Petrologic consequences of non-hydrostatic stress during subduction zone metamorphism

地下深处的岩石承受着高温和高压。岩石中矿物的化学成分可以记录岩石形成的温度和压力条件。不同的地质环境具有不同的温压特征。因此，地球科学家可以利用保存在矿物中的化学历史来揭示岩石形成的地质环境，例如在俯冲带或山带内。读取温度-压力记录的传统方法是假设岩石形成时在各个方向上的压力相等。然而，新兴的研究表明，岩石上的压力（或应力）可能随着方向的变化而变化。例如，地震和高海拔山脉的存在是定向应力发挥重要作用的环境，它对矿物之间发生的反应施加了根本的控制。这些反应包括释放水分的反应（脱水反应）。这些水可以削弱岩石，促进地震活动，或促使岩石融化，从而产生危险的火山，如雷尼尔山，以及世界上许多大型矿床。在地球的俯冲带及其周围，一个构造板块下沉到另一个板块下面的区域（例如，在俄勒冈州、华盛顿州和阿拉斯加州），自然灾害尤其严重。总之，为了更好地了解岩石记录，我们必须能够定量地评估不均匀应力对地质过程的影响。这项研究将需要博士和本科生的研究，以及通过耶鲁大学皮博迪自然历史博物馆的公共宣传项目。将测试与俯冲带变质岩有关的四个相互关联的关键概念。(1)固溶体矿物组成和晶体取向记录应力场。(2)颗粒尺度上的应力变化会导致矿物成分分带。(3)矿物内部的扩散可以产生影响扩散剖面形状的应力，从而影响变质和构造过程时间尺度的基于扩散的估计。(4)正应力可引发脱水反应、岩石弱化和潜在地震力矩释放。这些测试将涉及定量描述矿物不均匀应力所需的热力学和动力学方程的理论发展，以及在阿尔卑斯山、德国、希腊和挪威发掘的俯冲复合体的自然样品的应用。材料科学家F. C. larch<s:1>和J. W. Cahn具有里程碑意义的贡献将成为理论发展的基础。实地工作和样本收集将在挪威卑尔根弧进行；其他地方的样本已经在耶鲁大学的岩石学收藏中。将使用广泛的实验室技术，包括薄片岩石学、矿物晶体学取向的电子背散射衍射分析和矿物化学的电子探针微分析。最后，本研究将评估非静压应力对矿物界面稳定性、固溶体矿物组成和晶内扩散速率的影响。反过来，这将对基于扩散的时间尺度估计、重建变形、冷却和挖掘历史以及俯冲带的流体生成具有广泛的意义。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Rocks deep below Earth’s surface are subjected to high temperatures and intense pressures. The chemical compositions of the minerals in rocks can record the temperature–pressure conditions under which the rocks form. Different geologic environments are characterized by distinct temperature–pressure regimes. Consequently, geoscientists can use the chemical histories preserved in minerals to reveal geologic environments of rock formation, such as within a subduction zone or mountain belt. The traditional approach to reading the temperature–pressure record is to assume that the pressure on a rock was equal in all directions when it formed. However, emerging research demonstrates that pressure (or stress) on a rock may vary considerably with direction. For example, earthquakes and the existence of mountains that stand high above sea level are settings where directional stresses play an essential role by exerting fundamental controls on the reactions that take place among minerals. These include reactions that release water (dehydration reactions). This water can weaken rocks and facilitate seismic activity, or drive the rock melting that produces dangerous volcanoes such as Mt. Rainier as well as many of the world’s great ore deposits. The natural hazards are particularly acute in and around Earth’s subduction zones, regions where one tectonic plate descends beneath another (for example, in Oregon, Washington State, and Alaska). In summary, to obtain a better understanding of the rock record, we must be able to quantitatively assess the impact of unequal stresses on geologic processes. This study will entail Ph.D. and undergraduate student research as well as public outreach programs through the Yale University Peabody Museum of Natural History.Four key interrelated concepts relevant for metamorphic rocks in subduction zones will be tested. (1) Solid solution mineral compositions and crystallographic orientations record stress fields. (2) Stress variations at the grain scale can produce compositional zoning in minerals. (3) Diffusion within minerals can generate stresses that affect diffusion profile shapes and thus diffusion-based estimates for the timescales of metamorphic and tectonic processes. (4) Normal stress can trigger dehydration reactions, rock weakening, and potential seismic moment release. These tests will involve theoretical development of the thermodynamic and kinetic equations needed to quantitatively describe unequal stress in minerals, together with applications to natural samples from exhumed subduction complexes in the Alps, Germany, Greece, and Norway. The landmark contributions of materials scientists F. C. Larché and J. W. Cahn will be the basis for the theoretical development. Field work and sample collection will be done in the Bergen Arcs, Norway; samples from the other localities are already in the petrologic collections of Yale University. A wide array of laboratory techniques will be used, including thin section petrography, electron backscatter diffraction analysis of mineral crystallographic orientations, and electron-probe microanalysis of mineral chemistry. Ultimately, the research will evaluate the impact of non-hy¬drostatic stress on mineral interface stability, solid solution mineral compositions, and intracrystalline diffusion rates. This, in turn, will have broad implications for diffusion-based time¬scale estimation, reconstructing deformation, cooling, and exhumation histories, and fluid generation in subduction zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.