Application of the sphalerite cosmobarometer to group IAB iron meteorites

Sphalerite compositions from ten members of iron meteorite group IA and one member of group IB were determined and pressures (P350) calculated, assuming a blocking temperature for Fe diffusion of 350°C. Although three meteorites, Ogallala and Copiapo (IA) and Woodbine (IB), yielded clearly outlying values, the remaining eight members of group IA yielded pressures that were linearly correlated with 1(wt.% Ni) of the meteorite. The equation of the regression line is: 1(wt.% Ni) = 0.163 − 0.0102P350. Pressures calculated range from zero for Landes to 3.5 kbar for Toluca. The values for Woodbine, Copiapo and Ogallala appear to be due to the occurrence of sphalerite in Copiapo-type silicate inclusions. Sphalerite in these inclusions is believed to be only partially re-equilibrated from compositions that were stable in winonaite parental materials.