In-Situ Apatite Laser Ablation U-Th-Sm/He Dating, Methods and Challenges
The laser sample planchet sits on a computer-controlled Newport X-Y stage and holds up to 44 samples. On laser extraction line 2, diode laser heating is controlled by an in-line two-color pyrometer. The 44 position planchet is fixed while the x-y-z position of the laser beam deliver is computer controlled. The second quadrupole He mass spectrometry system consists of the following principle components: He Diffusion Experimental Line As discussed above, it is essential to accurately quantify the helium diffusion characteristics of mineral phases in order to use them for thermochronological purposes.
Inside the diffusion cell the Cu-foil wrapped sample is suspended by a K or J-type thermocouple and is heated by a V halogen bulb projected through a sapphire window. Precise temperature control is accomplished through a feedback mechanism involving a Watlow thermal controller and a phase-angle-fired Eurotherm power supply.
The Watlow thermal controllers are interfaced with the lab computer, allowing the helium diffusion experiments to be executed in a fully automated mode using LabView software. Most of our experimental efforts to date have concentrated on investigating the diffusion characteristics of a variety of new mineral phases, such as monazite, rutile, magnetite, fluorite, perovskite, or garnet.
The two aliquots yielded ages of Taken together these data suggest a remarkably 4. The Cfg of the K-feldspar Ar release data for this Arrhenius plots for these data are similar to previous sample is 0. The K-feldspar model suggests a slightly con- results for zircon Reiners et al. Details as in Figure 3a. On the basis of of post-high-T data.
White triangles and circles are single-grain zircon He ages. Gray and black triangles and circles are means of single-grain ages on each sample. White symbols are single-grain ages; black and gray symbols are means. Error bars on mean ages are two standard deviations of the single-grain ages.
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This can be expressed as: In high-T steps that approximate a single linear trend, and the some cases, however, it is difficult to resolve this from reported temperature of interest. Presentation of actual He release data and heating sched- therefore reference domain size in such an interpretation is that ules would help distinguish if this is a feature common for of the later steps, instead of the early steps. As shown later, this phases besides zircon and titanite. Thermal histories of Southwest Arm pluton. White circles are single-grain ages; black circles are means.
Error bars as in Figure 4. Here we investigate the plau- degassing. It is also noteworthy, for the modeling that fol- sibility of these potential origins, although we do not discuss lows, that for the grain sizes and heating schedules used the last one anisotropic diffusion beyond speculating that its here, the cumulative fraction of He released f from these results may be qualitatively similar to those predicted by a samples during all heating steps is between 0.
U-Th Zoning potential origins of the non-Arrhenius behavior. Several potential origins for the initial high diffusivity and Helium may be heterogeneously distributed in zircons by lower activation energy trends can be envisioned: It may be depleted near crystal rims by: However, the initial stages of most zircon experiments, including those in 4.
Radiation damage Reiners et al.
If this Radiation damage was suggested by Reiners et al. In this explanation, decreasing ap- caused by high U and Th concentrations there. Back-scattered parent diffusivity during step-heating would be due to progres- electron and cathodoluminesence imaging, as well as depth- sive restoration of crystallinity to amorphous or lattice-dam- profiling by laser-ablation ICP-MS, of zircons from Cornuco- aged zones in the course of each experiment. Progressive annealing during He diffusion served diffusivity changes, because all zircons, including those experiments was suggested partly on the basis of contrasting in the previous study Reiners et al.
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Closer inspection of these samples, however, raises U-Th concentrations, ages, and thermal histories, all samples doubts about this comparison, because zircons from the sample except one from Gold Butte show similar Arrhenius trends. In this case, the effects of radiation damage would resemble, and possibly be both empirically and mechanistically indistinguishable from, those of He release from multiple diffusion domains with varying size or other properties. Multidomain Models Some of the most salient features of multidomain diffusion models have been pointed out in studies of K-feldspar by Lovera et al.
For the specific case of He diffusion from titanite, Reiners and Farley e. One of the most general results is that, assuming no interaction between domains i. See text for derivation. This plot shows changes nomenon observed in our samples. Although highly simplified, this allows us to assess the temperature are negligible after this. Gas released from than radiation dosage of a bulk grain would predict, however.
This multidomain behavior would lead to apparently de- entire range of configurations that could produce Arrhenius creasing diffusivity in Arrhenius plots, as observed for zircon. However, consideration of several end- Radiation damage may be heterogeneously distributed at a member cases allows useful insight.
The most important con- range of scales. These considerations suggest, and spectroscopic diffusivity D, or domain size a.
Average Ea and D0 derived and microscopic studies confirm Sahama, ; Chakoumakos from the post-high-T heating steps of the diffusion experiments et al. These specific values are incon- et al.
In any case, between 0. The second and third sets are similar local high-damage zones would be expected to have relatively except gas proportions are See text for details of modeling. In domains on the bulk diffusivity of a zircon crystal would be most cases, several heating steps are required to completely minimal. This leads to two for the largest domain, the effective closure temperatures of subparallel trends, one for early and one for later steps. In the context mainly a function of the size of the small domain a relative to of this model, even if the smaller domains represented a sig- the larger one.
As suggested previously for ti- Fig. If a of the small domain is larger than this i. Thermochronologic Data of later steps. Mean zircon He ages of two samples degassing f far higher than observed in the real experiments Tracy Arm and Southwest Arm are highly concordant with Figs.sortmondrisifor.cf
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The Southwest Arm pluton also 0. Unfortunately, the Ar cooling model of sample KFR7 from the Although we have not explored the full range of parameter Te Kinga pluton does not extend to sufficiently low tempera- space, these examples and those of many other forward models tures to directly compare with the zircon He age. However, not shown here suggest that the two-domain model that pro- linear extrapolation of the KFR7 cooling model encompasses duces non-Arrhenius behavior in early degassing steps most the measured zircon He ages. This suggests that a contin- there is good agreement between zircon He ages and Ar cooling uum of domains with increasingly smaller proportions of gas in models for the Tracy Arm and Southwest Arm samples, this increasingly smaller domains would reproduce the observations only qualitatively supports the zircon He closure temperature of better, as is seen in K-feldspar multidomain models.
Trends from two real experiments are shown for comparison. Assuming that only 2. At least in the case of apatite, lower cooling rates 4. U-Th Zonation often lead to poorer reproducibility, however, because small differences in diffusivity from a variety of causes can greatly One potentially important source of age scatter in He dating magnify age differences Reiners and Farley, ; House et in general is heterogeneous intracrystalline U and Th distribu- al.
The direction and suggests that the underlying assumptions are robust and magnitude of age bias resulting from the U-Th zonation Fig. Southwest Arm pluton TK7. For depleted rims i. Figure 14 also shows that the magnitude of effects of rim depletion on corrected ages drops off quickly for thinner rims, but remains relatively high for thick rims. The magnitude of effects of rim enrichment on corrected ages drops off quickly for thicker rims, but remains relatively high for thin rims.
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If dated crystals from the TK7 sample Fig. Without knowing Te Kinga sample white diamonds. Electron microprobe and laser-abla- seen in Figure In these samples, the U-Th zonation Fig. To estimate Hourigan et al. Following laser profiling, we per- distance r from the rim of the crystal. Rims depleted in U-Th by factors of 2—20 i. Locations of spot analyses by electron microprobe, and concentrations in ppm are shown for U and Th, where concentrations were above detection limit ppm. Together with the results on zircon of Farley et al. This results in an 8. Non-Arrhenius behavior of He diffusion in the Southwest Arm pluton samples studied here.
If this high J. Terra Cognita 6th Int. This suggests that chronology and Isotope Geology 6, The transition erals are insignificant for bulk closure temperature and ther- from the crystalline to the metamict state. Science , — Although the relatively rapid cooling inferred from U.
In one case, single crystal ages show a large tute, Troy, New York, pp. General behavior as illustrated by Durango fluorapatite. Techniques, calibrations, and zonation and thus that the homogeneous U-Th assumption applications. However, characterization of intracrystalline U-zona- Farley K. Apatite, monazite, and xenotime. Acta 60, — We acknowledge Jeremy Farley K. Geology 29, 99 — In Potassium-Argon Dating eds. Schaeffer and microprobe assistance. This project was supported by NSF.
Analysis, scaling, geochemistry, and Allibone A. Cos- and gabbroic rocks from central Stewart Island, New Zealand. Acta 61, — — New Zealand Tertiary plutons in the northern Coast Mountains batholith. Radiometric Dating of Rocks Geochim. Acta 32, — Accumulation versus ther- dating investigation. Fission track dating, appar- Washington State University.
Tectonics 2, — Toward improved precision and accu- ical mirage of microtexture? AGU 84 46 , Fall Meet. Evidence of microstructural control on House M. Acta 63, Hurford A. South Central Switzerland and an age of vertical movement on the Reiners P. An example from the Bighorn Hurley P. Initial results from Fish A possible age method. Canyon Tuff and Gold Butte. South Island, New Zealand. Metamorphic history of the chim. Significance of activation energy determinations. A 83, 96 — A 83, — Oxford University Tagami T.
Earth tribution of the source. In Advances in Fission-Track Geochronology eds. De Corte , pp.