GEMOC ARC National Key Centre

Background

• The GAU contains:

• a Cameca SX-50 electron microprobe (SX-100 arrived in January 2003)

• a Hewlett Packard 4500 ICPMS (dedicated to U-Pb analysis)

• an Agilent 7500 ICPMS (industry collaboration)

• a custom-built UV laser microprobe, usable on either ICPMS

• two New Wave/Merchantek laser microprobes (266 nm and 213 nm) for the MC-ICPMS and ICPMS laboratories (industry collaboration)

• a New Wave/Merchantek excimer (193 nm) laser microprobe, based on a Lambda Physik OPTex laser

• a Nu Plasma multi-collector ICPMS

• a Spectro XLAB2000 energy-dispersive XRF with rocker-furnace sample preparation equipment

• a LECO H2O-CO2 analyser

• clean labs and sampling facilities provide infrastructure for ICPMS, XRF and isotopic analyses of small and/or low-level samples

• Experimental petrology laboratories in GEMOC include piston-cylinder presses (9, 15 and 40 kb), hydrothermal apparatus, and controlled atmosphere furnaces.

• The Centre for Isotope Studies provides access to thermal ionisation mass spectrometers for analysis of the Rb-Sr, Sm-Nd and U-Th-Pb systems, and extraction lines and gas-source mass-spectrometers for stable-isotope analysis of fluids and minerals.

The GEMOC Facility for Integrated Microanalysis (FIM) and Micro GIS development

GEMOC is continuing to develop a unique, world-class geochemical facility, based on in situ imaging and microanalysis of trace elements and isotopic ratios in minerals, rocks and fluids.  The Facility for Integrated Microanalysis now consists of four different types of analytical instrument, linked by a single sample positioning and referencing system to combine spot analysis with images of spatial variations in composition (“micro-GIS”).  All instruments in the FIM have been operating since mid-1999.  Major instruments will be replaced or upgraded in 2002-2004 through the $5.125 million DEST Infrastructure grant awarded to Macquarie University with the Universities of Newcastle, Sydney, Western Sydney and Wollongong as partners.

the facility provides:

• The capability to image both major- and trace-element distribution in a sample, as an interpretive tool and as the basis for higher pre

• Co-registration of images and spot data from different instruments, and use of digitised images to locate spots with a precision of better than 5 µm

• Analytical capability for most elements of the periodic table at ppm to sub-ppb levels

• In situ isotopic-ratio measurement for a range of elements, at the precision required for geologically useful results

• New approaches to data interpretation through application of micro-GIS principles

Electron Microprobe:  for imaging and point analysis of major and minor elements

Scanning Nuclear Microprobe:  for imaging and point analysis of trace elements at ppm levels

Laser-ablation ICPMS Microprobes:  for point analysis of a wide range of trace elements at low ppb levels

Multi-collector Sector ICPMS with laser microprobe:  for high-precision in situ analysis of isotopic ratios

Micro-GIS system:  A key aspect of the Facility is the co-registration of images and point analyses collected on all instruments.  All data for a sample, from any of the instruments or from a bench microscope, are in the same coordinate system and can be overlaid in the computer to enhance interpretation.  When fully developed, images from one instrument will be read into the computer of another instrument and used to guide the analysis.  Major-element maps from EMP, or trace-element maps from the nuclear microprobe, can be linked directly to images from petrographic or cathodoluminescence microscopes, BSE or SEM, or to spot analyses.  

current status

Electron microprobe (EMP):  The existing GEMOC EMP is a CAMECA SX50, installed in 1993; it routinely produces high-precision analyses of major and minor elements with a spatial resolution of one micron, as well as high-quality images of major-element (> 0.1 wt%) distribution over areas up to 45 x 45 mm, by stage-scanning with five fixed wavelength-dispersive spectrometers.  In early 1999 the EMP was upgraded with an energy-dispersive X-ray detector to allow rapid and simultaneous mapping of all major elements.  The EMP was replaced with a newer model that arived in late 2002 (see below).

Contemplating the Universe within the new Cameca electron microprobe.

Scanning nuclear microprobe (SNMP):  This instrument has been built by Dr C. G. Ryan (with GEMOC funding contribution) as a separate beam line on the HIAF particle accelerator at CSIRO, North Ryde.  The design incorporates several complementary types of detector, a new high-resolution probe-forming system and an innovative optical system, and provides both images of trace-element distribution and spot analyses, with a lateral resolution of 1-3 µm.  Current capabilities cover micro-PIXE, micro-PIGE and quantitative element imaging.  When fully developed, the capabilities will include:

(a)  micro-PIXE:  Proton Induced X-ray Emission (PIXE) using focused MeV proton beams provides non-destructive, simultaneous, quantitative microanalysis and imaging of the spatial distribution of trace elements with concentrations down to the ppm level.  The instrument provides simultaneous analysis and imaging of any elements present in a sample above the detection limits. Detection limits as low 0.2 ppm have been achieved in silicates, and 0.5 ppm in sulfides, extracted directly from the quantitative elemental images with 2 µm spatial resolution.

(b)  micro-PIGE:  Proton Induced Gamma-ray Emission (PIGE) spectra can be collected simultaneously with PIXE analysis.  The gamma-rays provide non-destructive analysis of light elements (eg Li, Be, B, F, Na) with detection limits of 10-200 ppm.  Other nuclear reaction techniques will provide analysis of anions (H, C, O, S, Cl) which are difficult to analyse by ICPMS and EMP.

(c)  CCM:  In Channelling-Contrast Microscopy (CCM), MeV energy beams of alpha particles or protons are channelled into the crystal planes and axes of minerals.  CCM can be used to yield images of lattice location information or to distinguish elements sited in inclusions from those in solid solution.  This capability can be applied to the trace levels detected using ion backscattering, micro-PIXE and micro-PIGE.

(d)  Quantitative True Elemental Imaging:  The SNMP provides trace-element imaging of areas up to 2.5 mm across; these images are digitised so that any pixel or larger area can be retrieved, and analysed separately for its element concentrations.  The Dynamic Analysis method developed by Dr Ryan enables live-time assembly of true elemental images that discriminate against pile-up, inter-element overlap, background and detector artefacts such as escape peaks and tailing.

Laser Ablation ICPMS microprobe (LAM-ICPMS):  The original GEMOC LAM was installed in December 1994 using a Perkin-Elmer ELAN 5100 ICPMS (later replaced by an ELAN 6000), attached to a UV laser ablation microprobe built for GEMOC by Memorial University, Newfoundland.  In 1999 the ICPMS was replaced by a Hewlett Packard 4500, and in 2000 an Agilent 7500 ICPMS was added.  These two instruments now routinely provide quantitative analyses of > 30 elements at sub-ppm levels in minerals, glasses and metals, as well as precise U-Pb dating of zircons.  Spatial resolution varies with the application, but typically is on the order of 30-40 µm.  The LAM is fitted with a computer-driven sample stage to provide co-registration of X-Y coordinates with the other instruments.  On-line data reduction with the GEMOC-developed “GLITTER” software enhances laboratory productivity and data interpretation; the software is marketed internationally by New Wave Research.

Rosa Maria Bomparola (University of Siena) using in situ Hf-isotope analysis to study granite genesis (see Research Highlights).

Multi-collector LAM-ICPMS microprobe (MC-LAM-ICPMS):  A fully-equipped Nu Plasma MC-ICPMS is an integral part of the Facility.  This instrument combines a laser ablation microsampler, an Ar-plasma ionisation source, and a multi-collector magnetic-sector mass spectrometer, to provide high-precision in situ analysis of isotope ratios in geological materials.  The MC-ICPMS also can be used in solution mode, with either a standard nebuliser or a desolvating nebuliser, to provide high-precision isotopic analysis of a wide range of elements, including many not accessible by standard thermal ionisation mass spectrometry.  A second Nu Plasma instrument was ordered early in 2003.

Applications in use and under development include:

Laser Analysis (in situ point analysis)

• Hf isotope analysis in zircon for studies of crustal generation, mantle evolution and crust-mantle interaction

• Re-Os dating of sulfides in mantle-derived xenoliths

• Nd isotope analysis in apatites, titanites and other REE-rich minerals

• Sr isotope analysis of carbonates, feldspars, apatites, pyroxenes

• Pb isotopes in sulfides and silicates

• Stable isotope ratios of Fe, Mg, Zn, Cu and other cations in appropriate minerals from hydrothermal systems and mantle rocks

Solution Analysis

• Re-Os — determination of mantle depletion ages and isochron ages in whole rocks, ilmenites and chromites; dating of sulfide assemblages in ore bodies

• Lu-Hf — crustal genesis, mantle metasomatism; Lu-Hf dating of garnet peridotites, eclogites, granulites; basalt genesis

• Rb-Sr, Sm-Nd, U-Pb, Pb-Pb — faster and simpler than TIMS; simplified low-blank chemistry, no time-dependent mass fractionation, hence greater precision
  

progress in 2002

1. Facility for Integrated Microanalysis

a.    Electron Microprobe:  After extensive testing at laboratories in France, an order was placed for a fully optioned Cameca SX-100 electron microprobe with five crystal spectrometers and an energy-dispersive spectrometer, to replace GEMOC’s aging, but still highly functional, SX-50 instrument.  The new instrument arrived in late December 2002, and was installed in January 2003.

b.     Laser-ablation ICPMS microprobe (LAM):  During 2002, the LAM laboratory produced large volumes of data for eight Macquarie PhD thesis projects, several projects carried out by international visitors and Honours students, in-house funded research projects and industry collaboration.  These projects included the analysis of trace elements in the minerals of mantle-derived rocks, in sulfide minerals (with James Cook University) and in a range of unusual matrices including laser crystals, teeth, stromatolites and otoliths (the ear bones of fish).  Over 5000 U-Pb analyses of zircons were carried out, related to projects (including TerraneChron^TM applications) in South America, Scandinavia, Antarctica, Italy, Iran, China and Australia.  The LAM laboratory also routinely provides data for projects related to mineral exploration (diamonds, base metals, Au), as a value-added service to the industry.  New developments included further improvements in the quantitative in situ analysis of diamonds, with the start of Sonal Rege’s PhD project, supported by DeBeers, on the trace-element signatures of diamonds (see Research Highlights).

The 4500 instrument is now dedicated primarily to U-Pb dating of zircons, where its stability and ease of operation provide a high throughput.

c.    MC-ICPMS:  A multi-collector magnetic sector ICPMS for in situ (laser-ablation) and solution analysis of isotopic ratios was installed in November 1998.  The instrument is the Nu Plasma, designed and manufactured by Nu Instruments of Wrexham, UK.  The instrument was producing good data only a few days after installation, and has continued to do so.  Merchantek EO (now New Wave Research) have provided a 266 nm UV laser microprobe (under a collaborative agreement; see below) for use with the MC-ICPMS and a 213 nm laser microprobe was purchased in 2000.  During 2002 the MC-ICPMS was fitted with a New Wave/Merchantek excimer (193 nm) laser microprobe, based on a Lamba Physik OPTex laser.  This has been used mainly for the analyis of Hf isotopes in zircon, where its different absorption characteristics have provided somewhat greater spatial resolution and beam intensity than were available using the 213 nm laser.

Major applications during 2002 (see Research Highlights) included the high-precision analysis of Hf in zircons to trace lithosphere evolution and magma-mixing histories in granitic rocks, the analysis of copper and iron isotope compositions in minerals from ore bodies, the analysis of Sr isotopes in carbonate fossils for stratigraphic control, and Re-Os dating of single grains of Fe-Ni sulfides in mantle-derived rocks.  We carried out a major Re-Os study of sulfide assemblages in xenoliths and xenocrysts from the Kaapvaal craton; other Re-Os studies included alpine-type peridotites from the Norwegian and Swedish Caledonides, and xenoliths from the Kerguelen oceanic plateau, eastern China and Taiwan.  A collaborative research project, with five industry sponsors, studied the stable-isotope variation in copper and iron in several ore deposits and was completed in 2002.  A study of U-Pb and Hf-isotope systematics of zircons in late-Proterozoic sediments in southern Norway provided a new view of the tectonics of southern Scandinavia.

A major development in 2002 was the in-situ analysis of Mg isotope compositions in olivine and other phases in mantle-derived peridotites, and the identification of significant fractionation apparently related to melting and metasomatism in the upper mantle (see Research Highlights).  Part of this work involved the definition of isotopic compositions for olivine in meteorites, using samples loaned by the West Australian Museum.

    The rapid growth in the use of the TerraneChron^TM application, with its use of in situ Hf-isotope analysis, by academia and industry (Research Highlights, coupled with the demand for in situ Re-Os analysis and stable isotope analysis, has led to competition for instrument time on the MC-ICPMS.  An order was placed early in 2003 for a second instrument, funded by the DEST infrastructure grant.

d.    Scanning Nuclear Microprobe:  Development of the SNMP in 2002 has concentrated on improving the quantitative trace-element imaging methods and developing the GeoPIXE II software for interactive exploration of the multielement datasets.  Quantitative proton induced X-ray emission (PIXE) imaging now includes user-friendly interfaces for full pixel-by-pixel matrix correction of images to produce quantitative images, even in cases involving strong matrix effects.  Differential absorption, the effect of strong contrasts in X-ray absorption between neighbouring pixels, is now corrected with an extension of this simple interface.  For example, this approach eliminates artefacts such as apparent enhancement of elements at the edge of a phase adjacent to a lighter phase with low absorption.  Non-linear effects are now being tackled.  The first success has been a general approach for treating pile-up and its effects on spectra and images.  Pile-up is inherently non-linear, related to the product of X-ray line intensities.  Hence, the nature of pile-up varies from pixel to pixel reflecting changing major element composition.  Methods have been developed to enable the modelling of PIXE spectra to account for this variation over an image area.  Recently, this has been extended to enable subtraction of pile-up related artefacts in elemental images.  Development of a general approach to proton induced gamma-ray emission (PIGE) imaging is continuing with a method for F and Na in place and evaluation for the quantitative analysis of these elements in fluid inclusions is underway.

e.    Laboratory development:  Funding from the DEST infrastructure grant will contribute toward the building of a new suite of clean-room laboratories on the second floor of building E5B, which will include facilities for the work on U-series chemistry to be carried out by Dr Simon Turner and his group starting in 2003.  At year’s end, planning was well advanced, and construction will begin in the first half of 2003. 

f.    Software:  Esmé van Achterbergh and Chris Ryan further refined the GLITTER (GEMOC Laser ICPMS Total Trace Element Reduction) software, our on-line interactive program featuring linked graphics and analysis tables.  This package provides the first real-time interactive data reduction for LAM-ICPMS analysis, allowing inspection and evaluation of each result before the next analysis spot is chosen.  In 2000, its capabilities were expanded to include on-line reduction of U-Pb data.  The use of GLITTER has greatly increased both the flexibility of analysis, and the productivity of the laboratory.  New Wave Research now market the software together with their laser microprobe equipment; GEMOC provides customer service and backup through Macquarie Research Limited.  Fourteen copies of GLITTER were sold worldwide in 2002, and the sales rate is increasing steadily, to the point of achieving industry-standard status.

2.  Laser development

GEMOC continues to benefit from an industry partnership with New Wave Research (formerly Merchantek EO), a major US manufacturer of laser ablation systems, which has made Macquarie its Alpha Test Site.  New Wave has donated their 266 nm Nd:YAG UV laser ablation sampling system to GEMOC and their new 213 nm system was delivered early in 2000.  Both lasers can be coupled to the Nu Plasma MC-ICPMS, allowing high precision isotope ratio determinations to be performed on minerals in situ.  The mobility of the probes has allowed them to be used on the quadrupole ICPMS instruments as well, in a range of applications.  A Merchantek/New Wave Research 193 nm excimer system based on a Lambda Physik OPTex laser was delivered in March 2002, but had to be replaced, and was finally commissioned late in the year.

During 2002, further tests were carried out on the stability and element-fractionation characteristics of the New Wave Research 213 nm laser, which showed that it produces less U/Pb fractionation than the 266 nm laser.  Its absorption characteristics also are superior, which allows the use of a smaller ablation pit for the same sensitivity, and gives longer and more stable runs.  As a result, the 213 nm laser is now used for most of the U-Pb work, especially where small grains are being analysed.

During a sabbatical leave at ETH, Zurich in 2002, Simon Jackson studied the processes that give rise to laser induced isotopic fractionation.  This work (see Research Highlights) showed that the particle size distribution within the sample aerosol has a profound effect on the accuracy and precision of measured isotope ratios due to isotopic fractionation during incomplete vaporisation of large particles (> ca 0.5 µm) in the ICP.  It was demonstrated that filtering large particles from the aerosol very substantially reduced the bias, and improved analytical precision by (1) reducing plasma noise caused by large particles, (2) reducing ablation time-dependent change in ratios related to changing particle size distribution.

3.  Energy Dispersive XRF

A Spectro XLAB2000 energy-dispersive X-ray spectrometer was installed in November 2000 in a joint venture with Tasman Resources.  Like the earlier model instrument installed in the laboratory at the ANU, this utilises the polarisation of scattered X-rays to substantially reduce backgrounds and enhance detection limits.  The XLAB2000 uses a specially designed 300 W palladium X-ray tube that improves its performance for the lighter trace elements, and also for major elements, relative to the earlier instrument.  In addition, this spectrometer is fitted with a silicon detector of a type recently developed that eliminates the low-energy “tail” from the lightest elements, and enables all major elements to be measured in a fused glass to levels below 0.01%.  The capability of the instrument to provide major element analyses of a quality at least comparable to the more expensive conventional crystal spectrometers has been thoroughly evaluated and confirmed.  This instrument will provide highest-quality data for major elements and for most trace elements to sub-ppm levels.  The operation of the equipment is enhanced by a 100 position sample loader, one of the first to be installed on a Spectro instrument, and the purchase of a rocker furnace for sample preparation.  During 2002 over 5000 samples were analysed for major and trace elements, providing data to student theses, in-house research projects, and industry collaborators.

4.  Solution ICPMS analysis

The Agilent  7500 ICPMS is regularly used to provide trace-element analyses of dissolved rock samples for the projects of GEMOC researchers and students, and external users, supplementing the data from the XRF.

The in situ analysis of the Rb-Sr, Lu-Hf, Sm-Nd and Re-Os systems by laser ablation microprobe has required the development of corrections for isobaric overlaps (eg 87Rb on 87Sr), and has demonstrated that these corrections can be done with very high precision in the Nu Plasma MC-ICPMS.  This has opened the possibility of simplifying the ion-exchange chemistry traditionally used to obtain clean element separations for standard mass-spectrometry analysis.  During the year we further developed and modified a scheme for the dissolution of rocks and separation of Sr, Nd, Hf and Pb for isotopic analysis using the MC-ICPMS in solution mode.  This provides precise whole-rock isotopic analyses that are faster, simpler and ultimately cheaper than those obtained by traditional methods.

During 2002 Dr Stuart Graham put a major effort into further development of methods for extracting Re and Os from rock samples and the analysis of the Os samples on the MC-ICPMS by sparging the oxidised Os directly into the ICPMS torch.  The method now routinely provides analyses with a precision and accuracy comparable to the best TIMS analyses, but much more rapidly.  The technique was applied to suites of mantle-derived peridotite and eclogite xenoliths from the Kaapvaal and Slave cratons, and to the Re-Os dating of sulfides from ore deposits. 

5.  Centre for Isotope Studies (CIS)

The Centre for Isotope Studies (CIS) is a consortium operated by the geoscience departments of the New South Wales Universities, CSIRO Exploration and Mining, and Petroleum Resources using jointly-purchased mass-spectrometers housed at the CSIRO in North Ryde.  The facility allows staff and students to obtain both radiogenic and stable isotopic analyses and uses technical staff jointly funded by the University members under an agreement arranged by Professor Peter Bergquist (Deputy Vice-Chancellor Research Macquarie University) and Dr Richard Flood (University Consortium Convenor).