V33E-01 INVITED
The pace of continental deformation and geochronology
Continental tectonics and geochronology have had a symbiotic relationship for many decades. A diverse range of chronometers has been applied to tectonic problems and to quantify rates of geological processes. These chronometers include U-Th-Pb, Ar-Ar, ZFTA, U-Th-He, and other systems and fall into geo-chronomters that date geological events (i.e. mineral crystallisation concurre3nt with igneous or metamorphic mineral growth) and thermo-chronometers (dating the time of cooling during exhumation, for example). With advances in linking mineral chemistry of accessory minerals to metamorphic reactions, it goes without saying that careful petrology, petrography, mineral chemistry and imaging are essential to arriving at an appropriate interpretation of U-Th-Pb dates of accessory minerals in metamorphic rocks (e.g. monazite, zircon, titanite). Examples from the Himalaya will be used to illustrate how ID-TIMS and intra-crystal SIMS and LA-ICP-MS U-Th-Pb dating can be used to elucidate P-T-t paths of metamorphic rocks during metamorphism and exhumation. By contrast, an emerging area of study involves using single detrital minerals to reconstruct provenance and patterns of erosion in mountain systems. Because these detrital minerals (mica, monazite, zircon, rutile, allanite, titanite, etc.) have been separated from their petrographic context, interpretations based upon ages of these minerals will be much less constrained and it is all the more important to ask ‘what do the dates mean?'. Further examples will be presented that attempt to explore whether a comprehensive approach to mineral provenance analysis in sedimentary materials (chronology, mineral chemistry, isotopes) can provide an accurate and useful snapshot of the geological characteristics of a portion of a mountain system undergoing erosion and exhumation. What can be reconstructed by this approach and what are its limitations? Of particular interest are monazite, rutile, garnet, and zircon which may provide pivotal information about the metamorphic – orogenic history. The limitations of the use of only zircon in such studies will be discussed.
V33E-02 INVITED
Disequilibrium Textures vs Equilibrium Modelling: Geochronology at the Crossroads
Observations made by electron microscopy show the processes affecting minerals at the atomic scale. The majority of reported analyses demonstrate chemical disequilibrium. A classic example are overgrowths of one mineral generation by a secondary one, which may be recognized on textural grounds. Disequilibrium recrystallization is promoted by water, which is everywhere on this planet (granites, contact aureoles, regional metamorphism, faults). It is mostly easier and energetically less costly to recrystallize a mineral at any temperature than to induce genuine volume diffusion in it. However, these observations are only relevant to geochronologists if chemical disequilibria are also accompanied by isotopic disequilibria. If a mineral mixture gives a mixed isotope record, then the interpretation of ages does not come cheap. If, on the contrary, diffusive reequilibration of the isotopic record is faster than that of chemical heterogeneities, then the petrology and microchemistry of a mineral could be ignored and its apparent age termed a "cooling age". First principle arguments and experimental data of the last decade concordantly show that the diffusivity of radiogenic isotopes is never higher than that of major elements forming the mineral structure. And indeed, end- member ages of mineral mixtures can be unravelled if the petrogenesis is understood. This was first shown by CL images of zircon grains (Gebauer et al, Schweiz Min Pet Mitt 68 (1988) 485-490). Similar progress was reported on monazite (Williams et al, Ann Rev Earth Planet Sci 35 (2007) 137-175), amphibole (Belluso et al, Eur J Mineral 12 (2000) 45-62), K-feldspar (Nyfeler et al, Schweiz Min Pet Mitt 78 (1998) 11-21), biotite (Villa et al, Water Rock Interaction 10 (2001) 1589-92). The mechanism for resetting the isotope record in nature thus seems more dependent on the availability of water to enhance disequilibrium recrystallization than on reaching a preset temperature. Intercomparison of laboratory release of Ar and Xe in Ba-fsp and Ba-mica require the same in vacuo degassing mechanism for hydrous and anhydrous silicates. This is not volume diffusion. Instead, it was observed to stem from a discrete structural rearrangement; fortunately, successive muscovite overgrowths do degas diathermally, so that successful dating of each mica generation is possible (Hetherington & Villa, GCA 71 (2007) 3336-47). In a parallel universe, diffusive equilibration is the basis for isotopic models providing numbers to be input into tectonic models. Is this the role of geochronology? With the coming of age of submicroscopic petrology, isotopic disequilibria can be put into context with petrogenetic disequilibria. This has opened up a much richer wealth of data on the P-T-A-X-d history of rocks, which in the long run will also be beneficial to those who now just ask for numbers.
V33E-03
Developing New Methods for Microsampling and Sm/Nd Dating of Zoned Garnet
Garnets provide one of the Earth Science community's most useful tools for studying rates, duration and timing of crustal processes. In this study we describe new techniques for fine sampling of multiple growth zones of garnet and Sm/Nd dating of each individual zone. We test these techniques on large (>5cm) garnets from a shear zone in the Tauern Window of Austria where we seek to quantify the growth history of garnet in a manner similar to dating tree rings. Microsampling permits a more precise quantification of duration, episodicity and kinetics of metamorphic reactions. Past studies of garnet growth duration – based on core and rim garnet ages – have been limited by sampling methods for extracting discrete, and accurate, growth zones. Modeling of radial growth symmetry in garnet shows that previous studies may underestimate garnet growth duration by as much as 50%. We are able to dramatically improve microsampling by using microdrilling guided by chemical maps of the garnet composition. This provides much improved precision and accuracy in sampling. By using chemical mapping of the garnet we can be sure that we are correctly sampling narrow (~500 micron wide) growth (i.e. age) zones rather than smearing and averaging multiple growth zones together. In principle, tens of growth zones (and ages) spanning the entire interval of garnet growth may be sampled and resolved. Microdrilled domains, the results of which are an ultrafine powder, are drilled and collected in water. Due to the adverse geochronological effect of unavoidable micro-inclusions in garnet, we have tested several partial dissolution techniques to cleanse the garnet of inclusions and yield higher 147Sm/144Nd and hence, more precise ages. Analysis of a finely crushed bulk Tauern Window garnet sample after HF/HClO3 cleansing indicates that 147Sm/144Nd at least as high as 0.89 is attainable in this particular sample, but cleansing efforts on microdrilled powders have thus far failed to yield such high ratios suggesting that the powders respond in unexpectedly different ways to our standard garnet cleansing procedures. Preliminary Sm/Nd age analysis of bulk garnet confirms a ~25Ma age for garnet growth. Progress in our inclusion cleansing procedures on microdrilled powders will be reported.
V33E-04
The role of electron microprobe mapping and dating in tectonic geochronology
Electron microprobe geochronology occupies a special niche within the spectrum of geochronological techniques and may be particularly relevant to the question, "What are we dating?" The technique was originally envisioned to be a low-cost, reconnaissance dating tool, opening low-resolution geochronology to a large number of researchers. However, more than a decade of research has shown that, when used in a reconnaissance fashion (i.e. using major-element analytical techniques for trace-element analysis) uncertainties are unsuitably large (several 10s of m.y. or more) for solving most tectonic problems. Using trace element analytical techniques (background modeling, interference correction, highly conductive coating, multi-analysis measurement, etc.) precision and accuracy are dramatically increased, but analysis time and cost are also increased, challenging the “quick, cheap, and easy” description. The power of microprobe geochronology comes from the spatial resolution and the natural integration with compositional data. High-resolution compositional mapping is valuable for all in-situ geochronology. Large area maps provide petrologic and textural context for chronometer phases; small scale maps illuminate the history of the chronometers themselves. Compositional maps associated with monazite are particularly informative, but examples from the East Athabasca granulite terrane using zircon, titanite, and rutile will be discussed. Most monazite crystals are 30μ or less and most have several compositional domains. Rim compositions and dates are particularly critical because they can commonly be tied to reactions and to matrix texture and fabric. Commonly, rims and internal sub domains are several microns in width and can only be analyzed by electron probe. Y has been widely used to tie monazite to Grt growth or breakdown, but current studies use a suite of trace and REE (Y, Sm, Nd, Ca, Eu, Gd, etc) to tie monazite into chemical reactions. A rapidly growing application involves detrital and authigenic monazite (and xenotime). Detrital grains that can be linked with source terrains can have very thin authigenic or metamorphic rims dating digenesis or early metamorphism. The rims involve reactions with monazite and surrounding phases that constrain depositional or metamorphic conditions and fluid compositions. Because concordancy cannot be tested, microprobe monazite applications in the Athabasca granulite terrain are strongest in combination with high-resolution U-Pb TIMS data. Microprobe dates constrain the age of specific deformation or metamorphic processes within the context of the overall high-precision geochronologic spectrum.
V33E-05 INVITED
Late Miocene coesite-eclogite exhumed in the Woodlark Rift, Papua New Guinea
One of the most exciting frontiers in the field of continental dynamics concerns the formation and exhumation of ultrahigh-pressure (UHP) rocks. The youngest exhumed UHP rock known on Earth was recently discovered in the lower plate of the D'Entrecasteaux Islands metamorphic core complexes within the Woodlark Rift of Papua New Guinea. This coesite-eclogite preserves a record following subduction to mantle depths (> 90 km). Unraveling its subduction and exhumation history requires establishing direct links between mineral assemblages and their isotopic ages, and "seeing through" the effects of possible overprinting thermal events caused by rifting, and propagation of the Woodlark Basin seafloor spreading system. A range of thermochronologic and petrologic techniques was used to unravel the P-T-t history of coesite-eclogite, including in situ ion microprobe techniques, and new thermometers based on [Zr] in rutile and [Ti] in zircon. Results indicate zircon crystallized at 650-675° C, below the closure temperature for Pb diffusion in zircon, at 7.9 ± 1.9 Ma. Rutile thermometry, using the calibration of Tomkins et al. (2007) that accounts for the pressure effect on the [Zr] in rutile, yielded temperatures of 695-743° C. These new thermometers provide more precise temperatures than those obtained previously using Grt-Cpx Fe-Mg exchange thermometers. Results suggest that zircon grew at a lower temperature than rutile, or that a pressure effect, not yet documented, exists for the [Ti] in zircon thermometer. During exhumation from depths > 90 km, the coesite-eclogite was partially retrogressed under amphibolite- facies conditions, pegmatite formed in strain shadows surrounding its amphibolite rind, and its quartzofeldspathic host underwent partial melting. 40Ar/39Ar muscovite ages from the pegmatite indicate that by 3.5 Ma temperatures had decreased to < 500° C. Apatite fission track data indicate that the eclogite host had cooled to below 120° C by 0.6 Ma. The integrated data set suggests an increase in apparent cooling rate (i.e., ~35° C/m.y. from 8 Ma to 3.5 Ma, ~135° C/m.y. from 3.5 Ma to present) during rapid (cm/yr) exhumation from UHP depths to the surface. Exhumation of Late Miocene coesite-eclogite within the Woodlark Rift requires reconsideration of the geological and tectonic evolution of the entire region to assess whether rifting has reactivated a former subduction thrust as a normal fault detachment system that is slipping in the direction of Woodlark-Australian plate motion.
V33E-06
Two-phase Neogene extension of the northwestern Basin and Range deduced from thermochronology of a single sample
We use a combination of apatite 4He/3He, (U-Th)/He, and fission-track thermochronology to date slip on the Surprise Valley Fault in northeastern California by analyzing a single granitic boulder from a conglomerate lens near the base of the adjacent Warner Range. Apatite from this sample yielded a fission-track age of 11.6 ± 1.4 Ma and a (U-Th)/He age of 3.12 ± 0.2 Ma. Regional geologic relationships indicate that this sample was buried to a depth of about 3.3 km prior to exhumation during slip on the Surprise Valley Fault. Apatite fission-track data indicate that the sample was fully reset (>120°C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for 4He/3He analysis; modeling of the resulting 4He distribution indicates cooling from >80°C to ~20°C between 3 and 1 Ma. Internally consistent time-temperature (t-T) solutions to the combined 4He/3He, (U-Th)/He, and fission-track data require two periods of cooling, consistent with non-continuous slip on the Surprise Valley Fault. Early cooling and fault slip took place between 14 and 8 Ma, and was followed by a more recent pulse of cooling at about 3 Ma. This timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the Basin and Range. These data demonstrate the resolving power of combined fission-track, (U-Th)/He, and 4He/3He thermochronometric data to extract high-resolution and ultra low temperature t-T information from a single sample close to the earth's surface.
V33E-07
Dating Ductile Deformation With Combined Lu-Hf and Ar-Ar Geochronology
Understanding continental deformation requires accurately dating the initiation and duration of discrete deformation events. We present Lu-Hf and Ar-Ar chronology from the Duraznos shear zone, a fundamental lithologic and structural boundary between the accreted Precordillera terrane and the proto-Andean margin in the western Sierras Pampeanas of Argentina. When combined with detailed petrography and thermobarometry, the ages date the initiation and duration of deformation within the shear zone. The hanging wall consists of meta-turbidites, meta-volcanic rocks and orthogneiss. The rocks contain the assemblage Amp±Ky±St-Grt-Bt±Ep-Pl-Qtz-Ms-Rt-Ilm and experienced one metamorphic event at conditions of ~9 kb and 650° C. Garnet is pre- to syntectonic with respect to the mylonitic fabric; amphibole and muscovite grew within the fabric and amphibole does not occur within garnet. From a single sample we obtained a Lu-Hf garnet age of 469±21 Ma, an Ar-Ar amphibole isochron age of 441±8 Ma and a muscovite plateau age of 436±6 Ma. The footwall consists of mafic and ultramafic intrusive rocks and meta-volcanics of the Pie de Palo complex and preserves two separate metamorphic events. The first event is preserved as distinct core domains within garnet and amphibole. The second event resulted in a mylonitic fabric with syn- to post-tectonic garnet growth preserved as distinct rims on the earlier garnet and as fine-grained matrix garnet. P-T conditions of the younger event determined from phases within the mylonitic fabric are ~9 kb and 550° C. A Lu-Hf garnet age of 1067±14 Ma from the mylonite within the footwall dates the older of the two metamorphic events. The second event is interpreted to be synchronous with or slightly post-date the Duraznos shear zone. We interpret the observed prograde metamorphism in the footwall and the ages in the hanging wall to record initiation of the shear zone post 469 Ma and progressive cooling of the hanging wall during thrusting over the Pie de Palo complex to 436 Ma. The combination of high and intermediate-T isotopic systems with thermobarometry and petrography provides a way to constrain the duration of deformation and identify distinct packages of crustal rocks that underwent a shared deep-crustal Ordovician event.