Monazite geochronology is a dating technique to study geological history using the mineral monazite. It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as igneous , sedimentary and hydrothermal rocks. The uniqueness of monazite geochronology comes from the high thermal resistance of monazite, which allows age information to be retained during the geological history. Also, textures of monazite crystals may represent certain type of events. Therefore, direct sampling techniques with high spatial resolution are required, in order to study these tiny zones individually, without damaging the textures and zonations. The advantage of monazite geochronology is the ability to relate monazite compositions with geological processes. Finding the ages of compositional zones can mean finding the ages of geological processes.
Electron Microprobe Dating of Monazite
Monazite rim formation was facilitated via dissolution—reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y 2 O 3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores.
U-Th-Pb chemical dating of monazites using the proton microprobe. Nuclear Instruments and Methods in Physics Research B: Beam Interactions with Materials.
Monazite is a light rare earth element LREE -bearing phosphate mineral. Crystals typically contain distinct chemical domains, each of which represent successive growth thru geologic history. Electron microprobe analysis can characterize the geometry and U-Th- total Pb age for each domain. This kind of data allow the growth of monazite to be related to geologic events affecting the host rock.
Monazite is common in pelitic and psammitic metamorphic rocks at greenschist facies and above where it is often recognized as inclusions in porphyroblasts but may also be in direct connection with the matrix. Locating monazite grains can be done on standard geological thin sections via x-ray compositional mapping. Figure 7A reveals a cerium x-ray map on a quartzite sample from the Cheyenne belt.
Peaks in cerium content may correlate to large monazite grains. Figure 8 shows compositional zonation on selected grains identified in figure 7. These individual domains likely represent successive generations of monazite growth and can be targeted for crystallization dates.
Investigation of the monazite chemical dating technique
Since the early twentieth century scientists have found ways to accurately measure geological time. The discovery of radioactivity in uranium by the French physicist, Henri Becquerel , in paved the way of measuring absolute time. Shortly after Becquerel’s find, Marie Curie , a French chemist, isolated another highly radioactive element, radium.
The realisation that radioactive materials emit rays indicated a constant change of those materials from one element to another.
The method does not provide the detail of isotopic methods, but results can be Electron microprobe U–Th–Pb monazite dating of the Transamazonian.
Kawakami, N. Nakano , F. Higashino, T. Hokada, Y. Osanai , M. Yuhara, P. Charusiri, H. Kamikubo, K. Yonemura, T. Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave Ma representing the timing of an upper amphibolite facies metamorphism. Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. Ma to Ma on the concordia.
Improving U Th Pb Electron Microprobe mineral dating
Monazite is an underutilized mineral in U—Pb geochronological studies of crustal rocks. It occurs as an accessory mineral in a wide variety of rocks, including granite, pegmatite, felsic volcanic ash, felsic gneiss, pelitic schist and gneiss of medium to high metamorphic grade, and low-grade metasedimentary rocks, and as a detrital mineral in clastic and metaclastic sediments. In geochronological applications, it can be used to date the crystallization of igneous rocks, determine the age of metamorphism in metamorphic rocks of variable metamorphic grade, and determine the age and neodymium isotopic characteristics of source materials of both igneous and sedimentary rocks.
It is particularly useful in the dating of peraluminous granitic rocks where zircon inheritance often precludes a precise U—Pb age for magmatic zircon. The U—Pb systematics of the mineral are not without complexity, however.
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known. Some do not change with time and form stable isotopes i.
The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes. Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces. These are released as radioactive particles there are many types. This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.
This radioactivity can be used for dating, since a radioactive ‘parent’ element decays into a stable ‘daughter’ element at a constant rate. For geological purposes, this is taken as one year. Another way of expressing this is the half-life period given the symbol T.
Geochronology is the science of providing ages of events in the history of the Earth and extraterrestrial material and of determining the temporal rates of geological processes by using a number of different dating methods. The ages can be absolute e. Most absolute dating methods rely on the analysis of radioactive isotopes and their radiogenic decay products.
A number of radioactive isotopes from different elements, such as uranium, thorium, rhenium, samarium, lutetium, rubidium and potassium are used for this purpose. Techniques exist to date practically all geological materials, from billions of years in age to historical records.
U–Th–Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method.
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The U—Pb geochronologic analysis of accessory minerals has played an important role in Earth and solar system science in constraining the ages of a wide variety of rocks and minerals.
In the laboratory, rock samples are crushed and the zircon grains are separated from the other minerals by heavy liquid and other mineral separation techniques. After being mounted, the crystals can be analyzed using an instrument such as a SHRIMP Sensitive High mass Resolution Ion MicroProbe which focuses a very narrow ion beam onto the grains so that mass spectrometers can measure the ratios of the isotopes vaporized from the targeted spot.
In this way, even different growth zones in individual crystals can be analyzed and thus “dated. An alternative procedure is to take all the zircon grains liberated from a rock sample, and if they are of uniform composition, chemically digest them into solution for standard mass spectrometer analysis. This dating method has become very popular for dealing with Precambrian terranes where it can often be difficult to resolve relationships between rock units and the geological history.
But just how good is this dating method?
This paper outlines an advanced procedure involving the chemical Th–U–total Pb isochron method (CHIME) dating of monazite using a field–emission electron.
Here we examine the control of major element chemistry in influencing the crystallization of monazite in granites Salihli and Turgutlu bodies and garnet-bearing metamorphic assemblages Bozdag and Bayindir nappes from the Menderes Massif, western Turkey. In S-type granites from the massif, the presence of monazite correlates to the CaO and Al 2 O 3 content of the whole rock. Granites with monazite only are low Ca 0.
As CaO increases from 2. However, examining data reported elsewhere for A-type granites, the correlation between major element chemistry and presence of monazite is likely restricted to S-type lithologies. Pelitic schists of the Menderes Massif show no correlation between major element chemistry and presence of monazite. One Bayindir nappe sample contains both prograde garnets and those affected significantly by diffusion.
These rocks have likely experienced a complicated multi-stage tectonic history, which influenced their current mineral assemblages. The presence of monazite in a metamorphic rock can be influenced by the number, duration, and nature of events that were experienced and the degree to which fluids were involved.
Canadian Journal of Earth Sciences
Geochronology involves understanding time in relation to geological events and processes. Geochronological investigations examine rocks, minerals, fossils and sediments. Absolute and relative dating approaches complement each other. Relative age determinations involve paleomagnetism and stable isotope ratio calculations, as well as stratigraphy.
Chemical dating of monazite by electron probe micro analyzer (EPMA) is a powerful and fast method, which provides reliable ages at moderate cost (Suzuki and.