Pyroxene Europium Valence Oxybarometer. Effects of Pyroxene Composition, Melt Composition and Crystallization Kinetics

Introduction: The behavior of multivalent elements such as iron, chromium, vanadium, and euro-pium in magmatic systems reflects the f O2 of the environment. The oxidation state of these elements will influence the mineral assemblage, crystallization sequence , and element partitioning[1]. The effect of f O2 on the relative proportions of divalent and trivalent Eu has proven to be a useful tool for estimating f O2 in a variety of magmatic systems [e.g., 2,3]. The behavior of Eu in pyroxene from martian basalts has been demonstrated to be an effective measure of f O2 [e.g., 4,5]. These studies demonstrate both the power of Eu va-lence as an oxybarometer and its dependence on a number of magmatic and mineralogical variables. Here, we evaluate the effect of pyroxene and melt composition, crystallization sequence, and crystalliza-tion kinetics on the behavior Eu in natural basalts from similar, reducing environments (IW-1) of the Moon and the HED parent body (presumably 4 Vesta) and assess the influence of these variables on Eu valence as an oxybarometer. Experimental Design and Analytical Approach: To compare Eu 2+ /Eu 3+ in lunar basalts and eucrites and to better understand the variables that may affect this indicator of f O2 in basalts, we selected two isochemical lunar pigeonite basalts (15058, 15499), a high-Ti lunar basalt (75035) and Pasamonte, an un-equilibrated eucrite. All of these basalts crystallized at an f O2 of approximately IW-1. Yet, they all have different cooling histories, crystallization histories, and pyroxene compositional trajectories within the pyrox-ene quadrilateral. The two lunar pigeonite basalts have experienced different cooling and crystallization histories [6]. Basalt 15499 cooled at a rate of 5 to 20°C/hour, whereas 15058 cooled at a rate of < 1°C/hour [6]. The high-Ti lunar basalt and the basalt clasts in Pasamonte represent basalts that had intermediate cooling rates [6,7]. All pyroxenes were BSE im-aged and compositionally mapped using scanning electron microscopy and electron microprobe. Imaging and major element analyses were used to strategically select a smaller subset of points for trace element ion microprobe analyses. A Cameca 4f ims at the Institute of Meteoritics (IOM), University of New Mexico was used to analyze Sm and Eu at all selected points.