The boron isotopic composition of tourmaline as a guide to fluid processes in the southwestern England orefield: An ion microprobe study
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Abstract Ion microprobe analyses of Li are presented for minerals of the Variscan granites and related rocks of west Cornwall. Together with electron microprobe data these provide insight into the behaviour of Li during the protracted late- and post-magmatic history of the granites. The role of the Mg-Fe tourmaline series is emphasized in this exploratory study.
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Recent work has shown the boron isotopic composition of tourmaline to be sensitive to a number of factors including boron source, waterrock interaction and temperature and pressure of formation (Palmer and Slack, 1989; Palmer et al, 1992). The boron isotopic composition of hydrothermal fluids has also been shown to be sensitive to boiling (Leeman et al, 1992). The boron isotopic composition of individual tourmalines from S.W. England has been determined in order to investigate both fluid source and fluid processes b y attempting to detect isotopic differences within crystals and between samples. Samples were analysed from three main rock types:granites, quartz-tourmaline (tourmalinite) and quartz-topaz-tourmaline (QTT) rocks and greisen(Sn,W)-related veins. Three main sample areas were used the St.Austell granite and its associated tourmalinites and QTT rocks at Roche and St. Mewan's Beacon, the roof zone of the Land's End granite exposed at Porth Ledden and the Cligga Head granite and sheeted vein system 9 Tourmalinites and QTT rocks are important as they are considered to represent the transition from magmatic to hydrothermal conditions (Manning, 1981;Bottrell and Yardley, 1988) 9
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The Cuonadong Sn–W–Be polymetallic deposit in the Himalayan leucogranite belt is a representative hydrothermal deposit. The role of fluid exsolution directly from magma and the fluid reaction with surrounding rocks for ore-forming element enrichment is still controversial. Tourmaline is a significant B-bearing mineral in the hydrothermal deposit, and its geochemical and B isotopic signatures can record the source and evolution of the ore-forming fluid. Two types of hydrothermal tourmaline in the hydrothermal quartz vein (Tur-1) and skarn (Tur-2) were used in this study. Both Tur-1 and Tur-2 have low X-site occupancy and mainly belong to the alkali group. Tur-1 plots in the schorl field, whereas Tur-2 is largely Mg-rich dravite. The B isotope analyses of Tur-1 have δ 11 B values of −13.7 to −13.2‰, whereas Tur-2 has higher δ 11 B values of −11.1 to −9.3‰. The distinct contact relationship and geochemical compositions suggest that Tur-1 in the hydrothermal vein was formed from a magmatic-hydrothermal fluid with little influence from surrounding rocks and had a genetic relationship with the Cuonadong leucogranite, whereas Tur-2 in the skarn involved more fluid from surrounding rocks with high δ 11 B values and strong metasomatic texture. The higher ore-forming element contents in Tur-2 than those in Tur-1 indicate that the reaction between the magmatic exsolution fluid and the surrounding rock is essential for the enrichment and precipitation of ore-forming elements.
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Electron-microprobe analyses (EMPA) and Mössbauer spectra (MS) have been acquired for natural tourmalines sampled near Crummock Water and Haweswater, English Lake District. In the former, the dravite-schorl series is prevalent, whereas in the latter, the dravite species is dominant. All tourmaline samples presently studied are chemically disordered. The compositional and MS data give an insight into the thermal and fluid evolution experienced by the tourmalinite rocks. The tourmalines from Crummock Water crystallized in a closed, low fO₂ environment, under the influence of a magmatic fluid. The tourmalines from Haweswater are of hydrothermal origin and were formed under low, variable T and low fO₂ conditions, from a fluid of constant composition.
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Abstract Lithium cannot be determined by electron microprobe, but it may be an essential component in tourmalinesupergroup minerals. Therefore, its estimation is important for structural formula calculation and nomenclature. In this paper, we present a method to estimate Li content in tourmaline from microprobe data based on a multiple linear-regression model, which is not reliant on a particular normalization scheme. The results derived from this model are reasonably accurate, particularly for low-Mg tourmalines (<2 wt.% MgO) with Li 2 O contents higher than ∼0.3 wt.%. Furthermore, it provides a better fitness compared with estimations of Li assuming that Li fills any cation deficiency at the Y site.
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