High precision Al–Mg systematics of forsterite-bearing Type B CAIs from CV3 chondrites

• Published in: Geochimica et cosmochimica acta 2017-03, Vol.201, p.65-82
• Main Authors: MacPherson, G.J., Bullock, E.S., Tenner, T.J., Nakashima, D., Kita, N.T., Ivanova, M.A., Krot, A.N., Petaev, M.I., Jacobsen, S.B.
• Format: Article
• Language: English
• Subjects: 26Al; CAI; Chronology; Early solar system; Forsterite-bearing Type B CAIs
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2016.12.006

In order to further elucidate possible temporal relationships between different varieties of calcium-, aluminum-rich inclusions (CAIs), we measured the aluminum–magnesium isotopic systematics of seven examples of the rare type known as forsterite-bearing Type B (FoB) inclusions from four different CV3 carbonaceous chondrites: Allende, Efremovka, NWA 3118, and Vigarano. The primary phases (forsterite, Al–Ti-rich diopside, spinel, melilite, and anorthite) in each inclusion were analyzed in situ using high-precision secondary ion mass-spectrometry (SIMS). In all cases, minerals with low Al/Mg ratios (all except anorthite) yield well-defined internal Al–Mg isochrons, with a range of initial 26Al/27Al ratios [(26Al/27Al)0] ranging from (5.30±0.22)×10−5 down to (4.17±0.43)×10−5. Anorthite in all cases is significantly disturbed relative to the isochrons defined by the other phases in the same CAIs, and in several cases contains no resolved excesses of radiogenic 26Mg (δ26Mg∗) even at 27Al/24Mg ratios greater than 1000. The fact that some FoBs preserve (26Al/27Al)0 of ∼5.2×10−5, close to the canonical value of (5.23±0.13)×10−5 inferred from bulk magnesium-isotope measurements of CV CAIs (B. Jacobsen et al., 2008), demonstrates that FoBs began forming very early, contemporaneous with other more-refractory CAIs. The range of (26Al/27Al)0 values further shows that FoBs continued to be reprocessed over ∼200,000years of nebular history, consistent with results obtained for other types of igneous CAIs in CV chondrites. The absence of any correlation between of CAI+FoB formation or reprocessing times with bulk composition or CAI type means that there is no temporal evolutionary sequence between the diverse CAI types. The initial δ26Mg∗ value in the most primitive FoB (SJ101) is significantly lower than the canonical solar system value of −0.040±0.029‰.