Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity
Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity
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DOI：
10.1016/0012-821x(84)90004-9
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发表时间：
1984-10
影响因子：
5.3
通讯作者：
A. Zindler;H. Staudigel;R. Batiza
中科院分区：
文献类型：
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作者：
A. Zindler;H. Staudigel;R. Batiza
东太平洋隆起6.8 m内、北纬9°~ 14°的年轻海山玄武岩的143nd /144Nd（0.51295 ~ 0.51321）、87Sr/86Sr（0.7025 ~ 0.7031）和（La/Sm）N（0.415 ~ 3.270）变化显著。在143nd /144Nd vs.87Sr/86Sr变化图上，Nd和Sr同位素比值呈反相关关系，形成与“地幔阵”大致平行的趋势。Nd和Sr同位素比值分别与（La/Sm）N呈负相关和正相关。在海底山观测到的地球化学变化几乎与在几百公里长的雷克雅内斯山脊或冰岛岛或夏威夷岛观测到的一样大，甚至更大。从一个特定的海山，海山6，样品显示了几乎整个观测范围的化学变化，提供了一个理想的机会来限制源地幔的非均质性。在所有可能的排列中，通过对主元素、微量元素、微量元素比值和同位素比值的比较，可以识别岩浆混合的系统特征。产生端元岩浆所需的源物质有：(1)典型的morb贫源橄榄岩；(2)相对丰富的物质，可能代表玄武岩熔体的古代地幔分离，俯冲洋壳的不完全混合残留物，或交代橄榄岩，如在圣保罗岩石或扎巴尔加德岛发现的。由于海底山靠近东太平洋隆起（EPR），源物质被认为是由海底山和邻近的东太平洋隆起下地幔中的紧密混合物组成的。在脊轴喷发的熔岩显示出小范围的同位素和不相容的微量元素组成，因为大程度的熔融和岩浆房的存在往往平均了大体积地幔的化学特征。如果假设的地幔物质在上地幔中普遍存在，具有大量级、小尺度的非均质性，那么从MOR拉斑岩到岛碱玄武岩的玄武岩化学变化可能反映的是采样差异，而不是整体地幔化学变化。
Basalts from young seamounts situated within 6.8 m.y. of the East Pacific Rise, between 9° and 14°N latitude, display significant variations in143Nd/144Nd (0.51295–0.51321),87Sr/86Sr (0.7025–0.7031), and(La/Sm)N(0.415–3.270). Nd and Sr isotope ratios are anti-correlated and form a trend roughly parallel to the “mantle array” on a143Nd/144Nd vs.87Sr/86Sr variation diagram. Nd and Sr isotope ratios display negative and positive correlations, respectively, with(La/Sm)N. The geochemical variations observed at the seamounts are nearly as great or greater than those observed over several hundred kilometers of the Reykjanes Ridge, or at the islands of Iceland or Hawaii.Samples from one particular seamount, Seamount 6, display nearly the entire observed range of chemical variations, offering an ideal opportunity to constrain the nature of heterogeneities in the source mantle. Systematics indicative of magma mixing are recognized when major elements, trace elements, trace element ratios, and isotope ratios are compared with each other in all possible permutations. The source materials required to produce the end-member magmas are: (1) a typical MORB-source-depleted peridotite; and (2) a relatively enriched material which may represent ancient mantle segregations of basaltic melt, incompletely mixed remnants of subducted ocean crust, or metasomatized peridotite such as that found at St. Paul's Rocks or Zabargad Island. Due to the proximity of the seamounts to the East Pacific Rise (EPR), the source materials are thought to comprise an intimate mixture in the mantle immediately underlying the seamounts and the adjacent EPR. Lavas erupted at the ridge axis display a small range of isotopic and incompatible trace element compositions because the large degrees of melting and presence of magma chambers tend to average the chemical characteristics of large volumes of mantle.If the postulated mantle materials, with large magnitude, small-scale heterogeneities, are ubiquitous in the upper mantle, chemical variations in basalts ranging from MOR tholeiites to island alkali basalts may reflect sampling differences rather than changes in bulk mantle chemistry.