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

W and Mo abundances in the bulk metals of 10 H, 5 L and 6 LL chondrite s were determined by instrumental neutron activation analysis (INAA). Partitioning of W and Mo between metal and non-metal phases was evalua ted by comparing Ni-normalized W and Mo abundances in the metal phases with those in the bulk samples. It is observed that W distributions d iffer significantly between equilibrated ordinary chondrites (EOCs) an d unequilibrated ordinary chondrites (UOCs). However, no correlation i s confirmed between the W distribution and the petrographic type for E OCs. This implies that the W partitioning among mineral phases can pro vide a clue for estimating the metamorphic temperature intervening bet ween EOCs and UOCs. The difference in W equilibrium temperatures was o bserved among H, L and LL chondrite groups and it may correspond to th e difference in cooling rates and subsequently in sizes of the chondri te parent bodies; LL chondrites have the lowest equilibrium temperatur e and, hence, have the largest parent body, whereas H chondrites have the highest equilibrium temperature and the smallest parent body. Mo/W abundance ratios remain constant in EOC metals, but are variable in U OC metals, suggesting that the W solid equilibrium has not been achiev ed in UOCs. The W and Mo distributions in UOCs still preserve the char acteristics of W and Mo in the nebula, which demonstrates that the cho ndritic metal was formed by melting highly oxidized precursors before or during the accretion of chondrite parent bodies.