EXPERIMENTAL PARTITIONING OF TC, MO, RU, AND RE BETWEEN SOLID AND LIQUID DURING CRYSTALLIZATION IN FE-NI-S

Abstract

Technetium isotopes 97Tc, 98Tc and 99Tc decay to 97Mo, 98Ru and 99Ru, with half-lives of 2.6 My, 4.1 My, and 0.21 My respectively. If there were early solar system processes that resulted in significant fractionation of Tc from the daughter elements, decay of extant Tc could have led to the creation of Mo and Ru isotopic heterogeneities. To assess the potential of metallic core crystallization to fractionate these elements, we examine the partitioning behavior of Tc relative to Re, Mo and Ru in the Fe-Ni-S system between solid metal and liquid metal alloy. The experimental evidence shows that Tc behaves more like the modestly compatible siderophile element Ru than the more highly compatible siderophile element Re, and that Tc is substantially more compatible than Mo. We also demonstrate a pressure effect in the partitioning of Mo during the crystallization of Fe-Ni-S melts. For a sulfur concentration in the liquid fraction of the core of 10 wt% (16.3 at%), the Jones and Malvin (1990) parameter is -ln(1-2 x 1.09 x 0.163) # 0.44, which yields: D(Re) # 4.1; D(Ru) # 2.3; D(Tc) # 1.7; D(Mo)Lo-P # 1.0;.and D(Mo)Hi-P # 0.5. Our results suggest that detectable Tc-induced isotopic anomalies (>=0.1 ε unit) in Ru and Mo could only be produced by unrealistically extreme degrees of crystallization of metal during asteroidal core fractionation, regardless of the time scales and initial Tc abundances involved.