Elemental analysis of individual fluid inclusions in minerals by Secondary Ion Mass Spectrometry (SIMS): Application to cation ratios of fluid inclusions in an Archaean mesothermal gold-quartz vein

Abstract

A SIMS technique using a CAMECA-4F Ion Microprobe has been developed to quantitatively analyse element ratios of individual, preselected fluid inclusions as small as 3 μm in diameter and as dilute as 0.37 mol% salts. The method, first attempted by Nambu et al. (1977), is destructive; fluid inclusions are opened under vacuum by sputtering through the host crystal with a 12.5 KeV beam of O − ions. Emitted secondary ions are analysed in a mass spectrometer, their intensities being proportional to the concentration of parent elements in the target inclusion. Whereas Nambu et al. (1977) analysed frozen inclusions, K Na and Ca Na ratios have been obtained in this study from inclusions in the liquid state, with a relative reproducibility ( 2σ x ̄ ) of 50–65 and 80–200%, respectively. The secondary ion yields of Na +, K +, and Ca + have been calibrated empirically using fluid inclusions synthesized with bulk salinities of 0.37–3.3 mol% and K Na and Ca Na ratios of 0.005–1.0. Thus individual gold-bearing fluid inclusions in quartz from the San Antonio Archaean gold deposit, Bissett, SE Manitoba, yield K/ Na = 0.036 + 0.04/−0.02 and Ca/ Na = 0.034 + 0.07/−0.03. The excellent spatial control of the primary ion beam permits individual inclusions to be opened without interfering with adjacent inclusions. The most powerful application of the method is therefore in the analysis of natural samples that contain multiple generations of fluid inclusions.