Magnetic measurements of the lunar crust and Apollo samples indicate that the Moon generated a dynamo magnetic field lasting from at least 4.2 until <2.5 billion years (Ga) ago. However, it has been unclear when the dynamo ceased. Here, we report paleomagnetic and 40 Ar/ 39 Ar studies showing that two lunar breccias cooled in a near-zero magnetic field (<0.1 μT) at 0.44 ± 0.01 and 0.91 ± 0.11 Ga ago, respectively. Combined with previous paleointensity estimates, this indicates that the lunar dynamo likely ceased sometime between ~1.92 and ~0.80 Ga ago. The protracted lifetime of the lunar magnetic field indicates that the late dynamo was likely powered by crystallization of the lunar core.

INTRODUCTION

The intensity of the present-day magnetic field across much of the lunar surface is <0.2 nT, indicating that the Moon currently does not have a global magnetic field (1). However, paleomagnetic measurements of Apollo samples indicate that the Moon once generated a core dynamo with surface field intensities of several tens of microtesla (comparable to that of Earth today) during the period 4.25 to 3.56 billion years (Ga) ago (2–7). Following this high-field epoch, the field declined by at least an order of magnitude by 3.2 Ga ago (8) and persisted in a weakened state (~5 μT) until at least 2.5 Ga ago (9). It has been unknown how long the dynamo persisted beyond this time. The youngest lunar paleointensity constraint is an upper limit of 7 μT at <7 million years (Ma) ago provided by an impact glass splash (10).

The time of the dynamo’s cessation has major implications for the mechanism of magnetic field generation as well as the thermal and mechanical evolution of the lunar interior (6). For example, a core dynamo powered purely by thermal convection (11–14) is thought to only be able to persist until ~3.5 Ga ago (15). In comparison, a mechanical dynamo driven by mantle precession (16) is thought to be sustainable until sometime between ~3.4 and 2.0 Ga ago for typical lunar physical parameters (6, 9). Alternatively, a thermochemical convection dynamo powered by core crystallization could power the dynamo even until close to the present time (15, 17).

To constrain the late history of the lunar magnetic field, we studied the paleomagnetism of the young Apollo 15 breccias 15465 and 15015 (18). During the assembly of these breccias, their clasts were welded together by 60 to 90 volume % matrix melt glass formed by impact melting of the local lunar regolith (section S1) (19). Breccia 15465 was a float sample collected from the rim of Spur crater and contains clasts with diameters ranging from <1 to ~80 mm that are dominantly regolith breccias (figs. S1 and S7). Our 40Ar/39Ar, 38Ar/36Ar, and 40Ar/36Ar measurements, combined with previous Ar analyses, indicate that the assembly of 15465 and the formation of its matrix glass most likely occurred at 0.44 ± 0.01 Ga ago (age ranges are 1 SD), while a ~20-mm-diameter regolith breccia clast formed at >3.4 Ga ago (section S7, table S20, and fig. S38). The latter clast contains a diversity of subclasts, including anorthosites, norites, and KREEP (potassium–rare earth element–phosphorus)–rich basalts that resemble the nearby Apollo 15 basalts (20, 21). Sample 15015 is a regolith breccia collected as float on the mare surface ~20 m from the Apollo 15 Lunar Module (19, 22). It contains <0.1- to 7-mm-diameter clasts in the form of rock, mineral, and glass fragments (figs. S2 and S11). Our 40Ar/39Ar, 38Ar/37Ar, and 40Ar/36Ar chronometry data indicate that 15015’s matrix glass likely formed at 0.91 ± 0.11 Ga ago, consistent with previous measurements (see section S7, table S21, and fig. S39). Combined with our thermal diffusion calculations (section S2), these data indicate that the matrix melt glass, clasts smaller than ~10 mm in diameter, and the thermally equilibrated exteriors of larger clasts should have recorded any ambient lunar magnetic field at ~0.4 Ga ago (15465) and ~0.9 Ga ago (15015).

Similar to previously studied regolith breccias (9), the ferromagnetic carriers in the glassy matrices of 15015 and 15645 have exceptional magnetic recording properties compared to most lunar rocks (section S6). Our electron microscopy observations indicate that the dominant ferromagnetic minerals in the glass matrix of 15465 are kamacite (α-Fe 1−x Ni x with x < ~0.04) and schreibersite (Fe 1−x Ni x ) 3 P with x ~ 0.1, while the aforementioned glassy regolith breccia clast contains mostly kamacite and martensite (α 2 -Fe 1−x Ni x with x ~ 0.08) (fig. S24). Previous analytical electron microscopy (AEM) studies of the matrix glass in 15015 found that it contains kamacite (23), while our electron microscopy observations identified both kamacite and schreibersite (fig. S28). Our hysteresis and isothermal remanent magnetization (IRM) and first-order reversal curve (FORC) measurements of the regolith breccia clast in 15465 and the glass matrix in 15015, along with the AEM data (23), indicate a dominantly single-vortex to superparamagnetic grain size, while the glass matrix in 15465 contains grains ranging from single domain to multidomain in size (figs. S25 to S27, S29, and S30).

Given the mineralogies and compositions of the ferromagnetic grains in 15465 and 15015, these grains should have acquired mostly a total thermoremanent magnetization (TRM) in any ambient magnetic field following the last major heating event (see section S6). The degree of crystallinity in the matrix glasses of the two breccias indicates that they cooled from the 780°C Curie point of kamacite to ambient surface temperatures over >24 hours (section S2 and fig. S4). Because this time scale exceeds the estimated lifetime of putative impact-generated fields from even the largest lunar impacts (24), the matrix glasses in these breccias should have only recorded any ambient dynamo field during cooling. The lack of microfractures in the glass matrices of both breccias (see figs. S1 and S2) constrains postcooling peak shock pressures to below ~3 GPa (25).