Trace Element Distribution and Mobility in Naturally-deformed Quartz
Abstract
We investigate the degree to which crystal plastic deformation affects trace element concentration and distribution in naturally-deformed quartz. The concentration and mobility of trace elements in deformed quartz is crucial to the effective application of many trace element thermobarometers, particularly the widely-cited Ti-in-quartz (TitaniQ).
We present chemical and fabric data from the Jurassic Cottonwood Springs granodiorite (CSG) in the Transverse Ranges of California. The CSG, containing an assemblage of Qz + Fsp (Pl + Kfs) + Bt + Hbl + Ttn + Zrn + opaques, intruded Proterozoic basement gneisses; these units were subsequently intruded by Cretaceous arc plutons. Discrete shear zones ranging in thickness from 10 cm - 2 m cut an otherwise magmatically foliated CSG, suggesting a single episode of subsolidus deformation. These shear zones, which are laterally continuous over 100+ m, are truncated by Late Cretaceous plutons. Shear zones within the CSG preserve a broad range of microstructures, grading from magmatic fabrics to high-strain, banded ultramylonites. Quartz preserves microstructures indicative of high temperature grain boundary migration near shear zone margins and lower temperature subgrain rotation and bulge nucleation within the central higher strain regions. Cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and electron backscatter diffraction (EBSD) are used to quantify the relationship between trace element concentration and quartz microstructure from the single grain to shear zone scale. Investigation by CL reveals significant variation in intensity of luminescence between recrystallized grains and magmatic grains or porphyroclasts, suggesting a variation in trace element content. We quantify this difference with detailed SIMS transects measuring concentrations of Al, Li, Ti, Ge, and Na. EBSD-based microstructure and texture analysis is used to determine recrystallization mechanisms, inter- and intragrain misorientations, and active slip systems.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMMR41B0057L
- Keywords:
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- 0545 Modeling;
- COMPUTATIONAL GEOPHYSICSDE: 1822 Geomechanics;
- HYDROLOGYDE: 5102 Acoustic properties;
- PHYSICAL PROPERTIES OF ROCKSDE: 5139 Transport properties;
- PHYSICAL PROPERTIES OF ROCKS