G.F. Suter

Quantitative pixe microanalysis of geological matemal using the CSIRO proton microprobe

Abstract The proton microprobe opens up new areas of geoscience research, that demand an efficient quantitative PIXE analysis method for trace element levels down to the limits of detection. The method developed at the HIAF laboratory reliably treats statistical fluctuations to provide quantitative PIXE microanalysis down to these levels. The software package (GEO-PIXE) that supports this standardless analysis procedure includes secondary X-ray fluorescence, and is structured to process the large numbers of analyses required in many geoscience applications. Analyses of standard rocks demonstrate the accuracy of the method, and highlight the problems associated with the use of standards in trace element analysis.

Ni in chrome pyrope garnets: a new geothermometer

Proton microprobe analyses of the minerals in garnet-peridotite xenoliths from kimberlites show that the partitioning of Ni between chrome pyrope garnet and olivine is strongly temperature(T)-dependent. The range of Ni contents in olivines is small relative to that in the analyzed garnets; a geothermometer therefore can be derived, based only on the Ni content of garnet. This allows estimation of T for single Cr-pyrope grains, such as the inclusions in diamonds, if these can be assumed to have equilibrated with olivine.

Trace-element zoning in garnets from sheared mantle xenoliths

Proton-microprobe analyses of garnets from sheared high-temperature ultramafic xenoliths reveal marked zonation of trace elements, paralleling trends in major and minor elements. Garnet rims (600–1000 μm wide) are enriched in Fe, Ti, Zr, Y and Ga, and either enriched or depleted in Cr, relative to cores. Zoning profiles for Ti and Zr are S-shaped and extend further into the grains than the Cr and Ga gradients. The profiles are consistent with the formation of Ti, Zr, Y-enriched garnet overgrowths, followed by diffusive equilibration between rim and core over years to hundreds of years. This enrichment in Fe, Ca, Al and incompatible elements is ascribed to melt infiltration and consequent melt-crystal exchange and garnet growth, shortly before eruption. Zr/Y is 1 to 2 in garnet cores but 4 to 5 in rims, and so the infiltrating melt may have been relatively alkalic. Major and trace element concentrations in such high-temperature sheared xenoliths are not likely to resemble those of primative mantle or of residual mantle depleted by melt extraction.

Conditions of diamond growth: a proton microprobe study of inclusions in West Australian diamonds

Crystalline primary inclusions in diamonds from the Argyle and Ellendale lamproites have been analyzed for Mn, Ni, Cu, Zn, Ga, Pb, Rb, Sr, Y, Zr, Nb, Ta, Ba and Mo by proton microprobe. Eclogite-suite inclusions dominate at Argyle and occur in equal proportions with peridotite-suite inclusions at Ellendale. Eclogitic phases present include garnet, omphacitic clinopyroxene, coesite, rutile, kyanite and sulfide. Eclogitic clinopyroxenes are commonly rich in K and contain 300–1060 ppm Sr and 3–70 ppm Zr: K/Rb increases with K content up to 1400 at 0.7–1.1% K. Rutiles have high Zr and Nb contents with Zr/Nb=1.5–4 and Nb/Ta ∼16. Of the peridotite-suite inclusions, olivine commonly contains > 10 ppm Sr and Mo; Cr-pyropes are depleted in Sr, Y and Zr, and enriched in Ni, relative to eclogitic garnets.Eclogite-suite diamonds grew in host rocks that were depleted in Mn, Ni and Cr, and enriched in Sr, Zn, Cu, Ga and Ti, relative to Type I eclogite xenoliths from the Roberts Victor Mine. Crystallization temperatures of the eclogite-suite diamonds, as determined by coexisting garnet and clinopyroxene from single diamonds, range from ∼1085 to ∼1575° C. Log KD (Cicpx/Cignt) varies linearly with 1/T for Zr, Sr and Ga in most of the same samples. This supports the validity of the temperature estimates; Argyle eclogite-suite diamonds have grown over a T range ≥400° C. Comparison with data from eclogite xenoliths in kimberlites suggests that KDSrand KDZrare mainly T-dependent, while KDGamay be both temperature-and pressuredependent. KDNi, KDCuand KDZnshow no T dependence in these samples.In several cases, significant major-and/or trace-element disequilibrium is observed between different grains of the same mineral, or between pyroxene and garnet, within single diamonds. This implies that these diamonds grew in an open system; inclusions trapped at different stages of growth record changes in major and trace-element composition occurring in the host rock. Diamond growth may have been controlled by a fluid flux which introduced or liberated carbon and modified the composition of the rock. The wide range of equilibration temperatures and the range of composition recorded in the inclusions of single diamonds suggest that a significant time interval was involved in diamond growth.

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.

Trace element geochemistry of ilmenite megacrysts from the Monastery kimberlite, South Africa

Abstract Ilmenite megacrysts in the Monastery kimberlite occur both as discrete monomineralic crystals and intergrown with all the other phases of the Cr-poor megacryst suite (cpx, opx, garnet, Fe-rich olivine, phlogopite and zircon). The ilmenites show systematic variations in trace element content which are interpreted in terms of a fractional crystallization model. Covariation of major and trace elements in the ilmenites with respect to their Nb content defines smooth curves, with breaks in the trends corresponding to changes in the inferred cumulate assemblage. Nb apparently behaved as an incompatible element in the megacryst magma throughout its crystallization history, and the Nb content of ilmenite serves as a useful fractionation index. After the appearance of ilmenite, the crystallization sequence of the Monastery megacryst suit is ilmenite+cpx+garnet+opx, followed by ilmenite+phlogopite, then ilmenite+zircon±phlogopite and finally by ilmenite+zircon+olivine+phlogopite. The incompatible behaviour of Nb indicates that ilmenite was overall never a predominant phase in the cumulate assemblage. Elevated Cr contents in late-stage ilmenites cannot be explained by the simple fractional crystallization model, and may require another process such as magma mixing or magma reaction with wall rock. The parent megacryst magma must have been highly magnesian and enriched in incompatible trace elements and in this respect may have been similar to meimechite. The late differentiate of this magma cannot be kimberlite, but must be undersaturated and high in iron, potassium, titanium and incompatible elements.

MICROBEAM AMS: PROSPECTS OF NEW GEOLOGICAL APPLICATIONS

Abstract AMS applications in geology have hitherto concentrated on the use of cosmogenic isotopes and rate in-situ produced isotopes for geomorphological and geophysical studies. Special features of AMS lend themselves to more general applications to other isotopes and geochronological systems. In-situ measurements in geochronology carried out with ion-microprobes are restricted by isobaric and molecular mass interferences to special systems where the problem is minimal. AMS can be used to alleviate this mass interference problem, and opens up the prospect of a less restrictive in-situ microanalysis for geochronology. At CSIRO, a microbeam AMS system designed to achieve this capability is under construction. With this system, several interesting applications such as the ReOs system became accessible more conveniently. The UPb system becomes accessible for hydrous minerals, and RbSr systems for Rb-rich samples. In addition, microbeam AMS allows determination of trace elements at lower levels than those accessible with the proton microprobe. This paper discusses these prospects and describes the AUSTRALIS system (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) being developed at CSIRO.

Selenium and its importance to the study of ore genesis: the theoretical basis and its application to volcanic-hosted massive sulfide deposits using pixeprobe analysis

Distribution coefficients for the partitioning of selenium between aqueous fluids and sulfide minerals have been estimated from existing thermodynamic data. These data indicate that selenium levels in pyrite increase at lower temperature for a given fluid H2SeH2S ratio. Consideration of the aqueous speciation of selenium and sulfur indicates that above 200°C, H2SeH2S approximates ΣSeΣS in most hydrothermal fluids. Using pixeprobe data for pyrite from eastern Australian volcanic-hosted massive sulfide deposits, the ΣSeΣS ratio of zinc-rich ores is below 1.5 × 10−6, whereas the ΣSeΣS ratio of copper-rich ores is 7–300 × 10−6. Using ΣSeΣS ratios of 10−7 for sea water and 10−4 for fluids with magmatic-derived sulfur, zinc-rich ores contain minimal ( 7%) magmatic-derived sulfur.

The proton microprobe: a revolution in mineral analysis

Abstract Application of the proton microprobe as a quantitative tool for trace-element microanalysis in the geosciences can be considered to have crossed the threshold of acceptability in a number of areas, particularly in igneous and metamorphic mineralogy and petrology. In the minerals industry, applications in base metal ore mineralogy provide new data useful for both processing and genetic studies. Applications in diamond and gold exploration are developing into new methods, with potential widespread acceptance. The paper will review a few case histories, and discuss the limitations of the current state-of-art and conditions conducive to widespread acceptance by geoscientists and by the minerals industry.

Microbeam recoil detection for the study of hydration of minerals

The elastic recoil detection technique in the glancing angle geometry is ideally suited for hydrogen content determination of thick geological samples, obviating the need for elaborate sample preparation such as that required for the transmission mode. With an appropriate scheme for locating the sample, the technique can be applied generally with a microbeam. Combined with simultaneous PIXE measurements, problems arising from uncertainties in beam charge collection are eliminated. The method is applied to the investigation of the hydration characteristics of silicates, in experimental petrological samples produced at high pressure and temperature, simulating lower crust and upper mantle conditions. Preliminary results show that the technique can be applied readily on a microscopic ( < 100 μm) scale for determination of H at a fraction of atomic percent level, at beam current levels that avoid radiation and thermal damage to the specimen.