Determining cooling rates of iron and stony-iron meteorites from measurements of Ni and Co at kamacite–taenite interfaces

Abstract

Analyses and modeling of Ni zoning in taenite in differentiated meteorites provide metallographic cooling rates at ∼500°C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100–6600°C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite–taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite–taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in taenite (Niγ) and kamacite (Niα) at kamacite–taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52wt.% while Niα decreases from 7 to 4wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetrataenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe–Ni–P phase diagram and Ni diffusion rates in taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using Niγ/Niα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion of Wasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe–Ni melt and crustal rocks, cooled at uniquely slow rates.