Noble gases in carbonatites of Sung Valley and Ambadongar: implications for trapped components

Abstract

Carbonates, apatites and magnetites from Sung Valley (107 Ma) and carbonates from Ambadongar (65 Ma), two carbonatitic complexes of India, have been analysed for noble gases by vacuum crushing and pyrolysis (apatites only). During vacuum crushing ²⁰Ne/²²Ne and ¹²⁹Xe/¹³²Xe of up to 12.1 and 1.06 respectively in Sung Valley and up to 10.3 and 1.06 respectively in Ambadongar have been measured. This clearly indicates presence of mantle noble gases. The ³He/⁴He are up to ≈ 9.01 ± 0.85 R_A for Sung Valley magnetites (where R_A is the atmospheric ³He/⁴He of 1.4 × 10^{- 6}). The ²¹Ne/²²Ne lie in the range of 0.031 to 0.069 for Sung Valley and 0.030 to 0.054 for Ambadongar. This is consistent with the presence of a MORB component admixed with crustal noble gases introduced into the lithospheric mantle during subduction. However, the crustal signatures are differently manifested in the carbonates and the late crystallising magnetites from Sung Valley owing to the relative variation of He/Ne ratios between the residual degassed magma (derived from the MORB source) and the vapour phase. As the relative He/Ne ratio of the magma increases progressively, assimilation of the crustal component from the lithosphere strongly affects the Ne, but not the He isotopic composition of the late crystallising magnetites. Ar and Xe compositions are also consistent with MORB contribution. Some dilution of magmatic signatures is clearly observed in the late stage ferrocarbonatites from Ambadongar and magnetites from Sung Valley, probably the effect of hydrothermal fluids bearing atmospheric gases. Unlike the Kola carbonatites which was dominated by plume component, both Sung Valley and Ambadongar have been generated during the waning stages of plume magmatism leading to dilution and replacement of plume signatures by increasing input from MORB and lithospheric components as the uprising plume induces their melting and subsequent entrainment.