Identification of Feo-rich Relict Olivines in Type Iia Chondrules Using Fe-mn
نویسندگان
چکیده
Introduction: Relict grains (i.e., grains that did not crystallize in situ from the host chondrule melt) provide a real physical record of chondrule precursor grains. Most relicts appear to be derived from previous generations of chondrules and hence represent evidence for recycling of material in the chondrule forming region(s). [1] estimated that at least 15% of chon-drules contain material that experienced at least two chondrule-forming events. Two types of relict grains are frequently encountered in chondrules: relict forsterite or enstatite grains in FeO-rich (type II) chon-drules and " dusty " olivine grains in FeO-poor (type I) chondrules [e.g., 1-3]. Moreover, [4] showed evidence that FeO-poor relicts exist in type I chondrules and [5] suggested that type II chondrules might contain ubiquitous FeO-rich relict grains. Below, we show how the latter can easily be identified via Fe-Mn systematics. Samples and method: As part of a detailed EPMA study of chondrules from pristine chondrites [6,7], we obtained random (3-10) olivine analyses of 5 type IIA chondrules in Kainsaz (CO3.2), 2 type IIA chondrules in MET00426 (CR3.0) and 6 type IIA chondrules each in MET00526 (L/LL3.05) and QUE97008 (L3.05). Additional analyses were collected on relict grains. Quantitative analyses were obtained on a JEOL 8200 electron microprobe with a focused beam, an accelerating voltage of 15 keV and a beam current of 20 nA. We also included previously published data in our study ([8], also see references in [6,7]). Results: Olivines from type IIA chondrules in CO and unequilibrated ordinary chondrites (UOC) have significantly different slopes in a Mn vs. Fe diagram [6,7], as indicated in Figs. 1d and 2b. Our recent assessment of previously published data of olivine zon-ing profiles in Semarkona type IIA chondrules [8] indicates that olivine grains which crystallized from the same chondrule melt typically fall on a single trendline in the Mn vs. Fe diagram [7]. Each chondrule may show a slightly different slope, but when a relict grain is encountered it plots in a significantly different region of the diagram. Most of the olivines we analyzed have compositions consistent with crystallization from the host chondrule when it was molten. However, based on their aberrant compositions, we recognized five FeO-rich olivine grains that we consider to be relicts, as well as several relict forsterites. Three of the FeO-rich relict grains are discussed below. In a Kainsaz type IIA chondrule (Fig. 1a), we found that the relict core of Grain …
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