High-pressure crystal structure and equation of state of ferromagnesian jeffbenite: implications for stability in the transition zone and uppermost lower mantle
High-pressure crystal structure and equation of state of ferromagnesian jeffbenite: implications for stability in the transition zone and uppermost lower mantle
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DOI：
10.1007/s00410-021-01850-0
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发表时间：
2021-10
影响因子：
3.5
通讯作者：
Fei Wang;E. Thompson;Dongzhou Zhang;E. Alp;Jiyong Zhao;J. Smyth;S. Jacobsen
中科院分区：
文献类型：
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作者：
Fei Wang;E. Thompson;Dongzhou Zhang;E. Alp;Jiyong Zhao;J. Smyth;S. Jacobsen
Jeffbenite，理想的Mg3 Al 2Si 3 O 12，已被确定为来自超过300 km深度的超深金刚石中的包裹体。虽然镁端成员jeffbenite在上地幔条件下具有有限的稳定性，含铁jeffbenite可能具有更广泛的P-T稳定性，延伸到过渡带或最上部的下地幔，将显着量的三价铁。利用基于同步辐射的单晶X射线衍射（XRD）和同步辐射穆斯堡尔谱（SMS）在高达29 GPa的压力下，我们报告了铁镁铁闪石（Mg2.32Al0.03Fe2+1.28Fe3+1.77Si2.85O12）中铁的晶体结构、可压缩性和可能的自旋跃迁。高压结构的细化表明，Fe 3+取代硅在T2网站，这与M2八面体共享的边缘，可能稳定jeffbenite在高压下，因为它增加了阳离子到阳离子之间的距离这些网站。尽管铁镁水硬铝石在30 GPa以下不会发生结构相变，但SMS超精细参数表明铁在22 GPa左右开始从高自旋（HS）到低自旋（LS）的电子自旋转变，这可能会增加其稳定性在高压下。将压力-容积数据拟合到三阶Birch-Murnaghan状态方程，得到V0 = 816.54（9），KT 0 = 181.54（1.39）和KT 0 = 2.76（14）。这些状态方程参数适用于超深金刚石包封压力的估算。铁镁硅铝榴石的密度和体积模量与铁铝榴石、铁铝榴石-镁铝榴石、钙钛榴石-镁铝榴石固溶体石榴石相近或更高，进一步表明硅铝榴石可能是地幔过渡带深部和下地幔上部重要的铁硅酸盐矿物。然而，未来的研究温度和氧化态对含铁铁的jeffbennite的稳定性和状态方程的影响仍然需要确定什么样的作用，如果有的话，jeffbennite在过渡带矿物学。
Jeffbenite, ideally Mg3Al2Si3O12, has been identified as inclusions in super-deep diamonds originating from depths that exceed 300 km. Although Mg-end member jeffbenite has limited stability at upper-mantle conditions, iron-bearing jeffbenite may have broaderP–Tstability that extends to the transition zone or uppermost lower mantle, incorporating significant amounts of ferric iron. Using synchrotron-based, single-crystal X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) at pressures up to 29 GPa, we report the crystal structure, compressibility, and likely spin transition of iron in ferromagnesian jeffbenite (Mg2.32Al0.03Fe2+1.28Fe3+1.77Si2.85O12). High-pressure structure refinements reveal that Fe3+substitution for Si in the T2 site, which shares edges with the M2 octahedron, likely stabilizes jeffbenite at high pressure, because it increases the cation-to-cation distance between these sites. Although ferromagnesian jeffbenite does not undergo a structural phase transition below 30 GPa, SMS hyperfine parameters suggest the onset of an electronic spin transition of iron from high-spin (HS) to low-spin (LS) at around 22 GPa, which may increase its stability at high pressures. Pressure–volume data were fit to a third order Birch–Murnaghan equation of state, resulting inV0= 816.54(9),KT0= 181.54(1.39), and= 2.76(14). These equation of state parameters are applicable to evaluating the encapsulation pressures of super-deep diamonds. The density and bulk modulus of ferromagnesian jeffbenite are similar to or higher than pyrope–almandine, pyrope–majorite, and skiagite–majorite solid solution garnets, further suggesting that jeffbenite may be an important ferric–iron silicate in the deeper parts of the mantle transition zone and uppermost lower mantle. However, future studies on the influence of temperature and oxidation state on the stability and equations of state of iron-bearing jeffbennite are still needed to determine what role, if any, jeffbenite plays in transition-zone mineralogy.