Figure F34. Molar Mg# vs. Ni concentration for peridotite samples from Sites 1270 and 1271. Whole-rock Mg#s are assumed to be close to original olivine Mg#s in all samples. The whole-rock Ni content in dunites is a good approximation for the Ni concentration in olivine in these samples. Ni concentrations in olivine in the harzburgites were estimated using an olivine/orthopyroxene Ni distribution coefficient of 4 at ~1250°C (Kelemen et al., 1998a), appropriate for igneous conditions beneath a mid-ocean ridge, and bounds of 10 and 30 wt% on the modal proportion of orthopyroxene. The Ni concentration in olivine in a sample of impregnated dunite was calculated in the same way. The olivine crystallization curve shows the composition of olivine in equilibrium with liquid during fractional crystallization of olivine from primitive mid-ocean-ridge basalt (MORB). The initial liquid composition was an estimated 10% melt of the MORB source, assuming polybaric incremental melting with an average pressure of 1 GPa (Kinzler and Grove, 1992, 1993). We used an olivine/liquid Fe/Mg K_d of 0.3 (Roeder and Emslie, 1970) and the dependence of the olivine/liquid Ni distribution coefficient on MgO content of the liquid determined by Hart and Davis (1978). Light gray symbols in the background are olivine compositions from the crust–mantle transition zone (MTZ) in the Samail and Wadi Tayin massifs of the Oman ophiolite (Godard et al., 2000; Koga et al., 2001). The Site 1271 dunites with the lowest Mg# have olivine Ni concentrations comparable to Ni in olivine in the most depleted residual harzburgites from Sites 1270 and 1271. These data are consistent with formation of the dunites by reaction between relatively low Mg# melt and residual mantle olivine and inconsistent with formation of the dunites as olivine cumulates during fractional crystallization of olivine from primitive MORB. Opx = orthopyroxene, harz = harzburgite.