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doi:10.2204/iodp.proc.324.102.2010

Geochemistry

Sampling and analysis of igneous rocks

Sample preparation

Representative samples of igneous rocks were analyzed for major and trace element concentrations during Expedition 324 using inductively coupled plasma–atomic emission spectroscopy (ICP-AES). Samples ranging in size from ~3 cm³ for cryptocrystalline to very fine grained rocks to as much as 10 cm³ for medium-grained rocks were cut from the core with a diamond saw blade. If possible, 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 from the drill and 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 in an ultrasonic bath. They were then washed for 10 min in Barnstead deionized water (~18 MΩ·cm), again 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 8000M mixer/mill or, for larger samples, a SPEX 8515 Shatterbox; the smallest samples were hand-ground in an agate mortar. It should be noted that among the elements analyzed, contamination from the tungsten carbide mills is negligible (Shipboard Scientific Party, 2003b). Some samples were amygdular and, before grinding, we hand-picked chips under a binocular microscope in order to obtain material that was as free of amygdules as possible. Hand-picking was also done for several clasts separated from the volcaniclastic rocks of Site U1348. After grinding, a 1000.0 ± 0.5 mg aliquant of the sample powder was weighed on a Mettler Toledo dual balance system and ignited to determine weight loss on ignition (LOI). Samples from Sites U1346–U1349 were ignited at 975°C for 4 h. Normally, a temperature >1000°C is achieved for LOI measurements, but the maximum temperature that the muffle furnace on the ship could reach during most of Expedition 324 was 975°C. For the Site U1350 samples, a temperature of 1025°C was attained, following addition of a step-up transformer to the furnace (see below). Estimated uncertainty on LOI values is ~0.2 mg (0.02 wt%).

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 aliquots 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. All samples and standards were weighed on the Cahn C-31 microbalance (designed to measure on board) with weighing errors estimated to be ±0.05 mg under relatively smooth sea-surface conditions.

Ten milliliters of 0.172 mM aqueous LiBr solution was added to the mixture of flux and rock powder as a nonwetting agent to prevent the cooled bead from sticking to the crucible. Samples from Sites U1346–U1349 were then fused individually in Pt-Au (95:5) crucibles for ~12 min at a maximum temperature of 1050°C in an internal-rotating induction furnace (Bead Sampler NT-2100). Because of a breakdown of this instrument late in the cruise, the Site U1350 samples were fused in the muffle furnace. Addition of a step-up transformer allowed the muffle furnace to reach a temperature of 1025°C. The furnace was heated to 900°C, 6–12 samples were inserted together, and the furnace temperature was increased as rapidly as possible to 1025°C. After 5 min at 1025°C, the samples were removed; the total time in the furnace at temperatures above 900°C was ~30 min.

After cooling, beads were transferred to high-density polyethylene (HDPE) bottles and dissolved in 50 mL of 10% (by volume) HNO₃; dissolution was 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 HDPE 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.

Analyses

Major (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 324 are provided in Table T5. For several elements, measurements were made at two wavelengths (e.g., Si at 251.611 and 288.158 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, 2003b) in 10% HNO₃. During the initial setup, BAS-140 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" sections of the site chapters 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 three times from the same dilute solution (i.e., in triplicate) within a given sample run. For elements measured at two wavelengths, we either used the wavelength giving the better calibration line in a given run or, if the calibration lines for both wavelengths were of similar quality, used the data for both and reported the average concentration.

A typical ICP-AES run (Table T6) included

• A set of 9 certified rock standards (BCR-2, BHVO-2, BIR-1, JA-3, JGb-1, JR-2, NBS-1c, SCO-1, and STM-1), analyzed at the beginning of each run;

• As many as 23 samples (unknowns) analyzed in triplicate;

• A drift-correcting standard (BHVO-2) analyzed in every fifth sample position and at the beginning and end of each run;

• Blank solutions, analyzed near the beginning and end of each run and, in the longer runs, at another point in the middle of the sequence; and

• Two "check" standards (BAS-140 and BAS-206) run as unknowns, each analyzed in triplicate at least twice during a run.

A 10% HNO₃ wash solution was run for 90 s between each analysis.

Data reduction

Following 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.

Replicate analyses of the basalt "check" standards BAS-140 and BAS-206 were used to estimate precision and provide an idea of accuracy for both major and trace elements. Table T7 summarizes the results and compares them with published data. For most elements, the Expedition 324 averages agree well with previously published values. However, our values for Cr in BAS-206 are considerably and consistently higher (135 ± 10 versus 83.7 ppm). At present, we have no explanation for this discrepancy, but we note that our Cr values for BAS-140 agree within error with published values (191 ± 9.7 and 186 ppm, respectively).

Individual analyses of both check standards and samples produced total major element weight percentages that vary from 100 wt% by as much as several percent. Probable causes include errors in weighing the sample (particularly in rougher seas) and/or flux powders and the relatively low temperature of ignition for most of the samples. To facilitate comparison of the Expedition 324 results with each other and with data from the literature, we normalized the measured major element values to 100 wt% totals.

Sedimentary carbon and carbonate analysis

Sediment and sedimentary rock samples were scraped to remove surface contamination, then freeze-dried to remove water and powdered to ensure complete homogenization. Carbonate content was determined by acidifying ~10 mg of bulk powder with 2 M HCl and measuring the CO₂ evolved. A UIC coulometer was employed for the measurement.

We determined total carbon content for the same samples by combustion of the sample at an initial temperature of 900°C in a Thermo Electron Corporation CHNS analyzer (Flash EA 1112 Series). The total carbon value was then used to calculate weight percent of total organic carbon by subtraction (i.e., the weight percent of carbon in the carbonate measured by coulometric analysis was subtracted from the total carbon content obtained with the CHNS analyzer).

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