Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic

Abstract

Abundant tourmaline occurs in the Lavicky leucogranite, Czech Republic as spherical to ovoid quartztourmaline orbicules, typically 5 to 7 cm in diameter. The tourmalines also occur as fine-grained quartztourmaline veins (<1 cm thick) that cut the orbicule-rich granite. Electron microprobe analyses reveal that tourmaline from the quartz-tourmaline orbicules is Fe-rich schorl with a range of Fe/(Fe+Mg) ratio 0.62 to 0.77 and Na/(Na+Ca) ratio 0.82 to 0.95. In contrast, tourmaline from quartz-tourmaline veins is Mg-rich dravite with a range of Fe/(Fe+Mg) ratio 0.23 to 0.45 and Na/(Na+Ca) ratio 0.67 to 0.90. Very low δ 11 B values of -37.3 to -32.1‰ are found in the tourmalines from the orbicules, whereas tourmalines from the veins display relatively higher δ 11 B values of -28.2 to -21.3‰. The overall large δ 11 B variation is suggested to reflect mixing of different boron sources and boron isotope fractionation during magmatic degassing and magmatic-hydrothermal evolution at late solidus to early subsolidus stages of granite crystallization. Only non-marine evaporites show very negative δ 11 B values (<-20‰) in all natural boron reservoirs, hence, the very low δ 11 B values of the quartz-tourmaline orbicules likely indicate a major contribution of boron from non-marine evaporites that probably exist in the magma source regions or assimilated into the magma during its ascent. Quartz-tourmaline orbicules may have formed during a transition from magmatic to hydrothermal processes, whereas the vein tourmalines formed by mixing of the exsolved magmatic-hydrothermal fluids with an external fluid rich in Ca and Mg and having higher δ 11 B than the exsolved magmatic fluids.

Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt, Namibia

“…Many empirical studies of B-isotopes in tourmaline from igneous and metamorphic rocks and from hydrothermal veins found significant variations in δ 11 B between early and late tourmaline generations, sometimes in single zoned crystals, which were interpreted in terms of temperaturedependent isotopic fractionation during crystal growth (Smith and Yardley, 1996;Chaussidon and Appel, 1997;Jiang and Palmer, 1998;Nakano and Nakamura, 2001;Matthews et al, 2003;Jiang et al, 2003; this paper). However, there are also examples of isotopic homogeneity in zoned or polyphase tourmaline growths that argue against significant fractionation effects (Tonarini et al, 1998;Marschall et al, 2006).…”

“…Such tourmaline-quartz segregations are known from several other boron-rich granites (e.g., Schust et al, 1970;Sinclair and Richardson, 1992;Taylor et al, 1992;Trumbull, 1993;Rozendaal and Bruwer, 1995;London, 1999;Jiang et al, 2003). Their origin is still not well understood, but it has been suggested that unmixing of a boron-rich late magmatic/early hydrothermal fluid from evolved granitic magma is involved (Samson and Sinclair, 1992;Taylor et al, 1992).…”

Reply to “Re-examination of the boron isotopic composition of tourmaline from the Lavicky granite, Czech Republic, by secondary ion mass spectrometry: back to normal” by H. R. Marschall and T. Ludwig: Critical comment on “Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic”

“…However, the analysis of boron isotope compositions in geological samples, in particular those with complex matrices such as tourmaline, is a challenging task (Gonfiantini et al, 2003;Tonarini et al, 2003). In 2003, we reported extremely negative boron isotope compositions of tourmaline from the Lavicky granite (Jiang et al, 2003), and these values have been proven to be wrong as they are not supported by new SIMS analysis on tourmalines from the same granite (Marschall and Ludwig, 2006). We thank Marschall and Ludwig for pointing out the problem, and will take this lesson into account in our future boron isotope studies.…”

“…Tourmaline samples from the Lavicky granite were decomposed using the acid digestion method (HF + HNO 3 ), purified through column chemistry, and the 11 B/ 10 B ratio determined using the N-TIMS method (Jiang et al, 2003). After publication of these extreme negative δ 11 B values for the Lavicky tourmalines, we noted that an abstract published in Geochemica et Cosmochimica Acta (2003, Suppl., A577) also reported very negative δ 11 B values from -41.1 to -9.2 per mil in tourmalines from the Altay No.…”

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New SIMS data (Marschall and Ludwig, 2006) indicate that some of the boron isotopic compositions of tourmaline from the Lavicky granite (Czech Republic) previously reported by us (Jiang et al, 2003) are erroneous. The problems are discussed in this paper.

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