Geochemistry
1.1Geochemistry
The term “geochemistry” was first used by the Swiss chemist Schönbein in 1838. You might guess, merely from the etymology of the word, that the field of geochemistry is somehow a marriage of the fields of geology and chemistry. That would be a good guess. But just how are chem- istry and geology combined within geochemistry; what is the relationship between them? Perhaps the best explanation would be to state that in geochemistry, we use the tools of chemistry to solve geological problems; that is, we use chemistry to understand the Earth and how it works. The Earth is part of a family of heavenly bodies, our Solar System, that formed simultaneously and are closely related. Hence, the realm of geochemistry extends beyond the Earth to encompass the entire Solar System. The goals of geochemistry are thus no different from those of other fields of earth science; just the approach differs. On the other hand, while geochemists have much in common with other chemists, their goals differ in fundamental ways. For example, our goals do not include elucidating the nature of chemical bonding or synthesizing new compounds, although these may often be of inter- est and use in geochemistry. Though geochemistry is a subdiscipline of earth science, it is a very broad topic. So broad in fact that no one can really master it all; geochemists invariably specialize in one or a few aspects, such as atmospheric chemistry, geochemical thermodynamics, isotope geochem- istry, marine chemistry, trace element geochemistry, soil chemistry, etc. Geochemistry has flourished in the quantitative approach that has dominated earth science in the second half of the
twentieth century. This quantitative approach has produced greater advances in the understanding of our planet in the last 50 years than in all of prior human history. The contri- butions of geochemistry to this advance have been simply enormous. Much of what we know about how the Earth and the Solar System formed has come from research on the chemistry of meteorites. Through geochemistry, we can quantify the geologic time scale. Through geochemistry, we can de- termine the depths and temperatures of magma chambers. Through geochemistry, mantle plumes were recognized. Through geochemistry, we know that sediments can be subducted into the mantle. Through geochemistry, we know the temperatures and pressures at which the various metamorphic rock types form and we can use this information, for example, to determine the throw on ancient faults. Through geochemistry, we know how much and how fast mountain belts have risen. Through geochemistry, we are learning how fast they are eroding. Through geochemistry, we are learning how and when the Earth’s crust formed. Through geochemistry, we are learning when the Earth’s atmosphere formed and how it has evolved. Through geochemistry, we are learning how the mantle convects. Through geochemistry, we are learning how cold the ice ages were and what caused them. The evidence of the earliest life, 3.8 gigayears (billion, or 109 years, which we will henceforth ab- breviate as Ga), is not fossilized remains, but chemical traces of life. Similarly, the tenuous evidence that life existed on Mars about the same time is also largely chemical. Not surprisingly, instruments for chemical analysis have been key part of probes sent to other heavenly bodies, including Venus, Mars, Jupiter. Geochemistry lies at the heart of environmental science and environmental concerns. Problems such as acid rain, the ozone hole, the greenhouse effect and global warming, water and
soil pollution are geochemical problems. Addressing these problems requires a knowledge of geochemis- try. Similarly, most of our non-renewable resources, such as metal ores and petroleum, form through geochemical processes. Locating new sources of these resources increasing requires geochemical ap- proaches. In summary, every aspect of earth science has been advanced through geochemistry. Though we will rarely discuss it in this book, geochemistry, like much of science, is very much driven by technology. Technology has given modern geochemists tools that allow them to study the Earth in ways that pioneers of the field could not have dreamed possible. The electron microprobe allows us to analyze mineral grains on the scale of microns in minutes; the electron microscope allowsus to view the same minerals on almost the atomic scale. Techniques such as X-ray diffraction, nu- clear magnetic resonance, and Raman and infrared spectroscopy allow us to examine atomic ordering and bonding in natural materials. Mass spectrometers allow us to determine the age of rocks and the temperature of ancient seas. Ion probes allow us to do these things on micron scale samples. Analyti- cal techniques such as X-ray fluorescence and inductively coupled plasma spectrometry allow us to perform in minutes analyses that would days using “classical” techniques. All this is done with greater precision and accuracy than was possible just a few decades ago. Mega-computers with giga- hertz of power and gigabytes of memory allow us to perform in seconds thermodynamic calculations that would have taken years or lifetimes half a century ago; the tera-computers just around the corner will offer us even more power. New instruments and analytical techniques now being developed promise even greater sensitivity, speed, accuracy, and precision. Together, these advances will bring us ever closer to our goal understanding the
Earth and its cosmic environment.
The Earth’s internal heat is responsible for tectonic processes, which tend to deform the surface of the planet, producing topographic highs and lows. The internal heat has two parts. Some fraction of the heat, estimated to be between 25% and 75%, originated from the gravitational energy released when the Earth formed. The other fraction of internal heat is produced by the decay of radioactive elements, principally uranium, thorium, and potassium, in the Earth. The Earth’s internal heat slowly decays over geologic time as it migrates to the surface and is radiated away into space. It is this migration of heat out of the Earth that drives tectonic processes. Heat causes both the outer core and the mantle to convect, as hot regions rise and cold regions sink. Convection within the outer core gives rise to the Earth’s magnetic field, and may have other, as yet not understood, geologic conse- quences. Convection in the mantle is responsible for deformation of the Earth’s crust as well as vol- canism. The great revolution in earth science in the 1960’s centered on the realization that the outer part of the Earth was divided into a number of “plates” that moved relative to one and other. Most tectonic processes, as well as most volcanism, occur at the boundaries between these plates. The outer part of the Earth, roughly the outer 100 km or so, is cool enough (<1000 ° C) that it is rigid. This rigid outer layer is known as the lithosphere and comprises both the crust and the outermost mantle (Figure1.10). The mantle below the lithosphere is hot enough (and under sufficient confining pressure) that it flows, albeit extremely slowly, when stressed. This part of the mantle is known as the astheno- sphere. Temperature differences in the mantle create stresses that produce convective flow. It is this flow that drives the
motion of the lithospheric plates. The motion of the plates is extremely slow, a few tens of centimeters per year at most and generally much less. Nevertheless, on geologic time scales they are sufficient to continually reshape the surface of the Earth, creating the Atlantic Ocean, for example, in the last 200 million years. Rather than thinking of plate motion as being driven by mantle convection, it would be more cor- rect to think of plate motion as part of mantle convection. Where plates move apart, mantle rises to fill the gap. As the mantle does so, it melts. The melt rises to the surface as magma and creates new oceanic crust at volcanos along mid-ocean ridges (Figure 1.10). Mid-ocean ridges, such as the East Pa- cific Rise and the Mid-Atlantic Ridge, thus mark divergent plate boundaries. As the oceanic crust moves away from the mid-ocean ridge it cools, along with the mantle immediately below it. This cooling produces a steadily thickening lithosphere. As this lithosphere cools, it contracts and its density increases. Because of this contraction, the depth of the ocean floor increases away from the mid-ocean ridge.
“地球化学”最初是在1838由瑞士化学家Schönbein使用。你可能会想，仅仅是从这个词的词源，即地球化学在某种程度上是一个地质化学领域的东西。但就如何在化学研究和地质地球化学相结合的；它们之间的关系是什么？最好的解释可能会对一个国家的，地球化学，利用化学工具解决地质问题；那就是，我们用化学的理解地球和它是如何工作的。地球是一个家庭的天体的一部分，我们的太阳系，这同时形成，是密切相关的。因此，地球化学领域超越地球包括整个太阳系。地球化学的目标，从而从这些地球科学的其它领域没有什么不同，只是方法不同。另一方面，当地球化学家与其他化学家多的共同点，他们的目标在不同的基本方式。例如，我们的目标不包括阐明化学键或合成新的化合物的性质，但这些往往是跨专业和利用地球化学。虽然地球化学是一门地球科学，这是一个非常
广泛的话题。事实上，没有人能真正掌握这一切那么宽；地球化学家总是专注于一个或几个方面，如大气化学，化学热力学，化学，同位素地球化学，海洋化学，微量元素地球化学，土壤化学，地球化学，在第二十世纪下半叶地球科学为主定量方法的兴盛。这种定量的方法产生了更大的进步，在我们这个星球的理解在过去的50年。用地球化学的贡献这是巨大的进步。我们所知道的关于地球和太阳系的形成来自于对陨石的化学研究。通过地球化学，我们可以量化地质时间尺度。通过地球化学，我们可以确定深度和岩浆的温度。通过地球化学，地幔柱被确认。通过地球化学，我们知道，沉积物可俯冲到地幔。通过地球化学，我们知道温度和压力的各种变质岩类的形式，我们可以利用这些信息，例如，确定古断层断距。通过地球化学，我们知道如何快速山带上升。通过地球化学，我们学习他们是如何快速侵蚀。通过地球化学，我们学习如何在地球的地壳形成。通过地球化学，我们学习的时候，地球的大气层形成以及它是如何发展的。通过地球化学，我们还学习了地幔对流。通过地球化学，我们学习寒冷的冰河时期，是什么使他们这样。最早的生命的证据，不是化石，而是生命的化学物质。同样的，脆弱的证据表明火星上存在生命的同时也在很大程度上化学。毫不奇怪，化学分析仪器已被发送到其他天体，探讨关键部分包括金星，火星，木星。地球化学的核心是环境科学和环境问题。如酸雨，臭氧空洞，温室效应和全球变暖，水和土壤污染地球化学问题。解决这些问题需要了解尝试地球。同样的，我们大部分的不可再生资源，如金属矿石和石油，通过地球化学过程的形式。这些新资源的增加源定位方法需要地球化学应用。总之，地球科学的各个方面一直通过先进的地球化学。虽然我们很少会在这本书中，讨论地球化学。技术是现代地球化学的工具，让学习的方式，这个领域的先驱，也不能想象可能的地球。电子探针分析允许我们在微米尺度的矿物颗粒分析；电子显微镜几乎可以看到相同的矿物在原子尺度上。技术，如X射线衍射，清晰的磁共振拉曼光谱和红外光谱，并允许我们研究原子有序和自然材料的粘接。质谱仪使我们能够确定岩石的年龄和古海洋的温度。离子探针允许我们在微米尺度的样品做这些事情。分析化学技术，如X射线荧光光谱和电感耦合等离子体质谱法允许我们进行分析，将天分钟用“经典”技术。所有这一切都是以更高的精度和准确度可能比仅仅几十年前。随着千
兆电力和内存，允许我们进行热力学计算，在几秒钟内将采取年甚至一生半个世纪前，赫兹的巨型计算机；万亿次计算机指日可待将为我们提供更多的权力。新的仪器和分析技术正在开发更具有敏感性，速度，准确度，精密。在一起，这些进展将使我们更接近我们的目标理解地球的宇宙环境。
地球内部的热量是构造的过程，这往往使行星的表面，产生地形的高点和低点。内部的热量有两部分。一部分的热，估计在25%和75%之间，起源于地球形成时释放的引力能。内部热量的其他部分是由放射性元素衰变产生的，主要是铀，钍，钾。地球内部的热量慢慢衰变在地质时间上它迁移到表面，发射进入太空。正是这种迁移的热量从地球驱动的构造过程。热引起的外核和地幔对流，热的地区和寒冷地区的上升下沉。在外核对流产生地球磁场，可能有其他的。在地幔对流负责地壳变形。伟大的革命在地球科学在1960年代以实现地球的外层部分分为多个“板块”的移动相对于一等。大多数的构造过程，以及火山活动，发生在这些板块之间的边界。地球的外层部分，大约外100公里左右，足够酷热（＜1000°C），它是刚性的。这种刚性的外层被称为岩石圈包括地壳和地幔的最外层。地幔岩石圈之下足够热（和足够的围压下），它流动，尽管非常缓慢，但相当强。在地幔温度的差异造成，产生对流的流动应力。正是这种流驱动的岩石圈板块的运动。该板块的运动非常缓慢，几十年最多，一般较少厘米。然而，他们足以不断重塑地球表面的地质尺度，例如，创建大西洋海洋，在过去的2亿年。而不是思维的板块运动是地幔对流的驱动下，它会更直接认为板块运动是地幔对流的一部分。再分开，地幔上升到填补空白。由于地幔的如此融化。熔体上升到岩浆表面产生沿洋中脊火山新的洋壳。洋中脊特定上升。大西洋中脊，因此标记不同的板块边界。作为海洋地壳运动远离洋中脊的冷却，伴随着地幔下降。这产生了一个不断增厚岩石圈冷却。这岩石圈冷却，它和其密度的增大而增大。由于这种收缩，海底的深度的增加，远离洋中脊。
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