FIU Discovery

Garnets are major silicates from the upper mantle to the transition zone. Elastic properties of garnets are essential to interpret the variation of seismic velocities at different depths and construct a model of the Earth’s composition. Due to the chemical flexibility at octahedron sites of the crystal structures, garnets usually exist with multiple components and have many composition variations. Pyrope is an important member in the garnet group. Fe²⁺-Mg²⁺ substitution in pyrope is one of the common solid solutions. We have synthesized and measured three synthetic solid solutions samples (Py₈₃Alm₁₇, Py₅₄Alm₄₆ and Py₃₀Alm₇₀). Equations of state yielded their isothermal bulk moduli K₀ to be 172(4)GPa, 174(2)GPa, and 183(2)GPa, respectively, which confirmed that almandine content (Fe²⁺ substitution) increased the bulk modulus of the garnet. A relation between the bulk modulus and the almandine mole fraction (n) was derived to be K₀ = 170 + 15 n, showing it is a nearly ideal mixing model.

Another factor that also significantly influences the elasticity of pyrope is water. Water is transported to the deep Earth by subduction slabs and mainly exists in nominally anhydrous minerals (NAM) as hydroxyl (OH^-). Its content in minerals varies as depth increases. We therefore investigated pressure influence on water solubility in pyrope. A suite of pyrope single crystals was synthesized in a water-saturated environment at 6, 7, 9 and 12GPa and water was characterized by FTIR. IR spectra showed a typical peak at 3630 cm^-1. At 9 and 12GPa, new peaks at 3572 cm^-1 and 3504 cm^-1 appeared and indicated that a new substitution mechanism, other than hydrogarnet substitution SiO₄=(OH)₄, was adopted in the pyrope crystal structure. Water solubility in pyrope reached 0.2wt% at 7GPa. From 4-7GPa, water solubility increased. At 9GPa, water content dropped to 0.07wt% and increased to 0.3wt% at 12GPa, where a cubic to tetragonal phase transition was observed. Water showed a weakening effect on the bulk moduli of hydrous pyrope. Their bulk moduli were 166GPa, 173GPa and 161GPa with water contents of 0.07wt%, 0.1wt% and 0.2wt%, respectively. An approximate linear relationship was proposed about the bulk modulus as a function of water content.