Thellier-Thellier Paleointensity of the Lunar Core Dynamo

Friday, 19 December 2014
Clement R Suavet1, Benjamin P Weiss1, Eduardo Andrade Lima1, Sonia M Tikoo1,2, Roger R Fu1, Huapei Wang1, Jun Wang3 and Yu-chen K Chen-Wiegart3, (1)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (2)University of California Berkeley, Earth and Planetary Science, Berkeley, CA, United States, (3)Brookhaven National Laboratory, Photon Sciences Directorate, Upton, NY, United States
A number of paleomagnetic studies based on Alternating Field (AF) demagnetization of lunar samples have recently shown that the Moon had a dynamo magnetic field of several tens of μT at 4.2 Ga, 3.72 Ga, 3.56 Ga, and that the field had declined to below a few μT by 3.2 Ga. Although uncertainties associated with AF-derived paleointensity estimates are up to a factor of 3, these values are too high to be explained by current lunar dynamo models: based on estimates of the power available to drive a dynamo in the early history of the Moon, it is expected that the field intensity should have been of the order of a few μT. Thermal demagnetization-based techniques such as the Thellier-Thellier paleointensity method have much lower uncertainties on the paleofield, but attempts have consistently failed due to alteration of the metal-bearing lunar samples when heated. We have recently designed the first system to conduct thermal demagnetization with oxygen fugacity control using mixtures of H2 and CO2to mitigate alteration. We are applying this method to the following lunar samples:

- Regolith breccia 15498. Impact melt from this breccia acquired a magnetization at 1.0-1.3 Ga. We conducted a Thellier-Thellier paleointensity experiment in a controlled atmosphere with oxygen fugacity at IW-1 log(atm). pTRM checks indicate that alteration is negligible up to 500°C. A paleointensity of 3.2 μT is obtained for the origin-trending high-temperature (>250°C) component. This is consistent with estimates based on AF-demagnetization data.

- Troctolite 76535. A single plagioclase crystal from 4.2 Gyr-old troctolite 76535 was thermally demagnetized in a controlled atmosphere with oxygen fugacity at IW-1 log(atm). The synchrotron transmission X-ray microscopy and hysteresis parameters show that the major magnetization carriers are fine-grained pseudo-single domain metal inclusions. Due to the small size and weak magnetization of the sample (natural remanent magnetization (NRM) ~5x10-12 Am2), measurements are performed with a SQUID microscope. The NRM is stable in direction and intensity up to 450°C, and rapidly decays between 450°C and 550°C. Partial TRM (pTRM) acquisitions were performed at 450°C and 550°C. Based on this limited data, a paleofield of at least 45 μT is obtained, which is consistent with estimates given by AF methods.