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doi:10.2204/iodp.proc.330.102.2012 GeochemistrySampling and analysis of igneous rocksSample preparationRepresentative samples of igneous rocks were analyzed for major and trace element concentrations during Expedition 330 using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). Samples ranging in size from ~2 to ~8 cm3 were cut from the core with a diamond saw blade. A thin section billet was taken from the same or adjacent interval. All outer surfaces were ground on a diamond-impregnated disk to remove altered rinds and surface contamination derived from the drill or saw. Each sample was then placed in a beaker containing trace-metal-grade methanol and washed ultrasonically for 15 min. The methanol was decanted, and the samples were washed in deionized water for 10 min and then in Barnstead deionized water (~18 MΩ·cm) for 10 min in an ultrasonic bath. The cleaned pieces were dried for 10–12 h at 110°C. The cleaned, dried samples were crushed to <1 cm chips between two disks of Delrin plastic in a hydraulic press. The chips were then ground to a fine powder in tungsten carbide in a SPEX 8515 Shatterbox. Some samples were amygdular, so before grinding we handpicked chips under a binocular microscope to obtain material as free of amygdules as possible. After grinding, an aliquant of the sample powder was weighed on a Mettler Toledo balance and ignited to determine weight loss on ignition (LOI). Samples were ignited at 930°–960°C for 4 h. For samples from Sites U1372–U1374, the amount of sample weighed for the LOI measurement was 1000.0 ± 0.5 mg. Estimated relative uncertainty on LOI values for these samples is ~14% on the basis of duplicate measurements. For samples from Sites U1375–U1377, a 5000.0 ± 0.5 mg aliquant was used, and the estimated relative uncertainty on LOI values is ~4%. ODP Technical Note 29 (Murray et al., 2000) describes in detail the shipboard procedure for digestion of rocks and ICP-AES analysis of samples. The following protocol is an abbreviated form of this procedure with minor modifications. After determination of LOI, 100.0 ± 0.2 mg splits of the ignited whole-rock powders were weighed and mixed with 400.0 ± 0.5 mg of LiBO2 flux that had been preweighed on shore. Standard rock powders and full procedural blanks were included with unknowns in each ICP-AES run (note that among the elements analyzed, contamination from the tungsten carbide mills is negligible; Shipboard Scientific Party, 2003). All samples and standards were weighed on a Cahn C-31 microbalance (designed to measure at sea) with weighing errors estimated to be ±0.05 mg under relatively smooth sea-surface conditions. To prevent the cooled bead from sticking to the crucible, 10 mL of 0.172 mM aqueous LiBr solution was added to the mixture of flux and rock powder as a nonwetting agent. Samples were then fused individually in Pt-Au (95:5) crucibles for ~12 min at a maximum temperature of 1050°C in an internally rotating induction furnace (Bead Sampler NT-2100). After cooling, beads were transferred to high-density polypropylene bottles and dissolved in 50 mL of 10% (by volume) HNO3, aided by shaking with a Burrell wrist-action bottle shaker for 1 h. Following digestion of the bead, the solution was passed through a 0.45 µm filter into a clean 60 mL wide-mouth high-density polypropylene bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 10% HNO3 to bring the total volume to 20 mL. The final solution-to-sample dilution factor was ~4000. For standards, stock standard solutions were placed in an ultrasonic bath for 1 h prior to final dilution to ensure a homogeneous solution. AnalysesMajor (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Ba, Sr, Zr, Y, V, Sc, Cu, Zn, Co, Cr, and Ni) element concentrations of standards and samples were determined with a Teledyne Leeman Labs Prodigy ICP-AES instrument. Wavelengths used for sample analysis during Expedition 330 are provided in Table T6. For several elements, measurements were made at more than one wavelength (e.g., Si at 250.690 and 251.611 nm). The plasma was ignited at least 30 min before each run of samples to allow the instrument to warm up and stabilize. A zero-order search was then performed to check the mechanical zero of the diffraction grating. After the zero-order search, the mechanical step positions of emission lines were tuned by automatically searching with a 0.002 nm window across each emission peak using basalt laboratory standards BAS-140 (Bach et al., 1996) or BAS-206 (Shipboard Scientific Party, 2003) dissolved in 10% HNO3. During the initial setup, a BAS-140 solution was used to select an emission profile for each peak to determine peak-to-background intensities and set the locations of background levels for each element. The Prodigy software uses these background locations to calculate the net intensity for each emission line. Photomultiplier voltage was optimized by automatically adjusting the gain for each element using BAS-140. The ICP-AES data presented in the “Geochemistry” section of each Expedition 330 site chapter were acquired using the Gaussian mode of the Prodigy software. This mode fits a curve to points across a peak and integrates the area under the curve for each element measured. Each sample was analyzed four times from the same dilute solution (i.e., in quadruplicate) within a given sample run. For elements measured at more than one wavelength, we either used the wavelength giving the best calibration line in a given run or, if the calibration lines for more than one wavelength were of similar quality, used the data for each and reported the average concentration. A typical ICP-AES run (Table T7) included
Data reductionFollowing each run of the instrument, the measured raw-intensity values were transferred to a data file, corrected for instrument drift, and then corrected for the procedural blank. Drift correction was applied to each element by linear interpolation between the drift-monitoring solutions run every fifth analysis. After drift correction and blank subtraction, a calibration line for each element was calculated using the results for the certified rock standards. Element concentrations in the samples were then calculated from the relevant calibration lines. The United States Geological Survey Hawaiian basalt standard BHVO-2 is the closest in composition to the Expedition 330 igneous rocks, and replicate measurements of this standard analyzed as an unknown were used to estimate precision and provide an idea of accuracy for both major and trace elements. Table T8 summarizes the results and compares them with recommended values (Govindaraju, 1994), showing that for most elements, Expedition 330 values agree very well with published values. Individual analyses of both standards and samples produced total volatile-free major element weight percentages that vary from 100 wt% by as much as several percent. Possible causes include some combination of errors in weighing the sample (particularly in rougher seas) and/or flux powders (although weighed on land, weighing errors are nevertheless possible), variability in the dilutions (which were done volumetrically), and the duration and relatively low temperature of ignition. To facilitate comparison of Expedition 330 results with each other and with data from the literature, we normalized the measured major element values to 100 wt% totals. Semiquantitative X-ray fluorescence analysisA portable X-ray fluorescence (XRF) spectrometer, the Thermo Scientific Niton XL3 Analyzer, was installed aboard the ship for evaluation during Expedition 330. This instrument provides relatively rapid measurement of concentration for a number of elements in either a 3 or 8 mm diameter area of a sample (X-ray “spot” analysis). For a preliminary report of the results of this evaluation, see XL3_EVAL.PDF in XRF in “Supplementary material.” In brief, the usefulness of the instrument for reliably distinguishing between different eruptive units via measurement of sawn core faces, cubes, or slabs is restricted to sites with a fairly large range of compositional variation. However, the instrument was found to be quite valuable for material identification in sedimentary intervals and zones of alteration. Interpretation of several core intervals was aided by the identification of ferromanganese material, phosphate, and Fe-poor or Fe-rich carbonates that were present in associations involving two or more phases. These materials could not be identified, or at least not with certainty, by visual or microscopic observation, but the handheld XRF instrument identified them readily. Sedimentary carbon, carbonate, and nitrogen analysisSediment samples were taken from the interiors of cores with autoclaved cut-tip syringes. They were then freeze-dried to remove water and powdered to ensure homogenization. Carbonate content was determined by acidifying ~10 mg of bulk powder with 2M HCl and measuring the CO2 evolved, all of which was assumed to be derived from CaCO3. A UIC coulometer was employed for the measurement. The weight percent of total inorganic carbon was calculated by dividing the CaCO3 content in weight percent by 8.33. We determined total carbon content for the same samples by combusting the sample at an initial temperature of 900°C in a Flash EA-1112 Series Thermo Electron Corporation CHNS analyzer (CHNS stands for carbon-hydrogen-nitrogen-sulfur). The total carbon value was then used to calculate weight percent of total organic carbon via subtraction. That is, the weight percent of inorganic carbon derived from the carbonate measured by coulometric analysis was subtracted from the total carbon content obtained with the CHNS analyzer. |
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