The Essentials of Alkali Metals: A review of the field and advances towards a multi-element method

Abstract

Alkali metal isotopes have become valuable geochemical tools in the last 20 years. Their simple chemistry with only one oxidation state, moderately volatile nature and lithophile nature make them ideal candidates to investigate processes across different settings. Their applications have varied from indicator of nebular and planetary differentiation, to weathering and anthropogenic input tracer. This development in the three multi- isotope alkali metals Li, K and Rb, can be mostly attributed to the success of multi- collector inductively coupled plasma mass spectrometry (MC-ICP-MS). At the advent of this method, sample purity was not considered paramount. However soon after, column chromatography became an essential part of sample preparation to ensure high purity. This study takes a multi-element approach to chromatography. The column chromatography protocol outlined here enables the collection of the three alkali metals with low backgrounds and high recovery rate. The chromatography is flexible to allow scaling up for larger sample sizes and modification to collect a single alkali metal or other element of interest. In this thesis, Li and Rb MC-ICP-MS capabilities at the Research School of Earth Sciences (ANU) were expanded, with particular focus on Rb. A literature comparable long term precision of in-house reference solution against itself was achieved, delta = +-0.06 on Rb. Wash protocols, sample-reference mismatch and matrix effects were investigated for Rb; along with some terrestrial samples (basalt, granite, sediments and an aqueous metamorphism mineral). The wash protocol outlined is a clean, potent and easy solution to persistent memory buildup; which may be trialed on other alkali metals. The key findings of this thesis are the following: Sample-reference mismatch is not of significant concern, with diminishing effect towards higher concentrations. Results point in general to matrix effects at Element/Rb > 1, with few exceptions. Terrestrial Rb isotopic compositions hint towards aqueous alteration fractionating heavier values, in agreement with current literature. Show more