Understanding Fluid Behavior in Monviso Using Chlorite-Bearing Altered Eclogites from the Lower Shear Zone
Abstract
Fluids released during subduction are important because they are responsible for geochemical cycling. Major and trace element variations within rock samples exhumed from paleo-subduction zones can provide useful information in determining the extent of fluid-rock interactions and reconstructing subduction/exhumation processes. In the Western Alps, the Monviso ophiolite complex contains chlorite-bearing eclogite units within a shear zone; as a hydrous phase, chlorite can provide direct information about fluid processes and compositions during subduction. This study focuses on two of these chlorite-eclogites; sample 1 is an altered eclogite block from the eclogite mélange unit, and sample 2 represents a block to rind sample from the edge of one of these blocks with abundant chlorite in the rind. Sample 1 contains varied chlorite replacement textures within the same thin section; chlorite is seen both coexisting with and replacing garnets and omphacite in different regions, as well as features that appear vein-like. Selections from different replacement regions and textures were analyzed using an electron microprobe and laser ablation ICP-MS for variations in major and trace elements between textures, and for sample 2, in a transect from block to rind. Results from sample 1 indicate that vein chlorite is enriched in Mg, Ni, and Cr and depleted in Li relative to other chlorite textures. Ni and Cr in particular are enriched in chlorite in all settings compared to non-hydrous phases. Compositions of replacement chlorite do not reflect the composition of original minerals; chlorite replacing garnet does not show enrichment in Y, Ti, or REEs; chlorite replacing omphacite does not show enrichment in Sr. In contrast to sample 1, chlorites in sample 2 show strong depletions in Ni and Ti, weak depletion in Mg, and enrichment in Fe in the rind compared to the block. These depletions and enrichments suggest different fluid sources producing the chlorite in the two samples. These findings suggest that fluids in sample 1 were likely sourced from ultramafic material, while the source of fluids in sample 2 is less clear. Evidence suggests that generally fluid immobile elements (Ni, Cr, REE) were introduced to and removed from the samples through fluid.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0136B
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY