E. van Achterbergh

The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites

Zircon megacrysts represent a late stage in the crystallisation of the magmas that produced the low-Cr megacryst suite (Ol1Opx1Cpx1Gnt1Ilm1Phl1Zir) found in many kimberlites, and may carry information on the sources of the parent magmas and the interaction of these magmas with the cratonic lithosphere. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zircons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of mantle-derived Hf over a long time span. Most values of «Hf fall between 0 and 110, but zircon suites from several kimberlites range down to «Hf 52 16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial «Hf-«Nd array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3- 0.5) greater than estimated Bulk Silicate Earth. The depleted and metasomatised lherzolites and harzburgites that make up much of the Archean lithospheric mantle have Lu/Hf ratios (#0.15) low enough to account for the lowest «Hf observed in the zircons, over time spans of 1-3.5 Ga. We therefore suggest that the magmas from which the kimberlitic zircons crystallised were derived from Depleted Mantle or OIB-type sources, and developed negative «Hf through reaction with the subcontinental lithospheric mantle. Copyright

The NAC nuclear microprobe facility

The NAC NMP facility is discussed, showing that it is a well characterised analytical tool, and some novel features are incorporated, such as on-demand beam deflection and a new lid for the sample chamber.

Quantitative, high sensitivity, high resolution, nuclear microprobe imaging of fluids, melts and minerals

Samples of fluids and melts trapped and preserved as inclusions in growing minerals or healed fractures provide unique windows on a range of geological processes from mantle melting and metasomatism through to economic ore formation and remobilization. Recent advances in nuclear microprobe (NMP) technology at the CSIRO provide powerful tools for the study of these inclusions and associated mineral assemblages. These tools include a new NMP designed for high resolution and high sensitivity, PIXE analytical methods for quantitative imaging and analysis, and simultaneous PIGE imaging. The quantitative imaging and analysis methods are based on the dynamic analysis approach, which generates a fast matrix transform for projection of list-mode PIXE data onto pure elemental images. Recent advances provide rapid pixel-by-pixel correction for matrix and absorption effects in different (mineral) compositions across the image area to yield true quantitative images. These methods are combined in a software package called GeoPIXE II.

Quantitative PIXE trace element imaging of minerals using the new CSIRO-GEMOC Nuclear Microprobe

Abstract The design of the new CSIRO–GEMOC Nuclear Microprobe for high sensitivity imaging of mineral samples, combined with a new GeoPIXE software environment, incorporating the dynamic analysis (DA) method for projecting quantitative PIXE trace element images, yields quantitative PIXE images at 1.8 μm spatial resolution with detection limits in silicates for extracted concentrations down to ∼0.2 ppm (Br). Detection limits (99% confidence level) down to 39 ppb (Ge) have been achieved in diamond. Advances in PIXE technique include the correction of DA images for the effects on PIXE yields of spatial variation of sample composition, interactive software tools for DA projection and concentration extraction, and their integration with simultaneous PIGE imaging for light elements in a new efficient PC environment. Application areas include the analysis and imaging of melt inclusions, sulfides from ancient and modern settings, and fluid inclusions in minerals.

Microanalysis of ore-forming fluids using the scanning proton microprobe

Abstract Much of the uncertainty in our early method of microanalysis of fluid inclusions using PIXE stemmed from the heterogeneous internal structure of each inclusion, which typically contains a vapor bubble and daughter crystals, and the non-uniform beam dose distribution. This paper reports on recent work to improve the accuracy of quantitative analysis through the use of beam-scanning techniques which provide a controlled beam-dose distribution. Improvements have been made to the PIXE yield modelling to better account for large vapor bubbles, and all fluid inclusion analysis routines have been incorporated as standard features of the GeoPIXE software package. The method has been tested using synthetic fluid inclusions in quartz (solutions of Na, K, Zn, Rb and Cs chlorides). The results demonstrated an accuracy improved to ∼10–15%. Examples show the application of the method for the analysis of ore-forming fluids in magmatic hydrothermal systems.

Magmatic inclusions in the search for natural silicate-salt melt immiscibility: methodology and examples

Immiscible phase separation during the cooling and crystallisation of magmas is an inherently fugitive phenomenon and melt inclusions may provide the only remaining evidence of this process. We detail those features of such inclusions that can both prove the existence of immiscible phase separation, and constrain the compositional signature of the process. To do so requires the combination of traditional methods (petrographic examination, microthermometry, etc.) with state of the art microbeam analytical techniques (laser Raman spectroscopy and proton-induced X-ray emission). Examples of inclusions in phenocrysts from barren and mineralised rocks are provided to illustrate the approach and validate the interpretations.

Nuclear microprobe analysis of melt inclusions in minerals: Windows on metasomatic processes in the earth's mantle

Abstract Rare samples of melts and fluids, responsible for metasomatic change and evolution of the earths upper mantle, and preserved as melt inclusions, pose special problems to quantitative analysis. Elements such as Zr, Nb, Th and the rare-earth elements (REE) can be highly concentrated into rare minor phases beneath the surface and easily overlooked using surface analysis techniques. A more accurate approach uses PIXE and the deep penetration of MeV protons to sample inclusion content, including minor quench phases, to ∼40 μm depth. A method has been developed using the CSIRO–GEMOC nuclear microprobe to image the minor and trace element components of melt inclusions, and integrate all contributions across and into each inclusion, using the dynamic analysis method. Examples show that this approach can quantitatively image spatial variation in component elements at 1.8 μm spatial resolution (at 8 nA) and at levels of ∼2 ppm and determine melt composition with detection sensitivities down to 0.2 ppm.

Overlap corrected on-line PIXE imaging using the proton microprobe

Abstract Conventional methods for PIXE imaging have used the counts recorded in a simple energy window set on an X-ray peak to approximate the image of an element. This method produces image artefacts due to the overlap of the X-ray lines of interfering elements, detector artefacts, such as peak-tails, pile-up and escape peaks, and continuum background. This paper reports on continuing development to solve this problem, based on the construction of a matrix transformation that transforms directly from PIXE spectrum vector to elemental concentration vector. This transformation provides a fast method for extracting on-line quantitative estimates of the concentration of a sample while still under the proton beam; the method has been called Dynamic Analysis to reflect this capability. If the spectrum is replaced by a single count in a channel, an event recorded at a point in a raster scan of the proton beam over a target, then the resultant concentration vector is the set of increments to make to all elemental images at that pixel. PIXE images accumulated in this way are inherently (i) quantitative (accumulated in ppm μC), (ii) overlap-resolved and background-subtracted, and (iii) can be formed directly on-line. The method is under development at the CSIRO and in routine use at the NAC using a simple procedure involving the fitting of a preliminary scan spectrum to build the transform matrix; spectrum fitting and matrix construction are part of the GeoPIXE software package. This paper outlines the method, reports various tests of it, and presents recent examples of its application to major and trace element imaging in geology.