Wark-lovering Rims at the Nanometer Scale: a Transmission Electron Microscopy Study

Introduction: Wark-Lovering rims (WLR) consist of a sequence of mono-mineral layers a few microme-ters thick (hibonite, perovskite, spinel, melilite, anor-thite, pyroxene, olivine) surrounding most coarse-grained Ca-Al-rich inclusions (CAIs) [1]. Despite numerous studies, their formation is still not well understood. Based on their mineralogy [1], their rare earth element (REE) content [2] or their Mg isotopic signatures [e.g., 3], several scenarios of formation have been proposed, including (i) condensation, (ii) flash-heating and/or (iii) reaction of the CAI with a Mg-Si-rich reservoir (gaseous or solid) [e.g. 1-5]. Recent Al/Mg isotope dating suggests that formation of WLR postdated the formation of the CAIs [6]. Finally, recent O isotopic measurements showed that minerals of the WLR have a 16 O enrichment similar to that of the CAI interior [7, 8], suggesting that they probably formed in the same 16 O rich-reservoir. Despite their very fine-grained mineralogy (mineral layers are a few µm thick), no transmission electron microscopy (TEM) studies have been performed on WLR. This was especially due to the difficulty of preparing thin sections of specific locations of a CAI useful for mineralogical observations. In this work, we studied the mineralogy at the nanometer scale of the WLR of a CAI from the reduced CV3 chondrite Leo-ville. TEM sections were prepared using the Focused Ion Beam (FIB) technique [9] as it allows the observation of specific sites of an object already characterized for mineralogy, petrology or isotopic compositions. Mineralogy and chemistry at the nm to µm scale (e.g., exact chemical composition of individual sub-µm minerals , microtextures of each mineral layer or crystallographic relationships between the mineral layers) will give new information about the formation of the WLR. This should help to better understand their formation and thus the conditions that prevailed in the early solar nebula and the physical processes responsible for the first step of dust formation from the solar nebula gas. Sample preparation: USNM 3537-2 is a coarse-grained spheroidal type B1 CAI of about 7 mm in diameter with a flat type I REE pattern [10]. It presents a fairly continuous WLR of ~ 40 to 80 µm [11]. In order to facilitate the sample preparation, a thin part of the WLR has been chosen and was studied by scanning electron microscopy (SEM) in order to be able to compare TEM and SEM results. We also chose this part as it was showing the whole mineralogical sequence and a variety …