The application of laser ablation ICP-MS to the analysis of volcanic glass shards from tephra deposits: bulk glass and single shard analysis

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been applied to the chemical analysis of fine-grained (125–250 μm) volcanic glass shards separated from tephra deposits. This has been used both for bulk sample analysis and for the analysis of individual shards. Initial work concentrated on the use of an infra-red (IR) laser operating at 1064 nm, which gave craters of the order of 200 μm and was suitable for the analysis of bulk samples. This technique requires of the order of 80 μg of sample to determine a full suite of trace elements. Modification of the laser optics to enable operation in the ultra-violet (UV, at 266 nm) enables craters between 5 and 50 μm diameter to be produced, and the UV laser couples better with glass than the IR laser. We have applied this UV laser system to the analysis of single shards from Miocene tephra deposits from the Ruby Range in south-west Montana. Detection limits are below 1 ppm for a wide range of petrogenetically significant elements, but are critically dependent upon operating conditions. Calibration is achieved using synthetic multi-element glasses, with internal standardisation provided from electron probe analyses. Analysis of single shards provides a wide range of data from a single sample, enabling (i) magmatic evolution to be discerned within one eruption and (ii) the identification of separate populations of shards within one deposit which may not be apparent from the electron probe data. In this paper we will present a summary of the techniques used for both bulk sample and single shard analysis and compare some new bulk analyses with analyses of glass derived from other analytical methods.