In the course of their scientific expeditions, geologists discovered a strange rock with a distinctive blue color, which was not widely found at first, and was only exposed in a few places. Just as azure is only possible when fired in rainy weather, the blue color of this rock has undergone an extraordinary process. So how exactly does this blue come about? Let's take a look.
01 What kind of blue do you want: blue amphibole + cyanoglucane
Because of the blue color of this rock, and the minerals in the rock are often arranged in a certain direction, showing a scaly metamorphic structure or a fibrous metamorphic structure and a flake structure, it is named blue schist. This directional arrangement is a common rock structure of metamorphic rocks, and blueschist is one of the metamorphic rocks, which is related to the directional pressure during the metamorphic process. The reason why blue schist is blue is because there are some special minerals in it: such as blue amphibole and cyanoglucane. In addition to this, it contains anderyl, aragonite, jadeite and Deere amphibolite, a mineral combination that indicates that the physical conditions for the production of blueschist are high pressure and low temperatures. Experiments have been carried out at high temperatures and pressures, and the results show that the formation of this rock requires extremely high pressures (more than 4-6 kbar) and relatively low temperatures (below 400 °C). This has led scientists to think, normally, such high pressure conditions, it is easy to think of the depths of the mantle, but the higher the pressure, the deeper the corresponding depth, and there should be a higher temperature, so how did the high pressure and low temperature conditions appear? It all starts with an active Earth.
Fig.1 Blue schist hand specimen
02 Active Earth: Colliding Plates
We know that the earth can be divided into crust, mantle and core from the outside to the inside, according to the change in the propagation speed of seismic waves at different depths of the ground. In the interior of the earth, that is, in the mantle and the core of the earth, a large amount of heat energy is stored, so we often say that the earth has a hot "heart". Like a heart that is constantly beating, the planet we live on is constantly active. Within the upper mantle there is a slow-flowing asthenosphere, which is a mixture of solid and a small amount of liquid (1%-10%), in a semi-viscous state. The part of the mantle above the asthenosphere is called the lithospheric mantle, and together with the earth's crust, it is called the lithosphere.
When the lithosphere is large enough, it is called a "plate", and it is made up of many oceanic and continental plates. These plates are not closely connected to each other, and there are certain fissures. They are carried on top of the asthenosphere in slow motion, sometimes separating and sometimes colliding. Since the density of different plates is not the same, when two plates move in opposite directions, a "subduction" can occur, which can be understood as a dense plate, and when it collides with another lighter plate, the heavier plate is inserted below the lighter plate. In general, the density of oceanic crust is much greater than that of continental crust, so the subduction of oceanic crust into continental crust was also first known. The subduction between the two continental plates has also been further understood with the continuous exploration of geologists.
Fig.2 Schematic diagram of the Earth's interior
Figure 3 Schematic diagram of the plate
03 Unique processing plant: dehydration and pressurization slowly heat up
During subduction, the previously cooler lithosphere is inserted into the hot mantle. The subduction channel is like a factory, reconstructing the original rock to produce new rock. As the depth of subduction increases, the pressure increases, the minerals undergo metamorphism, deformation, and some water-bearing minerals will undergo dehydration, as a result, the original rock morphology and mineral structure no longer exist, and a series of new rocks are produced, which we call metamorphic rocks. Depending on the temperature and pressure, the composition of the original rock will be different, and the type of new rock will also be different.
Generally speaking, the higher the pressure conditions correspond to the higher temperatures, so what is the reason for the blue schist to be born in a high-pressure environment but the temperature is very low? Scientists provide a reasonable explanation for this, the cold plate is obliquely inserted into the mantle during downward subduction, and the pressure continues to increase with depth, but the heating process lags relatively behind due to the poor thermal conductivity of the rock, so that the thermal structure of "low geothermal gradient" is formed, which provides a suitable tectonic environment for the formation of blue schist.
Fig.4. Schematic diagram of the subduction plant
(郑永飞等,2016;Tatsumi,2005)
04 Why is blue schist so young
Do you know? Even if blue schist has a history of millions of years, it is not very old in the colorful rock world, considering that the oldest zircon found in the metamorphic conglomerate of Jack Hills in Australia is 4.4 billion years old! Blue schist can only be regarded as a small great-grandchild in front of it.
There is a lack of ancient blue schist in the world, and the oldest blue schist is only 800 ~ 700 Ma in age. Initially, scientists speculated about the following reasons for the absence of ancient blue schist: (1) there was no plate tectonics before the Neoproterozoic, and the low geothermal gradient environment required for the formation of blue schist was missing; (2) plate tectonics existed before the Neoproterozoic, but the earth was very hot at that time, and the subduction zone was also higher than the modern geothermal gradient, so blue schist was not produced; (3) The ancient earth rocks are more Mg-rich, and the Mg-rich system is difficult to produce blueschist even under low geothermal gradients. (4) The ancient blue schist has been eroded away during its long geological history. The preservation of the geological record has now been largely eliminated. Because a large number of amphibolite and greenstone belts are preserved in the ancient earth, the density and hardness of mafic blueschist are greater than those of these rocks, and the probability of being preserved theoretically will be greater.
At present, the focus of the debate is mainly on the existence of modern plate tectonics before the Neoproterozoic, which requires a combination of evidence such as blue schist, dolerite, ophiolite, and ultra-high-pressure landmass. The in-depth study of blue schist is of great significance to the beginning time of plate tectonics, and the mystery has not yet been solved, and the story of the earth is still unfolding!
bibliography
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