Rare-earth elements and isotope ratios at DSDP Leg 82 Holes

We report 48 analyses of rare-earth elements (REE) and 15 143Nd/144Nd and 87Sr/86Sr analyses for basalts from the eight holes drilled during Leg 82. Discrete and distinct REE patterns and 143Nd/144Nd ratios characterize the eight holes, with little variation observed downhole except in Holes 561 and 558, thus suggesting dominantly long-term temporal and large-scale spatial variations in the mantle source of these basalts beneath the Mid-Atlantic Ridge over the last 35 Ma of its spreading activity. There is a good inverse correlation between 143Nd/144Nd and (La/Sm)EF with one exception in Hole 558 (approximately 35 Ma), the latter suggesting a recent (35 Ma) light REE depletion event, perhaps caused by dynamic or fractional melting. Short-term temporal and small-scale spatial mantle source variability is also evident in Hole 561 (approximately 18 Ma), which has rapid fluctuations in REE patterns and 143Nd/144Nd ratios (suggesting rapid transfer of magma from the time of melting) and is evidence contrary to the presence of a well-mixed magma chamber at this particular site and time.
The mantle source variations noted can be interpreted within two extreme models. The first model invokes a convecting mantle depleted in large ion lithophile elements (LILE) and containing lumps (or veins) of LILE-enriched material of various shapes and sizes, passively and randomly distributed throughout. A second more restrictive model considers the interaction of fixed mantle plumes and the LILE-depleted asthenosphere flowing towards a migrating Mid- Atlantic Ridge (MAR) axis. With the exception of Hole 558 and the uncertainties of reconstructions of absolute plate movements in the region, the observed variations can be explained by two hot spots; the nearly ridge-centered Azores hot spot (plume) and another hot spot located beneath the African plate that may be affecting the source of basalts currently erupting at the MAR axis at 35°N and which, in the past, would have produced the New England chain of seamounts on the North American plate and (later) the Atlantis-Great Meteor chain on the African plate.
Basalts erupted south of the Hayes Fracture Zone have not been affected by either of these two hot spots over the last 35 Ma and appear to have been continuously derived from the LILE-depleted source. Subaxial flow downridge from the Azores plume appears to have started 9 Ma, on the basis of the southward converging V-shaped time-transgressive ridges branching from the Pico and Corves Island, or not earlier than 16 Ma, on the basis of the geochemical results. Variations within Hole 558 remains unexplained by the latter model, unless we hypothesize a third hot spot.

本次研究报道了82航次（Leg 82）钻探的8个钻孔玄武岩的48件稀土元素（rare-earth elements, REE）分析数据，以及15组143Nd/144Nd及87Sr/86Sr同位素比值分析数据。8个钻孔的玄武岩均具有独特且离散的稀土元素配分模式与143Nd/144Nd同位素比值特征，除561号和558号钻孔外，其余钻孔的井下地球化学变化均极小；这表明在大西洋中脊（Mid-Atlantic Ridge, MAR）过去35 Ma的扩张活动期间，其下方玄武岩的幔源源区主要存在长期时间尺度与大尺度空间尺度的地球化学变化。

143Nd/144Nd与(La/Sm)EF之间存在显著的负相关关系，仅558号钻孔（约35 Ma）为例外；该例外指示了一次发生在约35 Ma的轻稀土元素亏损事件，其成因可能与动态熔融或分离熔融作用有关。561号钻孔（约18 Ma）中也呈现出短期时间尺度与小尺度空间尺度的幔源源区不均一性，其稀土元素配分模式与143Nd/144Nd比值存在快速波动（指示熔融阶段的岩浆发生了快速运移），这与该研究位点及对应时期存在充分混合岩浆房的假说相悖。

上述观测到的幔源源区地球化学变化可通过两种端元模型进行解释。第一种模型假定存在一个亏损大离子亲石元素（large ion lithophile elements, LILE）的对流地幔，其中被动且随机分布着不同形态与规模的、富集LILE的团块（或脉体）。第二种限制更为严格的模型则考虑了固定地幔柱与流向迁移中大西洋中脊（MAR）轴部的亏损LILE软流圈之间的相互作用。除558号钻孔的异常以及该区域绝对板块运动重建存在不确定性外，观测到的地球化学变化可通过两个热点进行解释：一是几乎以中脊为中心的亚速尔群岛热点（地幔柱），另一个则位于非洲板块下方，其可能影响当前在35°N处大西洋中脊轴部喷发的玄武岩源区；该热点在过去曾于北美板块上形成新英格兰海山链，后续又在非洲板块上形成亚特兰蒂斯-大梅特罗海山链。

在过去35 Ma内，海斯断裂带（Hayes Fracture Zone）以南喷发的玄武岩未受这两个热点的任何一方影响，其源区似乎持续来自亏损LILE的地幔源区。基于从皮克岛与科尔武岛分支出来、向南汇聚的V形穿时海岭，亚速尔群岛地幔柱的轴下顺脊流似乎始于9 Ma；而根据地球化学结果，该事件的发生时间不早于16 Ma。除非我们假设存在第三个地幔柱，否则后述模型无法解释558号钻孔内部的地球化学变化。