EARLY SOLAR SYSTEM FRACTIONATION OF METAL AND SILICATE REVEALED IN CR CHONDRITES
The CR chondrite meteorite Acfer139 was found in Algeria in 1990. A 14x5x4mm sample of this weathered stone was imaged by synchrotron computed tomography (CT) at the Advanced Photon Source. Igneously layered chondrules are not uncommon in CR chondrites, but CT revealed a ~5mm diameter chondrule with three successive layers of concentric FeO-poor silicate and quenched melt rimmed with metal. Further CT imaging at the American Museum of Natural History (AMNH) allowed thresholding and segmentation of the metal into discrete shells with nearly metal-free silicate between them. Serial sectioning through the chondrule allowed detailed chemical, modal, and textural analysis by EPMA and optical methods. Innermost olivine and metal grains record a higher temperature of equilibration with nebular vapor than outer layers. This object records sequential, separate silicate - metal accretion at the mm scale.
The CR chondrite Renazzo fell in Italy in 1824. An AMNH sample of Renazzo contains objects probably older than chondrules, and isotopically primitive, called amoeboid olivine aggregates (AOAs). Two such objects contain numerous 20-50 mm metal nuggets all rimmed by 10-20 mm layers of compact 2-4 mm olivine grains. These unmelted but perhaps sintered assemblages also appear to record a very early fractionation of silicate (olivine) from metal.
The metal - silicate fractionation recorded in these objects, and evidence from other primitive chondrites, allow speculation based on the dynamical behavior of dense metal relative to silicates. Could there exist dynamical regimes in the early nebula that could concentrate one material relative to the other, at the size scale recorded in these components of CR chondrites? Could that concentration be preserved at the location and scale of Mercury?