V31G-01 INVITED
Re-Os Geochronology of Organic-Rich Shales; Placing Absolute Time Pins in Ancient Sedimentary Basins
Methodological advancements in the past 5 years have allowed the 187Re-187Os isotope system to be used to reliably determine depositional ages for organic-rich shales. Clastic sedimentary rocks, in general, do not yield depositional ages using other radioactive isotope methods. In some cases, Re-Os ages with ± 1% uncertainty (2σ, derived from isochron regression analysis) can be determined. This method of directly dating shales is cross-calibrated to ages derived from volcanic rocks using the U-Pb zircon geochronometer to within 0.5%, and is widely applicable within Phanerozoic and Proterozoic sedimentary basins. Within Proterozoic sedimentary basins, the lack of available biostratigraphic control for correlation and determination of relative geologic age highlights the importance of the Re-Os method as an absolute timekeeper. Using the Re-Os method, we have been able to place precise, absolute age constraints within several thick Proterozoic sedimentary basins to better constrain the timing of important global events and processes. The last 200 million years of Proterozoic time was marked by a series of critical events in Earth history, including multiple large scale glaciations that were followed by the appearance of Ediacaran macroscopic metazoans . Though significant advances have been made in our ability to correlate sedimentary rocks of this time period using chemostratigraphy, the global database of high-precision geochronologic data is still limited. Re-Os dating of four separate black shale horizons from three Australian Proterozoic basins shows that the oldest known episode of Neoproterozoic glaciation (the Sturtian) terminated shortly prior to 645-660 Ma. These results show that the type Sturtian glaciation in Australia is significantly younger than putative global counterparts to which it has been correlated, and demonstrates diachronous deposition of these glacial units, which could not result from a global glacial event. In middle Proterozoic time, Mo isotopes in black shales record the nature and extent of Proterozoic deep ocean anoxia. The Velkerri Formation of northern Australia contains three mature organic-rich shales that are the source of prokaryotic cyanobacterial-rich and eukaryotic-poor biomarkers, bitumens and fluid inclusion – hosted oils in nearby rocks, but the age of the Velkerri Formation remains poorly constrained. Our Re-Os dates of 1361 ± 21 Ma and 1417 ± 29 Ma directly constrain the depositional age of the uppermost and lowermost (containing live oil) organic-rich intervals of the Velkerri Formation, respectively. Functionality of the Re- Os shale geochronometer requires some oxygenation of seawater coupled with suboxic/anoxic bottom water conditions for reductive capture of the elements. Currently, the oldest demonstrably accurate Re-Os shale age we have determined is 2500 Ma, suggesting that some oxygenation of Earth's oceans was underway by this time.
V31G-02
Promise and Pitfalls of Lu/Hf-Sm/Nd Garnet Geochronology
Our ability to routinely measure Lu-Hf and Sm-Nd isotopes in garnet allows broad new applications in geochronology, petrology, and tectonics. However, applications of these data can be limited by challenges in interpreting the petrologic record and preparing garnets for analysis. Here, we examine petrologic and chemical pitfalls encountered in garnet geochronology. Petrologic factors influencing trace element compositions in garnet include reactions that modify REE availability and partitioning (1,2), kinetically limited transfer of REEs to garnet (3), and bulk compositional heterogeneities (4). Interpreting the effects of these processes on Sm/Nd and Lu/Hf ages requires characterizing REE zonation prior to isotope analysis and age interpretation. Because garnet fractions are traditionally picked from crushed samples without regard to intracrystalline origins or chemistries, isochrons will represent mixtures derived to varying degrees from all periods of garnet growth. While measured zoning might generally indicate what garnet portion dominates the Lu/Hf or Sm/Nd budget, traditional mineral separation will rarely realize the chronologic potential afforded by high precision Hf and Nd isotope measurements. The potential use of alternative techniques, such as microsampling, necessitates selective digestion and/or leaching to eliminate inclusions within garnet. For Sm/Nd geochronology, H2SO4 leaching removes LREE-rich phosphates (e.g. apatite), but not silicates (e.g. epidote), precluding Sm-Nd dating of some rocks. For Lu/Hf geochronology, ubiquitous zircon microinclusions (c. 1 μm) can significantly disrupt age determinations. Microinclusions cannot be detected optically or separated physically, requiring selective chemical digestion. If complete digestion methods, such as bomb digestion, are used for garnet fractions, then "common Hf" from zircon will be contained in final solutions. These mixed analyses are of dubious utility and will fall into one of two categories based upon inclusion reaction history (5). In "best case" scenarios, zircon will be quasi-co-genetic with garnet formation and all phases will reflect the same Hf pool available during metamorphism. In this case, these "garnet" fractions may retain age information, but will yield underestimated Lu/Hf ratios that severely degrade isochron precision. However, "worst case" scenarios for these mixed-phase analyses will occur when zircon is inherited from prior events, such that garnet analyses represent two, unequilibrated Hf pools. Here, Lu/Hf isotopic analyses do not yield accurate age information on garnet growth as individual isotopic analyses reflect several, unrelated petrologic events. To avoid systematic introduction of errors of this type due to improper digestion procedures, garnet dissolutions should occur via low-pressure, hot-plate style digestion in Teflon beakers. Here, chemical conditions are unlikely to incorporate significant zircon-derived Hf within final solutions, ensuring that Lu/Hf ratios primarily reflect garnet compositions. 1. King et al., 2004, Geochem. Geophys. Geosys. 10.1029/ 2004GC000746; 2.Corrie and Kohn, 2008, J. Metam. Geol. in press; 3.Skora et al., 2006, Contr. Min. Pet. 152, 703-720; 4.Carlson, 2002, Amer.Mineral. 87, 185-204; 3. 5.Scherer et al., 2000, Geochim. Cosmochim. Acta 64, 3413-3432.
V31G-03
Long-term slip rates of the Elsinore-Laguna Salada fault, southern California, by U-series Dating of Pedogenic Carbonate in Progressively Offset Alluvial fan Remnants.
The Elsinore-Laguna Salada (ELS) fault is one of the principal strands of the San Andreas fault system in southern California, however its seismic potential is often de-emphasized due to previous estimates of a low slip rate. Nevertheless, the fault zone has produced two historic earthquakes over M6, with the 1892 event estimated at >M7; thus further investigation of the long-term slip rate on the ELS fault is warranted. On the western slopes of the Coyote Mountains (CM), southwest Imperial Valley, a series of alluvial fans are progressively offset by the Elsinore fault. These fans can be correlated to their source drainages via distinctive clast assemblages, thereby defining measurable offsets on the fault. Dating of the CM fans (to compute slip rates), however, is challenging. Organic materials appropriate for C-14 dating are rare or absent in the arid, oxidizing environment. Cosmogenic surface exposure techniques are limited by the absence of suitable sample materials and are inapplicable to numerous buried fan remnants that are otherwise excellent strain markers. Pedogenic carbonate datable by U-series, however, occurs in CM soil profiles, ubiquitously developed in fan gravels, and is apparent in deposits as young as ~1 ka. In CM gravels 10's ka and older, carbonate forms continuous, dense, yellow coatings up to 3 mm thick on the undersides of clasts. Powdery white carbonate may completely engulf clasts, but is not dateable. Carefully selected samples of dense, innermost carbonate lamina weighing 10's of milligrams and analyzed by TIMS, are geochemically favorable for precise U-series dating (e.g., U = 1-1.5 ppm, median 238U/232Th ~ 7), and yield reproducible ages for coatings from the same microstratigraphic horizon (e.g., 48.2 ± 2.7 and 49.9 ± 2.2 ka), indicating that U-Th systems have remained closed and that inherited coatings, though present, have been avoided. Accordingly, U-series on pedogenic carbonate provides reliable minimum ages for deposition of host landforms, thereby facilitating determination of maximum bounds on corresponding slip rates. Results to date show that pedogenic carbonate dating in the CM has a useful range of at least 140 ka, thus progressively offset geomorphic surfaces in the CM study area afford the opportunity to examine the pattern of slip on the Elsinore fault over time scales from circa 10 to >100 ka.
V31G-04
New high-precision 40Ar/39Ar ages and geochemical data from the greater Siberian large igneous province: The Biggest gets Bigger.
The Siberian Traps (ST) represent the remnants of the largest Phanerozoic continental flood basalt province, with an estimated original size of 4 x 106 km2 and an original combined volume of at least 2 x 106 km3 (Milanovskiy, 1976). The province may also be responsible for the Permo-Triassic mass extinction at 251 Ma. Despite recent intensive research on the province, the extent and duration of extrusive and related magmatism are still controversial. Although several areas surrounding the Siberian craton have been attributed to the ST volcanic activity, the full extent remains conjectural as precise age determinations and chemical correlations between units are missing. Basaltic, gabbroic and rhyolitic rocks occur throughout the West Siberian Basin beneath a thick succession of Mesozoic and Cenozoic sediments. Thicknesses of the lava sequences vary but exceed 2 km in places. Areas with basalt and dolerite rocks supposedly related to the ST are also reported further to the north of the Siberian craton, on the Taimyr Peninsula, along the eastern border of the Urals (Chelyabinsk) and in the polar Urals (Vorkuta), and to the south within the Kuznetsk Basin (Kuzbass) and Semeitau (Kazakhstan) areas. We have obtained widespread sample material from across Siberia, and show that the majority of basalts and gabbros have ages indistinguishable from the Traps on the craton. Plagioclase (from basalt) and biotite (from gabbros) have 40Ar/39Ar ages of 248.7 ± 0.6 Ma to 251.7 ± 0.6 Ma (relative to FC sanidine at 28.02 Ma). Early to mid-Triassic ages obtained on Chelyabinsk basalts demonstrate that volcanic activity in Siberia occurred in at least two episodes and that not all volcanism may be contemporaneous with the main Traps activity. An assessment of published and new geochemical and isotopic data demonstrate that basalts from the West Siberian Basin, and the Chelyabinsk, Kuzbass, Vorkuta regions have chemical characteristics typical of evolved, crustally-contaminated continental flood basalts (e.g., low Mg#, negative Nb anomaly) showing affinities with the Nadezhdinsky suite of the main Traps from the Noril'sk area. The Nadezhdinsky suite is known to immediately precede the supposedly main pulse of volcanism that extruded over large areas of the craton. Our data emphasize that basalts surrounding the ST exposed on the Siberian craton are likely to be a part of a greater Siberian LIP, but it is not yet possible, with the currently available data, to closely define the duration of activity. It also stresses the importance of correlating igneous units in Siberia using both chemical and radiometric age determinations.
V31G-05
40Ar/39Ar Dating of Volcanic Glass
Application of the 40Ar/39Ar method to volcanic glasses has been somewhat stigmatized following several studies demonstrating secondary mobility of K and Ar. Much of the stigma is unwarranted, however, since most studies only impugned the reliability of the K-Ar and 40Ar/39Ar techniques when applied to glass shards rather than obsidian clasts with low surface area to volume ratios. We provide further evidence for problematic K loss and/or 39Ar recoil ejection from glass shards in 40Ar/39Ar step heating results for comagmatic feldspars and shards. In an extreme case, the plateau age of the feldspars (0.17 ± 0.03 Ma at 2σ) is significantly younger than the plateau age of the glass (0.85 ± 0.05 Ma at 2σ). If the feldspar age is reasonably interpreted as the eruption age of the ash, it is likely that the glass shards experienced K and/or 39Ar loss. Electron microprobe analyses of the glass shards have low totals (~93%) and no systematic lateral variability (i.e., diffusion gradients) in K, suggesting that the lengthscale of the glass shards is smaller than the lengthscale of K diffusion. Obsidian clasts should not be as susceptible to K loss since any hydrated (K-depleted) volume represents a small fraction of the total material and can often be physically removed prior to analysis. Samples described here are detrital obsidian clasts from the Afar region of Ethiopia. Evidence from Fourier Transform Infrared Spectroscopy (FTIR), and previous work by Anovitz (1999), confirm that the scale of water and potassium mobility are often small in comparison to the size of obsidian clasts but large enough to effect the bulk composition of glass shards. This expectation is confirmed in another tuff wherein comagmatic obsidian clasts and sanidine phenocrysts yield indistinguishable 40Ar/39Ar ages of 4.4 Ma High abundances of non-radiogenic 40Ar, and kinetic fractionation of Ar isotopes during quenching and/or laboratory degassing resulting in incomplete equilibration between atmospheric and magmatic argon, may also hinder accurate 40Ar/39Ar geochronology of volcanic glasses. Clasts derived from single flows (as determined by extrusion age and trace element geochemistry) display variations as much as 3-4 orders of magnitude in atmospheric 40Ar concentrations. The clasts were likely sourced from different parts of the flows with varying proximity to the surface and thus differing thermal and atmospheric uptake histories. Because radiogenic and non-radiogenic components of 40Ar are energetically indistinct in glass, most samples fail to yield isochrons due to limited range in 40Ar: 39Ar: 36Ar. Most yield plateau ages, whose validity rests on the assumption of atmospheric initial 40Ar/36Ar. Some samples yield inverse isochrons with sub-atmospheric 40Ar/36Ar intercepts; unirradiated subsamples of these same samples also have sub-atmospheric 38Ar/36Ar ratios that are too high to be explained by mass fractionation. While the effects of non-radiogenic 40Ar remain poorly understood, this sample set yielded 32 out of 41 clasts having plateau and isochron ages within 2σ error of each other, and potentially problematic samples are generally identified with non-atmospheric isochron 40Ar/36Ar intercepts.
V31G-06
Inside CA-TIMS Zircon Analysis: the Interplay Among Natural Radiation Damage, Annealing, Solubility, and U-Pb Isotopic Systematics
The CA-TIMS method of U-Pb zircon analysis (Mattinson, 2005) has demonstrated remarkable effectiveness at isolation of closed-system, concordant zircon by selective removal of zircon domains that have lost Pb. However, our understanding of exactly how CA-TIMS works has been far from complete. Here we report analysis of a series of Sri Lankan zircon crystals with a range of U+Th concentrations. Natural radiation damage (D-alpha, units = 10E18 alpha-decays/g) in these samples ranges from ca. 0.9 (lightly damaged) to ca. 10.6 (totally metamict). The zircon grains are unzoned, greatly simplifying interpretation of the relationships among radiation damage, annealing, solubility, and U-Pb systematics. We made Raman and X-ray powder diffraction measurements before and after CA-TIMS annealing treatment (1,000 °C, 48 hrs). A subset of the annealed zircon samples was then dissolved in a series of up to 24 partial dissolution steps at progressively increasing temperatures from 80-215 °C; all using 50% HF for 12 hrs, and with complete U-Pb analysis of each step. Annealing at 1,000 °C has resulted in significant but still incomplete recovery of the radiation damage. Raman spectra indicate ca. 70% reconstitution of the short-range order, based on FWHM measurements. XRD indicates ca. 85% recovery of the long-range order based on unit cell dimensions. This is consistent with transmission electron microscope results; the remaining amorphous volume fraction appeared insignificant whereas there is still notable disorder of the (dominating) crystalline zircon fraction. For completely metamict zircon, there was no recovery of zircon structure, only a mixture of amorphous material and nano-scale ZrO2. Dissolution behavior correlates closely to original (and thus, residual) radiation damage. The originally completely metamict zircon completely dissolved at 80 °C. A D-alpha = 6 zircon was completely dissolved after the 140 °C step. In contrast, after the 160 °C step, a D-alpha = 4.7 zircon was only 17% dissolved, and D-alpha = 3 – 1.66 zircon samples were only 1.5 – 3% dissolved. Thus, the highly selective removal of badly damaged zircon (most likely to have been affected by Pb loss) by early CA-TIMS step(s) evidently is primarily due to the proportional amount of post-annealing residual radiation damage. In terms of isotopic systematics, specifically CA-TIMS plateau behavior, we observe a transition between original D-alpha = ca. 6 and above (badly damaged to totally metamict) and D-alpha = ca. 4.7 and below. The former do not yield plateau results for 206Pb/238U ages, but the latter do. The transition is close to a post-annealing damage level equal to the "first percolation point" where amorphous domains become interconnected.
V31G-07
Magma Chamber Dynamics From High-Precision U-Pb Geochronology of Micro-Sampled Chemical Domains
Dating of zircon by the U-Pb ID-TIMS method at precision of +/- 0.1% (per single date, internal error) often highlights subtle intra- and intergrain variability that in some cases complicates determination of simple igneous crystallization or eruption ages. The complexity is largely due to extended periods of zircon growth in magmatic systems. We present high-precision U-Pb dates from three magmatic systems: the Bishop Tuff, the Fish Canyon Tuff, and the Half Dome granodiorite. Timescales of magmatic evolution were obtained by dating of micro- sampled chemical domains identified by CL and BSE images of internal zoning and U, Th, and Hf concentrations. Contrasting zircon growth histories allow for the recognition of three end-members. (1) Rapid crystallization and eruption. Dates from 17 of 19 zircon grains from the Bishop Tuff are equivalent at the millennial scale and overlap with the 40Ar/39Ar sanidine age, requiring that most zircon crystallized immediately before eruption. (2) Zoned grains reflecting episodic growth. Rims on zircon from the Fish Canyon Tuff are up to 300 kyr younger than cores and 1% older than the 40Ar/39Ar sanidine age. This bias is identical to the typical ca. 1% bias between U-Pb and 40Ar/39Ar ages, suggesting that the rims formed immediately before eruption. (3) Protracted growth in large episodically replenished magmatic systems. Rims on zircon from the Half Dome granodiorite are up to 1.5 myr younger than cores and several million years older than the youngest titanite, indicating a long period of mineral growth below the closure temperature for Pb diffusion in titanite. Mineral growth in a handsample of granodiorite spans nearly the entire duration of emplacement of the Tuolumne Intrusive Suite, suggesting that growth occurred in crystal mushes in large magma chambers rather than small bodies that were emplaced incrementally. Because the results from Half Dome contrast with data from well-known silicic eruptive centers, we suggest that many plutons reflect protracted post-eruptive evolution as non-eruptive crystal mushes and pre- eruptive residence for large silicic magma chambers is 300 kyr or less. High-spatial resolution ID-TIMS dating of zircon from silicic systems offers the advantage of determining both eruption ages and magmatic histories.
V31G-08 INVITED
High-Precision Marine Sr Isotope Geochronology in Deep Time: Permian Tuffs and Conodonts
Stratigraphic sections of the Southern Urals containing abundant and well-preserved fauna for precise biostratigraphic correlation and common instratified volcanic ash beds dated by U-Pb zircon geochronology offer a unique opportunity to constrain a temporally accurate Late Pennsylvanian-Early Permian seawater Sr curve. The 87Sr/86Sr compositions of conodonts (biogenic apatite) were measured by high-precision thermal ionization mass spectrometry following rigorous pretreatment protocols, and plotted within an age model calibrated by 13 high-precision U-Pb zircon ash bed ages. The resulting seawater Sr curve shows a significant reduction in data scatter by comparison to earlier curves (Denison et al., 1994; Veizer et al., 1999; Bruckschen et al., 1999; Korte et al., 2006), suggesting that our conodont pre-dissolution treatment was highly effective for retrieving the original seawater Sr signal. The relatively flat Late Moscovian through mid-Ghzelian seawater Sr curve of this study is generally consistent with that of Bruckschen et al. (1999). Beginning in the mid-Ghzelian, our data define a decreasing trend in 87Sr/86Sr through the mid-Sakmarian, consistent with the data of Korte et al. (2006). By combining our high precision 87Sr/86Sr measurements and U-Pb age calibration, the resolution of Sr isotope geochronology approaches 0.5 Ma in this interval. This highly resolved seawater 87Sr/86Sr record obtained for the Late Moscovian through mid-Sakmarian will aid in global carbonate chemostratigraphic correlation and contribute to our understanding of the timing of Late Paleozoic glacial and tectonic events. References: Bruckschen, P., Oesmann, S., Veizer, J., 1999. Isotope stratigraphy of the European Carboniferous: proxy signals for ocean chemistry, climate and tectonics. Chemical Geology 161, p. 127-163. Denison, R.E., Koepnick, R.B., Burke, W.H., Hetherington, E.A., Fletcher, A., 1994. Construction of the Mississippian, Pennsylvanian and Permian seawater 87Sr/86Sr curve. Chemical Geology 112, p.145-167. Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, J., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, C., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, ´13C and δ18O evolution of Phanerozoic seawater. Chemical Geology 161, p. 59-88. Korte, C., Jasper, T., Kozur, H.W., Veizer, J., 2006. 87Sr/86Sr record of Permian seawater. Palaeogeography, Pala3eoclimatology, Palaeoecology 240, p. 89-107.