Identification of Relict Forsterite Grains in Forsterite-rich Chondrules

Introduction: Many chondrules clearly contain relict grains that result from incomplete melting of precursor material. Most discussions of relict grains have focused on grains that are easy to identify because they have compositions significantly different from their host chondrules, including forsterite in type II chondrules and dusty olivine in type I chondrules [1]. However, in order to fully understand the extent of chondrule recycling as well as the extent of melting during chondrule formation, it is necessary to be able to determine whether relict grains with compositions similar to those of the host chondrule are also present. Recent studies by Wasson and Rubin [2] have argued that such grains are common in type II chondrules, although this has been disputed [3]. Here we address the problem of how to identify relict forsterite grains in forsterite-rich chondrules. We report the results of a study of forsterite in type I chondrules in which we used cathodoluminescence (CL) to help identify grains that are distinguishable from host chondrule grains. Technique: We studied eight chondrules that had been separated from the Mokoia CV3 chondrite [4]. All of these are type I porphyritic chondrules that contain olivine with Fa <1 mole%: higher Fa contents quench CL. We carried out CL surveys of each chon-drule as well as electron microprobe analysis of selected grains. Our CL observations are limited to intensity variations; we have no spectral information. Results: Before we can identify relicts we need to understand the nature of CL variations in all grains. The CL distribution in individual forsterite grains in type I chondrules varies significantly. CL intensity is largely controlled by the concentrations of Al and Ti, and essentially maps the Al, Ti distribution in individual grains [5,6]. Al and Ti are frequently decoupled from Ca. Variations in CL distribution. We interpret CL distribution patterns as follows [7]: 1) homogeneous CL intensity across a grain results from chemical equili-bration during chondrule formation; 2) smooth decreases in CL intensity from cores to edges of grains, common in Al-rich chondrules, probably result from open system fractional condensation (OSFC) [6, 8]; 3) oscillations in CL intensity from cores to edges of grains can be the result of either disequilibrium during growth or episodic overgrowths onto relict grains; 4) heterogeneous CL distributions with randomly distributed patches of bright intensity are the result of dis-equilibrium growth from melts.