Magma storage conditions and differentiation of the mafic Lower Pollara volcanics, Salina Island, Aeolian Islands, Italy: implications for the formation conditions of shoshonites and potassic rocks

作者：Beermann Oliver^*; Holtz Francois; Duesterhoeft Erik

来源：Contributions to Mineralogy and Petrology, 2017, 172(5): 37.

DOI：10.1007/s00410-017-1363-z

• Aeolian Island volcanism
• Salina Island
• Lower Pollara eruption
• Magma storage conditions
• Magma differentiation
• Shoshonites
• Potassic rocks

Crystallization experiments of basaltic andesite mafic endmember from the 24 ka Lower Pollara eruption (Salina, Aeolian Islands, Italy) were investigated at 200 MPa, 950-1100 degrees C, in the H2O activity (aH(2)O) range similar to 0.3 to 1, and at two ranges of oxygen fugacity (fO(2)) between similar to FMQ to FMQ+1 and similar to FMQ+2 to FMQ+3.3 (log bars, FMQ is fayalite-magnetite-quartz). Comparison of the produced phase assemblages and phase compositions with the natural sample reveals that the storage conditions were similar to 1050 degrees C, similar to 2.8 wt% H2O in the melt (aH(2)O similar to 0.5), and relatively oxidizing (similar to FMQ+2.5). The composition of plagioclase in the groundmass indicates a period of cooling to <= 950 degrees C. The overall differentiation trends of the Salina volcanics can be explained by fractional crystallization close to H2O saturated conditions (similar to 5 wt% H2O in the melt at 200 MPa) and most likely by accumulation of plagioclase, i.e., in basaltic andesites, and by various degree of mixing-mingling between the corresponding differentiates. The slightly elevated K2O contents of the most mafic basaltic andesites that can be found in the lowermost unit of the Lower Pollara pyroclastics reveal earlier processes of moderately hydrous fractional crystallization at higher temperature (>similar to 1050 degrees C). Fractional crystallization with decreasing influence of H2O causes a moderate decrease of MgO and a significant increase of K2O relative to SiO2 in the residual liquids. It is exemplarily shown that the crystallization of SiO2-rich phases at high temperature and low aH(2)O of only moderately K2O-rich calc-alkaline basalts can produce shoshonitic and high potassic rocks similar to those of Stromboli and Volcano. This suggests that the observed transition from calc-alkaline to shoshonitic and high potassic volcanism at the Aeolian Arc over time can be initiated by a general increase of magmatic temperatures and a decrease of aH(2)O in response to the extensional tectonics and related increase of heat flow and declining influence of slab-derived fluids.

• 出版日期2017-5