IN-SITU OXYGEN ISOTOPE STUDY OF TOURMALINE AND QUARTZ: INSIGHTS INTO THE PROGRADE TEMPERATURE HISTORY

Oxygen isotopic studies are petrologically important tools for understanding, in part, fluid-rock interactions, sources of fluids, temperatures of formation and petrogenetic processes. Development of in-situ techniques for oxygen isotopes, pioneered by J.W. Valley, furthers their utility. Extending in-situ oxygen isotopic analyses to tourmaline (tur), a mineral with widespread utility for recording signatures of its geologic evolution, opens an additional avenue for tourmaline’s petrogenetic utility and for determining the prograde thermal history during tourmaline growth.

In-situ oxygen isotopes were measured in an Archean tourmaline from a quartz-rich staurolite zone (500-550° C, 3-4 kbar) metasediment from the Jardine Metasedimentary Sequence in the western Beartooth Mtns, MT, USA, using newly developed tur standards for the schorl-dravite series (Marger et al. 2017). The tourmaline contains a detrital core of schorl composition (X_Mg = 0.09) with two discernable stages of dravitic overgrowths (X_Mg = 0.53–0.58; 0.61-0.65). Oxygen isotopic values of the detrital core vary from 7.2 to 7.4 ‰ i.e. are homogeneous within error (0.3‰), while the metamorphic rim compositions of tourmaline vary from 7.8 to 8.6 ‰. These variations confirm the slow diffusion of oxygen in tourmaline. Quartz (qz) measured from inclusions and matrix grains differ: from 12.4‰ to 14.8 ‰, and 11.4 ‰ to 11.8 ‰, respectively. Temperatures (T) calculated for tur-qz pairs (Matthews et al. 2003) matching tur near qz inclusions and tur(rim)-qz(matrix) yield Ts from ca. 350°C to 490°C, lower than the calculated peak temperature. This may be due to isotopic disequilibrium, incorrect calibration of the tur-qz isotope equilibrium, spatial resolution of the outermost tur rim or that B was no longer available to the system causing cessation of tur growth prior to peak T. These apparent Ts calculated from matrix-rim pairs are about 150°C higher then those obtained from inclusions, confirming that qz inclusions maintain their isotopic composition, and preserve a record of the prograde history. This study demonstrates that careful calibration of SIMS analysis of oxygen isotopes will open a new, promising avenue for petrologic P-T-studies and provide a method to determine the prograde processes during metamorphism.