GEMOC - Department of EPS - Faculty of Science

The GEMOC Facility for Integrated Microanalysis

W.L. Griffin^1,2, N.J. Pearson¹, C.G. Ryan² and Suzanne Y. O'Reilly¹

1. ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Macquarie University, NSW 2109, Australia
2. CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia

Geochemical studies of the crust and mantle are at the core of GEMOC's strategic plan, and the maximum amount of information can be obtained where geochemical data on a rock sample can be placed in a mineralogical and spatial context. To meet these needs, GEMOC is developing a unique geochemical facility, based on in-situ imaging and microanalysis of trace elements and isotopic ratios in minerals and rocks. The Facility for Integrated Microanalysis (FIM) consists of a number different analytical instruments, linked by a single sample positioning/referencing system to combine spot analysis with images of spatial variations in composition ("micro-GIS").

The FIM provides:

The capability to image major- and trace-element distribution in a sample, both as an interpretive tool and as the basis for higher precision spot analysis of trace element concentrations and isotopic ratios.

Co-registration of images and spot data from different instruments; use of images to locate spots with a precision of better than 5 microns (micro-GIS).

Analytical capability for most elements of the periodic table at the ppm-ppb level.

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

The FIM consists of four major items of equipment, and the micro-GIS system:

(1) Electron Microprobe: (CAMECA SX-50) with full five-spectrometer WDS and EDS capabilities, for spot analysis and imaging of major- and minor elements over areas up to 45 x 45 mm with a spatial resolution of one micron.

(2) Scanning Nuclear Microprobe (SNMP): a dedicated beam line on the HIAF tandem electrostatic accelerator at CSIRO Exploration and Mining, North Ryde. The SNMP incorporates several complementary types of detector, a new high-resolution probe-forming system and an innovative optical system, and is designed to provide both images of trace-element distribution and spot analyses, with a lateral resolution of 1-3 microns. Capabilities include:

micro-PIXE: Proton Induced X-ray Emission (PIXE) using focused MeV proton beams provides non-destructive, simultaneous, quantitative microanalysis of trace elements with concentrations down to the ppm level.

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. PIGE also provides analysis of anions (H, C, O, S, Cl) which are difficult to analyse by ICPMS and EMP.

Channeling-Contrast Microscopy (CCM): MeV-energy beams of alpha particles or protons are channelled into the crystal planes and axes of minerals, 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.

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 at the CSIRO uses a matrix transform to project elemental concentrations directly from a PIXE spectrum. This 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. This technique can also generate light-element images in live time from PIGE data collected simultaneously with PIXE.

(3) Laser Ablation ICPMS microprobe (LAM): The GEMOC LAM uses a Perkin-Elmer ELAN 6000 ICPMS, attached to a UV laser ablation microprobe built for GEMOC by Memorial University, Newfoundland. The instrument routinely provides quantitative analyses of >25 elements at sub-ppm levels in minerals, glasses and metals. Spatial resolution varies with the application, but typically is on the order of 30-40 microns, and a typical analysis takes ca 5 minutes. The LAM uses a computer-driven sample stage to provide co-registration of X-Y coordinates with the other instruments. GEMOC's unique GLITTER software provides real-time interactive on-line data reduction for the LAM-ICPMS.

(4) Multi-collector magnetic-sector LA-ICPMS microprobe (MC-LAM-ICPMS):

The recently delivered Nu Plasma MC-ICPMS combines a laser ablation microsampler, an Ar-plasma ionisation source, and a multi-collector magnetic sector mass spectrometer into an instrument specifically designed for high-precision in-situ analysis of isotope ratios in geological materials. Projected applications include U-Pb dating of zircons and other U-bearing phases, Hf-isotope analysis of zircon for crustal-genesis studies, Sr-isotope analysis of carbonates, apatites and clinopyroxenes in mantle xenoliths, and studies of stable isotope ratios of lighter elements in ore-forming systems.

(5) Micro-GIS system: An important aspect of the Facility for Integrated Microanalysis 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 taken in the same coordinate system and can be brought together or overlaid in the computer to enhance interpretation. In the near future, we will be able to read images from one instrument into the computer of another instrument and use the image to guide the analysis. For example, we will be able to direct the LAM into a feature observed in a micro-PIXE image, to obtain sub-ppm trace element data or isotopic analysis at that point. Trace-element distributions across a feature observed on the EMP can be imaged using micro-PIXE and micro-PIGE on the nuclear microprobe, and both may be compared directly with optical images, including those obtained in other instruments such as the cathodoluminescence microscope.