31. SR-ND-PB ISOTOPE GEOCHEMISTRY OF LEG 144 WEST PACIFIC GUYOTS: IMPLICATIONS FOR THE GEOCHEMICAL EVOLUTION OF THE "SOPITA" MANTLE ANOMALY1
31. SR-ND-PB ISOTOPE GEOCHEMISTRY OF LEG 144 WEST PACIFIC GUYOTS: IMPLICATIONS FOR THE GEOCHEMICAL EVOLUTION OF THE "SOPITA" MANTLE ANOMALY1
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DOI：
10.2973/odp.proc.sr.144.031.1995
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
A. Koppers;H. Staudigel;D. Christie;J. Dieu;M. Pringle
中科院分区：
文献类型：
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作者：
A. Koppers;H. Staudigel;D. Christie;J. Dieu;M. Pringle
西太平洋海山省（WPSP）内5个白垩纪盖奥特（Limalok、Lo-En、Wodejebato、MIT和Takuyo-Daisan）的Sr、Nd和Pb同位素组成表明，它们起源于地球化学富集的地幔源区。它们的特点是铅同位素变化大（206Pb/204Pb = 18.58 - 19.80，2u7Pb/204Pb = 15.55 - 15.68，Sr（~（87）Sr/~（86）Sr = 0.70308 - 0.70486）和Nd（~（143）Nd/~（144）Nd = 0.512670.51307）的中等变化。白垩纪盖奥特研究的数据是在WPSP的整体同位素范围内，并类似于目前活跃的热点南太平洋同位素和热异常（SOPITA）所产生的海洋玄武岩。WPSP玄武岩显示出较大的同位素变异，它们包括具有富集地幔源（EMI和EMU）或206Pb/204Pb高而87Sr/86Sr低（HIMU）的地幔源的同位素特征的样品。这些观察结果表明，WPSP和SOPITA玄武岩可能是由相同的异质地幔源产生的：要么是同位素不同，与其他海洋和大陆火山地区更多样化，要么是通过混合推断为这些其他地区的不同地幔成分而产生的。我们加强了以前的论点的寿命同位素不寻常的SOPITA热点，特别是Rurutu-Ratak-Marcus-Wake海山链（RRMW）。我们来自WPSP的数据使现有数据库翻了一番，使我们能够检查RRMW链在1.1亿年的总时间跨度内的时间同位素演化，不包括尚未取样的25 - 50 Ma时间段。RRMW同位素数据表明：（1）总体同位素方差随时间变化是恒定的;（2）Sr和Nd方差小于SOPITA和WPSP的总方差，而Pb成分显示出类似的大方差;（3）SOPITA的同位素组成，特别是Rurutu火山岩，在过去的25百万年内逐渐发生变化。由以HIMU地幔成分为主的地幔源区转变为EM型地幔源区。后者表现为206Pb/204Pb、7/4Pb和Δ 8/4Pb比值的降低，而87Sr/86Sr比值的升高。在90至70 Ma期间，Ratak-Marcus-Wake链的同位素组成也可能出现类似的演变，尽管后者的趋势被WPSP地区较低密度的采样所掩盖。
The Sr, Nd, and Pb isotopic compositions of five Cretaceous guyots (Limalok, Lo-En, Wodejebato, MIT, and Takuyo-Daisan) within the West Pacific Seamount Province (WPSP) indicate that they originated from geochemically enriched mantle sources. They are characterized by large isotopic variations in Pb (206Pb/204Pb = 18.58-19.80, 2u7Pb/204Pb = 15.55-15.68, and 208Pb/204Pb _ 38.68-39.54) and by moderate variations in Sr (87Sr/86Sr = 0.70308-0.70486) and Nd ( l43Nd/l44Nd = 0.512670.51307). Data from the Cretaceous guyots studied are within the overall isotopic range of the WPSP and are similar to those of oceanic basalts produced by currently active hotspots of the South Pacific isotopic and thermal anomaly (SOPITA). The WPSP basalts show a large isotopic variance, and they include samples with isotopic signatures of either enriched mantle sources (EMI and EMU), or mantle sources high in 206Pb/204Pb and low in 87Sr/86Sr (HIMU). These observations suggest that the WPSP and SOPITA basaltic rocks may have been produced from the same heterogeneous mantle source: either one that is isotopically distinct and more diverse from other oceanic and continental volcanic regions, or one that is produced by mixing the different mantle components inferred for these other regions. We strengthen previous arguments for the longevity of the isotopically unusual character of the SOPITA hotspots, in particular, for the Rurutu-Ratak-Marcus-Wake Seamount Chain (RRMW). Our data from the WPSP doubles the existing database and allows us to examine the temporal isotopic evolution of the RRMW Chain over a total time span of 110 m.y., excluding the time period between 25 and 50 Ma that has not been sampled yet. The RRMW isotopic data suggest that (1) the overall isotopic variance is constant over time; (2) the Sr and Nd variances are smaller than the total variance for the SOPITA and WPSP, whereas the Pb compositions show similar, large variances; and (3) the isotopic composition of the SOPITA, and specifically the Rurutu volcanics, gradually changed within the last 25 m.y. from a mantle source dominated by the HIMU mantle component to an EM-type mantle source. The latter is indicated by a decrease in 206Pb/204Pb, 7/4Pb, and Δ8/4Pb paralleled by an increase in 87Sr/86Sr. A similar evolution in isotopic composition may also be revealed from the Ratak-Marcus-Wake Chain over the period from 90 through 70 Ma, although this latter trend is obscured by a less dense sampling in the WPSP area.