Anomalously high δD and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China
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Hydrogen isotopic compositions of four amphibole grains from three pieces of lherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94‰ to +46‰, some of which are much higher than the highest δD (+8‰) reported previously for mantle materials. The heterogeneities of D/H ratios within single grains have been observed, the variation of δD is up to 80‰ on the scale of less than 400 μm. No correlation between hydrogen isotopic ratios and hydrogen contents can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the fact that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should be heterogeneous and result in the observed large variable and anomalously high δD values of amphiboles. We suggested that such metasomatic fluids could be related to magma degassing in the mantle source. Based on the D-H diffusion data and the scale of hydrogen isotope heterogeneities, it was inferred that the mantle metasomatism took place soon before the eruption of host magma.
Keywordshydrogen isotope heterogeneity high δD amphibole
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- 1.Deloule, E., France-Lanord, C., Albarede, F., D/H analysis of minerals by ion probe (eds. Taylor, H. P., O’Neil, J. R., Kaplan, I. R.), Stable Isotope Geochemistry: A Tribute to Samuel Epstein, The Geochemical Society, Special Publication No.3, 1991, 53 – 62.Google Scholar
- 6.Boettcher, A. L., O’Neil, J. R., Stable isotope, chemical and petrographic studies of high pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites, Am. J. Sci., 1980, 280A: 594.Google Scholar
- 8.Kyser, T. K., Stable isotope variations in the mantle stable isotopes in high temperature geological processes (eds. Valley, J. W., Taylor, H. P., O’Neil, J. R.), Mineral. Soc. Am., 1986(2): 141.Google Scholar
- 10.Ingrin, J., Skogby H., Hydrogen in nominally anhydrous upper mantle minerals: Concentration levels and implications, Eur. J. Mineral., 2000, 12: 543.Google Scholar
- 12.Dyar, D. M., Martin, S. V., Mackwell, S. J. et al., Crystal chemistry of Fe3+, H+ and D/H in mantle-derived augite from Dish Hill: Implications for alteration during transport (eds. Dyar, M. D., McCammon, C., Schaefer, M. W.), Mineral Spectroscopy: A tribute to Rogers G Burns, The Geochemical Society, Special Publication No. 5, 1996, 287–301.Google Scholar
- 14.Xu, X. S., O’Reilly, S. Y., Griffin, W. L. et al., The nature of the Cenozoic lithosphere at Nushan, eastern China (eds. Flower, F. J., Chung, S. L., Lo, C. H.), Mantle dynamics and plate interactions in east China, Geodynamics Series 27, Am. Geophy. Union, Washington D C, 1998, 167–196.Google Scholar
- 15.Menzies, M. A., Rogers, N., Tindle, A. et al., Metasomatic and enrichment processes in lithospheric peridotites, an effect of asthenosphere-lithosphere interaction, Mantle Metasomatism, London (eds. Menzies, M. A.), 1987, 313–360.Google Scholar
- 18.Taylor, B. E., Isotopic variation of C, H and S stable isotopes in high temperature geological processes Mineral Soc. Am. (eds. Valley, J. W., Taylor, H. P., O’Neil, J. R.), 1986(2): 185.Google Scholar
- 22.Graham, C. M., Harmon, R. S., Sheppard, S. M. F., Experimental hydrogen isotope studies: hydrogen isotope exchange between amphibole and water, Am. Mineral., 1984, 69: 128.Google Scholar