Stability of Al-bearing superhydrous phase B at the mantle transition zone and the uppermost lower mantle
Stability of Al-bearing superhydrous phase B at the mantle transition zone and the uppermost lower mantle
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DOI：
10.2138/am-2018-6499
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发表时间：
2018-07
影响因子：
3.1
通讯作者：
S. Kakizawa;T. Inoue;Hideto Nakano;M. Kuroda;N. Sakamoto;H. Yurimoto
中科院分区：
文献类型：
--
作者：
S. Kakizawa;T. Inoue;Hideto Nakano;M. Kuroda;N. Sakamoto;H. Yurimoto
摘要测定了含al超水B相在20 ~ 24 GPa和1400 ~ 2000℃的稳定性和化学组成，探讨了地幔过渡带和上下地幔在接近地幔地热温度下的水输运机制。超水相B含有大量的Al2O3，从14 wt%到32 wt%不等，即使在2000℃和大约20-24 GPa的压力下，含al的超水相B仍然保持稳定。在20 ~ 24 GPa和1600℃温度下，存在两种不同化学组成的超水相B。当温度和压力升高到1600℃时，Al2O3和H2O含量增加，MgO和SiO2含量降低。在1600℃以上，随着温度的升高，MgO和Al2O3含量增加，SiO2和H2O含量降低。我们发现了两种取代机制：(1)2Mg2+ + Si4+;;)当下2Al3+ + 2H+ +□Mg （Mg位点空缺）（2Mg2+ = Al3+ + H+ +□Mg）：(Si4+ = Al3+ + H+) = 1:1, (2) Si4+ + 16H+换回4Mg2+ + 4Al3+。含al超水相B的最大H2O含量为11.1(3)wt%，是mg端元的1.9倍。两个共存的超水相B值的晶体结构预计彼此略有不同。结果表明，与软锰矿（如绿泥石）相比，含Al过水B相在典型的地幔过渡带和下地幔温度下可以稳定存在于高Al含量的俯冲板块中。因此，即使在典型的地幔地热中，水也可以通过俯冲板块中含al的超水B相输送到下地幔。
Abstract We determined the stability and chemical composition of Al-bearing superhydrous phase B at 20–24 GPa and 1400–2000 °C to discuss the mechanism of water transport in the mantle transition zone and uppermost lower mantle at temperatures close to the mantle geotherm. Superhydrous phase B contained significant amounts of Al2O3, from 14 to 32 wt%, and Al-bearing superhydrous phase B remained stable, even at 2000 °C and pressures of approximately 20–24 GPa. Moreover, two types of superhydrous phase B with different chemical compositions coexisted at 20–24 GPa and 1600 °C. The Al2O3 and H2O contents increased, and the MgO and SiO2 contents decreased as the pressure and temperature increased up to 1600 °C. Above 1600 °C, the MgO and Al2O3 contents increased, and the SiO2 and H2O contents decreased as the temperature increased. We found two substitution mechanisms: (1) 2Mg2+ + Si4+ ⇄ 2Al3+ + 2H+ + □Mg (Mg site vacancy) (2Mg2+ = Al3+ + H+ + □Mg):(Si4+ = Al3+ + H+) = 1:1, (2) Si4+ + 16H+ ⇆ 4Mg2+ + 4Al3+. The maximum H2O content of Al-bearing superhydrous phase B is 11.1(3) wt%, which is ~ 1.9 times larger than that of the Mg-end-member. The crystal structures of the two coexisting superhydrous phase B values are expected to be slightly different from each other. The present results indicate that Al-bearing superhydrous phase B can be stable in a subducted slab with a high Al content compared to pyrolite (e.g, chlorite) at temperatures typical of the mantle transition zone and the lower mantle. Thus, water can be transported to the lower mantle by Al-bearing superhydrous phase B in the subducting slab, even at the typical mantle geotherm.