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K-Ar/Ar-Ar techniques K (potassium) – lithophile element, in many minerals - has 3 isotopes: 39 K:93.10% 40 K: 0.0119%--> radioactive --> branched decay.

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Prezentace na téma: "K-Ar/Ar-Ar techniques K (potassium) – lithophile element, in many minerals - has 3 isotopes: 39 K:93.10% 40 K: 0.0119%--> radioactive --> branched decay."— Transkript prezentace:

1 K-Ar/Ar-Ar techniques K (potassium) – lithophile element, in many minerals - has 3 isotopes: 39 K:93.10% 40 K: %--> radioactive --> branched decay --> 40 Ar ( Ar = x y -1 ; 11%) 41 K: 6.88% --> 40 Ca ( Ca = x y -1 ; 89%) Athmospheric Ar has also 3 isotopes: 36 Ar, 38 Ar and 40 Ar Halflife ( Ar ) : Ga Age equation: 40 Ar/ 36 Ar atm.= 295.5“spike” radiogenní atmosf. kontaminace

2 Conventional K-Ar technique K analysis: total K is measured chemically (isotope dilution-mass spectrometry, AAS or optical emission spectrometry, flame photometry) Ar analysis: is done on another aliquot of the same sample, degassing in a high-vacuum system, purification from CO 2, H 2 O, analysis in a gas mass spectrometer by isotope dilution ( 38 Ar) chemicky nutné úplné odplynění vzorku

3 Problems of the conventional technique: Ar components in a geological sample: 1) athmospheric Ar -- 2) radiogenic Ar -- 3) extraneous parentless Ar (excess Ar) Ar loss through diffusion:- at moderate temperatures already - Ar loss is mostly incomplete - ages get too young - cannot be quantified by conventional method Excess Ar:- Ar of radiogenic, non-athmospheric origin - very common in high-pressure minerals - cannot be controlled by conventional technique - ages get too old Ar isochron diagram: initial Ar isotopic ratio = athmospheric mixing line between air and sample only if intercept ≠ 295.5, loss or excess can be detected The two aliquots for K and Ar analysis may not be strictly identical? iniciální (nadbytečný) Ar 40 Ar/ 36 Ar atm.= 295.5

4 Ar-Ar technique - 1 Goal: measuring mother and daughter isotopes on the same aliquot Technique: transformation 39 K -- (n, p) Ar by neutron irradiation ozáření vzorku v jaderném reaktoru proudem rychlých neutronů The irradiation also produces other isotopes of Ar, e.g. from Cl and Ca – correction procedures Unknown samples are irradiated together with standard of known age to calibrate for neutron flux Undisturbed age pattern: no post-crystallisation Ar loss through diffusion no excess Ar exact and accurate age = “plateau age” Stepwise degassing: sequential degassing with increasing temperature metoda postupného zahřívání apparent 39 Ar/ 40 Ar age (Ma) ploché části spektra (plató) – nenarušená část vzorku (či T)

5 Ar-Ar technique - 2 Disturbed age patterns: no plateau, no age information So-called “hump-shaped” patterns are considered to be characteristic for excess Ar Partial loss of radiogenic Ar through diffusion (due to a thermal influence, metamorphism) yields diffusion patterns with decreasing age from core to rim The same pattern may be produced by mixing two components of different age! jádro okraj

6 Ar-Ar technique - 3 metoda laserové ablace uvolnění Ar pomocí laseru měření malých vzorků stanovení stáří jednotlivých domén zrn minerálů přímo v hornině (výbrus)

7 Blokující teplota – teplota uzavření izotopického systému closure T, blocking T, T C = T kdy již neprobíhá („negligible“) výměna mateřských a dceřiných izotopů s okolím T C specifická pro určitý materiál (minerál) a izotopický systém lze ji určit experimentálně v laboratoři či empiricky (e.g. kontaktní aureoly) stáří vypočtené radiometrickým datováním = čas kdy hornina či minerál ochladl na T C (geochronologické hodiny začínají tikat – dceřinný prvek se akumuluje)

8 The concept of system closure: blocking temperatures -1 If ambient temperatures are high enough, the radiogenic daughter products undergo diffusion and leave the lattice The behaviour of the daughter isotope is determined by ==> each mineral has a blocking temperature, ==> the blocking temperature is characteristic for each mineral, each isotopic system, and depends on cooling rate ==> K-Ar and Rb-Sr systems of biotite, muscovite or hornblende date cooling. Q = activation energy k = Boltzmann constant T = temperature How can we determine cooling temperatures? 1. Numerical solution after Dodson (1973) 2. Profile through contact aureole of intrusive (Hart, 1964) 3. Temperature and age distribution in a vertical section of a borehole 4. Age distribution in metamorphic aureole (Jäger et al., 1967) Dodson M.H. (1973) Closure temperature on cooling geochronological and petrological systems. Contrib. Mineral. Petrol. 40, Hart S.R. (1964) The petrology and isotopic-mineral age relations of a contact zone in the Front Range, Colorado Jäger E., Niggli E. & Wenk E. (1967) Rb-Sr Altersbestimmungen an Glimmern der Zentralalpen. Beitr. geol. Karte Schweiz, nr. 134 objemová difuze – difuzní konstanta difuzní koeficient

9 The blocking temperature is characteristic for each mineral and each isotopic system. We can therefore plot mineral ages with decreasing closure temperature: cooling curve: Blocking temperatures -2 Closure temperatures: biotite (K-Ar, Rb-Sr): 300°C muscovite K-Ar: °C muscovite Rb-Sr: 500°C hornblende K-Ar: 550°C cooling rate – rychlost chladnutí Application of cooling curves: transformation into exhumation curves! Reconstructing rates of exhumation to conclude on the tectonic process! thermochronologie - stáří, trvání tepelných událostí v geol. historii (zahřívání, chladnutí) (T ze složení minerálů)

10 Blocking temperatures -3 We would like to have information on the widest possible temperature range: 900 to 50°C Zircon and monazite have “blocking temperatures” above geologically realistic temperatures (except for some granulites) and thus record crystallization ages in any case. Minerals with highest “blocking temperatures” show slowest diffusion behaviour. They react slowest to changing ambient conditions and may record ancient evolutionary stages: zircon U-Pb Exhumation, erosion, basin evolution, neotectonic movements are modelled using apatite fission-track and U-Th/He age data. > 900°C 750°C 350°C 300°C

11 T C pour différentes méthodes thermochronologiques Ar/ 39 Ar 238 U Fission TrackU/Th-HeExposure dating Hornblende Muscovite Biotite Alkali feldspars Evaporites ? Glauconite ? ? feldspath volcanique (sanidine - high albite) feldspath plutonique (perthite) Zircon ? ? Apatite Titanite ? ? Quartz, muscovite, sanidine Apatite Hematite specular Titanite ? ? Zircon ? ? Garnet ? ? Surface Qtz, olivine, Mn crusts

12 U-Pb technique The U-Th-Pb system has three independent decay systems: 238 U --> 206 Pbhalflife: 4.47 Ga = x a U --> 207 Pbhalflife: Ga = x a Th --> 208 Pbhalflife: Ga = x a -1 The parent isotopes do not decay directly into the stable daughter isotopes of Pb: 238 U decay series mother daughter intermediate nuclide: mother and daughter!

13 U-Pb technique: rutile xenotime monazite Other minerals used for U-Pb dating are: baddeleyite, titanite, allanite, columbite, uraninite (pichblende), thorite and many more Datable is any mineral that hosts more than 5 ppm of U and has low levels of common lead zirkon ppm U monazit ppm titanit ppm U xenotim ppm minerals to date zircon

14 U-Pb technique: Pb isotopic composition Example of a 250 Ma-old zircon from the Swiss Alps with some 500 ppm U: why is there so much 206 Pb and so little 207 Pb present? The 232 Th- 208 Pb dating technique is disregarded We have three possibilities to calculate an age: 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 Pb Zircon with 500 ppm U: ca. 20 ppm Pb rad. after 300 Ma ca. 20 pg Pb in 1 µg zircon good analysis is possible! 1 pg = g How do we calculate the radiogenic 207 Pb/ 206 Pb ratio? Pb neradiogenní (common) single grain

15 U-Pb technique: conventional U-Pb dating (ID-TIMS) Selection of zircons (rozlišení různých populací, čerstvá zrna, případně abraze) - dissolution (HF-HNO- 3 – x dní) - addition of spike (isotope dilution) - chemical separation (ion exchange column chemistry) - measurement on thermal ionisation mass spectrometer Very precise long procedure, expensive equipment, high level of training required Zircon laboratory at ETH Zürich

16 U-Pb technique: the Concordia diagram Minerals that are high in U and poor in initial (common) lead can be plotted in a so-called concordia diagram Allows graphical and mathematical solution of 2-component mixtures Concordia curve Discordant data point metoda konkordií konkordantní stáří (uzavřený systém) diskordantní stáří ztráta olova stáří krystalizace horní intercept

17 U-Pb technique: lead loss Ztráta olova Lead loss may occur during a thermal event (difuze +/- rekrystalizace) Is enhanced by an advanced metamict state of the zircon lattice Metamictization occurs through lattice damage during alpha decay and recoil of the decayed nuclide The age of crystallization and overprinting may be reconstructed by calculating “intercept ages” Who is ALF? horní intercept dolní intercept (L.I.) U.I. 207 Pb/ 235 U

18 U-Pb technique: lead inheritance The discordia is in this case a 2-component mixing line. It cannot be distinguished a priori from a lead loss line: knowledge of regional geology and zircon internal texture is required cathodoluminescence picture Přítomnost zděděné složky v zirkonech zděděná složka zděděné jádro

19 U-Pb technique: complex systems Normal case in metamorphic rocks (unfortunately): combination of lead loss, inheritance, crystallization and even “recent” lead loss Solution to the problem: Spatially resolved in-situ spot dating by ion microprobe Zircon Acasta gneiss, Canada 4.02 Ga old, with spots of an ion microprobe “recent” lead loss

20 U-Pb technique: ion microprobe dating Sensitive High-Resolution Ion MicroProbeWhat is SHRIMP? -

21 U-Pb technique: ion microprobe versus conventional dating přesnost, míra diskordanceprostor. rozliš. < 25µm

22 U-Pb technique: refractory behaviour of zircon Zircon has very slow diffusion for Pb, ages are not rejuvenated, old growth events are recorded forever (inheritance). No “closure temperature”, at least not at geologically significant temperatures. Zircons may therefore record a long evolutionary history of crustal growth in one area. The oldest zircons are 4.41 Ga old! In some cases it may be difficult to date the last crystallization (recrystallization, melting...) event of the rock, which is hosting the zircons Zircon is an interesting accessory phase but it does not necessarily reflect the evolution of the host rock.

23 U-Pb technique: single zircon evaporation  jedno zrno zirkonu na Re vlákno  postupné odpařování zrna zirkonu  analýza uvolněného radiogenního Pb (MS)  rychlá a levná metoda (Kober 1987, CMP) Nevýhody:  výsledná 207 Pb/ 206 Pb stáří – neumožňuje kontrolovat inheritanci (tzn. OK jen pro konkordantní zirkon)  menší přesnost než konvenční datování Metoda odpařování jednotlivých zirkonových zrn (Kotková et al. 1996)

24 U-Pb monazite systematics: excess of 230 Th Granulite-facies rocks in the Ivrea Zone Single monazite grains show variable 206 Pb excess (due to 230 Th fractionation --> see next slide) and largely variable age! What are they dating? Vavra G. & Schaltegger U. (1999) Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone. Contrib. Mineral. Petrol. 134, Search for another mineral to date precisely the peak of metamorphism: ---> monazite (Ce, La, U, Th) PO 4

25 206 Pb excess in monazite due to 230 Th disequilibrium Monazite may have elevated Th/U up to 40, thus enriching not only 232 Th, but also 230 Th

26 U-Pb monazite systematics: excess of 230 Th Vavra G. & Schaltegger U. (1999) Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone. Contrib. Mineral. Petrol. 134, Monazites with normal discordancy and with “reversed” discordancy in the same sample! - monazite closure at T>800°C yields normal discordant and concordant monazite - monazite growth at T <800°C yields reversely discordant monazite points with 230 Th disequilibrium Obviously very complex behaviour!

27 Monazite: as complex as zircon! Chemical U-Th-Pb dating of monazite by electron microprobe Cocherie A., Legendre O., Peucat J.J. & Kouamelan A.N. (1998) Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total lead determinations: Implications for lead behaviour in monazite. Geochim. Cosmochim. Acta 62,

28 Search for a rock-forming mineral to date precisely the peak of metamorphism: ---> e.g. staurolite Stepwise leaching U-Pb and Pb-Pb dating of staurolite in metamorphic rocks: - U-Pb systems are often disturbed - Pb-Pb ages are too imprecise - isotope systematics unknown (crystallization, cooling?) - different sources of lead during leaching Frei R., Biino G.G. & Prospert C. (1995) Dating a Variscan pressure- temperature loop with staurolite. Geology 23,

29 147 Sm 143 Nd celkové magmatické a metamorfované horniny (mafické, ultramafické), kombinace s minerály či minerální izochrony 87 Rb 87 Sr granitoidy a jejich metamorfované ekvivalenty 40 K 40 Ar datování termální (magmat. i metamorfní) i sediment. historie minerálů/hornin mladé vulkanické horniny (e.g. oceánské dno) – rychlé chladnutí vs. pomale chladnoucí plutonity či metamorfity – stáří chladnutí sedimentární horniny - i detritické (Ms) či diagenetické (glaukonit) minerály U Pb stáří frakcionovaných hornin, magmatické a metamorfní události sedimenty - provenience    Využití jednotlivých izotopických systémů v geologii 

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