Comparison between LA-ICP-MS and EPMA analysis of trace elements in diamonds

Eight elements were measured in twenty-eight microinclusion-bearing diamonds using both Electron Probe Micro-Analyzer (EPMA) and the cellulose-calibrated Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) methods. Orthopyroxene microinclusions (< 1μm) found in one of the diamonds have very similar Si atoms per formula unit and Mg/(Mg + Fe) ratios to those of larger orthopyroxene inclusions in diamonds, indicating that the EPMA analysis of the major elements in individual mineral and fluid microinclusions is accurate to better than 15%. For the fluid-bearing diamonds, very good correlations were found between the element/Fe ratios determined by EPMA and LA-ICP-MS for Mg, Ca, Na and K and most diamonds fall on or close to the 1:1 line, validating the accuracy of both techniques. Al/Fe, Ti/Fe and Ba/Fe ratios show good to moderate correlations. LA-ICP-MS analyses of four coated diamonds show that concentrations in the microinclusion-bearing coat are higher than those in the clear core by two orders of magnitude or more. Since most interferences from C-N-O-H molecular ions can be corrected by reference to analyses of pure synthetic diamond, and fewer interferences are expected for the heavier trace elements, the assembled information suggests that LA-ICP-MS technique combined with the cellulose calibration method provides accurate trace-element analyses of diamonds and allows compositional characterization of fluids trapped in them.11 page(s

Evaluation of perchloric versus nitric acid digestion for precise determination of trace and ultra trace elements by ICP-MS

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Trace element and isotopic composition of GJ-red zircon standard by laser ablation

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Granite, gabbro and mafic microgranular enclaves in the Gejiu area, Yunnan Province, China: a case of two-stage mixing of crust- and mantle-derived magmas

Geochronological, geochemical, whole-rock Sr-Nd, and zircon Hf isotopic analyses were carried out on the Jiasha Gabbro, mafic microgranular enclaves (MME) and host Longchahe Granite samples from the Gejiu area in the southeast Yunnan province, SW China, with the aim of characterizing their petrogenesis. Compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin. The Longchahe Granite largely comprises metaluminous quartz monzonite with distinctive K-feldspar megacrysts, but also contains a minor component of peraluminous leucogranite. The MME have spheroidal to elongated/lenticular shapes with sharp, crenulated and occasionally diffuse contacts with the host granite, which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma. Field observations, geochronology, geochemistry, Sr-Nd and zircon Hf isotopic compositions point to a complex petrogenesis for this granite-MME-gabbro association. Zircon Pb-206/U-238 ages determined by LA-ICP-MS for a mafic enclave, its host granite and the gabbro body are 83.1 ± 0.9 Ma, 83.1 ± 0.4 Ma and 83.2 ± 0.4 Ma, respectively, indicating coeval crystallization of these igneous rock units. Crystal fractionation processes can explain much of the compositional diversity of the Jiasha Gabbro. The geochemical features of the gabbro, such as high Mg^# (up to 70) and Cr (up to 327 ppm), enrichment in LILEs (e.g., Rb, Ba, K₂O) and LREEs, and depletion in HFSE (e.g., Nb, Ta, Ti), together with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.708-0.709 and negative εNd(t) values (-5.23 to -6.45), indicate they were derived from a mantle source that had undergone previous enrichment, possibly by subduction components. The Longchahe Granite has a large range of SiO₂ (59.87-74.94 wt%), is distinctly alkaline in composition, and has Sr-Nd-Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr)ᵢ > 0.712, εNd(t) = -6.93 to -7.62 and εHf(t) = -5.8 to -9.9) that are indicative of derivation from a crustal source. However, the most primitive rocks of Longchahe Granite are compositionally distinct from any feasible crustal melt. We interpret the spectrum of rock types of the Longchahe Granite to have formed via mixing between crustally derived peraluminous leucogranite magma and mantle-derived magma of similar heritage to the Jiasha Gabbro. We speculate that this mixing event occurred early in the magmatic history of these rocks at relatively high temperature and/or deep in the crust to allow efficient physical mixing of magmas. Saturation and accumulation of K-feldspar and zircon in the mixed magma is invoked to explain the megacrystic K-feldspar and elevated K₂O and Zr content of some of the granitic rocks. A later episode of magma mixing/mingling is preserved as the MME that have geochemical and isotopic compositions that, for the most part, are intermediate between the granite and the gabbro. The MME are interpreted to be fractionated melts of mafic magma related to gabbro that were subsequently injected into the cooler, partly crystalline granitic magma. Mingling and mixing processes within the convectively dynamic upper crustal magma chamber resulting in a hybrid (MME) magma. During this second mixing episode, element interdiffusion, rather than bulk physical mixing, is interpreted to be the dominant mixing process