Analyses of primitive meteorites and cometary samples have shown that the solar nebula must have experienced a phase of large-scale outward transport of small refractory grains as well as homogenization of initially spatially heterogeneous short-lived isotopes .
The stable oxygen isotopes , however , were able to remain spatially heterogenous at the \sim 6 % level .
One promising mechanism for achieving these disparate goals is the mixing and transport associated with a marginally gravitationally unstable ( MGU ) disk , a likely cause of FU Orionis events in young low-mass stars .
Several new sets of MGU models are presented that explore mixing and transport in disks with varied masses ( 0.016 to 0.13 M _ { \odot } ) around stars with varied masses ( 0.1 to 1 M _ { \odot } ) and varied initial Q stability minima ( 1.8 to 3.1 ) .
The results show that MGU disks are able to rapidly ( within \sim 10 ^ { 4 } yr ) achieve large-scale transport and homogenization of initially spatially heterogeneous distributions of disk grains or gas .
In addition , the models show that while single-shot injection heterogeneity is reduced to a relatively low level ( \sim 1 % ) , as required for early solar system chronometry , continuous injection of the sort associated with the generation of stable oxygen isotope fractionations by UV photolysis leads to a sustained , relatively high level ( \sim 10 % ) of heterogeneity , in agreement with the oxygen isotope data .
These models support the suggestion that the protosun may have experienced at least one FU Orionis-like outburst , which produced several of the signatures left behind in primitive chondrites and comets .