Computer Science
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.119..271d&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 119, no. 3, p. 271-281
Computer Science
4
Abundance, Argon Isotopes, Cosmology, Earth (Planet), Earth Crust, Garnets, Helium Isotopes, Rare Gases, Carbon Dioxide, Crustal Fractures, Fractionation, Nucleogenesis, Outgassing, Planetary Evolution
Scientific paper
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 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 He-3/He-4 ratios are above the typical value of old crustal rocks, which is about 0.01 - 0.02 times the atmospheric ratio (R(sub a)). Fluid inclusions from the crushed garnet sample contain helium with the highest He-3/He-4 ratio of 2.34 +/- 0.36 R(sub a). The He-3 excess is mantle derived and cannot be explained by a cosmogenic or nucleogenic He-3-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 He-4, Ne-21, Ne-22, and Ar-40, and atmospheric contamination. 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 hugh amount of CO2 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.
Dunai Tibor J.
Touret Jacques L. R.
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