The role of symmetry-breaking strains on quartz inclusions in anisotropic hosts: Implications for Raman elastic geobarometry
The role of symmetry-breaking strains on quartz inclusions in anisotropic hosts: Implications for Raman elastic geobarometry
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对称破缺应变对各向异性基质中石英包裹体的作用：对拉曼弹性地压测量的影响
DOI：
10.1016/j.lithos.2022.106716
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发表时间：
2022
期刊：
影响因子：
3.5
通讯作者：
Alvaro, M.
中科院分区：
文献类型：
--
作者：
Murri, M.;Gonzalez, J.P.;Mazzucchelli, M.L.;Prencipe, M.;Mihailova, B.;Angel, R.J.;Alvaro, M.
矿物基质-包裹体系统的拉曼弹性地质压力法通过测量包裹体的拉曼波数位移来确定作用在包裹体上的应变。计算的夹杂物应变，然后可以在弹性模型中使用，以计算包埋的压力和温度条件。这种方法经常应用于主体包裹体系统，其中主体几乎是弹性各向同性的（例如石榴石），而包裹体是弹性各向异性的（例如石英和锆石）。在这种情况下，当在流体静力条件下发生包埋时，主体将在夹杂物上施加各向同性应变，这反过来将产生非流体静力应力。在这种情况下，包裹体矿物的对称性得以保持，并且包裹体中的应变可以通过使用声子模式Grüneisen张量方法的拉曼光谱来测量。然而，当主体-包裹体系统完全各向异性时，例如当石英包裹体被捕获在锆石主体内时，这是因为，由于主体施加在夹杂物上的外部各向异性应变场，夹杂物的对称性可能被破坏，这又将改变声子模式。因此，我们计算了应变状态的石英包裹体中捕获的锆石主机在多个方向和在各种地质相关的压力和温度条件。然后，我们进行了从头算Hartree-Fock/密度泛函理论（HF/DFT）模拟α-石英在这些应变状态。这些HF/DFT模拟表明，在拉曼模式的位置的变化所产生的应变，预计对称破碎的石英夹杂物中的锆石通常是类似的，如果石英夹杂物保持真正的三角对称。因此，使用三角声子模Grüneisen张量来确定包裹体应变不会导致计算锆石中石英包裹体包封压力的地质学显著误差。
Raman elastic geobarometry for mineral host-inclusion systems is used to determine the strains acting on an inclusion still entrapped in its host by measuring its Raman wavenumber shifts which are interpreted through the phonon-mode Grüneisen tensors of the inclusion phase. The calculated inclusion strains can then be used in an elastic model to calculate the pressure and temperature conditions of entrapment. This method is applied frequently to host inclusion systems where the host is almost elastically isotropic (e.g. garnet) and the inclusion is elastically anisotropic (e.g. quartz and zircon). In this case, when the entrapment occurs under hydrostatic conditions the host will impose isotropic strains on the inclusion which in turn will develop non-hydrostatic stress. In this scenario the symmetry of the inclusion mineral is preserved and the strains in the inclusion can be measured via Raman spectroscopy using the phonon-mode Grüneisen tensor approach.However, a more complex situation arises when the host-inclusion system is fully anisotropic, such as when a quartz inclusion is entrapped within a zircon host, because the symmetry of the inclusion can be broken due to the external anisotropic strain field imposed on the inclusion by the host, which in turn will modify the phonon modes. We therefore calculated the strain states of quartz inclusions entrapped in zircon hosts in multiple orientations and at various geologically relevant pressure and temperature conditions. We then performed ab initio Hartree-Fock/Density Functional Theory (HF/DFT) simulations on α-quartz in these strain states. These HF/DFT simulations show that the changes in the positions of the Raman modes produced by strains that are expected for symmetry broken quartz inclusions in zircon are generally similar to those that would be seen if the quartz inclusions remained truly trigonal in symmetry. Therefore, the use of the trigonal phonon-mode Grüneisen tensor to determine the inclusion strains does not lead to geologically significant errors in calculated quartz inclusion entrapment pressures in zircon.