Three Dimensional Magma Dynamics in Large Sills

大基台中的三维岩浆动力学

• 批准号: 9418513
• 负责人: Bruce Marsh
• 金额: $ 34.5万
• 依托单位: Johns Hopkins University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1995
• 资助国家: 美国
• 起止时间: 1995-05-01 至 1998-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9418513&HistoricalAwards=false
• 关键词: Three; Dimensional; Magma; Dynamics; Large

This award supports a project to study the emplacement and solidification of sills in the Dry Valleys region of Antarctica and to compare their histories to similar sheet-like magma bodies on Easter Island and Reunion Island. If the three-dimensional history of emplacement and solidification of sheet-like magmatic bodies could be learned through direct observation, a great advance could be made in understanding the evolution of Earth. Sheet-like bodies of magma may well be responsible for delivering and shaping both the oceanic and continental crusts. Beneath ocean ridges, for example, relatively narrow (~1 - 4 kilometers), thin (100 - 300 meters) ribbons or sills of magma deliver, through a delicate balance between emplacement and solidification processes, the basic structure of the oceanic crust, which forms some 60% of Earth's surface. And basically similar solidification processes in sheets of underplating basaltic magma and high-level invading granitic magma go a long way to delivering and shaping the continental crust through remobilization and sorting of earlier magmas. Learning the dynamic history of sheet-like magmas has, however, proven exceedingly elusive for two simple reasons: lack of vast continuous rock exposures and lack of extensive, clear internal markers to reveal the temporal and spatial history of solidification. Internal layerings are plentiful in the large, well known layered intrusions, but there is little sign of the nature and style of magma filling (e.g., was filling periodic or steady? at discrete points or along fissures? slow or fast? etc.). The many large diabase sills throughout the world broadly show how filling occurred, but rarely do they show any internal indicators or markers detailing the filling and solidification processes themselves. And when a marker, as in the form of trails of large, distinct, previously formed crystals, does occur it rarely can be traced out in three dimensions due to poor natural exposures. T he Basement Sill of Antarctica changes all this. It is thick (300 meters), vast (greater than 5,000 cubic kilometers), superbly exposed, and above all carries a distinct internal marker in the form of a tongue of large (1 - 6 millimeters) orthopyroxene crystals. Moreover, this orthopyroxene tongue varies systematically in thickness, crystal size, and position within the sill reflecting both the filling locations and the rate of filling itself. The documentation of this body and its three dimensional dynamics may well prove to be the "Rosetta Stone" of sheet-like magmatic systems. It is of fundamental importance and is central to this project. The detailed processes that give rise beneath ocean ridges to the structure of the oceanic crust also may well be gained from this study. And the seeds of initiation of continental crust, through Solidification Front Instability and the production of silicic lenses, is also clear in the Basement Sill. As a comparative study, a search for evidence in the lavas of Easter Island may well also establish the importance of this process in volcanic systems.

该奖项支持一个项目，研究南极洲干燥山谷地区的岩床就位和固化，并将它们的历史与复活节岛和留尼汪岛上类似片状岩浆体的历史进行比较。如果能够通过直接观测了解板状岩浆体侵位和固化的三维历史，将会对地球演化的认识有很大的进步。片状的岩浆很可能负责输送和塑造大洋和大陆的地壳。例如，在洋脊之下，相对狭窄(约1-4公里)、薄(100-300米)的岩浆带或岩浆底板，通过侵位和凝固过程之间的微妙平衡，提供了洋壳的基本结构，形成了地球表面的约60%。底侵玄武岩岩浆和高级侵入花岗岩浆的凝固过程基本相似，通过早期岩浆的重新活化和分选，在很大程度上输送和塑造了大陆地壳。然而，由于两个简单的原因，研究片状岩浆的动态历史被证明是极其难以捉摸的：缺乏大量连续的岩石暴露，以及缺乏广泛、明确的内部标志来揭示凝固的时间和空间历史。在众所周知的大型层状侵入体中，内部层很丰富，但几乎没有迹象表明岩浆充填的性质和风格(例如，充填是周期性的还是稳定的？是在离散的点上还是沿着裂缝？慢点还是快点？等)。世界各地的许多大型辉绿岩岩床广泛地显示了充填是如何发生的，但它们很少显示任何详细说明充填和凝固过程本身的内部指示器或标记。当一个标记，如以前形成的大的、不同的晶体的痕迹形式出现时，由于糟糕的自然曝光，它很少在三维中被追踪到。南极洲的地下室山丘改变了这一切。它厚(300米)，大(超过5000立方公里)，极其暴露，最重要的是，它带有一个明显的内部标志，其形式是一个大(1-6毫米)斜方辉石晶体的舌形。此外，这种斜方辉石舌头在厚度、晶体大小和在基座中的位置上都有系统地变化，反映了填充位置和填充速度。对这个天体及其三维动力学的记录很可能被证明是片状岩浆系统的“罗塞塔石”。它具有根本性的重要性，是该项目的核心。从这项研究中还可以获得洋脊下形成洋壳结构的详细过程。通过凝固锋不稳定和硅质透镜体的产生，陆壳形成的种子也在基底海槽中清晰可见。作为一项比较研究，在复活节岛熔岩中寻找证据也很可能确定这一过程在火山系统中的重要性。