1 page handout paces etal v02 - usu
TRANSCRIPT
James B. Paces, U.S. Geological Survey, Denver, CO, [email protected] Elizabeth M. Niespolo, U.C. Berkeley and Berkeley Geochronology Center, Berkeley, CA, [email protected] Warren D. Sharp, Berkeley Geochronology Center, Berkeley, CA, [email protected]
SPEED DATING!: U-‐Th dating of hydrogenic & biogenic materials
1. Introduction and Application The U-‐Th dating method is applicable to a diverse range of hydrogenic and biogenic materials formed during the last 500 ka and has been widely used in geomorphic, tectonic, paleoclimate, paleohydrologic, and archaeologic studies. Calcite, aragonite, and dolomite from speleothems, corals, soil carbonate, tufa, travertine, and fault-‐related veins are most commonly used. Opal, sulfates, phosphates, and hydroxides (including ice) can also be dated, as can fossil bones, teeth, and ratite eggshells when enriched with secondary U. Dating of hydrogenic material relies on the large fractionation between U and Th in most near-‐surface water: U is relatively soluble while Th is not. Consequently, materials precipitated under these conditions typically incorporate U (notably 238U and 234U) but little or no 230Th (the alpha-‐decay progeny of 234U), thus starting the U-‐Th “clock”. Amenable material can yield precise Quaternary dates (better than ±1%), and because ages depend only on intrinsic sample properties, both newly collected and archival materials can be dated. Simple assumptions inherent to radioactive decay allow levels of accuracy to approach precision, and the system includes inherent checks on the reliability of resulting dates. A key assumption is that samples remain closed to U and Th after formation, or in the case of bones, teeth and eggshells, since U uptake. This assumption is likely valid if U-‐Th ages preserve micro-‐ or macro-‐stratigraphic order, and if samples of coeval material produce concordant dates. Visual criteria applied during subsampling increase the likelihood of success. A second assumption is that initial 230Th is absent or can be quantitatively subtracted using common Th corrections or isochron techniques. Contributions of 230Th from detrital sources can be reduced by careful selection of small (0.1-‐30 mg), “clean” subsamples. The veracity of U-‐Th dates can be further assessed by evaluating age versus initial 234U/238U results for a suite of related samples. In situ methods using laser ablation or ion probe analyses are capable of high spatial resolution (10-‐100 µm) and can provide increased accuracy for ages of materials with slow growth rates, albeit at reduced precision.
2. Age Range Suitable samples range in age from a few years to a few hundred thousand years. – Long-‐lived radioactive parent 238U (4.5 Ga t½) – Intermediate-‐lived radioactive progeny include 234U (246 ka t½) & 230Th (75 ka t½) The dating technique is useful up to ~750 ka for ideal samples; to ~500 ka for most samples. Eventually (within 5 – 10 half lives) constant activity ratios of unity are reached: [234U/238U] = [230Th/238U] = [230Th/234U] = 1.0 Any process that results in radioactive DISEQUILIBRIUM can restart the 230Th/U clock.
3. Field Supplies & Sampling Methods
Field Sampling: See section 4. For details on sample integrity before collecting samples!! Tools for sampling: Dependent on sample type •Generally: Rock hammer, hand lens, steel blade, hand-‐held core drill (± pickaxe, trowel) •Clean sample bags and supplies to record the context •If limited by export restrictions, use dilute (1-‐5%) hydrochloric acid (HCl) to test the purity of carbonate (if applicable), before shipping samples to the lab (use PPE)
If material is sampled to provide an age on stratigraphically related materials (e.g., fossils, archaeology, or another depositional event), the collector should: •Characterize stratigraphic relationships between the sample and the related material. Did the material precipitate during, or after sedimentary deposition? Document with evidence (e.g., GPS, photos, drawings). •Collect sample intact (not crushed or powdered) Laboratory microsampling: •Samples weights of 0.1-‐100 mg are typical. •Cut and polished slabs can inform sampling by revealing microstructures (particularly useful for in situ techniques). •Binocular microscopes and hand-‐held abraders or micro-‐mills allow controlled sampling.
Above: Polished slab of pedogenic carbonate rind dated with in situ techniques; U-‐series results revealed precise chronology of pedogenic growth.
James B. Paces, U.S. Geological Survey, Denver, CO, [email protected] Elizabeth M. Niespolo, U.C. Berkeley and Berkeley Geochronology Center, Berkeley, CA, [email protected] Warren D. Sharp, Berkeley Geochronology Center, Berkeley, CA, [email protected]
SPEED DATING!: U-‐Th dating of hydrogenic & biogenic materials
5. Laboratories
U.S.A. (not necessarily comprehensive):
U.S.G.S. Denver (Jim Paces)
Berkeley Geochronology Center (Warren Sharp)
U. California Irvine (Kathleen Johnson)
U. Florida (Andrea Dutton)
U Hawaii – Manoa (Ken Rubin)
U Kansas (Doug Walker, Noah McLean)
Massachusetts Institute of Technology (David McGhee)
U. Miami (Ali Pourmand)
U. Minnesota (Larry Edwards, Hai Chang)
U. New Mexico (Yemane Asmerom, Victor Polyak)
Stanford University (Kate Maher)
Texas A&M University (Franco Marcantonio)
U. Texas – Austin (Jay Banner, Staci Loewy)
U. Texas – El Paso (Lin Ma)
Woods Hole Oceanographic Institution (Laura Robinson)
In addition, there are many International facilities!
Selected References: Eds. Bourdon, B., et al., 2003, Uranium-‐series geochemistry, Reviews in
Mineralogy and Geochemistry 52, Mineralogical Society of America (Ed. Ribbe, P.H.)
Dickin, A.P. “U-‐Series Dating.” Radiogenic Isotope Geology, Cambridge University Press, 2005, 324-‐352.
Topical references:
Bone: Pike, A.W.G., et al., 2003, U-‐series dating of bone using the diffusion-‐adsorption model, GCA 66/24 4723-‐4286.
Corals: Hibbert, F.D. et al, 2016, Coral indicators of past sea-‐level change: A global repository of U-‐series dated benchmarks, QSR 145 1-‐56.
Corals, Speleothems: Scholz, D., & Hoffman, D., 2008, 230Th/U dating of fossil corals and speleothems, QSJ 57/1-‐2, 52-‐76.
Faults: Nuriel, P., et al., 2012, U-‐Th dating of striated fault planes, Geology 40/7, 647-‐650.
Earthquakes: Kagan, E.J., et al., Dating large infrequent earthquakes by damaged cave deposits, Geology 33/4, 261-‐264.
Eggshells: Niespolo, E.M., et al., 2017, U-‐Th burial dating of ostrich eggshells: a novel approach to dating African archaeological sequences beyond the 14C limit, GSA Annual Meeting (visit this poster tomorrow!)
Pedogenic carbonate: Sharp, W.D., U-‐Series dating of pedogenic carbonates, Encyclopedia of Scientific Dating Methods (Eds. Rink, W.J., Thompson, J.), 2014.
Opal: Paces, J.B., et al., 2004, Improved spatial resolution for U-‐series dating of opal at Yucca Mountain, Nevada, USA, using ion-‐microprobe and microdigestion methods, GCA 68/7, 1591-‐1606.
Teeth: Duval, M., et al., 2011, High resolution LA-‐ICP-‐MS mapping of U and Th isotopes in an early Pleistocene equid tooth from Fuente Nueva-‐3 (Orce, Andalusia, Spain), Quat. Geochron. 6 458-‐467.
Travertine: Uysal, I.T., et al., 2007, U-‐series dating and geochemical tracing of late Quaternary travertine in co-‐seismic fissures, EPSL 257/3-‐4, 450-‐462.
4. Sample Integrity and Considerations
Materials containing primary U include: •Carbonates (soil, lacustrine, spring, cave, vein fillings, marine cements, corals);
•Opaline silica •Evaporite minerals •Fe-‐Mn oxy-‐hydroxides, oxides, phosphates •Apatite, zircon, other high-‐U igneous minerals; Volcanic rocks (using isochrons)
Biogenic materials that readily incorporate diagenetic U: •Bones (specimens with thick cortical sections preferred) •Teeth •Eggshells (ostrich, emu, other ratites) Materials for 230Th/U dating should: •Preserve primary textures •Have minimal visible detritus •Lack evidence of alteration, recrystallization, or secondary fluid interaction
Presence of detritus (including clay) imposes large initial-‐230Th corrections and can limit the utility of U-‐series dates.
U-‐series techniques can be applied to archival samples! Ask yourself:
•Can you place the sample in its original field locality precisely? •Has the sample been stored in a neutral environment? •Does the sample meet the above criteria for quality U-‐Th dating material? •Is the sample likely to be younger than ~500 ka? If yes to all of these: You may have a U-‐Th dating sample in your archive. Seek out an expert!
Left: cleaned section of carbonate rhizolith; Right: Fault gouge Post-‐collection laboratory processing: •Requires careful protocols and caution to avoid contamination from detritus during sample preparation. Tool use during sample preparation requires cleaning with acids and/or organic solvents in between uses and sealed storage to avoid dust collection.
•Mass spectrometry is performed on a TIMS or ICP-‐MS instrument. Sample prep will vary depending on which instrument is used to measure and how the sample is introduced. Laboratories with these instruments capable of measuring U-‐series isotopes should have their own facilities equipped to properly prepare samples for mass spectrometry.