Zircon is a key mineral in geochronology because of its chemical and physical durability and tendency to incorporate radioactive trace elements such as U and Th. Quantifying the partitioning of the actinide elements is critical to constrain initial non-secular equilibrium amounts of 230 Th in zircon. An excess or deficit of 206 Pb will be produced from such an initial excess/deficit of 230 Th from the secular equilibrium condition, which influences the calculated 206 Pb/ 238 U age (Schärer, 1984; Mattinson, 1973). However, there is no standard way to calculate Th/U partitioning ratios when applying age corrections to young igneous systems, making uncertainties hard to estimate. To address this problem, zircon was synthesized in oneatmosphere experiments using basaltic andesite, andesite, and rhyolite starting materials, doped with Zr, U, and Th. Different experimental temperatures and oxygen fugacity conditions (ΔQFM-4 to ΔQFM+4) were explored to examine non-equilibrium U and Th partitioning. U and Th concentrations in zircon crystals and coexisting melt were analyzed through EPMA. In our study, we specifically quantify the effects of sector zoning, fractional crystallization, oxygen fugacity, melt composition, and temperature on actinide element partitioning. By combing experimental and natural zircon data, we find temperature has the primary control on the partitioning of U and Th in the zircon and there is a negative relationship between partition coefficient of actinide elements and zircon crystallization temperatures. The calibrated equation can be directly applied to the 230 Th correction for an improvement in the accuracy of Thcorrected 206 Pb/ 238 U dates using estimates of the zircon crystallization temperatures.

The Moon generated a long-lived core dynamo magnetic field, with intensities at least episodically reaching ~10­­–100 µT during the period prior to ~3.56 Ga. While magnetic anomalies observed within impact basins are likely attributable to the presence of impactor-added metal, other anomalies such as those associated with lunar swirls are not as conclusively linked to exogenic materials. This has led to the hypothesis that some anomalies may be related to magmatic features such as dikes, sills, and laccoliths. However, basalts returned from the Apollo missions are magnetized too weakly to produce the required magnetization intensities (>0.5 A/m). Here we test the hypothesis that subsolidus reduction of ilmenite within or adjacent to slowly cooled mafic intrusive bodies could locally enhance metallic FeNi contents within the lunar crust. We find that reduction within hypabyssal dikes with high-Ti or low-Ti mare basalt compositions can produce sufficient FeNi grains to carry the minimum >0.5 A/m magnetization intensity inferred for swirls, especially if ambient fields are >10 μT or if fine-grained Fe-Ni metals in the pseudo-single domain grain size range are formed. Therefore, it is plausible that the magnetic sources responsible for long sublinear swirls like Reiner Gamma and Airy may be magmatic in origin. Our study highlights that the domain state of the magnetic carriers is an under-appreciated factor in controlling a rock’s magnetization intensity. The results of this study will help guide interpretations of lunar crustal field data acquired by future rovers that will traverse lunar magnetic anomalies.