Bekaert David V, Gazel Esteban, Turner Stephen, Behn Mark D, de Moor J Marten, Zahirovic Sabin, Manea Vlad C, Hoernle Kaj, Fischer Tobias P, Hammerstrom Alexander, Seltzer Alan M, Kulongoski Justin T, Patel Bina S, Schrenk Matthew O, Halldórsson Sæmundur A, Nakagawa Mayuko, Ramírez Carlos J, Krantz John A, Yücel Mustafa, Ballentine Christopher J, Giovannelli Donato, Lloyd Karen G, Barry Peter H
Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;
Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853.
Proc Natl Acad Sci U S A. 2021 Nov 23;118(47). doi: 10.1073/pnas.2110997118.
It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high He/He (up to 8.9R) in low-temperature (≤50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source He/He >10.3R (and potentially up to 26R, similar to Galápagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1R). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high He/He values in central Panama are likely derived from the infiltration of a Galápagos plume-like mantle through a slab window that opened ∼8 Mya. Two potential transport mechanisms can explain the connection between the Galápagos plume and the slab window: 1) sublithospheric transport of Galápagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galápagos plume material blown by a "mantle wind" toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galápagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits.
地幔柱是地球深部富含地球化学物质向上涌升的主要通道，这一点已得到充分证实。然而，浅部侧向流动过程能够将富集的地幔物质从垂直的地幔柱通道分散至远（>1000公里）处的方式和程度，仍受到很大限制。在此，我们报告了来自中美洲南部边缘（CAM）65种热液流体的氦和碳同位素数据，这些数据显示，巴拿马中部低温（≤50°C）且与活火山活动无关的地热泉中，氦/氦比值（高达8.9R）显著偏高。经过放射性校正后，这些数据表明地幔源氦/氦比值>10.3R（可能高达26R，与加拉帕戈斯热点熔岩相似），明显高于上地幔范围（8±1R）。熔岩地球化学（铅同位素、铌/铀和铈/铅）以及地球物理约束表明，巴拿马中部高氦/氦值可能源于类似加拉帕戈斯地幔柱的物质通过约800万年前打开的板片窗的渗透。两种潜在的传输机制可以解释加拉帕戈斯地幔柱与板片窗之间的联系：1）加拉帕戈斯地幔柱物质通过沿巴拿马断裂带的岩石圈变薄引导的岩石圈以下传输；2）加拉帕戈斯地幔柱物质被“地幔风”吹向CAM的主动上涌。我们提出了一个全球地幔流模型，支持第二种机制，即加拉帕戈斯地幔柱物质的大部分向东传输发生在浅部软流圈。这些发现强调了侧向地幔流将地幔地球化学不均一性从地幔柱通道传输数千公里的潜力。
