A NOBLE-GAS STUDY OF A GRANULITE SAMPLE FROM THE NILGIRI HILLS, SOUTHERN INDIA - IMPLICATIONS FOR GRANULITE FORMATION

The He and Ar isotopic composition of several mineral separates from a 2.5 Ga old enderbite sample from the Nilgiri Hills, southern India we re 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 h eating of the minerals. All measured He-3/He-4 ratios are above the ty pical value of old crustal rocks, which is about 0.01-0.02 times the a tmospheric ratio (R(a)). Fluid inclusions from the crushed garnet samp le contain helium with the highest He-3/He-4 ratio of 2.34 +/- 0.36 R( a). The He-3 excess is mantle derived and cannot be explained by a cos mogenic or nucleogenic He-3-rich component. The elemental and isotopic ratios of He, Ne and Ar in all minerals can be explained by a combina tion of elemental fractionation during diffusional loss of noble gases , production of radiogenic/nucleogenic He-4, Ne-21, Ne-22 and Ar-40, a nd atmospheric contamination. Depending on the assumptions made for th e evolution of the He trapped in the garnet fluid inclusions, we obtai n 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 en derbite sample represents a former mantle-derived tonalitic magma, whi ch exsolved its volatiles upon crystallization in the lower crust. Lib eration of a huge amount of CO2 by similar intrusions, together with t he heat provided by them, might be responsible for the dehydration of the former amphibolite facies rocks to the north bordering the Nilgiri Hills.