ARTICLE IN PRESS. Measuring 87 Sr/ 86 Sr variations in minerals and groundmass from basalts using LA-MC-ICPMS

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1 DTD 5 Chemical Geology xx (2004) xxx xxx Measuring 87 Sr/ 86 Sr variations in minerals and groundmass from basalts using LA-MC-ICPMS Frank C. Ramos a, *, John A. Wolff b, Darren L. Tollstrup a a Department of Earth Sciences, University of California, Santa Cruz, Santa Cruz, CA, 95064, USA b Department of Geology, Washington State University, Pullman, WA , USA Received 28 May 2003; accepted 25 June 2004 Abstract We outline a technique which uses laser ablation (LA) sampling and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) to obtain relatively accurate in situ Sr isotope ratios in a range of common geologic materials. We have undertaken 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. To demonstrate the accuracy of this technique, we have analyzed minerals, including marine carbonate, plagioclase, and clinopyroxene, which offer differing concentrations of interfering elements. Unlike most previous studies, we have chosen samples that are not abnormally enriched in Sr to show the potential utility of this technique. In addition, we offer the first in situ LA-MC-ICPMS analyses of fine-grained basaltic and basaltic andesite groundmass, which are critically dependent on accounting for interfering Rb and REEs. We also address potential complications and pitfalls associated with the technique and LA-MC-ICPMS in general. Current results are more accurate than any previously attained on a wider range of materials and will allow for detailed assessments of additional interferences, baseline inaccuracies, and potential pit-related or material-related fractionation. The accuracy and reproducibility of Sr isotope ratios obtained with this technique will allow for the identification of processes involved in magmagenesis, the determination of crystal residence ages, and aid in studies involving biological materials such as otoliths, bones and teeth. D 2004 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Strontium isotopes; Basalts; In situ analyses; Microanalysis 1. Introduction * Corresponding author. Tel.: ; fax: address: framos@es.ucsc.edu (F.C. Ramos). Technological advances in both laser ablation (LA) sampling systems (Jefferies et al., 1998; Gunther and Heinrich, 1999; Gunther, 2001) and inductively coupled plasma mass spectrometry /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.chemgeo CHEMGE-14458; No of Pages 24

2 2 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx (ICPMS) now allow for in situ analysis of isotope ratios in a range of geological and biological materials (e.g., Latkoczy et al., 1998; Stirling et al., 2000; Thorrold and Shuttleworth, 2000; Jackson et al., 2001; Machado and Simonetti, 2001; Bacon et al., 2004). Specifically, the combination of laser ablation sampling with multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) has greatly enhanced our ability to more accurately measure Sr isotopes in situ. The potential of such measurements to address geological problems was demonstrated by early workers (Christensen et al., 1995; Davidson et al., 2001) and is also apparent from alternative, more time-consuming microsampling techniques such as microdrilling (Davidson et al., 1998; Tepley et al., 1999; Muller et al., 2000). Typically, the barrier to more widespread use of in situ laser ablation analysis has been its inability to attain similar accuracy of Sr isotope measurements to those obtained by thermal ionization mass spectrometry (TIMS). The benefits of laser ablation analyses, however, include spatial control and rapid Sr isotope ratio measurements, offering significant time savings compared to alternative techniques. It has only been recently that analyses utilizing laser ablation sampling have approached the level of accuracy needed in many geological investigations, although this accuracy is highly variable depending on the materials targeted (Waight et al., 2002). Incremental advances now allow the utility of in situ Sr isotope ratio measurements by LA-MC-ICPMS to be realized, although most investigators have focused on materials that have low abundances of interferants (elements or molecules that have masses that overlap those of Sr isotopes) and high Sr contents (Davidson et al., 2001; Bizzarro et al., 2003; Schmidberger et al., 2003). We introduce a laser ablation analysis technique in which in situ Sr isotopes can be measured with accuracies that in some cases approach those obtained by TIMS in a range of materials, including carbonate, plagioclase, clinopyroxene, and low Rb/Sr fine-grained basaltic groundmass. We emphasize that the technique can be used to successfully measure Sr isotopes in minerals, such as plagioclase (Sr=700 to 900 ppm) from tholeiitic basalts, that are not unusually enriched in Sr. Hence, the method can be applied to a wide variety of igneous rocks. 2. Analytical procedures Procedures were developed using a commercially available New Wavek UP213 nm Nd:YAG Laser Ablation System (Roy and Neufeld, 2004; Jackson, 2001) in conjunction with a double focusing Thermo- Finnigan Neptunek MC-ICPMS equipped with 9 Faraday collectors and V resistors (operating parameters are given in Tables 1 and 2). The UP213 nm laser has uniform laser energy distribution and does not suffer from many of the sample-laser coupling problems encountered with its 266 nm predecessor (Gunther, 2001). In addition, the Neptune offers high sensitivities, low backgrounds, and extremely stable electronics. All measurement and technique discussions refer to these two systems specifically, although many of the same technical aspects can be integrated into methods using comparable equipment UP213 nm Nd:YAG laser The methodology of laser ablation sampling is driven by three main factors: (1) attaining high Sr Table 1 Operating parameters of the ThermoFinnigan Neptunek MC- ICPMS RF power 1200 W Argon cooling gas flow rate 15 l/min Auxiliary gas flow rate 0.8 l/min Interface cones Nickel Acceleration voltage 10 kv Ion-lens settings Optimized for maximum sensitivity and optimal peak shape Mass resolution 400 Mass analyzer pressure mbar Detection system Nine Faraday collectors Sampling mode 3 or 10 blocks of 108 s integrations for laser and solution analysis, respectively Background/baseline 3 min on peak in 2.0% HNO 3 determination Nebuliser Glass cyclonic spray chamber fitted with a Micromist PFA nebuliser Uptake mode Free aspiration Sample uptake rate 50 Al/min Typical sensitivity on 88 Sr V/ppm (10 11 V resistors) Ar sample gas flow rate l/min, optimized to maximize 88 Sr signal Beam dispersion 25 (Dispersion Quad)

3 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx 3 signals, (2) accurately accounting for interfering elements and molecules, and (3) minimizing elemental and potential isotopic fractionations generated from the laser ablation process, including fractionation effects generated from the laser pit. We typically sample Am or Am btroughs,q or 80 Am diameter pits. We prefer to use troughs in order to maximize Sr signals while minimizing the depth of the laser pit. To attain typical results (e.g., 87 Sr/ 86 Sr 2 S.E. of b ), the sampling trough is usually from 70 to 130 Am deep depending on the target material. Although analyte intensities decrease as pits deepen, large pit-induced fractionations are not generally observed until trough depths exceed ~200 Am. We use helium as a carrier gas (~0.9 l/min, optimized during every analytical session) flushed through the laser ablation sample cell (Eggins et al., 1998), and mix this gas/sample mixture with ~0.7 l/min of argon prior to introduction of sample into the plasma. Laser ablation Sr isotope analyses consume much more sample than LA trace element analyses (e.g., Kent et al., 2002)as 88 Sr beam intensities must exceed ~1 V to attain reasonable measurement precisions. Smaller amounts can be measured depending on the precision required to address specific scientific problems. For example, an analysis of a Sr-poor phase such as clinopyroxene from a tholeiitic basalt (typically b100 ppm Sr) is characterized by errors that are ~4 times greater than for a relatively Sr-rich phase such as plagioclase. Thus, the likely dynamic range of Sr isotope variation in the problem to be addressed must be considered in assessing the applicability of LA-MC-ICPMS Sr isotope analyses Sample solution analyses To evaluate the accuracy and reproducibility of Sr isotope measurements, we measured ~1 ml aliquots of a 100 ppb solution of NBS987 Sr carbonate dissolved in 2% HNO 3 (~100 ng total Sr consumed/measurement). Samples obtained from microdrilling ~400 Am diameter holes were also analyzed using either TIMS or MC-ICPMS to compare with LA-MC-ICPMS results from the same samples. Microdrill samples, dissolved using distilled HF, HNO 3, and HCl in teflon digestion vessels, were purified using cation exchange chromatography (Ramos, 1992) with total process blanks of V150 pg. Analyses of sample and standard solutions consist of 100 total ratios with each ratio measured in 8 s integrations. Baselines are measured on-peak while free aspirating clean 2% HNO 3 using a 50 Al/min nebulizer. Typical 88 Sr backgrounds climbed from b1 mv to generally V5 mv during an analytical session but were relatively constant during a single analysis. 88 Sr beam intensities ranged from 4 to 7 V (100 ppb solutions) depending on the cone and torch configurations used. All 9 Faraday collectors were used to collect isotope signals and the resulting data were reduced as described in the Data reduction section Laser ablation analyses Samples targeted for LA-MC-ICPMS analyses were obtained from (1) cutting ~3 mm thick disks from 2.5 cm diameter cores drilled from hand samples, (2) thin-section billets, and (3) single grains separated from crushed whole rocks and mounted in 2.5 cm rings filled with epoxy. All samples were then polished to a 1 Am finish and cleaned in alcohol. Analyses of laser ablated samples consist of 30 ratios with each ratio measured in 8 s integrations. We attempted to attain maximum 88 Sr signals with the laser parameters outlined in Table 2. For a typical plagioclase (~600 ppm Sr), a Am trough yielded ~7 Ag of Sr. Calculated abundances of Sr consumed during the laser ablation analysis was ~0.2 Ag (compared to ~0.1 Ag Sr for 100 ppb Sr solutions), indicating that only ~3% of the ablated Sr was successfully transferred and ionized in the plasma. Baselines for analyses were measured on-peak for ~180 s while flushing the sample chamber with He mixed with Ar as in a normal analysis (no aspirated Table 2 Operating parameters of the UP213 New Wavek laser Wavelength 213 nm Spot dimensions (aperture mode) Am trough or Am trough or 80 Am diameter spot Energy density ~7 10 J Pulse rate 10 Hz Carrier gas Helium Stage speed Average ~70 Am/s He auxiliary gas flow rate (for analysis of Sr isotopes in plagioclase) 0.85 to 0.95 l/min, optimized every run session

4 4 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx solutions were introduced during LA analyses), but without firing the laser. Typical baselines were V5 mv ( 88 Sr) depending on the length of the analytical session, with the bulk of background Sr signals originating from material accumulation on the sample and skimmer cones. 3. Interferences The most challenging aspect of obtaining accurate analyses of Sr isotopes using LA sampling lies in accounting for elements and molecules that interfere with Sr isotope masses. This is no different for solution measurements with the exception that most solutions undergo chromatographic purification to reduce the amounts of interfering elements. For LA analyses, much greater amounts of interfering elements and molecules must be accounted for, so it is critical that corrections for such interferences are highly effective. There are many potential interfering elements and molecules which could overlap Sr isotope masses (Waight et al., 2002; Bizzarro et al., 2003; Schmidberger et al., 2003). We focus on critical interferences that influence 87 Sr/ 86 Sr by N For Sr isotopes in LA samples presented here, critical interferences include Kr, Rb, and doubly charged Er and Yb. In addition, we evaluate Ca dimers and Ca argides, Fe dioxides, Ga and Zn oxides, and doubly charged Hf, but find most of these to have only limited impact on Sr isotope measurements of the minerals and groundmass in this study. The collector array (Table 3) reflects our best efforts to monitor these critical interferences while still measuring all Sr isotope masses. Below we summarize potential interfering elements/masses encountered during Sr isotope analyses and evaluate their impact on measured Sr isotope ratios NBS987 Sr carbonate standard solutions Results encompassing a one-year period of 100 ppb NBS987 Sr solution analyses are presented in Fig. 1. Similar to LA analyses, raw intensities were exported and reduced offline as described in the Data reduction section. The means of NBS987 solutions are F37 (2r) and F90 (2r) for 87 Sr/ 86 Sr and 84 Sr/ 86 Sr, respectively, which are similar to previously determined values (Thirlwall, 1991; Royse et al., 1998). The 87 Sr/ 86 Sr mean is ~ lower than interference uncorrected data, but is more appropriate for comparative purposes as all laser data are reduced offline using the same data reduction program. Results demonstrate that the Finnigan- Neptunek generates highly accurate and reproducible results for 100 ppb NBS987 Sr solutions and attest to the outstanding overall performance of the mass spectrometer. These results also offer a foundation on which we can evaluate the impact of potential interferences (note that half-mass on-peak baselines for pure NBS987 Sr solutions were considered to be noise on the baseline, see below) Ca dimers and Ca argides Ca dimers (e.g., 44 Ca 43 Ca + ) have been shown to interfere with Sr isotope masses during secondary ionization mass spectrometry (SIMS) measurements of carbonate and aragonite (Weber et al., 2004). In addition to Ca dimers, Waight et al. (2002) suggest that Ca argides (e.g., 44 Ca 40 Ar + ), 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). We have undertaken a series of tests to evaluate this possibility using 100 ppb NBS987 Sr solutions doped with Sr-purified and Table 3 Collector block configuration of the ThermoFinnigan Neptunek MC-ICPMS used for both solution and LA-MC-ICPMS Sr isotope analysis Collector L4 L3 L2 L1 C H1 H2 H3 H4 Mass Isotope of interest 83 Kr 11.5% 167 Er Sr 0.56% 85 Rb 72.2% 171 Yb Sr 9.86% 173 Yb Sr 7.00% 88 Sr 82.6% Isobaric interferences 84 Kr 57.0% 86 Kr 17.3% 87 Rb 27.8% Er Er Er Er 2+ Yb Yb Yb Yb Yb Yb 2+ Monitored species and interferences affecting the Sr masses are also illustrated along with natural abundances for Sr, Rb and Kr.

5 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx 5 Fig. 1. Measured 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios of 100 ppb NBS987 Sr solutions normalized to 86 Sr/ 88 Sr= Error bars for individual analyses are 2 S.E. (2 standard in-run errors). Mean values of measured 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios are represented by solid horizontal lines with F2r errors represented by dashed horizontal lines. Results cover an analytical period of 12 months and all results have been reduced using the same data reduction program used on LA analyses. Results indicate that highly accurate and reproducible Sr isotopes ratios can be obtained using the Neptune MC-ICPMS. Sr-unpurified ICP standard Ca (Fig. 2A; Ca used in Sr-purified Ca-doped solutions was purified using Sr microchemistry from Wolff et al., 1999). We attempted to measure Ca-dimer ratios directly using pure Ca solutions but Ca-dimer intensities were so low that variable on-peak blanks, resulting from large plasma loads, generated highly variable Ca-dimer ratios. Thus, we instead analyzed Ca-doped Sr NBS987 solutions. Results for accompanying 100 ppb NBS987 Sr standard solution analyses without Ca are included for comparative purposes. Sr-purified and Sr-unpurified Ca-doped Sr solutions, which have Ca/Sr ratios that range from 50 to 550, reflect similar 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios indicating that the ICP standard Ca is highly pure in regard to Sr. Results for Sr-purified Cadoped solutions and undoped NBS987 Sr solutions (five total runs each) are identical, although Srpurified Ca-doped solutions have a wider overall range of 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios, as indicated by greater 2r values. In contrast to Waight et al. (2002), these results suggest a very limited role for interfering Ca dimers and Ca argides. As a result, we have not corrected laser ablation results for either of these species Fe dioxides Fe dioxides (e.g., 54 Fe 16 O + 2 ) may also interfere with Sr isotope masses (Schmidberger et al., 2003) in minerals characterized by high Fe/Sr ratios (e.g., clinopyroxene). To evaluate this possibility we have analyzed 100 ppb NBS987 Sr solutions doped with unpurified ICP standard Fe. Fe-doped solutions have Fe/Sr ratios that range from 50 to 200. Measured Sr isotope ratios in these solutions are compared to undoped NBS987 Sr solutions in Fig. 2B. As with Ca molecular species, results indicate only a limited role for interfering Fe dioxides. As a result, we have not corrected laser ablation results for these Fe species. We note, however, that Sr isotope ratios in both Fe- and Ca-doped solutions have broader overall ranges (i.e., larger 2r) as

6 6 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Fig. 2. Measured 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios of Ca- and Fe-doped 100 ppb NBS987 Sr solutions. (A) Sr isotope ratios of Sr-unpurified Cadoped solutions (unfilled squares) and Sr-purified Ca-doped solutions (filled squares) are similar suggesting that the ICP standard Ca is highly pure. Error bars for individual analyses are 2 S.E. and mean 87 Sr/ 86 Sr and 84 Sr/ 86 Sr values of purified Ca-doped solutions are shown with 2r errors (n=5) and represented by solid horizontal lines. Gray fields represent results for undoped NBS987 Sr solutions analyzed with these Cadoped NBS987 Sr solutions. Both Sr-purified and Sr-unpurified Ca-doped solutions cluster near ratios measured for undoped 100 ppb NBS987 Sr solutions suggesting that Ca dimers and Ca-argides do not significantly alter Sr isotope ratios in solutions with Ca/Sr ranging from 50 to 550. (B) Sr isotope ratios of unpurified Fe-doped NBS987 Sr solutions (unfilled squares) with Fe/Sr ratios of 50 to 200. Error bars and gray fields are the same as in (A) and the mean values are represented by solid horizontal lines. Results for Fe-doped solutions cluster near ratios measured for undoped 100 ppb NBS987 Sr solutions suggesting that Fe dioxides do not significantly alter Sr isotope ratios. compared to undoped NBS987 Sr solutions. We attribute these variations to the greater overall loads (e.g., Ca ranging from 5 to 55 ppm in Ca-doped NBS987 Sr solutions) introduced into the plasma which are not present in undoped NBS987 Sr solutions.

7 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Ga and Zn oxides Ga and Zn are elements that form oxides (e.g., 71 Ga 16 O +, 68 Zn 16 O + ) which may also interfere with Sr isotope masses. Although these elements are not enriched in many minerals, they are present in basaltic groundmass. We have undertaken tests to evaluate potential interferences of Ga and Zn oxides on Sr isotopes using Ga- and Zn-doped 100 ppb NBS987 Sr solutions. Sr isotope ratios of Ga-doped 100 ppb NBS987 Sr solutions with Sr/Ga of 20 and 10 are indistinguishable from undoped NBS987 Sr solutions. In addition, results for Zn-doped 100 ppb NBS987 Sr solutions with Sr/Zn of 5 and 1 are also indistinguishable from undoped NBS987 Sr solutions. These Sr/Ga and Sr/Zn ratios encompass the values in our groundmass samples and, as a result, we have not corrected Sr isotopes for either of these oxide species Erbium Erbium is a rare earth element that forms both singly (Er + ) and doubly charged (Er 2+ ) ions in the plasma. Singly charged Er ions appear at whole-mass positions (z162) that are much greater than Sr isotope masses and are thus not problematic during Sr isotope analyses. Doubly charged Er ions, however, appear at the same masses as singly charged Sr, Rb, and Kr isotopes due to equivalent mass/charge ratios (e.g., 162 Er 2+ appears at mass 81; Longerich and Diegor, 2001). Erbium has two isotopes that are problematic, 168 Er 2+ which interferes with 84 Sr and 170 Er 2+ which interferes with 85 Rb. Fig. 3 illustrates the effects of increasing Er contents (Sr/Er=12.5 to 200) on Eruncorrected Sr isotope ratios of 100 ppb NBS987 Sr solutions. Firstly, 87 Sr/ 86 Sr ratios decrease with increasing Er as a result of inappropriate Rb corrections. Rubidium has two isotopes, 85 Rb and 87 Rb, of which 87 Rb interferes with 87 Sr. During Sr isotope analyses, ions measured at mass 85 are assumed to be 85 Rb and used to calculate the number of 87 Rb ions contributing to the total ions measured at mass 87. Any 87 Rb counts are then subtracted from the total counts measured at mass 87. The remaining ions are assumed to be 87 Sr and a Rb-corrected 87 Sr/ 86 Sr ratio is determined. When 170 Er 2+ is present, it appears at mass 85. During a Sr isotope analysis, 170 Er 2+ ions are incorrectly assumed to be 85 Rb ions, and an inappropriate 87 Rb subtraction is applied. This Rb correction drives 87 Sr/ 86 Sr ratios lower with increasing Er contents. Secondly, 84 Sr/ 86 Sr ratios increase with increasing Er as a result of 168 Er 2+ ions, which appear at their half-mass of 84. During Sr isotope analyses, ions measured at mass 84 are assumed to be 84 Sr. Thus, the presence of 168 Er 2+ has the effect of artificially increasing 84 Sr/ 86 Sr ratios. With increasing Er contents, 84 Sr/ 86 Sr ratios increase substantially (Fig. 3). The interfering effects of doubly charged 168 Er 2+ and 170 Er 2+ ions can be accounted for in two ways. The 167 Er 2+ ion intensity measured at mass 83.5 during a Sr isotope analysis can be used in conjunction with the 167 Er 2+ / 168 Er 2+ and 170 Er 2+ / 168 Er 2+ ratios determined on previously analyzed 1 ppm Er solutions (without Sr) to calculate 168 Er 2+ and 170 Er 2+ contributions to the 84 and 85 masses, respectively. Alternatively, these same contributions can be determined using the 167 Er 2+ intensity and natural Er isotope ratios (Chartier et al., 1999) corrected for the effects of mass spectrometer induced mass bias (we refer to these ratios as mass bias uncorrected Er 2+ ratios). We use the 171 Yb 2+ / 173 Yb 2+ ratio measured during the Sr isotope analysis (see below) and compare this with the natural 171 Yb/ 173 Yb ratio (Segal et al., 2003) to determine a per amu mass bias factor. This factor can then be applied to natural Er ratios to determine mass bias uncorrected Er isotope ratios. These mass bias uncorrected Er 2+ ratios and the 167 Er 2+ intensity can then be used to calculate 168 Er 2+ and 170 Er 2+ contributions to the 84 and 85 masses, and any contributions from these species can be removed prior to applying an accurate Rb correction and determining 84 Sr/ 86 Sr and 87 Sr/ 86 Sr ratios. It is important to note that discrepancies between these two correction procedures (and those for Yb 2+ ) may result from subtle misalignments of the Faraday collectors (see Beam dispersion for a more detailed discussion). Fig. 3 illustrates the effect of Er 2+ corrections on Er-doped 100 ppb NBS987 Sr solutions Ytterbium Ytterbium also forms doubly charged ions that appear at singly charged Sr and Rb masses (~1.8% of

8 8 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Fig. 3. Uncorrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (filled squares) and corrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (unfilled squares) of Er-doped 100 ppb NBS987 Sr solutions with Sr/Er of 12.5 to S.E. error bars are not shown and 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios have been normalized to using the measured 86 Sr/ 88 Sr ratios. Solid horizontal lines represent mean values of solutions (undoped NBS987 Sr solutions) as in Fig Sr/ 86 Sr ratios decrease as the result of 170 Er 2+ ions and consequent inappropriate Rb corrections while 84 Sr/ 86 Sr ratios increase as a result of 168 Er 2+ ions which are assumed to be 84 Sr. Subtle variations in 86 Sr/ 88 Sr ratios are also seen although Er 2+ ions are not present at either the 86 or 88 masses suggesting that 86 Sr/ 88 Sr variation result from instability of the mass spectrometer or differential loading of the plasma. Unfilled squares illustrate the effectiveness of Er 2+ corrections in solutions with Sr/Er down to ~10. Yb ions are doubly charged in pure 1 ppm Yb ICP concentration standard solutions). Yb has five isotopes that appear as doubly charged ions at masses 84, 85, 86, 87, and 88. Four of the five add directly to Sr isotopes at these masses and the fifth ( 170 Yb 2+ ) generates inappropriate Rb corrections (see Er discussion). Fig. 4 illustrates the effect of increasing Yb contents (Sr/Yb=12.5 to 200) on the Sr isotope ratios of 100 ppb NBS987 solutions. 87 Sr/ 86 Sr and 86 Sr/ 88 Sr ratios increase with increasing Yb contents and the 84 Sr/ 86 Sr ratio decreases. In order to correct for the effects of Yb 2+ ions, we monitor both 171 Yb 2+ and 173 Yb 2+, at masses 85.5 and 86.5, respectively. Similar to Er, we can use the 173 Yb 2+ intensity and Yb 2+ isotope ratios measured on 1 ppb Yb solutions to determine Yb 2+ ion contributions to Sr and Rb masses. Alternatively, these same contributions can be determined using the 173 Yb 2+ intensity and natural Yb isotope ratios (Segal et al., 2003) corrected for the effects of mass spectrometer induced mass bias as reflected in the 171 Yb 2+ / 173 Yb 2+ ratio similar to that done for Er 2+.

9 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx 9 Fig. 4. Uncorrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (filled squares) and corrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (unfilled squares) of Yb-doped 100 ppb NBS987 Sr solutions with Sr/Yb of 12.5 to S.E. error bars are not shown and solid lines are the same as in Fig Sr/ 86 Sr ratios increase as the result of the addition of Yb 2+ ions with a 174 Yb 2+ / 176 Yb 2+ ratio greater than ~ In addition, 170 Yb 2+ ions generate inappropriate Rb corrections that do not compensate for 174 Yb 2+ additions. 84 Sr/ 86 Sr ratios decrease as a result of adding Yb 2+ of a low 168 Yb 2+ / 172 Yb 2+ ratio. The presence of Yb 2+ ions also subtly increase 86 Sr/ 88 Sr ratios, compromising the accuracy of 86 Sr/ 88 Sr normalization. Unfilled squares illustrate the effectiveness of Yb corrections in solution with Sr/Er up to ~10. Contributions from Yb 2+ ions to the Sr and Rb masses are then calculated using these mass bias uncorrected Yb 2+ ratios and the 173 Yb 2+ intensity. These contributions can then be subtracted from the measured intensities of Sr and Rb, an accurate Rb correction can be applied, and 87 Sr/ 86 Sr and 86 Sr/ 88 Sr ratios can be determined. The effectiveness of Yb 2+ corrections is illustrated in Fig. 4. In addition, the 171 Yb 2+ / 173 Yb 2+ ratio is typically constant and similar to ratios measured on 1 ppm solutions, confirming that ions appearing at half-mass positions are indeed REE 2+ ions. In addition to assessing the effects of Er and Yb on Sr isotopes individually, we have also undertaken experiments to evaluate the combined effects of Er and Yb, and the effectiveness of corrections on Erand Yb-doped 100 ppb NBS987 Sr solutions. Fig. 5 illustrates both uncorrected and corrected Sr isotope ratios for these solutions and attests to the high degree to which corrections can account for the interfering effects of Er and Yb. Similar to Er and Yb, doubly charged Hf isotopes (e.g., 176 Hf 2+ ) may also interfere with Sr isotope masses. To evaluate this, we doped 100 ppb NBS987

10 10 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Fig. 5. Uncorrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (filled squares) and corrected 87 Sr/ 86 Sr, 84 Sr/ 86 Sr, and 86 Sr/ 88 Sr ratios (unfilled squares) of Er- and Yb-doped 100 ppb NBS987 Sr solutions with Sr/Er and Sr/Yb of 12.5 to S.E. error bars are not shown and solid lines are the same as in Fig Sr/ 86 Sr ratios generally decrease from the combined effect of Er 2+ and Yb 2+ additions and inappropriate Rb corrections. In contrast, 84 Sr/ 86 Sr ratios increase as do 86 Sr/ 88 Sr ratios, although 86 Sr/ 88 Sr increases are subtle. Corrections for these interferences are effective in accounting for the combined effects of doubly charged Er and Yb ions in 87 Sr/ 86 Sr, 84 Sr/ 86 Sr at Sr/Er and Sr/Yb down to ~10. Sr with JMC475 Hf in Sr/Hf ratios of 10 and 0.2. Results for Sr isotope ratios of Hf-doped solutions were indistinguishable from undoped NBS987 Sr solutions. As a result, we have not corrected Sr isotopes for doubly charged Hf isotopes Krypton Krypton has two isotopes that interfere with Sr isotope masses, 84 Kr and 86 Kr. Kr is present in low levels as an impurity in the argon gas supplied to the torch, the sample introduction system, and the torch cooling system of the MC-ICPMS (typically V1 mv 83 Kr during an analysis). Kr is also present in the helium transfer gas used during laser ablation (V3 mv 83 Kr). We account for the interfering effects of Kr by subtracting on-peak blanks measured prior to firing the laser. These blanks account for Kr contributions from both the sample introduction system of the MC-ICPMS and the transfer gas as Kr signals are stable over individual analytical sessions.

11 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Rubidium Rubidium has long been known to limit the types of samples that can be analyzed without sample purification (see Dickin, 1995). As with TIMS, the presence of significant amounts of Rb can limit the materials that can be successfully analyzed for Sr isotopes by LA. We use the measured 86 Sr/ 88 Sr ratio and the natural 85 Rb/ 87 Rb ratio to calculate a mass bias uncorrected 85 Rb/ 87 Rb ratio. We then use this mass bias uncorrected 85 Rb/ 87 Rb ratio and the 85 Rb intensity measured during the Sr isotope analysis to calculate the 87 Rb contribution to mass 87. This contribution is then removed prior to calculating Sr isotope ratios. We have evaluated the interfering effects of 87 Rb on Rb-doped NBS987 Sr solutions. Corrections fail to accurately account for 87 Rb in NBS987 Sr solutions with Rb/Sr N Beam dispersion To accomplish interference monitoring while measuring all Sr and Rb isotope masses with the Faraday array illustrated in Table 3, we disperse the beam using a quadrupole lens located on the collector side of the magnet. The collector design of the Neptune does not allow for the H3 H4 and L3 L4 Faraday collectors to be positioned at Sr whole-mass collector spacing. The collector design of the Neptune incorporates a bar between the H3 H4 and L3 L4 collectors to passively position these collectors using the adjacent H3 and L3 collectors. This bar limits how close the collectors can get to each other. We, however, require the use of the H4 and L4 collectors at 1 Sr mass unit spacing (Table 3), thus we use beam dispersion to increase the separation between masses so that all Faraday collectors can be used to monitor half-mass ( 167 Er 2+, 171 Yb 2+ and 173 Yb 2+ ) and wholemass positions. All solution and LA results presented here use the collector configuration illustrated in Table 3, the machine parameters in Table 1, and beam dispersion. As a result of mass and charge differences, the mass positions in which doubly charged Er and Yb and singly charged Rb and Sr ions appear are slightly different. Doubly charged Er and Yb ions appear at masses shifted toward the high mass side (~0.05 amu) which only overlap ~50% of the high mass side of the singly charged ion peaks (i.e., the high mass half of the peak flat). If the Faraday cups are not perfectly aligned on the overlapping portions of the Sr + Rb + and Er 2+ Yb 2+ masses, non-natural Er and Yb ratios will be measured on 1 ppm Er and Yb solutions. Thus, it is critical that peaks are measured on the overlapping portion of the combined singly charged and doubly charged peak. If done correctly, doubly charged ions will have natural ratios ensuring accurate doubly charged REE corrections. 4. Data reduction Raw beam intensities, minus on-peak baselines, were exported to an independent data reduction program that sequentially strips the effects of interfering elements from raw beam intensities and recalculates resulting Sr isotope ratios. Prior to stripping, a tailing correction was used on all raw intensities measured. This correction used the measured abundance sensitivity on the Faraday array at 88 Sr (~1 ppm at 1 amu and ~3 ppm at 0.5 amu). For most LA measurements, tailing corrections at 1 ppm for 88 Sr tailing onto 87 Sr changed the 87 Sr/ 86 Sr by V Tailing corrections were, however, more significant in correcting 0.5 amu tailing of Sr peaks onto Er 2+ and Yb 2+ half-mass monitor peaks (e.g., 87 Sr tailing onto 173 Yb 2+ at mass 86.5). Most analyses were affected by less than using both half- and whole-mass tail corrections. The effects of interfering elements were accounted for in the following order: (1) Kr, (2) Yb 2+, (3) Er 2+, and (4) Rb. If any of the monitor masses for these elements were negative, no corrections were made. For our purposes, negative intensities were considered noise on the baseline and were manifest as beam intensities of generally V0.002 mv. If negative intensities z 0.01 mv were measured, on-peak baselines were repeated. It is critical that the sequence of corrections is maintained as some interfering elements also interfere with other interfering elements (e.g., 170 Er 2+ and 85 Rb; 168 Er 2+ and 84 Kr). First, Kr interferences were accounted for using on-peak baseline measurements. For LA, on-peak baselines were measured under analytical operating conditions using all gas flows but without firing the laser. Thus, we assume that any addition to relevant peaks from

12 12 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Kr is accounted for by subtracting on-peak baselines with the normal gas amounts being flushed through the system for both LA and solution analyses. Second, Yb 2+ was stripped from raw intensities using measured Yb 2+ isotope ratios uncorrected for mass bias (i.e., measured ratios fully affected by mass bias and not normalized to natural ratios) and the 173 Yb 2+ intensity. Third, intensities were then corrected for the influence of Er 2+ using Er 2+ ratios uncorrected for mass bias and the 167 Er 2+ intensity. Resulting beam intensities were then corrected for Rb using a 85 Rb/ 87 Rb ratio uncorrected for mass bias (calculated using the measured 86 Sr/ 88 Sr ratio) and the 85 Rb intensity. This procedure was used for all solution and LA analyses and is the most accurate method of internally correcting each Sr isotope ratio measured during the analysis. The resulting interference-corrected Sr isotope ratios were calculated and exponentially corrected to 86 Sr/ 88 Sr= Ratios were then compared to measurements from microdrilled samples purified using column chromatography to assess the accuracy of Sr isotope measurements using LA-MC-ICPMS. 5. Results 5.1. Marine carbonate Results for our in-house marine carbonate (a modern clam shell) are presented in Table 4 and Fig. 6. This shell has ~1000 ppm Sr and Rb/ SrV0.001; the accepted 87 Sr/ 86 Sr value for modern seawater is F (Dia et al., 1992). Means for 87 Sr/ 86 Sr and 84 Sr/ 86 Sr acquired by LA- MC-ICPMS are F (2r) and F (2r), respectively. Gray fields in Fig. 6 represent MC-ICPMS Sr isotope results for two samples of this marine carbonate that were dissolved and purified using Sr microchemistry (Wolff et al., 1999). Purified 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios averaged and , respectively (n=2). 87 Sr/ 86 Sr ratios of purified and LA samples are identical and overlap values for modern seawater. These results demonstrate that both accurate and reproducible 87 Sr/ 86 Sr ratios can be attained on carbonate using LA-MC-ICPMS. In contrast, 84 Sr/ 86 Sr ratios are ~0.3% lower (D 84 Sr/ 86 Sr=0.0002) Table 4 Sr isotope results and measured 88 Sr, 85 Rb, and 173 Yb 2+ intensities for in-house marine carbonate reference Sample name/type 88 Sr 85 Rb 173 Yb Sr/ 86 SrF2S.E. 87 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. Sampling method (V) (mv) (mv) uncorrected corrected uncorrected corrected Marine carbonate laser ablation Trough# F F F F3 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F2 Trough# F F F F3 Trough# F F F F2 Trough# F F F F2 Trough# F F F F3 Trough# F F F F2 Trough# F F F F4 Trough# F F F F2 Microdrill/purified MC-ICPMS Hole # F F1 Hole # F F1

13 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx 13 Fig Sr/ 86 Sr and 84 Sr/ 86 Sr of 16 analyses of our in-house marine carbonate reference. Filled squares represent analyses obtained by LA-MC-ICPMS with respective 2 S.E. error bars. Mean values of measured 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios are represented by solid horizontal lines with mean values shown. Gray field represents Sr isotope ratios of purified samples (with maximum 2 S.E. range) analyzed using MC-ICPMS (n=2). Results indicate that highly accurate and reproducible 87 Sr/ 86 Sr ratios can be obtained on carbonate using LA-MC-ICPMS. Results for 84 Sr/ 86 Sr are lower (0.0002) than purified samples and more variable overall. for LA samples than equivalent ratios obtained on purified solutions Plagioclase Results for two plagioclase samples are presented in Table 5 and Figs. 7 and 8. The first is a ~3.5 mm plagioclase phenocryst from the Miocene Grande Ronde Formation (~16 Ma) of the Columbia River Basalt Group. The second is a ~10 mm plagioclase phenocryst from a late Quaternary alkali basalt flow at Pisgah Crater (~20 ka) California. In both cases, age corrections for 87 Rb 87 Sr decay are negligible. The Grande Ronde plagioclase is homogenous in both Sr (~900 ppm) and 87 Sr/ 86 Sr, and has Rb/Sr~0.003 (Tollstrup, 2003). The Sr content is at the high end of what would normally be expected for plagioclase in a tholeiitic basalt, but much less than that of Sr-rich plagioclase grains analyzed in previous LA-MC- ICPMS studies (N1500 ppm, Davidson et al., 2001; Waight et al., 2002). It has been sampled using nine Am laser troughs and two microdrill holes. Results are shown in Fig. 7 with gray fields reflecting purified microdrilled samples analyzed by MC- ICPMS. LA analyses are highly reproducible (within 2 S.E.) with a mean 87 Sr/ 86 Sr of F (2r). Two microdrill samples, purified using column chromatography, have an average 87 Sr/ 86 Sr of F Both microdrilled/purified and LA results offer high degrees of reproducibility although 87 Sr/ 86 Sr ratios acquired using LA sampling are ~ (D 87 Sr/ 86 Sr) higher than microdrilled samples. Elevated ratios in LA results compared to solution results were also observed by Waight et al. (2002), but accuracy here is much greater (D 87 Sr/ 86 Sr= vs ). In addition, more accurate 84 Sr/ 86 Sr ratios are also obtained (D 84 Sr/ 86 Sr lower by ~0.0003). The Pisgah Crater plagioclase (Fig. 8) is from a crustally contaminated alkali basalt flow (Ramos, 2000; Ramos and Reid, 2004). This phenocryst exhibits Sr concentrations and 87 Sr/ 86 Sr ratios that vary systematically from core to rim. 87 Sr/ 86 Sr variation within the crystal was first characterized using purified microdrill samples analyzed by TIMS (gray squares in Fig. 8). Sr concentrations determined by isotope dilution vary from 850 ppm (core) to 720 ppm (rim). The crystal core has an 87 Sr/ 86 Sr= , increasing to at the rim. Following initial characterization, we used both LA troughs and pits to analyze this grain. Troughs were Am and sampled material between the two microdrill holes nearest the rim of the plagioclase. Pits were 80 Am in diameter and sampled plagioclase between the second and third microdrill holes nearer to the core (Fig. 8). Results clearly demonstrate the ability of LA analyses to track small internal 87 Sr/ 86 Sr variations in plagioclase (Fig. 8). In addition, 84 Sr/ 86 Sr ratios are relatively constant and similar to values of the Grande

14 14 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Table 5 Sr isotope ratios and 88 Sr, 85 Rb, and 173 Yb 2+ intensities of Grande Ronde and Pisgah Crater plagioclases Sample/type 88 Sr 85 Rb 173 Yb Sr/ 86 SrF2S.E. 87 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. Sampling method (V) (mv) (mv) uncorrected corrected uncorrected corrected Grande Ronde Plagioclase laser ablation Trough# F F F F2 Trough# F F F F1 Trough# F F F F1 Trough# F F F F1 Trough# F F F F2 Trough# F F F F2 Trough# F F F F1 Trough# F F F F4 Trough# F F F F1 microdrill/purified/ MC-ICPMS Hole #1 Rim F F1 Hole#2 Core F F1 Pisgah Crater Plagioclase laser ablation Trough# F F F F3 Trough# F F F F3 Trough# F F F F3 Trough# F F F F4 Trough# F F F F6 Trough# F F F F4 Trough# F F F F3 Trough# F F F F3 Trough# F F F F3 Trough# F F F F3 Trough# F F F F3 Trough# F F F F4 Trough# F F F F5 Trough# F F F F3 Trough# F F F F4 Trough# F F F F3 Trough# F F F F4 Trough# F F F F4 Trough# F F F F3 Trough# F F F F3 Pit# F F F F5 Pit# F F F F5 Pit# F F F F8 Pit# F F F F5 Pit# F F F F6 Pit# F F F F7 Pit# F F F F5 Pit# F F F F5 Pit# F F F F3 Pit# F F F F3 Pit# F F F F7 Pit# F F F F5

15 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx 15 Table 5 (continued) Sample/type 88 Sr 85 Rb 173 Yb Sr/ 86 SrF2S.E. 87 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. 84 Sr/ 86 SrF2S.E. Sampling method (V) (mv) (mv) uncorrected corrected uncorrected corrected Microdrill/purified/TIMS Hole# F F1 Hole# F F1 Hole# F F1 Ronde plagioclase. Although LA 87 Sr/ 86 Sr ratios are again slightly higher than microdrill/tims values (compare the leftmost five troughs to the rim microdrill hole in Fig. 8), they accurately track the changes expected between microdrill holes. The 80 Am pits exhibit poorer measurement precision compared to troughs, mainly due to lower Sr signals for pits Clinopyroxene Single mineral grains of clinopyroxene, separated from a late Quaternary northern Grand Canyon alkali basalt with 8% MgO (V100 ka, Ramos, 2000), were analyzed using Am LA troughs and by Fig Sr/ 86 Sr and 84 Sr/ 86 Sr ratios of a rim to core to rim transect of Grande Ronde plagioclase. Filled squares represent analyses obtained by LA-MC-ICPMS with associated 2 S.E. error bars. Gray fields represent Sr isotope ratios of purified solutions with maximum 2 S.E. range obtained through microdrilling and gray squares represent individual analyses and 2 S.E. error bars. Mean values of measured 87 Sr/ 86 Sr and 84 Sr/ 86 Sr ratios in Grande Ronde plagioclase are represented by solid horizontal lines. Means of solution and LA results suggest that LA 87 Sr/ 86 Sr ratios are ~ higher than equivalent solution analyses. In addition, 84 Sr/ 86 Sr analyses are ~ lower than solution values, similar to values obtained on marine carbonate. Fig Sr/ 86 Sr and 84 Sr/ 86 Sr of core to rim analyses of Pisgah Crater plagioclase. Filled squares with 2 S.E. error bars are analyses obtained by LA-MC-ICPMS using troughs and pits (as indicated) and gray squares (and gray field) are analyses of purified solutions obtained through microdrilling and TIMS. Note that Sr isotope ratios obtained by LA sampling track 87 Sr/ 86 Sr isotope variations determined by microdrilling. Mean values of measured 84 Sr/ 86 Sr ratios are represented by the solid horizontal line. In addition, 84 Sr/ 86 Sr analyses are ~ lower than solution values (similar to values obtained on Grande Ronde plagioclase and marine carbonate) and do not vary with 87 Sr/ 86 Sr ratios.

16 16 F.C. Ramos et al. / Chemical Geology xx (2004) xxx xxx Table 6 Sr isotope ratios and 88 Sr, 85 Rb, and 173 Yb 2+ intensities of Northern Grand Canyon clinopyroxenes, Pisgah Crater fine-grained groundmass, and Cascade basaltic andesite plagioclases and groundmass Sample name/type 88 Sr 85 Rb 173 Yb Sr/ 86 SrF2 S.E. 87 Sr/ 86 SrF2 S.E. 84 Sr/ 86 SrF2 S.E. 84 Sr/ 86 SrF2 S.E. Sampling method (V) (mv) (mv) uncorrected corrected uncorrected corrected Northern Grand Canyon clinopyroxene Grain 7 laser ablation Trough# F F16 Trough# F F12 Trough# F F14 Trough# F F19 Microdrill/purified/MC-ICPMS Hole # F F2 Grain 6 laser ablation Trough# F F18 Trough# F F23 Trough# F F17 Trough# F F16 Microdrill/purifed/MC-ICPMS Hole # F F5 Whole grain digestion purified/tims Grain# F F6 Grain# F F6 Pisgah Crater fine-grained groundmass laser ablation Trough# F F F F10 Trough# F F F F10 Pit# F F F F20 Pit# F F F F17 Pit# F F F F12 Microdrill/purified/ TIMS Hole# F F1 Hole# F F1 Hole# F F1 Hole# F F1 Hole# F F1 Cascades basaltic andesite plagioclase laser ablation Grain 1 trough F F F F1 Grain 2 trough F F F F1 Grain 3 trough F F F F1 Grain 4 trough F F F F2 Fine-grained groundmass Trough# F F F F1 Whole rock/purified/mc-icpms F F F1