The origin and history of the metal and sulfide components of chondrules

Fourteen siderophile and other non-lithophile elements determined in 31 Semarkona (LL3.0) chondrules by neutron activation analysis are severely fractionated relative to lithophile elements. Their chondrule/whole-rock abundance ratios vary by factors of up to 1000; the mean ratio is ~0.2. Non-refractory siderophile abundance patterns in Ni-rich chondrules are smooth functions of volatility and in Ni-poor chondrules patterns are more irregular. Refractory siderophile elements are often fractionated from Ni; they covary, confirming the presence of a refractory metal component. The chalcophile element Se correlates with Br and siderophile elements. Zinc is uniformly low and uncorrelated with other elements. Most metal and sulfide in chondrules was probably present in the solar nebula before chondrule formation; most siderophile and chalcophile elements were in these materials. Some Fe was also in silicates, as were minor amounts of Ni, Co, Au, Ge and possibly Se. The amount of metal formed by reduction during chondrule melting was minor. The common metal component in chondrules is similar to, and may be the same as the common component involved in the metal/silicate fractionation of the ordinary chondrite groups. Chondrules are depleted in metal chiefly because they sampled metal-poor precursor assemblages. Metal segregation during the molten period and subsequent loss was a minor process that may be responsible for most surface craters on chondrules.