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Periodic Table--Aluminum

Aluminum has nine isotopes whose mass numbers range from 23 to 30. Only 27Al (stable isotope) and 26Al (radioactive isotope; t1/2 = 0.72x106 yr) occur naturally. 26Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminum isotopes have found practical applications in dating marine sediments, Mn-nodules, glacial ice, quartz in rock exposures, and the terrestrial age of meteorites. The ratio of 26Al to 10Be has been used to study the role of transport, deposition, sediment storage and burial times, and erosion on 105 to 106 yr timescales.

Cosmogenic 26Al were first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. After falling to Earth, atmospheric shielding protects meteorite fragments from further 26Al production, and its decay can then be used to determine a terrestrial age (Dicken, 1995).

Erosion rates calculated using 26Al of sediments are consistent with long-term rates of denudation estimated by using the volume of basin fills, the depth of basalt incision, and fission track analysis. Granger and Kirchner (1994 a,b) demonstrate that measurements of 10Be and 26Al in sediments currently being deposited on two small, northeastern California alluvial fans can be used to calculate basin scale erosion rates similar to those deduced considering fan volume and age. In situ produced 10Be and 26Al measured in quartz-rich sediment samples collected from several tectonically and climatically diverse environments have been shown to reflect basin-scale rates of erosion (Bierman, 1995). Brown et al. (1992) used a series of 10Be and 26Al measurements in a core to estimate erosion rates for quartz sandstone in the Quartermain Mountains. Nishiizumi et al. (1991) used 26Al and 10Be abundances to calculate maximum steady-state erosion rates. Measurement of nearly 60 samples of granite from the inselbergs of south central Australia (Bierman and Turner, 1995) shows that the tops of these landforms are eroding at rates only slightly higher than some Antarctic surfaces.

The preferential decay of 26Al during sediment storage and transport is well documented. The differences in 10Be/26Al ratios can also be used to understand qualitatively the transport history of some sediments. These studies suggest that measurement of 26Al in sediments may be a new tool for determining long-term rates of landscape change at the scale of drainage basins and mountain belts.

Source of text: This review was assembled by Eric Caldwell, primarily from Dicken (1995), Bierman et al. (1998) and Faure (1986).

References
• Bierman, P. and Turner, J. (1995). "10Be and 26Al evidence for exceptionally low rates of Australian bedrock erosion and the likely existence of pre-Pleistocene landscapes." Quaternary Res., 44: 378-382.
• Bierman, P.R., Albrecht, A., Bothner, M., Brown, E.T., Bullen, T., Gray, L.B. and Trupin, L. (1998). "Erosion, Weathering and Sedimentation." In: C. Kendall and J.J. McDonnell (Eds), Isotope Tracers in Catchment Hydrology. Elsevier, Amsterdam, pp. 647-678.
• Brown, E.T., Brook, E.J., Raisbeck, G.M., Yiou, F. and Kurz, M.D. (1992). "Effective attenuation of cosmic rays producing 10Be and 26Al in Quartz: Implications for exposure dating." Geophys. Res. Lett., 19, 4: 369-372.
• Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University Press, New York, 452 p.
• Evans, J.C., Rancitelli, L.A. and Reeves, J.H. (1979). "26Al content of Antarctic meteorites: implications for terrestrial ages and bombardment history." Proc. 10th Lunar Planet. Sci. Conf., pp. 1061-1072.
• Evans, J.C. and Reeves, J.H. (1987). "26Al survey of Antarctic meteorites." Earth Planet. Sci. Lett., 82: 223-230.
• Faure, G. (1986). Principles of Isotope Geology, Second Edition. John Wiley and Sons, New York. 589 p.
• Granger, D.E. and Kirchner, J.W. (1994a). "Erosional response to tectonic forcing inferred from cosmogenic isotopes in alluvial sediment." EOS Transactions, American Geophysical Union, 75, 44: 287.
• Granger, D.E. and Kirchner, J.W. (1994b). "Estimating catchment-wide denudation rates from cosmogenic isotope concentrations in alluvial sediment: Fort Sage Mountains, California." In: M.A. Lanphere, G.B. Dalrymple and B.D. Turrin (Eds.), Abstracts of the Eighth International Conference on Geochronology, Cosmochronology, and Isotope Geology, U.S. Geological Survey Circular-1107. U.S. Geological Survey Circular, pp. 116.
• Lal, D. (1985). "On the study of continental erosion rates and cycles using cosmogenic 10Be and 26Al and other isotopes." Dating Young Sediments, pp. 285-298.
• Nishiizumi, K., Kohl, C.P., Arnold, J.R., Klein, J., Fink, D. and Middleton, R. (1991). "Cosmic-ray produced 10Be and 26Al in Antarctic rocks: exposure and erosion history." Earth Planet. Sci. Lett., 104: 440-454.
• Nishiizumi, K., Kohl, C.P., Shoemaker J.R., Arnold, J.R., Klein, J., Fink, D. and Middleton, R. (1991). "In situ 10Be and 26Al exposure ages at Meteor Crater, Arizona." Geochim. et Cosmochim. Acta, 55: 2699-2703.
• Nishiizumi, K., Kohl, C.P., Arnold, J.R., Dorn, R., Klein, J., Fink, D., Middleton, R. and Lal, D. (1993). "Role of in situ cosmogenic nuclides 10Be and 26Al in the study of diverse geomorphic processes." Earth Surf. Proc. Landforms, 18: 407-425.
Related Links
• Periodic Table
• Fundamentals of Stable Isotope Geochemistry
• General References
• Isotope Publications
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