| Abstract: |
Introduction: Marshall (1961) suggested that water molecules contained in glass are capable of breaking the Si-O-Si and Al-O-Al cross bonds by adding hydroxyl groups to an Si or Al atom. In 1966, Friedman et a1. suggested that perlite (hydrated glass from obsidian) can form during diagenesis at measurable rates and applied Fisk's Law to describe the depth of water penetration as a function of environmental temperature at given glass compositions. In 1969, Morgenstein described the alteration process of palagonitization from sideromelane (basaltic glass) and developed a linear reaction, base-exchange-limited, which conformed to observed products during halmyrolysis (ocean weathering). Morgenstein and Riley (1975:152) described an empirical method for obtaining a "general" rate of hydration for basaltic glass (sideromelane) in Hawaii and have related that method to the theoretical development of hydration of acid glass (obsidian). More recently, Jakobsson and Moore (1986) reported empirical rates of hydration of basaltic glass based upon measured temperature and palagonite rinds at Surtsey volcano, Iceland. Their data shed light on the earlier work of Morgenstein and Riley (1975). Morgenstein (1969) showed that the hydration of basaltic glass in deep-sea sediments at temperatures approximating 5 °C was 2.91 microns-per-thousand years (approximately 0.003 microns-per-year). Hekinian and Hoffert (1975) obtained a hydration rate of 0.003 /μm-per-year (3.0 /μm-per-thousand years) for sideromelane to palagonite transition for deep-sea Atlantic rift zone samples at temperatures of approximately 4°C. The Atlantic data were acquired by fission-track dating of basalt samples. Submarine basalts and basaltic glass are fairly uniform in chemical composition (Table 1). Deep ocean bottom-water temperatures and for the most part water composition are also quite invariant
consequently, ocean-floor volcanics provide reasonable natural controls on the variables affecting volcanic-glass hydration. For these reasons and the agreement of rates of basaltic-glass hydration in the literature, a hydration rate for ocean-floor sideromelane of 0.003 μm-per-year at 4 °C is assumed to be a reasonable and valid value.
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