Combination of multiple thermochronometric systems into a single inverse modeling framework: concepts, pitfalls, and opportunities
Combination of multiple thermochronometric and related techniques is a natural way to leverage the growing tool kit at geologists' disposal to extract more detailed information bearing on a research problem. However, experience indicates that this approach often cuts both ways. In many cases, when we combine thermochronometric systems we are often really testing our understanding and calibration of each of them. In essence, we are posing a hypothesis that we understand each system well, and that our characterization of them is robust for the particular geological conditions and processes that are being investigated. The most demanding case occurs when we combine systems into a single framework for thermal history inverse modeling. The basic approach of minimizing a function that quantifies misfit of measured data to model calculations requires that we specify not only a characterization and calibration of each system, but also a realistic assessment of measurement uncertainties and their propagation, to provide a stable baseline for weighting among systems. This is especially the case when the systems employed have overlapping ranges of temperature sensitivity, requiring that their predictions be more harmonious than required for "connect the dots" situations. The development of the apatite fission-track and apatite (U-Th)/He systems provides an instructive case. Active comparison of these systems helped reveal gaps in our understanding of each, with the result that our understanding of them has progressed considerably, though, importantly, significant questions remain for both. The next stage in development will consist of integrating both familiar and new systems, such as zircon fission-track, (U-Th)/He on a wider variety of minerals, and alpha recoil tracks in micas, into a unified framework. All of these systems have been studied far less intensively than apatite, and it is inevitable that our incomplete knowledge of them will impact attempts to utilize them in tandem for solving practical geologic problems. However, these seeming setbacks will in many cases be opportunities to rapidly test and improve our understanding of these systems and the physical and chemical processes that underlie them.
Beyond Apatite (U-Th)/He thermochronometry: Detailed low-T thermal history reconstructions and applicability to a wider range of geodynamic environments
Low-T thermochronometric dating techniques have proven to be powerful tools to constrain the cooling of rocks exhumed by a wide-range of tectonic and geologic processes and are widely used to elucidate time- temperature and exhumation histories of mountain belts, metamorphic terranes, and sedimentary basins. The approach is fundamentally based on the fact that rocks cool during tectonic or erosional unroofing such that the timing, rate, and magnitude of exhumation can be quantified by low-temperature thermochronometric data. While (U-Th)/He dating of apatite (Tc of ∼55- 70°C) and zircon (∼190°C) are now a well- established thermochronometric techniques, (U-Th)/He dating of other refractory and metamorphic minerals such titanite (∼200°C), monazite (∼180-240°C), rutile (∼220°C), magnetite (∼250°C), and other silicate and oxide phases have been developed as new tools and have attracted significant attention. These novel tools and the potential of multi-mineral and multi- thermochronometer dating offer new and exciting possibilities of resolving different portions of a rock's low-T thermal history and their overall thermal history in greater detail. The combination of multiple (U-Th)/He thermochronometers has the potential to quantify the t-T path between 4°C and 250°C. Furthermore, expansion of low-T thermochronometry to wider range of lithologies and geological environments through development of new mineral phases offers exciting new tools for a more detailed understanding of tectonic and thermal processes in different geodynamic environments. A more complete understanding of He diffusion kinetics, careful integration with petrologic and mineralogical context, and advanced numerical modeling of thermochronometric data have significantly improved our ability to more fully elucidate and quantify thermal processes in response to structural, petrologic, and erosional processes. Development and refinement of new thermochronometers and systematic incorporation of new analytical techniques is enabling us to derive high-resolution reconstructions of tectonic processes in different geodynamic settings.
Discriminating Fision-Track Ages of Low-Retentive Zircons using micro-Raman Spectroscopy
In orogenic systems, rocks containing zircon are exhumed to the surface through erosion and tectonic processes. When a heterogeneous suite of detrital zircon with a wide range of radiation damage is heated, the fission tracks anneal, but the radiation damage from alpha recoil is more robust and is partly retained even at very high temperatures (amphibolite grade). Upon cooling driven by exhumation, rocks with zircon cool as they pass through the effective closure temperature, and fission tracks begin to accumulate. This effective closure temperature is a function of internal radiation damage, and the cooling rate, but for zircon grains in a given rock, the only significant variable is radiation damage. Our hypothesis is that low-retentive zircon (LRZ) grains have high levels of remnant radiation damage, and these grains have the lowest effective closure temperature. As such, in a rock exhumed in an orogenic belt, these LRZ have the youngest fission track ages because they were the last ones to pass through closure. More retentive grains, with lower radiation damage, have higher annealing temperatures and in some cases are more resistant to annealing. These other grains tend to cause the grain-age distribution to become overdispersed. We have developed a technique of using Raman wavenumber of individual FT dated grains to determine the remnant radiation damage so that we can isolate the LRZ and reduce grain-age dispersion. We then apply this to buried and exhumed rocks from the Hellenic trench. The phyllite-quartzite unit (PQU) in Greece represents Cretaceous sediment with Mesozoic-Paleozoic zircon cooling ages (~100-400 Ma) that have been heated to greenschist-blueschist facies and exhumed in the Hellenic forearc between 21-7 Ma. In general, the normal ZFT age distribution is overdispersed for these samples, and this overdispersion reflects heterogeneous annealing and variable closure during exhumation to the surface. As an example, a partial annealed quartzite with an overdispersed zircon grain population age having a central age of 24.8 Ma and 2 binomial peaks at 9.2 Ma and 84.4 Ma implies that it has been annealed by the latest thermal event at 9.2 Ma. By reducing grain-age dispersion through the Raman discrimination of LRZ technique, we determined the latest thermal event with the binomial peak age of 7.5 Ma. In our technique, we used mineral properties revealed by Raman spectroscopy on zircons having a wide range of radiation damage, to isolate grains with high damage that have had all fission tracks annealed during burial. This Raman discrimination allows calculation of a ZFT-LR age ('low retentive'), which allows for a dramatic reduction in grain-age dispersion, and gives greater insight into the tectonic significance of the cooling ages.
Advances in U-Th-Pb-He Double Dating Techniques and Applications in Diamond Exploration
Zircon entrained within kimberlite deposits should have distinctive U-Th-Pb-He signatures compared to those found in the host terrane. To investigate the application of zircon double dating to kimberlite diamond exploration, we performed (U-Th)/He and SHRIMP U/Pb double dating analysis of zircon from the Sacramore kimberlite pipe located in the Merlin field in the Northern Territory of Australia and of detrital zircon from a regional sample of the kimberlitic host, the Bukalara Sandstone. The Sacramore zircon U/Pb age (n=14) ranged from 1541-2433 Ma, consistent with the Mesoproterozoic formation of the North Australian Craton and indicating that the kimberlitic zircon is of xenocrystic origin. (U- Th)/He thermochronometry of these kimberlite zircon xenocrysts (n=33) yielded a mean weighted average age of 368±4 Ma (2σ), concordant with a previously determined phlogopite Rb-Sr age of 367±4 Ma for the Merlin field. The U/Pb age (1472-2939 Ma; n=41) of detrital zircon from the Bukalara Sandstone is statistically indistinguishable from that of the kimberlite zircon xenocrysts, while the detrital (U-Th)/He ages range from 459 to 1279 Ma. A bivariate age density distribution approach (Sircombe 2006, Geochem Geophys Geosyst V7) using the open-licence R statistical package allows 3D visualization of double-dated zircon populations. The consistently young (U-Th)/He ages of the kimberlite zircon xenocrysts distinguish them from surficial detrital zircon. This geochemical feature could have application for regional diamond exploration in tropical and sub-tropical climates where standard kimberlite indicator minerals (e.g., Cr-pyrope, Cr-diopside, picroilmenite, chromite) are prone to destruction by chemical weathering. Up to 40% of detrital zircon grains obtained from streams draining the Merlin kimberlite field have (U-Th)/He ages comparable to those obtained from the Sacramore pipe.
Erosion Rates Over Multiple Timescales: The Power and Perils of Integrated Detrital- Mineral Thermochronology and Cosmogenic-Nuclide Dating
In recent years, numerous studies have employed detrital-mineral thermochronology and cosmogenic nuclide dating of active drainage basins to estimate basin-wide erosion rates. Because isotopic thermochronometers have different closure temperatures depending on the diffusivity of the radiogenic daughter product in the dated mineral, each thermochronometer, in principle, yields an integrated erosion rate for a different interval of the evolutionary history of the basin. In studies of active orogenic systems with relatively high erosion rates, these timescales may vary over tens of millions to millions of years. Cosmogenic-nuclide analyses yield information on even younger (nominally millennial) timescales. The integration of both data types, particularly when source-region lithologies contain a wide variety of datable minerals, is a powerful way to deduce variations in erosion rate through time but only if a substantial set of starting assumptions are valid. Some of these starting assumptions relate to erosional process (e.g., uniform erosion across the basin). Other relate to the character of the eroded material (e.g., the uniform distribution of dated material in the source region). Still others relate to the sample selection and analytical process itself (e.g., the number of analyzed grains in a thermochronologic dataset). Fortunately, we can minimize the probability that the last category of assumptions might be invalid through optimal collecting and data acquisition strategies, and we can evaluate the likelihood of validity of the ensemble of other assumptions through exploratory data analysis. This talk is a brief review of available techniques and a prospectus regarding opportunities for future improvements.
Fake Statistically Valid Isotopic Ages in Impact Crater Geochronology
Precise dating of impact structures is crucial in several fundamental aspects, such as correlating effects on the bio- and geosphere caused by these catastrophic processes. Among the 176 listed impact structures , only 25 have a stated age precision better than ± 2%. Statistical investigation of these 25 ages showed that 11 ages are accurate, 12 are at best ambiguous, and 2 are not well characterized . In this study, we show that even with statistically valid isotope ages, the age of an impact can be "missed" by several hundred millions of years. We present a new 40Ar/39Ar plateau age of 444 ± 4 Ma for the Acraman structure (real age ∼590 Ma ) and four plateau ages ranging from 81.07 ± 0.76 Ma to 74.6 ± 1.5 Ma for the Brent structure (estimated real age ∼453 Ma ). In addition, we discuss a 40Ar/39Ar plateau age of 994 ± 11, recently obtained by  on the Dhala structure (real age ∼2.0 Ga ). Despite careful sample preparations (single grain handpicking and HF leaching, in order to remove alteration phases), these results are much younger than the impact ages. Petrographic observations show that Acraman and Dhala grain separates all have an orange color and show evidence of alteration. This suggests that these ages are the results of hydrothermal events that triggered intensive 40Ar* loss and crystallization of secondary phases. More intriguing are the Brent samples (glassy melt rocks obtained from a drill core) that appeared very fresh under the microscope. The Brent glass might be a Cretaceous pseudotachylite generated by a late adjustment of the structure and/or by a local earthquake. Because we know the approximate age of the craters with stratigraphic evidences, these outliers are easy to identify. However, this is a red flag for any uncritical interpretation of isotopic ages (including e.g., 40Ar/39Ar, U/Pb, or U-Th/He ). In this paper, we encourage a multi-technique approach (i.e., isotopic, stratigraphic, paleogeographic [7,8]) and cross- calibrations in order to obtain both accurate and precise impact ages.  Earth Impact Database, Univ. New Brunswick, Canada (accessed Feb 28, 2009),  Jourdan et al., submitted to EPSL,  Baldwin et al., AJES 1991,  Grieve, Impact structures in Canada, GEOText 5, Geol. Assoc. Canada, 2006,  Jourdan et al., LPSC 39, 2008.,  van Soest et al., LPSC 40, 2009,  Schmieder et al., Geol. Mag. 145, 2008,  Buchner et al., LPSC 40, 2009.
Thermotectonic Evidence for Two-stage Extension on the La Grange Fault, Eastern Klamath Mountains
The accretionary fabric of the Klamath Mountains of northern California and southern Oregon has been overprinted by multiple episodes of Cretaceous and Cenozoic deformation. Although offering a potentially significant record of plate boundary processes at the western margin of North America, episodic reactivation of structures and extensive erosion (in many cases to crystalline basement) leave the distribution and magnitude of recent activity across this region often difficult to isolate. Combined fission track and (U-Th)/He analysis of 8 apatite samples collected along a transect crossing the footwall of the La Grange Fault in the Eastern Klamath Mountains has revealed new insight into the repeated activation history of this low-angled detachment structure. Both fission track and (U-Th)/He ages display subtle variation correlated to the extension lineation preserved on exposed remnants of the fault, but the structural implications of either independent dataset are ambiguous. When the complementary thermochronological constraint of the two is explicitly linked through integrated modelling, however, the combined ages and fission track length variation observed require two discrete episodes of extension on this and/or low-angled precursor structures. Applying this insight allows the character and magnitude of the multiple episodes of deformation superimposed on this structure to be resolved, providing better correlation of its kinematic development to regional tectonic history. The earlier event spanned a period that may have begun as early as the mid-Cretaceous, and continued to c. 80 Ma, terminating while currently exposed samples were still 2-3 km below the surface. Final exhumation of the footwall block occurred in a second discrete extensional episode initiating at c. 21 Ma. This appears to have initially reactivated the existing low-angled structure, although later development of new high-angle normal faults in the south of the footwall block may have contributed to the overall exhumation experienced.