ANALYTICAL TECHNIQUES
Mineral compositions were determined using energy dispersive analysis (EDS) on a JEOL 8600 Superprobe at the Geology Department, University of Otago. Specimens were in the form of carbon-coated thin sections. Operating conditions were: accelerating voltage 15 kv, specimen current of 1.10-9 A, beam diameter 10-20 m (depending on the stability and grain size of the mineral concerned), counting time 200 seconds. Data were processed by a Moran Scientific (Australia) system calibrated against a selection of natural and synthetic minerals. Typically at least 4 grains of each mineral were analysed in each thin section, although where compositional zoning was detected, mineral analyses were more numerous and undertaken at a closer spacing.
Geochemical analyses were determined by XRF techniques by Actlabs, with methodology described at
Trace elements in calcite were determined using a UP213 Nd:YAG 213 nm laser ablation system (New Wave Research, Fremont, CA, USA) coupled to an Agilent 7500cs quadrupole inductively coupled mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) at the Centre for Trace Element Analysis at the University of Otago. Calcite was hand picked, mounted in an epoxy briquette and polished. Laser ablation was conducted at 50% power at 5 Hz repetition rate using a spot diameter of 55 µm. Ablated material was carried by He gas from the sample cell, mixed with Ar and inlet into the ICP-MS. Data for 45 mass peaks were collected in time-resolved mode with one point per peak and integration times of 10 ms for a total scan time of 530 ms. Background (laser off) data were acquired for 20 s followed by 40 s with the laser on, giving about 100 mass scans from a spot approximately 20 µm deep. A purge time of at least 120 s is allowed between each spot analysis to permit a return to background signal levels. Each set of three tracks on two unknowns was bracketed by analyses of standard glass NIST 610.
Raw mass peak count rates were backgrounds subtracted, corrected for mass bias drift and converted to concentrations (in parts per million) by reference to standard glass NIST 610 using an offline spreadsheet. After triggering, it takes 5-10 s for a steady signal to be reached, so these initial data were excluded from the calculations. Trace-element concentrations were obtained by normalising count rates for each element to those for 43Ca in the sample and standard using known Ca and trace element concentrations in NIST SRM 610 (Pearce et al. 1997) and assuming CaO to be stoichiometric in calcite with a concentration of 56 wt%.
Strontium isotopic compositions were determined using the same laser coupled to a Nu Plasma HR multiple-collector inductively coupled mass spectrometer (MC-ICP--MS) (Nu Instruments, Wrexham, UK) housed in the Centre for Trace Element Analysis at the University of Otago , and the same calcite samples mounted in a polished epoxy briquette as for the trace element analysis described above. Instrumental conditions were optimised daily for maximal intensity and stability of 88Sr signals by ablating NIST SRM 610 and an in-house calcite standard (Tridacna sp. clam shell). Following a gentle pre-ablation to remove surface contamination, each calcite specimen was analysed using a 100 µm spot size at 70% power and a 10 Hz repetition rate. Signal sizes were typically 0.8 to 12 x 10-11 Afor 88Sr. Ablated material was carried by He gas from the sample cell, mixed with Ar and inlet into the MC-ICP-MS. For isotope analysis, masses 82, 83, 84, 85, 86, 87, 88 and 89 were simultaneously measured in Faraday collectors in static mode (Woodhead et al. 2005). Data were collected at 0.2 s intervals by using the time-resolved analysis software supplied by Nu Instruments. Prior to the sample ablation and after the pre-ablation step, on-peak background signals were measured for 60 s. The clam shell standard, with a seawater ratio of 87Sr/86Sr = 0.70917 ± 2 (2sigma of the mean; Goldstein & Jacobsen 1987), was analysed several times at the beginning of each analytical session and also once per every three to six calcite ablations, in order to check analytical accuracy and reproducibility.
Data were processed off-line using a spreadsheet programme for isobaric interference corrections and instrumental mass bias correction. Interferences of Kr (a trace impurity found in the Ar and He carrier gases) were removed by subtracting on-peak background signals from sample signals. Interference of 87Rb on 87Sr was corrected based on the mass 85 signals, corresponding to 85Rb, assuming 87Rb/85Rb = 0.3855. Interferences from Ca argides/dimers on Sr masses 84 (44Ca40Ar/44Ca40Ca), 86 (46Ca40Ar/46Ca40Ca. and 88 (48Ca40Ar/48Ca40Ca) were subtracted by monitoring the signal levels of 42Ca40Ar/42Ca40Ca at the mass 82 and calculating appropriate interference levels (Woodhead et al. 2005). Correction for mass bias was by referencing to the 86Sr/88Sr ratio of 0.1194 and using the exponential law (Russell et al. 1978). The veracity of our data reduction procedure was assessed by monitoring the 84Sr/86Sr ratio (≈ 0.0565)(Woodhead et al. 2005).
Carbon and oxygen isotopes were determined on the calcite following conversion to CO2 by an adaptation of the method of McCrea (1950). Aliquots of crushed rock (for the calcite-bearing syenites) or mineral separate (for marble or carbonatite) containing ~200 mg calcite were sealed in 12 mL exetainers and the air flushed out with helium. A few drops of 103% H3PO4 was added to each exetainer and the samples allowed to react for >6 hours at 25C. The isotopic composition of the CO2 liberated was measured by continuous flow mass spectrometry using a GasBench interfaced to a Thermo Finnigan DeltaPLUSXP (Thermo Finnigan, Bremen). The samples were calibrated to international scales using NBS18 and NBS19 calcite reference materials and were bracketed with two secondary laboratory standards. The precision of the results was ± 0.2‰ for 18O and ± 0.1‰ for 13C based on the reproducibility of the calcite standards.
Calcite concentrates used for oxygen and carbon isotope analysis were dissolved in specpure dilute HCl. Pb was purified from solutions by standard double-pass anion exchange chemistry using BioRad AG 1-X8 (200 – 400 mesh) resin and HBr-HCl media. Total procedural blank concentrations (<0.1 ng) were <0.3% of the sample Pb concentrations and thus negligible.
Purified samples were dissolved in 2% HNO3 and mixed with a Tl solution (NIST 997) in 2% HNO3 to achieve a Pb/Tl concentration ratio of approximately 10. Instrumentation is Nu Plasma HR coupled to a DSN-100 (Nu Instruments) sample desolvation system. Masses 202 (Hg), 203 (Tl), 204 (Pb and Hg), 205 (Tl), 206 (Pb), 207 (Pb) and 208 (Pb) were simultaneously measured in Faraday collectors in static mode. Each analysis consisted of 4 blocks of 20 scans (5 s integration per scan) with each block preceded by a 30 s baseline determination using ESA-deflected signals. Sample Pb signal levels were kept around 6 to 9 10–11 A for 208Pb. Data were processed off-line using a spreadsheet programme for interference correction and instrumental mass bias correction.
Interference of 204Hg on 204Pb was removed based on the 202Hg signal levels and assuming a natural ratio of 204Hg/202Hg = 0.2299 (Rosman and Taylor 1998). Instrumental mass bias was corrected by using an empirical external normalisation procedure (Maréchal et al. 1999, Woodhead 2002) which does not assume equal mass bias between the analyte (Pb) and the normalisation standard (Tl). Mass bias was characterised each day by analysing a set of Pb isotopic standard solutions (NIST 981; prepared in 2% HNO3) mixed with the NIST 997 Tl solution at Pb/Tl ≈ 10. We adopted the following isotopic compositions for NIST 981 (Todt et al. 1996): 208Pb/204Pb = 36.7006, 207Pb/204Pb = 15.4891, and 206Pb/204Pb = 16.9356.
Zircons from calcite syenite Kay 10 were separated by crushing, heavy liquid and hand picking techniques and were analysed by LA-ICP-MS techniques at the Research School of Earth Sciences, Australian National University, using methodology and processing techniques described by Scott & Palin (2008).
REFERENCES
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