Formation and evolution of magmas due to sediment subduction: a case study of the submarine Paphsanias volcano, Aegean arc
Final Report Abstract
The project allowed the first sampling and observation of the submarine volcanism west of Methana and six different volcanic structures were found, five of which were sampled successfully. We report the first data on submarine samples including major and trace element abundances and Sr-Nd-Pb isotope ratios for basaltic to andesitic lavas of the submarine Paphsanias volcanic field, which are the most primitive of the Aegean island arc with MgO up to 9 wt.% and Ni >100 ppm. Melt inclusions trapped in olivine and plagioclase display a wide range in composition from 45 to 75 wt.% SiO2. MELTs modeling shows that the differentiated lavas are formed by fractional crystallization from the mafic melt inclusions with MgO of ~14 wt.%, +1 to +2 QFM, 2 – 4 wt.% H2O at about 5 kbar. MORB-normalized trace element patterns of the Paphsanias and Methana lavas overlap and show typical island arc patterns with an enrichment of fluid mobile elements and pronounced negative anomalies of Nb, Ta and Ti. Positive peaks of Sr, K, Pb and a flat pattern of heavy rare earth elements resemble those of sediment composition from the Hellenic trench. Unaltered Paphsanias lavas have low 143Nd/144Nd (0.51253 - 0.51265), radiogenic 87 Sr/86Sr (0.70541 - 0.70646) and 206Pb/204Pb (18.682 - 18.859) ratios that overlap with isotope signatures of Methana. Trends of isotope ratios with incompatible element ratios of Th/Nb, Sr/Nd, Ba/Nb, Th/Nd, Nb/Yb suggest that the observed lava compositions originated from a two-stage process. Compared to depleted mantle, increased Sr/Nd and decreased Nd-isotopes in the most primitive lavas indicate an origin from a mantle that was metasomatized by a fluid and a sediment melt. Assimilation of local crust and fractional crystallization of plagioclase decrease Sr/Nd and 143Nd/144Nd at a concurrent increase of 87 Sr/86Sr, Th/Nd and Th/Nb towards the composition of rhyolites. Thus, magmas erupting in the Aegean Arc reflect sediment subduction and also assimilation of basement rocks during ascent, making conclusions regarding the primitive mantle source difficult. Although whole rock data of Paphsanias and Methana completely overlap, the examination of primitive olivine and trapped Cr-spinel inclusions reveal different source compositions leading to variable Fo, Ni, and Al contents and Cr# in these minerals.
Publications
- (2017) Poseidon Berichte - Paphsanias Volcano Cruise POS512, 25.04.2017 – 06.05.2017, Heraklion (Griechenland) – Heraklion (Griechenland), 124 pp.
Haase, K., Beier, C., Bodendorfer, M., Cuno, P., Huusmann, H., Lampridou, D., Karagiannakis, E., Matthiessen, T., Pieper, M., Storch, B. and Wölki, D.
(See online at https://doi.org/10.3289/CR_POS512) - (2018). Evolution and genesis of calc-alkaline magmas at the Paphsanias Volcanic Field, Aegean Arc. GeoBonn2018, Bonn, DE
Günther, T., Wölki, D., Storch, B., Beier, C., and Haase, K.
