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doi:10.2204/iodp.proc.309312.204.2009

Methods and materials

Ten major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) were analyzed by XRF (Rigaku RIX 3000) at Niigata University following the analytical procedures of Takahashi and Shuto (1997) (Table T1). For trace element analyses (Sc, V, Cr, Co, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Ta, Pb, Th, and U), 159 samples were dissolved by HF-HNO₃ acid digestion following Takazawa et al. (2003). A total of 62 samples were also dissolved and analyzed by alkali fusion. Solutions were analyzed using an Agilent 7500a ICP-MS at Niigata University, calibrated using the reference values for BHVO-1 (basalt; Hawaii, U.S. Geological Survey [USGS]) of Eggins et al. (1997). Although we did not have enough sample powder to analyze all samples, selected trace elements (V, Cr, Ni, Rb, Sr, Y, Zr, Nb, Ba, Pb, and Th) were also determined by XRF to cross-check the ICP-MS data where possible.

This approach identified that for many samples cored during Expedition 312 and three samples cored during Expedition 309, there had not been complete dissolution during HF-HNO₃ attack, most probably because of the incomplete dissolution of acid-resistant minerals such as zircon and/or titanite. Therefore, all samples from Expedition 312 and three samples from Expedition 309 were dissolved by alkali fusion (Roser et al., 2000). In a variation of the Roser et al. (2000) method we dissolved fluxed samples in concentrated nitric acid as opposed to perchloric acid. Sample powders were dissolved in concentrated HF-HNO₃ using Pt crucibles and evaporated. Afterward, Na₂CO₃ alkali flux was added and heated in an electric furnace. The fluxed samples are dissolved in concentrated HNO₃, HCl, and ultrapure water. The samples were finally diluted ~20,000 times.

To check the accuracy of analyses, GSJ (Geological Survey of Japan) reference samples were analyzed by XRF. Major element data of JB-1a (basalt; Kitamatsuura), JB-2 (basalt; Oshima), and JB-3 (basalt; Fuji) are shown in Table T2. For trace element analyses, USGS Geochemical Reference Standard W-2 (diabase; Virginia) was dissolved and analyzed with every group of samples (Table T3). Major element and trace element abundances of shipboard standards BAS-206 and BAS-312 were also analyzed (Table T3).

For major elements, relative deviation (RD) values, an estimate of accuracy, between this study and recommended values are very low (Table T2). For trace elements, relative standard deviation (RSD) values, an estimate of precision, are <5% except for Sc, Co, Ni, Pb, and U. Trace element RD values of the average results from this study are less than ±4% for five sets of W-2 analyses compared to the recommended values of Eggins et al. (1997). RSD values of Zr, Hf, Th, and U that were dissolved by alkali fusion are <5%, but RD values for the recommended value (Eggins et al., 1997) are slightly higher (10%) for eight sets of sample analyses.

Table T3 shows average (five analyses each) values of BAS-206 and BAS-312, RSD values and average standard deviation values for this study. RSD for most elements are <5% except for V, Cr, Ni, Cs, Ba, and Pb.

Sc analyses were strongly affected by a high blank of up to 8% of BHVO-1 counts per second, and therefore RSD values of Sc were high. Pb blanks were also high, possibly because of contamination by Pb from plastic solution storage vessels. The contamination of Pb from plastic vessels is demonstrated by the correlation between residence time in the plastic vessels and measured Pb abundances in the 5% HNO₃ solutions. Hence, Pb analyses also show highly dispersed results.

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