Magnetism of a very young lunar glass

Abstract

Recent paleomagnetic studies of Apollo samples have established that a core dynamo existed on the Moon from at least 4.2 to 3.56 billion years (Ga). Because there is no lunar dynamo today, a longstanding mystery has been the origin of magnetization in very young lunar samples (<~200 million years old (Ma)). Possible sources of this magnetization include transient fields generated by meteoroid impacts, remanent fields from nearby rocks magnetized during an earlier dynamo epoch, a weak late dynamo, and spontaneous remanence formed in a near-zero field. To further understand the source of the magnetization in young lunar samples, we conducted paleomagnetic, petrographic, and 40 Ar/ 39 Ar geochronometry analyses on a young impact melt glass rind from the exterior of~3.35 Ga mare basalt 12017. Cosmic ray track densities and our 40 Ar/ 39 Ar and cosmogenic 38 Ar analyses constrain the glass formation age to be <7 Ma and most likely <20 thousand years (kyr), making it likely the youngest extraterrestrial sample yet studied with paleomagnetic methods. Despite its relatively high fidelity magnetic recording properties compared to most lunar rocks, we find that the glass carries no stable primary natural remanent magnetization and that it formed in a field <~7 μT (with a 2 σ upper limit of <11 μT). Given the poor magnetic recording properties of the majority of lunar samples, this provides further evidence that many or perhaps even all previous paleointensity estimates for ≤1.5 Ga rocks are upper limits on the true paleofield and therefore require neither a protracted strong (>10 μT) core dynamo field nor impact-generated fields.

The Case Against an Early Lunar Dynamo Powered by Core Convection

“…Second, we consider those scenarios that yield a long‐lived, low‐intensity magnetic field that may have persisted until 200 Ma. Such a magnetic field is expected to have a nominal intensity of ~5 μT or less after 3.56 Ga (Buz et al, ; Tikoo et al, , ). Figure and Table summarize the maximum magnetic field surface intensities generated for a suite of lunar convective dynamo cases with varying durations and contributions of radioactive energy.…”

“…A majority of the high‐intensity paleomagnetic records are found to have been acquired between 3.85 and 3.56 Ga, although the earliest evidence of remanent magnetization detected within the sample suite requires a dynamo to have been initiated no later than ~4.25 Ga (Cournède et al, ; Garrick‐Bethell et al, , ; Shea et al, ; Suavet et al, ). Paleomagnetic signatures recorded by younger samples indicate that lunar surface fields decreased in intensity to less than ~10 μT by 3.2 Ga (Buz et al, ; Tikoo et al, ). This significantly reduced surface field represents a very rapid decline of the lunar core dynamo.…”

Constraining the Decline of the Lunar Dynamo Field at ≈3.1 Ga Through Paleomagnetic Analyses of Apollo 12 Mare Basalts

“…Italicized samples are not used in this study and are included in this table for completeness. (Suavet et al, 2013;Tikoo et al, 2014) Tikoo et al (2012Tikoo et al ( , 2014; Buz et al (2015) sized grains) and could therefore be capable of acquiring and retaining records of weaker lunar fields, such as those expected to be present during the initial period of field decline and the subsequent low field epoch.…”

A two-billion-year history for the lunar dynamo

“…However, the poor magnetic recording properties ( 7 , 8 , 27 ) and complex thermal and deformational histories of most lunar samples ( 6 , 7 ) as well as the rarity of young (<3.2 Ga) Apollo igneous samples have thus far hindered efforts to establish when the dynamo ultimately ceased. Although some Apollo-era studies have suggested that lunar samples as young as ~200 million years old (Ma) formed in lunar paleofields of ~1 to ~10 μT ( 28 ), most of these values are likely upper limits given the samples’ magnetic recording fidelities ( 8 , 27 , 29 ). Furthermore, it has been proposed that impact-generated plasmas could generate transient magnetic fields [lasting up to ~1 day for large basin-forming impacts ( 30 ) or <~1 s for the small impacts after 3.7 Ga ( 8 )] that could magnetize shocked and quickly cooled rocks throughout lunar history ( 31 , 32 ).…”