Abstract
The pyrope (Prp)–almandine (Alm) solid solutions are the most fundamental garnet components on the Earth, and both the quartz inclusions in garnet (QuiG) barometry and the garnet inclusions in diamond barometry need to be constrained by the thermoelastic parameters of Prp-Alm solid solution garnets. Here, we report the thermoelastic properties of a series of synthetic Prp-Alm solid solutions based on the high-pressure and high-temperature (HP–HT)in situsynchrotron single-crystal x-ray diffraction (SCXRD) experiments up to ∼20 GPa and 700 K, using diamond anvil cell (DAC). Fitting the SCXRD data by the Birch-Murnaghan equation of state (BM-EoS) and the thermal-pressure EoS, we obtain the thermoelastic parameters of Prp-Alm solid solution garnets, including bulk modulus (K0), its pressure derivative (K′0), and the thermal expansion coefficient (α0). TheK0along the Prp-Alm solid solution changes linearly with Prp content within their uncertainties and can be expressed byK0(GPa) = 181.0(8) – 0.11(1)Xprp(R2= 0.91,Xprpis the Prp mole fraction andK′0fixed at 4). Our result indicates that the compressibility of the Prp-Alm solid solution increases with the increasing Prp content. However, the thermal expansion coefficient of Prp-Alm solid solution at ambient pressure shows a non-linear trend with Prp content and can be expressed byα0(10−5K−1) = 2.7 (1) + 3.0 (5)XPrp−3.2 (4)X2Prp(R2= 0.985). It shows that the Prp-Alm solid solution with intermediate composition has a larger thermal expansion coefficient than those close to the endmembers at ambient conditions. Furthermore, we also evaluated the influence of thermoelastic properties of the Prp-Alm solid solution on the entrapment pressure (Pe) estimation for two types of elastic geobarometers. Our results indicate that the garnet component may significantly influence entrapment pressure, and among the thermoelastic parameters of garnet, the thermal expansion coefficient has the main effect on the estimation ofPe.
Highlights
Diamond is the only direct sample obtained from inaccessible portions of Earth (Nestola et al, 2019) and the subduction zone metamorphism controls many global geochemical cycles and the lithosphere (Ashley et al, 2016)
The P–V relations of these Prp-Alm solid solutions have been determined by fitting their High pressure and room temperature (HP–RT) data to the third-order Birch-Murnaghan EoS, which is given in the following form: P
For most of the ferromagnesian silicates and carbonates that have been studied so far, the bulk modulus appears to increase with increasing Fe content, such as garnet (Huang and Chen, 2014), olivine (Speziale et al, 2004), and siderite (Zhang J. et al, 1998), which is inconsistent with bulk modulus–volume systematics
Summary
Diamond is the only direct sample obtained from inaccessible portions of Earth (Nestola et al, 2019) and the subduction zone metamorphism controls many global geochemical cycles and the lithosphere (Ashley et al, 2016). Estimating the pressure and temperature of these minerals or rocks provides fundamental data for studying many such geological processes, but their determination remains extremely controversial (Bebout, 2007; Tajčmanová et al, 2021). Part of the difficulty in determining the pressure of rocks is the lack of typical pressure-dependent mineral equilibria, especially for the diamond-inclusion system (Ashley et al, 2016; Nestola et al, 2017)
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