BASALTS OF NEAR CONSTANT COMPOSITION ERUPTED OVER ~600,000 YEARS IN THE SANTORINI VOLCANIC FIELD, AEGEAN SEA, GREECE

Volcanism on Santorini, Hellenic arc, is related to northeasterly subduction of the African plate at > 4 cm/yr. Calc-alkaline lavas have erupted mainly from four volcanic centers (Akrotiri, Micro Profitis Ilias, Megalo Vouno, and Skaros) over the past 600,000 years. The lavas range in composition from basalt to rhyodacite, and are the products of fractional crystallization combined with assimilation and mixing in intra-crustal magma chambers located at depths of ~7-14 km. Geochemical data indicate that lavas erupted from each center lie on different liquid lines of descent (LLD's) reflecting different intracrustal evolutionary histories. The LLD's define linear arrays on plots such as K2O versus SiO2 that intersect at low values of these oxides. The LLD's are well described by first order polynomials allowing the composition(s) of the parental basalts to be calculated. The concentration ranges of SiO2 (50.4-51.2) and K2O (0.49-0.60) are tightly constrained and are about equal to the range of values expected from analytical uncertainties. The concentrations of representative LILE, HFSE, and REE calculated from regressions versus K2O and/or SiO2 also fall within a narrow range about equal to that expected from analytical uncertainties. This strongly suggests that all magmas erupted on Santorini were derived from a common parental magma or from parental magmas showing a very small range of composition. This requires that the parental magmas have remained essentially constant in composition over 600,000 years. Preliminary results indicate that T, P and pre-eruptive water contents of the parent magmas have also remained nearly constant over this time period. The results imply that magmas produced in the sub-arc wedge have near invariant compositions, which is remarkable given current understanding of magma transfer within volcanic arcs. The geochemical characteristics of the parental magmas are consistent with derivation from a depleted (harzburgitic) mantle source. Comparison with data for other calc-alkaline magmas indicates that this source was similar to that of oceanic island arc basalts, and is characterized by a strong MORB signature. Geochemical variation shown by Santorini magmas reflects the importance of intra-crustal evolutionary processes rather than heterogeneities in the mantle source.