Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body

Abstract

AbstractPrecise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ18O that vary from −4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), −3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), −3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and −3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ18O value (+2.60‰) observed in a BO chondrule and a minimum δ18O value (−4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ∆17O values that range from −5.65‰ to −3.25‰ (PO), −4.60‰ to −2.80‰ (POP), −4.95‰ to −3.00‰ (BO), −5.30‰ to −3.20‰ (PP), and −4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.