Cooling Histories from Helium Thermochronometry

For the last decade my group has worked to develop techniques for establishing very low temperature cooling histories of rock masses. Thermal histories are important for example for studying tectonics, especially associated with mountain range exhumation, for paleogeomorphology, and for resource exploration (e.g., hydrocarbon maturation). The technique we have been developing is based on the radioactive decay of uranium and thorium to 4He, a dating method proposed more than a century ago but never widely utilized. The key to the He thermochronometry method is the recognition that at elevated temperatures characteristic of the crust at a depth of a few km, helium diffuses from common U,Th-bearing minerals as rapidly as it is supplied by radioactive decay. As rocks cool, the rate of helium diffusion drops exponentially. As a result He-ages record time since cooling, rather than time since mineral growth. In the case of apatite (calcium fluorophosphate), lab experiments44 and field observations36,47 demonstrate that  above about 80oC,  helium is almost instantly lost, while it is quantitatively retained by 40oC19.  By measuring He, U and Th concentrations in apatite grains we can calculate how long it has been since the crystal cooled through the critical interval 40-85o.Other minerals we have investigated include zircon60, titanite38,76, and monazite65. Details of the techniques we use, including laser extraction, and a review of He-datable minerals and their temperature sensitivities have been published 48,65,66.
 

Because of its widespread occurrence and sensitivity to very low temperatures (about 25oC cooler than the apatite fission track method), the apatite (U-Th)/He dating method has now been applied in many different places and with many different objectives. The most straightforward applications are in tectonics, in which the timing of fault motion can be deduced, e.g., in the White Mountains of California47. Along with Brian Wernicke, Jason Saleeby and several students and post-docs we have investigated helium ages of the Sierra Nevada26, 33, 88 (above, at the Kern-Kaweah Divide) as part of a major program for understanding the paleogeomorphology of this range. The basis for interpreting cooling ages in terms of paleogeomorphology and surface processes was recently reviewed68. Other detailed studies include work  in the Coast Mountains of  British Columbia 52,83 ,87 and on meteoretic phosphates which constrain the early history of asteroidal parent bodies71.

Most recently we have been developing a different approach to thermochronometry, by measuring the 4He concentration profile in mineral grains. Consider a sample that has cooled quickly, with no time for He diffusion. That sample will have a concentration profile that is unmodified by diffusion, i.e., ignoring other phenomena the 4He profile will be "square". In contrast a sample that has resided at temperatures where diffusion is active will have a "rounded" concentration profile, that is, lower concentrations at the grain edge, where diffusive loss occurs, than in the grain interior. We have forward-modelled the effect, and find that the concentration profile, coupled with the absolute (U-Th)/He age, can provide very restrictive limits on cooling history74. It is not presently possible to directly measure the 4He concentration at the requisite spatial scale, so we have developed an alternative approach. By irradiating samples with 100+ MeV protons at the Northeast Proton Therapy Center, we can transmute some target elements into the rare isotope of helium - 3He. This isotope will be uniformly distributed within the grain. By subjecting the irradiated sample to step heating, in which the helium is progressivly degassed from the sample, we can image the 4He/3He ratio within the grain, and hence the 4He distribution within the grain. We have applied this technique in several different ways, including a destiled study of glacial incision in the Coast Mountains75, 87.


In the summer of 2004 former post-doctoral fellow Marin Clark (now at U. Michigan) and I sampled granites across the Tibetan plateau to better assess when this region achieved its high elevation. The picture at right is of the Kunlun Shan, on the northern margin of the plateau. More pictures from our work in China and Tibet can be found here.

Many students and post-docs have worked with me on (U-Th)/He projects over the years. Several that have continued this line of research have websites:

Peter Reiners at Yale
Danny Stockli at Kansas
Jim Spotila at Virginia Tech
Todd Ehlers at Michigan
Marin Clark at Michigan

David Shuster at BGC

 

 

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Cosmic Dust in Seafloor Sediments

Geochemical investigations using the element Iridium have demonstrated that the major species extinction horizon that marks the end of the Cretaceous Era (65 million years ago) was caused by the catastrophic impact of a large extraterrestrial object with the Earth. This event demonstrates that the Earth's biological and climatic systems are very sensitive to the accretion of material from space. While Iridium provides a tool for recognizing large, episodic impact events, it is not very sensitive to the large and temporally continuous accretion of cosmic dust (photomicrograph, left, showing 10 micron grain) submillimeter grains which rain down at a rate of about 40 million kg per year.

We have been working to determine the accretion rate of cosmic dust back through time, by analyzing the rare isotope 3He in deep sea sediments. This isotope is extremely abundant in extraterrestrial matter, and in most seafloor sediments more than 95% of the 3He derives from cosmic dust48. My results reveal striking variations in the cosmic dust accretion rate with time, which, intriguingly, are temporally correlated with both large impact events15 and global glacial cycles17. I am presently working to understand whether these correlations indicate causality, or are simply coincidental. Confirmation of a causal relationship would provide critical insights to the behavior and sensitivity of the Earth system to extraterrestrial events. My most recent work seems to suggest the occurrence of a comet shower - a period of strongly enhanced cometary activity, possibly caused by gravitational perturbations associated with passage of a star close to our solar system - at 36 million years ago34. The K/T boundary impact was NOT associated with such a shower, nor is there any indication of comet shower periodicity, at least in the period 30 to 74 Ma49.

A more recent application of this technique is to use extraterrestrial 3He abundances as an indicator of sedimentation rate (high sedimentation rates yield low 3He concentrations for a given extraterrestrial flux, and vice versa), particularly for assessing the pace of rapid climate change events in the distant geologic past. Using this technique we can constrain the duration of the K/T boundary clay to just 20 kyrs49! Similar work provided new constraints on the duration and temporal progression of the Paleocene-Eocene Thermal Maximum, a very large and rapid climate excusion that occurred 55 million years ago70.

Several claims of extraterrestrial impacts at the Permian-Triassic Boundary have been made, some based on detection of extremely high 3He concentrations in boundary sediments. Sometimes the 3He is thought to be encapsulated in fullerenes. Although we have repeatedly tried 56,82 we have been unable to confirm high 3He levels in any Permian-Triassic boundary sediments, casting doubt on the earlier reports.

In contrast there is abundant evidence from 3He and from direct observations of the asteroid belt for a massive asteroid collision 8.2 Million years ago. This collision produced the Veritas asteroid family and greatly enhanced the flux of extraterrestrial dust to Earth for a several million year period89.

 

Participants in my group include:

Sujoy Mukhopadhyay now at Harvard

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Isotope Geochemistry of Ocean Island and Mid-Ocean Ridge Basalts


Lavas erupted at ocean island volcanos (such as Robinson Crusoe island5, photo) have diverse chemical and isotopic compositions which constrain the evolution of the Earth's interior and atmosphere, the return of material to the Earth's interior at subduction zones, and the process of formation of volcanic rocks. I am particularly interested in the isotopes of helium, which show that the Earth's interior has experienced variable degrees of outgassing to form the atmosphere. We have recently investigated the He isotopic composition of lavas from Papua New Guinea24, the North American Basin and Range Province, Kauai (Hawaii)67, and American2 and Western Samoa14.


The heavier noble gases (neon, argon, krypton, xenon) also have an interesting story to tell about planetary evolution29. Unlike helium, which is not retained by the Earth's gravitational field, these gases have accumulated in the atmosphere from volcanic emissions throughout the history of the Earth. By comparing the composition of the atmosphere with the Earth's interior it is possible to constrain when and how the atmosphere formed. The key to this field is high precision, low-blank analyses, and this is the driving force behind the laser extraction system in my laboratory. We have measured the Ne-Ar-Xe composition of CO2-dominated fluid inclusions in some extraordinary xenoliths (pieces of the mantle entrained in lavas) from Samoa3 (photomicrograph at right).
 

Also in my lab Pete Burnard (now at Nancy) and Dr. David Graham at OSU recently completed on a systematic analysis of He-Ne-Ar-CO2 in mid-ocean ridge glasses from the Southeast Indian Ridge to better understand the source composition and petrogenetic behavior of the noble gases80.

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New Techniques for Surface Exposure Dating

Surface exposure dating provides critical information on a range of Earth science problems including ages of glacial features, erosion rates, and rates of fault motion. The technique relies on the production of rare isotopes produced by interactions of cosmic rays with target nuclei in rocks within ~ 1 meter of Earth's surface. The most commonly used "cosmogenic isotopes" are radionuclides (10Be, 26Al, 36Cl) that require extensive chemical purification and accelerator mass spectrometry for analysis. In contrast cosmogenic noble gases, especially 3He, require far less labor intensive purification and a simple sector-field mass spectrometer. Thus cosmogenic noble gases offer the advantage of faster and less expensive data acquisition. However cosmogenic 3He is now routinely analyzed only in olivine and pyroxene because only these minerals are known to retain 3He against diffusive loss and have tolerably low non-cosmogenic 3He background levels. We recently began work to develop cosmogenic 3He dating of additional minerals which are known to retain He at Earth surface conditions, especially the common accessory phases apatite, sphene, and zircon. The challenge of working with these minerals is their small grain size, which causes a variety of nuclear effects associated with the long stopping range of 3He in minerals. In addition the presence of high concentrations of 4He in these uranium rich minerals demands high abundance sensitivity for the 3He measurement. Despite these complications we have obtained excellent results in some settings. For example, this image shows cosmogenic 3He in the accessory phases separated from a tuff in Bolivia. This work was done in collaboration with Julie Libarkin at Ohio University. Work continues with Doug Burbank at UCSB and Caltech graduate student Willy Amidon.

 

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