In situ laser microprobe techniques for stable isotope analysis

In situ laser extraction techniques have been developed for determining the stable isotope ratios in carbonates, silicates, oxides and sulfides with high spatial resolution. All of these methods use the laser as a heat source to extract the appropriate gas. Carbonate analyses are made by decomposing the carbonate at elevated temperatures to liberate CO₂. Oxygen is extracted from silicates by heating the sample surface in a halogen fluoride atmosphere, and sulfur is extracted from sulfides by heating in either an O₂ or halogen fluoride atmosphere. The analytical precision for carbonate analyses isσ = ±0.2‰and±0.1‰ forδ¹⁸C andδ¹⁸O, respectively. Higher scatter in some samples is probably due to sample heterogeneities. In situ silicate analyses have a precision ofσ = ±0.1‰0.6‰ depending upon sample material and heterogeneity. The measured isotopic ratio may be different from the accepted value, but is insensitive to changing operating conditions. Published results for in situ analyses of sulfides have a precision of< ±0.3‰ (1σ). Theδ³⁴S-values obtained with the laser method are less than accepted values, and vary according to the sulfide.

A new level of spatial resolution is possible with the in situ laser methods. Oxygen isotope variations of10‰ are found to exist at a scale of< 1mm in greenschist-grade quartz veins. Carbon and oxygen isotopic ratios may be quite heterogeneous in hydrothermal calcites and low-grade marbles. High-grade marbles are isotopically homogeneous. Determinations of isotopic variations at the sub-millimeter scale may be used as an indicator of fluid sources and mechanisms of isotopic exchange.