Refine
Has Fulltext
- no (2) (remove)
Document Type
- Article (2)
Language
- English (2)
Is part of the Bibliography
- yes (2)
Keywords
- SIMS (2) (remove)
Institute
- Institut für Geowissenschaften (2) (remove)
Raman spectroscopic quantification of tetrahedral boron in synthetic aluminum-rich tourmaline
(2021)
The Raman spectra of five B-[4]-bearing tourmalines of different composition synthesized at 700 degrees C/4.0 GPa (including first-time synthesis of Na-Li-B-[4]-tourmaline, Ca-Li-B-[4]-tourmaline, and Ca-bearing square-B-[4]-tourmaline) reveal a strong correlation between the tetrahedral boron content and the summed relative intensity of all OH-stretching bands between 3300-3430 cm(-1). The band shift to low wavenumbers is explained by strong O3-H center dot center dot center dot O5 hydrogen bridge bonding. Applying the regression equation to natural B-[4]-bearing tourmaline from the Koralpe (Austria) reproduces the EMPA-derived value perfectly [EMPA: 0.67(12) B-[4] pfu vs. Raman: 0.66(13) B-[4] pfu]. This demonstrates that Raman spectroscopy provides a fast and easy-to-use tool for the quantification of tetrahedral boron in tourmaline. The knowledge of the amount of tetrahedral boron in tourmaline has important implications for the better understanding and modeling of B-isotope fractionation between tourmaline and fluid/melt, widely used as a tracer of mass transfer processes.
The Devonian Las Chacras-Potrerillos batholith comprises six nested monzonitic to granitic intrusions with metaluminous to weakly peraluminous composition and a Sr-Nd isotopic signature indicating a dominantly juvenile mantle-derived source. The chemically most evolved units in the southern batholith contain a large number of intra-granitic, pod-shaped tourmaline-bearing pegmatites. This study uses in situ chemical and boron isotopic analyses of tourmaline from nine of these pegmatites to discuss their relationship to the respective host intrusions and the implications of their B-isotope composition for the source and evolution of the magmas. The tourmalines reveal a diversity in element composition (e.g., FeO, MgO, TiO2, CaO, MnO, F) which distinguishes individual pegmatites from one another. However, all have a narrow 5 11 B range of -13.7 to -10.5%0 (n = 100) which indicates a relatively uniform magmatic system and similar temperature conditions during tourmaline crystallization. The average delta(11) B value of -11.7%0 is typical for S-type granites and is within the range reported for peraluminous granites. pegmatites, and metamorphic units of the Ordovician basement into which the Las Chacras-Potrerillos batholith intruded. The B-isotope evidence argues for a crustal boron source like that of the Ordovician basement, in contrast to the metaluminous to weakly peraluminous composition and juvenile initial Sr and Nd isotope ratios of the Las Chacras-Potrerillos batholith magmas. We propose that the boron was not derived from the magma source region but was incorporated from dehydration melting of elastic metasedimentary rocks higher up in the crustal column.