Chemical analysis data derived from NERC grant NE/I025573/1, When on Earth did modern plate tectonics begin?

Earth is a dynamic planet, for the simple reason that it is still cooling down from the heat of accretion and subsequent decay of radioactive elements. The main mechanism by which it loses heat is plate tectonics, a theory that has been widely accepted since the 1970s. The Earth is formed of a dense metallic core surrounded by a partially molten silicate mantle which itself is capped by a buoyant crust, either continental or oceanic. We live on the continental crust which largely exists above sea level. The ocean crust forms the floors of oceans and is only rarely exposed. The ocean crust forms by mantle melting at mid ocean ridges, such as the mid Atlantic ridge upon which sits the volcanic island of Iceland. New crust is constantly formed, forcing the older crust to spread outwards and oceans to grow larger. As the ocean crust spreads away from the ridge, it cools and becomes denser. Eventually it interacts with a continent, made of less dense material. The ocean crust is driven beneath the continent back into the mantle, a process known as subduction. Volcanoes form along the continental margin above the subduction zone and at least some of this activity results in addition of new continental crust. This may have been the main process responsible for initial formation and subsequent evolution of our continents. It can be observed now around the margin of the Pacific Ocean, where widespread volcanism is known as the "Ring of Fire". However, not all oceans can continue to grow! The Atlantic Ocean has stopped getting bigger as a response to the continued growth of the Pacific. Eventually, an ocean will close completely and the surrounding continents will collide, resulting in a linear mountain chain. A good example is the Himalaya, where India has collided with Asia. This whole process known as plate tectonics has a profound affect on our planet, providing us with land on which to live, seas in which to fish, freshwater to drink and our complex weather patterns. It is also a regulator of our climate since weathering of continental rocks results in drawdown of CO2 to the deep sea where it is stored. Understanding plate tectonics is central to Earth and Environmental Scientists. There are still important details that we know little about, such as how and when it began. This proposal seeks to investigate this by a novel study of critical rocks that characterise plate tectonics, in particular those that result from subduction. When ocean crust is subducted, increasing pressure and temperature change it into denser rock. As the Earth has evolved, the exact pressure and temperature conditions of this "metamorphism" have also changed. We propose to study this by using minerals that form within ocean crust during subduction. The rocks themselves are often destroyed by erosion, but tiny crystals of a robust mineral called rutile (titanium dioxide) can survive to be found in sediments derived from them. By dating these and using their chemical composition as a fingerprint, we can work out the pressure and temperature within the eroded subduction zone. Similarly, the volcanic rocks that form during subduction have changed through time. These are also often destroyed by erosion so that the exposed record may not be representative. Another robust mineral known as zircon (zirconium silicate) often survives the weathering and ends up alongside rutile in the younger sediments. Using similar methods with zircon we can also investigate changing styles of magmatism throughout Earth's history. . Currently the magmatic record implies that modern subduction began around 2500 million years ago, yet the metamorphic record implies a later start of around 700 million years ago. Our novel approach will test this. We will be able to say whether the younger date is correct and the older marks a different kind of plate tectonics, or whether the older date does indeed represent the onset of modern plate tectonics, and the exposed rock record is biased.

地球，一颗充满活力的行星，其之所以如此，乃因其正处于由积聚之热能以及放射性元素后续衰变所导致的冷却过程中。地球散热的主要机制为板块构造，这一理论自20世纪70年代以来已被广泛接受。地球由一个致密的金属核心、一个部分熔融的硅酸盐地幔以及覆盖其上的浮动的地壳（无论是大陆性地壳还是海洋性地壳）所构成。我们居住在大陆性地壳之上，大部分地区均位于海平面之上。海洋地壳构成了海洋的底部，且很少露出地表。海洋地壳的形成是通过地幔在中洋脊处（如冰岛所在的北大西洋脊）的熔融而成。新地壳的不断形成迫使旧地壳向外扩展，从而使海洋面积增大。当海洋地壳从中洋脊向外扩展时，它会冷却并变得更加致密。最终，它将与由密度较低物质构成的陆地相遇，海洋地壳被驱动至大陆下方，进入地幔，这一过程被称为俯冲。火山沿着大陆边缘在俯冲带上方形成，至少有一部分这种活动会导致新大陆地壳的形成。这可能正是我们大陆初始形成及其后续演化的主要过程。现在，在太平洋边缘可以观察到这一过程，那里广泛的火山活动被称为“火环”。然而，并非所有海洋都能持续扩张！大西洋由于太平洋的持续扩张而停止了扩张。最终，一个海洋将完全闭合，周围的陆地将发生碰撞，形成线性山脉。喜马拉雅山脉就是一个很好的例子，印度与亚洲在此相撞。这一整个过程，即板块构造，对我们的地球产生了深远的影响，为我们提供了居住的土地、捕鱼的海洋、可饮用的淡水和复杂的气候模式。它还是我们气候的调节者，因为大陆岩石的风化作用会导致二氧化碳向深海中的沉积，并在那里储存。对板块构造的理解对于地球和环境科学家至关重要。尽管如此，我们对于一些重要细节仍所知甚少，例如其起始方式和时间。本提议旨在通过一项关于表征板块构造的关键岩石的新颖研究来调查这一点，特别是那些由俯冲产生的岩石。当海洋地壳发生俯冲时，不断增加的压力和温度将其转变为密度更大的岩石。随着地球的演化，这种“变质作用”的确切压力和温度条件也发生了变化。我们建议通过研究在俯冲过程中形成于海洋地壳中的矿物来研究这一点。岩石本身往往会被侵蚀破坏，但一种称为二氧化钛的坚固矿物的微小晶体（即锐钛矿）能够幸存，并在源自它们的沉积物中被发现。通过对这些晶体进行年代测定，并利用其化学组成作为指纹，我们可以推断出已侵蚀的俯冲带中的压力和温度。同样，在俯冲过程中形成的火山岩石也随时间发生了变化。这些岩石也常常被侵蚀破坏，因此暴露的记录可能并不具有代表性。另一种称为锆石（硅酸锆）的坚固矿物往往能够经受住风化，最终与锐钛矿一起出现在较新的沉积物中。通过使用类似的方法对锆石进行研究，我们还可以调查地球历史上岩浆活动风格的演变。目前，岩浆记录表明，现代俯冲始于约25亿年前，而变质记录则暗示了大约7亿年后的开始。我们新颖的方法将对此进行检验。我们将能够确定较晚的日期是否正确，而较早的标记则代表了一种不同的板块构造，或者是否较早的日期确实代表了现代板块构造的开始，而暴露的岩石记录存在偏差。