Delivery of deep-sourced, volatile-rich plume material to the global ridge system
Delivery of deep-sourced, volatile-rich plume material to the global ridge system
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DOI：
10.1016/j.epsl.2018.07.028
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发表时间：
2018-10-01
影响因子：
5.3
通讯作者：
Richards, Mark A.
中科院分区：
文献类型：
--
作者：
Gibson, Sally A.;Richards, Mark A.
全球大洋中脊（莫尔）系统是地幔挥发物放气的主要场所。通过大洋中脊玄武岩喷发释放的H2O量沿着全球海岭系统变化，在与地幔柱相互作用的地点最大。这些深源热异常影响了大约三分之一的MORs -反映在不相容微量元素，同位素特征和隆起的山脊地形（过度熔化）的富集-但所涉及的物理机制是有争议的。“标准模型”涉及固态流动相互作用，其中活跃的上升流羽流影响大洋中脊产生的发散上升流，因此与正常的扩张脊相比，融化发生在更高的压力和更大的数量。然而，这个模型并不能解释某些神秘的特征，包括从活跃的地幔柱辐射到附近的莫尔的线性火山脊。其中的例子有沃尔夫-达尔文线性构造（加拉帕戈斯）、罗德里格斯海脊（留尼汪岛）、发现海脊（发现），以及与亚速尔群岛和伊斯特-萨拉斯-戈麦斯热点有关的许多较小的海脊状结构。从我们的研究中得到的一个重要观察结果是，具有异常高的H2O含量（高达1.3wt.%）的分馏校正MORE的稀土元素反演熔融（INVMEL）程序中的新算法，使我们能够模拟羽状隆起的相互作用，通过混合的挥发性轴承熔体在主动上涌和被动驱动的角流过程中产生的组合物。我们从这些经验模型中的发现表明，在地幔柱-洋脊相互作用的位置，中等富集的MORB（0.2- 0.4wt.%）H2O）的结果混合形成的熔体：（一）主动上升的羽流物质的最小深度为35公里;和（二）被动熔融在较浅的深度下的脊。最富挥发性的MORB（0.4- 1.3wt.% H2O）可通过进一步添加高达25%的“深”小馏分羽流茎熔体形成，所述羽流茎熔体含有>3 wt.%水我们建议，这些挥发物丰富的熔体直接输送到附近的莫尔段通过压力诱导，高度通道化的流嵌入在一个更广泛的“水坑”的主要是固态羽流材料，扩散板下的重力流。这解释了地球化学和水深测量中的短波长变化（超过10公里），这是叠加在更大的（许多100公里）的“腰部宽度”的羽流影响的海岭。熔融通道可能构成一个主要的输送机制，挥发物从羽流茎附近的MORs，并在某些情况下，在表面表示为火山线性构造和脊。通过两相（熔体基质）制度的交付小部分含水熔体从羽茎脊意味着一个平行的，双峰运输系统涉及在网站的羽脊相互作用。我们估计，深源富含挥发分的熔体在火山线性构造下方的通道中的侵位速率很高，涉及数千km（3）/Ma。由于地幔柱占MORs熔体生产的一半以上，我们的研究结果对我们理解地球深部挥发性循环具有重要意义。(C)2018作者由爱思唯尔公司出版
The global mid-ocean ridge (MOR) system represents a major site for outgassing of volatiles from Earth's mantle. The amount of H2O released via eruption of mid-ocean ridge basalts varies along the global ridge system and greatest at sites of interaction with mantle plumes. These deep-sourced thermal anomalies affect approximately one-third of all MORs - as reflected in enrichment of incompatible trace elements, isotope signatures and elevated ridge topography (excess melting) - but the physical mechanisms involved are controversial. The "standard model" involves solid-state flow interaction, wherein an actively upwelling plume influences the divergent upwelling generated by a mid-ocean ridge so that melting occurs at higher pressures and in greater amounts than at a normal spreading ridge. This model does not explain, however, certain enigmatic features including linear volcanic ridges radiating from the active plume to the nearby MOR. Examples of these are the Wolf-Darwin lineament (Galapagos), Rodrigues Ridge (La Reunion), Discovery Ridge (Discovery), and numerous smaller ridge-like structures associated with the Azores and Easter-Salas y Gomez hot spots. An important observation from our study is that fractionation-corrected MORE with exceptionally-high H2O contents (up to 1.3 wt.%) are found in close proximity to intersections of long-lived plume-related volcanic lineaments with spreading centres.New algorithms in the rare-earth element inversion melting (INVMEL) program allow us to simulate plume-ridge interactions by mixing the compositions of volatile-bearing melts generated during both active upwelling and passively-driven corner-flow. Our findings from these empirical models suggest that at sites of plume-ridge interaction, moderately-enriched MORBs (with 0.2-0.4 wt.% H2O) result from mixing of melts formed by: (i) active upwelling of plume material to minimum depths of 35 km; and (ii) those generated by passive melting at shallower depths beneath the ridge. The most volatile rich MORB (0.4-1.3 wt.% H2O) may form by the further addition of up to 25% of "deep" small-fraction plume stem melts that contain >3 wt.% H2O. We propose that these volatile-rich melts are transported directly to nearby MOR segments via pressure-induced, highly-channelised flow embedded within a broader "puddle" of mostly solid-state plume material, spreading beneath the plate as a gravity flow. This accounts for the short wavelength variability (over 10s of km) in geochemistry and bathymetry that is superimposed on the much larger (many 100s of km) "waist width" of plume-influenced ridge.Melt channels may constitute a primary delivery mechanism for volatiles from plume stems to nearby MORs and, in some instances, be expressed at the surface as volcanic lineaments and ridges. The delivery of small-fraction hydrous melts from plume stems to ridges via a two-phase (melt-matrix) regime implies that a parallel, bimodal transport system is involved at sites of plume-ridge interaction. We estimate that the rate of emplacement of deep-sourced volatile-rich melts in channels beneath the volcanic lineaments is high and involves 10s of thousands of km(3)/Ma. Since mantle plumes account for more than half of the melt production at MORs our findings have important implications for our understanding of deep Earth volatile cycling. (C) 2018 The Authors. Published by Elsevier B.V.