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Magnesium and silicon isotopic profiles across melilite grains in two type B1 and two type B2 calcium-aluminum-rich inclusions (CAIs) reveal differing but constant enrichments in heavy isotopes everywhere except ≤1000 μm from the CAI margins. There is no close correlation in the B1s or the B2s between isotopic composition and åkermanite content of the melilite, a measure of progressive igneous crystallization, yet such a correlation might be expected in a type B2: without a melilite mantle (as in B1s) to seal the interior off and prevent further evaporation, the melt would have maintained communication with the external gas. These observations indicate a model in which B1s and B2s solidified under differing conditions. The B2s solidified under lower hydrogen pressures ( [ABSTRACT FROM AUTHOR]

Abstract: Type B CAIs are subdivided into B1s, with well-developed melilite mantles, and B2s, with randomly distributed melilite. Despite intensive study, the origin of the characteristic melilite mantle of the B1s remains unclear. Recently, we proposed that formation of the melilite mantle is caused by depletion of the droplet surface in volatile magnesium and silicon due to higher evaporation rates of volatile species compared to their slow diffusion rates in the melt, thus making possible crystallization of melilite at the edge of the CAI first, followed by its crystallization in the central parts at lower temperatures. Here, we present the results of an experimental study that aimed to reproduce the texture observed in natural Type B CAIs. First, we experimentally determined crystallization temperatures of melilite for three melt compositions, which, combined with literature data, allowed us to find a simple relationship between the melt composition, crystallization temperature, and composition of first crystallizing melilite. Second, we conducted a series of evaporation and cooling experiments exposing CAI-like melts to gas mixtures with different oxygen fugacities . Cooling of the molten droplets in gases with resulted in crystallization of randomly distributed melilite, while under more reducing conditions, melilite mantles have been formed. Chemical profiles through samples quenched right before melilite started to crystallize showed no chemical gradients in samples exposed to relatively oxidizing gases , while the near-surface parts of the samples exposed to very reducing gases were depleted in volatile MgO and SiO2, and enriched in refractory Al2O3. Using these experimental results and the fact that the evaporation rate of magnesium and silicon from CAI-like melts is proportional to , we estimate that Type B1 CAIs could be formed by evaporation of a partially molten precursor in a gas of solar composition with . Type B2 CAIs could form by slower evaporation of the same precursors in the same gas with . [Copyright &y& Elsevier]

Abstract: High precision SIMS 26Al–26Mg isotope analyses were performed for a pristine Type B1 CAI Leoville 3535-1 on multiple mineral phases that include aluminum-rich zoned melilite mantle (Åk20–Åk60), magnesium-rich melilite (Åk70), fassaite, spinel and anorthite in the core. The data yield a well-defined internal isochron with an inferred initial 26Al/27Al ratio of (5.002±0.065)×10−5, which is lower than those of bulk CAIs and pristine fine-grained CAIs. Assuming homogeneous distribution of 26Al in the early solar system, Leoville 3535-1 formed ∼50ka after the time corresponding to the bulk CAI isochron. One anorthite analysis near the grain boundary adjacent to melilite shows sub-μm-scale heterogeneous magnesium distribution, though the 26Al–26Mg data plot on the isochron regression. Thus, the internal 26Al–26Mg system of the CAI remained closed since the last melting event that crystallized anorthite. High precision magnesium isotope analyses of magnesium-rich minerals (fassaite, magnesium-rich melilite, and spinel) show a small amount of scatter from the regression line (∼0.1‰) beyond analytical uncertainties. Most spinel and some fassaite data are systematically displaced below and above the regression line, respectively. It is likely that spinel remained unmelted at the time of the last melting event, while fassaite and anorthite crystallized from a partial melt. The regression line made by fassaite and anorthite data shows an initial 26Al/27Al ratio indistinguishable from that using all data. Mass dependent fractionation of magnesium isotopes in spinel is the same as those in melilite and fassaite (δ25Mg∼5‰), indicating that magnesium isotope fractionation, presumably from evaporative magnesium loss, predated the last melting event. Thus a majority of the CAI appears to have escaped magnesium isotope exchange with nebular gas during the last melting event, except for the outer rim of the melilite mantle that shows slightly lower δ25Mg values. The 26Al–26Mg systematics of this Leoville Type B CAI suggest it experienced a final melting event at ∼50ka after the initial Al–Mg fractionation event. This may correspond to melting during the active protostar stage (known as “Class I”), during which sporadic high temperature heating events repeatedly affected refractory solids in the solar nebula. [Copyright &y& Elsevier]