Pan-African metamorphism and prolonged slow cooling of high pressure granulites in Tanzania, East Africa. Precambrian Research, Full text not available from this repository. Abstract U-Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamorphosed during a Pan-African tectonothermal episode. These mineral ages range from to Ma and indicate that peak metamorphic conditions were diachronous in the different granulite domains. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East- and West-Gondwana as the cause of granulite facies metamorphism. The high pressure granulites of eastern Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting.
University of Tasmania, Australia
Our discussions are based on both palaeomagnetic constraints and on geological correlations of basement provinces, orogenic histories, sedimentary provenance, the development of continental rifts and passive margins, and the record of mantle plume events. In our preferred Rodinia model, the assembly process features the accretion or collision of continental blocks around the margin of Laurentia. Like the supercontinent Pangaea, Rodinia lasted about million years after complete assembly. Mantle avalanches, caused by the sinking of stagnated slabs accumulated at the mantle transition zone surrounding the supercontinent, plus thermal insulation by the supercontinent, led to the formation of a mantle superswell or superplume beneath Rodinia 40—60 million years after the completion of its assembly.
As a result, widespread continental rifting occurred between ca. Like its assembly, the break-up of Rodinia occurred diachronously.
New U-Pb metamorphic titanite dating of a foliated Archaean granite sample (ca. Ga) suggests that the onset of ductile deformation within the Lupa Goldfield occurred at ca. Ga, and some ca. 40 Myr prior to auriferous and brittle-ductile mylonitic shear zones at ca. Ga.
Departments Migratory tectonic switching, western Svecofennian orogen, central Sweden: Geological features common to both the adjacent domains are found at Forsmark, which, consequently, is a key area for understanding the tectonic evolution of the Svecofennian orogen in this region. New SIMS and TIMS geochronological data from several samples in a limited area at Forsmark constrain the timing of igneous activity, ductile deformation and metamorphism in this area.
More The Forsmark area, in the western part of the Svecofennian orogen, central Sweden, is situated between two major Palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity, ductile deformation and metamorphism. The older and most voluminous plutonic suite intruded at 1. The younger and less voluminous hypabyssal suite intruded at 1.
Common lead-corrected laser ablation ICP-MS U-Pb systematics and geochronology of titanite
This study reports data chiefly from titanite, but also from rutile and apatite, obtained using the laser ablation LA -ICP-MS methodology on polished thick sections in order to retain as much petrologic information as possible, and allowing trace element analyses from adjacent areas to the U—Pb analyses. The samples analysed come from Svecofennian intermediate to acid volcanic rocks of the Porphyry Group within the major iron ore province of Norrbotten, northern Sweden, where titanite is a common phase associated with the mineralisation.
However, the data are a priori accurate as they are geologically reasonable. Apatite shows reverse discordance, which could be explained by a number of scenarios. The titanite grains studied show complex internal structures.
U–Pb geochronology using laser ablation ICP–MS is a fairly recently introduced technique and thus far its development has centred on zircon dating. Using titanite as an example, the application of laser ablation ICP–MS to U–Pb dating of common Pb-bearing accessory phases is presented.
Cenozoic volcanic rocks show two clusters of compositions compilation in Lucassen et al. The regional distribution of the clusters indicates Pb provinciality Mamani et al. The unradiogenic Midproterozoic Arequipa source is already an endmember in the compositional array of the Early Palaeozoic basement rocks. The spatial distribution and related to the huge area still sparse geochronological data does not unequivocally allow to distinguish between continuous high temperatures or episodes of thermal relaxation.
In the eastern Sierras Pampeanas, metamorphism and magmatic activity ceased in the Early Ordovician e. Fowler and magmatism was still active. The occurrence of different concordant U—Pb ages on titanite in rock units without differences in the metamorphic grade or tectonic contacts has been interpreted as reset of the U—Pb clock by localized ductile deformation and recrystallization of the titanite during persistent high T Lucassen and Becchio Element and isotope chemical composition and crustal residence The chemical affinities of Early Palaeozoic metasedimentary basement and granitoid intrusions, older Neoproterozoic to Cambrian sedimentary rocks and Ordovician sedimentary rocks indicate crustal recycling in sedimentary—magmatic cycles e.
The new chemical data fit this scheme perfectly Fig. At the outcrop level, these felsic rocks are dominant and juvenile Early Palaeozoic additions to the crust, i. The small volumes of mafic magmatic and metamorphic rocks have chemical arc signatures and variable contributions from the felsic crust e. The plagioclase-rich orthogneiss and amphibolite from Sierra de Maz resemble compositional features of other metaigneous rocks from Sierra de Maz Casquet et al.
The Sm—Nd isotope system is used to constrain crustal residence ages. The distribution of the single-stage TDM ages of Early to Late Palaeozoic metamorphic, magmatic and sedimentary rocks shows a prominent peak between 1.
Table of Contents
Il decadimento radioattivo[ modifica modifica wikitesto ] Esempio di un decadimento radioattivo , dal Piombo Pb al Piombo Pb. Alcuni nuclidi sono instabili: Molte sostanze radioattive decadono da un isotopo fino ad un isotopo stabile figlio attraverso una serie di passaggi conosciuta come catena di decadimento. Gli isotopi utili per la datazione radiometrica sono quelli con vita media da poche migliaia di anni fino a miliardi di anni.
For the U–Pb dating of apatites, the mass resolution was set to ~ at Pb for U–Pb analyses. The magnet was cyclically peak-stepped from mass u to mass u as follows; 40 Ca 2 31 P 16 O 3 +, Hg +, Pb +, Pb +, Pb +, Pb +, PbH +, .
Zircons occur in three morphologic populations: Cathodoluminescence CL imaging of the three populations shows simple concentric oscillatory zoning. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter BSE images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths.
Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis EMPA clearly distinguishes the two populations. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In brown grains, oscillatory zoned cores formed at ?? Colorless grains and overgrowths on brown grains yield an age of ??
However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny.
These isotopic data indicate that titanite, like zircon, can contain multiple age components.
U-pb dating definition relationship
At the time that Darwin’s On the Origin of Species was published, the earth was “scientifically” determined to be million years old. By , it was found to be 1. In , science firmly established that the earth was 3. Finally in , it was discovered that the earth is “really” 4. In these early studies the order of sedimentary rocks and structures were used to date geologic time periods and events in a relative way. At first, the use of “key” diagnostic fossils was used to compare different areas of the geologic column.
There has been controversy over the number and timing of orogenies in the Precambrian Moine block in the Scottish Caledonides since the earliest radiometric dating in the s. This work challenges a recent hypothesis, that this sector of the Laurentian margin was subjected to continuous crustal extension between > and Ma.
It you were trying to date the crystallisation of the parental magma, most likely U-Pb dating on zircon or perhaps monazite if present. Both minerals occur commonly as igneous minerals in granite and are relatively resistant to metamorphism, so stand a reasonable chance of retaining the crystallization age. The U-Pb system is also a long-lived decay system, so is appropriate to the timescale.. If you were trying to date the age of the ‘stuff’ the granite was made from i.
If you were trying to date the cooling history of the granite, a combination of U-Pb on minerals such as titanite , Ar-Ar and Rb-Sr dating. The Rb-Sr and K-Ar which Ar-Ar dating is based upon and the Lu-Hf system are also long-lived systems, which are appropriate to the timescalse but typically record different information from the U-Pb system in zircon.
In the case of the Rb-Sr and K-Ar system, they are prone to disturbance by heating, which makes them useful for dating the cooling and uplift history of rocks.
Problems with the U-Pb Radioisotope Dating Methods—1. Common Pb
Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. The final decay product, lead Pb , is stable and can no longer undergo spontaneous radioactive decay. All ordinary matter is made up of combinations of chemical elements , each with its own atomic number , indicating the number of protons in the atomic nucleus.
2. Full Text Geochronology of Hydrothermal Processes Leading to the Formation of the Au-U Mineralization at the Rompas Prospect, Perapohja Belt, Northern Finland: Application of Paired U-Pb Dating of Uraninite and Re-Os Dating of Molybdenite to the Identification of Multiple Hydrothermal Events in a Metamorphic Terrane.
Providing customized analytical solutions at the highest standards of quality assurance and quality control. Geochronology U-Pb Dating Ar-Ar Dating Lu-Hf Dating U-Pb Dating Samples for U-Pb dating are processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator.
Four binocular microscope work stations are available for sample picking. The external morphology of mineral grains for analysis can be documented by SEM, and internal structure can be examined in polished grain mounts by cathodoluminescence imaging. We have dated rocks from Pliocene to Archean in age, for clients from universities, government and industry. For this work, both instruments employ an analogue single Daly collector. U and Pb are loaded together on an outgassed zone-refined Re filament, and run separately in peak-hopping mode.
Data reduction is done with U-Pbr, an excel-based routine based on the error estimate algorithms published by Schmitz and Shoene Ludwig, is used for final plotting of data. Zircons are routinely analyzed from igneous rocks as well as detrital zircon or stream sediment samples.
U-pb dating of titanite farming
The isochron method Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists. These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements. In this case, it is a big advantage to present the data in a form in which the abundance of both the parent and daughter isotopes are given with respect to the abundance of the initial background daughter.
Thermochronology, cosmogenic isotopes and dating of young sedimentary rocks Part 8: Usage of U-Pb and U-Th geochronology in sedimentary environment.
Decay routes[ edit ] The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays. The term U—Pb dating normally implies the coupled use of both decay schemes in the ‘concordia diagram’ see below. However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method.
Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. This is termed the lead—lead dating method. Clair Cameron Patterson , an American geochemist who pioneered studies of uranium—lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth.
University of Tasmania, Australia
Volume 69 , September , Pages Review Pegmatites and aplites: Their genetic and applied ore geology Author links open overlay panel Harald G. Dill Show more https:
In-situ LA-ICP-MS U-Pd dating for the titanite from the shallow and deep orebodies yielded ± Ma and ± Ma to ± Ma, consistent with the age of the buried diorite in the area.
The giant Beiya Au skarn deposit over tonnes Au metal reserve is located in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt. The deposit is the largest Au skarn deposit and third largest Au deposit in China. In this paper, we present garnet U-Pb ages and trace element geochemical data from two types of skarn-related U-rich garnet from Beiya, and discuss their implications on skarn metallogenesis.
Based on optical characteristics and major element compositions, the older Grt I and the younger Grt II both belong to the grossular-andradite solid solution are distinguishable: These ages represent the timing of the garnet crystallization, and are consistent with published alteration ages within error, which suggests that the technique presented provides a reliable alternative in dating skarn alteration. From the early prograde skarn Grt I to late prograde skarn Grt II stage, the ore-forming fluids may have changed from a relatively reduced acidic to a relatively oxidized neutral pH condition.
Introduction Garnet is a common mineral in metamorphic rocks and hydrothermally-altered rocks such as skarn.