Using trace element geochemistry of carbonate minerals to track metamorphic processes: Case studies from the Castner Marble, Texas and the thermal aureole of the Ballachulish Igneous Complex, Scotland

Date

2020-05

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Abstract

Many metamorphic studies rely on porphyroblasts for understanding the conditions and timing of metamorphism. Most work on understanding porphyroblast growth and element transport to growing porphyroblasts have focused on homogenous matrices due to easier modeling and lack of initial compositional gradients. This leaves questions on how porphyroblasts growing in heterogeneous matrices are influenced by chemical gradients in the heterogeneous system. As metamorphic reactions, porphyroblast forming or otherwise, are taking place, new minerals appear, and others may become unstable and breakdown. Some minerals (e.g. quartz and carbonates) however are present throughout a rocks P-T-t history and if reacting, via participation and/or recrystallization, provide an opportunity to track element transport and mobility in the system through the duration of a metamorphic cycle. The research in this study utilizes a combination of electron microscopy, major and trace element geochemistry, and isotope geochemistry to understand the potential for carbonate minerals to track metamorphic reactions and processes in metacarbonates. The Castner Marble is a layered marble-hornfels unit with garnet porphyroblasts of varying morphologies, both within marble layers and along the marble-hornfels interface. These garnets grew via interface-controlled nucleation and growth in an environment where there were no compositional gradients for the major elements and no nucleation suppression surrounding the growing porphyroblasts. Trace element data from recrystallized calcites in depletion haloes surrounding garnets indicates the LREEs were being transported to the growing porphyroblasts via intragranular diffusion. This indicates that there was a divergence between the transport mechanism for the major elements and the trace elements. Stable isotopic data from carbonates and garnets from various locations in the matrix and textures demonstrates there was isotopic disequilibrium in the system. The stable isotope data also suggests fluids present during porphyroblast growth were internally derived and not from an external source. Metacarbonates from the thermal aureole of the Ballachulish Igneous Complex provide a case study where the geochemistry of carbonate minerals, dolomite and calcite, were evaluated across a temperature gradient at constant pressures in a system with well-defined reactions and mineral assemblages. The trace element abundances in carbonates show a decrease in elemental abundance with increasing temperature. This decrease in trace element abundance of carbonate minerals can be correlated with both major and accessory phases reactions occurring in the rock unit. This indicates that the trace element geochemistry of carbonates from this locality can be used to track reactions happening in the rocks.

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Keywords

Carbonate, Trace element geochemistry, Scotland

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