Geochemistry of basalts at DSDP Leg 73 Holes

During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 51) and 522B (Anomaly 16) have ages of about 12 m.y. and about 38 m.y., respectively.
The major- and trace- (including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption.
In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts.
In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it.
Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of about 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick, about 74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks.
The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

深海钻探计划（Deep Sea Drilling Project, DSDP）第73航次（南大西洋海域）期间，研究人员在519A、520、522B和524号钻孔中钻遇了玄武质枕状熔岩、熔岩流及岩床。古地磁数据显示，519A号钻孔（对应磁异常51）和522B号钻孔（对应磁异常16）所获玄武岩的年龄分别约为12百万年和38百万年。

519A号钻孔玄武岩（总厚度27米）的常量及微量元素（含稀土元素）组成特征表明，这类玄武岩属于典型的正常型洋中脊玄武岩（normal-type mid-ocean-ridge basalts, N-type MORB），其成分与大西洋中脊其他正常段产出的橄榄石拉斑玄武岩重叠。不相容元素（或低亲岩浆元素，如钛Ti、钒V、钇Y及锆Zr）的光谱特征与稀土元素丰度均证实，这些玄武岩是喷发前橄榄石、尖晶石及斜长石发生低压分异作用的产物。

520号钻孔总钻进深度达10.5米，仅回收了1.7米的玄武岩。这些枕状玄武岩由中等程度分异的（正常型洋中脊玄武岩型）拉斑玄武质岩浆结晶形成。

522B号钻孔共钻穿19米厚的玄武质枕状熔岩与熔岩流。研究人员根据玻璃质边缘及细淬火结构划分出13个冷却单元。与519A和520号钻孔不同，522B号钻孔所在海岭段的玄武岩可划分为两个主要的拉斑玄武岩组：（1）第1至12冷却单元，（2）第13冷却单元。该段玄武岩同样属于正常型洋中脊玄武岩，但整体分异程度高于519A和520号钻孔的玄武岩。最底部的玄武岩（第13冷却单元）成分最为原始，构成了与上方12个单元中分异程度更高的玄武岩截然不同的成分组。

524号钻孔钻于沃尔维斯海岭（Walvis Ridge）南翼，因此获取了南大西洋海底更为复杂区域的样品。研究人员钻遇了三套与火山碎屑沉积物互层的玄武岩岩石组合。自上而下，这三套岩石组合分别为：碱性玄武质枕状熔岩；厚度16米、年龄约65百万年的碱性辉绿岩床（依据钾氩测年及浮游有孔虫定年结果）；以及另一套厚度约9米、年龄约74百万年的拉斑质岩床，其成分与上覆的碱性岩石差异显著。

524号钻孔的碱性玄武岩的化学特征与大西洋中脊脊峰以东约400公里处的特里斯坦-达库尼亚群火山的玄武质熔岩极为相似。据此，沃尔维斯海岭可被合理地解释为一条热点碱性火山链。