OXYGEN ISOTOPE MEASUREMENTS OF ALBITE-QUARTZ-ZEOLITE MINERAL ASSEMBLAGES, HOKONUI HILLS, SOUTHLAND, NEW ZEALAND

Abstract--The oxygen isotopes of albite, quartz, and zeolites from the Hokonui Hills, New Zealand, constrain crystallization temperatures and the type of pore fluids present during diagenesis. A section of altered vitric tufts in this region contains an extremely sharp reaction boundary between a heulandite-chlorite assemblage containing fresh detrital plagiodase and a laumontite-albite-quartz ssemblage. A laumontite vein follows the local joint pattern and forms the reaction boundary, suggesting that lau-montitization occurred as a result of fracturing and increased fluid flow during uplift. The albite (~80 = + 15.0)-quartz (6~80 = + 19.9 to + 20.5) geothermometer constrains the temperature of alteration between 145 ~ and 170"(2 with a pore water 6isO of + 1.8 to + 3.5. The tuff was buried to an estimated maximum temperature ofabout 225*(2, indicating that alteration occurred after maximum burial. Framework oxygen was extracted from zeolites by reaction with CIF3 after the zeolites were thermally dehydrated in a vacuum. Laumontite was dehydrated at300"C, and stilbite at 150"C. The precision of the method is typically about + 0.45 ~ Fractionation curves for dehydrated zeolites are based on a general expression from the literature for feldspars, which depends only on the Si/A1 ratio of the mineral. Measured 51sO values for laumontite in the groundmass of the altered tuff were + 14.4~/~. The laumontite-quartz pair constrains the temperature to between 139 " and 162"C, in excellent agreement with the albite-quart