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26 Al‐ 26 Mg isotope systematics of the first solids in the early solar system
Author(s) -
Kita Noriko T.,
Yin QingZhu,
MacPherson Glenn J.,
Ushikubo Takayuki,
Jacobsen Benjamin,
Nagashima Kazuhide,
Kurahashi Erika,
Krot Alexander N.,
Jacobsen Stein B.
Publication year - 2013
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/maps.12141
Subject(s) - isochron dating , chondrite , isochron , isotopes of magnesium , igneous rock , protoplanetary disk , geology , meteorite , mineralogy , analytical chemistry (journal) , chemistry , magnesium , geochemistry , astrobiology , astrophysics , physics , organic chemistry , chromatography , planet
Abstract High‐precision bulk aluminum‐magnesium isotope measurements of calcium‐aluminum‐rich inclusions ( CAI s) from CV carbonaceous chondrites in several laboratories define a bulk 26 Al‐ 26 Mg isochron with an inferred initial 26 Al/ 27 Al ratio of approximately 5.25 × 10 −5 , named the canonical ratio. Nonigneous CV CAI s yield well‐defined internal 26 Al‐ 26 Mg isochrons consistent with the canonical value. These observations indicate that the canonical 26 Al/ 27 Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI ‐forming region. The internal isochrons of igneous CV CAI s show a range of inferred initial 26 Al/ 27 Al ratios, (4.2–5.2) × 10 −5 , indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial 26 Al/ 27 Al ratios is also obtained from the internal isochrons of many CAI s (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAI s and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CV s, are significantly depleted in 26 Al. At least some of these refractory objects may have formed prior to injection of 26 Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium‐isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of 26 Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial 26 Al/ 27 Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAI s with the canonical 26 Al/ 27 Al ratio.