Paper No. 3
Presentation Time: 1:45 PM
A SAMPLE CLEANING METHOD FOR HIGH PRECISION SIMS MEASUREMENT OF CARBON CONTAINING SPECIES IN VOLCANIC MELT INCLUSIONS AND OTHER NATURAL MATERIALS
The study of volcanic melt inclusions (MI) has recently gained popularity for the determination of pre-eruptive volatile constituents of magmas. The formation of MI occur at high pressures, allowing volatile elements and compounds (H2O, CO2, S, Cl, and F) to be trapped in the MI. The concentration of the volatile components provides information regarding the magmatic system such as the depth at which the MI was formed and the likelihood of an eruption. Due to their typically low concentrations (less than 1%), and the need for spatially precise microanalysis, the characterization of these volatile components can be difficult without advanced techniques. Secondary ion mass spectrometry (SIMS), provides concentration analysis and micro-scale spatial precision and has become essential to the characterization of volcanic MI. However, due to its excellent detection limits, SIMS requires careful sample preparation to avoid data skewing from contamination, especially considering that many of the volatile components of interest in MI are ubiquitous. Previous SIMS characterization results have been unreliable due to the amount of carbon contamination from sample preparation and mounting, which skews the quantification of carbon in the sample. This study focuses on creating reproducible and repeatable SIMS results by development and application of a method for carbon contamination removal. The following cleaning methods have been chosen to minimize the carbon contamination: 1) polishing at various grits sizes and 2) wet and dry chemical etches. Like most geological samples, MI have natural variability; therefore, the contamination removal steps were performed on pure synthetic silicon and quartz specimens to minimize the variation seen in natural specimens. SIMS analysis of the synthetic materials was performed to test the effectiveness of the cleaning steps on specimens mounted using the two most common SIMS mounting methods (epoxy and indium). After identifying the most efficient cleaning method MI assemblages were cleaned and analyzed to test the effectiveness and achievable precision. While this presentation focuses on MI, the cleaning method developed will be generally applicable to other carbon measurements that suffer from poor repeatability due to carbon contamination.