Minor and Trace Element Zoning in Pallasite Olivine: Modeling Pallasite Thermal History

Introduction: Pallasites are stony-iron meteorites that consist of comparable volumes of olivine and Fe-Ni metal. It is widely believed that pallasites were formed at the interface of the metallic core and peridotitic mantle in differentiated asteroid(s) [1,2]. Fe-Ni metal in pallasites exhibits Widmanstätten structure indicative of slow cooling (~10 C/yr) [3,4]. These metallographic cooling rates are consistent with formation in the deep interior of ~100 km-sized asteroids [4]. Pallasite olivine had been described as chemically homogeneous in the major elements [3]. However, pallasite olivines do have significant chemical gradients in minor elements [5-8]. If we assume those gradients were developed by diffusion, we can estimate cooling rates of pallasites from concentration profiles of olivine grains. Miyamoto et al. [6,7] used concentration profiles measured with the electron microprobe to estimate cooling rates of a few to tens of degrees per year for pallasite olivines. Such fast cooling rates are in conflict with cooling rates estimated from Fe-Ni metallography. In order to understand the pallasite cooling history, we obtained precise chemical concentration profiles of the pallasite olivine and performed numerical computer calculations for cation diffusion in olivines. Sample and Experiments: Potted butt samples of Albin and Springwater, polished slices of Esquel and Imilac, and a polished thin-section of Eagle Station were used for this study. To ensure that the grains studied were cut through their centers, larger olivines with convex shapes were selected. When grains were thick enough to observe the three-dimensional shapes, olivines with perpendicular edge to the cut surface were selected. Chemical concentration gradients were obtained using Cameca IMS 6f ion probe at Arizona State University. All samples were carbon coated. The primary O beam of 2-4 nA was accelerated by 9 kV to yield a 5-10 μm spot. The secondary-ion mass spectrometer was operated with a mass resolving power of m/∆m ~3500. Secondary ions were counted using an electron multiplier at mass peaks of Al, Si, Ca, MgO, Ti, V, Cr, Mn, Fe, Co and Ni. Peak count ratios were converted to element ratios using correction factors derived from San Carlos olivine and were converted to ppm wt. supposing that atomic abundances of Fe + Mg are constant in the olivine structure. Count rates for Ca and V tend to decrease during an analysis. For these elements, we only use data from the last few measurement cycles to reduce effects of possible contamination from the sample surface. Results: Ca, Cr and Co are elements with obvious downward concentration gradients toward the olivine-metal boundary, although Ca zoning in Albin, Imilac and Springwater olivine is not severe (Table 1). Profiles of Ca, Cr and Co sometimes become very steep as they approach the olivine-metal boundary (Fig. 1). Cr, Al, and Ti profiles have correlated irregular peaks superimposed on the curved profiles. V and Ni show relatively flat profiles with minor downward zoning near the olivine-metal boundary. Mn show flat profiles with slight upward zoning near the olivine-metal boundary.