| Summary: | Cr-spinel [(Mg, Fe<sup>2+</sup>)(Cr, Al, Fe<sup>3+</sup>)<sub>2</sub>O<sub>4</sub>)] is a common mineral in the ultramafic core of the Duke Island complex in southeastern Alaska, US. Cr-spinel grains with an unmixed texture have been observed in dunite and wehrlite of the complex. Inhomogeneous Cr-spinel with a ratio of Cr/(Al + Cr + Fe<sup>3+</sup>) <0.37 is prominent in dunite. The inhomogeneous Cr-spinel consists of two completely different compositions: Al-rich Cr-spinel, and Fe<sup>3+</sup>-rich Cr-spinel with a wide range of Cr content (from 11.8 wt.% to 28.6 wt.% Cr<sub>2</sub>O<sub>3</sub>). The unmixed texture is complex, and three subtypes of inhomogeneous Cr-spinel are recognized: <i>Type</i><i>B<sub>1</sub> Cr-spinel</i> showing complete separation, crystallographically oriented <i>type B<sub>2</sub> Cr-spinel</i>, and irregular Al-rich Cr-spinel rimmed <i>type B<sub>3</sub> Cr-spinel</i>. The unmixed texture was achieved by an unmixing process at around 600 °C due to the miscibility gap of spinel between Al-rich and Fe<sup>3+</sup>-rich phases. The unmixed patterns of inhomogeneous Cr-spinel are controlled by the initial chemical composition, grain size of the initial spinel, and the cooling process. We propose that the initial composition of inhomogeneous Cr-spinel was formed by the interaction of high-temperature fluid and olivine; Cr-spinel that experienced unmixing may be a useful proxy to unveil the activity of high-temperature fluid in the formation of Alaskan-type complexes.
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