Beryllium-10 is produced in carbonate rocks by spallation of both carbon and oxygen. Successful measurement of 10Be in carbonates would allow high-resolution surface exposure dating due to a high production rate, as well as the ease of preparing large samples. In addition, the simultaneous measurement of 10Be and 36Cl would allow burial dating over timescales of 105 years.
Previous work in meteorites has demonstrated that the production rate of 10Be from C is 2-4 higher than from O. The production rate at sea-level, high-latitude is thus expected to be 9-14 at/g/yr in calcite. However, recent work by Braucher et al. (2005) shows production rates a factor of 3 higher.
To determine production rates in calcite, we measured 10Be in three samples collected from a Roman-age marble quarry near Carrara, Italy. Despite thorough sample preparation, only one of the samples yielded a production rate near the expected value. The other two showed high levels of contamination with meteoric 10Be that was incorporated into the calcite crystal. Inspection of the rocks under ultraviolet light revealed that the two contaminated samples contained zones of brightly fluorescent fulvic compounds within recrystallized calcite. The uncontaminated sample showed no fluorescence. We hypothesize that 10Be is transported into the marble while adsorbed onto fulvic compounds derived from soil dripwater. Laboratory tracer experiments confirmed that organic-rich water transports beryllium through a crushed calcite medium.
If we assume that our lowest measured sample contained no meteoric 10Be, and that the marble quarry is 2000 years old, then we obtain a SLHL production rate of ~12 at/g/yr. This value indicates 10Be production from C is 3.6 times higher than from O, in good agreement with expectation but inconsistent with the values reported by Braucher et al. (2005).
Successful measurement of 10Be in carbonate rocks will require efficient identification and separation of secondary carbonate, even deep beneath the rock surface. One potential solution is to use the mineral dolomite rather than calcite, which dissolves less readily and is not precipitated as a secondary mineral. Primary and secondary minerals are thus easily separated by selective dissolution. This method shows significant promise for dating marble and dolomite quarries and buildings in archaeology.
References: Braucher, R., Benedetti, L., Bourles, D. L., Brown, E. T., and Chardon, D., 2005, Geochimica et Cosmochimica Acta, v. 69, p. 1473-1478.
See more of New Developments in Terrestrial Cosmogenic Nuclide Research
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)