CI origins

B type Bennu, like Cb Ryugu, corresponds to CI meteors sent down here from Above - like local CM, CO and maybe CT.

Now Maria Schönbächler et al. have formally published this, as reported in Switzerland Wednesday.

When the CIs accreted, they accreted with water ice. That "when", for Ryugu and arguably for Bennu, could have been as early as 2My after CAI formation. They take those "calcium-aluminum-rich inclusion" chondrites to be the eldest of them all, I assume marking that 4.567Gy age we keep seeing. Our CI, and I assume CY also, would precede CM and CO (and CV). By this time, Jupiter had grown to 20 Earth masses.

The paper accepts that Bennu is rubble from a parent. But, without naming 142 Polana. Polana presently rolls around 2.4 AU. This is well within the 4-5 AU ice line today, and assuredly in CM times then, therefore has migrated. The paper's "figure 5" speaks of something called "ureilites" to be native down at ~2.5 AU, sooty achondrite rocks from some differentiated parent destroyed 5 My after CAI beyond Vesta.

Such chaos may account for why the authors seem cautious about assigning general CI/CY (for asteroids, B/Cb) distance from our (very) young sun at this time. And they don't speculate on the Grand Tack; this may be because they must admit hazy understanding of asteroids beyond Jupiter. CI was once mooted as far as 7 AU, assumed for instance here (slushball Ceres remains, I think, a migrate from up there). Other papers have put CO out 3.7 AU so, hey, why not for CI.