Halogens in Earth Sciences: analysis and applications

Abstract

The halogens (Cl, Br, I and F) influence and trace the distribution of other volatile species and incompatible elements amongst the Earth’s geochemical reservoirs. Halogens play an important role in the transport and precipitation of ore-forming metals and are key for the exploration of energy critical elements. The concentrations of halogens in hydrothermal fluids and volcanic gases represent valuable tools for monitoring sub-volcanic processes at active volcanoes. This research developed a new Laser Ablation-ICP-MS method for the determination of Cl and Br in apatite, scapolite and silicate glasses. The technique offers rapid, spatially resolved data acquisition using common instrumentation and the commercially available data reduction software Iolite. Protocols were developed using five well known apatite occurrences, four gem quality scapolites and fourteen widely distributed silicate glasses. Cross-calibration of selected samples was achieved using combustion ion chromatography (CIC). Results are compared with available published data obtained using LA-ICP-MS, SIMS, EMPA, TXRF, INAA and the Noble Gas method to offer insights into reference material homogeneity, polyatomic interferences and recommendations for analytical ‘best practice’.

 

Halogen data were acquired using a Teledyne Photon Machines Analyte G2 193 nm Excimer laser ablation system coupled to a Thermo Scientific iCAP Qc ICP-MS. Extended analyte dwell times of up to 250 ms were employed to improve sensitivity and signal stability. Duty cycles of 350 – 400 ms can be achieved though the selection of a restricted analyte list (e.g. 35Cl, 81Br and 43Ca or 29Si as internal standard), with external standardisation using matrix matched reference materials. The implementation of a new interference correction, integrated within the data reduction scheme, provides limits of determination as low as 100 µg/g Cl and 10 µg/g Br.

 

The application of halogen systematics to research in the Earth Sciences has expanded rapidly over the past decade. This research method demonstrates how halogen geochemistry is able to provide valuable insights into the volatile inventory of un-degassed mantle melts, sediment provenance and tracing recycled subduction reservoirs in the mantle.

Biography

John Caulfield completed his PhD at Macquarie University (Sydney) where he conducted research into the timescales of mantle metasomatism, melt transport and magma differentiation in the intra-oceanic Tonga arc. He has worked extensively in interdisciplinary research teams to develop conceptual models for groundwater flow in poorly productive hard rock aquifers, and to investigate the environmental impacts of Unconventional Gas Exploration and Extraction (fracking). His current research focuses on the origin of geochemically anomalous enriched magmas in complex convergent margin settings, and constraining the changes in the mode of melt generation in subduction zones during the Neoarchean.