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U-Th-Pb Geo/Thermochronology U-Th-Pb Geo/Thermochronology Blair Schoene, John Cottle, Michael Eddy [email protected], [email protected], [email protected] U Method Overview 238 The U-Th-Pb radioisotope sytem is the basis for one of Pb/ the most important geochronometers in use today. It 206 exploits three independent radiogenic isotopes 238U (t1/2=), 235U (t1/2=), and 232Th (t1/2=) that together can be used to date events from the beginning of the solar system to the Pleistocene. This versatility, along with the ubiquity of minerals containing high U and Th concentrations (i.e., zircon, titanite, monazite, baddeleyite, apatite, rutile, etc.), has led to the use of U-Th-Pb geo/ thermochronology in the study of igneous, metamorphic, and sedimentary systems. 207Pb/235U U-Th-Pb geochronology is also used as the standard by Weatherill concordia plot showing U-Pb zircon geochronologic analysis by ID-TIMS. which all other geochronologic methods are compared The dual 238U -> 206Pb and 235U -> 207Pb decay systems permit a check for closed system and in some cases has been used to calibrate decay behavior. The curve (blue line) where both systems agree is called concordia. constants for other radioisotope systems. Recent advances in analytical techniques have improved Sampling and Mineral Separation precision of U-Th-Pb age determinations, reduced the The closure temperatures (temperature below which sample volume needed for analysis, and reduced diffusion of Pb is negligible) of high-U minerals span a analysis time such that U-Th-Pb geochronology has large range from >900˚C for zircon to 400˚C for apatite. become the most widely used chronometer in the study Therefore, the sampling strategy and mineral separation of deep time. techniques used will depend on the individual project’s goals. Nevertheless, most projects will require sampling 600 a rock in the field, crushing or disaggregating the rock to U–Pb liberate individual minerals, and density and magnetic 500 40Ar/39Ar + K–Ar separation techniques to isolate the mineral of interest. A Sm–Nd key component of all these steps is carefully avoiding 400 Rb–Sr sample contamination through careful treatment of the U/Th–He sample in the field to keeping a clean laboratory for 300 crushing and mineral separation. Helpful information Number of papers 200 for how zircon (the most commonly used U-Pb chro- nometer) is separated can be found at 100 earth.boisestate.edu/isotope/labshare. Closure Temperatures of Common U-Pb Chronometers 1970 1975 1980 1985 1990 1995 2000 2005 2010 Publication year Zircon >900˚C Number of publications with U-Pb geochronology in the title by year, showing the recent Baddeleyite >900˚C growth of this technique. The data was extracted from the Web of Knowledge and the Monazite >800˚C figure is from Chapter Four of the Treatise on Geochemistry (Schoene, 2014). Rutile 400-600˚C One advantage of U-Th-Pb geo-/ thermochronology is Titanite 600-800˚C that the three radioisotope systems 238U -> 206Pb, 235U -> Apatite 400-500˚C 207Pb, and 232Th -> 208Pb can be used as a check that the system has remained closed since the crystallization of 207Pb/206Pb date (Ma) the mineral of interest. This can be done on several concordia diagrams by ensuring that both dates provide the same age, or are ‘concordant’. A fourth isotopic system can be used by combining the 206Pb/238U and 207 235 2800 Pb/ U age equations and assuming a constant 2700 terrestrial 235U/238U. This system utilizes only the 2600 207 206 2500 Pb/ Pb ratio and is useful when the target mineral 2400 has experienced recent “Pb-loss” (preferential loss of Pb 2300 100 µm 2200 from the target mineral through zones of high radiation 2100 damage) as this process does not affect Pb isotope Cathodoluminescence (CL) image (L) and isotopic age map (R) of a complexly zoned systematics. zircon. Zircon is a powerful U-Th-Pb chronometer often recording multiple geologic events, the timing of which can be extracted to high precision and accuracy using modern mass spectrometry techniques. Analytical Techniques Petrochronology The most commonly used methods are isotope An exciting aspect of modern U-Pb geo-/ dilution-thermal ionization mass spectrometry thermochronology is the possibility of combinining (ID-TIMS), secondary ion mass spectrometry (SIMS), U-Pb dates with the trace element composition of the and laser ablation-inductively coupled plasma-mass analyzed mineral. This can be done simultaneously spectrometry (LA-ICP-MS). Each technique has during LA-ICP-MS analyses by splitting the volume of advantages and disadvantages in analysis duration, ablated material and feeding it into two mass cost, precision, and volume of sample material needed. spectrometers using the laser ablation split stream ID-TIMS is the most expensive and time (LASS) method or by analyzing aliquots of the dissolved consuming method (3-4 hours + lengthy sample prepa material produced during ID-TIMS analyses using the ration), and involves dissolution of part, or all, of the TIMS-TEA method. These analyses can measure REE or target mineral. However, the technique provides the trace element patterns that can be linked to phase most accurate and precise (typically <0.1% for zircon) assemblages that were present during the mineral’s dates possible and data produced by this technique are crystallization or can measure isotopic data that can used to calibrate standard reference materials and the shed light on the mineral’s origin (i.e., Hf isotopes in geologic timescale. When the highest precision analyses zircon). Ultimately, the goal is to provide the most are needed, this is the go-to method. petrologic context possible for each U-Pb age. SIMS and LA-ICP-MS analyses are much less destructive and are typically done by ablating the target mineral’s surface with either a focused ion beam (SIMS) ellipses are 2 or a high energy laser (LA-ICP-MS) either on separated Pb crystals or directly in polished petrographic thin 0.058 206 sections. Both techniques routinely produce dates with moderate precision (~2-4% for zircon) and are relatively Pb/ fast (30 minutes: SIMS; 1-2 minutes: LA-ICP-MS). These 207 techniques are also relatively inexpensive. They are best suited to analysis of complex minerals with many 0.056 small-scale growth domains or projects where age 430 precision of 1-2% will not limit geologic interpretations. 420 In any case, the best results will be obtained 2.8 410 400 through collaboration with U-Th-Pb geochronologists 390 who can help figure out which method is most 0.054 appropriate and assess the limitations of a given dataset and ensure analysis quality. Dy (a) Isotope dilution – thermal ionization mass spectrometry 0.2 Gd 238U/206Pb 88.35 ±0.06 14.6 15.0 15.4 15.8 16.2 U+Pb Tracer HF Tera-Wasserburg concordia plot from Kylander-Clark et al. (Chemical Geology, 2013) Zircon ACID showing LASS data from zircon extracted from the Midsund Bruk eclogite. Older dates Dissolution have low Dy/Gd, while younger dates have high Dy/Gd.. These distinct populations Ion exchange separation likely reflect zircon growth during breakdown and/or growth of different minerals. z29 Onto filament U+Pb Distinct CL zones in one of the analyzed zircons further supports the interpretation that μ 100 m 65.04 ±0.07 in TIMS these two different geochemical signatures represent distinct growth events. Analysis time: ~3–4 h (b) Secondary ion mass spectrometry Into mass spectrometer Useful Websites Analysis time: ~30 min UO+ U+ + −8000 V www.earth-time.org Ion beam Pb + + Organization focused on calibrating Earth history by Zr2O HfSi increasing inter-laboratory reproducibility. This website is a great resource for learnng about high- 50 μm +8000 V precision ID-TIMS geochronology. (c) Laser ablation inductively coupled plasma mass spectrometry www.plasmage.org Organization focused on increasing precision and 50 μm Laser beam accuracy in LA-ICP-MS analyses. This website is a U+Pb + great resource for information regarding best He carrier gas Ablated particles everything else practices for LA-ICP-MS analyses. Zircon Into ICP-MS Analysis time: £2 min Sample chamber www.earthscope.org/research/geochronology Program for graduate students interested in being A comparison of different U-Pb geochronologic techniques showing the general analytical connected with a geochronology laboratory and setup, analysis time, and the sample volume used. (A) Typical workflow for ID-TIMS analyses of zircon. (B) Typical geometry for production of secondary ions during SIMS applying for funding to visit that lab and get data. analyses with an SEM image showing pit size. (C) Typical geometry for LA-ICP-MS analyses and SEM image of ablation pits. This figure is from Chapter Four of the Treatise on Geochemistry (Schoene, 2014)..
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