Temperature and pressure dependence of Ni partitioning between olivine and high-MgO silicate melts
Abstract
Mantle melting that produces ocean island basalts (OIBs) takes place at temperatures (T) and pressures (P) significantly higher than the conditions at which they erupt or are intruded in the crust/shallow upper mantle [1]. To the degree that the olivine (ol)-liquid (liq) nickel partition coefficient depends on T and P, it is important that models used to describe ol-liq Ni partitioning during mantle melting include data from experiments at elevated T and P. The expressions can then be used in models which aim to reproduce the wide range of Ni contents measured in primitive phenocrysts from OIBs [2]. Available data on Ni partitioning is dominated by 1-atm experiments in which T and liquid composition are highly correlated, making it difficult to separate the effects of these variables on the observed variations in Ni partitioning between ol and liq based on 1-atm experiments alone [3]. <br/> We conducted experiments on a mixture of MORB and olivine at 1 atm (1400°C) and 1-3 GPa (1450-1550°C). We moderated the loss of Ni from the silicate melt to the Pt-enclosed graphite capsule by surrounding the chip of MORB glass with powdered olivine and sintering the assembly at a T below the solidus of the MORB chip. The data presented in this work is from a series of reversed experiments where T and P were increased in such a way that the liquid composition remained approximately constant (MgO ~ 17 wt. %), effectively isolating the effects of T and P from those of liquid composition on the ol-liq partition coefficient. The resulting partition coefficient decreases from ~5 to 3.8 (by wt) as the temperature increases from 1400 to 1550°C. The rate of decrease of the Ni partition coefficient measured in these experiments (~0.5/100°C) is less than that of recent models, which predict a decrease of ~0.1/100°C [4]. Using the results of our experiments we fit a thermodynamic expression to describe the ol-liq Ni-Mg exchange equilibrium as a function of both T and liquid composition. Using our model in conjunction with predictions of partial melting (BATCH [5]), we predict that when a deep partial melt of depleted mantle [6] (8% melt from 3.6 GPa) is brought to low P (0.1 GPa) and allowed to crystallize, the low P olivines will have significantly higher NiO contents (~0.44 wt %) than those in the mantle source (0.33 wt %), reflecting the decrease in the partition coefficient from the mantle source to the near-surface crystallization conditions . Estimates of solid and liquid component partial molar volumes suggests that P has only a small effect on the Ni-Mg exchange and thus that T and liquid composition are the most important variables over the P-T range spanned by this calculation. The effect of temperature on Ni partitioning can account for olivine phenocrysts with higher NiO contents than those in normal mantle peridotites, and thus may contribute to the high-NiO contents of many olivine phenocrysts from Hawaii [2]. <br/> [1] Putirka (2005) G3, 6. [2] Sobolev et al. (2005) Nature 434, 590-597. [3] Hart & Davis (1978) EPSL 40, 203-219. [4] Li & Ripley (2010) Chem Geo 275, 129-138. [5] Longhi (2002) G3, 3. [6] Workman & Hart (2005) ESPL 231, 53-72.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V13F..03M
- Keywords:
-
- 1037 GEOCHEMISTRY / Magma genesis and partial melting;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology