Chemistry of accessory chromian spinel in serpentinites from the Penjwen ophiolite rocks, Zagros thrust zone,

Chromian spinel is a widespread accessory mineral in ultramafic rocks of the Penjwen ophiolite, occurring in two modes; fresh grains in dunite rocks that crop out in the eastern part and altered grains in serpentinized peridotites in the western part of the Penjwen ophiolite. Unaltered grains are generally primary phase optically and chemically homogeneous with uniform high Cr# at ~ 0.72 and low Mg# at 0.3. HighCr#, low-Al2O3, low-TiO2 chromites have the geochemical characteristic of chromites preserved in arc lavas and specifically overlap the field of chromites in boninites. . The cores of altered spinel grains from serpentinized harzburgite appear to retain their original igneous chemistry and are similar to fresh chromian spinel grains. The outer rim of these grains shows different degrees of alteration. Four criteria characterized the alteration process of Cr-spinel in serpentinised peridotite rocks of the Penjwen ophiolite. First, optical inhomogeneity on the scale of micrometers; chromian spinel is deep reddish brown in thin section, but opaque due to replacement by magnetite along cracks and grain boundaries. Second, chemical zonation with a decrease in Mg, Al, and Cr, increase in Fe and significant modification of minor elements (Si, Ti and Mn) towards the fractures and grain margins. Third, significant amount of SiO2 analyzed in Cr-spinel. Although Si does not go into the crystal structure of spinel, the microprobe analysis detected sub ordinary amount of SiO2. This suggests the presence of submicroscopic silicate phase situated at the vacancy of dissolution elements (Mg, Al, and Cr). Fourth, Ferritchromite rims and chlorite aureoles. Altered spinels deviate from primitive Cr-spinel, by sharp increase in Fe# and Cr#. The chemical changes reflect hydrothermal metamorphic reactions between spinels and the surrounding serpentinized silicate matrix. Introduction Chromiam spinel is common in small quantities in peridotites, and its compositions have often been used to determine the petrogenetic conditions of evolving mafic magmas (Irvine, 1965). In igneous rocks the chromiam spinel compositions are sensitive to melt composition, crystallization pressure, and the degree of partial melting in the mantle source region (Dick & Bullen, 1984). More recently it has also been recognized that