Premium
Al‐Mg systematics of hibonite‐bearing Ca,Al‐rich inclusions from Ningqiang
Author(s) -
HSU Weibiao,
GUAN Yunbin,
WANG Ying
Publication year - 2011
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2011.01185.x
Subject(s) - melilite , chondrite , allende meteorite , carbonaceous chondrite , murchison meteorite , geology , meteorite , mineralogy , geochemistry , chemistry , spinel , astrobiology , physics , paleontology
– Hibonite‐bearing Ca,Al‐rich inclusions (CAIs) usually occur in CM and CH chondrites and possess petrographic and isotopic characteristics distinctive from other typical CAIs. Despite their highly refractory nature, most hibonite‐bearing CAIs have little or no 26 Mg excess (the decay product of 26 Al), but do show wide variations of Ca and Ti isotopic anomalies. A few spinel‐hibonite spherules preserve evidence of live 26 Al with an inferred 26 Al/ 27 Al close to the canonical value. The bimodal distribution of 26 Al abundances in hibonite‐bearing CAIs has inspired several interpretations regarding the origin of short‐lived nuclides and the evolution of the solar nebula. Herein we show that hibonite‐bearing CAIs from Ningqiang, an ungrouped carbonaceous chondrite, also provide evidence for a bimodal distribution of 26 Al. Two hibonite aggregates and two hibonite‐pyroxene spherules show no 26 Mg excesses, corresponding to inferred 26 Al/ 27 Al < 8 × 10 −6 . Two hibonite‐melilite spherules are indistinguishable from each other in terms of chemistry and mineralogy but have different Mg isotopic compositions. Hibonite and melilite in one of them display positive 26 Mg excesses (up to 25‰) that are correlated with Al/Mg with an inferred 26 Al/ 27 Al of (5.5 ± 0.6) × 10 −5 . The other one contains normal Mg isotopes with an inferred 26 Al/ 27 Al < 3.4 × 10 −6 . Hibonite in a hibonite‐spinel fragment displays large 26 Mg excesses (up to 38‰) that correlate with Al/Mg, with an inferred 26 Al/ 27 Al of (4.5 ± 0.8) × 10 −5 . Prolonged formation duration and thermal alteration of hibonite‐bearing CAIs seem to be inconsistent with petrological and isotopic observations of Ningqiang. Our results support the theory of formation of 26 Al‐free/poor hibonite‐bearing CAIs prior to the injection of 26 Al into the solar nebula from a nearby stellar source.