QUANTIFYING SHOCK MODIFICATION OF THE LUNAR PALEOINTENSITY RECORD

Paleomagnetic and crustal magnetism studies indicate that the Moon generated a core dynamo field that existed between at least 4.25 billion years ago (Ga) and 2.5 Ga. During the period from ~3.85 Ga to 3.56 Ga, lunar surface fields reached intensities of ~40-110 µT that are comparable to the modern Earth’s field. However, there appears to be an order-of-magnitude dispersion in paleointensity values obtained from rocks that formed within this period. Because most Apollo samples were collected as regolith float, it is possible that shock pressures from impacts that excavated these rocks caused them to become fully or partially demagnetized. To determine the amount of magnetization lost from lunar rocks after exposure to modest pressures (<1.8 GPa), we considered a series of pressure demagnetization experiments on several Apollo samples that were initially imparted with laboratory remanences. In addition, we studied how pressure demagnetization affected paleointensity results obtained from our rocks. We found that pressures up to 1.8 GPa may demagnetize lunar rocks by up to ~30%, and that demagnetization effects were largely confined to low coercivity (<20 mT) grains. Because the low coercivity range of magnetization (<20 mT) is often excluded when processing data from commonly utilized room-temperature paleointensity methods, the effect of shock demagnetization on the published lunar paleointensity record is limited (typically <10%). As such, our results suggest that large variations in lunar paleointensity values obtained from similarly aged samples are unlikely to result from shock demagnetization, and are likely related to other factors such as changes in dynamo field intensity, poor magnetic recording properties of lunar rocks, issues involving the calibration of room-temperature paleointensity methods, uncertainties associated with sample ages, or thermal remagnetization of rocks after the lunar dynamo declined.