Expedition 391 Preliminary Report: Walvis Ridge Hotspot-猫眼课题宝

DOI：

10.14379/iodp.pr.391.2022

发表时间：

2022-05

期刊：

International Ocean Discovery Program Preliminary Report

影响因子：

通讯作者：

中科院分区：

其他

文献类型：

作者：

W. Sager;K. Hoernle;T. Höfig;A. Avery;R. Bhutani;D. M. Buchs;C. A. Carvallo;C. Class;Y. Dai;G. Dalla Valle;A. V. Del Gaudio;K. Gaastra;S. Han;D. Heaton;S. Homrighausen;Y. Kubota;C. F. Li;W. R. Nelson;E. Petrou;K. E. Potter;S. Pujatti;J. Scholpp;M. Pincus;S. Fielding;J. Shervais;S. Thoram;S. M. Tikoo-Schantz;X. J. Wang;M. Widdowson;M. Tshiningayamwe;Scientists the Expedition

热点轨迹（海山、山脊和其他火山结构的准线性链）提供了板块运动以及地幔地球动力学、岩浆通量和地幔源成分的重要记录。特里斯坦-高夫-鲸湾山脊 (TGW) 热点路径从特里斯坦-达库尼亚和高夫的活火山岛延伸，穿过居约省，然后沿着鲸湾山脊到达埃滕德卡洪水玄武岩省，形成了最突出和最复杂的全球热点路径之一。 TGW 热点轨迹显示出紧密的线性年龄进展，其中年龄从岛屿到溢流玄武岩（覆盖约 135 My）逐渐增加。太平洋轨道是简单的海山链，通常被比作珍珠链，而太平洋轨道则不同，TGW 轨道交替出现陡峭的窄脊、海洋高原、近平行的线性山脊和海山链，以及看似随机分散的海山区域。该轨迹显示了过去约 70 My 的同位素分区。地带出现在轨道中部附近，就在它分裂成两到三条山脊和居约型海山之前。较老的山脊还覆盖着年龄渐进的晚期火山活动，在初次喷发后约 30-40 My 处就位，并具有独特的同位素组成。 391 号探险队的计划是在六个地点进行钻探，其中三个位于沃尔维斯海岭，三个位于海山（盖奥特）省，以收集火成岩，以更好地了解轨道大厦的形成、热点的时间和地球化学演化，以及火山大厦形成的古纬度的变化。为了应对船上爆发的 2019 冠状病毒病 (COVID-19) 病毒，在推迟 18 天后，第 391 号探险队继续在四个拟议地点进行钻探：瓦尔维斯海脊东部瓦尔迪维亚浅滩周围的三个地点、山脊内的海洋高原以及中心轨道居奥下侧的一个地点，中心轨道位于特里斯坦子轨道（从终点延伸）之间的山脊。鲸湾山脊至特里斯坦达库尼亚岛）和高夫次轨道（从鲸湾山脊延伸至高夫岛）。在 U1575 地点钻了一个洞，该地点位于瓦尔迪维亚浅滩以北鲸湾山脊东北部的低处。在此位置，对 209.9 m 的沉积物和 122.4 m 的火成岩基底进行了取芯。后者由 10 个海底熔岩单元组成，包括枕状、叶状、片状和大量熔岩流，其中最厚约 21 m。大多数熔岩是拉斑岩，但也发现了一些碱性玄武岩。火成岩序列的一部分由低钛玄武岩组成，这种玄武岩很不寻常，因为它们出现在 Etendeka 溢流玄武岩中，但之前在鲸湾海岭上从未发现过。在瓦尔迪维亚浅滩西侧 U1576 站点钻了两个孔。第一个洞由于在穿透火成岩基底后不久有点堵塞而被终止。 U1576A 孔发现了约 380 m 厚的沉积剖面，主要由白垩组成，覆盖了从古新世到晚白垩世（坎帕阶）几乎完整的序列。这些沉积物显示出短期和长期的周期性颜色变化，这意味着天文强迫和长期的古环境变化。火成岩基底产生了 11 个海底熔岩单元，从枕状熔岩到块状熔岩流，其成分从拉斑玄武岩到玄武安山岩不等，这是从 TGW 轨迹中首次发现这种成分。这些单元由七个沉积白垩单元隔开，厚度范围为 0.1 至 11.6 m，这意味着沉积和熔岩喷发的长期相互作用。 U1577 遗址位于瓦尔迪维亚浅滩的最东侧，取芯穿透了 154 m 厚的沉积层，其底部约 108 m 是马斯特里赫特阶-坎帕阶（可能是桑托阶）白垩，带有玻璃质火山灰层。火成基底取心仅在沉积物-玄武岩接触面以下 39.1 m 处进行，回收了三个巨大的海底拉斑玄武岩熔岩流，厚度分别为 4.1、15.5 和 >19.1 m。 U1577 和 U1576 地点的古地磁数据表明，它们的火山基底分别在白垩纪正常超纪元结束之前和之后不久的 Chron 33r 期间形成。生物地层学和古地磁数据表明，瓦尔迪维亚浅滩的年龄呈东西向演变，向西变得更年轻。 U1578 地点位于居奥中心轨道上，提供了一个长而多样的火成岩部分。在对主要由始新世和古新世白垩组成的 184.3 m 远洋碳酸盐沉积物进行取芯后，U1578A 孔对 302.1 m 火成岩基底进行了取芯。地下室熔岩主要是枕头状的，但散布着片状熔岩和大量熔岩流。熔岩成分主要是碱性玄武岩和一些夏威夷岩。几个层段含有丰富的橄榄石，一些枕状堆由钛含量非常高的玄武岩组成。火成岩序列被 10 个由白垩和火山碎屑组成的沉积互层中断，厚度范围为 0.46 至 10.19 m。古地磁数据显示，洞底上方约 100 m 处基底磁极性发生了变化。将磁性地层学与生物地层学数据相结合，推断火成岩部分的跨度 >1 My。在 U1575、U1576 和 U1578 地点发现了枕状熔岩边缘的大量玻璃。尽管火成岩渗透量仅为计划量的三分之二，但第 391 次探险期间的钻探获得的样本显然将有助于更深入地了解特里斯坦-高夫热点及其轨迹的演化。相对新鲜且回收率良好的玄武岩将为地球化学、地质年代学和古地磁研究提供充足的样品。晚白垩世和早新生代白垩纪的良好恢复为古环境研究提供了样本。 © 2022 作者。版权所有。

Hotspot tracks (quasilinear chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are simple chains of seamounts that are often compared to chains of pearls, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts, and areas of what appear to be randomly dispersed seamounts. The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. The older ridge is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamount (guyot) province, to gather igneous rocks to better understand the formation of track edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard outbreak of the coronavirus disease 2019 (COVID-19) virus, Expedition 391 proceeded to drill at four of the proposed sites: three sites on the eastern Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the island of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to the island of Gough). One hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The latter comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first hole was terminated because a bit jammed shortly after penetrating igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Paleocene to Late Cretaceous (Campanian). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement yielded 11 submarine lava units ranging from pillows to massive flows, which have compositions varying from tholeiitic basalt to basaltic andesite, the first occurrence of this composition recovered from the TGW track. These units are separated by seven sedimentary chalk units that range in thickness from 0.1 to 11.6 m, implying a long-term interplay of sedimentation and lava eruptions. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154 m thick sedimentary section, the bottom ~108 m of which is Maastrichtian–Campanian (possibly Santonian) chalk with vitric tephra layers. Igneous basement coring progressed only 39.1 m below the sediment-basalt contact, recovering three massive submarine tholeiite basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomag etic data from Sites U1577 and U1576 indicate that their volcanic basements formed just before the end of the Cretaceous Normal Superchron and during Chron 33r, shortly afterward, respectively. Biostratigraphic and paleomagnetic data suggest an east–west age progression across Valdivia Bank, becoming younger westward. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk, Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine, and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging in thickness from 0.46 to 10.19 m. Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Abundant glass from pillow lava margins was recovered at Sites U1575, U1576, and U1578. Although the igneous penetration was only two-thirds of the planned amount, drilling during Expedition 391 obtained samples that clearly will lead to a deeper understanding of the evolution of the Tristan-Gough hotspot and its track. Relatively fresh basalts with good recovery will provide ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study. © 2022 Authors. All rights reserved.

Expedition 391 Preliminary Report: Walvis Ridge Hotspot

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