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The mineralogy and chemistry of secondary phases observed in altered basalts from the Nazca plate provide a record of temporal variations in the processes that accompany the aging of oceanic crust. The earliest formed phase in vein and vesicle fillings is Fe- and Mg-rich, Al-poor saponite; it is the most abundant alteration mineral in younger rocks dredged from near the East Pacific Rise, and it lines many fractures and vesicles in much older pillow basalts recovered from the plate. Saponite formation is enhanced by low pH values, high water-rock ratios, and Mg- and Fe-rich solutions, conditions that persist until fractures and other pore spaces become closed and the hydrothermally induced circulation becomes negligible. During this phase of alteration, constituents derived from high-temperature leaching of underlying holocrystalline diabasic and gabbroic rocks are redistributed, both into secondary minerals found in overlying pillow basalts and into sea water. Holocrystalline, upper layer 2 basalts containing abundant smectite are richer in Mg, Na, K, and Ti and poorer in Ca relative to unaltered fresh glasses. During the waning of the hydrothermal system, lower temperatures and slightly alkaline and oxidizing solutions are indicated by the formation of calcite, ferric oxides and hydroxides, and celadonite. Calcite commonly fills all remaining void spaces in hydrothermally altered rocks. If such hydrothermally altered crust is re-exposed to oxygenated sea water by tectonic uplift or by other means, intense low-temperature oxidation can dramatically affect its bulk chemical composition and secondary mineralogy. Earlier formed saponites and ferrosaponites are destroyed and replaced by celadonite and ferric oxides, and bulk-rock compositions show pronounced losses in Mg and enrichments in Fe and K relative to fresh glasses. It is argued that secondary mineral assemblages in altered layer 2 crustal rocks are very sensitive indicators of conditions of alteration. With few exceptions, evidence of nonoxidative, hydrothermal alteration is ubiquitous in upper layer 2 crustal rocks recovered from many different locations on the Nazca plate.

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