Oxygen Isotope Measurements of Albite-Quartz-Zeolite Mineral Assemblages, Hokonui Hills, Southland, New Zealand

Abstract

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 heulanditechlorite assemblage containing fresh detrital plagiodase and a laumontite-albite-quartz assemblage. A laumontite vein follows the local joint pattern and forms the reaction boundary, suggesting that laumontitization 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 of about 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 at 300"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-quartz pair, and supporting the petrographic observation of co-crystallizing albite-laumontite.Oxygen isotope values for fracture-filling laumontite in the vitric tuff, as well as those for groundmass and vein laumontite from other parts of the stratigraphic section, cluster around + 14.5, suggesting that laumontite probably crystallized under similar conditions throughout much of the section. Oxygen isotope values for stilbite veins from various parts of the section indicate that this mineral crystallized at lower temperatures than the laumontite, for a given fluid isotopic composition, in agreement with the observed cross-cutting of laumontite by stilbite.

Dehydration veins in diagenetic and very‐low‐grade metamorphic rocks: features of the crustal seismogenic zone and their significance to mineral facies

“…Jointcontrolled replacement of heulandite by laumontite together with microcrystalline quartz, K-feldspar and albite has been described by Boles & Coombs (1975) and Stallard & Boles (1989) for one of the stratigraphically lowest Occurrences of heulandite. Clastic plagioclase is simultaneously albitized.…”

“…Stallard & Boles (1989) interpret oxygen isotope data on quartz-albite pairs as constraining the alteration to a temperature in the range 145-170" C. The quartz-laumontite pair suggests a temperature in the range 139-162°C. Based on their position in the stratigraphic section, however, these rocks are believed to have been exposed to a maximum temperature of up to 225°C when at their maximum burial, which probably exceeded 9 km.…”

Isotope Geochemistry of Zeolites

“…In the absence of experimental data, Stallard and Boles (1989) developed a semiempirical expression relating zeolite framework-water fractionation to the Si/Al ratio of the Karlsson and Clayton (1990b); 2: Feng and Savin (1993b); 3: Noto and Kusakabe (1997a).…”

“…For example, their predicted analcime (or wairakite) fractionation is too low by 1‰ and 1.8‰ at 250°C and 25°C, respectively. The semi-empirical expression of Stallard and Boles (1989) can therefore be used to estimate isotopic fractionation for zeolites where experimental data are lacking. Another semi-empirical approach is based on bond counting.…”

Willsher Group and geology of the Triassic Kaka Point coastal section, south‐east Otago, New Zealand

“…Diffusively migrating fluids may have been responsible. Stallard & Boles (1989) and Boles (1991) showed that in the Southland Syncline substantial mineralogical reconstitution has taken place, not at the maximum depths of burial and temperatures to which the sediments were subjected, but at lower temperatures related to post-dating folding and uplift. Similar effects may be anticipated in the Willsher Group with low temperatures further impeding the approach to equilibrium mineral assemblages.…”