DIFFUSION OF TRACE ELEMENTS IN FENI METAL: APPLICATION TO ZONED METAL GRAINS IN CHONDRITES

Abstract

We have measured diffusion coefficients for P, Cr, Co, Ni, Cu, Ga, Ge, Ru, Pd, Ir, and Au in Fe metal from 1150 to 1400°C and at 1 bar and 10 kbar. Diffusion couples were prepared from high-purity Fe metal and metal from the IIA iron meteorite Coahuila (single crystal kamacite) or the pallasite Springwater (polycrystalline kamacite) and held at run conditions for 3.5 to 123 h. Diffusion profiles were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or the electron microprobe. Many elements were measured from the same experimental runs so interelemental comparisons are improved over other data sets in which data for different elements come from different experiments. Some literature diffusion coefficients (D) for Ni and Co in taenite can be up to a factor of 3 higher for Ni than Co, yet our results show no difference (e.g., DNi and DCo ∼ 2.2 × 10-15 m2/s at 1150°C). Thus, diffusion of Ni and Co in single crystal taenite will not measurably fractionate the Ni/Co ratio. On the other hand, the large difference in DNi and DIr (DIr is ∼5 times lower) and the similarity of DNi and DRu at all temperatures investigated indicates that Ni/Ir and Ni/Ru ratios in zoned metal grains will be useful discriminators of processes controlled by diffusion vs. volatility. In zoned metal grains in primitive chondrites, deviations of the Ni/Ru and Ni/Ir ratios from a condensation curve are opposite to a diffusion-controlled process, but consistent with a volatility-controlled process. The new multielement diffusion coefficients will also be useful in evaluating a variety of other processes in planetary science.