IMPLICATIONS OF MANTLE PLUME STRUCTURE FOR THE EVOLUTION OF FLOOD BASALTS
IMPLICATIONS OF MANTLE PLUME STRUCTURE FOR THE EVOLUTION OF FLOOD BASALTS
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DOI：
10.1016/0012-821x(90)90072-6
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发表时间：
1990-07-01
影响因子：
5.3
通讯作者：
GRIFFITHS, RW
中科院分区：
文献类型：
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作者：
CAMPBELL, IH;GRIFFITHS, RW
Morgan[1,2]认为大陆洪水玄武岩是新地幔柱的第一次火山表达。粘性液体的实验研究表明，新的或“开始”的羽流应该由一个巨大的球茎头和一个狭窄的喂料管道组成。对地幔柱流动[3]的分析表明，如果地幔柱上升是由热浮力驱动的，那么在上升过程中，头部会夹带周围的地幔。头部冷却并扩大，并形成源地幔和夹带地幔的成分分区。来自羽流源的未受污染的、相对较热的物质继续沿着尾部导管向上流动，使得羽流轴的温度高于头部其余部分的温度。我们通过比较两个洪水玄武岩省（德干和卡鲁）的物理和化学特征与动力学模型的预测来探讨这种羽流结构的含义。这两个省的年代学、构造学和地球化学都很符合起始羽流假说。我们将大陆洪水火山活动的突然爆发和短暂持续时间（在2000-2500公里宽的等量区域内）归因于羽流头部的融化，其随后的下降归因于羽流尾部融化的窄火山活动链（约200公里宽）。预计地表隆起500-1000米，但在火山活动的主要时期开始之前，由于羽头的横向扩张，地面下沉。随后，随着岩浆从地幔中逸出并负荷地球表面，预计会出现一段加剧的沉降期，随后随着地幔柱的热异常逐渐衰减，会出现109年的缓慢沉降。火山活动的时间和持续时间尚未得到确定的预测，但预计活动将以爆发的形式开始，并在20 Ma数量级的总时间内迅速消失，这与测年结果一致，测年结果表明，每个省的大部分岩浆是在2-3 Ma内喷出的，随后在5-10 Ma内喷出的岩浆量较小。该模型预测，与大陆洪水玄武岩相关的高温旋辉质熔体来自于地幔柱轴处的热的、相对未受污染的地幔，而体积更大的拉斑玄武岩是由地幔柱头部较冷的混合地幔熔体产生的。这就解释了为什么卡鲁和德干的苦荞岩强烈富集高度不相容元素，与oib型源的熔融一致，而伴生的玄武岩则弱富集不相容元素，与oib -下地幔混合源的衍生一致。
Morgan [1,2] suggested that continental flood basalts appear as the first volcanic expression of new mantle plumes. Experimental studies in viscous liquids have shown that new, or “starting”, plumes should consist of a large bulbous head followed by a narrow feeder conduit. Analysis of the plume flow [3] indicates that, if the plume ascent is driven by thermal buoyancy, the head will entrain the surrounding mantle as it rises. The head cools and enlarges, and develops a compositional zonation of source and entrained mantle. Uncontaminated, relatively hot material from the plume source continues to flow up the trailing conduit making the temperature of the plume axis greater than that of the remainder of the head. We explore the implications of this plume structure by comparing the physical and chemical characteristics of two flood basalt provinces (the Deccan and Karoo) with predictions of the dynamical model.The chronology, tectonics and geochemistry of the two provinces all fit well with the starting plume hypothesis. We attribute the sudden onset and short duration of continental flood volcanism, over an equant area 2000–2500 km across, to melting the plume head and its subsequent decline to a narrow chain of volcanic activity, ∼ 200 km wide, to melting in the plume tail. A surface uplift of 500–1000 m is predicted but this gives way to subsidence due to lateral spreading of the plume head before the onset of the main period of volcanism. A period of enhanced subsidence is then predicted to occur as magma escapes from the mantle and loads the earth's surface, followed by slow subsidence over 109years as the plume's thermal anomaly gradually decays. The timing and duration of volcanism has not been predicted with certainty, but activity is expected to begin as a burst and to die away rapidly over a total time of order 20 Ma, in agreement with dating which indicates that the bulk of the magmas in each province were ejected within 2–3 Ma and followed by smaller volumes over a further 5–10 Ma. The model predicts that the high-temperature picritic melts associated with continental flood basalts are derived from hot, relatively uncontaminated plume-source mantle at the plume axis and that the more voluminous tholeiitic basalts are produced by melting of cooler hybrid mantle in the plume head. This explains for why the picrites of the Karoo and Deccan are strongly enriched in highly incompatible elements, consistent with melting of an OIB-type source, whereas the associated basalts are weakly enriched in incompatible elements, consistent with derivation from a mixed OIB-lower-mantle source.