Further evidence for early lunar magnetism from troctolite 76535

Abstract

The earliest history of the lunar dynamo is largely unknown and has important implications for the thermal state of the Moon and the physics of dynamo generation. The lunar sample with the oldest known paleomagnetic record is the 4.25 billion year old (Ga) troctolite 76535. Previous studies of unoriented subsamples of 76535 found evidence for a dynamo field with a paleointensity of several tens of microteslas. However, the lack of mutual subsample orientation prevented a demonstration that the magnetization was unidirectional, a key property of thermoremanent magnetization. Here we report further alternating field demagnetization on three mutually oriented subsamples of 76535, as well as new pressure remanent magnetization experiments to help rule out shock magnetization. We also describe new 40Ar/39Ar thermochronometry and cosmogenic neon measurements that better constrain the rock's thermal history. Although the rock is unbrecciated, unshocked, and slowly cooled, its demagnetization behavior is not ideal due to spurious remanence acquisition. Despite this limitation, all three subsamples record a high coercivity magnetization oriented in nearly the same direction, implying that they were magnetized by a unidirectional field on the Moon. We find no evidence for shock remanence, and our thermochronometry calculations show no significant reheating events since 4249 ± 12 million years ago (Ma). We infer a field paleointensity of approximately 20–40 μT, supporting the previous conclusion that a lunar dynamo existed at 4.25 Ga. The timing of this field supports an early dynamo powered by thermal or thermochemical core convection and/or a mechanical dynamo but marginally excludes a dynamo delayed by thermal blanketing from radiogenic element‐rich magma ocean cumulates.

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…The magnetic intensity estimations scaled from our calculated CMB heat flux (e.g., Christensen et al, 2009) can be compared to the observed paleomagnetic record ( Figure S11; supporting information). The predicted paleointensities cannot reach the measured paleointensities of 20-40 μT (Evans et al, 2018;Garrick-Bethell et al, 2016) even with a large core size (D150v1e-4V1e21E100Core410 in Figure S11). However, the timing of the stimulated core dynamo we predict does coincide with an inferred periods of lunar paleomagnetism at 4.25 Ga .…”

“…The timescales of lunar cumulate mantle overturn may provide useful information for understanding lunar paleomagnetic records (Garrick‐Bethell et al, ; Lawrence et al, ). Cold downwellings during overturn were hypothesized to create a pulse of heat flux at the core‐mantle boundary (CMB) that might power a transient lunar core dynamo (Weiss & Tikoo, ).…”

Is the Lunar Magnetic Field Correlated With Gravity or topography?

“…The Moon does not have a core‐generated magnetic field at present, but surface and orbital magnetic field measurements show that portions of the lunar crust are strongly magnetized (Dyal et al, ; Hood et al, ; Tsunakawa et al, ). Some lunar samples collected on the Moon's surface also are magnetized, and paleomagnetic analyses of these samples suggest that a long‐lived core dynamo could have operated from ∼4.25 Ga (Garrick‐Bethell et al, , ) to somewhere between ∼2.5 and 1 Ga (Mighani et al, ; Tikoo et al, ). The surface intensity of this early dynamo field is predicted to be 10s of μT and to have decreased by an order of magnitude after 3.56 Ga (Suavet et al, ; Tikoo et al, ).…”

The Lunar Paleo‐Magnetosphere: Implications for the Accumulation of Polar Volatile Deposits

“…Several decades of lunar sample analysis and remote sensing studies have established that the Moon once possessed a dynamo (Fuller & Cisowski, ; Weiss & Tikoo, ). This dynamo may have existed from ~4.25 Ga to perhaps as recently as 1.0 Ga, with paleofields ranging from ~5 to 100 μT at the surface (Garrick‐Bethell et al, ; Tikoo et al, ). To date, there has been no analysis of how this dynamo field would have interacted with the early solar wind.…”