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.

沃尔维斯海岭（Walvis Ridge）上的深海钻探计划（Deep Sea Drilling Project，DSDP）525A、528和527号钻孔揭露的基底，由海底玄武岩流、枕状玄武岩及少量夹层沉积物组成。这些钻孔分布于一个北北西-南南西走向的海岭地块的脊顶、西北侧翼中下部，该地块曾与古洋中脊近乎平行（Rabinowitz与LaBrecque，1979，doi:10.1029/JB084iB11p05973；Moore等，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70百万年前，与安哥拉盆地紧邻的洋壳年龄一致，且符合古洋中脊的形成环境（Moore等，1983）。玄武岩类型从脊顶的无斑石英拉斑玄武岩，过渡至侧翼的富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存在系统性差异。诸多元素与同位素比值对呈现出系统性变化趋势，一端为脊顶玄武岩，另一端为富斑晶的侧翼玄武岩。

脊顶玄武岩具有较低的¹⁴³Nd/¹⁴⁴Nd（0.51238）、²⁰⁶Pb/²⁰⁴Pb（17.54）、²⁰⁷Pb/²⁰⁴Pb（15.47）与²⁰⁸Pb/²⁰⁴Pb（38.14）比值，以及较高的⁸⁷Sr/⁸⁶Sr（0.70512）比值，表明其源自钕/钐、铷/锶及铀/铅富集的古老地幔源区。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，但钕与铅同位素比值显著更低。可将同位素比值变化趋势外推至侧翼玄武岩之外，其同位素比值呈现为¹⁴³Nd/¹⁴⁴Nd（0.51270）、²⁰⁶Pb/²⁰⁴Pb（18.32）、²⁰⁷Pb/²⁰⁴Pb（15.52）与²⁰⁸Pb/²⁰⁴Pb（38.77）更高，而⁸⁷Sr/⁸⁶Sr（0.70417）更低，对应钕/钐、铷/锶及铀/铅亏损程度逐渐升高的源区成分。这些同位素相关关系既符合亏损端元与富集端元熔体的混合作用，也符合非均质性、不同程度富集的地幔源区的部分熔融过程。但实测的锆-钡-铌-钇元素间相关关系，与任何简单的双端元岩浆混合模型均不相符——该类模型通常对应下地幔柱穿过上地幔的过程。不相容元素与铅同位素体系特征同样表明，该玄武岩未受到亏损型（N型）洋中脊玄武岩（Mid-Ocean Ridge Basalt，MORB）物质或其地幔源区的显著混染。在我们优选的岩石成因模型中，沃尔维斯海岭玄武岩源自类似富集型（E型）洋中脊玄武岩源区的地幔部分熔融，该源区因少量熔体与交代流体的加入而发生了小型非均质性改造。