Thermal equation of state of post-aragonite CaCO3-Pmmn
Thermal equation of state of post-aragonite CaCO3-Pmmn
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DOI：
10.2138/am-2020-7279
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发表时间：
2020-09
影响因子：
3.1
通讯作者：
M. Lv;Jiachao Liu;E. Greenberg;V. Prakapenka;Susannah M Dorfman
中科院分区：
文献类型：
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作者：
M. Lv;Jiachao Liu;E. Greenberg;V. Prakapenka;Susannah M Dorfman
摘要碳酸钙（CaCO 3）是地球表面最丰富的碳酸盐之一，通过俯冲作用将碳输送到地球内部。虽然一些岩石学的观察支持保存在冷板下地幔深处的碳酸钙，地球物理性质和稳定性的碳酸钙在这些深度是未知的，部分原因是复杂的多晶相变和缺乏限制的热力学性质。在这里，我们测量了CaCO 3-Pmmn的热状态方程，CaCO 3的稳定多晶型物通过下地幔的大部分，使用激光加热的金刚石砧室中的同步加速器X射线衍射高达75 GPa和2200 K。用三阶Birch-Murnaghan状态方程拟合了CaCO 3-Pmmn的室温压缩数据，得到KT 0 = 146.7（±1.9）GPa和K 0 = 3.4（±0.1），其中V0固定为从头计算确定的值97.76 × 10 - 3。高温压缩数据与零压热膨胀αT = a0 + a1 T一致，其中a0 = 4.3（±0.3）×10-5 K-1，a1 = 0.8（±0.2）×10-8 K-2，体积模量的温度导数（Δ KT/Δ T）P = -0.021（±0.001）GPa/K;在德拜温度θ0 = 631 K时，通过从头计算预测了Grüneisen参数γ0 = 1.94（±0.02），体积无关常数q = 1.9（±0.3）。利用这些新确定的热力学参数，密度和体积声速的CaCO 3-Pmmn和（Ca，Mg）-碳酸盐的榴辉岩定量模拟从30至80 GPa沿着冷板地热。假设碳酸盐均匀地混合到板中，结果表明，在俯冲板中碳酸盐的存在是不可能被地震观测检测到的，碳酸盐提供的浮力对板动力学的影响可以忽略不计。
Abstract Calcium carbonate (CaCO3) is one of the most abundant carbonates on Earth’s surface and transports carbon to Earth’s interior via subduction. Although some petrological observations support the preservation of CaCO3 in cold slabs to lower mantle depths, the geophysical properties and stability of CaCO3 at these depths are not known, due in part to complicated polymorphic phase transitions and lack of constraints on thermodynamic properties. Here we measured thermal equation of state of CaCO3-Pmmn, the stable polymorph of CaCO3 through much of the lower mantle, using synchrotron X‑ray diffraction in a laser-heated diamond-anvil cell up to 75 GPa and 2200 K. The room-temperature compression data for CaCO3-Pmmn are fit with third-order Birch-Murnaghan equation of state, yielding KT0 = 146.7 (±1.9) GPa and K0 = 3.4(±0.1) with V0 fixed to the value determined by ab initio calculation, 97.76 Å3. High-temperature compression data are consistent with zero-pressure thermal expansion αT = a0 + a1T with a0 = 4.3(±0.3)×10-5 K-1, a1 = 0.8(±0.2)×10-8 K-2, temperature derivative of the bulk modulus (∂KT/∂T)P = –0.021(±0.001) GPa/K; the Grüneisen parameter γ0 = 1.94(±0.02), and the volume independent constant q = 1.9(±0.3) at a fixed Debye temperature θ0 = 631 K predicted via ab initio calculation. Using these newly determined thermodynamic parameters, the density and bulk sound velocity of CaCO3-Pmmn and (Ca,Mg)-carbonate-bearing eclogite are quantitatively modeled from 30 to 80 GPa along a cold slab geotherm. With the assumption that carbonates are homogeneously mixed into the slab, the results indicate the presence of carbonates in the subducted slab is unlikely to be detected by seismic observations, and the buoyancy provided by carbonates has a negligible effect on slab dynamics.