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Thermal equation of state of Cr-pyrope: implications for entrapment pressure of Cr-pyrope inclusion in diamond


Cr-pyrope is one of the most abundant mineral inclusions in peridotitic diamonds and its thermal equation of state (EoS) is a prerequisite for accurately determining its entrapment pressure (Pe), which is important to understanding the physicochemical environment of diamond formation. We present in situ single-crystal X-ray diffraction (XRD) experimental results of five natural Cr-pyropes (Cr# = 0.3–22.4, Cr# = Cr/(Cr + Al)) at high pressure (P), high temperature (T), and high PT up to 13.2 GPa and 950 K. The obtained P–volume(V)–T data were used to derive EoS parameters. The results indicated that the compressional behaviors of these Cr-pyropes are close (up to 2.8% difference), but the differences between their thermal expansivities are up to 8.8%. The thermal expansivities of the Cr-pyropes are significantly higher than that of end-member garnets (pyrope, almandine, and grossular) obtained by room-P high-T XRD, but they are consistent with the end-member thermal expansivities obtained by high PT XRD. To investigate the compositional effects on the estimation of Pe of Cr-pyrope, the obtained EoS parameters were used to calculate the Pe in diamond. The results indicated that the variation in thermal expansion behavior plays a more significant role in influencing the Pe in comparison with the compressional behavior. In addition, the Pe of the Cr-pyropes are compared with the Pe of end-member garnets (pyrope, almandine, grossular, and uvarovite), which indicates that the low-Cr (Cr# = 0.3–4.4) pyropes are closer to pyrope in Pe while the high-Cr (Cr# = 9.3–22.4) pyropes are closer to grossular.

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This project was funded by the National Natural Science Foundation of China (41802043, U2032118, and 42172048), the Chinese Academy of Sciences “Light of West China” Program (2019), the Youth Innovation Promotion Association CAS (Dawei Fan, 2018434), and the Science and Technology Foundation of Guizhou Province (QKHJC-ZK[2021]ZD042). Guangzhong Yang acknowledges the financial support from “The geological research project of Guizhou province—The research of the placer anomaly and mineralogical characteristics of pyrope in Xiawengshao area, Shibing County (2016-02)”. The experiments were performed at GeoSoilEnviroCARS (13-BM-D), Advanced Photon Source (APS), and Argonne National Laboratory. The use of the gas-loading system is supported by COMPRES and GeoSoilEnviroCARS. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1634415) and the Department of Energy—Geosciences (DE-FG02-94ER14466). COMPRES is under NSF Cooperative Agreement EAR-1661511. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We thank Dr. Peter Eng for kindly providing the micro heater for room-P high-T experiments, the editor (Prof. Dante Canil) for handling this manuscript, and Dr. Robert Myhill and the other anonymous reviewer for the thorough reviews, which help to improve this manuscript. The authors declare no competing financial interests.

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Correspondence to Jingui Xu.

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Communicated by Dante Canil.

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Xu, J., Fan, D., Li, B. et al. Thermal equation of state of Cr-pyrope: implications for entrapment pressure of Cr-pyrope inclusion in diamond. Contrib Mineral Petrol 177, 69 (2022).

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  • Cr-pyrope
  • Thermal equation of state
  • Diamond
  • Entrapment pressure