We measured Ni partitioning between olivine and melt, D ol=liq Ni , in experiments on mid-ocean ridge basalt (MORB) encapsulated in olivine at pressures from 1atm to 3•0 GPa and temperatures from 1400 to 15508C. We present a series of experiments where the temperature (T) at each pressure (P) was selected so that the liquid composition remained approximately constant over the entire P^T range.This approach allowed us to investigate the effects of Tand P on D ol=liq Ni , independent of substantial changes in liquid composition. Our experiments show that for a liquid with $18 wt % MgO, D ol=liq Ni

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Figure S1 Supplemental Information Figure S1. Mn-Mg olivine-liquid exchange coefficients vs. 10 4 /T(K), (a) and vs. MgO content of experimental liquids (in wt. %), (b). Constant-composition series experiments from this work show no staticallysignificant temperature dependence (see Supplemental Information Fig. S2), but our work, and highprecision experiments from the literature, show a slight compositional effect on the exchange coefficient. Solid black line in (b) is a bisquare-weighted fit to both the low-error literature data and data from this work (Equation 1, main text). Note that axes are scaled such that 22 experiments are not shown

We report the results of experiments designed to separate the effects of temperature and pressure from liquid composition on the partitioning of Ni between olivine and liquid, . Experiments were performed from 1300 to 1600 °C and 1 atm to 3.0 GPa, using mid-ocean ridge basalt (MORB) glass surrounded by powdered olivine in graphite–Pt double capsules at high pressure and powdered MORB in crucibles fabricated from single crystals of San Carlos olivine at one atmosphere. In these experiments, pressure and temperature were varied in such a way that we produced a series of liquids, each with an approximately constant composition (~12, ~15, and ~21 wt% MgO). Previously, we used a similar approach to show that for a liquid with ~18 wt% MgO is a strong function of temperature. Combining the new data presented here with our previous results allows us to separate the effects of temperature from composition. We fit our data based on a Ni–Mg exchange reaction, which yields Each subset of constant composition experiments displays roughly the same temperature dependence of (i.e.,) as previously reported for liquids with ~18 wt% MgO. Fitting new data presented here (15 experiments) in conjunction with our 13 previously published experiments (those with ~18 wt% MgO in the silicate liquid) to the above expression gives  = 3641 ± 396 (K) and  = − 1.597 ± 0.229. Adding data from the literature yields  = 4505 ± 196 (K) and  = − 2.075 ± 0.120, a set of coefficients that leads to a predictive equation for applicable to a wide range of melt compositions. We use the results of our work to model the melting of peridotite beneath lithosphere of varying thickness and show that: (1) a positive correlation between NiO in magnesian olivine phenocrysts and lithospheric thickness is expected given a temperature-dependent and (2) the magnitude of the slope for natural samples is consistent with our experimentally determined temperature dependence. Alternative processes to generate the positive correlation between NiO in magnesian olivines and lithospheric thickness, such as the melting of olivine-free pyroxenite, are possible, but they are not required to explain the observed correlation of NiO concentration in initially crystallizing olivine with lithospheric thickness.Electronic supplementary materialThe online version of this article (doi:10.1007/s00410-016-1319-8) contains supplementary material, which is available to authorized users.