Article reproduced from "Micro-region analysis"
Abstract: The pulsating mineralization of the millennium scale is constrained by hydrothermal quartz, the ancient altitude of the plateau is inverted, and the ultra-high plateau that exists briefly due to plate fragmentation is explored.
Millennial pulses of ore formation and an extra-high Tibetan Plateau
1. Does the stone also have a heartbeat?
When it comes to medium-acidic magmatic rocks, most people may be very unfamiliar, in fact, they cover most of the land of the planet, with hard granite floors and violent volcanic eruptions into our lives, and also gave birth to metal resources such as gold, copper and iron, providing the raw materials needed for industrialization.
Geologists work with these stones all day long, trying to understand how old they are, where they come from, and what secrets they hide. One of the questions that has always fascinated me is, does the stone have a heartbeat? For higher creatures such as man, the heartbeat is innate, and we have long been accustomed to its existence. But cold stones also have a heartbeat? This may sound a bit of a fantasy.
To answer this question, we mainly studied a mineral called quartz (Figure 1). Its chemical composition is silica, which is most common on white beaches and is often used to make glass. When quartz is pure and beautiful enough, it is called crystal and is made into jewelry, utensils, ornaments, etc.
Figure 1, quartz of all kinds
ENJOY THE ROCK
Regularly grown quartz leaves a ring band like a tree ring (Figure 2), which can be clearly seen with the help of an electron microscope. Using the idea of counting tree rings, reading the quartz from the core to the edge of the ring, we can understand how the quartz grew up and what it went through in the process.
Figure 2, rings of trees and ring bands of quartz under an electron microscope
By measuring the oxygen isotopes of quartz on a per-loop basis, the information recorded during the growth of the quartz can be inverted. Quartz grown in hydrothermal systems of magma, and its oxygen isotopes are generally very stable. When the oxygen isotopes of hydrothermal systems change significantly, such as rainwater, they are recorded by quartz, which is the basis for the study of the growth process using oxygen isotopes.
The oxygen isotope of magmatic fluids is about 8‰, so the greater the difference in the value of rainwater, the easier it is to identify this process. Rainfall has a very interesting feature, that is, its oxygen isotope decreases with elevation. For example, rainfall in Lhasa in winter may be as low as -30 per thousand, while in Beijing it may be as low as -10 per thousand. Clearly, there are inherent advantages to this study of samples at higher elevations at the time of formation.
The sample we studied was taken from a silicone copper mine called Zhibula in the east of Lhasa, adjacent to the Dragon Porphyry copper mine that I studied in my doctoral dissertation. These quartz crystals are generally centimeter-sized and exhibit beautiful ring bands under an electron microscope that contain four stages of the growth process (Figure 3).
Figure 3, The ring band and four-stage growth process of Zhibura quartz
Using an instrument called secondary ion mass spectrometry, isotopic information about quartz can be obtained at a scale of 20 microns, which is about one-fifth the diameter of a hair. During the course of the experiment, it was found that each analyzed quartz crystal experienced a sudden decrease in oxygen isotopes and a rise again late in growth (Figure 4). This mutation is recorded in the range of about the width of a human hair, so accurately finding a target area of dozens of microns in an area of a few millimeters or even centimeters requires good luck and patience.
We interpret the abrupt decrease in oxygen isotopes in stages 2-3 as rain addition, while the subsequent rise represents a new phase of magmatic fluid injection. These data suggest that the quartz grows as it records one rain and at least two magmatic fluid injections. Is the rhythmic rise and fall of oxygen isotopes very similar to the heartbeat?
Figure 4, Isotope evolution of oxygen recorded in The Labra Quartz
2. Perceive the heart rate of the stone
When we do an ECG in the hospital, a key parameter of the heartbeat is frequency. Therefore, after successfully recognizing the heartbeat, determining the heart rate becomes an important issue. Quartz contains little radioactive elements that can be used for dating, making it difficult to perform accurate isotopic dating of quartz. To determine the heart rate of the quartz, we used a relative dating method called diffusion dating. This uses the ubiquitous diffusion effect, such as Wang Anshi's "remote knowledge is not snow, for there is dark incense" smell of plum incense, it is mainly transmitted through diffusion.
Diffusion chronology is an old and young dating technique, and the booming micro-region analysis technology has made it come alive in recent years. The aforementioned quartz ring belt is mainly controlled by trace elements such as aluminum, titanium and lithium, and the brighter the ring belt, the higher the content of aluminum and other elements.
Affected by diffusion, these elements diffuse from areas with high levels to areas with low levels. The degree of diffusion is mainly controlled by temperature and time, which forms the theoretical basis of diffusion chronology. If the time is long enough, these ring bands will be smoothed out. Unlike radioisotope dating, which gives the time point at which a process occurs, diffusion dating gives the duration of the process.
Figure 5, Quartz Al content distribution characteristics and diffusion simulation results
The ideal state of diffusion chronology is to measure elements with high content and rapid diffusion. The hydrothermal quartz, titanium and lithium we studied, although diffused quickly, were too low to be accurately measured, so the only option was aluminum with higher content but extremely slow diffusion. The diffusion of aluminum in hydrothermal quartz usually occurs at the micron scale, which is about one percent of the diameter of a hair, so it is generally considered that the aluminum in quartz under hydrothermal conditions is basically non-diffuse. Using image scanning technology from nanoion probes (Figure 5), we successfully achieved aluminum content testing at the 0.1 micron scale, seeing for the first time the diffusion profile of aluminum in quartz at low temperatures (Figure 5).
Through simulation calculations, the duration of the above heartbeat is about several hundred years. At this point, the question of whether the stone has a heartbeat and what the heart rate is has been answered. Finding the engine that controls the heartbeat has become a new topic that needs to be answered by further research.
3. Measure the height of the plateau
As mentioned earlier, the oxygen isotope of rainwater is related to altitude, and this correlation can be used to study paleoal elevation. After noticing that hydrothermal quartz recorded very low oxygen isotope composition, we quickly realized that this could be a new paleoaltimeter. Based on a simple two-terminal element hybrid model, the oxygen isotope composition of rainwater was calculated, and the altitude was further calculated. The results show that 17 million years ago, the ancient altitude of the area where Mebra was located was about 6 km. In other words, there was once a plateau that was ~1 km higher than it is now.
Figure 6, Ancient elevation and possible dynamic mechanisms recorded by The Zymbora quartz
So, what exactly did the plateau go through that caused it to grow taller and then become shorter? We speculate that the Indian plate, which is subducting below the Eurasian plate, broke around 17 million years ago, and the upper cladding plate was lifted due to buoyancy (Figure 6).
Acknowledgements and original message:
This research is mainly supported by the National Key R&D Program project "New Technology of Transformative In situ Analysis for Mineral Deposits Research" and the National Natural Science Foundation of China Outstanding Youth Science Foundation.
李扬, Mark Allen, 李献华. Millennial pulses of ore formation and an extra-high Tibetan Plateau. Geology 10.1130/g49911.1 (2022).
Edit: Yu Yan
Review: Li Yang, Li Xianhua