Significance and predictive value of the SiO2/Al2O3 ratio in explosively erupted components of alkaline-mafic pyroclastic deposits

A fundamental geochemical signature in a high proportion of volcanic rock components is the SiO2/Al2O3 ratio. The process of fractional crystallization at depth causes predictable, systematic differentiation of SiO2/Al2O3 between crystallizing minerals and the accompanying residual melt, as evidenced in lava mineralogy, experimental and theoretical systems. In pyroclastic deposits, fragmented magmatic minerals and glassy components (scoria, pumice, fiammae, shards, melt inclusions) record a range of these SiO2/Al2O3 signatures, including representation of the most evolved residual melt fraction as assessed by petrographic context. When placed into a fractional crystallization framework, the appropriate, analysed melt signature of SiO2/Al2O3 then provides an opportunity to pinpoint magma evolution conditions at depth, immediately prior to explosive eruption. Surface processes of alteration in the hydrothermal to diagenetic range are also systematic and predictable, most notably for zeolitization of volcanic glass, long established as closely sustaining the SiO2/Al2O3 ratio (r2 = 0.94 for product-reactant pairs). The relationship therefore presents an opportunity to link zeolitic ‘proxyglass’ signatures of SiO2/Al2O3 to deep magmatic processes in the same way as for fresh glass, above. The opportunity is important because zeolites typically constitute up to ~80% of components in Italian pyroclastic deposits (Langella et al., 2001), representing substantial proportions of the record of erupted former melts. In the case of the pervasively zeolitized Tufo Lionato deposit of Colli Albani (the caldera-forming Villa Senni eruption), magmatic mineral constituents (leucite, diopside, mica, nepheline) quantified by bulk sample XRD closely match those of the expected fractionating system. The alteration assemblage comprises 42% chabazite, 22% phillipsite, 2.3% analcime, and minor smectite. Proxy-glass SiO2/Al2O3 at 2.05 was determined from 114 microprobe analyses of chabazite and phillipsite. When compared with the whole rock SiO2/Al2O3 signature of 2.68 for the associated pre-caldera lava, the residual melt signature for Tufo Lionato is consistent with evolution by magmatic fractionation to a greater extent than evidenced by scarce fresh glass. Reconstruction of this residual melt, its crystallizing minerals and physical parameters at the point of explosive eruption, was enabled by magmatic modelling and successfully validated by comparison with published analyses, especially for clinopyroxene compositions. Thus, a link from surface processes in deposits to processes and conditions at depth occurring immediately prior to explosive eruption, is not only possible but also uniquely informative, demonstrating the significance and predictive value of the SiO2/Al2O3 ratio across disciplines. Langella, A., Cappelletti, P. & de’Gennaro, R. (2001): Zeolites in closed hydrologic systems. In: Bish, D.L. and Ming, D.W. (eds.) Rev. Min. Geochem. 45, 235-260.