A noble gas study of a granulite sample from the Nilgiri Hills, southern India: implications for granulite formation

Abstract

The He and Ar isotopic composition of several mineral separates from a 2.5 Ga old enderbite sample from the Nilgiri Hills, southern India were analyzed. The sample contains abundant high-density carbonic fluid inclusions, which have been well characterized in previous studies [1,2] on this particular sample. Garnet contains synmetamorphic primary fluid inclusions, while quartz and plagioclase contain re-equilibrated secondary inclusions. The noble gases were extracted by crushing or heating of the minerals. All measured 3He/ 4He ratios are above the typical value of old crustal rocks, which is about 0.01–0.02 times the atmospheric ratio ( R a). Fluid inclusions from the crushed garnet sample contain helium with the highest 3He/ 4He ratio of 2.34 ± 0.36 R a. The He excess is mantle derived and cannot be explained by a cosmogenic or nucleogenic 3He-rich component. The elemental and isotopic ratios of He, Ne and Ar in all minerals can be explained by a combination of elemental fractionation during diffusional loss of noble gases, production of radiogenic/nucleogenic 4He, 21Ne, 22Ne and 40Ar, and atmospheric contamination. Depending on the assumptions made for the evolution of the He trapped in the garnet fluid inclusions, we obtain mixing ratios between mantle-derived He and crustal He ranging from 1 : 5.5 to essentially pure mantle He. However, probably pure mantle He was trapped in the minerals 2.5 Ga ago. These data suggest that the enderbite sample represents a former mantle-derived tonalitic magma, which exsolved its volatiles upon crystallization in the lower crust. Liberation of a huge amount of CO 2 by similar intrusions, together with the heat provided by them, might be responsible for the dehydration of the former amphibolite facies rocks to the north bordering the Nilgiri Hills.