Geochemistry of basalts from IODP site U1365: Implications for magmatism and mantle source signatures of the mid-Cretaceous Osbourn Trough
Geochemistry of basalts from IODP site U1365: Implications for magmatism and mantle source signatures of the mid-Cretaceous Osbourn Trough
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IODP 站点 U1365 玄武岩的地球化学：对白垩纪中期奥斯本海槽岩浆作用和地幔源特征的影响
DOI：
10.1016/j.lithos.2012.04.014
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发表时间：
2012-07
期刊：
影响因子：
3.5
通讯作者：
Inagaki, Fumio
中科院分区：
文献类型：
--
作者：
Li, He;Zarikian, Carlos;D'Hondt, Steven;Inagaki, Fumio
综合海洋钻探计划现场U1365钻入了西南太平洋地壳的地下室，该地壳形成于白垩纪中期的奥斯本海槽，该海槽将马尼希基高原和Hikurangi高原(大马尼希基高原)分开。该点玄武岩的地球化学特征对了解中白垩世奥斯本海槽的岩浆作用和地幔来源具有重要意义。回收的新鲜玄武岩为低钾拉斑玄武岩(N)和亏损(D)洋中脊玄武岩(MORB)。它们的微量元素和锶-钕同位素组成指示了一个太平洋类型的地幔来源，而不是来自附近的路易斯维尔海山链或大马尼希基高原的任何重大影响。尽管在奥斯本海槽的初始阶段，在奥斯本海槽下方出现了一个羽流头部，但它的重要性可以用奥斯本海槽向南长距离(1000公里)的迁移来解释。U1365点熔岩的喷发顺序从底部的低镁(6.9wt.%)N-MORB到高镁(8wt.%~9.5wt.%)D-MORB，再到中镁(7.3wt.%~8.2wt.%)N-MORB，并伴随着岩浆在岩浆库中的混合。D-MORB组熔岩的熔融度高于N-MORB组熔岩的熔融度，这是因为它们的结晶校正后的二氧化钛、Na2O和CaO的浓度相对于MgO=7.8wt.%。高镁D-MORB熔岩的主要元素组成与尖晶石-橄榄岩带在~3.1~2 GPa地幔压力区间内的部分熔融一致。N-MORB和D-MORB在锶-钕同位素上的显著重叠表明，两组间的化学差异来自于地幔熔融过程。根据与东太平洋隆起熔岩的对比，发现地幔熔融程度与扩张速率呈正相关，我们认为奥斯本海槽可能具有约140 mm/年的全扩张速率。因此，奥斯本海槽缓慢的脊状轴向地貌应该是已灭绝的快速海脊的特征。
The Integrated Ocean Drilling Program site U1365 drilled into the basement of the southwest Pacific crust formed from the mid-Cretaceous Osbourn Trough that rifted apart the Manihiki and Hikurangi Plateaus (the Greater Manihiki). The basalt geochemistry at this site is crucial for understanding the magmatic processes and mantle source of the mid-Cretaceous Osbourn Trough. The recovered fresh basalts were low-K tholeiitic normal (N) and depleted (D) mid-ocean ridge basalt (MORB). Their trace element and Sr–Nd isotope compositions indicate a Pacific-type mantle source rather than any significant influences from the nearby Louisville Seamount Chain or from the Greater Manihiki Plateau. Despite the presence of a plume head underneath the Osbourn Trough at its initial stage, the insignificance of a plume head could be explained by the long-distance (>1000km) southward migration of the Osbourn Trough. Lavas at site U1365 vary from low-MgO (<6.9wt.%) N-MORB at the bottom to high-MgO (8wt.% to 9.5wt.%) D-MORB and, then, to medium-MgO (7.3wt.% to 8.2wt.%) N-MORB according to their eruption sequences, which was accompanied by magma mixing in the magma reservoir. The D-MORB group lavas have higher melting degrees than those of N-MORB group based on their concentrations of TiO2, Na2O and CaO corrected for crystallization relative to MgO=7.8wt.%. The major element compositions of the high-MgO D-MORB lavas were consistent with partial melting in the spinel–peridotite zone over a pressure interval from ~3.1GPa to 2GPa in the mantle. The significant overlap of N-MORB and D-MORB in Sr–Nd isotopes suggests that chemical differences between the two groups were derived from the mantle melting processes. Based on comparison with lavas from the East Pacific Rise where a positive correlation of mantle melting degree vs. spreading rate is shown, we suggest that the Osbourn Trough might have a full spreading rate of ~140mm/yr. Thus, the slow ridge-like axial morphology of the Osbourn Trough should be a character of an extinct fast ridge.
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