Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal

Abstract

Few studies have focused on gem-quality tourmaline acting as a petrogenetic recorder, and the colour genesis of pink elbaite is still controversial. We carry out in situ major, trace element and boron isotope composition analyses on a single tourmaline crystal. This crystal is characterized by sudden transformation from colourless to pink, which can represent full pegmatite magma evolution. According to the analysis results, all spots are divided into alkali groups according to X-site occupancy and subdivided into elbaite series. The pink part accommodates higher concentrations of volatile and incompatible elements. The result is most consistent with successive pegmatite evolution in which the colourless part crystallized from the early stage, while the pink part crystallized from the late stage. The relatively consistent δ11B value between the colourless and the pink part suggests no fluid exsolution occurred during pegmatite evolution. The slight increase of δ11B values within the pink part and the colourless part may be due to mica crystallization. The combination of (Li++Mn2+) (Al3++Xvac)-1 and the exclusive positive linear relationship of Mn2+ vs. Ti4+ indicate that Mn2+ is the main cause of pink, while Mn2+-Ti4+ intervalence charge transfer also plays an important role.

In-situ boron isotope and chemical composition of tourmaline in the Gyirong pegmatite, southern Tibet: Implications for petrogenesis and magma source

“…In this experiment, the ablating diameter for the LA-ICP-MS trace element analysis was set at 35 μm on the equivalent locations where the EPMA had been conducted. The detailed testing procedures and conditions were similar to those described by Zheng and Chen (2021). Data processing was performed offline using ICPMSDataCal software (Liu et al, 2008).…”