Geochemistry of basalts from ODP Leg 49, North Atlantic Ocean

The geochemistry of basalts recovered from seven sites in the North Atlantic is described with particular reference to minor elements. Three sites (407, 408, and 409) along the same mantle flow line, transverse to the Reykjanes Ridge at about 63°N, provide information on the composition of basalts erupted over a 34-m.y. interval between 2.3 and 36 m.y. ago. At Site 410, at 45°N, penetration into 10 m.y.-old crust west of the ridge axis permits comparisons with young basalts dredged from the median valley at 45°N. Three sites in the FAMOUS area at about 36°N provided material from very young (1 m.y.) basaltic crust (Site 411), and material to test the geochemical coherence of basalts of different ages (1.5 and 3.5 m.y.) on either side of a fracture zone (Sites 412 and 413). These sites complement earlier data from dredged and drilled sites (Leg 37) in the FAMOUS area.
At Site 407, four geochemically distinct basalt units occur, with different normative and rare-earth element (REE) characteristics, and there is a clear correlation with magnetic stratigraphy. Yet there is a remarkable consistency in incompatible element ratios between these units, indicating derivation from an essentially similar mantle source. The basalts from the younger sites, 408 and 409, show a similar range of normative and REE variation, but incompatible element ratios are identical to those at Site 407, indicating that basalts at all three sites were produced from a mantle source which was geochemically relatively uniform. Rare-earth differences between the basalts can be interpreted in terms of variations in the degree and depth of partial melting causing HREE (+Y) retention in the source, although there may be some inter-site differences with respect to REE.
A similar picture is presented at 45°N. Apparently a range of tholeiitic, transitional, and alkalic basalts were being erupted 10 m.y. ago, which have almost identical geochemical characteristics to those recently erupted in the median valley at 45°N. Incompatible element ratios are markedly different from those recorded at the Reykjanes Ridge. Basalts recovered from the FAMOUS sites are geochemically similar to previous samples recovered from the FAMOUS area, and their incompatible element ratios are similar, but not identical, to those at 45°N. However, total trace element levels are consistently lower than in 45°N basalts, which might imply smaller degrees of partial melting and/or greater depths of magma generation at 45°N, or higher trace element levels in the mantle source at 45°N.
Few of the basalts recovered on Leg 49 have the geochemical characteristics of typical "MORB" (e.g., Nazca Plate, Leg 34). The data strongly support models invoking geochemical inhomogeneity in the source regions of basalts produced at the Mid-Atlantic Ridge. However, the data also introduce an additional time factor into such models and demonstrate the uniformity of the mantle source at a particular ridge sector (over periods in excess of 30 m.y.), while emphasizing the marked differences along the ridge. Mixing models invoking "depleted" and "enriched" mantle sources would seem to be inadequate to account for the observed variations.

本文描述了从北大西洋7个站位采获的玄武岩地球化学特征，重点关注次要元素（minor elements）。其中3个站位（407、408和409）位于同一条地幔流线上，垂直于北纬约63°处的雷克雅内斯海岭，提供了2.3至36百万年前、跨度达34百万年的时间间隔内喷发的玄武岩成分信息。位于北纬45°的410站位钻穿了海岭轴以西10百万年历史的地壳，可与北纬45°中央谷拖网采获的年轻玄武岩进行对比。北纬约36°的FAMOUS区域内的3个站位，提供了极年轻（1百万年）玄武质地壳的样品（411站位），以及用于检验断裂带两侧不同年龄（1.5和3.5百万年）玄武岩地球化学一致性的样品（412和413站位）。这些站位补充了此前FAMOUS区域内拖网采获和钻探站位（第37航次）的相关数据。
在407站位，存在4个地球化学特征迥异的玄武岩单元，各自具有不同的标准矿物和稀土元素（Rare-earth Element, REE）特征，且与磁性地层学存在明确的相关性。但这些单元之间的不相容元素比值却表现出显著的一致性，表明其源自本质上相似的地幔源区。408和409这两个较年轻站位的玄武岩，呈现出相似的标准矿物与稀土元素变化范围，但不相容元素比值与407站位完全一致，说明这三个站位的玄武岩均产自地球化学性质相对均一的地幔源区。玄武岩间的稀土元素差异可通过部分熔融程度和深度的变化来解释——这类变化会导致源区内重稀土元素（HREE）与钇（Y）发生滞留，不过在稀土元素特征上仍可能存在站位间差异。
北纬45°区域的情况与此类似。显然，10百万年前曾喷发过一系列拉斑玄武岩、过渡型玄武岩与碱性玄武岩，其地球化学特征与近日在北纬45°中央谷喷发的玄武岩几乎完全一致。该区域的不相容元素比值与雷克雅内斯海岭的记录存在显著差异。从FAMOUS站位采获的玄武岩，其地球化学特征与此前从该区域采获的样品相似，不相容元素比值也与北纬45°的样品相近，但并不完全相同。不过，其总微量元素含量始终低于北纬45°的玄武岩，这或许意味着北纬45°处的部分熔融程度更低、岩浆生成深度更大，或是该区域地幔源区的微量元素含量更高。
第49航次采获的玄武岩中，极少具备典型“洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）”的地球化学特征（例如纳斯卡板块的第34航次样品）。这些数据有力支持了大西洋中脊产出的玄武岩源区存在地球化学不均一性的模型。然而，这些数据还为这类模型引入了额外的时间维度，证明了特定洋脊段的地幔源区在超过30百万年的时间尺度上保持均一，同时凸显了洋脊沿线的显著差异。采用“亏损”与“富集”地幔源区的混合模型，似乎无法合理解释观测到的地球化学变化。