Magnetism of a very young lunar glass

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 (<~200million years old (Ma)). Possible sources of this magnetization include transient fields generated bymeteoroid 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 Ar/Ar geochronometry analyses on a young impact melt glass rind from the exterior of ~3.35Ga mare basalt 12017. Cosmic ray track densities and our Ar/Ar and cosmogenic Ar analyses constrain the glass formation age to be <7Ma 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 <~7μT (with a 2 σ upper limit of<11μT). Given the poormagnetic recording properties of themajority of lunar samples, this provides further evidence that many or perhaps even all previous paleointensity estimates for ≤1.5Ga rocks are upper limits on the true paleofield and therefore require neither a protracted strong (>10μT) core dynamo field nor impact-generated fields.