Noble gases in South Indian carbonatites: trapped and in situ components

Abstract

We have studied noble gases in carbonates and apatites from three carbonatites of South India, namely Hogenakal (2400 Ma), Sevattur (770 Ma) and Khambamettuu (523 Ma) by vacuum crushing. Apatite has also been analysed by pyrolysis. Vacuum crushing mostly releases the trapped gas components. The ratios ²¹Ne/²⁰Ne, ²²Ne/²⁰Ne and ⁴⁰Ar/³⁶Ar increase with progressive crushing due to preservation of different composition gases in smaller inclusions released in later steps. This heterogeneity of isotopic composition of fluid inclusions is a consequence of the involvement of magmas carrying different noble gas signatures. The inclusions with lower ratios suggest the presence of a subducted atmospheric component, while the higher ²¹Ne/²⁰Ne, ²²Ne/²⁰Ne and ⁴⁰Ar/³⁶Ar can be attributed to the presence of an enriched lithospheric mantle component. In addition, very minor trapped gases from less degassed, deeper mantle may also be present but overprinted by lithospheric and/or nucleogenic components. We propose that these carbonatites were generated only in an advanced stage of magmatism when this lithospheric component overwhelmed any contribution from the deeper mantle source. The lithospheric mantle underwent enrichment during an ancient subduction process through mantle metasomatism manifested in nucleogenic/radiogenic isotopic ratios of ²¹Ne/²⁰Ne, ²²Ne/²⁰Ne and ⁴⁰Ar/³⁶Ar. The apatites analysed by pyrolysis clearly show nucleogenic ²¹Ne from ¹⁸O(α,n) reaction. We have demonstrated the potential of using U,Th-²¹Ne systematics as a thermo-chronometer in conjunction with the established U,Th-⁴He and U-¹³⁶Xe clocks. While for Hogenakal, the U,Th-²¹Ne age of 845 ± 127 Ma is in agreement with the age of emplacement of other adjacent younger carbonatites, syenites and alkali granites, for the Sevattur apatite (738 ± 111 Ma) it indicates the crystallisation age.