The gravitational and aerodynamic sorting of meteoritic chondrules and metal: Experimental results with implications for chondritic meteorites

Abstract

Chondritic meteorites appear to have undergone metal‐silicate fractionations and chondrule size sorting that resulted in unique distributions for each chondrite class. We present evidence from laboratory experiments for size sorting of meteoritic chondrules and metal by fluidization during parent body degassing caused by evaporation and sublimation of water in the parent bodies. Experiments using chondritic analogs in an upward flowing stream of air indicate that metal can travel upward in a fluidized bed, resulting in a metal‐rich surface and metal‐poor regions at depth. When the outgassing velocity drops below the minimum fluidization velocity for chondrules, metal grains still suspended in the gas travel upward through the pore spaces between chondrules, provided the size ratio between chondrules and metal is >3. These experiments also suggest that chondrule size sorting can occur but with less efficiency than metal‐silicate sorting. Fluidization can account for the sorting observed in enstatite chondrites, suggesting that only one parent body is required for EH and EL chondrites. Ordinary chondrite components are also sorted, with H chondrites deriving from locations near the surface of the parent body, while L and LL chondrites originate from greater regolith depths. The upward migration of metal to asteroid surfaces may help explain the reddened spectral features of asteroids likely to be the parent bodies of ordinary chondrites.