Serendibite, Ca1.93Na0.08Mg2.58Al5.03B1.53Si2.71O20, was recently discovered in a lens of B-rich calc-silicate rock metamorphosed at 6–7 kbar, 650–700 °C in the Central Metasedimentary Belt, Grenville Province. Based on paragenesis and tourmaline composition, which were used to monitor changes in fluid composition, the following stages of mineralization are recognized: (1) a prograde assemblage consisting of K-feldspar, tourmaline [T1 with XCa = Ca/(Ca + Na) = 0.54, T2 with XCa = 0.71], and calcite, inferred from relicts in scapolite; (2A) a peak metamorphic assemblage of aluminous diopside, serendibite, lesser phlogopite, and local scapolite (Me62); (2B) continued formation of phlogopite around serendibite in calcite pockets although serendibite was stable; and (3) high-temperature breakdown of serendibite to uvite (T3; XCa = 0.82) + spinel + calcite, and of aluminous diopside to pargasite. Three generations of idiomorphic magnesian tourmaline that crystallized in calcite pockets (T4 to T6) recorded fluid evolution as follows: (A) XCa ≈ 0.5 and relatively moderate Ti concentrations (generation T4); (B) increase in Na relative to Ca, XCa ≈ 0.3 (generation T5), with the rare occurrence of oligoclase (XCa = 0.17) and relatively low Ti concentrations; and (C) XCa ≈ 0.5, relatively high Ti concentrations (generation T6). The final stage is localized, low-temperature alteration to fine-grained phyllosilicates.

The boron isotopic composition of peak metamorphic serendibite (δ11B = +4.3 ± 1.7‰) is lower than that of prograde tourmaline (δ11B = +10.6 ± 2.3‰) from which it formed. The fractionation difference between serendibite and metamorphic fluid (with B derived from prograde tourmaline), Δ11BSrd−Tur = δ11BSrd − δ11BTur = −6.3‰, is similar to the ab initio calculated fractionation factor between serendibite and fluid (Δ11BSrd−fluid = −7.6 ± 1.4‰ at 727 °C). Tetrahedral boron coordination in serendibite is largely responsible for this fractionation. High-temperature uvite replacement of serendibite is significantly higher in δ11B relative to serendibite (+10.6‰ versus +4.3‰), which is attributed to buffering of boron isotopes by serendibite in local equilibrium with uvite. Tourmalines formed in calcite pockets range from +8‰ to almost +12‰ on average, while the core of a T4 tourmaline has δ11B = +3.4‰. This low δ11B value is attributed to a temporary departure from equilibrium due to fluid influx. The boron isotope signature of prograde tourmaline suggests a marine evaporite source.

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