Mineralogical changes upon heating in the Millbillillie meteorite: Implications for paleointensity determination in Apollo samples

Abstract

Measuring the intensity of the Moon's ancient magnetic field is an important goal of lunar sample analysis. Paleointensity estimates using thermal methods (Thellier–Thellier) raise the problem of sample alteration, especially for lunar samples carrying sulfides. To address this problem, we made real-time measurements of sample magnetization during heating with a three-axis vibrating sample magnetometer (the Triaxe, LeGoff and Gallet, 2004), in the hope that rapid heating would minimize alteration (Coe et al., 2014). We studied the Millbillillie meteorite, which has a lunar-like mineralogy. We find that after repeated heating phases to ∼600°C, we form pyrrhotite, magnetite, and magnetic phases with low (200–270°C) Curie temperatures. These low-temperature phases appear after pyrrhotite has apparently been destroyed by subsequent heating phases, and their mineralogy is unknown. These results have implications for the paleomagnetic study of any extraterrestrial samples with even small amounts of sulfides, and further experiments are required to understand them.