Kenneth A. Farley

The effects of long alpha-stopping distances on (U-Th)/He ages

A mathematical framework for quantitative evaluation of alpha-stopping effects on (U-Th)/He ages has been developed. Alpha stopping ranges in the ^(238)U, ^(235)U, and ^(232)Th chains vary between ∼10 and ∼30 μm, depending on decay energy and density/composition of the stopping medium. In the case of U- and Th-rich accessory minerals (e.g. apatite, zircon, titanite), the dominant effect of long stopping distances is alpha ejection to adjacent minerals. For grains smaller than a few hundred microns in minimum dimension, ejection effects will cause measured helium ages to substantially underestimate true ages. For example, a sphere of 100 μm radius retains only ∼82% of its alphas. For a homogeneous distribution of parent nuclides, the fraction of alphas ejected is ∼ 1/4 of the mean alpha range multiplied by the crystal surface to volume ratio, independent of geometry. Removal of the outer 20 μm of a crystal prior to dating eliminates the region which has experienced alpha loss, but may lead to erroneous ages when crystals are strongly zoned with respect to uranium and thorium. By careful characterization of four sieved apatite separates from a single sample, we show that it is possible to accurately correct (U-Th)/He ages for alpha ejection even when ejection exceeds 35% of total decays. Our results are useful for identifying the size and shape of grains which are best suited for (U-Th)/He dating and provide the basis for correcting ages when ejection effects are significant. This work underscores that meaningful (U-Th)/He ages require either large crystals, or correction of measured ages for alpha ejection.

Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite

High-precision stepped-heating experiments were performed to better characterize helium diffusion from apatite using Durango fluorapatite as a model system. At temperatures below 265°C, helium diffusion from this apatite is a simple, thermally activated process that is independent of the cumulative fraction of helium released and also of the heating schedule used. Across a factor of ∼4 in grain size, helium diffusivity scales with the inverse square of grain radius, implying that the physical grain is the diffusion domain. Measurements on crystallographically oriented thick sections indicate that helium diffusivity in Durango apatite is nearly isotropic. The best estimate of the activation energy for He diffusion from this apatite is E_a = 33±0.5 kcal/mol, with log(D_0) = 1.5±0.6 cm^2/s. The implied He closure temperature for a grain of 100 μm radius is 68°C assuming a 10°C/Myr cooling rate; this figure varies by ±5°C for grains ranging from 50 to 150 μm radius. When this apatite is heated to temperatures from 265 to 400°C, a progressive and irreversible change in He diffusion behavior occurs: Both the activation energy and frequency factor are reduced. This transition in behavior coincides closely with progressive annealing of radiation damage in Durango apatite, suggesting that defects and defect annealing play a role in the diffusivity of helium through apatite.

Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U‐Th)/He thermochronology

High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperature-time paths inferred from ^(40)Ar/^(39)Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar ^(40)Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling ( 30°–50°C/m.y.) coincident with exhumation from inferred depths of ∼8–10 km, at denudation rates of 1–2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period (∼3°C/m.y.), suggesting limited exhumation (1–2 km). However, these samples record a slight increase in cooling rate (from <1 to ∼3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene (∼5–12 Ma).

Modeling of the temperature sensitivity of the apatite (U–Th)/He thermochronometer

Apatite (U–Th)/He apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion. To characterize the response of apatite He ages to thermal histories involving partial He retention, we explored solutions to the He production–diffusion equation. Under thermally static conditions, the analytical solution to this equation, coupled with published diffusivity data, demonstrates that the zone of partial He retention extends from about ∼40°C to ∼85°C. This zone lies at temperatures ∼35°C cooler than the analogous fission track partial annealing zone. He ages within the partial retention zone ultimately achieve a balance between He production and loss, yielding a steady state age. Both the ultimate age and the time it takes to achieve this age are temperature dependent. For example, an apatite held at 75°C equilibrates to an age of ∼2 Ma after ∼17 Myr, regardless of whether equilibrium is approached from a higher or a lower initial He age. For representative dynamic thermal histories, we evaluated apatite He ages using a numerical solution to the ingrowth–diffusion equation. The results illustrate the sensitivity of He ages to various geologic histories and are useful for understanding He age–elevation relationships and for testing time–temperature paths derived from apatite fission track length distributions. In addition, although He diffuses rapidly from apatite at shallow crustal temperatures, modeling of ambient temperature fluctuations indicates that He ages are nearly unaffected by surficial processes.

Apatite (U–Th)/He thermochronometry: methods and applications to problems in tectonic and surface processes

In the last decade apatite (U–Th)/He thermochronometry has emerged as an important tool for quantifying the cooling history of rocks as they pass through the upper 1–3 km of the crust. The low closure temperature of this technique (∼70°C) has gained the interest of geomorphologists and tectonocists because it is applicable to interdisciplinary studies in landform evolution, structural geology, and geodynamics. We discuss current analytical techniques, the temperature calibration of the method, and sample quality considerations. Results from 1D, 2D and 3D thermo-kinematic numerical models are used to illustrate applications of He thermochronometry to problems in tectonics and landform evolution.

Helium diffusion and low-temperature thermochronometry of apatite

To investigate the potential of the (U-Th)/He system for low-temperature thermochronometry, we have studied helium diffusion and have measured helium ages on Durango fluorapatite and on apatites from a gabbro and two tonalites from the Peninsular Ranges Batholith. Diffusivity at moderate to very low temperatures (as low as 80°C) was measured to high analytical precision using long duration incremental outgassing experiments. All four apatites displayed remarkably similar helium diffusion behavior. Helium loss apparently occurs via volume diffusion from subgrain domains (<60 μm) which are nearly identical in size in all samples. At temperatures below 290°C, diffusivity obeys a highly linear Arrhenius relationship with an implied activation energy of about 36 kcal/mol. Above this temperature, diffusivity deviates from linearity toward lower activation energies. This transition does not arise from multiple diffusion domains, but rather from a reversible change in the physical mechanism of helium diffusion. For thermochronometric purposes the high-temperature diffusion behavior is largely irrelevant because essentially no helium is retained over geologic time at temperatures above 290°C. Using the results from the low-temperature regime, all samples yield helium closure temperatures in the range 75 ± 7°C. This value is independent of chemical composition and grain size of the apatites, suggesting that a single closure temperature may apply to a wide range of samples. The (U-Th)/He ages of these apatites (17–120 Ma) range from a small fraction to nearly 100% of the crystallization age of their host rocks, and are consistent with a low-temperature thermochronometric interpretation. These results strongly support previous suggestions that (U-Th)/He dating of apatite can provide high precision chronometry of very low temperature geological events.

Binary mixing of enriched and undegassed (primitive?) mantle components (He, Sr, Nd, Pb) in Samoan lavas

We have measured He, Sr, Nd, and Pb isotope ratios and Rb, Sr, Sm, and Nd concentrations in stratigraphically controlled lavas from the Pago shield volcano on Tutuila, American Samoa. We interpret these lavas as products of mixing between two isotopically extreme mantle constituents. The first is a highly enriched component with very high Sr isotope ratios, low ^3He/^4He ratios, and high “Δ(7/4)” and “Δ(8/4)” Pb isotopic characteristics. This is probably recently recycled ( 24R_A) and intermediate Sr, Nd, and Pb isotopic ratios. This material was derived from a largely undegassed mantle source, and, in conjunction with data from several other ocean islands, provides strong evidence for the existence of a high ^3He/^4He ratio mantle end member (primitive helium mantle, PHEM) with consistent Sr, Nd, and Pb isotopic characteristics: near bulk-earth ^(87)Sr/^(86)Sr (0.7042–0.7052) and ^(143)Nd/^(144)Nd (0.51265–0.51280,e_(Nd) = +0.2to+3.2), and radiogenic Pb (^(206)Pb/^(204)Pb∼ 18.5–19.0, ^(207)Pb/^(204)Pb∼ 15.5–15.57, ^(208)Pb/^(204)Pb∼ 38.4–39.2). Although this material cannot have been derived from a reservoir completely closed to elemental fractionation for the full 4.55 Ga duration of Earths history, it may indicate the presence of a highly primitive mantle source. Erratic temporal variations in the isotopic composition of individual flows indicate sporadic and variable mixing of these two sources. We interpret these results using a model in which high ^3He/^4He ratio plume material, rising intermittently from a lower-mantle source, intercepts and melts recycled crustal matter in the upper mantle or lithosphere and erupts as a binary mixture of PHEM and the so-called “EM” components derived from this recycled material.

Dating topography of the Sierra Nevada, California, using apatite (U–Th)/He ages

The upward motion of rock masses relative to the Earths surface has been documented for most of the main mountain belts using thermochronological and petrological techniques. More fundamental to the physical processes of mountain building, however, is the motion of the Earths surface itself, which remains elusive. Here we describe a technique for estimating the age of topographic relief by mapping the low-temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium, thorium and helium contents. The technique exploits horizontal variations in temperature in the shallow crust that result from range-normal river drainages,, because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges. Our results from the Sierra Nevada, California, indicate that two of the modern transverse drainages, the Kings and the San Joaquin, had developed deep canyons by the Late Cretaceous period, suggesting that the high topography of the range is ∼50–60 million years older than generally thought.

Oxygen isotope constraints on the sources of Hawaiian volcanism

We have measured oxygen isotope ratios in 99 separates of olivine and 14 separates of plagioclase or glass from Hawaiian lavas. These data confirm that the source(s) of some Hawaiian basalts are lower in δ^(18)O than peridotite xenoliths and the source region for mid-ocean ridge basalts (MORB). Our data document correlations between oxygen and radiogenic isotope ratios and consistent differences in δ^(18)O between volcanoes. Low values of δ^(18)O are associated with a ‘depleted’ component that is relatively high in ^(206)Pb/^(204)Pb, low in ^3He/^4He, and anomalously low in ^(207)Pb/^(204)Pb relative to ^(206)Pb/^(204)Pb. This component is preferentially sampled in lavas from the so-called Kea trend volcanoes (Kilauea, Mauna Kea, Kohala and Haleakala). Low δ^(18)O values in the ‘Kea’ component suggest that it is hydrothermally altered oceanic crust. The similarity of the Kea end member to Pacific MORB in terms of Sr, Nd, and Pb isotope ratios further suggests that this component is assimilated from the local Pacific plate in subcrustal magma chambers. Anomalous ^(206)Pb/^(204)Pb-^(207)Pb/^(204)Pb relationships indicate recent enrichment in U/Pb in this component and further support the hypothesis that this component is young ( < 10^8 yr) Pacific crust. The isotopic distinctions between Loa and Kea trend volcanoes implies a systematic difference in the magma supply and plumbing systems of volcanoes on these two trends. Samples from Lanai and Koolau have ‘enriched’ radiogenic isotope compositions (radiogenic Sr and non-radiogenic Nd and Pb) and higher δ^(18)O than typical upper mantle values, suggesting the incorporation of recycled sediment and/or oceanic crust in their sources. Other isotopic end members to Hawaiian lavas (e.g., high ^3He/^4He and post-erosional lavas) have δ^(18)O values within the range typical of the upper mantle.

He diffusion and (U–Th)/He thermochronometry of zircon: initial results from Fish Canyon Tuff and Gold Butte

To evaluate the potential of (U–Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U–Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U–Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature-dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (E_a) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum E_a for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (T_c, for a cooling rate of 10 °C/myr) is about 190 °C. Age–paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5–11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age–depth profile suggests a He T_c of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral.