To expand the newly developed ARM glasses as reference materials for in situ microanalysis of isotope ratios and iron oxidation state by a variety of techniques such as SIMS, LA‐MC‐ICP‐MS and EPMA, we report Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb isotope data and Fe 2+ /ΣFe ratios for these glasses. The data were mainly obtained by TIMS, MC‐ICP‐MS, IR‐MS and wet‐chemistry colorimetric techniques. The quality of these data was cross‐checked by comparing different techniques or by comparing the results from different laboratories using the same technique. All three glasses appear to be homogeneous with respect to the investigated isotope ratios (except for B in ARM‐3) and Fe 2+ /ΣFe ratios at the scale of sampling volume and level of the analytical precision of each technique. The homogeneity of Li‐B‐O‐Nd‐Pb isotope ratios at the microscale (30–120 μm) was estimated using LA‐MC‐ICP‐MS and SIMS techniques. We also present new EPMA major element data obtained using three different instruments for the glasses. The determination of reference values for the major elements and their uncertainties at the 95% confidence level closely followed ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. The ARM glasses may be particularly useful as reference materials for in situ isotope ratio analysis.
Abstract Basaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐ 3 He/ 4 He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.
Abstract Myanmar, the third largest global tin supplier, is an important component of the Southeast Asian tin province. We have conducted laser ablation-inductively coupled plasma-mass spectrometry U-Pb dating of cassiterite, wolframite, and zircon and Re-Os dating of molybdenite from six primary and two placer Sn deposits in Myanmar. A combination of our geochronological data with previous studies revealed that three episodes of Sn mineralization in the Western tin belt of Southeast Asia formed during the closure of multiple Tethys oceans, namely the Late Triassic (~218 Ma) mineralization in a collisional setting after closure of the Paleo-Tethys, the Early Cretaceous (~124–107 Ma) mineralization during subduction of the Meso-Tethys, and the Late Cretaceous to Eocene (~90–42 Ma) mineralization related to the Neo-Tethys subduction. Recurrent Sn mineralization is recorded not only in the Western tin belt but also in the Central and Eastern tin belts in Southeast Asia. Compilation of currently available cassiterite U-Pb ages from all over the world revealed that durations of regional Sn mineralization events are typically in the range of ~5–30 m.y., whereas the Neo-Tethys subduction in Southeast Asia generated prolonged Sn mineralization lasting up to ~50 m.y. The Southeast Asian tin province, as a whole, has the longest cumulative episodes of mineralization, compared to other Sn provinces. The Sn mineralization ceased in the late Eocene when the tectonic setting changed from Neo-Tethys subduction to dextral motion along a series of strike-slip faults and extrusion of the Indochina block in Southeast Asia.
Abstract Several important processes in the petrogenesis of granite are still debated due to a poor understanding of complex interactions between minerals during the melting and melt segregation processes. To promote an improved understanding of the mineral-melt relationships, we present a systematic petrographic and geochemical analysis for melanosome and leucosome samples from the Triassic Jindong migmatite, South China. Petrographic observations and zircon U-Pb geochronology indicate that the Jindong migmatite was formed through water-fluxed melting of the Early Paleozoic gneissic granite (437 ± 2 Ma) during the Triassic (238 ± 1 Ma), with the production of melt dominated by the breakdown of K-feldspar, plagioclase, and quartz. The Jindong leucosomes may be divided into lenticular and net-structured types. Muscovite, plagioclase, and K-feldspar in the net-structured leucosome show higher Rb and much lower Ba and Sr contents than those in the lenticular leucosome. This may be attributed to the elevation of Rb and decreasing Ba and Sr abundances in melts during the segregation process due to early fractional crystallization of K-feldspar and plagioclase. These leucosomes show negative correlation between εNd(t) and P2O5, reflecting increasing dissolution of low-εNd(t) apatite during the melting process. The continuous dissolution of apatite caused saturation of monazite and xenotime in melt, resulting in the growth of monazite and xenotime around apatite in the melanosome. This process led to a sharp decrease of Th, Y, and REE with increasing P2O5 in the leucosome samples. This complex interplay of accessory mineral reactions in the source impacts REE geochemistry and Nd isotope ratios of granites. As the granites worldwide exhibit similar compositional and isotopic patterns to the Jindong leucosomes, we suggest that both the melting and melt segregation processes strongly control the granitic melt compositions.
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