Incorporation of high amounts of Na in ringwoodite: Possible implications for transport of alkali into lower mantle
Incorporation of high amounts of Na in ringwoodite: Possible implications for transport of alkali into lower mantle
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DOI：
10.2138/am-2016-5570
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发表时间：
2016-01-01
影响因子：
3.1
通讯作者：
Irifune, Tetsuo
中科院分区：
文献类型：
--
作者：
Bindi, Luca;Tamarova, Anastasia;Irifune, Tetsuo
本文报道了MgSiO_3-Na_2CO_3-Al_2O_3体系中富钠的环磷钙石和硼镁石在24 GPa和1700 ℃下的共存现象。在我们的实验中，林伍德石结合了高达4.4wt%的Na 2 O，Na与Si一起进入八面体位置，根据机制：Mg 2 + -> 2/3 Na（+）+ 1/3Si（4+）。晶胞体积随Na含量的增加沿着增大。一个类似的行为被观察到的钠轴承bridgmanite的晶胞体积，虽然钠掺入到这种结构的机制仍然未知，因为缺乏足够的晶体学数据。Na 2 O在林伍德石中相对于硼镁石是相容的，分配系数（D）为5（+5/-4），但在林伍德石中相对于富碳酸盐熔体/流体是不相容的，D值在0.5和0.1之间。铝是高度富集在bridgmanite相对于其他共存相。深部过渡带中碳酸盐岩熔体交代作用可能导致钠的局部富集，而林伍德岩可能是深部过渡带中钠的重要容矿体。交代地幔的后续对流或俯冲作用可能导致上地幔和下地幔中碱性元素的富集。
Here we report on the coexistence between Na-rich ringwoodite and bridgmanite in the system MgSiO3-Na2CO3-Al2O3 at 24 GPa and 1700 degrees C. In our experiments ringwoodite incorporates up to 4.4 wt% Na2O, with Na entering the octahedral site together with Si, according to the mechanism: Mg2+ -> 2/3Na(+) + 1/3Si(4+). The volume of the unit cell increases along with the Na content. A similar behavior is observed for the unit-cell volume of Na-bearing bridgmanite, although the mechanism of Na incorporation into this structure remains unknown because of the lack of sufficient crystallographic data. Na2O is compatible in ringwoodite relative to bridgmanite with a partition coefficient (D) of 5 (+5/-4), but is incompatible in ringwoodite relative to carbonate-rich melt/fluid, with the D value ranging between 0.5 and 0.1. Al is highly enriched in bridgmanite relative to the other coexisting phases. Carbonatitic melt metasomatism in the deep transition zone may lead to local Na-enrichment, and ringwoodite may be an important host for Na in the deep transition zone. Subsequent convection or subduction of metasomatized mantle may lead to enrichment of alkaline elements in the upper and lower mantle.