During Leg 185, two methods were used to examine basement and sediment geochemistry: XRF and atomic absorption (AA) spectrometry.
Samples considered by the shipboard party to be representative of individual lithologic units or of unusual composition were analyzed for major and selected trace elements by XRF. We used a fully automated wavelength-dispersive ARL8420 XRF system, equipped with a 3-kW generator and a Rh-anode X-ray tube, to determine the major- and trace-element abundances in the samples. Analytical conditions used are given in Table T5. The spectrometer is calibrated using a suite of 30 well-analyzed reference standards for basement rocks and 20 for sediments. The values recommended by Govindaraju (1989) are used for all elements. The analytical errors (standard deviation) based on the average of replicate shipboard analyses of the U.S. Geological Survey reference standard BHVO-1 are better than 0.75% for all major elements except for N, K, and Mn, which are <2.60% (Table T5).
After cutting by either a water-cooled diamond circular saw or a 1-in-diameter diamond drill, the samples were sandblasted with alumina grit to remove saw marks or any unwanted material. The average sample taken weighed ~22 g. After sandblasting, the samples were cleaned in an ultrasonic bath in methanol and deionized water for 10 min each, followed by drying at 110°C. Larger pieces (~20 cm3) were reduced to <1 cm diameter by crushing between two disks of Delron plastic in a hydraulic press. The samples were then ground for ~1-5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. Contamination of the samples with Nb during grinding was investigated before the start of Leg 152, and none was detected (Larsen, Saunders, Clift, et al., 1994), although significant contamination occurred for W, Co, and Ta (Thompson and Bankston, 1970).
Samples were prepared for major-element analysis using fused lithium tetraborate glass disks doped with lanthanum oxide as a heavy absorber (Norrish and Hutton, 1969). The discs were prepared from 600 mg of rock powder that had been ignited for 4 hr at ~1025°C and mixed with 7.2 g of dry flux consisting of 80% lithium tetraborate and 20% lanthanum oxide. This mixture, with 20 mL of LiBr (8.6 M) added to prevent adhesion to the Pt-Au crucible, was then melted in air at 1100°C for ~10 min with constant agitation to ensure thorough mixing and then cooled. The 12:1 flux to sample ratio and the use of the heavy absorber made matrix effects insignificant over the normal range of igneous rock compositions. Hence, the relationship between X-ray intensity and element concentration was linear.
We determined trace elements with pressed-powder pellets. For the basement, these were made by mixing 5 g of rock powder with ~2 mL of a solution of Chemplex polymer in methylene chloride (100 mg/cm3) and then pressing the mixture into an aluminum cap under a load of 7 T. For the sediments, we used 6.0 g of powder. A pellet made with 5 or 6 g of powder should be infinitely thick at the shortest wavelengths used in the analysis. X-ray intensities were corrected for line overlap and interelement absorption effects. The latter corrections were based on the relationship between the mass absorption coefficient and the intensity of the Rh-K Compton scatter line (Reynolds, 1963, 1967; Walker, 1973). We measured loss on ignition from weighed powders, which were heated for 4 hr at 1025°C, then reweighed.
For more details on calibration, see the "XRF Analyses" sections in the "Explanatory Notes" chapters of Becker, Sakai, et al. (1988, pp. 9 and 10) and Dick, Erzinger, Stokking, et al. (1992, pp. 13-18).
Hole 801C basalt samples were analyzed for K by AA spectrometry in order to provide higher resolution data than that provided by the more complete and time-consuming XRF analyses. Potassium was measured because it is sensitive to basalt alteration and is a main contributor to the natural gamma radiation (NGR) measurements of both the downhole logs and the MST. Thus, we were interested in obtaining a higher resolution data set for K on targeted samples and intervals to better understand the length scales of basalt alteration and to help calibrate the NGR measurements. The results of this study are described in "Discussion" in "Igneous Petrology and Geochemistry" and "Physical Properties," both in the "Site 801" chapter. There are several advantages to the AA method over the XRF protocol: (1) smaller sample sizes can be used (<0.1 g), as opposed to the >3 g required by XRF, thus allowing higher resolution sampling; (2) detection limits are lower; and (3) the prepared solutions may be used for shore-based inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) analyses of a greater suite of major and trace elements.
For the shipboard AA analyses, small chips of rock samples were taken on the sampling table, cleaned in an ultrasonic bath in nanopure water (DI), and oven dried at 105°C for several hours. These sample chips were subsequently crushed between Delron plastic dishes and powdered by hand in an agate mortar and pestle. Approximately 0.10 ± 0.01 g of dried sample powder was used in the analysis. The precise weight was recorded to four decimal places and was used in calculations of final individual dilution factors on a per sample basis. The precision parameter on the Scientech Balance Control was set to 0.0005 to provide an effective precision of weighing to better than 1% of the measured value. The precision of the weighing was aided by the calm seas of the balmy, tropical climate at Site 801 during this leg.
The sample powder was poured directly into 6 mL of 8 N (35%) nitric acid in a Teflon beaker, into which was added 2 mL of concentrated HF acid. Both small (3 mL) and large (100 mL) Teflon beakers were used for the dissolution, with no difference in decomposition quality. Samples were heated and dried on a hot plate to a hard residue at ~60°C over 5-8 hr. The commonly yellow-colored residue resulting from the drydown was redissolved in 6 mL of 8 N (35%) nitric acid for ~30 min. Even at this stage, a yellow residue was commonly visible. The 6 mL of acid solution, along with the residue, was diluted in a 60 mL acid-cleaned bottle by adding 12 mL of DI. This primary dilution was achieved by successive DI additions of 3 mL each to thoroughly clean the Teflon beaker. During this dilution, the yellow residues dissolved. From this initial dilution, 3 mL was taken and added to 18 mL of 0.35% CsCl solution to arrive at a final aliquot for AA analysis. This dilution worked well for the lowest concentration samples only, with the remaining ones requiring a further 11-fold dilution. The final dilution factor in the majority of the samples, therefore, was ~13,860-fold.
The AA was operated in flame emission mode, using the 766.5-nm wavelength and a slit width of 0.2 nm. The AA conditions, and the use of the CsCl matrix, were selected on the basis of the operating conditions suggested by Gieskes et al. (1991) for the analysis of interstitial water.
Blanks and standard reference materials (SRMs) were prepared along with the samples. Several SRMs and natural samples were analyzed in replicate to assess procedural and analytical precision, as well as for a final correction. Precision was quantified to be better than 10% of the measured value. Matrix matched (in terms of the nitric acid and CsCl concentrations) calibration standards at K concentrations of 0, 0.1, 0.2, 0.5, and 1 ppm were used. The calibration was fit with a second order polynomial equation over this concentration range with a correlation coefficient (r2) >0.995. Data from all unknown samples, known samples (previously analyzed by XRF), and SRMs were analyzed, and the data were reduced in this fashion.
Accuracy was also assessed by the analyses of the SRMs. As long as the same pipettes and all other laboratory apparatus were used during the preparation of the SRMs and the unknowns, then uncertainties caused by the volumetric (as opposed to mass) dilutions and vagaries of the dissolution procedure were minimized. Comparison of the AA results of the analysis and the accepted values of the SRMs shows a strongly linear relationship, with the shipboard measurements agreeing to within 10% of the accepted values. The shipboard values were corrected by this offset to arrive at the final concentrations.