Hydrothermal alteration patterns exposed in the sheeted dike complex at Pito Deep




Heft, Kerri Laura

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This thesis documents hydrothermal alteration patterns exposed along major fault scarps at Pito Deep, a tectonic window into the upper ocean crust formed at the very fast-spreading (>140 mm/yr) East Pacific Rise. Two main study areas were examined, each covering at least five lateral kms across the sheeted dike complex, exposing > four km of relief. and revealing >70,000 years of spreading history. Hydrothermal alteration patterns reveal vertical and lateral variation in the degree of alteration, dominant secondary mineral assemblages, peak temperatures of alteration, and metal depletion on the scale of 10s to 100s of meters. Amphibole and chlorite are the most common secondary minerals and replace clinopyroxene, interstitial zones, and plagioclase. Mineral assemblages indicate alteration temperatures of 250-450°C were common throughout most of the sheeted dike complex. Geothermometry indicates a range in alteration temperatures from 902-505°C and 350-60°C for amphibole-plagioclase and chlorite thermometry respectively. The highest temperatures are recorded in both deformed and undeformed dikes indicating that deformation was not always synchronous with peak alteration temperatures.



ocean bottom, Pacific Ocean, hydrothermal alteration