GEMOC ARC National Key Centre

LASER ABLATION MICROANALYSIS AT ANU

Stephen Eggins¹ and Richard Arculus¹.

1. GEMOC ANU

The Helex laser ablation microanalysis system, a ANU development jointly undertaken between the Department of Geology and Research School of Earth Sciences, is providing new opportunities to address a wide range of geochemical processes and problems, particularly in regard to the evolution and metallogeny of subduction zone magmas. The Helex system, based on precision laser micromachining technology using near vacuum ultraviolet (193nm) light, is able to vaporise and sample stoichiometrically virtually any material (aluminosilicate, sulfide, carbonate, oxide or metal) and to sweep the condensed particulates via a gas stream into an Ar plasma for analysis by emission spectroscopy (ICPES) or mass spectrometry (ICPMS). The Helex system can sample sites of any shape within the dimensions 400µm by 1mm and as small as 10µm, and has the capability to analyse any line profile or array pattern under computer controlled rastering. This inherent flexiblity affords an unrivalled range of bulk, site microsampling and mulitdimensional compositional profiling analytical tasks. Moreover, the superior sensitivity attained with the Helex system enables low and sub ppb level detection limits for virtually any element, even on very tiny spots (<50µm), with the simultaineous analysis of 20 to 30 elements requiring only 1 minute. Several outstanding examples where the Helex system has been applied to subduction zone magmas include:

MARIANA AND IZU-BONIN ARC ASH RECORDS

Measurement of a set of petrogenetic trace elements in microscopic basaltic to rhyolitic glass shards recovered as ash layers from ODP drilling sites outboard of the Mariana and Izu Bonin arcs has enabled Colleen Bryant and Richard Arculus to construct a complete spatial and temporal chronology of the development of these arc system since their inception in the Eocene. Distinctive breaks and subtle evolution in glass shard chemistry, that accompany the initiation, development and cessation of discrete intervals of back arc basin spreading, are providing a picture of the dynamics of subduction zone systems and enabling fundamental aspects of magma generation in subduction zones to be understood.

IN-SITU PGE, GOLD AND RHENIUM ANALYSIS

Ultratrace levels of Platinum group elements, Re and Au, are able to be analysed in situ using the Helex system. This capability has been used to establish the occurrence of anomallously high levels of many of these elements in basaltic glasses from subduction zones compared to other tectonic settings. In addition, the analysis of silicate and oxide phenocrysts, metal and sulfide phases, is enabling the emergence of a coherent picture of the behaviour of these elements in evolving magmatic systems.

TRACE ELEMENTS IN TRAPPED MELT AND FLUID INCLUSIONS

The spatial resolution and sensitivity of the Helex systems enables tiny glass, brine, or fluid inclusions trapped within host crystals to be analysed for trace lithophile, chalcophile and siderophile elements. In the case of glass inclusions in phenocryst phases, we are able to 'see into' the chemical inventories and behaviour of elements within magma reservoirs at depth unhindered by the unavoidable loss of a magmatic volatile phase and loss of metals that affect erupted magmas. To date, high levels of dissolved sulfur and certain ore forming metals are being documented in subduction zone magmas, and evidence is present for their systematic loss from these magma systems at depth. Further studies are being aimed at elucidating systematics in the behaviour and concentrations of these elements as magma system evolve across the range of compositions observed in subduction zones.

Acknowledgements: Michael Shelley and Les Kinsley have been instrumental in the development and commissioning of the prototype Helex laser ablation system at RSES, ANU, and with current construction and development of the next generation system for installation at the Department of Geology, ANU. Colleen Bryant kindly provided the Mariana and Izu-Bonin ash data and her interpretations for presentation.