The duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal

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 [Göpel 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).