(Tables 1-3) Pb and Sr isotopic data for sediments and rocks from DSDP holes in the Mariana island-arc system

Analyses of the isotopic composition of Pb in (1) western Pacific Ocean sediments [Jurassic(?) to Pleistocene in age, including clays and biogenic oozes], (2) Pacific Ocean basaltic rocks, (3) Mariana frontal arc volcanic rocks (Eocene to Miocene), and (4) Mariana active arc volcanic rocks [Pliocene (?) to Holocene] indicate that Pacific Ocean sediments could not have been a significant component of the source material for the Mariana arc volcanic rocks. Calculations involving the average concentrations and isotopic compositions of Pb in oceanic sediments, sea-floor basaltic rocks, and the Mariana arc volcanic rocks suggest that the sediment component must have been less than 1 percent of this source material.
The Pb isotopic compositions of the Mariana arc volcanic rocks lie, within experimental error, along the trend of available Pacific Ocean basalt analyses in versus 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams. Isotopic analyses of Pb in Pacific Ocean sediments do not lie along this trend; they have higher 207Pb/204Pb and 208Pb/204Pb values for comparable 206Pb/204Pb ratios. Clayey sediments generally have higher 208Pb/204Pb and 207Pb/204Pb ratios than biogenic oozes regardless of the age of the sediment.
Comparison of combined Sr and Pb isotopic analyses for (1) mantle-derived materials erupted through oceanic crust, (2) altered ocean-floor basaltic rocks, and (3) volcanic rocks from oceanic island arcs suggests that the Mariana arc volcanic rocks were derived, at least in part, from altered Pacific lithosphere subducted beneath the Mariana arc.
Unaltered basalts from the Mariana inter-arc basin (Mariana Trough) have Pb and Sr isotopic compositions that are very similar to those reported for some Hawaiian volcanic rocks but distinct from Mariana active and frontal arc compositions. These observations, in addition to existing major-and trace-element data, support a mantle origin for the interarc basin volcanic rocks. Dacites dredged from the Mariana remnant arc (South Honshu Ridge) have Pb isotopic compositions that are within experimental error of the active-arc analyses, consistent with a genetic relation.

对（1）西太平洋沉积物（疑似侏罗纪至更新世，包含黏土与生物成因软泥）、（2）太平洋玄武岩、（3）马里亚纳前锋弧火山岩（始新世至中新世）以及（4）马里亚纳活动弧火山岩（疑似上新世至全新世）中的铅同位素组成开展分析，结果表明太平洋沉积物并非马里亚纳弧火山岩源区物质的主要组成部分。通过对海洋沉积物、洋底玄武岩以及马里亚纳弧火山岩中铅的平均浓度与同位素组成进行计算可知，沉积物组分在该源区物质中的占比不足1%。
马里亚纳弧火山岩的铅同位素组成在实验误差范围内，于207Pb/204Pb与206Pb/204Pb、208Pb/204Pb与206Pb/204Pb的同位素相关图解中，均落在现有太平洋玄武岩分析数据的演化趋势线上。太平洋沉积物的铅同位素分析结果则不沿该趋势分布；在206Pb/204Pb比值相近的情况下，其207Pb/204Pb与208Pb/204Pb值更高。无论沉积物的地质年代如何，黏土质沉积物的208Pb/204Pb与207Pb/204Pb比值通常均高于生物成因软泥。
对三类样品的锶与铅同位素联合分析结果进行对比：（1）通过洋壳喷发的幔源物质、（2）蚀变洋底玄武岩、（3）大洋岛弧火山岩，结果表明马里亚纳弧火山岩至少部分源自俯冲至马里亚纳弧下方的蚀变太平洋岩石圈。
采自马里亚纳弧间盆地（马里亚纳海槽）的未蚀变玄武岩，其铅与锶同位素组成与部分已报道的夏威夷火山岩数据极为相似，但与马里亚纳活动弧及前锋弧的同位素组成存在显著差异。结合现有主量与微量元素数据，上述观测结果支持弧间盆地火山岩源自地幔的成因观点。从马里亚纳残余弧（本州南脊）拖网打捞的英安岩，其铅同位素组成在实验误差范围内与活动弧的分析结果一致，证实二者存在成因关联。