D.R. Cousens

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.

SNIP, a statistics-sensitive background treatment for the quantitative analysis of PIXE spectra in geoscience applications

Abstract Statistical fluctuations in X-ray spectra must be treated properly for reliable quantitative PIXE analysis. A background approximation that provides reliable treatment of fluctuations, the Statistics-sensitive Non-linear Iterative Peak-clipping (SNIP) algorithm, is described. Monte Carlo simulation demonstrates the stability of this background approximation, and hence the deduced trace element concentrations, over a wide range of counting statistics.

Proton microprobe-determined partitioning of Nb, Ta, Zr, Sr and Y between garnet, clinopyroxene and basaltic magma at high pressure and temperature

Abstract Application of a proton microprobe to experimentally produced mineral-liquid pairs has enabled determination of Nb and Ta partition coefficients ( D ) between garnet, clinopyroxene, and a Nb- and Ta-enriched hydrous basaltic liquid at 2.5 GPa, 1100°C. For garnet D Nb =0.02, D Ta =0.06, and for clinopyroxene D Nb =0.005 and D Ta =0.013. Both minerals significantly fractionate Nb and Ta from each other and may have an important role in modifying the Nb/Ta ratio of evolving magmas from that of their parent magma or their source region. For eclogite melting up to 40% change could occur in the Nb/Ta ratio. Additional partition coefficients obtained using the proton microprobe include values of: for garnet, D Zr (0.4–0.7), D Sr ( D Y (>2.5–9); for clinopyroxene, D Zr (0.10), D Sr (0.06) and D Y (0.9); and for orthopyroxene D Zr (0.18), D Sr (0.04) and D Y (0.18). These were determined for natural element abundance levels, and are consistent with data obtained for artificially enriched compositions, verifying Henrys law behaviour and the valid application of these data to geochemical modelling of natural systems.

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.

Quantitative analysis of PIXE spectra in geoscience applications

Abstract Geoscience research using the proton microprobe demands an accurate analysis method for all element concentrations, particularly those at low levels where the proton probe is providing unique trace element data. A method, and its integration into the GEO-PIXE suite of computer codes, is described which meets these requirements, with particular emphasis on elements near the detection limit. Results using the method are compared with accepted analyses of standard rocks and electron microprobe analyses. Monte Carlo tests indicate that the method is accurate for low level trace elements near the detection limit, and for demonstrably different spectra for which standards comparisons cannot be readily made.

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.

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.

Application of the proton microprobe in mineral exploration and processing

Abstract The in situ high-sensitivity multielement detection capability of PIXE, combined with μm spatial resolution opens up new possibilities in mineral-related research. Trace element data obtained using the proton microprobe provide new tools in exploration for diamond and precious metals, ore genesis studies as well as in the processing of precious-metal-bearing ores. Introduction of this new methodology to the minerals industry however is controlled by economic factors which must be taken into consideration.

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.