BORON ISOTOPE FRACTIONATION BETWEEN SERENDIBITE AND TOURMALINE IN CALC-SILICATE ROCKS OF THE PORTAGE-DU-FORT GROUP MARBLE, PONTIAC REGIONAL COUNTY MUNICIPALITY, QUéBEC, CANADA

Hawthorne’s contributions include crystal chemistry of rare borosilicates, e.g., prismatine. Another such is serendibite, which was recently discovered in a lens of B-rich calc-silicate rock metamorphosed at 8 kbar, 750°C in the Central Metasedimentary Belt, Grenville Province, only the 14^th locality worldwide. Based on paragenesis and tourmaline composition to monitor changes in fluid composition, the following stages of mineralization are recognized: (1) a prograde assemblage consisting of dominant K-feldspar and subordinate tourmaline (T1 with X_Ca = Ca/(Ca+Na) = 0.55) and calcite; (2) a peak metamorphic assemblage of aluminous diopside, serendibite, Ca_1.93Na_0.08Mg_2.56Al_4.97B_1.52Si_2.77O₂₀, lesser phlogopite, and local scapolite (Me₆₂); (3) continued formation of phlogopite around serendibite in calcite pockets although serendibite was stable. An idiomorphic tourmaline (T2) included in Me₆₂ indicates high Ca activity (X_Ca = 0.71); (4) high-temperature breakdown of serendibite to uvite (X_Ca = 0.79) + spinel + calcite, and diopside to pargasite. Idiomorphic tourmaline crystallized in calcite pockets. In stage (5) three more generations of magnesian tourmaline (T3 to T5) recorded fluid evolution as follows: (a) X_Ca ≈ 0.5 and relatively moderate Ti activity (T3); (b) increase in Na activity, X_Ca ≈ 0.3 (T4), with the rare occurrence of oligoclase and relatively low Ti activity; and (c) X_Ca≈ 0.5, relatively high Ti activity (T5). The final stage is low-temperature alteration to fine-grained phyllosilicates.

Tourmaline δ¹¹B decreases from +11.6±1.2‰ (T3) to +5.8±1.8‰ (T5), resulting in a decrease of isotopic fractionation between serendibite (δ¹¹B = –5.4 ± 0.38 ‰) and tourmaline, Δ¹¹B (Tur-Srd) = δ¹¹B (Tur) - δ¹¹B (Srd) from 17.0‰ to 10.2 ‰. In comparison, fractionation calculated using state-of-the-art ab initio methods, is 7.1 ± 1.3‰ for dravite and 6.7 ± 1.3‰ for uvite at 727°C, implying isotopic equilibrium was not attained with T3-T5. However, the calculated fractionation should be compared with δ¹¹B of idiomorphic uvite (T2) crystallizing approximately coevally with serendibite during peak conditions. The decrease in tourmaline δ¹¹B on the retrograde path is attributed to decreasing fluid δ¹¹B during later stages of metamorphism, probably caused by the progressive destruction of serendibite.