NO8-1，我们从“词--句--篇章”来谈谈这篇文章。
原文：
Geol**ists have long known that the Earth‘s mantle is heter**eneous， but its spatial arrangement remains unresolved—is the mantle essentially layered or irregularly heter**eneous? The best evidence for the layered-mantle thesis is the well-established fact that volcanic rocks found on oceanic islands， islands believed to result from mantle plumes arising from the lower mantle，are composed of material fundamentally different from that of the midocean ridge system， whose source， most geol**ists contend， is the upper mantle.Some geol**ists， however， on the basis of observations concerning mantle xenoliths， argue that the mantle is not layered， but that heter**eneity is created by fluids rich in “incompatible elements” (elements tending toward liquid rather than solid state) percolating upward and transforming portions of the upper mantle irregularly， accordingto the vagaries of the fluids’ pathways. We believe， perhaps unimaginatively， that this debate can be resolved through further study， and that the underexplored midocean ridge system is the key.
一、难词
spatial： a.1.空间的，太空的;2.存在(或者发生)于宇宙空间的
plume： n.1.羽状物2.柱，地柱
xenolith：俘虏岩
percolate:v.1.渗透2.扩散到，弥漫于
portions： n.一部分
二、难句
1.the best evidence for the layered-mantle thesis is the well-established fact that volcanic rocks found on oceanic islands，islands believed to result from mantle plumes arising from the lower mantle，are composed of material fundamentally different from that of the mid-ocean ridge system， whose source，mostgeol**ists contend，is the upper mantle.
对于地幔分层论点来说，最好的证据乃这样一个确认的事实，即在那些海洋岛屿——这些岛屿据信是源于由下层地幔升上来的地幔柱状溶岩流(mantle plume)——上发现的火山岩石，是由与海洋中部山脊系统的物质根本不同的物质构成的，而这一海洋中部山脊系统的成因，大多数地质学家论辩道，为上部地幔。
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