DETECTING DIAGENESIS: ~（87）Sr/~（86）Sr ANALYSIS OF ARCHAEOLOGICAL BIOAPATITES BY LA-MC-ICPMS TO ASSESS DIAGENETIC UPTAKE OF Sr IN ENAMEL

摘要

Sr isotope analysis is a popular method for characterising mobility and migrations in archaeologicalpopulations. The method has its basis in the geological variability in the 87Sr/86Sr ratio of Sr inrocks as a function of age and initial Rb/Sr ratio and the observation that as this geological Sr iscycled into the food chain via soils and plants the mass fractionation corrected ratio of 87Sr to 86Srin the biologically available Sr is preserved. In mammals, Sr is concentrated in bones and teethwhere it replaces Ca in the (bio)-apatite lattice and thus calcified tissues preserve a record of wheredietary Sr was being sourced during mineralisation. When this analysis is applied to archaeologicalpopulations and with suitable characterisation of biologically available 87Sr/86Sr this allowsarchaeologists to determine the presence allochthonous individuals in populations and assessmobility and migrations.The basis of archaeological strontium isotope analysis is that the strontium that is analysed in bonesand teeth is of biogenic origin and that this Sr has not been contaminated by exogeneous Sr fromthe burial environment. Bone and tooth dentine have been shown to be problematic for recoveringbiogenic Sr isotope signals however enamel has been shown to be relatively resistant to diagenesis.Nevertheless, tooth enamel is chemically similar to bone and dentine (both mineralogicallyapproximate carbonated hydroxyapatite) and is in contact with the same burial environment.When considering the possibility of diagenetic uptake of strontium into archaeological enamel onemay consider that diagenetic strontium may not be equally distributed across the whole thicknessof an enamel section, with greater amounts of diagenetic strontium at the edges of the enamelcompared to the enamel core. Thus, it is desirable to work with a techniques that can detectdiagenetic Sr in spatially resolved manner with a spatial resolution in the 100 μm range.Furthermore, detection of diagenetic Sr in enamel is more compelling if one can measure both theSr concentration and the 87Sr/86Sr ratio of altered enamel relative to pristine enamel. This, incombination with samples where one can make a priori assumptions about either the diagenetic orbiogenic 87Sr/86Sr with a high degree of certainty allows for conclusive identification of diageneticSr in enamel.LA-MC-ICPMS would appear to be ideally suited to detecting diagenetic Sr in archaeologicalenamel and is able to determine Sr concentrations and 87Sr/86Sr to respectable precisions at spatialresolutions in the ~ 100 μm range. However, making accurate measurements of 87Sr/86Sr inbio-apatites by LA-MC-ICPMS is an extremely challenging analysis which is beset by a complexseries of interferences on the isotopes of interest.Interference species are numerous but come principally from the gases used to generate the plasmaion source (Kr+) and the from the sample matrix being ablated. Sample derived interferences rangein nature from singly charged isobaric interferences (87Rb+) and doubly charge rare earth elements(e.g., 174Yb++)which are present in the singly charged Sr mass range through to more exotic plasma species such as Ca and Ar dimers (~（40）Ca,~（40）Ar-~（44）Ca~+). The final problematic interference for ~（87）Sr/~（86）Sranalyses is 40Ca(40Ar)31P16O+ on 87Sr+ which is responsible for small yet consistent offsets in~（87）Sr/~（86）Sr on teeth measured by LA-MC-ICPMS relative to the known values.In this presentation we describe the analytical complications of undertaking 87Sr/86Sr analysis bylaser ablation, how tuning of the plasma conditions and modifications to the ICP interface canimprove the accuracy of analysis and the need for well characterised standards with a range of Srconcentrations. We then present the results of ~（87）Sr/~（86）Sr analysis of a series of Pleistocene age largemammal teeth recovered from the North Sea and use these teeth to assess diagenetic strontiumuptake into archaeological enamel.