Elemental and Sr Isotope Investigations of Human Tooth Enamel by Laser Ablation-(MC)ICP-MS: Successes and Pitfalls & Anthropological Applications Antonio Simonetti Dept. Civil & Environmental Engineering & Earth Sciences University of Notre Dame, Notre Dame, Indiana 46556 USA Sr isotope investigations •  Tracing magma/mantle processes •  Tracing ancient civilization migrations in sites of Archaeological interest •  Hydrothermal activity/diagenetic processes •  Groundwater research Radioactive Decay - The Basic Equations Total number of daughter atoms in system undergoing decay is: D = Do + D* where D = total; Do = original; D* = number produced by decay  As D* = N (eλt – 1), then: D = Do + N (eλt - 1) λ = decay constant t = age of rock, mineral Basic equation for age determination of rocks & minerals. Writing decay equation using a ‘real” example, such as the decay of 87Rb to 87Sr: 87Sr = 87Sro + 87Rb (eλt – 1) However: Much easier and more meaningful to measure the ratio of two isotopes rather than the absolute abundance of one (using a MC-ICP-MS instrument). Radioactive Decay - The Basic Equations Therefore, 87Sr is normalized to a non-radiogenic isotope, i.e. 86Sr. Thus, the useful form of the decay equation is: Radioactive Decay - The Basic Equations 87 Sr 86 Sr = 87 Sr 86 Sr ! " # $ % & Initial + 87 Rb 86 Sr (eλt −1) BACKGROUND •  Water/sediments/rocks contain elements that have radiogenic isotopes that formed by the decay of their long-lived radioactive parent nuclides in the rocks of the continental crust. •  Thus, the isotopic composition of these elements (e.g., Sr) in water and soils depend on the age and parent-daughter ratios of the bedrock exposed to weathering in the drainage basins of the continents. Sr isotope compositions – Soils/ SurfaceWater/ Upper crust •  Depends upon: –  the 87Sr/86Sr ratios and Sr concentrations of each rock type present in drainage basin; –  the area of surface exposure of the different rock types; –  the susceptibility to chemical weathering of the minerals contained within the rocks; –  mixing of water derived from different rock types within streams entering the basin 87Sr/86Sr compositions - Advantages •  Unlike chemical compositions, Sr isotope ratios are NOT fractionated/varied by: –  Changes in temperature, pH, etc; –  Biological activity –  However, Sr isotopes can monitor effectively mixing between different components Sr isotope compositions – terrestrial reservoirs 87Sr/86Sr isotope compositions of: •  Present-day MORB (Mid-Ocean Ridge Basalt) = 0.7020 – 0.7025 •  Old (>2.7 billion year-old) granite = >0.7200 •  Present-day seawater has a 87Sr/86Sr value of 0.7092 87Sr/86Sr compositions – Analytical Considerations Outline •  Ramos et al. (2004) undertook a thorough evaluation of potential elemental and molecular interferences including Ca dimers and Ca argides, Fe dioxides, Ga and Zn oxides, doubly charged REEs and Hf, and singly charged Kr and Rb. •  Critical interferences include Kr, Rb, and doubly charged Er and Yb ions, while molecular species have only a limited impact on Sr isotope ratios. •  Demonstrate the accuracy with analyzed minerals, including marine carbonate, plagioclase, and clinopyroxene, which offer differing concentrations of interfering elements. •  Address potential complications and pitfalls associated with the technique and LA-MC-ICPMS in general. Collector Configuration – Ramos et al. (2004) Collector Configuration – Paton et al. (2007) Collector Configuration – In-situ Sr, University of Notre Dame Methodology (Ramos et al. 2004) •  UP213 nm laser ablation system coupled to Neptune MC-ICP-MS •  Employed rastering – troughs of 160 x 500 microns, or 80 x 500 microns (using a 80 micron spot size) •  Depth of penetration ~ 70 to 130 microns •  He gas was flushed into laser ablation cell at a rate of ~0.90 L/min Methodology (Ramos et al. 2004) •  Sample (Ar) gas flow rate was ~0.7 L/min •  88Sr ion signal – minimum value of ~1.0 volt •  Generate precision of <0.00005 – standard error 2 sigma level on the 87Sr/86Sr ratio •  Baseline measurements were conducted “onpeak” for 180 seconds Interferences •  Ca dimers (e.g., 44Ca43Ca+ ) have been shown to interfere with Sr isotope masses during secondary ionization mass spectrometry (SIMS) measurements of carbonate and aragonite (Weber et al., 2004 ). •  Waight et al. (2002) suggest that Ca argides (e.g., 44Ca40Ar+ ), present as a result of Ca ionization in the argon plasma, also interfere with Sr isotope masses when analyzing materials characterized by high Ca/Sr ratios such as carbonate (~500) and plagioclase (~50–200). Ramos et al. (2004) Ramos et al. (2004) Interferences •  Erbium (Er): •  Forms singly- (Er+) and doubly-charged (Er2+) ions in plasma; the latter are problematic since mass (m)/charge (z) of Er2+ ions overlaps that of Rb, Sr and Kr •  168Er2+ overlaps 84Sr and 170Er2+ overlaps 85Rb Ramos et al. (2004) Interferences •  Ytterbium (Yb): Ramos et al. (2004) Anthropological Investigations Sr isotope analysis of tooth enamel – why? •  Mature dental enamel is substantially denser and less porous than other skeletal tissues- stable and more resistant to structural and chemical change; •  Sr isotope ratios measured in dental enamel reflect various periods of life dependent on the tooth type sampled, ranging from the time in utero to approximately sixteen years of age; •  Sr incorporated is more likely an average of several months or years of strontium ingestion due to the long residence time in the body; Study Area & Regional Geology Buzon et al. (2007, J. Archaeol. Sci., 34:1391-1401) Advantages of (MC)-ICP-MS instrumentation •  Typical Sr isotope analysis by TIMS (Thermal Ionization Mass Spectrometry) takes ~1.5 to 2 hours to complete •  Typical Sr isotope analysis by solution-mode MC-ICP-MS takes ~15 minutes (up to 8 times faster) with little (if any) detriment to the quality of the individual measurements •  Trace element analyses conducted either by solution- or laser ablation modes using a quadrupole ICP-MS instrument also consist of relatively rapid measurements (few minutes) •  Sr isotope measurements by LA-MC-ICP-MS are also extremely rapid (minutes); however are they accurate?? Analytical Methods (details in Simonetti et al., 2008. Archaeometry) •  Quadrupole-ICP-MS (Perkin Elmer ELAN6000): –  Trace element abundances via both solution mode & laser ablation analysis •  Multi-collector-ICP-MS (NuPlasma Instrument): –  Sr isotope measurements via both solution mode & laser ablation analysis •  New Wave Research UP213 laser ablation system 160 µm Laser Ablation Trace Element Analysis – Brief Outline •  NIST SRM 612 international glass standard used for external calibration – with normalization of intensities to 43Ca •  GLITTER® laser ablation software – data reduction, concentration determinations, detection limits, internal uncertainties •  Validation of elemental abundances verified with ‘internal’ standard – Durango Apatite •  Similar analytical protocol and internal standard as described by Trotter & Eggins (2006, Chem. Geol.) •  Durango Apatite: Ca5(PO4)3(F, Cl, OH) •  Enamel - hydroxyapatite: 3Ca3(PO4)2.CaX ; where X = F, Cl, CO2, OH Comparison laser ablation vs. solution mode – Durango Apatite -40 -30 -20 -10 0 10 20 30 40 Mg Mn Fe Ge Rb Sr Y Sb La Ce Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Pb U Elements %Difference Durango Apatite – This study vs. Trotter & Eggins (2006) 1 10 100 1000 10000 100000 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Chondritenormalized Durango - Canberra Australia Durango - average U of A Comparison trace element abundances – Laser ablation (LA) vs Solution Mode (SM) for enamel 1000 10000 100000 1000 10000 100000 10 100 1000 10 100 1000 Sr(ppm)-LA Sr (ppm) - SM A 1:1 0 1 10 100 0 1 10 100 Cu(ppm)-LA Cu (ppm) - SM 1:1 B D 1:1 0.0 0.1 1.0 0.0 0.1 1.0 C 1:1 Rb (ppm) - SM Rb(ppm)-LA Mg (ppm) - SM Mg(ppm)-LA Simonetti et al. (2008) Pb, Zn, Fe MC-ICP-MS Sr Isotope Analyses •  Monitor isobaric interferences: 84Kr, 86Kr, 85Rb→87Rb, 40Ar+(16O)3→88O •  Monitor invariant 84Sr/86Sr values → 0.0565 in nature –  Schmidberger et al. (2003, Chem. Geol.); Bizzarro et al. (2003, Geochem. Cosmochim. Acta) •  Laser spot size used was 160 microns NIST SRM 987 Sr isotope standard solution-mode - 100 ppb solution 0.71016 0.71018 0.71020 0.71022 0.71024 0.71026 0.71028 0.71030 0.71032 0 2 4 6 8 10 12 14 8786 Sr/Sr Analysis Average = 0.710242 ± 41 (2 )σ STDEV 84Sr/86Sr= 0.0565 ±4 (2σ STDEV) 0.710245 - accepted Laser ablation Sr isotope measurements – Modern-day Coral 0.7090 0.7091 0.7092 0.7093 0.7094 0.7095 0 10 20 30 Analysis 8786 Sr/Sr Weighted mean = 0.70924 ±1.4 (2σ) 84Sr/86Sr= 0.0565 ±1 (2σ STDEV) Comparison 87Sr/86Sr isotope values – LA- vs SM-MC-ICP-MSSample# 87 86 Sr/ Sr 0 10 20 30 40 50 60 0.7070 0.7075 0.7080 0.7085 0.7090 0.7095 0.7100 0.7105 LA-MC-ICP-MS SM-MC-ICP-MS Simonetti et al. (2008) What is the cause of the offset? Rb? 0.000 0.001 0.002 0.003 0.000 0.002 0.004 0.006 0.008 0.010 87 86 Rb/ Sr DifferenceSr/Sr:LA-SM 8786 87Rb isobaric interference with 87Sr However, no correlation between offset & Rb/Sr values What is the cause of the offset?? REEs + Y? •  Solution mode-ICP-MS analysis of all of the enamel samples investigated in this study indicate extremely low concentrations of REEs and Y •  These extremely low concentrations were confirmed by subsequent laser ablation analyses (i.e. all below detection limit) Average values (n= 37 samples) ppm Y 0.02 La 0.008 Ce 0.012 Nd 0.007 Sm