Diamond potential of southern Indian kimberlites using bulk-rock trace elements and REE mineral chemistry, along with insights from mantle metasomatism

First row transition elements (Sc to Zn) in kimberlites, compositions of olivine, accompanied with REE mineral chemistry of garnets and clinopyroxenes from published data are used to study the diamond potential of kimberlites from the Southern Indian Diamond Province. The Wajrakarur kimberlites show lower Ti, Y, Ga, Mn, Fe, Sc, Zn, V and Cu, and higher Ni and Cr compared to the non-prospective Narayanpet kimberlites. The compositions of mantle minerals i.e. olivine, garnet and clinopyroxene have been studied as indicators to strong mantle metasomatism in the Eastern Dharwar Craton. Olivines with low Mg# and high Ti associated with Ti-rich metasomatism and poor diamond potential belong to Narayanpet kimberlites and p-5 kimberlite from the Wajrakarur Kimberlite Field (WKF). Subcalcic harzburgite garnets (G10D) from the diamond stability field showing sinusoidal REEN patterns belong to p-9 and p-10 kimberlites from the WKF. These are indicative of depleted nature and are potentially diamondiferous. Whereas olivines, garnets and clinopyroxenes from kimberlites displaying modified or normal REEN patterns are indicative of strong mantle metasomatism (i.e. enrichment in HFSE such as Ti, Y, Ga, Yb, and Lu). These have been previously associated with diamond destruction events. Subcalcic harzburgite garnets (G10D) from the diamond stability field are indicators of clinopyroxene-absent, depleted cratonic peridotites. These react with carbonate-rich metasomatic fluids under specific redox conditions to form diamonds during the mineralization event. Carbonate-rich metasomatic fluids with geochemical signatures e.g. high LREE/HREE, Nb/La, Ca/Al, moderate to low Zr/Hf and low Ti/Eu ratios are responsible for the diamond formation event. Lithospheric diamond formation is confined to depths greater than 135 km within the depleted mantle, characterized by high-Mg olivines (Mg# > 90) and harzburgitic garnets from the diamond stability field. However, diamonds remain susceptible to dissolution through intense carbonate and Fe-Ti metasomatism at depths exceeding 150 km.