Montesano, Giovanna (2024) Secondary mineralization processes in pyroclastic rocks of Surtsey (ICDP-SUSTAIN Project). [Tesi di dottorato]

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Abstract

The alteration of basaltic glass and in situ mineral growth are key processes that affect the chemical characteristics of very shallow portions of the Earth’s oceanic crust. Since the end of the volcanic eruptions in 1967, these processes have occurred at the basaltic island of Surtsey (SW Iceland), which is one of the Earth’s best-monitored volcanic sites. The island of Surtsey formed over a near continuous 3.5-year effusive and explosive basaltic eruptions from 1963 to 1967. Subaerial and submarine tuff samples belong to three drill cores acquired during the Surtsey Underwater volcanic System for Thermophiles, Alteration processes and Innovative Concretes (SUSTAIN) project, sponsored by the International Continental Drilling Program (ICDP). They have been characterized in this study throughout minero-petrographic and chemical analysis, including optical microscopy (OM), qualitative and quantitative X-ray powder diffraction (XRPD) analysis performed on both bulk samples, separated clay fraction and on single crystals, as well as Field Emission Scanning Electron Microscopy (FESEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS). The analyzed volcanic products consist mainly of vesicular hypocrystalline tuffs; their constituents vary from angular to slightly or even wholly rounded ash to lapilli size pyroclasts. Vesicles have variably size and shape and are dispersed into a cineritic matrix. Loose plagioclase and olivine crystals are also present. The glassy groundmass of the pyroclastic components generally appears deeply altered into palagonite, albeit preserving fresh basaltic glass (sideromelane). Clay minerals are very common in Surtsey deposits, occurring as a late authigenic maturation stage of palagonite, as alteration rims around olivine or as a complete pseudomorphic replacement of olivine crystals. Pores and vesicles (small and rounded vs. bigger, respectively), as well as void spaces in the fine ash matrix, are filled by the precipitation of secondary phases, mainly calcium and aluminium silicate hydrate (zeolites, tobermorite and xonotlite), together with minor carbonates and sulphates. Therefore, the island represents one of the most suitable sites to investigate the formation of these authigenic secondary phases. The integrated analyses revealed that: i)the multi-stage alteration of basaltic glass and the secondary mineral precipitations from interstitial fluids as pore or vesicle fillings occurs as a rapid and low-temperature process through different intervening stages and sequences of crystallization; ii)different extent of alteration depends not only on temperature but also and especially on the chemical evolution of pore fluids at the millimiter-to-meter scale, suggesting that interaction of pore fluids with changing reactive components in microenvironments could be as just as influential as the temperature in determining the reaction pathways that produce subsequent secondary phases; iii)the presence of clay minerals, characterized for the first time in Surtsey deposits through a clay fraction separation technique, which show a prevailing smectitic component as well as chlorite and serpentine; iv)the presence of chabazite, so far recognized only in the 1979 drill core; v)the coexistence of phillipsite and merlinoite, previously identified only as phillipsite, through the use of a non-conventional analytical technique (i.e., single crystal diffraction analysis); vi)the presence of xonotlite, despite temperature is too low with respect to the formation temperatures so far reported; vii)the presence of a Si-rich still unknown phase, never detected before in previous studies on Surtsey deposits.

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