Zeolite and Albitized Plagioclase Crystallization from Oxygen Isotopes

Mary L. Stallard, James R. Boles

Zeolites and albitized plagioclase are widespread in volcanogenic sediments. ^dgr¹⁸O ratios have been measured in natural laumontite, heulandite, stilbite, albitized plagioclase, and quartz. Framework oxygen values for zeolites are measured after removing zeolitic water. Zeolite-water fractionation coefficients are estimated from an expression for framework aluminosilicates by O'Neil and Taylor.

The classic burial sequence of Southland, New Zealand, comprises 10 km of Triassic-Jurassic age, marine volcaniclastic sediments. Once deeply buried heulandite tuff with fresh plagioclase has been altered across a laumontite-filled fracture to laumontite, albite, and quartz. Albitized plagioclase (+0.9 ^pmil_SMOW) has significantly different oxygen values than its calcic plagioclase precursor (+15.0 ^pmil_SMOW) demonstrating a solution-reprecipitation mechanism for albitization.

The laumontite fracture-fill and the laumontite in tuff have similar isotopic ratios (+15 ^pmil and +14.5 ^pmil,respectively), suggesting contemporaneous formation. Albite (+15 ^pmil) and quartz (+19.9-20.5 ^pmil), which presumably crystallized with the laumontite, constrain the alteration temperature to 140° to 170°C in the presence of water with a calculated ^dgr¹⁸O of +1.8 to +3.5 ^pmil. These temperatures are considerably lower than the estimated burial temperature of 225°C and suggest the reactions proceeded in response to rock fracturing during the Cretaceous uplift, rather than during initial burial.

Oxygen isotopes for laumontite in veins and pore-fillings from various parts of the section (+14.5 to 15 ^pmil) suggest similar laumontization conditions throughout the area. Laumontite and stilbite veins high in the section are 1.5 to 3 ^pmil lighter than deeply buried samples, possibly reflecting the influx of istopically light meteoric water during uplift.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.