CRYSTAL SIZE DISTRIBUTION EVIDENCE FOR MAGMA RECHARGE AT STROMBOLI VOLCANO

Subaerial activity at Stromboli over the last 100,000 yr. has produced a wide range of compositions including calc-alkaline (CA), high-K calc-alkaline (HKCA), shoshonitic (SHO) and leucite-bearing shoshonitic (KS). Seven different periods of activity have been recognized: Paleostromboli I, II, III, Scari, Vancori, Neostromboli, and Recent. Focusing on the Neostromboli (KS) and Recent (SHO) groups, lavas have holocrystalline, porphyritic to seriate textures, with phenocryst:groundmass ratios between 60:40 and 35:65. Phenocryst phases are plagioclase, clinopyroxene, and olivine, with plagioclase as the most abundant phase. Most crystals are euhedral to subhedral with the larger crystals being more equant and smaller crystals being more acicular. Disequilibrium textures are present in plagioclase crystals in abundance and range from mottled or sieve-textured cores to dusty rims. Many crystals are characterized by zones that display disequilibrium textures interspersed with zones that reflect equilibrium crystal growth. Such intergrowths may be evidence of multiple recharge/mixing events during these magmatic phases. Mineral compositions and trace element data support this suggestion. Crystal size distribution (CSD) data were collected on rocks from the Neostromboli and Recent suites to better elucidate recharge processes at Stromboli. Using images from a petrographic microscope and NIH imaging software, between 1000 and 2500 crystals per sample were measured and reduced into CSD plots. For each sample, CSD data plot as a single linear array in ln (N) vs. L space, suggesting that each crystal population grew in a relatively homogeneous thermal environment. CSD slopes and intercepts range from –2.6 to –12.2, and 4.0 to 7.9, respectively. For reasonable growth rates (10^-9—10^-10 mm/sec), preliminary estimates of crystal residence times range from decades to centuries. Integration of textural and CSD data suggest that during their growth, crystals in these lavas are exposed to multiple recharge events, but little thermal contrast is introduced by mixing of recharge and standing magmas. This may indicate that recharge at Stromboli occurs quasi-continuously. Further work may help to elucidate distinctions in CSD plots for magmatic systems that experience episodic vs. continuous recharge.