The Role of Lithosphere Thickness in the Formation of Ocean Islands and Seamounts: Contrasts between the Louisville and Emperor-Hawaiian Hotspot Trails
The Role of Lithosphere Thickness in the Formation of Ocean Islands and Seamounts: Contrasts between the Louisville and Emperor-Hawaiian Hotspot Trails
复制标题
岩石圈厚度在海洋岛屿和海山形成中的作用：路易斯维尔和皇帝夏威夷热点路径之间的对比
DOI：
10.1093/petrology/egaa111
复制
发表时间：
2020
影响因子：
3.9
通讯作者：
Fitton J
中科院分区：
文献类型：
--
作者：
Fitton J
夏威夷-皇帝和路易斯维尔海山形成了地球上两条最突出的时间进步热点小径。两者在岩石圈上形成的时间间隔相似，年龄和厚度范围也相似。夏威夷-帝王海山很大，目前岩浆生产率似乎正在增加。相比之下，路易斯维尔海山更小，踪迹似乎正在减弱。我们提供了在综合大洋钻探计划第330次考察期间钻探的五座较老的路易斯维尔海山(74-50 Ma)的新的常量元素和微量元素数据，并将这些数据与相同年龄的皇帝海山的已发表数据进行了比较。尽管在路易斯维尔海山的三座形成盾牌的火山岩中钻探得很深，但我们的数据证实了早期基于疏浚样本的研究结果，即路易斯维尔海山由非常均匀的碱性玄武岩组成。玄武岩成分可用以石榴石-二辉橄榄岩为主的地幔的∼1·5-3 %部分熔融来模拟，其成分与安通爪哇高原地幔源区的成分相似。在皇帝海山上挖掘和钻探得到的岩石样品的成分从拉斑玄武岩到碱性玄武岩，需要更大的熔融程度(2-10 %)和尖晶石-石榴石-二辉橄榄岩来源。我们用一个简单的减压熔融模型表明，在不同厚度的岩石圈之下，潜在温度为15 0 0 °C的地幔熔融可以解释皇帝海山形成屏蔽拉斑玄武岩的成分，而屏蔽后碱性玄武岩需要较低的温度(13 0 0~14 0 0 °C)。这与夏威夷-帝王形成的岩浆来自较热的地幔热柱轴线，以及来自较冷的热柱鞘的屏蔽后岩浆的派生是一致的，因为海山漂离了热柱轴线。路易斯维尔海山玄武岩的成分在海山形成时与岩石圈厚度没有明显的变化，这与我们的减压熔融模型的预测相反。岩石圈厚度的这种缺乏影响是大多数海洋岛屿的玄武岩的特征。如果路易斯维尔海山是由地幔脱水融化而形成的，那么这个问题就可以得到解决。地幔在较冷的羽流中含有少量的水。在相对较冷的地幔热柱中(Tp= 1350-1400 °C)的水合熔融可以产生少量熔体，然后通过增加粘度达到干燥的地幔固相线而进一步熔融而被抑制。羽流未能到达干燥的地幔固体线或岩石圈底部，这意味着产生的岩浆将具有相同的成分，而与岩石圈厚度无关。皇帝海山和夏威夷群岛之下较热的地幔热柱(TP≈ 15 0 0 °C)在熔融开始前具有较低的粘度，熔融程度较大，并向岩石圈底部减压。因此，我们的减压融化模型可以潜在地解释皇帝和路易斯维尔海山的组成。岩石圈对几乎所有海洋岛屿的玄武岩成分没有明显的控制，这表明脱水熔融是规则，夏威夷群岛是例外。或者，许多大洋岛屿可能不是地幔热柱的产物，而可能是在一般的上地幔对流中，由橄榄岩组成的非均质地幔来源的减压熔融形成的，其中橄榄岩含有碳化和硅质过饱和榴辉岩的离散实体。
The Hawaii–Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii–Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace-element data from five of the older (74–50 Ma) Louisville seamounts drilled during Integrated Ocean Drilling Program (IODP) Expedition 330 and compare these with published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1·5–3 % partial melting of a dominantly garnet-lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10 %) and spinel- to garnet-lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500 °C under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, whereas post-shield alkali basalt requires a lower temperature (1300–1400 °C). This is consistent with the derivation of Hawaii–Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp= 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands are the exception. Alternatively, many ocean islands may not be the product of mantle plumes but may instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.