Katherine J. Dobson

A diode laser system for heating minerals for (U‐Th)/He chronometry

We have developed a diode laser (25 W, 808 nm) system for He extraction from minerals for (U-Th)/He chronometry. The laser beam is delivered via a 600 μm fiber cable and focused using a binocular microscope. Temperatures necessary for He release from apatite (500–600°C) and zircon (1100–1300°C) encapsulated in Pt-foil tubes are attained by heating to 0.5 W for 30 s and 1.25–2.5 W for 20 min, respectively, using a defocused beam. Heating at these powers does not result in measurable U and/or Th loss from apatite, as noted by the preservation of the distinct Th/U in multiple splits of two different Durango apatite crystals. Analyses of Durango and the California Institute of Technology internal standard apatite 97MR22 yield (U-Th)/He ages of 32.8 ± 1.8 Ma (1σ, n = 11) and 4.6 ± 0.5 (1σ, n = 5), respectively, well within accepted ages. The (U-Th)/He age and Th/U of five Fish Canyon Tuff zircon aliquots yield 29.3 ± 2.2 Ma (1σ) and 0.6 ± 0.03, respectively, and are indistinguishable from ages produced by resistance furnace He extraction. Heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals (e.g., hornblende, biotite, muscovite, garnet) and basalt groundmass, suggesting that diode lasers offer a cheap, small, low-maintenance alternative to Nd:YAG and Ar ion lasers for 40Ar/39Ar, cosmogenic noble gas, and stable isotope studies.

Rapid early Miocene exhumation of the Ladakh batholith, western Himalaya

Zircon, apatite (U-Th)/He, and apatite fission-track age data record a rapid cooling event in the Ladakh batholith of northwest India ca. 22 Ma. A combination of inverse and forward modeling of the data confirms this qualitative interpretation. Combining the thermochronometric data with structural evidence, we propose that exhumation was due to south-directed thrusting of the batholith along a north-dipping structure, coupled with erosion to bring the rocks to the surface. The rapid exhumation recorded in Ladakh is contemporaneous with exhumation of the High Himalaya. This focused surface denudation and structural shortening north of the Indus suture zone in early Miocene time implies that the actively deforming and eroding Himalayan thrust wedge extended farther north than channel flow models currently predict.

Quantitative constraints on mid- to shallow-crustal processes using the zircon (U–Th)/He thermochronometer

Abstract Despite the potential of zircon He thermochronometry for constraining rock thermal histories, it remains less commonly exploited than the apatite He chronometer. In part, this is due to the more challenging analytical techniques required to extract He, U and Th. Here we present a new method for the routine determination of zircon (U–Th)/He ages, and demonstrate how it can be used to constrain thermal histories and to quantify cooling in different tectonic settings. We present zircon (U–Th)/He ages that place a firm upper limit on the extent of denudation-induced cooling (c. 3 km) on the SE Australian passive margin; a region where synrift apatite fission-track and apatite (U–Th)/He ages have previously prevented quantitative constraint. We have also used the zircon (U–Th)/He thermochronometer to quantify the cooling of early Tertiary mafic plutons from Skye, Scotland, where the rate and timing of cooling cannot be determined using other thermochronometers.

Constraining the post-emplacement evolution of the Hebridean Igneous Province (HIP) using low-temperature thermochronology: how long has the HIP been cool?

Abstract: The thermal history of the Hebridean Igneous Province has been determined through the application of low-temperature thermochronology to the four central complexes in the province. The zircon (U–Th)/He age (59.4 ± 3.3 Ma, 1σ, n = 18) and the ages from each complex (60.7 ± 2.3 Ma for Skye; 58.0 ± 0.4 Ma for Mull; 55.9 ± 3.2 Ma for Ardnamurchan; 55.6 ± 2.7 Ma for Rum) are indistinguishable from the crystallization ages. Apatite fission-track ages (61.2–57.2 Ma, mean of 59.3 ± 3.4 Ma) from the major plutonic units also overlap crystallization ages, implying that on a regional scale the Hebridean Igneous Province cooled rapidly to near-surface temperatures immediately after emplacement. However, apatite fission-track ages and track lengths and apatite (U–Th)/He ages from some small-volume intrusions in the Skye and Rum central complexes identify localized mid-Eocene (45–47 Ma) cooling. Forward and inverse modelling suggests a discrete heating–cooling event at c. 47 Ma, which may have been caused by structurally controlled localized advection of heat above shallow emplacement. This is the first suggestion of Eocene magmatism in the Hebridean Igneous Province.

How Many Years Can a Mountain Exist before it's Washed to the Sea? Or, Bob Dylan's Theory of Landscape Evolution

Abstract The question of the persistence of topography is not just a line in one of the most famous songs ever written, it is also one of the long-standing questions in geomorphology. Mountains created by tectonic processes hundreds of millions of years ago still stand proud in the landscape, posing intriguing questions of how topography can persist for such a long time, and if the mountains we see today are the result of more recent processes, and therefore not a direct relict of the original topography. Although models have been proposed to explain the antiquity of topography, the only way to test these models is by providing denudation rates over millions of year time scales. Such data can be uniquely provided by low temperature thermochronometers. In this paper we provide a brief review of these techniques and an example from western Scotland where the thermochronometers have been applied to determine the age of first order topography. The results indicate that although denudation rates have been temporally and spatially variable in the last ∼300 Ma, the macro-topography has not changed significantly.