Abstract

Tubes, 3–l0 μm in diameter and up to 3 mm long, and linear arrays of spherical and crystallographically faceted fluid inclusions, 5–35 μm in diameter, were produced in single crystals of San Carlos olivine (Fo89) annealed in the presence of CO2 and CO2-H2O fluids at 2–3 GPa and 1200–1400 °C in a piston-cylinder apparatus utilizing NaCl-BN sample assemblies. Experiments conducted under identical conditions in the same apparatus utilizing NaCl + pyrex + alumina sample assemblies did not produce tubes or fluid inclusions. Subsequent oxidation of one specimen to reveal the dislocation substructure showed that inclusions that can be traced to tubes in the process of necking down and linear inclusion arrays (representing healed tubes) are connected by single and multiple curvilinear dislocations. These observations suggest that the tubes formed on growth dislocations that existed in the olivine crystals prior to annealing. The formation of tubes is sensitive to chemical environment, and the growth rate of tubes is dependent on temperature. The tubes are interpreted to be etch tunnels, based on their similarity to etch tunnels produced in synthetic and natural quartz by HF etching and hydrothermal treatment. The most commonly accepted origin for etch tunnels in quartz is that growth dislocations segregate impurities (H or Al) that locally enhance the solubility of the crystal in the fluid. It appears that etch tunnels in olivine are produced by a similar mechanism.

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