Chemical and boron isotopic composition of magmatic and hydrothermal tourmalines from the Sinceni granite–pegmatite system in Swaziland

Abstract

The Archean Sinceni granite is a geologically and geochemically well-studied example of the transition from granite to pegmatite, with subsequent development of hydrothermal alteration in metatonalite wallrocks. Tourmaline occurs in all settings of this system, from the granite to the pegmatites and in the hydrothermal rocks. The chemical and B-isotopic composition of tourmalines was studied by electron- and ion microprobe to understand how tourmaline composition varies during the stages of magmatic to hydrothermal development. The chemical composition of tourmaline from each rock type (granite, pegmatite, hydrothermal vein in metatonalite) is distinctive. In terms of their Na/Na+Ca and Fe/Fe+Mg ratios, the tourmalines correlate with the whole-rock ratios of the corresponding host rock. Tourmalines from the pegmatites show significantly higher Al contents than those from the parental granite, and a correspondingly greater degree of alkali-site vacancies (ca. 30% in pegmatite, 15–20% in granite). Tourmalines in granite have a narrow range of isotopic composition ( δ 11 B , mean −15.7‰, σ=1.3), which is considered to approximate the composition of the magma from which they crystallized. Tourmalines from aplitic zones in pegmatite have the same range of boron isotopic composition as the granite (mean −15.2‰, σ=1.7). In contrast, tourmaline from a coarse, zoned pegmatite and hydrothermal tourmalines are isotopically lighter ( δ 11 B values of −16.6 to −23.0‰). These differences are consistent with a model of isotopic fractionation due to fluid exsolution. Tourmaline–fluid fractionation factors allow us to estimate that the exsolved fluid had a δ 11 B composition of about −13‰. The B-isotopic compositions of hydrothermal tourmalines are consistent with formation from this fluid at temperatures of 350 to 500°C, which are in line with temperatures from fluid inclusion microthermometry.