T25 Advances in Mineralogy, Crystallography, and Petrology: In Honor of Frank C. Hawthorne, 2013 Roebling Medalist

Sunday, 27 October 2013
8:00 AM - 12:00 PM
Room 203

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

Paper number: T25-10
Program: Technical Sessions
Time: 10:25 AM – 10:40 AM

Authors

Abstract

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 14th 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 XCa = Ca/(Ca+Na) = 0.55) and calcite; (2) a peak metamorphic assemblage of aluminous diopside, serendibite, Ca1.93Na0.08Mg2.56Al4.97B1.52Si2.77O20, 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 (XCa = 0.71); (4) high-temperature breakdown of serendibite to uvite (XCa = 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) XCa ≈ 0.5 and relatively moderate Ti activity (T3); (b) increase in Na activity, XCa ≈ 0.3 (T4), with the rare occurrence of oligoclase and relatively low Ti activity; and (c) XCa≈ 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.