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1. First experimental evaluation of zircon-in-garnet elastic thermobarometry at high pressure

   Koch, M M; Thomas, J B; Gonzalez, J P (May 2025, Geological Society of America)

   Phase equilibrium modeling and conventional thermobarometry are rooted in the assumption that chemical equilibrium is closely attained during metamorphism to determine the pressure-temperature (P-T) histories of metamorphic rocks. Alternatively, elastic thermobarometry, a complimentary method for P-T calculations, does not require chemical equilibrium constraints and is instead based on the elastic response of mineral inclusions and their host mineral upon exhumation. The most commonly used host-inclusion system is the quartz-in-garnet (QuiG) barometer, which has been experimentally evaluated and applied to many natural samples. Zircon-in-garnet (ZiG) is often cited as a host-inclusion pair that can be used for thermometry but has had far less application in natural case studies and has been observed to reset in atmospheric-pressure heating experiments (Campomenosi et al., 2023; Zhong et al., 2024). We performed piston-cylinder experiments entrapping synthetic zircons in almandine garnet at conditions of 700–900 °C and 2.0–3.0 GPa to experimentally evaluate the reliability of ZiG. We used Raman spectroscopy to measure the 356, 439, 975, and 1008 cm-1 phonon modes of synthesized zircon inclusions and calculated their peak shifts relative to a free synthetic zircon crystal. We subsequently calculated remnant inclusion strains, stresses, and pressures (Pinc) using the zircon phonon mode Grüneisen tensor approach (Angel et al., 2019) and elastic stiffness tensor (Cij) of zircon. ZiG host-inclusion pairs synthesized at pressures >2.0 GPa yield Pinc values within one standard deviation of expected values, corresponding to entrapment temperatures (Ttrap) within ~15 °C of experimental temperatures. experiments run at 2.0 GPa yield Pinc values that are ~0.08 GPa systematically higher than expected; this corresponds to Ttrap values that are ~60 °C higher than the experimental conditions. We hypothesize that systematic Pinc discrepancies in 2.0 GPa experiments potentially derived from slight errors in the zircon or garnet equations of state (EoS) and/or Cij tensors. Zircon, in particular, readily substitutes trace elements into its crystalline structure and can be altered by radiation damage, which, even in slight quantities, can greatly affect both the EoS and Cij relative to pristine zircon. Nonetheless, application of ZiG and QuiG elastic thermobarometry together is a potentially powerful tool for constraining the P-T conditions of metamorphic events independent of assumed chemical equilibrium. Angel, R.J., Murri, M., Mihailova, B., and Alvaro, M., 2019, Stress, strain, and Raman shifts: Zeitschrift für Kristallographie, v. 234, p. 129–140. Campomenosi, N., Angel, R.J., Alvaro, M., and Mihailova, B., 2023, Resetting of zircon inclusions in garnet: Implications for elastic thermobarometry: Geology, v. 51, p. 23–27, https://doi.org/10.1130/G50431.1. Zhong, X., Wallis, D., Kingsbery, P., and John, T., 2024, The effect of aqueous fluid on viscous relaxation of garnet and modification of inclusion pressures after entrapment: Earth and Planetary Science Letters, https://doi.org/10.1016/j.epsl.2024.118713. 

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2. Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

   https://doi.org/10.5194/se-12-1335-2021  

   Cisneros, Miguel; Barnes, Jaime D.; Behr, Whitney M.; Kotowski, Alissa J.; Stockli, Daniel F.; Soukis, Konstantinos (January 2021, Solid Earth)

   Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes. 

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3. Effect of structural water on the elasticity of orthopyroxene

   https://doi.org/10.2138/am-2021-7843  

   Hou, Mingqiang; Zhou, Wen-Yi; Hao, Ming; Hua, Florian Tian-Siang; Kung, Jennifer; Zhang, Dongzhou; Dera, Przemyslaw K.; Zhang, Jin S. (April 2022, American Mineralogist)

   Abstract As a major nominally anhydrous mineral (NAM) in the Earth’s upper mantle, orthopyroxene could host up to several hundred parts per million H2O in its crystal structure and transport the H2O to the deep Earth. To study the effect of structural H2O on the elasticity of orthopyroxene, we have measured the single-crystal elasticity of Mg1.991Al0.065Si1.951O6 with 842–900 ppm H2O and 1.64 ± 0.20 wt% Al2O3 at ambient conditions using Brillouin spectroscopy. The best-fit single-crystal elastic moduli (Cijs), bulk (KS0), and shear (G0) modulus of the hydrous Al-bearing orthopyroxene were determined as: C11 = 235(2) GPa, C22 = 173(2) GPa, C33 = 222(2) GPa, C44 = 86(1) GPa, C55 = 82(1) GPa, C66 = 82(1) GPa, C12 = 75(3) GPa, C13 = 67(2) GPa, and C23 = 49(2) GPa, KS0 = 111(2) GPa, and G0 = 78(1) GPa. Systematic analysis based on the results presented in this and previous studies suggests that the incorporation of 842–900 ppm H2O would increase C13 by 12.0(7)% and decrease C23 by 8.6(8)%. The effects on C11, C22, C33, C44, C66, KS0, and VP are subtle if not negligible when considering the uncertainties. The C55, C12, G0, and VS are not affected by the presence of structural H2O. Although laboratory experiments show that Fe,Al-bearing orthopyroxenes can host up to 0.8 wt% H2O in its structure, future high-pressure-temperature elasticity measurements on orthopyroxene with higher H2O content are needed to help better quantify this effect. 

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4. First experimental evaluation of zircon-in-garnet elastic thermobarometry at high pressures

   https://doi.org/10.1130/G54299.1  

   Koch, Megan M; Gonzalez, Joseph P; Angel, Ross J; Thomas, Jay B (March 2026, Geology)

   Abstract Zircon inclusions in garnet (ZiG) can be used as an elastic thermobarometer to estimate the pressure-temperature (P-T) conditions of geologic processes. Although this method is being increasingly applied as a thermometer to natural samples, experimental evaluation of ZiG is limited and has not been investigated at geologically relevant pressures. We present the first high-pressure assessment of the reliability of ZiG thermobarometry by crystallizing almandine with zircon inclusions in piston-cylinder experiments between 700–900 °C and 2.0–3.0 GPa. Zircons entrapped in experiments at >2.0 GPa yield residual inclusion pressures within 1σ of predicted values and yield entrapment temperatures within 17–30 °C of experimental T. Zircon inclusions in experiments performed at 2.0 GPa yield average inclusion pressures that are ~0.09 GPa higher than predicted values and therefore yield entrapment temperatures 65–73 °C higher than the experimental T. Evaluation of inclusion pressure trends shows that ZiG host-inclusion systems did not undergo non-elastic deformation during experimental exhumation, confirming that ZiG elastic thermobarometry can accurately record P-T conditions of garnet crystallization so long as zircon inclusions do not go into tension post-entrapment. Our high-pressure experiments suggest that the ZiG host-inclusion system may be applicable to a variety of geologic settings and processes, such as subduction zone metamorphism, partial melting, and contact metamorphism. 

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5. High-pressure single-crystal elasticity and thermal equation of state of omphacite and their implications for the seismic properties of eclogite in the Earth's interior

   https://doi.org/10.1029/2018JB016964  

   Hao, Ming; Zhang, Jin S.; Pierotti, Caroline E.; Ren, Zhiyuan; Zhang, D. (February 2019, Journal of Geophysical Research: Solid Earth)

   Omphacite is a major mineral phase of eclogite, which provides the main driving force for the slab subduction into the Earth's interior. We have measured the single‐crystal elastic moduli of omphacite at high pressures for the first time up to 18 GPa at ambient temperature using Brillouin spectroscopy. A least‐squares fit of the velocity‐pressure data to the third‐order finite strain equation of state yields KS0'=4.5(3), G0'=1.6(1) with ρ0=3.34(1) g/cm3, KS0=123(3) GPa, and G0=74(2) GPa. In addition, the synchrotron single‐crystal X‐ray diffraction data have been collected up to 18 GPa and 700 K. The fitting to Holland‐Powell thermal‐pressure equation of state yields KT0'=4.6(5) and α0=2.7(8) ×10‐5 K‐1. Based on the obtained thermoelastic parameters of omphacite, the anisotropic seismic velocities of eclogite are modeled and compared with pyrolite between 200 to 500 km. The largest contrast between the eclogite and pyrolite in terms of seismic properties is observed between ~310 to 410 km. 

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