Introduction to U-Pb geochronology

Transcription

1 Introduction to U-Pb geochronology with a focus on high-precision ID-TIMS Blair Schoene Earthscope GSA geochronology shortcourse

2 Introduction to U-Pb geochronology, with a focus on high-precision ID-TIMS Outline: 1. The basics decay chains, dates, and data visualization 2. Geochemistry of U and Pb - what materials can we date? 3. Analytical techniques 4. Focus on high-precision U-Pb geochronology 1. Methodology 2. Case studies

3 Decay of U and Th to Pb U 92 Pa U 206 Pb + 8α + 6β - + Q; λ 238 = e-10 a U Neutron number (N) Th Th 228Th 208 Pb + 6α + 4β - + Q; λ 232 = e-11 a Th 207 Pb + 7α + 4β - + Q; λ 235 = e-10 a Pa 34 ka 234U 246 ka 235U 0.7 Ga 234Pa 230Th 232Th 231Th 234Th 75 ka 14 Ga 238U 4.5 Ga Ac Ac 228Ac 22 a Atomic number (Z) Ra Ra 224Ra Fr Fr Rn Rn 219Rn 220Rn 222Rn At At 218At 219At Po Po 211Po 212Po 214Po 215Po 216Po 218Po Bi Bi 211Bi 212Bi 214Bi 215Bi Pb Pb 207Pb 208Pb 210Pb 22 a 211Pb 212Pb 214Pb Tl Tl 207Tl 208Tl 210Tl Hg Hg 226Ra 1.62 ka 228Ra 230Th 75 ka indicates beta decay (to isotope diagonal in indicated direction and the same mass) ~Half-life (only indicated if >10 a) indicates alpha decay (to isotope diagonal in indicated direction and 4 a.m.u. less)

4 Three isochron equations for the three systems # # # 206 Pb 207 Pb 208 Pb total total total = # = # = # 206 Pb 207 Pb 208 Pb init. init. init. + # + # + # 238 U 235 U 232 Th now now now ( e λ 238 t 1) ( e λ 235 t 1) ( e λ 232 t 1) (1) (2) (3) plus one extra: # # 207 Pb 206 Pb total total # # 207 Pb 206 Pb init. init. ( ) ( ) λ 1 = e 235 t 1 e λ 238 t 1 (4) slope of the isochron: ( ) ( ) λ e 235 t 1 e λ 238 t 1

5 Correcting for initial daughter product (common Pb) # 206 Pb total = # 206 Pb init. + # 238 U now ( e λ 238 t 1) 1) ignore it because there is so much radiogenic Pb relative to Pb c (either because the mineral is old or U-rich) 2) use isochron methods to solve for the composition of Pb c (if the minerals meet the requirements of an isochron) 3) use a co-existing low-u phase to measure the composition of Pb c 4) estimate it using a bulk earth Pb evolution model (e.g. Stacey and Kramers)

6 testing closed-system behavior: the concordia diagram 238 U 206 Pb t 1/ 2 4.5Gyr 235 U 207 Pb t 1/ 2 0.7Gyr 206 Pb 238 U = exp λ 238t ( ) Pb 235 U = exp λ 235t ( ) 1 note that common lead correction is already made

7 Using the concordia diagram The wetherhill concordia diagram The Tera- Wasserburg concordia diagram

8 What materials can we date? Chemistry of U, Th and Pb

9 Geochemistry of U, Th and Pb U 4+ Th Å 1.10 Å Pb Å

10 Minerals used in U-Th-Pb dating Mineral Formula U content Th/U Common Pb Rock (ppm) (ppm) Type Zircon Zr SiO >10, < 1 most Titanite (sphene) CaTiOSiO k,c,a,m,ig, mp, gp,hv, gn,sk Monazite (Ce,La,Th)PO >50, < 10 mp,sg, hv,gp Xenotime YPO 4 5,000-30, < 5 gp,sg Thorite ThSiO 4 > 50,000 huge < 2 gp,sg Allanite (Ca,Ce) 2 (Fe +2,Fe +3 ) ig,gp,sk Al 2 O OH[Si 2 O 7 ] [SiO 4 ] Perovskite (Ca,Na,Fe +2,Ce) < 2-90 k,c (Ti,Nb)O 3 Baddeleyite ZrO <0.2 < 5 k,c,um, m,a Rutile TiO 2 < < 2-10 gp,gn, hv Apatite Ca 5 (PO 4 ) 3 (OH,F,Cl) < 5-30 most k=kimberlite, c = carbonatite, a=alkaline, m = mafic, ig = I-type granitoids, sg = s-type granitoids, mp = metapelites, hv=hydrothermal veins, gp=granitic pegmatites, leucogranites, sk=skarn

11 U- Pb geochronology analyecal techniques sample weight (µg) sample weight (ng) A 2.0 ID-TIMS publication year B 4.0 SHRIMP/SIMS publication year reported precision () reported precision () laser spot diameter (µm) C 1995 publication year LA-ICPMS reported precision ()

12 Imaging of chemical zoning important for guiding ID-TIMS geochronology Zircon with inherited cores (to be avoided or microsampled) Zircon without cores (to be dated or microsampled) CL images Detection of age domains in complex zircon Slide courtesy of J. Crowley

13 CA-ID-TIMS U-Pb on zircon Zircon in grain mount 100 µm Separate U + Pb from other elements Load onto filament, put into mass spectrometer TIMS lab at Princeton

14 A thermal ionization mass spectrometer (TIMS) analyzer An IsotopX Phoenix62 at Princeton University magnet Footprint is ~2 x 1 m source

15 Why is precision so good with TIMS? 1. Stable ion beams for long periods of time: lots of data ~ 4 hrs

16 Why is precision so good with TIMS? Cartoon courtesy of D. Condon 2. Isotope dilution allows us to measured Pb and U separately, and thus not worry about interelemental fractionation during measurements (which is a limiting factor in precision of other techniques). How many red and blue balls are there in the grey box if you don t know the size of the box?

17 Slide courtesy of D. Condon What is isotope dilution? Measure the ratio of the reds to blue this is what mass spectrometers do well Answer: Red/blue = 1.00 Problem: cannot measure U and Pb at the same time in a TIMS, so you need moles, not ratios

18 What is isotope dilution? Take 100 yellow balls and mix them into the box thoroughly then remeasure the ratios of all the balls measure: Yellow/red = 0.05 So how many blue balls are there? If you mix a tracer solution containing both yellow U and Pb into your sample, and measure them separately, then you know moles of each accuracy of date then depends on how well you know the ratio of Pb and U in your tracer solution

19 Application 1: calibration of the geologic timescale and earth history

20 HIST ANOM. CHRON. C31 C32 C M0r M1 M5 M10 M12 M14 M16 M18 M20 M22 M25 M29 M3 RAPID POLARITY CHANGES 30 C30 C C1 C2 C2A C3 C3A C4 C4A C6 C6A C6B C6C C7 C8 C9 C10 C11 C12 C13 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C7A C5 C5A C5B C5C C5D C5E 2 2A 3 3A 4 4A 5 5B 5A 5C 6 6A 6B A 6C 5D 5E 30 C30 PALEOZOIC PERMIAN DEVONIAN ORDOVICIAN SILURIAN MISSIS- SIPPIAN PENNSYL- VANIAN CAMBRIAN CARBONIFEROUS AGE (Ma) EPOCH AGE PICKS (Ma) PERIOD GZHELIAN KASIMOVIAN MOSCOVIAN BASHKIRIAN SERPUKHOVIAN VISEAN TOURNAISIAN FAMENNIAN FRASNIAN GIVETIAN EIFELIAN EMSIAN PRAGIAN LOCHKOVIAN 315 PRECAMBRIAN PROTEROZOIC ARCHEAN AGE (Ma) EON ERA BDY. AGES (Ma) Lopingian MIDDLE Guadalupian Cisuralian LLANDO- VERY EARLY EARLY FURON- GIAN Epoch 3 Epoch 2 TERRE- NEUVIAN LATE LUDLOW LATE MIDDLE WENLOCK MESOZOIC TRIASSIC JURASSIC CRETACEOUS AGE (Ma) EPOCH AGE PICKS (Ma) MAGNETIC POLARITY PERIOD PERIOD LATE EARLY LATE EARLY MIDDLE LATE EARLY MIDDLE MAASTRICHTIAN CAMPANIAN SANTONIAN CONIACIAN TURONIAN CENOMANIAN ALBIAN APTIAN BARREMIAN HAUTERIVIAN VALANGINIAN BERRIASIAN TITHONIAN KIMMERIDGIAN OXFORDIAN CALLOVIAN BATHONIAN BAJOCIAN AALENIAN TOARCIAN PLIENSBACHIAN SINEMURIAN HETTANGIAN NORIAN RHAETIAN CARNIAN LADINIAN ANISIAN OLENEKIAN INDUAN RAPID POLARITY CHANGES CENOZOIC AGE (Ma) EPOCH AGE PICKS (Ma) MAGNETIC POLARITY PERIOD HIST. ANOM. CHRON. QUATER- NARY PLEISTOCENE* MIOCENE OLIGOCENE EOCENE PALEOCENE PLIOCENE PIACENZIAN ZANCLEAN MESSINIAN TORTONIAN SERRAVALLIAN LANGHIAN BURDIGALIAN AQUITANIAN CHATTIAN RUPELIAN PRIABONIAN BARTONIAN LUTETIAN YPRESIAN DANIAN THANETIAN SELANDIAN CALABRIAN HOLOCENE PALEOGENE NEOGENE GELASIAN CHANGHSINGIAN WORDIAN ROADIAN WUCHIAPINGIAN CAPITANIAN KUNGURIAN ASSELIAN SAKMARIAN ARTINSKIAN PRIDOLI LUDFORDIAN GORSTIAN HOMERIAN RHUDDANIAN TELYCHIAN AERONIAN SHEINWOODIAN HIRNANTIAN SANDBIAN KATIAN DARRIWILIAN DAPINGIAN AGE 10 JIANGSHANIAN PAIBIAN GUZHANGIAN DRUMIAN AGE 5 AGE 4 AGE 3 AGE 2 FORTUNIAN FLOIAN TREMADOCIAN EDIACARAN CRYOGENIAN TONIAN STENIAN ECTASIAN CALYMMIAN STATHERIAN OROSIRIAN RHYACIAN SIDERIAN NEOPRO- TEROZOIC MESOPRO- TEROZOIC PALEOPRO- TEROZOIC NEOARCHEAN MESO- ARCHEAN PALEO- ARCHEAN EOARCHEAN HADEAN EARLY EARLY MIDDLE MIDDLE LATE LATE GSA GEOLOGIC TIME SCALE v. 4.0

21 When precision and accuracy really matter. Examples of ashbed geochronology from the stratigraphic record.

22 Why the need for higher precision? Volcanism exenceon environment Schoene et al., 2010a

23 ApplicaEon 2: evolueon of magmaec systems What are the rates of mass and heat transport in the crust? What are the rheological properees of the crust during orogenesis? What are Emescales of melt generaeon, storage and transport in the lithosphere? How are batholiths made? Why do super volcano erupeons occur? Pb/ 238 U zircon date (Ma) km N μm a PFS μm

24 IntegraEon of ID- TIMS with mineral chemistry helps generate petrologic models First do laser ablation for zircon geochemistry, then do ID-TIMS U-Pb geochron Eu/Eu* T O C (TiZR) Group A Group B Alder Creek Rhyolite Age (Ma) A B Ti-in zircon T (ºC) Ti-in zircon T (ºC) A B Age (Ma) Eu/Eu* Truncations HRT TIMS ages (all) HRT TIMS ages (young) HRT 40 Ar/ 39 Ar age (young) SRB TIMS ages (all) C Huckleberry Ridge Tuff } } Overgrowths HRT SRB CD1 Grain interiors Melt Inclusion CD2 CD Core-rim tie line Dark bands Rivera et al (2013) Rivera et al (2014) relative probability

25 Integration of ID-TIMS with mineral chemistry helps generate petrologic models U-Pb TIMS-TEA (trace element analysis) Pb/ 238 U date (Ma) A Wotzlaw et al., 2013 Fish Canyon Tuff Nutras Creek Dacite Andesite enclave Yb/Dy B Number ε Hf Th/U confidence level. Yellow and orange bars are the calculated compositional ranges f zircons in equilibrium with interstitial glass and whole rocks, respectively, on the basis of xperimentally determined partition coefficients (Rubatto and Hermann, 2007) and published lass and whole-rock analyses (Bachmann et al., 2002, 2005). Inset in B shows histogram of f isotopic compositions (expressed as ε Hf ) for Fish Canyon and andesite enclave zircons. Schoene et al., 2010

26 06 Application 3: calibrating the Archean plagioclase at the highest pressures of plagiocla existence (Fig. 3). The prograde garnet generatio this study is indeed calcic (35 40 grossular) and co pure sodic endmember albitic plagioclase (An3 1 reaction is steeply orientated in P Tspace in the area the high-pressure plagioclase phase boundary. number Mg# (Mg/Mg þ Fe) in garnet scales invers ture away from this phase boundary. The low M prograde garnet in these rock compositions (Mg# Field observations, structural geology, and petrology of the same rocks have resulted in very different tectonic models for Archean terranes M.J. Van Kranendonk et al. / Precambrian Research 131 (2004) g. 19. Schematic model of partial convective overturn for the ast Pilbara. (A) Stage 1 ( Ma): generation of a gravitionally stable, layered crust through emplacement of a sheeted TG sill complex into greenstones of the Coonterunah and lower Warrawoona Groups. Domes are initiated at felsic volcanic erupvan(b)kranendonk etma): al., eruption 2004 of the 5 8 km ve centres. Stage 2 ( ick Euro Basalt instigates an inverted crustal density profile and vertical tectonics in the Pilbara craton melting of TTG gneisses (magmatic diapirism) into the cores of progressively evolving domes (e.g. MEGC and CDGC; Fig. 12). In this model, the partial melts of the TTG were forced laterally out from underneath areas of thicker, sinking greenstones and rose up preexisting topographic perturbations in the sheeted sill complex that were being steepened by the combination of greenstone sinking and transfer of granitoid mass (Fig. 19b; Pawley et al., this issue). The partial melts accumulated upwards and were emplaced as weakly deformed plutons into the cores of rising granitoid complexes where they crystallised on cooling and inhibited completion of the overturn process. The transfer of heat into the cores of the domes by this process, within greenstones of lower thermal conductivity, accounts for the thermal anomaly associated with the domes as documented by Bickle et al. (1985) (cf. Allen and Chamberlain, 1989). Once established, the dome-and-basin geometry was re-activated and amplified by subsequent thermotectonic events, regardless of tectonic origin, at punctuated intervals throughout the history of the craton. Different granitoid complexes contain different proportions of the different aged-suites (Van Kranendonk et al., 2002), suggesting that individual domes experienced the dominant component of uplift at different times, similar to the way in which salt diapirs rise (Jackson et al., 1990). Different domes rose to different crustal levels depending on the amount of melt that accumulated and was transported upwards in the domes. The tectonic events that drove episodes Zegers et al., overturn 1999 include thermal inof partial convective cubation at Ma (Sandiford et al., 2004), Core-complex/extensional Tectonics in the Pilbara craton Figure 4 Geodynamic sketch summarizing the inferre of the southern Barberton terrane, and the associated evolution. a c, Circles lettered A, B and C correspond (Ts) formation of the Tjakastad schist belt10, the ISZ sam and the Schapenburg greenstone belt13, respectively. IF, Subduction/accretion in the Kaapvaal craton Moyen et al., 2006

27 One of the less well known rheological parameters is the yield stress τy. Our default value was rather arbitrarily chosen to be τy = 200 MPa. In a first experiment, we reduce τy to 100 MPa. Fig. 3 shows the resulting effect on the subduction dynamics for different mantle temperatures. For ΔTpot = 0 and 100 K, this subduction completely, but its rather (once every 0.75 Ma in this case) on We also performed calculations the yield stress: τy = 400 MPa (res 1 GPa (Fig. 4). Such large yield s representative for Peierl's creep a deformation mechanism (Karato, Numerical modeling can make predictions for tectonics if one makes it hotter N. Thébaud, P.F. Rey / Precambrian Research 229 (2013) But can we test these models with only structural geometries, finite strain and geochronology with ±10-20 Myr uncertainties? Fig. 4. Initial settings (top box) and snapshots of the numerical experiment from t to t myr following the emplacement of the Kelly Group at t myr. The history from t0 is t myr is not shown. This period corresponds to the progressive emplacement of the greenstone cover, and to a warming up of the crust. During this time there is with little to no deformation. Thickness variations of the lower Warrawoona greenstone speedup initiation of sagduction. Two circular red markers record the horizontal versus vertical motions as well as the magnitude of shortening in the greenstone cover. From t to t myr, gravity-driven shortening above the downwelling region is >60. Blue shading shows post yielding plastic strain. Arrows pointing at passive vertical markers in the basement document the deformation pattern. vertical tectonics in the Pilbara craton Thebaud and Rey, 2013 Subduction/accretion Van Hunen and Van der Berg., 2008

28 Reducing age uncertainties using the 207 Pb/ 206 Pb chronometer MaWnson 1987

29 Reducing age uncertainties using the 207 Pb/ 206 Pb chronometer Using the 207 Pb/ 206 Pb date, uncertainees on low- N weighted- mean of are possible

30 Obtaining high-precision dates on Archean rocks is possible and necessary 12 A 96 B 3236 C Ar/ 39 Ar date (Ma) Aucanquilcha Volcanic Cluster, Chile Pb/ 238 U date (Ma) Tuolumne Batholith, California Pb/ 206 Pb date (Ma) Usutu intrusive suite, Swaziland Comparison between dates from Phanerozoic and Archean igneous rocks

31 Further reading (review papers) on ID-TIMS U-Pb geochronology: Bowring, S. A., and Schmitz, M. D., 2003, High- precision U- Pb zircon geochronology and the straegraphic record, in Hanchar, J. M., and Hoskin, P. W. O., eds., Zircon, Volume 53: Washington, D.C., Mineralogical Society of America, p Bowring, S. A., Schoene, B., Crowley, J. L., Ramezani, J., and Condon, D. C., 2006, High- precision U- Pb zircon geochronology and the straegraphic record: progress and promise, in Olszewski, T., ed., Geochronology: Emerging OpportuniEes, Paleontological Society Short Course, Volume 12: Philidelphia, PA, The Paleontological Society p Parrish, R. R., and Noble, S. R., 2003, Zircon U- Th- Pb geochronology by isotope dilueon thermal ionizaeon mass spectrometry (ID- TIMS), in Hanchar, J. M., and Hoskin, P. W. O., eds., Zircon, Volume 53: Washington, D.C., Mineralogical Society of America, p Schoene, B., 2014, U- Th- Pb geochronology, in Rudnick, R., ed., TreaEse on Geochemistry, Volume 4.10: Oxford, U.K., Elsevier, p Corfu MaWnson Schaltegger