Knowledge of the intense heat and pressure inthe mantle led researchers to hypothesize in the late 1960s that ocean crustoriginates as tiny amounts of liquid rock known as melt—almost as though the solid rocks were “sweating.” Even a minuscule releaseof pressure (because of material rising from its original position) causes meltto form in microscopic pores deep within the mantle rock. Explaining how therock sweat gets to the surface was more difficult. Melt is less dense than themantle rocks in which it forms, so it will constantly try to migrate upward,toward regions oflower pressure. But what laboratory experiments revealedabout the chemical composition of melt did not seem to match up with thecomposition of rock samples collected from the mid-ocean ridges, where eruptedmelt hardens. Using specialized equipment to heat and squeeze crystalsfrom mantle rocks in the laboratory, investigators learned that the chemical compositionof melt in the mantle varies depending on the depth at which it forms; the compositionis 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 meltrises, it dissolves one kind of mineral, orthopyroxene, and precipitates, or leaves behind, another mineral, olivine. Researcherscould thus infer that the higher in the mantle melt formed, the moreorthopyroxene it would dissolve, and the more olivine it would leave behind.(melt上升时， 溶解Ort产生Oli, 所以melthigher， 溶解的Ort越多，产生的/留在身后的Oli也越多) Comparing these experimental findingswith lava samples from the mid-ocean ridges revealed that almost all of them have the composition of melts thatformed 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 preservingthe composition appropriate for a greater depth. If melt rose slowly in smallpores in the rock, as researchers suspected, it would be logical to assume thatall 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 lavasamples suggests their source melt migrated through the uppermost 45 kilometers of the mantle withoutdissolving 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 ananswer: the melt must make the last leg of its upward journey along enormous cracks. Open cracks wouldallow the melt to rise so rapidly that it would not have time to interact withthe surrounding rock, nor would melt in the core of the crack ever touch thesides. 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 meltmight sometimes be enough to fracture the solid rock above, like an icebreaker shipforcing its way through polar pack ice. Adolphe Nicolas of the University ofMontpellier in France and his colleagues discovered tantalizing evidence forsuch cracks while examining unusual rock formations called ophiolites. Typically,when oceanic crust gets old and cold, it becomes so dense that it sinks backinto the mantle along deep trenches known as subduction zones, such as thosethat encircle the Pacific Ocean. Ophiolites, on the other hand, are thick sectionsof old seafloor and adjacent, underlying mantle that are thrust up ontocontinents when two of the planet’s tectonic plates collide. A famous example,located in the Sultanate of Oman, was exposed during the ongoing collision ofthe Arabian and Eurasian plates. In this and other ophiolites, Nicolas’s teamfound unusual, light-colored veins called dikes, which they interpreted ascracks in which melt had crystallized before reaching the seafloor. The problemwith this interpretation was that the dikes are filled with rock thatcrystallized from a melt that formed in the uppermost reaches of the mantle,not below 45 kilometers, where most mid-ocean ridge lavas originate. Inaddition, the icebreaker scenario may not work well for the melting regionunder mid-ocean ridges: below about 10 kilometers, the hot mantle tends to flowlike caramel left too long in the sun, rather than cracking easily.
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