Chemical studies of h chondrites: III. Regolith evolution of the fayetteville chondrite parent

Abstract

Data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, and Zn in consortium samples of solar-gas- and track-rich dark matrix, and dark and light clasts in the Fayetteville H chondrite regolith breccia were determined by radiochemical neutron activation analysis. The matrix proved chemically homogeneous at a level never before observed in an ordinary chondrite. Most dark clasts had similar mean trace element concentrations as matrix but were compositionally much more heterogeneous. Two clasts were unusual, one having apparently formed from impact-melted soil, the other representing a mixture of 90% matrix-10% C2M chondrite. The single light clast proved compositionally similar to H4–6 chondrite falls except for its higher contents of labile Bi, Cd, In, and Tl. Relative to matrix, all other samples seemed Cs-deficient. Other H chondrite regolith breccias studied in the past had shown peculiar Cs and/or Rb contents so that these alkalis seem diagnostic of such breccias. Dark portions of such breccias apparently represent compacted regolith soil produced by isochemical comminution of “normal” dark and light clasts. The dark clasts, in turn, likely represent primordial nebular condensate, although it is possible that they represent planetary “cold traps” that garnered labile elements generated elsewhere in the parent body.