    data file S2. 15498 Thellier-Thellier paleointensity data sets.

    References (66125)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Sample 15498
    • section S2. NRM behavior
    • section S3. Paleointensity
    • section S4. Rock magnetic properties
    • section S5. 40Ar/39Ar and 38Ar/37Ar thermochronology
    • fig. S1. Apollo 15 landing site and 15498 sampling context.
    • fig. S2. Sample 15498.
    • fig. S3. Backscattered scanning electron microscopy images of 15498 matrix showing absence of post-lithification microfracturing.
    • fig. S4. BSEM images of FeNi grains in 15498.
    • fig. S5. Equal-area stereographic projections of LC/LT and MC/MT magnetization components observed for peripheral subsamples of 15498.
    • fig. S6. AF demagnetization of sample 15498,282a over the range of the HC component.
    • fig. S7. Thellier-Thellier paleointensity experiments for subsamples 15498,313k1 and 15498,313k2 following the IZZI variant.
    • fig. S8. Paleointensity fidelity limit tests for 15498.
    • fig. S9. FORC distribution for sample 15498,287b1.
    • fig. S10. Rock magnetic experiments on 15498,282a.
    • fig. S11. PRM acquisition by 15498 subsample 15498,282a.
    • fig. S12. VRM decay experiment on sample 15498,282c.
    • fig. S13. The predicted effects of 600 Ma of solar heating at the lunar surface, calculated using the 15498 MP-MDD model.
    • fig. S14. Arrhenius plots with calculated diffusion coefficients for 39Ar and 37Ar released during the first 20 release steps.
    • fig. S15. Schematic depicting time-temperature conditions underlying our thermochronological models.
    • fig. S16. 15498 MP-MDD model predictions for diffusion of 40Ar* resulting from impact heating at 2000 Ma (to temperatures ranging between 450° and 675°C), followed by daytime heating to an effective mean temperature of 69°C after 600 Ma.
    • fig. S17. 15498 MP-MDD model predictions for diffusion of 40Ar* resulting from impact heating at various times in lunar history (to temperatures ranging between 450° and 675°C), followed by daytime heating to effective mean temperatures ranging between 35° and 56°C after 600 Ma.
    • fig. S18. 15498 MP-MDD model age spectra incorporating diffusion of 40Ar* resulting from impact heating at 650 Ma (to temperatures ranging between 450° and 675°C), followed by daytime heating to an effective mean temperature of 25°C after 600 Ma.
    • table S1. WDS measurements of metal grains in 15498 thin sections 298 and 299.
    • table S2A. NRM components identified for interior matrix glass subsamples of 15498.
    • table S2B. NRM components identified for peripheral matrix glass subsamples of 15498.
    • table S2C. Fisher mean component directions derived from 15498 data in table S2A.
    • table S3A. Thellier-Thellier paleointensity determinations for 15498 subsamples.
    • table S3B. Comparison of pTRM and pTRM check values for 15498 subsamples.
    • table S3C. ARM paleointensity determinations for 15498 subsamples.
    • table S3D. IRM paleointensity determinations for 15498 subsamples.
    • table S4. Rock magnetic and hysteresis parameters.
    • table S5. Anisotropy of ARM (85-mT ac field with 0.01-mT dc field).
    • table S6. Complete 40Ar/39Ar incremental heating results.
    • table S7. Oxide weight percent compositions of K-bearing phases in basalt clast 15498-282-1.
    • table S8. Summary of MP-MDD model parameters with cosmogenic 38Ar production rates for 15498.
    • table S9. Summary of 40Ar/39Ar chronology for 15498.
    • table S10. Reduced χ2 misfit statistics for best-fit thermochronometry models for a variety of breccia formation ages.
    • References (66–125)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • data file S1 (.txt format). 15498 demagnetization data sets.
    • data file S2 (.txt format). 15498 Thellier-Thellier paleointensity data sets.

    Files in this Data Supplement: