Cosmogenic 3he helium age dating

The He values, which will allow students to contribute insight on the geochemical evolution of the San Francisco volcanic field and provide insight into magmatic processes.

Students will be strongly encouraged to prepare an abstract for a subsequent professional meeting and play a leading role in the eventual publication of project outcomes in peer-reviewed journals.

Large (50%) decreases in sensitivity with ^4He amount arising from space charge effects were observed but can be corrected for using an isotope dilution–like technique in which ^3He spike is added to a sample midway through the mass spectrometric analysis.

Large amounts of ^4He also cause the time evolution of the ^3He signal to become steeper, degrading precision of the initial peak height determination from the intercept. Farley (2010), Mass spectrometric 3He measurement in 4He-rich phases: Techniques and limitations for cosmogenic 3He dating of zircon, apatite, and titanite, Geochem.

We demonstrate the viability of the U–Th/He technique on basalts with ages of 373 ka and 3400 ka previously dated by Ar–Ar, and show that concurrent measurements of U–Th–He technique is applied to a series of boulders and flows from Box Canyon near Hagerman, Idaho, to evaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode of formation of the canyon.

The U–Th/He ages of the Box Canyon basalts are between 86 ± 12 ka and 130 ± 12 ka (1-sigma errors) and suggest at least two periods of eruption. 95 ka Ar–Ar age previously assigned, and distinguish the Box Canyon basalts from nearby ca. Exposure ages of boulders within the canyon at the mouth, head and middle of Box Canyon are 21 ± 1, 19 ± 3 and 48 ± 3 ka; scoured bedrock at the canyon head has an exposure age of 45 ± 5 ka.

Included are publications by current BGC Faculty arising from prior affiliations elsewhere. For reprints, please indicate the full citation (including author). Extremely rapid directional change during Matuyama-Brunhes geomagnetic polarity reversal.

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He exposure ages on olivine for a suite of basalt samples from the Snake River Plain, Idaho.

Student project tasks will include an initial phase of field work involving surficial mapping and sampling of scoria cones and lava flows, followed by preparation of mineral separates and laboratory measurement of helium isotopes in laboratories at the University of New Hampshire, Massachusetts Institute of Technology, and the Woods Hole Oceanographic Institution.

Students will then work on the interpretation of combined field observations and isotopic data to reconstruct a detailed chronology of volcanic events in the volcanic field.

Taken together we find that these effects preclude reliable analysis of samples with ^4He 1 μmol and that ^3He/^4He ratios of greater than ~5 × 10^(−10) are required to routinely measure ^3He to better than 20% precision.

We present some general considerations by which to assess the probability of success of measuring cosmogenic ^3He in these phases as a function of elevation, exposure age, and helium cooling age. Received 14 April 2010; accepted 29 July 2010; published 8 October 2010.

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