Internal ~(26)Al- ~(26)Mg isotope systematics of a Type B CAI: Remelting of refractory precursor solids

摘要

High precision SIMS ~(26)Al- ~(26)Mg isotope analyses were performed for a pristine Type B1 CAI Leoville 3535-1 on multiple mineral phases that include aluminum-rich zoned melilite mantle (?k _(20)-?k _(60)), magnesium-rich melilite (?k _(70)), fassaite, spinel and anorthite in the core. The data yield a well-defined internal isochron with an inferred initial ~(26)Al/ ~(27)Al 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 ~(26)Al 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 ~(26)Al- ~(26)Mg data plot on the isochron regression. Thus, the internal ~(26)Al- ~(26)Mg 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 ~(26)Al/ ~(27)Al ratio indistinguishable from that using all data. Mass dependent fractionation of magnesium isotopes in spinel is the same as those in melilite and fassaite (δ ~(25)Mg~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 δ ~(25)Mg values.The ~(26)Al- ~(26)Mg 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.