High-frequency isotopic variations in the Mauna Kea tholeiitic basalt sequence: Melt zone dispersivity and chromatography
High-frequency isotopic variations in the Mauna Kea tholeiitic basalt sequence: Melt zone dispersivity and chromatography
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DOI：
10.1029/95jb03494
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发表时间：
1996-05-10
影响因子：
3.9
通讯作者：
DePaolo, DJ
中科院分区：
文献类型：
--
作者：
DePaolo, DJ
分析了夏威夷科学钻探项目取芯的拉斑玄武岩Sr、Nd、Pb和He同位素比值测量的地层控制序列，以限制夏威夷下熔融生产带的分散性和色谱效率。使用简化模型的同位素信号通过熔融区的岩浆分离的浮力驱动的多孔流从上涌的固体传输的数据进行解释。假设一个恒定的熔岩积累率为620米厚的部分分析玄武岩，120 kyr的熔岩积累的总持续时间是基于Ar-Ar测年。通过傅立叶分析确定了熔岩序列中同位素变化的振幅-“周期”谱，并与地幔岩浆源的预测振幅-周期谱进行了比较，该预测基于假设的地幔振幅-波长谱和20 cm/yr的地幔柱上涌速度。地幔中的同位素变化在熔岩记录中似乎大大减弱。如果衰减是由于熔融区中的流体动力学分散，则熔融区分散度估计在100-1000 m的范围内，主要取决于熔体-基质速度对比度的假设值。分散性是熔融区有效粒度的粗略测量;大值，虽然严格来说只是上限，表明熔融区的渗透性是不均匀的，存在一定量的通道发育。在莫纳克亚山的部分中，He-3/He-4和Pb-206/Pb-204的比值描述了相互绘制的回旋环，与熔融区中He和Pb的显着色谱分离一致。同位素信号色谱的证据是一致的多孔模型的预测，并表明熔体保持适度接近化学平衡的固体，因为它分离。模拟结果表明，如果变化周期为10（5）年量级，熔融带中的色谱作用通常会导致熔岩记录中观测到的同位素比值-比值关系与地幔源区中的同位素比值-比值关系严重异相;对于大于10（6）年的周期，比例接近于“同相”，而对于小于10（6）年的周期，熔岩记录是绝望地混乱的。
A stratigraphically controlled sequence of Sr, Nd, Pb, and He isotope ratio measurements on tholeiitic basalt cored by the Hawaii Scientific Drilling Project is analyzed to constrain the dispersivity and chromatographic efficiency of the melt-producing zone under Hawaii. The data are interpreted using a simplified model for the transport of isotopic signals through the melt zone as magma separates from the upwelling solid by buoyancy-driven porous flow. A constant lava accumulation rate is assumed for the 620-m-thick section of analyzed basalt; a total duration of lava accumulation of 120 kyr is based on Ar-Ar dating. The amplitude-''period'' spectrum of the isotopic variations in the lava sequence is determined by Fourier analysis and compared with a predicted amplitude-period spectrum for the mantle magma sources, based on an assumed amplitude-wavelength spectrum for the mantle and a plume upwelling velocity of 20 cm/yr. The isotopic variations in the mantle appear to be substantially attenuated in the lava record. If the attenuation is due to hydrodynamic dispersion in the melting zone, then the melt zone dispersivity is estimated to be in the range 100-1000 m, depending mainly on the value assumed for the melt-matrix velocity contrast. Dispersivity is a crude measure of effective grain size in the melt zone; the large values, although strictly only upper limits, suggest that permeability in the melt zone is inhomogeneous and some amount of channel development is present. The ratios He-3/He-4 and Pb-206/Pb-204 in the Mauna Kea section describe convoluted loops when plotted against one another, consistent with significant chromatographic separation of He and Pb in the melt zone. The evidence for chromatography of isotopic signals is consistent with the porous how model predictions and indicates that the melt is maintained moderately close to chemical equilibrium with the solid as it separates. The modeling suggests that generally, chromatography in the melting zone causes isotope ratio-ratio relationships observed in the lava record to be severely out-of-phase with those present in the mantle source if the period of the variability is of order 10(5) years; for periods greater than 10(6) years the ratios are close to being ''in phase,'' whereas for periods less than 10(6) years the lava record is hopelessly scrambled.