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doi:10.2204/iodp.proc.309312.202.2009

Methods and materials

Onboard the JOIDES Resolution, selected representative whole-rock samples (shipboard AES samples) were cleaned by grinding off the outer surfaces with a diamond-impregnated disk to remove surface contamination by saw marks and altered rinds resulting from drilling. After sequential ultrasonication in trace-metal grade methanol, deionized water, and nanopure water, rocks were dried for 10–12 h at 110°C. The dry, clean samples were fragmented to small chips by crushing them between disks of Delrin plastic in a hydraulic press. The rock chips were then ground to a fine powder in a tungsten carbide mill.

Expedition 309/312 POOL samples were prepared at the National Oceanography Centre, University of Southampton (United Kingdom). Following a comprehensive cleaning procedure, samples were crushed and powdered in a chrome-steel mill.

All samples were digested and analyzed in a clean laboratory. All reagents used were distilled and 18.2 MΩ ultrapure water was used. All gabbroic samples and those samples not completely digested on a hot plate were digested in high-pressure bombs. Accurately weighed samples (100 mg) were loaded into polytetrafluoroethylene (PTFE) cups and 1 mL 15.8N HNO₃ and 2 mL 24N HF were added. Samples were dried on a hot plate at 150°C to evaporate the SiF₄. Then 3 mL 12N HCl, 1 ml 16N HNO₃, and 4 mL 24N HF were added to the cups, which were then capped and placed in steel jackets and left in an oven at 180°C for 24 h. Samples were transferred to Savillex PTFE PFA beakers and dried to incipient dryness. After adding 4 mL 6N HCl, the Savillex beakers were placed on a hot plate at 100°–120°C and dried to incipient dryness. Concentrated HNO₃ (2 mL) was added and the solution dried; this step was repeated two more times. After adding 4 mL of 8N HNO₃ to the beakers, the beakers were capped and left on a hot plate at 100°C until the samples were completely redissolved. Sample powders of basalt and dike rock were digested on a hot plate at 150°C using mixed HF and HNO₃ in Savillex PFTE PFA beakers and then dried on the hot plate at the same temperature. When the solution was completely dried, 2 mL 6N HCl was added to the beakers and the beakers were placed on hot plate to dry completely. After adding 4 mL 8N HNO₃, the beakers were capped and placed on a hot plate at ~100°C for 5–12 h. After transferring the sample solutions into acid cleaned polypropylene bottles, a known weight of internal standard solution was added and then diluted with water to a dilution factor of ~1000. The resulting solutions contained 2% HNO₃ and had a nominal internal standard concentration of 10 ppb. The internal standards used were Rh, In, Tm, Re, and Bi and enriched isotopes ⁶Li, ⁶¹Ni, ⁸⁴Sr, and ¹⁴⁵Nd, the mass of which covers the entire mass spectrum of the 35 analytes. This multiple internal standards technique provides the ability to monitor and correct the complex mass-dependent fractionations encountered in inductively coupled plasma–mass spectrometry (ICP-MS) multielement analysis (Eggins et al., 1997). The samples were analyzed with a Varian ICP-MS at University of Houston (USA).

Data reduction was performed offline. During the course of an analytical run and with the introduction of different sample matrixes, it is very common that the instrument sensitivity (defined as the number of counts per second obtained for a given concentration unit, e.g., cps/ppm) will drift. Accurate and precise data can only be obtained if the drift can be monitored and corrected. Raw intensities are corrected for drift, including mass-dependent drift, using combined external and internal standards. The United States Geological Survey (USGS) standard BHVO-2 was applied as an unknown sample to monitor the analytical precision and accuracy for each run. The elements Li, Be, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Pb, Th, and U were analyzed. Oxide interferences for rare earth elements (REEs), Hf, and Ta were corrected by applying a correction factor determined by the analysis of a 5 ppb pure Nd solution prior to each analytical run. Details of this technique is described by Hollocher (2008). Analytical precision represented by relative standard deviation is typically better than 5% (1.3%–4.6%), except for Pb which is 8.7% (Table T1). Analytical accuracy represented by the difference between analyzed and referenced concentrations in percentage is generally better than 5%, obtained by 57 replicate analyses (Table T1). Contamination introduced from crushing and powdering may cause significant bias of the accuracy. Therefore, the precision and accuracy evaluated from the repeated analysis of USGS standard BHVO-2 (Table T1) only refer to the dissolution and ICP-MS procedures.

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