Redox state of the Dharwar craton root as inferred from eclogite and peridotite sourced mantle cargo, with implications for kimberlite and lamproite magma formation

Abstract

Despite over 400 occurrences of kimberlites and related rocks in India, mantle-derived xenoliths are known only from a few occurrences. This paucity of mantle-derived xenoliths in Indian kimberlites has hampered investigations of the subcontinental lithospheric mantle (SCLM). Using a valuable selection of the rare xenolith inventory, we here report Fe³⁺/ΣFe measurements for garnets using the electron microprobe (EPMA) flank method, targeting six mantle eclogite xenoliths (KL2 pipe) and fourteen peridotitic garnet xenocrysts (P9 and P10 hypabyssal intrusions) from the Wajrakarur kimberlite field (WKF) on the Eastern Dharwar craton (EDC). These data provide some of the first direct constraints on the oxygen fugacity (fO₂) of the lithospheric mantle beneath the Indian subcontinent. The measured Fe³⁺/ΣFe ratios vary between 0.02 and 0.05 (± 0.01) for the eclogite xenoliths and between 0.02 and 0.10 (± 0.01) for the peridotitic garnets. Calculated ΔlogfO2 values for the KL2 eclogites show a wide range from FMQ-3.9 to FMQ-0.9 (± 0.6), straddling the boundary between the diamond and carbonate stability fields. In terms of redox compositions, it appears that the KL2 eclogites are able to host diamond, which is consistent with the diamondiferous nature of this particular WKF locality and the presence of eclogitic garnet inclusions in diamonds from the nearby TK4 kimberlite body. The peridotitic garnet xenocrysts from the P9 and P10 kimberlite bodies, which were entrained between ~ 125 and 170 km depth, reveal ΔlogfO2 values between FMQ-4.5 and FMQ-2.6 (± 0.9). Garnet xenocrysts with ‘normal’ REE patterns exhibit higher Fe³⁺/ΣFe ratios compared to garnets with ‘sinusoidal’ REE patterns. Importantly, the Fe³⁺/ΣFe ratios of garnet xenocrysts with ‘normal’ REE patterns (~ 125–160 km depth) correlate with metasomatic Ti–Y–Zr–V enrichment, which suggests metasomatism-driven oxidation of the cratonic mantle at mid-lithospheric depths. Such melt-related mantle metasomatism was probably diamond-destructive within the otherwise diamond-fertile lithospheric keel. The observed wide range of ΔlogfO2 values for the Dharwar cratonic mantle lithosphere allows for stabilization of various metasomatic phases (e.g., amphiboles, micas, carbonates) that may have formed (or concentrated in) distinctly different metasome assemblages within the continental root that underpins Peninsular India. Changing the relative contributions from such highly diverse volatile-rich metasomes may explain the spatiotemporal association of kimberlites and various diamond-bearing potassic magma types such as orangeites, ultramafic lamprophyres and lamproites, a scenario that is influenced by the redox composition of the Dharwar craton root.