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doi:10.2204/iodp.proc.309312.102.2006

Geochemistry

Analysis of basement fluid

Water samples collected with the water-sampling temperature probe were double-filtered through 0.45 µm sterile Acrodisc filters. Aliquots for future shore-based analyses were treated following established protocols (Table T8). Full geochemical characterization was undertaken on the overflow sample, whereas a more limited suite of analyses was performed on the Ti-coil sample (Table T8). Salinity and total dissolved solids were determined with an optical handheld refractometer (Reichert), and pH and alkalinity were measured by Gran titration with a Brinkmann pH electrode and a Metrohm autotitrator. Chloride concentrations were determined by titration with AgNO₃. Silica, phosphate, and ammonium measurements were carried out by colorimetry using a Milton Roy Spectronic spectrophotometer, following the analytical techniques described by Gieskes et al. (1991).

Sulfate, potassium, calcium, and magnesium concentrations were analyzed by ion chromatography using the Dionex DX 120 ion chromatograph. Major and minor cation concentrations (Li, B, Sr, Ba, Mn, and Fe) were determined using the Jobin-Yvon Ultrace ICP-AES following the procedure outlined by Murray et al. (2000). In preparation for analysis by ICP-AES, 0.5 mL aliquots of borehole fluid were acidified with 9.5 mL of nitric acid (HNO₃) and diluted 10-fold with a matrix solution (2.25% HNO₃ containing 9 ppm Y) for minor elements and diluted 50-fold for major elements. Analytical blanks were prepared in an identical manner by analyzing nanopure water acidified and diluted with a matrix solution to ensure a matrix match with the borehole fluid. Sodium was determined using a charge balance calculation, where Σ_cations = Σ_anions.

Dissolved organic carbon concentrations were measured using a TOC-5000A analyzer. Approximately 1.5 mL of the borehole fluid was removed from the overflow sample and quickly frozen to preserve the sample until an analysis could be made. Samples were diluted sixfold (1 mL of sample to 5 mL of nanopure water) and acidified to a pH of ~2 using 53 mL of 2N hydrochloric acid (HCl). Samples were then purged with purified air for 3 min (50 mL/min) and analyzed by triple injections of 25 µL of sample.

International Association of Physical Sciences of Organization (IAPSO) standard seawater was used for calibrating most techniques. The reproducibility of these analyses, expressed as a percent of the standard deviation (1 σ) divided by the average of several IAPSO values, is summarized in Table T9. Accuracy of individual analyses is within the standard deviation range accepted for IAPSO values.

Hard rock sampling and geochemical analyses

Sample preparation

Representative samples of igneous rocks were analyzed for major and trace elements during Expedition 309/312 using ICP-AES. Approximately 20 cm³ samples for cryptocrystalline to fine-grained rocks and as much as 50 cm³ samples for medium- to coarse-grained rocks were cut from the core with a diamond saw blade, and when possible, a thin section billet was also taken. All outer surfaces were ground on a diamond-impregnated disk to remove surface contamination by saw marks and altered rinds resulting from drilling. Each cleaned sample was placed in a beaker containing trace metal–grade methanol and ultrasonicated for 15 min. The methanol was decanted, the samples were ultrasonicated twice in deionized water for 10 min and ultrasonicated for 10 min in nanopure deionized water. The cleaned pieces were then dried for 10–12 h at 110°C.

The clean, dry whole-rock samples were crushed to <1 cm chips between two disks of Delrin plastic in a hydraulic press. The rock chips were then ground to a fine powder in a tungsten carbide mill in a SPEX 8510 shatterbox. After grinding, a 1.0000 ± 0.0005 g aliquot of the sample powder was weighed on a Mettler Toledo balance and ignited at 1025°C for 4 h to determine weight loss on ignition (LOI) with an estimated precision of 0.02 g (2 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 with minor changes and additions. 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. A check on grinding contamination contributed by the tungsten carbide mills (Table T10) was performed during Leg 206 and was found to be negligible (Shipboard Scientific Party, 2003b). All samples and standards were weighed on the Cahn C-31 microbalance (designed to measure on board) with weighing errors conservatively estimated to be ±0.02 mg.

A 10 mL aliquot of 0.172 mM aqueous LiBr solution was added to the flux and rock powder mixture as a nonwetting agent to prevent the cooled bead from sticking to the crucible. Samples were then individually fused in Pt-Au (95:5) crucibles for ~12 min at a maximum temperature of 1050°C in a Bead Sampler NT-2100 (internal rotating induction furnace). After cooling, beads were transferred to 125 mL high-density polypropylene bottles and dissolved in 50 mL 10% HNO₃, aided by shaking with a Burrell wrist-action bottle shaker for 1 h. Samples were ultrasonicated for ~1 h after shaking to ensure complete dissolution of the glass bead. After digestion of the glass 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 for this procedure was ~4000. During Expedition 312, stock standard solutions were ultrasonicated for 1 h prior to their final dilution and analysis to ensure a homogeneous solution.

Analyses

Major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Sc, V, Zn, Cr, Co, Ni, Cu, Sr, Y, Zr, Nb, and Ba) element concentrations of standards and samples were determined with the JY2000 Ultrace ICP-AES, which routinely measures wavelengths between ~100 and 800 nm. Specific analytical conditions for sample runs during Expedition 309/312 are provided in Table T11.

The JY2000 plasma was ignited at least 30 min before each sample 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 the BAS-140 standard (basalt interlaboratory standard created during Leg 140 in Hole 504B; Bach et al., 1996), or the BAS-206 standard (basalt interlaboratory standard created during Leg 206) prepared in 10% HNO₃. During the initial setup, an emission profile was selected for each peak, using BAS-140, to determine peak-to-background intensities and set the locations of background levels 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 BAS-140.

ICP-AES data presented in “Geochemistry” in “Expedition 309”and “Geochemistry” in “Expedition 312”(both in the “Site 1256” chapter) were acquired using either the Gaussian or Maximum mode of the Windows 5 JY2000 software. Gaussian mode fits a curve to points across a peak and integrates the area under the curve to determine element intensity; it was used for Si, Ti, Al, Fe, Ca, Na, K, Sc, V, Zn, Sr, Zr, Cu, and Nb. Maximum mode was used for elements with asymmetric emission peaks (Mn, Mg, P, Y, Cr, Ni, and Ba), and intensity was integrated using the maximum intensity detected. Each unknown sample was run at least twice (from the same dilute solution; i.e., in replicate), nonsequentially, within a given sample run. For Si and Nb, measurements were made at two wavelengths (Si = 251.611 and 288.158 nm; Nb = 269.706 and 309.418 nm) and for each analysis the peak with the better precision was used. Net intensities of the working blank for Nb were almost identical to the peak intensities of the standards and unknowns. Because a calibration line could not be established, Nb was excluded after Run 3 for Expedition 309 and measured but not reported for Expedition 312.

A typical hard rock ICP-AES run (Table T12) during Expedition 309 included

• A set of six certified rock standards (JA-3, JB-3, BHVO-2, AGV-1, BIR-1, and JGb-1) analyzed twice during each sample run;
• Up to 20 unknown samples run in replicate;
• A drift-correcting sample (BCR-2) spiked with Ni and Cr and analyzed every fourth sample position and at the beginning and end of each run;
• Blank solutions run near the beginning and end of each run; and
• A check standard (i.e., standard run as an unknown), typically BAS-140 or BAS-206.

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

Hard rock ICP-AES runs were slightly modified during Expedition 312 to include

• A set of three certified rock standards (BIR-1, JB-3, and JGb-1) having major and trace element compositions similar to the rock types recovered during Expedition 309 and those anticipated during Expedition 312;
• A set of four unknown samples run in replicate;
• A drift-correcting sample (BHVO-2) in every fourth sample position beginning with the first position of each run; and
• Interlaboratory standards run as unknowns that included, beginning with Run 3, BAS-140, BAS-148, BAS-206, and BAS-312.

The first two runs did not use the above running protocol and so were reanalyzed; only the data obtained during reanalysis are reported for these samples.

Data reduction

Following each sample run, raw intensities were transferred to a data file and all analyses were corrected first for drift and then for the full procedural blank. Drift correction was applied to each element by linear interpolation between drift-monitoring solutions run every fourth analysis. Following drift correction and blank subtraction, calibration curves were constructed based on six certified rock standards (JA-3, JB-3, BHVO-2, BIR-1, JGb-1, and BCR-2 run as the drift monitor) during Expedition 309. Expedition 312 calibration curves were constructed based on four certified rock standards (BIR-1, JB-3, JGb-1, and BHVO-2 run as the drift monitor). Unknown concentrations were then calculated from the calibration line.

Estimates of accuracy and precision of major and trace element analyses during Expedition 309 were based on replicate analyses of check standards (usually BAS-140 and BAS-206), the results of which are presented in Table T13. Run-to-run relative standard deviation by ICP-AES was generally ±3% for major elements and ±10% for trace elements. Exceptions typically occurred when the element in question was near background levels. Estimates of reproducibility during Expedition 312 were based on replicate analyses of interlaboratory rock standards (BAS-140, BAS-148, BAS-206, and BAS-312), the results of which are presented in Table T14. Relative standard deviation (i.e., precision) for the major elements is ±2% and for most trace elements is ±5%. Details regarding the precision for individual elements can be found in Table T14.

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