410 km discontinuity sharpness and the form of the olivine α-β phase diagram
410 km discontinuity sharpness and the form of the olivine α-β phase diagram
复制标题
410 km间断面锐度与橄榄石α-β相图的形式
DOI：
10.1111/j.1365-246x.1996.tb05292.x
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发表时间：
1996
影响因子：
2.8
通讯作者：
B. Wood
中科院分区：
文献类型：
--
作者：
G. Helffrich;B. Wood
摘要
根据不连续性反射率，410 公里地震不连续性的厚度估计低至 4 公里，据信是由于橄榄石中的 α-橄榄石β-改性尖晶石转变所致。然而，如果使用属性的线性插值进行建模，则地震估计的厚度会偏向于比真实变换间隔更窄的值。 5 km 线速度梯度产生的平均反射系数与基于橄榄石相图特征的 10 km 过渡间隔的平均反射系数相同。此外，相图的替代形式，同样与实验确定的铁镁分配一致，可以产生窄至 4 km 的真实转变间隔。这通过两种方式协调了相平衡和不连续厚度的地震测量之间的差异：（1）地震厚度估计太窄； (2) 考虑到现有的相平衡约束，允许窄的过渡间隔。结合最近关于 H2O 对不连续性特性影响的结果，410 km 不连续性反射率似乎对 H2O 浓度变化比对温度更敏感，这表明不连续性反射率变化反映了地幔化学的变化。
SUMMARY
Estimates of the thickness of the 410 km seismic discontinuity, believed to be due to the α-olivineβ-modified-spinel transformation in olivine, are as low as 4 km based on discontinuity reflectivity. The seismically estimated thickness is, however, biased to values narrower than the true transformation interval if linear interpolation of properties is used for modelling. A 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase-diagram features. Moreover, alternative forms of the phase diagram, equally consistent with experimentally determined iron-magnesium partitioning, can yield true transition intervals as narrow as 4 km. This reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways: (1) seismic thickness estimates are too narrow; and (2) narrow transition intervals are permissible given existing phase-equilibrium constraints. Incorporating recent results on the influence of H2O on discontinuity properties, it appears that 410 km discontinuity reflectivity is much more sensitive to varying H2O concentration than to temperature, suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry.