MICROANALYSIS OF OXYGEN ISOTOPE RATIOS I: MY FAVORITE MINERALS

For 75 years, oxygen isotope ratios (δ¹⁸O) in minerals have provided critical evidence to understand the thermal, fluid and kinetic histories of igneous, metamorphic and sedimentary rocks. However, minerals are commonly zoned at micron-scale due to complex processes of growth or alteration. Conventional analysis at mm-scale can thus result in average compositions, lost information and misinterpretation. Just as the electron microprobe surpassed wet chemistry (50 years ago) for most cation analysis, the need for in situ microanalysis of stable isotopes has long been evident.

SIMS (secondary ion mass spectrometry) stable isotope analyses (1-10 micron spot size) can be correlated with microscopy and other in situ techniques to reveal otherwise hidden correlations and detail. Over the past 30 years, precision, accuracy, spot size, and time/analysis have all improved dramatically. Spot-to-spot δ¹⁸O precision of 0.2‰ (2SD) is attainable in 3 minutes with care. For some studies, excellent precision is enough to elucidate trends/zoning and answer geologic questions but, well-matched reference materials (RMs) are necessary for accurate calibration to the VSMOW scale. For minerals with variable chemistry, matrix effects contribute to analytical bias by SIMS and vary predictably with composition. Calibration of a SIMS working curve with multiple RMs, if run during each SIMS session and correlated to in situ analysis of cations, allows accurate analysis of δ¹⁸O ±~0.5‰. Minerals showing limited solid solution can be accurate ±0.3‰.

Geological advancements are numerous. A few of my favorites include quartz overgrowths in sandstone; diagenetic carbonate cements; biocarbonates (see Valley- II, this meeting); zoned skarn garnets; and zircons ranging from 0 to 4400 Ma.