Abstract
Trace elements in Fe-rich basaltic systems are important for understanding petrogenesis on the Moon and layered intrusions on Earth. To investigate crystal-chemical controls on partitioning between clinopyroxenes and garnets in Fe-rich basalts, an experimental study was conducted utilizing two synthetic compositions, a ferrobasalt (Fe-rich, Al-poor) and an intermediate basalt (relatively more Al- and Mg-rich) at approximately one log unit below the iron-wüstite buffer. Experiments were run using piston cylinder and multi-anvil apparatuses at pressures of 1 to 5 GPa, testing partitioning behavior in phases that precipitated from the endmember haplobasalts and a 50-50 mixture. We find that Al substitution into the clinopyroxene tetrahedral site influences 1+, 3+ and 4+ cation partitioning, with more aluminous samples having the highest partition coefficients for large ion lithophile elements, trivalent rare earth elements (REEs), and high field strength elements. Low-Al ferrobasalt clinopyroxenes exhibit heavy REE (HREE) partition coefficient anomalies, suggesting HREE substitution onto the M1 site, and a positive Eu partitioning anomaly. In contrast, higher-Al intermediate basalt clinopyroxenes have negative Eu partitioning anomalies and no HREE partitioning anomaly. The almandine-type garnets produced in higher-pressure experiments provide novel constraints for lunar-relevant systems. Comparison with literature data suggests these Fe-rich garnets exhibit partitioning behavior similar to garnets in more magnesian systems. Experimentally determined partition coefficients and new predictive models are applied to the petrogenesis of Fe-rich lunar basalts to evaluate their potential origins. To explain Yb/Sm ratios of black glasses, successful models invoke a minor garnet component in their mantle sources.
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