Determining the Bulk Chemical Composition of Chondrules by Electron Microprobe: a Comparison of Different Approaches

Introduction: A precise knowledge of bulk chemical compositions of chondrules is not only essential for understanding the relationship between chondrules and matrix [e.g. 1], but also for constraining the chondrule forming mechanism [2]. Despite the given importance, relatively few data sets for chondrule bulk chemical compositions are available in the literature, especially for the most primitive chondrites [3]. Instrumental neutron activation analysis (INAA) and electron probe microanalysis (EPMA) are the two major methods used to obtain such datasets, but each technique has its disadvantage. Si cannot be measured by INAA and trace elements are difficult to obtain by EPMA. There is also some discussion on how representative the composition of a 2 dimensional section is versus the 3 dimensional composition [4,5]. Separating chondrules for INAA analysis is challenging, especially for meteorites with very small chondrules (e.g. CO chondrites). In such a case, EPMA is the most realistic routine method that can be used to obtain chondrule bulk compositions. In this abstract, we point out problems with various EPMA methods that have been used. We compare several different approaches for determining bulk compositions using EPMA, including: (1) broad beam or defocused beam analyses (DBA), (2) modal recombination analysis (MRA) using backscattered electron images and (3) MRA using element maps. Sample and analytical conditions: We measured the bulk composition of a type I chondrule from the Vigarano (CV3) chondrite (Fig. 1). An accelerating voltage of 15 keV was used for all analyses. Table 1 shows the beam current and probe diameter utilized for defocused beam analyses and point analyses on the different phases. Element maps were obtained using a beam current of 30 nA. For MRA, the area % of the different phases were determined using Scion Image Software (http://www.scioncorp.com/frames/fr_scion_ products.htm).