Ilmenite megacrysts from Dharwar craton kimberlites, and their P-T-X implications for the lithospheric mantle architecture beneath India

Investigating mantle-derived xenocrysts to probe the thick roots of cratons by in-situ analytical techniques has emerged as a powerful tool in petrology, especially in cases of xenolith-poor kimberlites. Whereas mantle-derived pyrope garnet, chrome-diopside and olivine have been used extensively, the petrogenetic information from ilmenite remained underutilised. We exploit kimberlite-borne ilmenite megacrysts (176 grains, >10 mm in size) from intrusions of the Kalyandurga cluster (KL1, KL3, KL4) and the CC4 intrusion of the Chigicherla cluster of the Wajrakarur Kimberlite Field (WKF) to probe the deep Dharwar craton lithosphere. In addition to petrographic and major element analyses, we provide the first-ever in-situ trace element data for selected ilmenite megacrysts (63 grains) from WKF intrusions. Approximately 45% of the ilmenite megacrysts display recrystallisation and typically show evidence of high-temperature deformation that possibly occurred at mantle depths. Neoblasts (recrystallised grains) show systematic morphological variations indicative of progressive recrystallisation due to static re-equilibration and annealing processes. The orientation of exsolution lamellae in the neoblasts, with respect to their host grains, indicates a shift in their lattice-preferred orientations due to recrystallisation. The exsolution lamellae consist of Ti-magnetite and rarely hematite. The ilmenite megacrysts are dominated by ilmenite-geikielite solid-solution, with minor hematite and negligible pyrophanite components (Ilm_46-66Gk_17-50Hem_2-23Pph_0-2). Rare pyrophanite-rich ilmenite (Ilm₃₂Gk₁Hem₃Pph₆₄) occurs as a monocrystalline grain. Most ilmenite megacrysts are homogeneous; however, a few grains display patchy zoning along their grain margins, leading to subtle heterogeneity with increasing trends in Mg, Ti, and Cr. The KL4 and CC4 ilmenites exhibit elevated Nb, Ta and Zr compared to kimberlite-borne ilmenite megacrysts from other localities worldwide, which may reflect HFSE enrichment in the ilmenite-forming melts. The ilmenite trace element compositions follow similar evolutionary trends to those of other kimberlite-borne ilmenite megacrysts from cratons worldwide, such as decreasing Ni and Cr at increasing Nb but nearly constant Nb/Ta, indicative of mineral-melt fractionation. The compositional and textural attributes of the ilmenite megacrysts studied align with their crystallisation from a proto-kimberlite melt over a wide range of depths within the cratonic mantle. The elevated MgO, Cr₂O₃, TiO₂, and Ni contents of the CC4 ilmenite megacrysts at low Fe₂O₃ and HFSE suggest a less evolved nature of the proto-kimberlite melt beneath the Chigicherla cluster compared to Kalyandurga. Our pressure and temperature calculations for the WKF ilmenite megacrysts demonstrate their extensive vertical (and lateral) distribution within the Dharwar craton mantle root. This finding supports a model of low-degree partial melt akin to proto-kimberlite residing within the deep extension of the cratonic keel, from where multiple kimberlite eruptions were sourced at ca. 1.1 Ga.