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doi:10.2204/iodp.proc.304305.102.2006

Geochemistry

During Expeditions 304 and 305, we performed chemical analyses of samples selected by the shipboard scientific party using inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and gas chromatography (GC). Various rock types, including basalt, diabase, and gabbroic and ultramafic rocks, were analyzed for major oxide and selected trace element concentrations. Sampling and analytical procedures were adapted from those developed during ODP Legs 147, 153, 176, 187, 197, 203, and 209, and the overall strategy was described in ODP Technical Note 29 by Murray et al. (2000). The shipboard analytical facilities are reviewed in the Leg 147 and 187 ODP Initial Reports volumes (Shipboard Scientific Party, 1993, 2001). The ICP-AES was first used during Leg 187, and additional details on hard rock analytical procedures are given in the Leg 197 and Leg 209 Initial Reports volumes (Shipboard Scientific Party, 2002, 2004]).

ICP-AES analyses of major and trace elements

Sample preparation

Selected representative samples were first cut with a diamond-impregnated saw blade and wet-ground on a diamond abrasive wheel to remove surface contamination. Samples were washed in an ultrasonic bath containing methanol for ~10 min, followed by three consecutive ~10 min washes in an ultrasonic bath containing nanopure deionized water. Samples were then dried for ~12 h in an oven at 110°C. The cleaned whole-rock samples (ideally ~20 cm³ in size) were reduced to fragments <1 cm in diameter by crushing them between two disks of Delrin plastic in a hydraulic press or in a percussion crusher, followed by grinding for ~5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. Approximately 1 g of sample powder was weighed on a Scientech balance and ignited at 1025°C for 4 h to determine weight loss on ignition (LOI).

Aliquots of 100 ± 0.5 mg of the ignited whole-rock powders were mixed with 400 ± 0.5 mg of lithium metaborate (LiBO₂) flux that had been preweighed on shore. All samples and standards were weighed on the Cahn Electrobalance under computer control. Weighing errors are conservatively estimated to be ±0.01 mg.

Mixtures of flux and rock powders were fused in Pt-Au crucibles at 1050°C for 5 min in a Bead Sampler NT-2100. Cooled beads were transferred to 125 mL polypropylene bottles and dissolved in 50 mL of 2.3M nitric acid (HNO₃) by shaking with a Burrell bottle shaker for 1 h. After dissolution of the glass bead, all of the solution was filtered to 0.45 µm into a clean 60 mL wide-mouth polypropylene bottle. Next, 2.5 mL of this solution was pipetted to a plastic vial and diluted with 17.5 mL of 2.3M HNO₃ to bring the total volume to 20 mL. This solution-to-sample dilution, used for major and trace elements, is 4000. Dilutions were conducted using a Brinkman digital dispenser (50 mL).

Analysis

Major and trace element concentrations of powder samples were determined with the JY2000 Ultrace ICP-AES. The JY2000 sequentially measures characteristic emission intensities (with wavelengths between ~100 and 800 nm). We routinely ran two element menus: major elements (P, Si, Mn, Fe, Mg, Ti, Ca, Al, Na, and K [in order as measured in the sequence]) and trace elements (Co, Ni, Cr, V, Cu, Zr, Sc, Y, Sr, and Ba [in order as measured in the sequence]). Certified international rock reference materials, calibration and drift solutions, and chemical procedure blanks were included with the unknown samples for each sample run. The elements analyzed, emission lines used, limits of detection, and specific analytical conditions during Expeditions 304 and 305 are provided in Table T7. Detection limits were calculated as being three times the positive blank intensity transferred (using procedure blanks of Runs 1–12). This method to calculate the limit of detection results in a very conservative estimate for the analytical capability for low concentrations.

The JY2000 plasma was ignited 30 min before each run to allow the instrument to warm up and stabilize. After the warm-up period, a zero-order search was 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 standard reference material BHVO-2 prepared in 2.3M HNO₃. During the initial setup, an emission profile was collected for each peak, using BHVO-2, to determine peak-to-background intensities and to set the locations of background points for each element. The JY2000 software uses these background locations to calculate the net intensity for each emission line. The photomultiplier voltage was optimized by automatically adjusting the gain for each element using the standard with the highest concentration for that element (either BHVO-2 or JP-1). Before each run, a profile of BHVO-2 was collected to assess the performance of the instrument from day to day. A typical analytical session for 12 samples lasted ~6–7 h, depending on the chosen element menu (major or trace) and the number of replicate analyses.

All ICP-AES data presented in the site chapter reports were acquired using the Gaussian analytical mode (mode 2) of the Windows JY2000 software (version 5.01). During Expedition 304, this mode was used to fit a Gaussian curve to 5 points, each measured for 1 s across a peak, and during Expedition 305, it was used to fit a Gaussian curve to 7 points, each measured for 0.5 s, across a peak. The fit was then integrated to determine the area under the curve. This mode requires about a factor of 3 more analysis time compared to simply measuring a single peak intensity, but it leads to considerable improvement in analytical precision (Shipboard Scientific Party, 2001). We used the concentric nebulizer for the JY2000 because it delivers a finer aerosol to the plasma and results in a more stable signal (Shipboard Scientific Party, 2001). Use of this nebulizer requires filtering the solutions, as well as somewhat greater dilution factors to reduce clogging.

In order to determine the optimum analysis conditions for elements in the various rock types sampled, three different nebulizers were used during Expedition 304 and their results were compared: a modified Babington glass nebulizer (V-Groove), a Burgener polymer Mira Mist nebulizer (Miramist), and a concentric glass nebulizer (Meinhard). The Meinhard and Miramist nebulizers deliver a finer aerosol to the plasma and result in a more stable signal. Use of these nebulizers requires filtering the solutions, and high dilution factors are needed to reduce clogging. Salting and clogging nevertheless occurred during ICP-AES runs, causing interference with the gas flow and nebulization pattern. Over a single set of analyses, the Miramist nebulizer exhibited reduced clogging and salting, but an inability to effectively remove deposits precluded its further use. We used the Meinhard for the JY2000 for most of the analyses during Expedition 304 and all the Expedition 305 analyses. During Expeditions 304 and 305, the instrument performance and stability were best when using shorter elemental menus for each run, operating at 4000-fold dilution, and running major and trace elements as separate analytical routines.

The ICP-AES runs included duplicates of the three certified rock reference materials, JP-1, BIR-1, and JA-3, uniformly distributed over the run. To monitor the sensitivity of the instrument, eight drift standards (BHVO-2) were measured at least every fifth position and used for a drift correction. During Expedition 305, for the analysis of Cr and Ni, the BHVO-2 drift standard was spiked with artificial Cr and Ni standards (SPEX; 1000 ppm in 2% HNO₃) to achieve a concentration of 2 ppm Cr and 0.7 ppm Ni. A procedure blank solution, derived from a LiBO₂ fusion without a sample, was measured twice, at the beginning and the end of the run. In between samples and standards, a 2.3N HNO₃ wash solution was run for 90 s to avoid cross contamination. A typical run of 30 positions included 2 blanks, 7 drift standards, 9 certified rock standards (primary and secondary), and 12 unknowns. Certified international rock reference materials (see below) were used to test analytical accuracy and reproducibility of the obtained data. ROA-3, a pyroxenite from the Ronda ultramafic massif (Remaïdi, 1993), Sample 305-U1309D-171R-4, 18–30 cm, a gabbro sampled during Expedition 305, and BAS-140, a shipboard diabase standard (Sparks and Zuleger, 1995; Bach et al., 1996), were used as secondary standards during ultramafic rock, gabbro, and basalt analyses, respectively.

Data reduction

Following each analytical session, the raw intensities were transferred to a data file and data reduction was completed using a spreadsheet to ensure proper control over standardization and drift correction. Once transferred, the average raw net intensities, derived from three single values, were recalculated if the precision indicated analytical outliers, which are usually related to the harsh environment in which the instrument was operated. Intensities were then corrected for the procedural blank. A drift correction was then applied to each element by linear interpolation between the drift-monitoring solutions run before and after a particular batch of samples. The interpolation factor for each sample is based on the time of the analysis. Following blank subtraction and drift correction, concentrations for each sample were calculated from the average intensity per unit concentration for the standards JP-1, BIR-1, and JA-3 (Geological Survey of Japan, 2003; U.S. Geological Survey, 2003). A blank was also included in the regression, with both its intensity and concentration set to zero.

Tables T8 and T9 show the analytical uncertainties based on international rock reference materials and samples during Expeditions 304 and 305, respectively. During Expedition 304, estimates of accuracy and precision for major and trace element analyses were based on replicate analyses of ROA-3, DTS-1, BHVO-2, BAS-140, and BIR-1 run as unknowns. During Expedition 305, they were based on replicate analyses of three international reference materials (JB-3, JGb-1, and DTS-1) (Geological Survey of Japan, 2003; U.S. Geological Survey, 2003) and Sample 305-U1309D-171R-4, 18–30 cm. Repeatability of major element measurements was typically >5% (1 relative standard deviation [RSD]), except for phosphorous. Trace element analyses are typically repeatable within 5% (1 RSD), except for phosphorous, cobalt, copper, and some values near the detection limit (see Table T7 for instrument detection limits).

Gas chromatography

To more fully investigate LOI for each sample, GC separation of sample volatiles was carried out using a Carlo Erba NA 1500 CHS analyzer, in which the respective gaseous oxides of C and H were quantitatively determined by a thermal conductivity detector following the procedure developed during Leg 209. The sample introduction system has a vertically mounted quartz tube containing small pellets of reduced Cu separated by a small amount of quartz wool from tungstic anhydride, which acts as a catalyst. Samples were dropped into the tube in tin boats and heated at 1010°C in the presence of oxygen for ~75 s. During this time, nitrogen, hydrogen, carbon, and sulfur released from the sample are oxidized and swept into the GC using a helium carrier gas. The sulfur from sulfide minerals is oxidized to SO₂, and SO₃ is released from any sulfate minerals present in the sample. Aliquots of sulfanilamide (C₅H₈N₂O₅S) weighing between 0.5 and 2.5 mg were used for primary calibration of the instrument. A single sample analysis required ~12 min. During this time, signal intensity at the detector was continuously recorded, and N, C, H, and S separated by the GC were measured sequentially at ~60, 120, 320, and 465 s. Following blank subtraction, concentrations for each sample were calculated from the integrated peak areas of the respective gases relative to those for the standard using a linear regression. The blank was included in the regression.

Sample analyses were performed on rock powder dried at 1100°C for 12 h. Sample sizes were typically 5–12 mg. The certified international rock reference material JP-1 (Imai et al., 1995; Geological Survey of Japan, 2003) and the internal laboratory standard 147 895D-10W (Puchelt et al., 1996) were routinely measured to monitor analytical accuracy and reproducibility. Typical analytical sessions included multiple analyses of each of these standard rocks and triplicates of each unknown sample. Results of the GC analyses for BAS-140 and JP-1 during Expedition 304 are presented in Table T10. Based on 12 runs, the reproducibility of JP-1 (N = 48 for CO₂; N = 49 for H₂O) and 147 895D-10W (N = 35 for CO₂; N = 44 for H₂O) was better than 6.5% for CO₂ and better than 4.0% for H₂O during Expedition 305. The average CO₂ and H₂O concentrations obtained for JP-1 and 147 895D-10W overlap published values (Imai et al., 1995; Puchelt et al., 1996; Geological Survey of Japan, 2003) within 2% and 4%, respectively. Detection limits taken from Expedition 304, calculated as three times the standard deviation of all the blank analyses, are 0.04 wt% for CO₂ and 0.15 wt% for H₂O. In comparing the GC volatile analyses to the LOI results, it is also important to bear in mind that conversion of Fe²⁺ to Fe³⁺ during ignition may produce a gain in weight that is 11.1% of the percentage of ferrous Fe contained in the sample. This can lead to LOI analyses that are less than the volatile concentrations determined by the GC analysis.

Complete sample analyses are given in the CHEMDATA.XLS file (see “Supplementary material”).

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