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| Titel |
Ge and Ga behaviour during subduction-related metamorphism of basic rocks |
| VerfasserIn |
Afifé El Korh, Beatrice Luais, Marie-Christine Boiron, Etienne Deloule |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094763
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| Publikation (Nr.) |
 EGU/EGU2014-10194.pdf |
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| Zusammenfassung |
| Ge and Ga are can be used as fluid tracers in hydrothermal systems, as their solubility in fluid is temperature dependant [1,2]. Physico-chemical conditions of hydrothermal processes would result either in Ge enrichment [3], or Ge depletion of the basaltic crust in presence of hydrothermal sulphides [4]. Variations in Ge and Ga concentrations were investigated in a series of HP¬-LT metabasites (peak P-T conditions: 1.7-2.3 GPa and 550-600°C) from the Ile de Groix (France), to examine their behaviour during the fluid-rock interactions and associated metamorphism related to subduction. The studied metabasites represent former MOR-type basalts, which underwent a pre-HP low-temperature hydrothermal alteration [5].
Whole rock Ga and Ge concentrations were analysed by SN-ICP-MS (± 8%; 1σ). The Ga abundances in eclogites and blueschists (21.0-21.6 ppm) are similar to the values measured in tholeiitic basalts (18-22 ppm; [6]), while they decrease slightly in the greenschist facies metabasites (16.9-20.5 ppm). The Ge abundances measured in the metabasites (1.2-2.1 ppm) are generally higher than those of tholeiitic basalts (1.4-1.5 ppm; [6]), suggesting Ge enrichment during the low-T hydrothermal alteration. Similarly to Ga, the decrease of the Ge abundances in greenschists (1.2-1.9 ppm) compared to blueschists and eclogites (1.6-2.0 ppm) indicate that a fraction of Ge and Ga left the metabasites during the fluid-rock interactions related to the retrograde metamorphism.
Ga and Ge abundances in minerals were obtained using LA-ICP-MS (± 8-15%; 1σ). Garnet and epidote are major phases in the metabasites. They are the main hosts for Ge (3.3-8.2 ppm in Grt; 2.4-12 ppm in Ep) and Ga (2.6-8.6 ppm in Grt; 33-112 ppm in Ep). Glaucophane and omphacite also contain significant amounts of Ga (3-15 ppm). The different generations of garnet and epidote formed during the prograde and retrograde metamorphic stages [5] can be distinguished using the Ge/Si and Ga/Al ratios. Ge/Si in garnet increases from blueschists to eclogites (prograde metamorphism), i.e. with increasing temperature from 500 to 550°C [5], in line with the Ge/Si increase in garnet rims. Ga/Al values remain relatively constant. By contrast, Ge/Si in garnet in greenschists (retrograde path) is similar to garnet in blueschists. In epidote, the Ga/Al ratios increase during the prograde path, while the Ge/Si decrease. At peak conditions (500-550°C [5]), Ge seems to be preferentially incorporated in garnet than in epidote. These results allow evaluating the Ga and Ge exchange behaviour during the various P-T conditions.
REFERENCES
[1] Pokrovski and Schott (1998). GCA 62, 1631-1642.
[2] Wood, S.A., Samson, I.M. (2006). Ore Geol. Rev. 28, 57-102.
[3] Luais, B. (2012). Chem. Geol. 334, 295-311.
[4] Malvin, D.J., Drake, M.J. (1976). GCA 51, 2117-2128.
[5] El Korh, A., Schmidt, S. Th., Ulianov, A., Potel, S. (2009). J. Petrol. 50, 1107-1148.
[6] De Argollo, R., Schilling, J. (1978). GCA 42, 623-630. |
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