Mg-ilmenite megacrysts from the Arkhangelsk kimberlites, Russia: Genesis and interaction with kimberlite melt and postkimberlite fluid

Abstract

In the present work we studied Mg-ilmenite megacrysts from the Arkhangelsk kimberlites (the Kepino kimberlite field and mantle xenoliths from the Grib pipe). On the basis of isotopic (Rb/Sr, Sm/Nd, δ18O) and trace-element data we argue that studied Mg-ilmenite megacrysts have a genetic relation to the “protokimberlitic” magma, which was parental to the host kimberlites. Rb-Sr ages measured on phlogopite from ilmenite-clinopyroxenite xenoliths and the host Grib kimberlite overlap within the error (384 Ma and 372 ± 8 Ma, respectively; Shevchenko et al., 2004) with our estimation of the Kotuga kimberlite emplacement (378 ± 25 Ma). Sr and Nd isotopic compositions of megacrysts are close to the isotopic composition of host kimberlites (Mg-ilmenites from kimberlites have 87Sr/86Sr(t = 384) = 0.7050–0.7063, ɛNd(t = 384) = + 1.7, +1.8, ilmenite from ilmenite-garnet clinopyroxenite xenolith has 87Sr/86St(t = 384) = 0.7049, ɛNd(t = 384) = +3.5). Oxygen isotopic composition of ilmenites (δ18O = +3.8–+4.5‰) is relatively “light” in comparison with the values for mantle minerals (δ18O = +5–+6‰). Taking into account ilmenite-melt isotope fractionation, these values of δ18O indicate that ilmenites could crystallize from the “protokimberlitic” melt. Temperatures and redox conditions during the formation of ilmenite reaction rims were estimated using ilmenite-rutile and titanomagnetite-ilmenite thermo-oxybarometers. New minerals within the rims crystallized at increasing oxygen fugacity and decreasing temperature. Spinels precipitated during the interaction of ilmenite with kimberlitic melt at T = 1000–1100°C and oxygen fugacity $$\Delta \log f_{O_2 }$$ [QFM] ≈ 1. Rims comprised with rutile and titanomagnetite crystallized at T ≈ 1100°C, $$\Delta \log f_{O_2 }$$ [NNO] ≈ 4 and T = 600–613°C, $$\Delta \log f_{O_2 }$$ [QFM] ≈ 3.7, respectively. Rutile lamellae within ilmenite grains from clinopyroxenitic xenolith were formed T ≥ 1000–1100°C and oxygen fugacity $$\Delta \log f_{O_2 }$$ [NNO] = −3.7. Since the pressure of clinopyroxene formation from this xenolith was estimated to be 45–53 kbar, redox conditions at 135–212 km depths could be close to $$\Delta \log f_{O_2 }$$ [NNO] = −3.7.