One advantageofU-Th-Pbgeo-/thermochronology is The U-Th-Pbradioisotopesytemisthebasisforoneof Method Overview of deeptime. become themostwidelyusedchronometer inthestudy analysis timesuchthatU-Th-Pbgeochronology has sample volumeneededforanalysis,andreduced precision ofU-Th-Pbagedeterminations,reduced the advances inanalyticaltechniqueshaveimproved constants forotherradioisotopesystems.Recent and insomecaseshasbeenusedtocalibratedecay which allothergeochronologic methodsare compared U-Th-Pb geochronology isalsousedasthestandard by igneous, metamorphic,andsedimentarysystems. U-Th-Pb geo/thermochronology inthestudyof baddeleyite, apatite,rutile, etc.),hasledtotheuseof concentrations (i.e.,zircon, titanite,monazite, ubiquity ofmineralscontaininghighUandTh system tothePleistocene.Thisversatility, alongwiththe used todateeventsfrom thebeginningofsolar (t 207 207 207 that thethree radioisotopesystems exploits three independentradiogenicisotopes the mostimportantgeochronometers inuse today. It systematics. damage) asthis process does not affect Pbisotope from thetarget mineralthrough zonesofhighradiation has experiencedrecent “Pb-loss”(preferential lossofPb terrestrial system canbeusedbycombiningthe the sameage,orare ‘concordant’. A fourthisotopic concordia diagramsbyensuringthatbothdatesprovide the mineralofinterest. Thiscanbedoneonseveral system hasremained closedsincethecrystallizationof Number ofpublicationswithU-Pbgeochronology inthetitlebyyear, showingtherecent figure isfrom ChapterFouroftheTreatise on Geochemistry(Schoene,2014). growth ofthistechnique.Thedatawasextractedfrom theWeb ofKnowledgeandthe 1/2

Pb/ Pb/ Pb, and Number of papers 100 200 300 400 500 600 =), 206 235 1970 235 Pb ratioandisusefulwhen thetarget mineral U ageequationsandassuming aconstant U (t 232 235 Th -> U/ 1975 1/2 =), and 238 U. Thissystemutilizesonly the 208 1980 Pb canbeusedasacheckthatthe U/Th–He Rb–Sr Sm–Nd 40 U–Pb Ar/ U-Th-Pb Geo/Thermochronology [email protected], [email protected], [email protected] 232 39 Th (t Publication year 1985 Ar +K–Ar 1/2 Blair Schoene, JohnCottle, Eddy Michael 1990 =) thattogethercanbe 238 1995 U -> 206 Pb/ 206 2000 238 Pb, U and 238 235 2005 U U -> 2010 Apatite Titanite Rutile Monazite Baddeleyite Zircon Cathodoluminescence (CL)image(L)andisotopicagemap(R) ofacomplexlyzoned modern mass spectrometry techniques. events, thetimingof which canbeextractedtohighprecision andaccuracyusing zircon. Zircon is a powerfulU-Th-Pbchronometer oftenrecording multiplegeologic Weatherill concordia plotshowingU-Pbzircon geochronologic analysisbyID-TIMS. earth.boisestate.edu/isotope/labshare. nometer) isseparatedcanbefoundat for howzircon (themostcommonlyusedU-Pbchro- crushing andmineralseparation.Helpfulinformation sample inthefieldtokeepingacleanlaboratoryfor sample contaminationthrough careful treatment ofthe key componentofallthesestepsiscarefully avoiding separation techniquestoisolatethemineralofinterest. A liberate individualminerals,anddensitymagnetic a rock inthefield,crushing ordisaggregating therock to goals. Nevertheless,mostprojects willrequire sampling techniques usedwilldependontheindividualproject’s Therefore, thesamplingstrategyandmineralseparation large rangefrom >900˚Cforzircon to400˚Cforapatite. diffusion ofPbisnegligible)high-Umineralsspana The closure temperatures (temperature belowwhich Sampling andMineralSeparation The dual behavior. Thecurve(blueline)where bothsystemsagree iscalledconcordia. Closure Temperatures ofCommonU-PbChronometers 100 206 238

µm Pb/ U 238 U -> 206 Pb and 235 U -> 207 Pb decaysystemspermitacheckforclosedsystem 2100 2200 2300 2400 2500 2600 2700 2800 400-500˚C 600-800˚C 400-600˚C >800˚C >900˚C >900˚C 207 date (Ma) Pb/ 207 Pb/ 206 Pb 235 U 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).