Peter K. Zeitler

U-Th-He dating of apatite: A potential thermochronometer

Abstract We found a gem quality crystal of Durango fluorapatite to have a 4He content consistent with complete retention of radiogenic helium since its formation at about 31 Ma. Isothermal heating and step-heating analysis reveal 4He loss to occur systematically by volume diffusion at low temperatures. The linear, low-temperature portion of the diffusion data yields an activation energy of 38.5 ± 8.1 kcal/mol and a frequency factor of In ( D 0 a 2 ) = 16.4 ± 2.8 sec −1 , corresponding to a closure temperature of 105°C ± 30°C (cooling rate 10°C/m.y.). It appears that U-Th-He dating of apatite might represent a useful new thermochronometer with a range similar to that of fission-track dating of apatite. From these results, we infer that a number of the too-young U-Th-He dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low-temperature thermal history of their host rocks

Saddle-shaped 40Ar39Ar age spectra from young, microstructurally complex potassium feldspars

Abstract A suite of young potassium feldspars show markedly saddle-shaped 40 Ar 39 Ar age spectra as a result of incorporating 10−10 to 10−9 mol/g of excess 40Ar. The minima of these age spectra record reasonable cooling ages, based on the known thermal history and geology of the samples. Acid etching of one sample indicates that excess 40Ar is concentrated near grain margins. The release of a substantial portion of this excess Ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40Ar. Anion vacancies have been proposed to act in this role in plagioclase, and we speculate that this is so in K-feldspar as well. Such a mechanism would explain the observation that relative to radiogenic 40Ar, excess 40Ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory. Oxygen diffusion provides an appropriate analogy for this phenomenon, being relatively fast under natural, hydrothermal conditions, but extremely slow in anhydrous environments such as an Ar-extraction system. TEM observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns, due to the presence of such microstructures as incoherent exsolution lamellae, dislocations, and stepped twins. TEM observations also reveal the presence in one sample of orthoclase enclaves in a microcline host.

Rates of late Cenozoic tectonism in the Vallecito–Fish Creek basin, western Imperial Valley, California

The kinetics of continental rifting are recorded in the late Neogene sediments of the western Imperial Valley, California. The Gulf of California opened in its present form about 4 m.y. ago, capturing the Colorado River. Some 5 km of deltaic and fluvial sediments accumulated in the Vallecito–Fish Creek area during the interval 4.3−0.9 m.y. ago. Initial sedimentation rates were 5.5 mm yr −1 , which diminished in an exponential fashion to 0.5 mm yr −1 . When isostatic adjustments are accounted for, this implies tectonic subsidence rates of 1.5 mm yr −1 , decreasing systematically to 0.1 mm yr −1 . After 0.9 m.y. ago, right-lateral shearing along the Elsinore fault zone cut across the basin, causing it to be tilted and uplifted at a mean rate of 5.9 mm yr −1 . The basin was concurrently rotated as a unit 35° clockwise. The dissected pediments and badlands that now characterize the area were formed and eroded in the past 0.9 m.y.

Ar diffusion in partially outgassed alkali feldspars: Insights from 40Ar39Ar analysis — Comments

Two microclines showing saddle-shaped 40Ar39Ar age spectra, a rapidly cooled orthoclase, and a sanidine from the Fish Canyon Tuff (Colorado, U.S.A.) were subjected to laboratory degassing in an attempt to better understand Ar diffusion systematics in alkali feldspars. The microclines and orthoclase show age spectra that confirm that these samples, and probably most alkali feldspars, comprise diffusion domains of widely varying grain size (greater than a factor of 10). The sanidine shows an age spectrum consistent with the range of grain sizes found in the dated mineral separate. Activation energies derived from the 39Ar release are correlated with intensity of outgassing as well as initial structural state. Despite evidence that much of the excess 40Ar contained in the microclines is situated near grain boundaries, dry degassing, even at 800°C, has little effect on this component. However, hydrothermal treatment at 400°C facilitates significant loss of this excess 40Ar, providing evidence that it is sited in anion vacancies. The fact that most alkali feldspars are likely to comprise a range of effective grain sizes for diffusion has important implications for the interpretation of age spectra, and will complicate the derivation of diffusion parameters from 39Ar released during step-heating.

Geochronological evidence for ∼ 530–550 Ma juxtaposition of two Proterozoic metamorphic terranes in the Musgrave Ranges, Central Australia

Contrasting temperature‐time paths from two Proterozoic metamorphic belts in the Musgrave Ranges, central Australia suggest that they represent district tectonothermal terranes. The thermal histories of these terranes converged about 530–550 Ma ago, as they were juxtaposed along the Woodroffe Thrust. Amphibolite fades metamorphism in the gneissic terrane north of the thrust is dated by zircon U‐Pb ages at ∼ 1600 Ma. Subsequently, the gneiss was intruded by granitoid at ∼ 1500 Ma, followed by a second metamorphic event which may be dated by a few zircon ages at ∼ 1400 Ma. Age maxima of ∼ 1300 Ma and ∼ 1260 Ma from steadily rising 40Ar‐39Ar spectra date cooling below closure temperatures of hornblende and K‐feldspar respectively, following the latter metamorphism. In contrast, zircon U‐Pb ages date granulite facies metamorphism in the terrane south of the Woodroffe Thrust at ∼ 1200 Ma. Post‐metamorphic cooling in the granulite terrane was slow with hornblende and biotite remaining open to argon diffusion un...

Ion-microprobe dating of zircon from quartz-graphite veins at the Bristol, New Hampshire, metamorphic hot spot

Detrital zircons entrained in hydrothermal quartz-graphite-rutile veins found near the Bristol, New Hampshire, metamorphic hot spot are overgrown by thin rims. Ion-microprobe analyses of these rims date their growth at 408 {plus minus} 6 Ma. These measurements quantitatively confirm textural evidence that the graphite veins were emplaced during peak metamorphism associated with the Acadian orogeny, and they provide a direct positive test of the hypothesis, based on petrological and stable-isotope evidence, that the hydrothermal systems responsible for the quartz-graphite veins were also responsible for the hot-spot metamorphism.

Dating late Pan-African cooling in the Uluguru granulite complex of Eastern Tanzania using the 40Ar39Ar technique

Abstract Age spectra measured by the 40 Arue5f8 39 Ar technique on hornblende, muscovite and K-feldspar from the Uluguru granulite complex of Eastern Tanzania indicate that following granulite facies metamorphism at ∼ 715 Ma the terrane cooled slowly, reaching a temperature of ∼475° C about 630 Ma ago. Subsequent cooling was even slower, reaching temperatures of ∼170° C about 420 Ma ago. Assuming a simple relationship between cooling rate and thermal gradient, the cooling history translates into an uplift path characterized by a phase of rapid uplift soon after granulite facies metamorphism followed by a period of slow uplift which began about 630 Ma ago. Such a history is consistent with model thermal histories of crustal segments undergoing thermal relaxation and isostatically-driven uplift following tectonic thickening events.

Discordant 40Ar39Ar ages from the Musgrave Ranges, central Australia: Implications for the significance of hornblende 40Ar39Ar spectra

Hornblende and biotite from granulites in the Musgrave Ranges, central Australia, yield variable 40Arue5f839Ar bulk ages even at the scale of a single outcrop. There is no obvious relationship between age, chemical composition, sample location, or textural characteristics of the minerals. We interpret the age variation as due to the incorporation of variable quantities of excess Ar. However, the age spectra of the minerals do not show the typical gain profiles commonly associated with excess Ar and isotope correlation diagrams do not indicate trapped Ar of non-atmospheric composition. Instead, some of the hornblendes and all the biotites show relatively simple age spectra similar to those commonly interpreted in terms of volume diffusion. These observations lead us to argue that, as is commonly the case with biotites, not all hornblende age spectra with simple patterns, similar to those predicted by diffusion theory, can be interpreted unambiguously in terms of geologically significant Ar concentration profiles. More specifically, we conclude that a relatively simple hornblende age spectrum does not necessarily indicate the absence of excess Ar. n nDespite their complexity, our results show that post-metamorphic cooling of the granulites was very slow. Although the incorporated excess Ar hinders the reconstruction of a more precise thermal history, the data show that following granulite-facies metamorphism ≈ 1200 Ma ago, the terrane did not cool below temperatures at which hornblende and biotite close to Ar diffusion until after ≈ 930 and ≈ 690 Ma, respectively.

Hydrothermal Graphite Veins and Acadian Granulite Facies Metamorphism, New Hampshire, USA

Hydrothermal graphite has been found over an area of 3,000 Km2 in New Hampshire, USA. Its pervasive presence records fluid flow of regional extent during metamorphism. Graphite precipitated from metamorphic fluids crosscuts and replaces Silurian and Devonian metasedimentary and metaplutonic rocks. The metamorphic grade of host rocks ranges from kyanite + staurolite, to sillimanite, sillimanite + alkali feldspar, or cordierite + sillimanite + alkali feldspar.