Geochemistry at DSDP Leg 82 Holes

The nine holes (556-564) drilled during DSDP Leg 82 in a region west and southwest of the Azores Platform (Fig. 1) exhibit a wide variety of chemical compositions that indicate a complex petrogenetic history involving crystal fractionation, magma mixing, complex melting, and mantle heterogeneity. The major element chemistry of each hole except Hole 557 is typical of mid-ocean ridge basalts (MORBs), whereas the trace element and rare earth element (REE) abundances and ratios are more variable, and show that both depleted Type I and enriched Type II basalts have been erupted in the region. Hole 556 (30-34 Ma), located near a flow line through the Azores Triple Junction, contains typically depleted basalts, whereas Hole 557 (18 Ma), located near the same flow line but closer to the Azores Platform, is a highly enriched FeTi basalt, indicating that the Azores hot-spot anomaly has existed in its present configuration for at least 18 Ma, but less than 30-34 Ma. Hole 558 (34-37 Ma), located near a flow line through the FAMOUS and Leg 37 sites, includes both Type I and II basalts. Although the differences in Zr/Nb and light REE/heavy REE ratios imply different mantle sources, the (La/Ce)ch (>1) and Nd isotopic ratios are almost the same, suggesting that the complex melting and pervasive, small-scale mantle heterogeneity may account for the variations in trace element and REE ratios observed in Hole 558 (and FAMOUS sites). Farther south, Hole 559 (34-37 Ma), contains enriched Type II basalts, whereas Hole 561 (14-17 Ma), located further east near the same flow line, contains Type I and II basalts. In this case, the (La/Ce)ch and Nd isotopic ratios are different, indicating two distinct mantle sources. Again, the existence along the same flow line of two holes exhibiting such different chemistry suggests that mantle heterogeneity may exist on a more pervasive and transient smaller scale. (Hole 560 was not sampled for this study because the single basalt clast recovered was used for shipboard analysis.) All of the remaining three holes (562, 563, 564), located along a flow line about 100 km south of the Hayes Fracture Zone (33°N), contain only depleted Type I basalts. The contrast in chemical compositions suggests that the Hayes Fracture Zone may act as a "domain" boundary between an area of fairly homogeneous, depleted Type I basalts to the south (Holes 562-564) and a region of complex, highly variable basalts to the north near the Azores hot-spot anomaly (Holes 556-561).深海钻探计划（Deep Sea Drilling Project, DSDP）第82航次在亚速尔平台（Azores Platform）西侧及西南侧区域（图1）钻取的9个钻孔（孔位编号556至564），其样品具有多样的化学成分，指示了复杂的岩石成因历史，涉及结晶分异、岩浆混合、复杂熔融以及幔源不均一性。除557号钻孔外，其余各钻孔的主量元素地球化学特征均符合洋中脊玄武岩（mid-ocean ridge basalts, MORBs）的典型特征；而微量元素与稀土元素（rare earth element, REE）的丰度及比值变化更为显著，表明该区域曾喷发过亏损型I型玄武岩与富集型II型玄武岩两类岩石。 位于穿过亚速尔三联点的流线附近的556号钻孔（形成时代为30~34 Ma），其样品以典型的亏损型玄武岩为主；而位于同一流线但更靠近亚速尔平台的557号钻孔（形成时代为18 Ma）则为高度富集的FeTi玄武岩，这表明亚速尔热柱异常至少在18 Ma以来便维持当前的分布格局，但其存在时限不足30~34 Ma。 位于穿过FAMOUS区域及第37航次钻孔位点的流线附近的558号钻孔（形成时代为34~37 Ma），同时产出I型与II型玄武岩。尽管Zr/Nb比值与轻稀土/重稀土比值的差异暗示其幔源源区存在差异，但(La/Ce)ch（>1）与钕同位素比值几乎一致，这表明复杂熔融过程与广泛分布的小型幔源不均一性，或许可以解释558号钻孔（以及FAMOUS区域钻孔）中观测到的微量元素与稀土元素比值的变化。 更向南的559号钻孔（34~37 Ma）仅产出富集型II型玄武岩；而位于同一流线更东侧的561号钻孔（14~17 Ma）则同时产出I型与II型玄武岩。在此情形下，(La/Ce)ch与钕同位素比值存在显著差异，指示了两个截然不同的幔源源区。同样，同一流线沿线的两个钻孔呈现出如此迥异的地球化学特征，表明幔源不均一性可能以更为广泛且短暂的小规模形式存在。（本研究未对560号钻孔开展采样，因仅回收的一块玄武岩岩屑已用于船载分析。） 剩余的3个钻孔（562、563、564）均沿位于海斯断裂带（Hayes Fracture Zone）以南约100 km的流线分布（北纬33°），其样品仅产出亏损型I型玄武岩。 上述化学成分的显著差异表明，海斯断裂带或许可作为一个“域边界”：南侧为相对均一的亏损型I型玄武岩分布区（562~564号钻孔），北侧则为受亚速尔热柱异常影响、玄武岩地球化学特征复杂且变化剧烈的区域（556~561号钻孔）。