Tutt Library

Venture Grant: Applications and Methods of (U-Th)/He dating: Taking Stock

Cover sheet

Venture Grant Application

This past summer I participated in the Keck Geology Consortium Bighorn Mountains research project with six Geology students from other colleges and universities, and two professors from Amherst College and Washington State University, Dr. Peter Crowley and Dr. Peter Reiners. Our group traveled to the Bighorn Mountains in north-central Wyoming to collect samples of granite in order to determine the age of the mountain range. These samples were analyzed and dated at the California Institute of Technology in Pasadena, CA from June 30th to July 21st. We processed the samples by separating the mineral apatite from the granitic matrix of the rock in order to date the uplift of the rocks with the low-temperature (U-Th)/He geochronometer. This newly developed technique quantifies the products of the radioactive decay of Uranium to Thorium in certain granitic minerals at a steady rate over geologic time. Since this process is temperature dependent (low temperatures correspond to shallow depths beneath the Earth’s surface) the length of time that decay has taken place correlates to how long ago the rocks being analyzed were uplifted from deep within the Earth to create the mountain range they constitute. Our completed analytical work will serve as the foundation for a distinctions thesis that I will complete and present to the CC Geology Department on April 1st, 2000.

The preliminary results of our research are being presented as an abstract to the American Geophysical Union (AGU) at their annual fall conference, taking place from December 13th to 17th in San Francisco, CA. As a co-author of the abstract, I am requesting funding from the Venture Grant Committee of The Colorado College to underwrite the costs of attending the meeting so that I may personally take part in the presentation of my abstract. Below is our abstract as it will be presented to the AGU in December as part of a special conference session on Thursday, December 16th entitled, "Applications and Methods of (U-Th)/He dating: Taking Stock."

Apatite (U-Th)/He Thermochronology of the Bighorn Mountains, Wyoming: Implications for Structural and Sedimentation History

Crowley, P D
pdcrowley@amherst.edu
Amherst College, Dept. of Geology, Amherst, MA 01002

Reiners, P W
Washington State University, Dept. of Geology, Pullman, WA 99164

Farley, K A
Caltech, Div. of Geological and Planetary Sciences, Pasadena, CA 91125

Brown, H E
University of Arizona, Dept. of Geoscience, Tucson, AZ 85721

Kaye, G D
Colorado College, Dept. of Geology, Colorado Springs, CO 80903

Reuter, J M
Carleton College, Dept. of Geology, Northfield, MN 55075

We measured apatite (U-Th)/He ages on 29 samples of Precambrian crystalline basement from the Bighorn mountains from transects that represent 1900 m of topographic relief but possibly less than 500 meters of structural relief beneath the basal Cambrian unconformity. The ages, representing the timing of cooling through low temperatures (65-70°C), generally cluster around 90 Ma, but range from 62 to 391 Ma. We measured the youngest ages (62-68 Ma) in the central core of the range and along its northwest flank, near the Five Springs area. These ages are consistent with sedimentologic evidence in adjacent basins indicating unroofing and cooling of the Bighorns in earliest Tertiary time. Older ages probably reflect the fact that much of the exposed basement in the Bighorns has not been deeply buried, and rocks have not been heated above about 85° C, since the Precambrian. Stratigraphic sections adjacent to the Bighorns suggest that prior to Cretaceous sedimentation, the range was buried beneath less than 1400 m of preCretaceous sedimentary rocks. Temperatures beneath this sedimentary pile were never high enough for helium to readily diffuse out of apatite. Cretaceous sedimentation buried the region a further 2.5 to 4 kilometers pushing the basement into or through the He partial retention zone. The wide range in (U-Th)/He ages from the Bighorns probably reflect variations in the thickness of these pre-exhumation sedimentary rocks; some areas, especially the southwest flank of the range near Shell, were only buried to shallow depths, yielding samples with early Paleozoic ages.

AGU Fall Meeting
San Francisco, CA, December 13-17, 1999

Budget Breakdown:

Airfare:
$373 round trip from Colorado Springs to San Francisco
Departing December 12th and Returning December 17th
(quote from Taylor Travel on 11/10/99)

Conference Fees:
$205 For Student Multi-Day Admission

Food:
$100 for food, at $20 per diem

Lodging:
I will secure my own lodging by sharing rooms with other attendees.

TOTAL: $678.00