Fingerprints of metamorphism in chromite: New insights from minor and trace elements

Abstract

A suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in chromite grains from ophiolitic chromitites subjected to high-pressure metamorphism defines a metamorphic signature. A two-stage process associated with the infiltration of fluids during retrograde metamorphism from eclogite- to amphibolite-facies has produced four types of chromites: (1) porous chromite strongly enriched in Cr and Fe2+ but depleted in Al and Mg, with abundant chlorite filling the pores; (2) non-porous chromite strongly enriched in Fe3+ (i.e., ferrian chromite); (3) partly altered chromite with primary cores surrounded by chlorite-bearing porous chromite; and (4) zoned chromite made up of primary cores surrounded by non-porous rims of ferrian chromite.Compared to spinels from unmetamorphosed chromitites the cores of partly altered chromites after primary high-Cr chromite are enriched in Zn, Co and Mn but strongly depleted in Ga, Ni and Sc. This distribution of minor- and trace-elements is related to a decrease in Mg# [Mg/(Mg+Fe2+)] and Al, produced by the crystallization of chlorite in the pores of porous chromite. Non-porous chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned chromites have cores depleted in Ga, Ni and Sc but are progressively enriched in Zn, Co and Mn as Mg# and Al decrease toward the rims; they have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian chromite. The complex variation of the minor- and trace-elements vs Fe3+/(Fe3++Fe2+) in the different types of chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims.The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear “unaltered” in terms of major elements. The use of the major elements as indicators of magmatic processes therefore must be linked to careful evaluation of metamorphic effects, using LA-ICP-MS analysis of minor and trace elements.