Geochemistry of Ontong Java Plateau basalts

Basement rocks from the Ontong Java Plateau are tholeiitic basalts that appear to record very high degrees of partial melting, much like those found today in the vicinity of Iceland. They display a limited range of incompatible element and isotopic variation, but small differences are apparent between sampled sites and between upper and lower groups of flows at Ocean Drilling Program Site 807.40Ar-39Ar ages of lavas from Site 807 and Deep Sea Drilling Project Site 289 are indistinguishable about an early Aptian mean of 122 Ma (as are preliminary data for the island of Malaita at the southern edge of the plateau), indicating that plateau-building eruptions ended more or less simultaneously at widely separated locations. Pb-Nd-Sr isotopes for lavas from Sites 289, 803, and 807, as well as southern Malaita, reflect a hotspot-like source with epsilon-Nd(T) = +4.0 to +6.3, (87Sr/86Sr)T = 0.70423-0.70339, and 206Pb/204Pb = 18.245-18.709 and possessing consistently greater 208Pb/204Pb for a given 206Pb/204Pb than Pacific MORB. The combination of hotspot-like mantle source, very high degrees of melting, and lack of a discernible age progression is best explained if the bulk of the plateau was constructed rapidly above a surfacing plume head, possibly that of the Louisville hotspot.
Basalt and feldspar separates indicate a substantially younger age of ~90 Ma for basement at Site 803; in addition, volcaniclastic layers of mid-Cenomanian through Coniacian age occur at DSDP Site 288, and beds of late Aptian-Albian age are found at Site 289. Therefore, at least some volcanism continued on the plateau for 30 m.y. or more. The basalts at Site 803 are chemically and isotopically very similar to those at the ~122 Ma sites, suggesting that hot plume-type mantle was present beneath the plateau for an extended period or at two different times.
Surviving seamounts of the Louisville Ridge formed between 70 and 0 Ma have much higher 206Pb/204Pb than any of the plateau basalts. Thus, assuming the Louisville hotspot was the source of the plateau lavas, a change in the hotspot's isotopic composition may have occurred between roughly 70 and 90 Ma; such a change may have accompanied the plume-head to plume-tail transition. Similar shifts from early, lower 206Pb/204Pb to subsequently higher 206Pb/204Pb values are found in several other oceanic plateau-hotspot and continental flood basalt-hotspot systems, and could reflect either a reduction in the supply of low 206Pb/204Pb mantle or an inability of some off-ridge plume-tails to melt refractory low 206Pb/204Pb material.

翁通爪哇海台（Ontong Java Plateau）的基底岩石为拉斑玄武岩（tholeiitic basalts），其记录了极高程度的部分熔融作用，与现今冰岛周边产出的玄武岩特征高度相似。该类玄武岩的不相容元素与同位素变化范围有限，但在采样站位之间，以及大洋钻探计划（Ocean Drilling Program, ODP）807站位的熔岩流上层组与下层组之间，存在显著的细微差异。来自807站位与深海钻探计划（Deep Sea Drilling Project, DSDP）289站位的熔岩的氩-40-氩-39（40Ar-39Ar）年龄与早阿普第期的平均年龄122 Ma无明显差异（海台南缘的马莱塔岛（Malaita）的初步测年数据亦呈现相同结果），这表明海台形成期的火山喷发在相距甚远的不同区域近乎同时结束。针对289、803、807站位以及马莱塔岛南部的熔岩开展的铅-钕-锶（Pb-Nd-Sr）同位素分析显示，其源区具有典型热点特征：εNd(T)值为+4.0至+6.3，(87Sr/86Sr)T值为0.70423~0.70339，206Pb/204Pb值为18.245~18.709；且在给定206Pb/204Pb值的情况下，其208Pb/204Pb值始终高于太平洋洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）。结合类热点地幔源区、极高程度的熔融作用以及未发现可识别的年龄演化序列这三项特征，若该海台主体是在抬升的地幔柱头部（plume head）之上快速形成，则该地幔柱极可能源自路易斯维尔热点（Louisville hotspot）。
对803站位基底的玄武岩与长石分离物测年结果显示，其年龄显著更年轻，约为90 Ma；此外，在DSDP 288站位发育有森诺曼期中期至康尼亚克期的火山碎屑岩层，289站位则发现了阿普第晚期-阿尔布期的岩层。由此可见，该海台上至少有部分火山活动持续了30百万年乃至更久。803站位的玄武岩在地球化学与同位素特征上，与年龄约122 Ma的站位的玄武岩高度相似，这表明热地幔柱型地幔在海台下方持续存在了较长时间，或是在两个不同时期出现过。
形成于70~0 Ma的路易斯维尔海岭（Louisville Ridge）现存海山的206Pb/204Pb值远高于该海台的所有玄武岩。因此，若假定该海台熔岩的源区为路易斯维尔热点，则该热点的同位素组成可能在约70~90 Ma之间发生了改变；这一变化可能伴随了地幔柱头部向地幔柱尾部（plume tail）的转变。在其他多个洋高原-热点体系与大陆溢流玄武岩（continental flood basalt）-热点体系中，均观测到了类似的同位素值变化：从早期较低的206Pb/204Pb值转变为后续较高的206Pb/204Pb值，这一现象可解释为低206Pb/204Pb的地幔供给减少，或是部分洋外地幔柱尾部无法熔融难熔的低206Pb/204Pb物质。