V32B-01 INVITED
Nonmonotonic (reheating) thermal histories from contrasting kinetics of multiple thermochronometers
Reheating events are often difficult to deduce in thermochronology, because the age resetting they cause can usually be modeled by varying the form of a presumably simpler monotonic cooling path (an exception to this is fission-track length modeling). However, reheating and full or partial resetting due to metamorphism, hydrothermal circulation, magmatism, wildfire, or (at least in the case of meteorites) impacts, are likely common in many settings. Such effects may be particularly important for samples that have resided for long periods at or near the surface with old cooling ages, where they are susceptible to brief, high-temperature events. Failure to recognize reheating may lead to erroneous tectonic interpretations. Nonmonotonic thermal histories may be resolved by using multiple thermochronometric systems with appropriately contrasting kinetic properties. At relatively high temperatures and short timescales, systems with different activation energy ( E), frequency factor ( D0) and domain size (a) display crossovers in diffusion (or annealing) rates that may be used to diagnose reheating episodes of particular intensity and duration. The most diagnostic effect of these kinetic crossovers are apparent "age inversions" in which systems with higher closure temperatures ( Tc) are more strongly reset (resulting in younger ages) than systems with lower Tc (e.g., apatite fission-track and He systems). In cases of complete resetting of the higher- Tc system and partial resetting of the lower- Tc system, reheating may be diagnosed and the intensity and duration of the event partially constrained. When both systems are partially reset, Dt/a2 of the reheating event can be calculated and used to estimate the specific form and timing of reheating thermal histories. Examples of high temperature thermochronometers with potentially useful kinetic crossovers include the Rb-Sr system in both biotite and muscovite coupled with many higher temperature systems such as Ar in hornblende and muscovite, Pb in titanite and apatite, Sm/Nd in garnet, and Rb-Sr in Kspar. The anomalously low E of the Rb- Sr system in micas makes these couplings useful for examining reheating events on the order of ~400-700 °C over timescales of 106-108 yr. At lower temperatures, systems with anomalously low E, such as Ar in maskelynite and He in basaltic glass may be coupled with higher E systems such as AFT, and He in apatite or goethite to diagnose reheating events at ~200-400 °C over timescales from seconds to ~102 yr. We applied these approaches to investigating reheating related to wildfire and meteorite impacts. We observe abundant FT-He age inversions in apatite from the outer surfaces of exposed bedrock and hillslope detritus, requiring heating from 200-450 °C over timescales of minutes to hours. Fluvial detrital apatite from the same catchments, however, only rarely carry such signatures, indicating fractionation of apatite from hillslopes to channels. In Martian meteorite ALH84001, Ar in maskelynite has a much lower E than He in phosphates, resulting in a kinetic crossover at roughly 150 °C (for equal fractional resetting of both systems). Taken together, the relatively large He losses from phosphate and low Ar losses from maskelynite require very long residence at extremely low temperatures, but at least one short duration (minutes to hours), high temperature heating event, which if it occurred only once (at 15 Ma) reached temperatures of ~400-450 °C.
V32B-02
Low-Temperature Thermochronology of Borehole and Surface Samples From the Wind River and Beartooth Laramide Ranges, Wyoming and Montana, USA
We dated borehole and surface samples from the Wind River and Beartooth Laramide-age, basement-cored uplifts of the Rocky Mountain foreland using the apatite (U-Th)/He (AHe) system. Comparison of these results to previously published apatite fission-track (AFT) data along with the incorporation of new He diffusion models (Shuster et al., 2006), reveals several new insights into, and poses new interpretational challenges for, the shallow exhumation histories of these ranges. Deep (2.2-2.8 km below surface) borehole samples from the Wind River Range have AHe ages of 9-12 Ma, and suggest at least 600 m of rapid exhumation during the Miocene. Shallower samples range from 35-66 Ma and are consistent with exhumation of a fossil partial retention zone. Previously-published apatite fission track (AFT) data from the same borehole show at least 2 km of rapid exhumation at ~45-38 Ma at depths where AHe ages are 9-50 Ma. This contrasts with the AHe ages which show slow exhumation between 12-66 Ma and have a trend on an age-elevation plot that appears to cut across the AFT age trend. Forward modeling of the cooling ages of these data using well-constrained thermal histories and conventional Durango apatite He diffusion data cannot explain these coupled AFT-AHe age-elevation relationships. However, modeling using diffusion kinetics of the Shuster et al. radiation-damage trapping model can explain the observed age trends, including the apparent presence of a 45-38 Ma exhumation event in the AFT data and its absence in the AHe data. In the model the shallow samples do not reach high enough temperatures for annealing of accumulated radiation damage, so He is trapped and ages are much older than predicted by conventional diffusion models. Previously-published AFT data from the Beartooth Range also show a large Laramide-age exhumation event, dated at 57-52 Ma. Similar to our observations from the Wind River Range, this event is not represented in our AHe results from borehole samples, which instead show slow cooling between at least 63-10 Ma. The trapping model predicts that the observed AHe age of a single apatite grain will be proportional to its effective Uranium content (eU), a proxy for radiation damage. Multiple single-grain replicates from a sample from the Wind River borehole are consistent with this, showing a strong correlation with eU. Although the trapping-diffusion model explains the coupled AFT-AHe data of borehole samples, surface samples from the Fremont Peak area in the Wind River Range have AHe ages that are older than the corresponding previously-published AFT ages over the 1.2 km elevation traverse sampled. AFT ages show ~1 km of rapid exhumation at ~62-58 Ma; corresponding AHe ages are as much as 20 Myr older. Although the radiation damage trapping model predicts that some AHe ages may be older than the corresponding AFT ages, thermal- diffusion forward models cannot explain these large age differences over such a large sampling interval, even if trapping model kinetic parameters are varied by 5%. Thus, discrepancies in AFT and AHe ages of these surficial samples remain problematic. The thermal histories required to approximate the borehole data require burial up to the end of the Cretaceous of ~3-4 km followed by at least two phases of cooling and exhumation. The first and larger cooling event of several tens of degrees (~3-4 km of exhumation) occurred during the Paleocene-Eocene, followed by a smaller cooling event of a few tens of degrees (~1 km of exhumation) during the Miocene.
V32B-03
Intercalibration of 3He and Other Cosmogenic Nuclide Production Rates In Multiple Mineral Phases
To extend the applicability of cosmogenic 3He dating beyond minerals restricted to mafic rocks (i.e., olivine and pyroxene), we have been assessing the suitability of 3He dating of additional minerals, e.g., zircon. A key aspect of this undertaking is the calibration of spallation production rates. Because the production rate of 3He varies from element to element in a fashion that is not yet well known, an empirical approach is necessary. Our recent work has focused on the Devil's Kitchen rhyolite, Coso Volcanic Field, SE California. This rhyolite has a 587 ka K/Ar age, which will be verified with a new 40Ar/39Ar age on sanidine. The attraction of this rhyolite is its extraordinary mineral assemblage arising from both the rhyolite itself and abundant mafic inclusions. From two individual rocks we obtained separates and measured 3He in the following minerals: olivine, clinopyroxene, orthopyroxene, garnet, zircon, apatite, hornblende, and ilmenite. In addition we intend to measure 21Ne in quartz and sanidine, 38Ar in sanidine, and 10Be and 26Al in quartz. We sampled two surfaces: one with tension gashes documenting an original flow surface, and a protruding rock fin. The sample with primary surface morphology is apparently uneroded, but preliminary 3He analyses of olivine suggest the surface has been buried for a substantial part of its history. Olivine 3He analyses from the fin indicate exposure ages consistent with the eruption age, so this sample may allow absolute calibration of production rates. In either case we can use these samples to inter-calibrate production rates in a large number of minerals using multiple cosmogenic isotopes.
V32B-04
NeAr Dating: New Dimensions for Ar-Ar Dating Using Nucleogenic Neon Isotopes
The neutron reactions that produce 37Ar from Ca, 38Ar from Cl and 39Ar from K form the very heart of the 40Ar/39Ar dating system. Not only can ages be derived, but much can be deduced from the effective mineral separation performed by step-heating analysis. However, the normal suite of elements detected using Ar isotopes cannot determine the presence of some minerals. Specifically, the absence of Na means that it is not possible in principle to uniquely determine the compositions of degassing feldpsars and the inability to measure Mg limits the discrimination of some mafic phases. Mineral and glass samples of known composition have been irradiated to determine the important nucleogenic Ne isotopes produced from F, Na and Mg. Mg produces two isotopes from the reactions 24Mg(n,α)21Ne and 25Mg(n,α)22Ne, with a production ratio for (22Ne/21Ne)Mg of about 0.25. For Na, the important reactions are 23Na(n,α)20F(β-)20Ne with a production ratio for (20Ne/22Ne)Na of about 5.3. The thermal neutron reaction for F is 19F(n,γ)20F(β-)20Ne with (20Ne)F/(39Ar)K equal to about 1.1. Because there are only 3 isotopes and 4 end member isotopic compositions, it is not possible to uniquely deconvolve the above nucleogenic sources along with atmospheric Ne. Fortunately, most unirradiated minerals analyzed have had extremely low levels of atmospheric Ne. A maximal correction for atmospheric Ne can be done assuming an atmospheric 20Ne/36Ar ratio. Measuring Ne isotopes along with Ar isotopes is challenging, requiring extra time and cryo-separation of the two species. In addition, there are unresolved issues dealing with the relative rates of Ne and Ar diffusion and Ne recoil effects. However, there is promise for the method for all whole-rock samples, amphiboles, feldspars and any mineral with expected complex exsolution textures. Examples of a variety of Ne-enhanced argon age spectra will be shown.
V32B-05
40Ar/39Ar Geochronology by Multi-collector Mass Spectrometry
The ushering in of a new generation of multi-collector mass spectrometers for 40Ar/39Ar geochronology represents a major technological advance, but what are the true benefits of making routine measurements with such machines versus the practical aspects of additional setup and calibration? Is argon multi-collection the way of the future? Recent experiences with a three collector Nu Instruments Noblesse mass spectrometer will be presented that address these questions. One immediate observation is that electronic stability improves average precision of the isotopic measurements. For example, on any given day 100 consecutive automated air pipette measurements yield 40Ar/36Ar ratios reproducible to 0.1%. One of the great advantages of multi-collection 40Ar/39Ar geochronology is the flexibility and ease of changing detector array configurations, which is useful when measuring irradiation packages with a range of K-content or grain size. For example, in situ UV laser ablation of micas or laser step-heating single grains of amphibole may be run by peak hopping on a single detector, while single grains of muscovite may be run by laser step-heating and analyzed by multi-collection. One of the drawbacks with multi-collection is the effort required with detector calibration and intercalibration. Source and detector discrimination and mass fractionation, however can be accomplished using air pipettes as an argon reference material. A series of experiments using sanidine standards have been performed that demonstrate the reproducibility of single collector and multi-collector measurements, with some as yet unexplained shifts using different detector arrays. Finally, several examples applying multi-collection 40Ar/39Ar data collection will be given that illustrate the utility of multi-collector measurements on a variety of mineral and rock materials.
V32B-06
Astronomical age of the Cretaceous-Tertiary (K-T) Boundary
Recent refinements of models for the motions of the planets, including the Earth-Moon system, have led to the realization that the calculated cyclical changes in Earth's orbital eccentricity may be approximately correct for the whole of the Cenozoic. This raises the possibility of an astronomically-tuned geological timescale that extends to, and perhaps beyond, the Cretaceous-Tertiary (K-T) boundary. In order to test the validity of these long numerical integrations, we compare calculations of Earth's orbital eccentricity 62-67 million years (Ma) ago with a well-documented succession of basinal limestones and marlstones at Zumaia in the Basque region of Spain. Previous work has shown that each limestone-marlstone couplet records one axial precession cycle (~21 ka). An obvious bundling of couplets defines 36 "short" (~100-ka) eccentricity cycles between a carbonate-rich interval, used previously to tie the Zumaia section to our calculations, and the K-T boundary. If we assume an uninterrupted succession of couplets, each 20.8 ka in duration, and step the amplitudes of the time series according to the color (white, pink, or red) of the carbonates, we retrieve a strong 102-ka eccentricity signal with spectral analysis. This permits other ties to be made between prominent features of the calculated time series and the observed rock record. On this basis, the K-T boundary is >65.83 or >65.84 Ma using the ~100 ka cycles, >65.88 Ma using the 20.8-ka precessional cycles, and ≥65.95 Ma using the metronomic 406-ka eccentricity cycle line frequency, all significantly older than the current consensus age of 65.5 Ma.
V32B-07
Combining Hf-W Ages, Cooling Rates, and Thermal Models to Estimate the Accretion Time of Iron Meteorite Parent Bodies
The 182Hf-182W short-lived chronometer has been widely used to date metal-silicate differentiation processes in the early Solar System. However the presence of cosmogenic effects from exposure to GCR can potentially hamper the use of this system for chronology purposes (e.g. [1,2]). These effects must be corrected for in order to calculate metal-silicate differentiation ages. In this study, high-precision W isotope measurements are presented for 32 iron meteorites from 8 magmatic and 2 non-magmatic groups. Exposure ages and pre- atmospheric size estimates are available for most of these samples [3]. Our precision is better than or comparable to the currently most precise literature data and our results agree with previous work [4]. All magmatic irons have ε182W equal within error to or more negative than the Solar System initial derived from a CAI isochron [5]. Iron meteorites from the same magmatic groups show variations in ε182W. These are most easily explained by exposure to cosmic rays in space. A correction method was developed to estimate pre-exposure ε182W for individual iron meteorite groups. Metal-silicate differentiation in most iron meteorite parent bodies must have occurred within 2 Myr of formation of refractory inclusions. For the first time, we combine 182Hf-182W ages with parent body sizes inferred from metallographic cooling rates in a thermal model to constrain the accretion time of iron meteorite parent bodies. The estimated accretion ages are within 1.5 Myr for most magmatic groups, and could be as early as 0.2 Myr after CAI formation. This is consistent with the study of Bottke et al. [6] who argued that iron meteorite parent bodies could represent an early generation of planetesimals formed in the inner region of the Solar System. [1] Masarik J. (1997) EPSL 152, 181-185. [2] Markowski A. et al. (2006) EPSL 250,104-115. [3] Voshage H. (1984) EPSL 71, 181-194. [4] Markowski A. et al. (2006) EPSL 242, 1-15. [5] Kleine T. et al. (2005) GCA 69, 5805-5818. [6] Bottke W. F. et al. (2006) Nature 439, 821-824.
V32B-08 INVITED
Advances in Isotope Cosmochemistry and High-Resolution Chronology Using Extinct Radionuclides
Recent advances in analytical techniques for high precision isotopic measurements (particularly with the newest generation multicollector ICPMS, TIMS and SIMS instruments) have resulted in significant improvements in the ability to resolve small relative time differences (ΔT <<1 My) between events occurring in the earliest history of the Solar System. This presentation will provide an overview, with some specific examples, of recent applications of high-resolution chronometers based on several different extinct radionuclides to meteorites and their components. Some particular issues that will be addressed include: (1) concordance, or lack thereof, between various high-resolution relative chronometers based on the extinct radionuclides and the absolute Pb- Pb chronometer, (2) presence of isotopic heterogeneities in meteoritic materials and their implications for the application of these chronometers, and (3) estimates of the abundances and distributions of short-lived radionuclides in the early Solar System and implications for the environment of formation of the Solar protoplanetary disk.