AUSTRALIS: A new tool for the study of isotopic systems and geochronology in mineral systems

AUSTRALIS (AMS for Ultra Sensitive TRAce eLement and Isotopic Studies) is a microbeam accelerator mass spectrometry (AMS) system designed for in situ microanalysis of geological samples for trace elements and radiogenic and stable isotope data. The AMS method eliminates molecular and isobaric interferences in in situ mass spectrometric measurements, opening up new opportunities in geochronology and tracer applications. Tests have been carried out for measurements of Pb, S and Os isotopes, conducted mainly at 1.5 MV accelerating voltage. In Pb and S tests, precision as high as 0.3‰ has been obtained, made possible by a fast isotope switching system to counter the effect of instabilities in the ion source and beam transport system. In trace‐element analysis, a detection limit for Au at the sub‐ppb level was obtained.     

Trace-element zonation in garnets from The Thumb: heating and melt infiltration below the Colorado Plateau

Proton microprobe (PIXE) analysis of garnet, pyroxene, and olivine for Zr, Y, Ga, Ge, Sr, Ni, Mn and Zn has been combined with electron-probe and petrographic analysis to interpret the histories of garnetperidotite xenoliths from the minette neck at The Thumb on the Colorado Plateau. Garnet in seven rocks contains 10–110 ppm Zr and 25–95 ppm Ni. Substantial parts of these ranges are preserved in single, zoned garnets (Zr, 25–90 ppm; Ni, 25–60 ppm). Pyroxene and olivine are more homogeneous and equilibrated more quickly than granet to changing temperatures and metasomatic fluxes. The distribution coefficient of Ti between pyroxene and garnet rims may be sufficiently sensitive to pressure to be used as a geobarometer. Zirconium and Ti appear to have behaved similarly during melt infiltration and diffusion within garnet. Nickel in garnet is a sensitive recorder of temperature. A temperature of 900° C or less calculated from Ni in the cores of large garnets in one rock is at least as cool as that calculated for the Archaean Kaapvaal craton at similar depth, and the low temperature may be due to cooling of the Plateau lithosphere by the subducted Farallon plate. The zonation of these garnets to Ni-enriched rims has been simulated numerically by heating 260° C at 0.02°/year, followed by overgrowth of a rim and short annealing. Garnet in another rock records a temperature decrease of about 70° C, but Ni is more homogeneous in garnets in the other five rocks. The diverse temperature histories are attributed to local melt-mantle interactions. Calculated pressures and temperatures of xenoliths from The Thumb form a grouping similar to those for high-temperature parts of inflected geotherms in other xenolith suites, and the similarity is evidence for both the reality and the transients nature of the calculated inflections. Garnet that is zoned in Zr, Y, Ti and other elements preserves evidences for grain growth in response to melt infiltration in four of seven rocks. The ranges of both major and trace elements in the xenolith suite may be due largely to enrichment processes following earlier depletion.     

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.     

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 K _D (C _i ^cpx /C _i ^gnt ) 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 K _D ^Sr and K _D ^Zr are mainly T-dependent, while K _D ^Ga may be both temperature-and pressuredependent. K _D ^Ni , K _D ^Cu and K _D ^Zn show 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.