THE DURATION OF ORDINARY CHONDRITE METAMORPHISM INFERRED FROM TUNGSTEN MICRODISTRIBUTION IN METAL

Abstract

Precise ratios of W and Re relative to Ir determined by laser ablation ICP-MS reveal that matrix metal in equilibrated ordinary chondrites (EOC) exhibits a correlated variation of these ratios. In unequilibrated ordinary chondrites (UOC), the W/Ir ratio varies by over two orders of magnitude (W/Ir=0.003-0.6), and shows no correlation with any other siderophile element. Most matrix metal in UOC is depleted in W relative to metal in EOC. Thus, W must enter metal during metamorphism. The correlation between W/Ir and Re/Ir is evident in type 4 chondrites. This implies that the abundance (and isotopic composition) of W is set in ordinary chondrite metal by petrologic type 4, and that the reductant must be exhausted to limit further isotopic exchange, as indicated by the 182Hf-182W ages of ordinary chondrite metal [Lee and Halliday, Science 274 (1996) 1876-1879]. This offers a means of dating the onset of metamorphic heating of OC parent bodies. A discrepancy in ages between 182Hf-182W [Lee and Halliday, Science 274 (1996) 1876-1879] and 207Pb-206Pb [Gopel et al., Earth Planet. Sci. Lett. 121 (1994) 153-171] (or 129I-129Xe [Brazzle et al., Geochim. Cosmochim. Acta 63 (1999) 739-760]) dating is real, and measures the interval of metamorphism: the time difference between reduction of W during the onset of heating and isotopic closure of Pb (or Xe) by cooling, e.g., ΔTm=2-12 Myr for H chondrites. The 187Re-187Os and 182Hf-182W ages are set by the same process, metamorphic equilibration, establishing an important link between an absolute chronometer (t1/2[187Re]=41.6 billion years) and a short-lived radioisotope system (t1/2[182Hf]=9 Myr).