4.5 billion years – this is the unimaginably large number that researchers today estimate to be the age of our Earth. In comparison, the age of geochronology (i.e., the science of the age of the Earth) seems rather modest: Although since the late Middle Ages there have been increasing doubts about the official church doctrine of world creation within 7 days or a world age of about 6,000 years, it would take until well into the twentieth century before scientists were able to develop direct methods for determining the absolute age of rocks.
The first pioneering step on this truly rocky road was taken by a Swedish geologist named Gerard De Geer, who, around the turn of the twentieth century, discovered how to infer the age of a rock by analyzing its annual layers. The discovery of radioactivity finally brought the breakthrough: Among other things, it was discovered that each radioactive isotope has a characteristic half-life such that when it decays, the proportions of the decay products shift predictably with time. If the amount of radiogenic isotopes present in a rock sample is a direct function of time, then the absolute age of the sample can be derived from their ratio! Thus, the radioactive substances naturally occurring in rocks, such as potassium-40 or uranium-235, became the midwives of modern geochronology: What time witnesses and calendar entries are for the historian, the quantity ratios of radioisotopes are for geochronologists.
In contrast to stratigraphy, which deals with relative temporal correlations of different rock layers, geochronology has its sights set on the big picture; its declared goal is the definition of an absolute geological time scale, or a complete "life course" of the Earth. The first phase of the Earth can be defined as the so-called Pre-archaic phase – when Earth was a formless, primordial soup about 4 billion years ago. This is followed by the Archean phase with the first continental crustal blocks and rock layers, followed by the Proterozoic, when an oxygen-containing atmosphere developed around the Earth. The age of visible life, the Phanerozoic, then began about 550 million years ago – and only for this comparatively small time window can researchers draw on continuous fossil records!
Depending on the time ranges to be investigated, geochronologists have to use different radiometric measurement methods. For example, analyses based on heavy isotopes such as uranium are suitable for age determinations in the range of more than one million years due to their long half-lives. For organic objects younger than 50,000 years, there is the so-called radiocarbon method, which is based on the decay series of C-14 formed by cosmic radiation in the higher atmosphere. With the help of these and numerous other evaluation methods, geochronologists can analyze the immeasurable abundance of rock puzzle pieces scattered around the world in order to verify and constantly refine their model of the Earth's development – a model that leaves the observer in awe of the incredible journey our planet has already taken.
But researchers are concerned with more than just nostalgia. Discovering when and how quickly events took place in the Earth’s history is the key to understanding how and why they happened. This, in turn, leads us to make realistic assumptions about what developments we can expect in the future. For example, it is useful to match our predictions regarding current climate change with sound studies of the natural pace, range, and causes of long-past climate variations on Earth. The Earth has already conducted countless "climate experiments" in its millions of years of history, so wouldn't it be unforgivable not to use our retrospective knowledge of the outcomes to assess the current situation or mitigate possible consequential damage?
Thanks to the tireless work of numerous geochronological research institutes, our knowledge about the formation of the Earth is constantly growing. Research teams from all over the world – be it the German Geoscience Center of the Helmholtz Foundation in Potsdam, the SUERC Institute in Glasgow or the Paleomagnetism and Geochronology Laboratory in Beijing – are carefully evaluating all available data and material sources. In journals such as GChron, published by the European Geoscience Union, researchers present their latest findings, always on the lookout for possible gaps in the Earth's life history.
The Berkeley Geochronology Center (BGC), a nonprofit research institute founded in 1994 in California, is also dedicated to documenting the Earth's history as completely as possible. But the BGC researchers are stretching the arc even further: With the help of state-of-the-art technology, they are dating the development of not only the Earth, but also our closer planetary neighbors – back to the earliest stages of our solar system, billions of years ago! On all seven continents of the Earth as well as on the Moon and Mars, BGC scientists are evaluating rocks and other materials in order to date all central events of Earth's or near-Earth history and to draw conclusions about phases of massive volcanism on Earth, drastic climate fluctuations or possible bombardments by meteorites. In this way, they can not only track down possible causes for the mass extinction of the dinosaurs, but also provide an increasingly stringent explanation for the development of the human species.
Given this mammoth task, it's no wonder that the BGC has an incredibly wide range of evaluation tools. There's a paleomagnetism lab, where the Earth's magnetic field is "turned off" to allow precision measurements of the fossil magnetism prevalent in rocks. There’s also a state-of-the-art Ar-40/Ar-39 lab with three fully automated gas extraction mass spectrometers used to date rocks and minerals between 2,000 and 4.6 billion years old.
The list could go on and on, with new equipment being added all the time. For example, Tim Becker, Lab Manager at BGC, reports, "We are currently working on a system for processing gas released from mineral samples. After the sample is heated with a laser, the escaping gas must be purified as well as possible so that only non-reactive gases enter the noble gas mass spectrometer for analysis." The system operates in an ultra-high vacuum, which is created using turbomolecular and ion pumps in a stainless steel structure baked at 250° C.
Becker and his colleagues have entrusted the challenging task of shielding the ultraclean interior of the purification system from the room atmosphere to about a dozen Series 54 and 57 VAT all-metal valves, several manual VAT valves with high-vacuum elastomer seals, and several VAT gate valves. The valves separate the different process volumes (laser heating and cleaning chamber, cryogenic concentration volume, pumps) and establish a clear line of separation between the gas processing system and the mass spectrometer itself. Becker explains the underlying challenge: "All these valves have to work absolutely reliably and must not contribute anything to the sample gas themselves. In my experience, VAT valves are best suited for this."
Their reliability is no coincidence: VAT all-metal valves are baked at up to 350° C during their manufacture, which gives them particularly high purity and extremely low outgassing. For this reason, BGC also relies on the highly efficient VAT valve technology for numerous other geochronological analysis instruments. According to Tim Becker, the cooperation between BGC and VAT is a long-standing success story: "For example, when we were developing a new high-precision mass spectrometer in 2010, it became apparent at a very early stage of development that impurities were accumulating in the interior of the instrument - too much to ensure the desired measurement accuracy. At that time, a VAT expert provided us with very valuable support in finding a solution. Ultimately, the key to solving the problem was a switch to gold-coated VATRING valve disc seals in all process-critical parts of the device. In the standard version, the hard-on-hard sealing gaskets are silver-coated, but in view of the enormously high precision requirements in this device, silver simply proved to be insufficiently inert. This finding may not sound particularly exciting to the interested layperson, but it caused quite a stir in the geochronology community at the time!"
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