Olivine as Probe Into Mantle Source, Magma Crystallisation and Lithospheric Thickness: Insights From the 2.37 Ga and 2.21 Ga Giant Mafic Dyke Swarms of the Dharwar Craton, India

ABSTRACT Paleoproterozoic dolerite dyke swarms (2.37 and 2.21 Ga) of the Dharwar Craton (DC) preserves early crystallising olivine phenocrysts that are imperative to magma generation, crystallisation process and nature of the subcontinental lithospheric mantle (SCLM), however a multi‐element approach using olivine is still lacking. This study provides a detailed major and trace element analyses of 70 olivine grains from the two dyke swarms examining their compositional zoning patterns, mantle source characterisation, crystallisation modelling, Al‐in‐olivine geothermometry and lithospheric thickness variations beneath the DC. Olivine grains are classified into unzoned (Mg# 78–67, 2.37 Ga; 84–62, 2.21Ga) and zoned varieties (Mg# 77–38, 2.37 Ga; 78–56, 2.21 Ga). Zoned olivines show continuous decrease in Ni, Cr, Al and Ca, coupled with increase in Mn, Co and Ti concentrations from core to rim, consistent with progressive fractional crystallisation and melt evolution. Increase in Ni/(Mg/Fe)/1000 (0.7–3.0), Ni/Co (1–8), Fe/Mn (68–95) values and a decrease in 100Mn/Fe values (1.5–1.0) in olivines from the 2.37 Ga dykes to the 2.21 Ga dykes may be interpreted as transition of the SCLM from a peridotite‐dominated composition with lower proportions of recycled oceanic crust to pyroxenitic composition with higher proportions of recycled oceanic crust at 2.21 Ga. However, distinctly high Ni, and low Ca concentrations of the 2.21 Ga dolerites with respect to their crystallisation modelling trends are more consistently explained by repeated magma recharge and advanced fractional crystallisation rather than the source composition alone. The NiO/MnO values in Fo 89 olivines of dyke swarms increase spatially from eastern (1.16–1.36) to western parts (1.88–2.92) of the craton independent of age and are best attributed to differences in lithospheric thicknesses. High olivine crystallisation temperatures (1466°C ± 25°), together with trace element and isotopic signatures, indicate a dominant plume influence for the mafic dykes, modified by interaction with the SCLM.