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This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA‐ICP‐MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses.

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic ('grey gneiss') and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of 'average' Archaean TTG, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle, notably in intraoceanic arc settings in the forearc and back-arc regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking and tectonic accretion of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, 'arc-like' crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.

A UV laser ablation microprobe coupled to an ICPMS has been used to determine trace element concentrations in solids with a spatial resolution of S O microns and detection limits ranging from 52 pg/g for Ni to S O ng/g fortheREE,Th,andU.Experimentsdesignedtooptimize laser operating conditions show that pulse rates of 4 Hz p d u c e a steady state signal with less inter-element fractionation per unit time than higher pulse rates (10-20 Hz). Comparisons of laser microprobe analyses of garnets and pyroxenes using the NIST 610 and 612 glasfesascalibrationstandards,with proton microprobe, solutionICPMS,INMandXRFdatashownosignificant matrix effects. Laser microprobe analyses of the NIST 610 and 612 glasses have a p d o n and accuracy of 2-5%, and error analysis shows that counting statistics and the precision on the internal standard concentration accountsfor the analytical uncertainty.The NISTglasses appearto be useful calibration materialsfor trace element analysis of geological materials by laser microprobe.A laser ablation microprobe coupled with an inductively coupled plasma mass spectrometer (ICPMS) holds considerable promise for the rapid and precise in siru determination of trace element abundances in geological materials. Advantages of the technique include a spatial resolution of < 0.1 mm, sub-ppm detection limits for a variety of elements, and rapid analysis times (typically < 5 minutes per point analysis). Although relatively new, the technique has been applied to the determination of trace element zonation in silicate and carbonate minerals, natural and experimental crystal-melt partition,coefficients, whole rock analyses using pressed pellets and fused glasses, determination of Pb isotopic ages of zircons, and analysis of fluid inclusions (e.g., 1-10).As yet there is no consensus on analytical procedures such as laser operating conditions or calibration protocols, reflecting to some degree the variety of applications suitable for laser ablation analysis (11). This paper describes the analytical procedures and instrumental operating conditions currently used by our laboratory to malyse minerals and glasses for trace element abundances by laser ablation ICPMS. We also present new electron microprobe analyses for major element abundances in the NIST 610.612.614, and 616 glasses, o w best estimates for values of 28 trace elements in these glasses, and a comparison of laser microprobe analyses of a suite of mantle-derived garnets and pyroxenes with results by other techniques including proton m i c r o p r o b d solution ICPMS. INSTRUMENTATION Laser ablation systemThe laser ablation system and supporting software was designed by Drs. Simon Jackson and Henry Longerich of Memorial University (2,5). and was installed at Macquarie University in December, 1994. The laser is a Continuum Surelite 1-20 Q-switched Nd:YAG laser with a fundamental infrared (IR) wavelength at 1064 nm and a pulse width of 5-7 nsec. l b o frequency doubling crystals provide second and fourth harmonics in the visible (VIS, 532 nm) and ultrav...

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