Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.在瓦尔维斯海岭（Walvis Ridge）的深海钻探计划（Deep Sea Drilling Project, DSDP）525A、528和527号钻孔中揭露的基底，由海底玄武岩流、枕状熔岩及少量夹层沉积物构成。这些钻孔分布于一个北北西-南南西（NNW-SSE）走向脊块的顶部、西北翼中段及下段，该脊块原本与古洋中脊近乎平行（Rabinowitz与LaBrecque, 1979, doi:10.1029/JB084iB11p05973；Moore等人, 1983, doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2）。这些玄武岩的喷发时代约为70百万年（m.y.），与安哥拉盆地邻近洋壳的形成时代一致，且符合古洋中脊处的成因机制（Moore等人, 1983）。玄武岩类型从海岭顶部的无斑晶石英拉斑玄武岩，变化至海岭翼部的高斜长石斑晶橄榄拉斑玄武岩，二者在不相容微量元素与同位素组成上呈现系统性差异。诸多元素及同位素比值对均呈现出系统性趋势：一端为海岭顶部的玄武岩，另一端则为高斑晶含量的海岭翼部玄武岩。 海岭顶部玄武岩的低143Nd/144Nd（0.51238）、206Pb/204Pb（17.54）、207Pb/204Pb（15.47）、208Pb/204Pb（38.14）比值，以及高87Sr/86Sr（0.70512）比值，表明其源区为经受Nd/Sm、Rb/Sr及Pb/U富集的古老地幔。该同位素特征与特里斯坦-达库尼亚群岛的碱性玄武岩相似，但Nd与Pb同位素比值显著更低。可将该同位素比值趋势外推至海岭翼部玄武岩之外：其143Nd/144Nd（0.51270）、206Pb/204Pb（18.32）、207Pb/204Pb（15.52）、208Pb/204Pb（38.77）比值更高，87Sr/86Sr（0.70417）比值更低，对应Nd/Sm、Rb/Sr及Pb/U亏损程度逐渐升高的源区组成。这些同位素相关性既可以用亏损端元与富集端元的熔体混合来解释，也可以通过非均质性且经历不同程度富集的地幔源区的部分熔融来阐释。然而，观测到的Zr-Ba-Nb-Y元素间相关关系，与任何简单的两组分岩浆混合模型均不相符——这类模型通常被用于解释下地幔柱穿过上地幔上升的过程。不相容微量元素与Pb同位素系统学特征，也排除了亏损型（N型）洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）物质或其地幔源区的广泛参与。在我们优选的岩石成因模型中，瓦尔维斯海岭玄武岩源于类似富集型（E型）洋中脊玄武岩源区的地幔部分熔融，该源区因少量熔体和交代流体的加入，在小尺度上发生了非均质性改造。