Composition and Thermal History of the Ivb Iron Meteorites

Introduction: Among the magmatic iron meteorites, group IVB irons have some unusual characteristics, such as high bulk Ni (15-18 wt%), high refractory elements (e.g., Ir), low volatile elements (e.g., Ga, Ge) [1, 2], fast cooling rates and a small parent body [3, 4]. IVB irons do not have a Widmanstatten pattern as in lower Ni IIIAB or IVA irons. Instead, IVB irons have a plessite structure with micron sized kamacite (α) spindles sometimes associated with phosphide (Ph). The microstructure is very different between low Ni and high Ni IVB irons. The low Ni IVB irons have very few kamacite spindles and phosphides while high Ni IVB irons have many more kamacite spindles and significant amounts of phosphides for the same size analysis area. Because of the small micron sized kamacite spindles, the classical metallographic cooling rate methods such as taenite (γ) central Ni vs. half width (Wood) method [5] or taenite (γ) Ni profile matching method [6] have not been applied. Cooling rates for IVB irons have been measured using the kamacite band width method [3, 4, 7]. The measured cooling rates are very fast, >1,000 K/Myr and do not vary with meteorite Ni content [4]. However, the measurement of kamacite band width is not accurate in this study since the orientation of the spindles with respect to the sample surface is unknown and the errors in individual cooling rates vary by a factor about 10 [4]. The cooling rates may not be accurate and the constancy of the measured cooling rates with Ni content may be in doubt [8]. Purpose: In order to understand the IVB thermal history and the nature of its parent body, we have remeasured the IVB cooling rates. We have measured Ni gradients using x-rays generated from thin sections of IVB irons in the electron microscope and have applied the taenite Ni profile matching method [6]. Method: We examined ten IVB irons which were also studied by [2]: Cape of Good Hope, Hoba, Iquique, Santa Clara, Skookum, Tawallah Valley, Tlacotepec, Warburton Range, Weaver Mountains, and Ternera. The samples were first prepared for optical microscopy and observed and analyzed using light optical microscopy and the Cameca SX-50 electron probe microanalyzer (EPMA). In order to determine the nucleation temperature of the kamacite spindles, it is necessary to have accurate bulk Ni and P contents for each meteorite. Although bulk Ni and P contents have been measured systematically [1, 2], there are significant differences between measured values of the P content. Therefore, our first step was to remeasure the bulk Ni and P content of each meteorite using area scans obtained with the EPMA. The bulk compositions of the major elements Fe, Ni, Co and P were measured in scan areas from 64,000 μm to 216,000 μm for each of the ten IVB irons by EPMA. Since few kamacite spindles and phosphides are present in the low Ni IVBs, the composition is more or less homogeneous and relatively small scan areas can be measured. For high Ni irons, the presence of larger amounts of kamacite and phosphide required larger x-ray scan areas to obtain a representative bulk composition. The second step was to measure Ni profiles in taenite and adjacent kamacite across kamacite-taenite interfaces as input to the Ni profile matching method. For each IVB iron suitable kamacite spindles and surrounding taenite were thinned for electron microscopy using a dual beam FEI DB-235 focused ion beam (FIB)/SEM instrument at Sandia National Laboratories. We measured the Ni profiles across each kamacite-taenite boundary by x-ray analysis using a FEI Tecnai F30ST field emission transmission analytical electron microscope (TEM-AEM) at Sandia National Laboratories. The measured Ni profiles are used to match the calculated Ni profiles for various cooling rates from the cooling rate simulation program [9]. Results and Discussion: The bulk Ni and P measured in ten IVB irons are plotted in Fig. 1.