Iron Isotope Composition of Allende and Chainpur Chondrules: Effects of Equilibration and Thermal History

Introduction: We have studied the petrography and Fe-isotope composition of seven chondrules, four from the Allende (CV3) carbonaceous chondrite and three from the Chainpur (LL3.4) ordinary chondrite. Three textural-chemical chondrule types are represented within this sample set: (1) BO, (2) RP and (3) PO/POP. These textural differences allow us to exa mine the Fe-isotope composition in material with different thermal histories. Fe-isotope measurements may help to constrain the chondrule forming process and determine the nature of chondrule precursor material. Fe is the most volatile major element found within chondrules [1], and thus the Fe-isotope composition may reflect evaporative loss of iron, if evaporation occurred by a Rayleigh fractionation process. Experimental Techniques: A Phillips XL-30 SEM was used for textural characterization of a polished subsample of each chondrule. Quantative silicate and metal phase analyis was undertaken using a Cameca SX-50 electron microprobe. Chondrule dissolution follows a two step HF-HClO4-HCl method [2]. Digests are purified using anion exchange chromatography, which separates out Fe, Cu and Zn. This procedure has been detailed elsewhere [3]. Fe-isotope compositons (δFe and δFe) were measured on a fixed resolution (m/ ∆m = 500) MC-ICP-MS (IsoProbe, Micromass, U.K.) with respect to the Fe-isotope standard IRMM-014, using the sample-standard bracketing method [3]. Correction for Cr on Fe is made by monitoring the Cr signal and applying an on-line mathematical correction at mass 54. Blank subtraction is undertaken off-line. The effect of ArOH background at mass 57 is reduced by analysing at relatively high concentrations [4]. Standard and sample solutions are analysed at approximately 10ppm concentration and are concentration matched to within 2%. Chondrule Petrography: Allende EM-1 (Figure 1A). Allende EM-1 is a barred olivine chondrule. Olivine bars range in comp osition from Fo88 to Fo97. The groundmass consists of plagioclase laths (An91 to An97) set in diopside (Wo47En50Fs3 to Wo36En59Fs5). Allende EM-2 (Figure 1B). Allende EM-2 is a radial pyroxene chondrule which is rimmed with forsteritic olivine of variable composition (Fo60 to Fo78). Pyroxene (enstatite) is homogeneous in comp osition (Wo2En93Fs5 to Wo9En86Fs5). Allende EM-3 (Figure 1C). Allende EM-3 is a porphyritic olivine chondrule with rare pyroxene phenocrysts set in an anorthitic groundmass (An89 to An95). All but the smallest olivine phenocrysts are zoned. Phenocryst cores comprise Fo82 to Fo86 whereas rims comprise Fo86 to Fo92. Smaller unzoned olivine phenocrysts approximate to rim Fo content, with compositions falling between Fo86 and Fo88. Diopside phenocrysts range from Wo46En50Fs4 to Wo31En64Fs5. Allende EM-4 (Figure 1D). Allende EM-4 is a porphyritic olivine chondrule. Olivines exhibit clear compositional delineation between cores and rims. In contrast to EM-3, the olivines in this chondrule have more magnesian cores (Fo94 to Fo99) and more iron rich rims (Fo89 to Fo93). Pyroxene (augite subcalcic augite) only occurs in the mesostasis and ranges in composition from Wo36En50Fs14 to Wo9En74Fs17. Individual pyroxene crystal compositions do not vary systematically in composition.