Figure F54. Estimated molar Mg# vs. Ni concentration in olivine in Site 1275 troctolites, compared to similar estimates for peridotite samples from Sites 1270, 1271, 1272, and 1274. Whole-rock Mg#s are assumed to be close to original olivine Mg#s in all samples. We estimated the olivine Ni concentrations in the troctolites based on the bulk compositions of samples with 0.23 wt% CO2. To do this, we calculated CIPW norms for the anhydrous CO2-free bulk composition. We then estimated the original Ni concentration in olivine, assuming Ni partitioning between olivine and pyroxenes was governed by partitioning at 1250°C, using relationships derived by Kelemen et al. (1998a), and that there is no Ni in plagioclase (or any other phase, including sulfide). 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 from Kelemen et al. (1998a) and bounds of 10 and 30 wt% on the modal proportion of orthopyroxene. 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 Kd 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 in dunites, impregnated dunites, and wehrlites from the crustmantle transition zone (MTZ) in the Samail and Wadi Tayin massifs of the Oman ophiolite (Godard et al., 2000; Koga et al., 2001). Like most Site 1271 dunites and some Oman MTZ dunites, the estimated olivine compositions in the Site 1275 troctolites have olivine Ni concentrations comparable to Ni in olivine in the most depleted residual harzburgites from Sites 1270 and 1271. These Mg#s are consistent with formation of the troctolites 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. Readers should keep in mind that the method of estimating olivine composition, using the bulk composition of highly altered troctolites, is uncertain, so this result is preliminary. Opx = orthopyroxene.
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