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

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 ∼200,000 years 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‰.