How is bromine extracted on earth
Thanks to new analysis methods, an international team of researchers with Innsbruck participation was able to gain new insights into the formation of the earth. Using meteorites, the scientists analyzed the occurrence of the volatile elements chlorine, iodine and bromine and found that previous models did not match the new data. The earth already “held onto” the so-called halogens in the first phases of its formation.
The formation of our solar system and thus the first moments in the history of our earth still pose many puzzles to research today. The distribution of the chemical elements in the primeval solar nebula, i.e. the material from which all the planets in our solar system were created, is of great interest, as this distribution still largely determines the properties inside the planets and thus also in the earth. "In order to understand what is happening inside the earth today, we have to look back into the past - ideally to the very beginning around 4.5 billion years ago," says Dr. Bastian Joachim from the Institute for Mineralogy and Petrography at the University of Innsbruck. Together with colleagues from the University of Manchester, University of Oxford and ETH Zurich, the researcher was able to gain new insights into this early stage of the formation of the earth in a current study that has now been published in the magazine "Nature". For this purpose, the research team reconstructed the chemical composition of certain meteorites with the help of innovative, extremely precise measuring methods. “A lot of research has already been done in this area, but it reached its limit at certain points due to the challenges in the measurement options,” explains Joachim.
New measurement methods
A new method developed at the University of Manchester allows highly sensitive measurements using mass spectrometers. These measurements enabled the researchers to determine elements that are only found in very small amounts in the respective samples. “In this study, we were interested in what are known as halogens. These are volatile, i.e. easily volatile elements, which include chlorine, iodine and bromine, among others. Volatiles influence, among other things, the degree of viscosity of rock melts in the earth's interior and thus, for example, the way in which a volcano erupts. On a much larger scale, volatiles influence the course of mass flows in the earth's mantle, which is known as mantle convection. These mass flows significantly shape the migration of the continents and the formation and disappearance of oceans, as well as the long-term cooling of our planet. Thus, volatiles can have dramatic effects on life on earth, ”explains Bastian Joachim. The method is called “Neutron Irradiation Noble gas mass spectrometry” (NI-NGMS). The samples are first irradiated with neutrons in a research reactor. This leads to the halogens being converted into noble gases. In turn, noble gas concentrations can be measured with the help of mass spectrometers with significantly higher accuracy than halogen concentrations. In the current study, the research team succeeded in determining the concentrations of the elements chlorine, bromine and iodine in meteorites with unprecedented accuracy. The values obtained do not agree with the previous assumptions about the original "equipment" of the solar system with these elements - which in turn raises new questions.
Meteorites from the hour of birth
Halogens belong to the class of elements that volatilize relatively quickly at high temperatures. “The beginning of our universe was extremely hot, followed by a gradual, slow cooling of the system. When such a colorful mixture cools down, of course not everything becomes solid at the same time: While the compounds that were solidified at first are rich in aluminum and calcium, the volatiles followed much later: That is why planets close to the sun are solid, those further away - for example Saturn - gaseous, ”describes Joachim. Nevertheless, halogens also occur deep in the earth's mantle and are more robust than previously thought, even in the extremely hot original conditions of our solar system, the research team shows with the new measurement methods. The researchers examined so-called chondrites - the type of meteorite that has largely retained its original shape since the early days of the solar system around 4.5 billion years ago. "They are the only window into the beginnings of the earth's history that we have: they contain the mixture of the original solar system", the mineralogist emphasizes the great importance of the small, cosmic stones. The problem with measuring halogens is their extremely low concentrations. “We're talking about two to four atoms per million atoms. It's incredibly difficult to prove. With the new measurement method we have now succeeded. "
With surprising results: The concentration of the halogens chlorine, iodine and bromine in the chondrites is much lower than was assumed in previous models. The peculiarity of this discovery becomes clear when you take a look at the history of the earth: The earth formed from a primordial fog and slowly began to cool down. “But this condition did not stay that way, as the earth was massively hit by meteorites in the further course. For example, a large impact led to the hurling out of huge masses, which then formed the moon. In the 'late veneer' theory, it is assumed that after the formation of its core, the earth was exposed to such massive meteorite impacts that it only gained about half of its mass ”, says Joachim. "Since the earth was theoretically much too hot in its original formation to hold halogens, it was previously assumed that these elements only came through these later impacts on the then somewhat cooled earth." The research team was able to confirm this assumption his results no longer confirm, on the contrary. “The amounts of chlorine, iodine and bromine that were additionally introduced into the earth via the meteorite impacts are less than the amount that is present on earth today.” Joachim and his colleagues conclude that these halogens have been present from the beginning must have been present - even if this should not actually be the case due to their properties. “So there has to be a mechanism that we don't understand yet. The earth "held on" to these elements and we don't know how, "says Joachim. Although the scientists have some theories, such as incorporation into very robust minerals, they see a need for further research to clarify a new riddle in the formation of our earth.
The project was funded by the European Research Council (ERC) FP7 ‘NOBLE’ grant no. 267692 funded.
Publication: Halogens in chondritic meteorites and terrestrial accretion. Patricia L. Clay, Ray Burgess, Henner Busemann, Lorraine Ruzié-Hamilton, Bastian Joachim, James M.D. Day, Christopher J. Ballentine. Nature 2017. DOI: 10.1038 / nature24625
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