Multiple mantle metasomatism scenarios in Kempirsai ophiolite, Kazakhstan: Evidence from the borehole peridotites

Harzburgite, amphibole-harzburgite, pyroxenite and dunite collected from the borehole 639 provide insights into the complex metasomatic modifications in the paleo-oceanic lithosphere of the Kempirsai ophiolite, Kazakhstan, hosting the world’s largest podiform chromite deposits. Three distinct metasomatic events have been identified in ultramafic rocks of the Kempirsai ophiolite, based on detailed petrographical observations and geochemical data. In the first stage, the mantle melting residues were percolated by SiO₂-rich melts, likely derived from the decompression melting of asthenosphere, forming the orthopyroxene-rich harzburgites. Orthopyroxene1 is observed to corrode and crosscut coarse olivine grains (olivine1), enclose them in crystallographic continuity with surrounding olivines, or occur along olivine grain boundaries. These various mineral relationships imply the crystallization of orthopyroxene1 through the consumption of olivine1. As the melt/rock ratio decreased with melts migrating upward, less coarse orthopyroxene1 was precipitated, but more irregular counterparts were observed to scatter along olivine1 boundaries in the shallow-level harzburgites. In the second stage, orthopyroxene1, particularly the orthopyroxene porphyroclasts, reacted with water-poor, SiO₂-unsaturated melts to crystallize clinopyroxene, orthopyroxene2 and olivine2, with occasional presence of amphibole. The rare earth element depleted patterns of clinopyroxene in both harzburgites and Amp-harzburgites (La/Yb: 0.01-0.16), together with the insignificant enrichment in fluid mobile elements, attest to the water-poor nature of the metasomatic melts. Subsequently, water-rich liquid-rock interaction at the third stage triggered the dissolution of previous metasomatic products (orthopyroxene1 and clinopyroxene) and the precipitation of mineral assemblages of amphibole+orthopyroxene2+olivine2±spinel±phlogopite. They either form veinlets that crosscut the coarse orthopyroxene1, mantle the orthopyroxene porphyroclasts, or form fine-grained polymineralic domains running parallel to the lineation of samples. This interaction is likely a syn-deformation metasomatism (869-991 ℃), as witnessed by the decrease in orthopyroxene1 and clinopyroxene modal contents as well as the increase in the proportion of amphibole with depth. The metasomatic agent is suggested to be alkaline- and water-rich liquids, which infiltrated the mantle peridotites as a reactive porous flow and enriched other silicate minerals in light rare earth elements and fluid-mobile elements, especially for those in the deep amphibole-harzburgites. We propose that the water-rich liquids derived from the dehydration of amphibolite of the metamorphic sole rocks, based on syn-deformation metasomatism and enrichment of fluid mobile elements. The liquids are also of Cl-rich nature, which could gradually enrich the fluids with Cr₂O₃, as suggested by the high Cr₂O₃ contents in amphibole in the mantle peridotites (up to 3.10 wt.%). The fluids could migrate upward to metasomatize mantle peridotites or trigger its partial melting, forming podiform chromitites containing similar Cr₂O₃-rich amphibole (up to 4.02 wt.%).