Geochemical and petrographical studies of volcanic products from the Quaternary Nisyros-Yali volcanic system in the easternmost part of the Hellenic arc (Greece) reveal insight into magma generating processes. A compositional gap between 61 and 68 wt % SiO2 is recognized. This compositional gap coincides with the stratigraphic distinction between pre-caldera and post-caldera volcanic units. Petrographic observations show that hydrous phases, such as amphibole, first occur in significant amounts in the first post-caldera lava, which additionally yields a second plagioclase generation. In addition, trace elements subdivide Nisyros-Yali volcanic units into a bimodal suite that also reflects the stratigraphic pre- and post-caldera sequence. Nd and Hf isotope data from Nisyros and Yali units vary between 0.512570 and 0.512675 and between 0.282795 and 0.282940, respectively. Their variation and the fact that they are distinct from the isotope compositions of MORB rule out an origin by pure differentiation and instead require assimilation of an old crustal component. Pb isotope ratios between 206Pb/204Pb = 18.647 ¿ 18.779, 207Pb/204Pb = 15.624 ¿ 15.652 and 208Pb/204Pb = 38.502 ¿ 38.720 support the Nd and Hf isotope signatures and reflect mixing of mantle material with an old upper crust equivalent. Abundant hydrous fluids are characterised by extremely mantle-like Sr isotope ratios between 0.703648 ¿ 0.704847 and probably by CO2-enrichment responsible for transporting Ba and Th. The occurrence of hydrated minerals (e.g., amphibole) in the first post-caldera unit with the lowermost 87Sr/86Sr ratio of 0.703648 can be interpreted as the result of the increased water activity in the source. The presence of two different plagioclase phenocryst generations in the first lava subsequent to the caldera-causing event is indicative for a longer storage time of this magma at a shallower level. A model capable of explaining these observations involves three evolutionary stages. First stage, assimilation of old European basement by a primitive magma of mantle origin (as modelled by Nd-Hf isotope systematics). This stage ended by an interruption in replenishment that led to an increase of crystallisation and, hence, an increase in viscosity, suppressing eruption. During this time gap, differentiation by fractional crystallisation led to enrichment of incompatible species, especially aqueous fluids, to silica depolymerisation and to a decrease in viscosity, finally enabling eruption again in the third stage.
Buettner, A., Kleinhanns, I., Rufer, D., Hunziker, J., Villa, I. (2005). Magma generation at the easternmost section of the Hellenic arc: Hf, Nd, Pb and Sr isotope geochemistry of Nisyros and Yali volcanoes (Greece). LITHOS, 83(1-2), 29-46 [10.1016/j.lithos.2005.01.001].
Magma generation at the easternmost section of the Hellenic arc: Hf, Nd, Pb and Sr isotope geochemistry of Nisyros and Yali volcanoes (Greece)
VILLA, IGOR MARIA
2005
Abstract
Geochemical and petrographical studies of volcanic products from the Quaternary Nisyros-Yali volcanic system in the easternmost part of the Hellenic arc (Greece) reveal insight into magma generating processes. A compositional gap between 61 and 68 wt % SiO2 is recognized. This compositional gap coincides with the stratigraphic distinction between pre-caldera and post-caldera volcanic units. Petrographic observations show that hydrous phases, such as amphibole, first occur in significant amounts in the first post-caldera lava, which additionally yields a second plagioclase generation. In addition, trace elements subdivide Nisyros-Yali volcanic units into a bimodal suite that also reflects the stratigraphic pre- and post-caldera sequence. Nd and Hf isotope data from Nisyros and Yali units vary between 0.512570 and 0.512675 and between 0.282795 and 0.282940, respectively. Their variation and the fact that they are distinct from the isotope compositions of MORB rule out an origin by pure differentiation and instead require assimilation of an old crustal component. Pb isotope ratios between 206Pb/204Pb = 18.647 ¿ 18.779, 207Pb/204Pb = 15.624 ¿ 15.652 and 208Pb/204Pb = 38.502 ¿ 38.720 support the Nd and Hf isotope signatures and reflect mixing of mantle material with an old upper crust equivalent. Abundant hydrous fluids are characterised by extremely mantle-like Sr isotope ratios between 0.703648 ¿ 0.704847 and probably by CO2-enrichment responsible for transporting Ba and Th. The occurrence of hydrated minerals (e.g., amphibole) in the first post-caldera unit with the lowermost 87Sr/86Sr ratio of 0.703648 can be interpreted as the result of the increased water activity in the source. The presence of two different plagioclase phenocryst generations in the first lava subsequent to the caldera-causing event is indicative for a longer storage time of this magma at a shallower level. A model capable of explaining these observations involves three evolutionary stages. First stage, assimilation of old European basement by a primitive magma of mantle origin (as modelled by Nd-Hf isotope systematics). This stage ended by an interruption in replenishment that led to an increase of crystallisation and, hence, an increase in viscosity, suppressing eruption. During this time gap, differentiation by fractional crystallisation led to enrichment of incompatible species, especially aqueous fluids, to silica depolymerisation and to a decrease in viscosity, finally enabling eruption again in the third stage.
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