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是这个吧？哈哈！还有原文！
2.2.1. ★火山熔岩的来由
V1 duke3d001 750, wade777, echosweet 700 & yueqianchen
关键词：45KM, Olivine, Orthopyroxene (referenced by gitarrelieber)。这篇文章的题目不难，狗的骨架也很清晰。
第一段讲火山爆发来源于Mantle中的Lava，而Lava来源于Melt ，Melt在向地表上升的过程中会与Mantle中的Rock反应并不断互相交换物质、变化结构，即吸收Orthopyroxene并排出Olivine。
第二段说一个跟理论不太相符的事情，一种海底里的lava sample，在距离地表45千米突然发现已经停止这种物质交换，Melt的结构不变了。一种假设是那里的Mantle太松散了，使Melt无法与他们接触并交换物质，但立即被否定了(因为45KM还很深东西都很软，没有裂缝)。另一种假设是Melt在之前的上升过程中已经吸收了足够的Orthopyroxene， 并将能排出的Olivine都排了，无法继续反应。
1 darkchoco 710是什么可以证明这种exchange的存在：熔岩的成分
2 gyz12 740 一道文章最后句定位：Olivine的用完了,exchange就停止了
3 gyz12 740 一道是选chemical composition为特征 sashimiyuki 720 V37 选“lab experiments” indicate 那个melt 的变化的，没有选chemical composition, 细节题定位后决定的，确认后到现在还没有深深后悔过
4 tianmo0512 是什么发生反应：选melt
5 feifeizoe 750 V39 文中什么情况下描述了那种正常的exchange：lab experiment中实现了那种现象
6 The author mention “the melt to rise so rapidly” in order to：提出了一种hypothesis，这种hypothesis在后面被反驳
(疑似)原文未缩减 gitarrelieber (sereneys 730 V40 基本确认)
节选自The Origin of the Land under the Sea (Scientific American Magazine @ February 2009)
Author: Peter B. Kelemen
Knowledge of the intense heat and pressure in the mantle led researchers to hypothesize in the late 1960s that ocean crust originates as tiny amounts of liquid rock known as melt—almost as though the solid rocks were “sweating.” Even a minuscule release of pressure (because of material rising from its original position) causes melt to form in microscopic pores deep within the mantle rock. Explaining how the rock sweat gets to the surface was more difficult. Melt is less dense than the mantle rocks in which it forms, so it will constantly try to migrate upward, toward regions of lower pressure. But what laboratory experiments revealed about the chemical composition of melt did not seem to match up with the composition of rock samples collected from the mid-ocean ridges, where eruptedmelt hardens. Using specialized equipment to heat and squeeze crystals from mantle rocks in the laboratory, investigators learned that the chemical composition of melt in the mantle varies depending on the depth at which it forms; the composition is controlled by an exchange of atoms between the melt and the minerals that makeup the solid rock it passes through. The experiments revealed that as melt rises, it dissolves one kind of mineral, orthopyroxene, and precipitates, or leaves behind, another mineral, olivine. Researchers could thus infer that the higher in the mantle melt formed, the more orthopyroxene it would dissolve, and the more olivine it would leave behind.(melt上升时， 溶解Ort产生Oli, 所以melt higher， 溶解的Ort越多，产生的/留在身后的Oli也越多) Comparing these experimental findings with lava samples from the mid-ocean ridges revealed that almost all of them have the composition of melts that formed at depths greater than 45kilometers. This conclusion spurred a lively debate about how meltis able to rise through tens of kilometers of overlying rock while preserving the composition appropriate for a greater depth. If melt rose slowly in smallpores in the rock, as researchers suspected, it would be logical to assume that all melts would reflect the composition of the fashallowest part of the mantle,at 10 kilometers or less. Yet the composition of most mid-ocean ridge lavas amples suggests their source melt migrated through the uppermost 45 kilometers of the mantle without dissolving any orthopyroxene from the surrounding rock. But how? （疑大概为狗狗第一段的背景内容）
In the early 1970s scientists proposed an answer: the melt must make the last leg of its upward journey along enormous cracks. Open cracks would allow the melt to rise so rapidly that it would not have time to interact with the surrounding rock, nor would melt in the core of the crack ever touch the sides. Although open cracks are not a natural feature of the upper mantle— the pressure is simply too great—some investigators suggested that the buoyant force of migrating melt might sometimes be enough to fracture the solid rock above, like an icebreaker ship forcing its way through polar pack ice. Adolphe Nicolas of the University of Montpellier in France and his colleagues discovered tantalizing evidence for such cracks while examining unusual rock formations called ophiolites. Typically, when oceanic crust gets old and cold, it becomes so dense that it sinks back into the mantle along deep trenches known as subduction zones, such as those that encircle the Pacific Ocean. Ophiolites, on the other hand, are thick sections of old seafloor and adjacent, underlying mantle that are thrust up onto continents when two of the planet’s tectonic plates collide. A famous example, located in the Sultanate of Oman, was exposed during the ongoing collision of the Arabian and Eurasian plates. In this and other ophiolites, Nicolas’s team found unusual, light-colored veins called dikes, which they interpreted as cracks in which melt had crystallized before reaching the seafloor. The problem with this interpretation was that the dikes are filled with rock that crystallized from a melt that formed in the uppermost reaches of the mantle, not below 45 kilometers, where most mid-ocean ridge lavas originate. In addition, the icebreaker scenario may not work well for the melting region under mid-ocean ridges: below about 10 kilometers, the hot mantle tends to flow like caramel left too long in the sun, rather than cracking easily.
To explain the ongoing mystery, I began working on an alternative hypothesis for lava transport in the melting region. In my dissertation in the late 1980s, I developed a chemical theory proposing that as rising melt dissolves orthopyroxene crystals, it precipitates a smaller amount of olivine, so that the net result is a greater volume of melt. Our calculations revealed how this dissolution process gradually enlarges the open spaces at the edges of solid crystals, creating larger pores and carving a more favorable pathway through which melt can flow. As the pores grow, they connect to form elongate channels. In turn, similar feedbacks drive the coalescence of several small tributaries to form larger channels. Indeed, our numerical models suggested that more than 90 percent of the melt is concentrated into less than 10 percent of the available area. That means millions of microscopic threads of flowing melt may eventually feed into only a few dozen, high porosity channels 100 meters or more wide. Even in the widest channels, many crystals of the original mantle rock remain intact, congesting the channels and inhibiting movement of the fluid. That is why melt flows slowly, at only a few centimeters a year. Over time, however, so much melt passes through the channels that all the soluble orthopyroxene crystals dissolve away, leaving only crystals of olivine and other minerals that the melt is unable to dissolve. As a result, the composition of the melt within such channels can no longer adjust to decreasing pressure and instead records the depth at which it last “saw” an orthopyroxene crystal. One of the most important implications of this process, called focused porous flow, is that only the melt at the edges of channels dissolves orthopyroxene from the surrounding rock; melt within the inner part of the conduit can rise unadulterated. |
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发表于 2013-3-14 05:14:32
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楼主
1：主要说科学家一直研究一种lava（又称C）是什么东西。有人认为C和一种I的物质是一样的。