Can be dated rocks with carbon

Isotope Geochemistry and Isotope Geology

The Isotope geology belongs to the interdisciplinary scientific fields between physics and geosciences. With its investigation possibilities, working approaches and methods, it plays a major role in all areas of modern geo-research and offers solutions and strategies for geo-research (e.g. for age determination) to which there is often no other alternative. The core processes and atomic properties are used to investigate geoscientific processes, e.g. in the formation of rocks and minerals, or in the hydro-, atmosphere or pedosphere. The processes examined can be placed in geological time, i.e. dated. The processing of chronometric questions in isotope geology belongs to the area of Geochronology. In addition, the physical properties of the particles and compounds involved in the processes can also be used to “color” chemical elements, compounds and groups of substances in order to study their behavior and their transport paths during the course of the investigated processes. This way of working is called the "tracer" approach and in the other important area of ​​isotope geology, the Isotope geochemistry used.

Both geochronology and isotope geochemistry use the analytical methods of nuclear physics and are strongly analytically oriented geodisciplines. You use labor-intensive and highly specialized methods of mass spectrometry as well as chemical trace or ultra-trace analysis to work on research questions on the predominantly very small quantities of substrate. In most cases, large-scale equipment must be available for this purpose. In addition, the use of clean room laboratory areas and the use of special sample preparations is often necessary and indispensable.

 

Geochronology and isotopic age determinations

Geochronology deals with the absolute geological calculation of time. In the 20th century, age determination methods based on physical and chemical processes were added to the classic methods for the relative determination of geological time periods, e.g. through sediment deposits (lithostratigraphy) and the occurrence of fossils (biostratigraphy). Mainly through the use of nuclear physical processes it became possible to determine the absolute age of minerals and rocks directly and to get calibration points for the geological time scales.

Isotopic age regulations

Most chemical elements have isotopes. With the same number of protons (atomic number, protons are positively charged nuclear particles), isotopes of an element have different numbers of neutrons (uncharged nuclear particles). Some isotopes are unstable and decay (alpha, beta, gamma decay) over time. An age can be derived from the balance of the daughter isotope and its parent isotope (hourglass model). Different periods of time can be dated with the different mother and daughter systems (age determination methods and datable geological periods) due to different rates of decay (half-lives). Half-life is the time in which the parent isotope has half decayed. Another possibility to determine material age is based on the production of unstable isotopes by cosmic rays. With known production and decay rates, an age can be determined from the amount of unstable daughter isotopes that are still present.

Isotopic age data and timescale

Basically, the point in time of a mineral or rock system at which it no longer loses any mother or daughter isotopes is dated. This can be the cooling below a certain temperature (cooling age) or the formation of a new mineral (formation age). If later chemically or thermally caused losses of the mother or daughter isotope occur, the isotope compositions change and the age information is modified. To interpret isotopic age data, additional information on geological history and the places where the rocks or minerals were formed are therefore necessary.

With these methods, processes in and on the earth such as magmatic events or the formation of deposits can be examined in more detail. With the geochemistry of the isotopes, additional information about the material dynamics of these processes can be obtained.

Time scales are created from the synthesis of various geological information and isotopic age data as independent calibration points. International conventions define names for the geological periods and for the respective boundaries (e.g. Paleozoic / Mesozoic, dating methods).

 

Examples of dating methods

For age dating, pure mineral or rock preparations are chemically and isotopically analyzed (flow chart for measuring technology). Using mass spectrometry, the relevant isotopes are separated on the basis of their different masses and the frequency ratios are measured. Age values ​​can be calculated from these isotope frequency ratios and possibly chemically determined element concentrations.

 

Zirconium dating with the uranium-lead method

The radioactive parent uranium isotopes 238U, 235U decay via different decay series to form the stable lead daughter isotopes 206Pb and 207Pb, respectively. In the case of uranium-containing minerals, the age can be derived from the frequency ratios of the uranium isotopes to the lead isotopes. Zirconia (cathodoluminescent photo of a zoned zirconium crystal) are suitable for dating because they are heat and weather resistant and they can store uranium and lead. In addition to zirconium populations, individual zirconium crystals in particular are analyzed (using an ion probe, evaporation).

If the measured frequency ratios of the lead isotopes to the uranium isotopes are plotted against each other, the measurement data for undisturbed age information lie on a so-called "concordia" curve (U-Pb concord diagram). For zircons that have lost lead as a result of a metamorphosis, the data points can form a straight line (discordia) that intersects the concordia curve at two points. The upper intersection corresponds to the rock crystallization and the lower to the metamorphic event. With the U-Pb method, both very old, e.g. archaic, and relatively young, e.g. Mesozoic rocks can be dated.

 

Mica dating with the potassium-argon method

The K-Ar method is based on the dual decay of the potassium isotope 40K into radiogenic 40Ca and radiogenic 40Ar. The age of rocks and minerals containing potassium can be determined on the basis of the potassium or argon concentrations and their isotope frequencies. As common, rock-forming minerals, potassium-rich mica (biotite, muscovite) and alkali feldspars are suitable for K-Ar dating.

The 40Ar / 39Ar technique represents an improved analytical variant of the K-Ar method with higher measurement precision. The radioactive argon isotope 39Ar is generated from the potassium isotope 39K by neutron activation. Thus, only argon isotope ratios need to be measured by mass spectrometry. The sample is degassed in partial steps at temperatures of around 400 ° C to 1300 ° C. For each step, an age value can be calculated, which is plotted against the proportion of the released argon isotope 39Ar (40Ar / 39Ar age determination). A rapid cooling of the mineral and a closed K-Ar isotope system can be deduced from a plateau-shaped stage degassing spectrum. With this measuring technique billions of years old rocks from the earliest times of the earth, meteorites, Paleozoic mountain formations, but also very young, several thousand years old volcanic rocks can be dated.

 

Isotope geochemistry with “radiogenic” and “stable” isotopes

The different isotopes of the elements open up a wide range of applications in geochemistry. The nuclides take part in the processes of material movement and redistribution, in the formation of rocks, minerals and deposits and in the cycles in the environment. The isotope distributions are used to “color” the elements or groups of substances to which they belong. Instead of the element distributions, one can also evaluate these “isotope colors” and their changes during the examined processes. Working with isotopes as a “tracer” of material movement in geological processes is often much more sensitive than working with element distributions and chemical analysis. Isotope tracers are therefore very important tools in geochemical projects.

There are two types of these tracer tools. The first group summarizes the nuclides that are influenced by the decay of primordial radionuclides. These include the isotopes of argon, strontium, neodymium, hafnium, osmium and lead, i.e. all elements characterized by an increase in radio isotopes. This group is therefore called "radiogenic isotopes" summarized. Strictly speaking, this group name is somewhat misleading because the isotope distributions of these elements almost always contain, in addition to the decay products of primordial radionuclides, also substantial amounts of accessory parts of the same as well as other isotopes of the element under consideration.

The second group summarizes all elements with relatively low mass numbers that are not influenced by radioactive decay, but in which one can nevertheless observe significant variations in their isotope distributions in nature with a correspondingly high analytical effort. Particularly frequently used members of this group are hydrogen, carbon, nitrogen, oxygen and sulfur. In addition to boron, lithium, magnesium, silicon, chlorine and calcium, elements such as iron, copper and zinc have recently been added to this group with the establishment of new analytical methods. This group is commonly referred to as "stable isotopes" guided. This group name is also somewhat misleading, as the previously mentioned “radiogenic isotopes” are mostly also stable (not radioactive). In this case, the group name means that there is no radioactive decay of primordial radionuclides involved.