T167 Metamorphic Petrology: New Approaches and Outcomes

Monday, 2 November 2015
1:30 PM - 5:30 PM
Room 317

MECHANISM OF STABLE ISOTOPE EXCHANGE IN HYDROTHERMAL OLIVINE-CALCITE VEINS, BERGELL, ITALY: PERSPECTIVES BASED ON OXYGEN ISOTOPE SIMS ANALYSIS

Paper number: T167-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

Olivine-calcite veins cross-cutting large (>50 m) dolomite xenoliths that were caught up in the Bergell tonalite are excellently exposed on Alpe Sissone, Italy. They were the focus of several petrologic and geochemical studies in the past (e.g., Bucher, 1998; Taylor and Bucher, 1986; Begue, 2008). A mm-sized open central vein is rimmed by 1-20 cm thick reaction rims, where recrystallized dolomite is replaced by forsterite and calcite. Traditional oxygen and carbon isotope analyses of calcite, olivine, and dolomite revealed mm-sized isotope fronts of several ‰ in contact with unreacted dolomite (Taylor and Bucher, 1986; Begué 2007). Apparent oxygen isotope equilibrium between olivine and calcite gives a formation temperature of 540°C, in agreement with estimates of the phase petrology.

SIMS oxygen isotope analysis of calcite is indeed homogeneous throughout the reaction zone (13-14 ‰, VSMOW); even grains touching unreacted dolomite at the rim of the reaction zone have the same values. SIMS-measured isotopic compositions of dolomite, in contrast, change from > 20 ‰ to roughly 14 ‰ at the contact over several 100 µm. Some grains are zoned, with heavy isotope compositions in the cores and lighter rims. Olivine compositions vary by up to 2 ‰, without apparent systematic zonation in the replacement zone. These data confirm that product minerals seem to precipitate in equilibrium with the fluid all through the reaction zone (olivine, calcite), while reactant minerals (dolomite) maintain their original composition, slowly exchanging with the intergranular fluid only. Additional studies are required to identify if this occurs by diffusion and/or domain-recrystallization. Hence fluid-rock interaction models need to be adapted to incorporate different kinetic isotope exchange mechanisms for precipitating minerals and reacting minerals.

Bégué F. (2007) Master Univ. Lausanne, CH

Bucher K. (1998) Min. Pet. 63, 151-171

Taylor B., Bucher-Nurminen K. (1986) GCA 50, 1267-1279