DISTRIBUTION OF W AND MO IN ORDINARY CHONDRITES AND IMPLICATIONS FOR NEBULAR AND PARENT BODY THERMAL PROCESSES

Abstract

W and Mo abundances in the bulk metals of 10 H, 5 L and 6 LL chondrites were determined by instrumental neutron activation analysis (INAA). Partitioning of W and Mo between metal and non-metal phases was evaluated by comparing Ni-normalized W and Mo abundances in the metal phases with those in the bulk samples. It is observed that W distributions differ significantly between equilibrated ordinary chondrites (EOCs) and unequilibrated ordinary chondrites (UOCs). However, no correlation is confirmed between the W distribution and the petrographic type for EOCs. This implies that the W partitioning among mineral phases can provide a clue for estimating the metamorphic temperature intervening between EOCs and UOCs. The difference in W equilibrium temperatures was observed among H, L and LL chondrite groups and it may correspond to the difference in cooling rates and subsequently in sizes of the chondrite parent bodies; LL chondrites have the lowest equilibrium temperature and, hence, have the largest parent body, whereas H chondrites have the highest equilibrium temperature and the smallest parent body. Mo abundance ratios remain constant in EOC metals, but are variable in UOC metals, suggesting that the W solid equilibrium has not been achieved in UOCs. The W and Mo distributions in UOCs still preserve the characteristics of W and Mo in the nebula, which demonstrates that the chondritic metal was formed by melting highly oxidized precursors before or during the accretion of chondrite parent bodies.