Abstract: Oxygen Isotope Studies on Authigenic Quartz using the VG Isolab 54 Microbeam Ion Probe

Patrick James McKeever, Stuart D. Burley, Ian C. Lyon

The origin of pore fluids from which authigenic minerals precipitate can be partly constrained from their stable oxygen isotope composition. Although quartz overgrowths form a significant cement in many sandstones their oxygen isotope composition is difficult to determine and the results are often limited in value because of the difficulty in obtaining a pure separate of overgrowths from the host sandstone. Furthermore, the assumption is made that each overgrowth separate is isotopically homogenous. One solution is to use in-situ determination of oxygen isotope composition whilst being able to locate a beam precisely on the sample. The VG 54 lsolab microbeam ion probe utilizes a Cs⁺ ion source to bombard the sample, generating a beam of secondary O^- ions that are accelerated to a multi-collection mass spectrometer.

Two beam transects have been performed across an area of zoned quartz cement. Beam size was 90-60 microns and the ^dgr¹⁸O values obtained range from -0.8 to 9.7% SMOW in the detrital grains, 13.5^pmil SMOW in the oldest area of overgrowth and up to 28^pmil SMOW for the youngest area of quartz cement. Reproducibility (2 ^sgr) is +/-1.O^pmil or better and the results represent an evolution of the ^dgr¹⁸O composition of the quartz cement of some 13^pmil. Subsequent improvements have reduced beam diameter to approximately 30 × 40 microns. Given the limitations of conventional oxygen isotope measurements on quartz cement, this new technique represents a significant advance in the study of quartz cements.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994