Thermodynamic constraints on the formation history of lodranites

Introduction: Primitive achondrites (IAB irons/winonaites; acapulcoites/lodranites; and brachinites) have chondritic compositions, but nonchondritic textures (from metamorphic through partially melted) [1]. Because of this, they offer a unique insight into the first stages of the differentiation process on asteroids. Acapulcoites and lodranites have textural and mineralogical evidence that they experienced varying amounts of partial melting during their history. They exhibit recrystallized textures. Acapulcoites typically have average grain sizes < 250μm, while lodranites are more coarse-grained (>300μm average grain size). The mineralogy is dominated by orthopyroxene and olivine followed by varying abundances of plagioclase, troilite, Fe-Ni metal, phosphates and chromite. Minor partial melting has been proposed for the acapulcoites based on their approximately chondritic abundances of plagioclase and troilite, the smaller grain size [2], and trace element compositions [3, 4]. Similar textural and chemical evidence indicates that Lodranites experienced higher degrees of partial melting [2,3,4]. The petrogenesis and formation history of the acapulcoite/lodranite parent body has been the topic of a number of recent abstracts and papers [5-9]. One of the main questions regarding the formation of the acapulcoites/lodranites is whether they experienced reduction during partial melting. In previous studies, we examined the thermodynamic properties of the winonaite/IAB group [10] and the acapulcoites [11]. In this study we look at these properties (closure temperature and oxygen fugacity) for the lodranites. Samples and Analytical Techniques: We examined two thin sections of Lodran (USNM 481-1, USNM 481-2), the type meteorite of the group. It is characterized by a course-grained, recrystallized texture [2, 12] and mineral compositions (Table 1) at the higher FeO-end of the range for acapulcoites. Minerals. Because of a lack of high-Ca pyroxene in the available sections, for this study we used chromite and olivine to determine oxygen fugacity. Chromite, although rare, exhibits two different morphologies that have different compositions (see below), as noted in a previous study of Lodran [13]. One type of chromite is found in association with metal and olivine and exhibits subhedral textures, while the other type of chromite is found as rounded blebs within olivine grains. Compositions. Mineral compositions for olivine and chromite (Table 1) were acquired with a Cameca SX-100 at The Natural History Museum. Operating conditions were 20kV accelerating voltage and 20nA beam current. Well-known minerals were used as standards and a company-supplied ZAF correction scheme was applied. Temperature and oxygen fugacity calculations. Closure temperature and oxygen fugacity were determined following the method described in [10]. In brief, we applied the olivine-chromite thermometer of [14]. Using these temperatures, oxygen fugacities were calculated based on the quartz-iron-fayalite buffer. The relevant buffer is expressed as follows: