Collaborative Research: Relating Bulk Composition to Seismic Properties in Crustal Rocks

地球的大陆地壳是矿物和其他自然资源的储存库。此外，地壳的化学成分控制其物质性质（例如，强度），从而影响地壳在地质时期的变形和演变方式。然而，虽然地壳成分在地球上？表层可以通过直接采样确定，而深层的成分必须通过间接方法如地球物理成像来推断。最常见的地球物理成像方法之一是使用地震波，地震波以不同的速度穿过不同的岩石类型。通过测量地震波的速度，地壳的三维内部结构可以使用类似于医学研究中的X射线计算机断层扫描（CT扫描）的方法来确定。在这个项目中，我们将采取地震速度模型的地球？的大陆地壳和反演他们推断地壳组成。这些组成模型将用于确定地壳强度的变化，以及具有不同自然资源潜力的地区。在过去的几十年里，通过NSF的Earthscope，GeoPRISMS和Cascadia Initiative等项目，已经可以获得大量表征大陆地壳的地震数据。包括来自受控源实验的同位置P波数据、来自接收器函数的VP/VS约束以及来自环境噪声层析成像的S波数据。这些数据集为今后研究大陆地壳的性质和组成奠定了基础。然而，尽管地震数据提供了丰富的信息，岩石成分和地震速度之间的定量关系缺失。在这个项目中，我们将开发一套新的工具，将地壳成分与地震速度联系起来。我们的方法分为四个步骤：（1）导出地壳成分空间的代表性数据库，（2）在各种平衡P、T、H2O和fO 2条件下对每种成分的平衡矿物组合进行热力学计算，（3）计算不同原位P、T条件下每种相组合的VP和VS，以及（4）使用所得到的数据集来建立地壳成分与地震速度之间的正演关系。这些数据将汇编成一个在线数据库，可供其他方面使用，以得出在特定范围的地壳条件下主要元素化学与地震速度之间的关系。该项目将支持麻省理工学院/WHOI联合项目学生William Shinevar。为了最大限度地利用成分反演的社区，我们将开发一个基于Web的Java小程序，让用户直接从自己的地震数据集反演主要元素的组成。

The Earth's continental crust is a reservoir for minerals and other natural resources. Moreover, the chemical composition of the crust controls its material properties (e.g., strength), and thus influences the way in which the crust deforms and evolves over geologic time. However, while crustal composition at the Earth?s surface can be determined by direct sampling, at depth composition must be inferred from indirect methods such as geophysical imaging. One of the most common geophysical imaging methods employs seismic waves, which travel at different speeds through different rock types. By measuring seismic wave speeds the 3-D internal structure of the crust can be determined using an approach similar to x-ray computed tomography (CT scan) in medical studies. In this project, we will take seismic velocity models for the Earth?s continental crust and invert them to infer crustal composition. These compositional models will then be used to determine variations in crustal strength, as well as regions with different natural resource potential.In the past decades a wealth of seismic data characterizing the continental crust have become available through NSF programs such as Earthscope, GeoPRISMS, and the Cascadia Initiative?including co-located P-wave data from controlled-source experiments, VP/VS constraints from receiver functions, and S-wave data from ambient noise tomography. These datasets provide the foundation for future research on the nature and composition of the continental crust. However, despite the abundance of information provided by the seismic data, a quantitative relationship between rock composition and seismic velocity is missing. In this project we will develop a new suite of tools to relate crustal composition to seismic velocity. Our approach is divided into four steps: (1) derive a representative database for crustal composition space, (2) perform thermodynamic calculations of the equilibrating mineral assemblages for each composition at various equilibrium P,T, H2O and fO2 conditions, (3) calculate VP and VS for each phase assemblage for different in situ P,T conditions, and (4) use the resulting dataset to establish forward relationships between crustal composition and seismic velocity. These data will be compiled into a publically available online database, which can be used by others to derive relationships between major element chemistry and seismic velocity under a specified range of crustal conditions. The project will support MIT/WHOI Joint Program student William Shinevar. To maximize the utility of the compositional inversion to the community we will develop a web-based Java applet to allow users to directly invert major element compositions from their own seismic datasets.