A geochemical link between plume head and tail volcanism
A geochemical link between plume head and tail volcanism
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羽流头部和尾部火山活动之间的地球化学联系
DOI：
10.7185/geochemlet.1742
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发表时间：
2017
影响因子：
3.9
通讯作者：
J. Day
中科院分区：
文献类型：
--
作者：
B. Peters;J. Day
doi: 10.7185/geochemlet.1742 地幔柱的地球动力学模型通常会调用初始的高容量羽流“头部”岩浆作用，然后是较低容量的羽流“尾部”。然而，以德干地盾等溢流玄武岩为代表的地幔柱头部和以留尼旺岛等海洋岛屿为代表的地幔柱尾部之间的地球化学联系是模糊的，挑战了地幔柱理论的经典观点。利用 Sr-Nd-Os 同位素数据，我们证明了留尼旺热点地区典型的羽流头部和尾部火山活动之间的地球化学联系。之前对留尼汪岛或其他全球热点地区的地球化学研究尚未明确表明类似的羽流首尾关系。这种联系是通过使用相容元素实现的，例如锇，它可以规避地壳或岩石圈岩浆同化带来的复杂性，因为与原生地幔熔体相比，这些元素在地壳中是稀缺的。我们计算了留尼汪岛原生岩浆的 Sr-Nd-Os 同位素组成，发现这些与德干原生岩浆的预测相同。我们的结果为时间稳定的地幔柱提供了地球化学证据，该地幔柱采样了与非洲大型低剪切速度省相关的原始储层，并且其遗产始于白垩纪-古近纪边界。 2017 年 4 月 5 日收稿 | 2017 年 9 月 28 日接受 |发表于 2017 年 11 月 3 日 1. 地球科学研究部，斯克里普斯海洋学研究所，加州大学圣地亚哥分校，拉霍亚，CA 92093，美国 # 现于卡内基科学研究所地磁系，华盛顿特区，20015 年，美国 * 通讯作者（电子邮件：bpeters@carnegiescience.edu） 信件 陆地板内“热点”的起源火山活动仍然存在争议。一种常见的模型表明，热点源自从地幔深处升起的热化学浮力地幔柱，引发了短暂的、大量的“溢流玄武岩”火山活动，这可能会产生附带的构造和环境后果（Richards等，1989；Hill，1991；Self等，2008）。其他模型认为热点岩浆作用是由响应构造过程的浅层熔化引发的（例如，Foulger 等，2015）。目前有三种观点支持板内岩浆作用的羽流起源。首先，独立于板块构造活动的连续的、年龄渐进的、大致线性的火山轨迹的形成可能需要活跃的地幔源，例如地幔柱。其次，地震层析成像表明一些板内火山中心下方存在深层、低剪切速度的上升流（French 和 Romanowicz，2015）。第三，许多现代热点的同位素特征与大陆和大洋中脊下面的地幔的同位素特征不同，并且随着时间的推移保持相对一致，这表明它们具有孤立的深层地幔源储层，不断补充浅层岩浆储层（Hofmann，1997）。德干-留尼汪热点地区是沿着线性、年龄渐进的火山轨迹研究羽流头部和尾部关系的最佳例子之一。那里有一个体积巨大的大陆溢流玄武岩 (CFB) 省，即德干地盾，通过抗震海底山脊与留尼旺岛活跃喷发的洋岛玄武岩 (OIB) 连接起来（图 1）。德干 CFB 和留尼旺岛 OIB 之间以及其他类似热点轨迹之间的遗传联系通常被地幔柱理论隐含地假设，并且是最近的地球动力学模型所要求的（Glišović 和 Forte，2017）。然而，这样的假设与古代地幔柱的预期物理后果不一致。例如，现代热流测量并没有记录德干地盾下方热输出的增加，预计古代地幔柱可能使大陆地壳和岩石圈变薄，并增加了从地幔到地表的传热（Roy 和 Rao，2000）。在此类情况下，地球化学是连接 CFB 和 OIB 岩浆作用的潜在诊断工具。如果发现溢流玄武岩和后来的“羽尾”岩浆源自成分不同的来源，那么就无法为地幔柱的长期存在提供明确的证据；然而，如果它们有一个与现代可到达地幔不同的共同地球化学来源，那么地幔中这种特征的持续存在将强烈暗示地幔柱的存在。几十年来利用亲石同位素系统进行的地球化学研究已经导致羽流头部和尾部之间的可疑关联。例如，德干玄武岩的 Sr-Nd 同位素成分偏离了留尼旺端元（例如，Peng 等人，1994 年的图 2），这可能是岩石圈或地壳物质广泛同化为 CFB 岩浆的结果。其他热点系统也观察到了这种效应（Gibson et al., 1995; Peate, 1997; Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2017) 5, 29-34 | doi: 10.7185/geochemlet.1742 30 图 1 西印度洋盆地的卫星测深图。德干的大致空中范围印度次大陆上的灰色区域显示了圈闭熔岩流。阴影区域中的数字对应于采样区域：1 – Kutch（样本 1-5）、2 – Saurashtra（样本 6-46）、3 – Pavagadh、Kalsubai、Amba Dongar 及周边地区（样本 48-54、63-78）、4 – Dhule 及周边地区（样本 55-62）、 5 – 孟买、西高止山脉和马哈拉施特拉邦（样本 79-115，MMF7），留尼旺热点的近似轨迹由陆地区域上的实线黑色箭头表示，与 2014 年 GEBCO 世界地图复制的底图，www.gebco.net。 2016），这意味着这可能是岩浆上升穿过岩石圈和地壳的不可避免的结果，有效地掩盖了原始岩浆特征。亲铁剂 187Re-187Os 同位素系统（t1/2 ≈ 42 Gyr）（Shirey 和 Walker，1998）提供了解决羽流头部和尾部火山活动之间地球化学联系的机会。铼相对不相容，而 Os 高度相容，导致地壳中具有高 Re/Os 和长期放射成因 187Os/188Os 特征（>1；Peucker-Ehrenbrink 和 Jahn，2001），而原始地幔相对于球粒陨石保留了较低的 Re/Os 和较少放射成因 187Os/188Os 特征（~0.1296） （0.126–0.128；Meisel 等人，1996；Day 等人，2016）。相比之下，长寿的亲石同位素系统（例如，Rb-Sr、Sm-Nd、Lu-Hf、U/Th-Pb）仅涉及不相容元素，这些元素在岩石圈熔体和地壳材料中相对于地幔衍生的岩浆富集。这解释了为什么溢流玄武岩的Sr-Nd-Pb同位素特征可以由岩石圈熔融和部分熔融地壳的贡献所主导(例如，Peng等，1994；Gibson等，1995)。然而，地幔 Os 同位素特征可能会在岩石圈或地壳不同程度的分化或同化中持续存在，从而使 187Os/188Os 成为检查地幔柱头部和尾部岩浆作用之间地球化学联系的诊断工具。尽管如此，之前的审查
doi: 10.7185/geochemlet.1742 Geodynamical models of mantle plumes often invoke initial, high volume plume ‘head’ magmatism, followed by lower volume plume ‘tails’. However, geochemical links between plume heads, represented by flood basalts such as the Deccan Traps, and plume tails, represented by ocean islands such as La Réunion, are ambiguous, challenging this classical view of mantle plume theory. Using Sr-Nd-Os isotope data, we demonstrate a geochemical link between archetypal plume head and tail volcanism in the Réunion hotspot. Similar plume head-tail relationships have not been definitively shown in previous geochemical studies for Réunion or other global hotspots. Such a link is enabled by use of compatible elements, such as Os, which can circumvent complexities introduced by magmatic assimilation of crust or lithosphere because these elements are scarce in crust compared to primary mantle melts. We calculate Sr-Nd-Os isotopic compositions for the Réunion primary magma and find these are identical to predictions for the Deccan primary magma. Our result provides geochemical evidence for a temporally stable mantle plume that samples a primitive reservoir associated with the African large low-shear-velocity province and with a heritage beginning at the Cretaceous-Palaeogene boundary. Received 5 April 2017 | Accepted 28 September 2017 | Published 3 November 2017 1. Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA # Now at Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA * Corresponding author (email: bpeters@carnegiescience.edu) Letter The origins of terrestrial intraplate “hotspot” volcanism remain controversial. One common model suggests that hotspots are derived from thermo-chemically buoyant mantle plumes that rise from the deep mantle, initiating short-lived, high volume ‘flood basalt’ volcanism that likely had collateral tectonic and environmental consequences (Richards et al., 1989; Hill, 1991; Self et al., 2008). Other models posit that hotspot magmatism is initiated by shallow melting in response to tectonic processes (e.g., Foulger et al., 2015). Three arguments currently favour a plume origin for intraplate magmatism. First, the formation of a continuous, age-progressive and roughly linear volcanic track independent of plate tectonic activity likely requires an active mantle source, such as a plume. Second, seismic tomography suggests deep, low-shear-velocity upwelling beneath some intraplate volcanic centres (French and Romanowicz, 2015). Third, the isotopic signatures of many modern hotspots are distinct from those of the mantle underlying continents and mid-ocean ridges and remain relatively consistent through time, suggesting that they have isolated, deep mantle source reservoirs that continually replenish shallow magma reservoirs (Hofmann, 1997). One of the best studied examples of plume head and tail relationships along a linear, age-progressive volcanic track is the Deccan-Réunion hotspot. There, a volumetrically massive continental flood basalt (CFB) province, the Deccan Traps, is linked by aseismic, submarine ridges to ocean island basalts (OIB) that actively erupt on the island of La Réunion (Fig. 1). A genetic link between Deccan CFB and Réunion OIB, and for other similar hotspot tracks, has often been implicitly assumed by mantle plume theory and is demanded by recent geodynamical models (Glišović and Forte, 2017). Such assumptions, however, are at odds with expected physical consequences of ancient mantle plumes; for example, modern heat flow measurements do not record increased thermal output beneath the Deccan Traps, where an ancient mantle plume might be expected to have thinned the continental crust and lithosphere and increased heat transfer from the mantle to the surface (Roy and Rao, 2000). In cases like these, geochemistry is a potentially diagnostic tool for linking CFB and OIB magmatism. If flood basalts and later ‘plume tail’ magmas are found to originate from compositionally distinct sources, it would provide no definitive evidence for a long lived mantle plume; however, if they share a common geochemical source that is distinct from the modern accessible mantle, then the persistence of such signatures in the mantle would strongly implicate the existence of a mantle plume. Decades of geochemical research utilising lithophile isotope systems have resulted in dubious associations between plume heads and tails. For example, Sr-Nd isotopic compositions of Deccan basalts deflect away from a Réunion end member (e.g., Fig. 2 of Peng et al., 1994), which may be a consequence of extensive assimilation of lithospheric or crustal material into CFB magmas. This effect has also been observed for other hotspot systems (Gibson et al., 1995; Peate, 1997; Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2017) 5, 29-34 | doi: 10.7185/geochemlet.1742 30 Figure 1 Satellite bathymetry map of the western Indian Ocean basin. Approximate aerial extent of Deccan Traps lava flows are shown by the gray fields on the Indian subcontinent. Numbers in the shaded region correspond to sampling regions: 1 – Kutch (samples 1-5), 2 – Saurashtra (samples 6-46), 3 – Pavagadh, Kalsubai, Amba Dongar and surrounds (samples 48-54, 63-78), 4 – Dhule and surrounds (samples 55-62), 5 – Mumbai, Western Ghats and coastal Maharashtra (samples 79-115, MMF7). Approximate trace of the Réunion hotspot is shown by the transparent black arrow, approximate plate motion vectors are shown by solid black arrows over land areas and are proportional to plate motions. Base map reproduced from the GEBCO world map 2014, www.gebco.net. Dale et al. 2009; Day, 2016), meaning that it may be an inevitable consequence of magma ascent through lithosphere and crust, effectively masking primary magmatic signatures. The siderophile 187Re-187Os isotope system (t1/2 ≈ 42 Gyr) (Shirey and Walker, 1998) provides an opportunity to resolve a geochemical link between plume head and tail volcanism. Rhenium is relatively incompatible while Os is highly compatible, leading to high Re/Os and long term radiogenic 187Os/188Os signatures in crust (>1; Peucker-Ehrenbrink and Jahn, 2001), while the primitive mantle preserves lower Re/ Os and less radiogenic 187Os/188Os signatures (~0.1296) relative to chondrites (0.126–0.128; Meisel et al., 1996; Day et al., 2016). In contrast, long lived lithophile isotope systems (e.g., Rb-Sr, Sm-Nd, Lu-Hf, U/Th-Pb) involve exclusively incompatible elements that are enriched in lithospheric melts and crustal materials relative to mantle-derived magmas. This explains why the Sr-Nd-Pb isotopic characteristics of flood basalts can be dominated by contributions from lithospheric melt and partially melted crust (e.g., Peng et al., 1994; Gibson et al., 1995). Mantle Os isotopic signatures, however, may persist through variable extents of differentiation or assimilation of lithosphere or crust, making 187Os/188Os a diagnostic tool for examining geochemical links between plume head and tail magmatism. Notwithstanding, previous examination of the