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Methods and materials

A total of 19 representative rock samples were selected to constrain the Shikoku Basin basalts drilled during Expedition 333. Special care was taken to remove surface contamination by saw marks and altered rinds resulting from drilling by grinding off the outer surfaces on a diamond-impregnated disk. The cleaned rock blocks were then ultrasonicated in trace-metal grade methanol, deionized water, and Milli-Q water (18.2 MΩ) to remove contamination during drilling and cutting. The cleaning procedures were done repeatedly until the silver nitrate solution test was negative to ensure the complete removal of seawater. The cleaned rocks were then dried for 10–12 h at 110°C. These dry and clean samples were fragmented to small chips by crushing them between two disks of Delrin plastic in a hydraulic press. The rock chips were then ground to a fine powder in an aluminum ceramic mill.

Analysis for major and minor element concentrations (Si, Al, Mg, Fe, Mn, Ti, Ca, Na, K, and P) were conducted on a Thermo-Jarrell Ash sequential inductively coupled plasma–atomic emission spectrometer (ICP-AES) at the University of Houston (TX, USA) using methods described in Lytwyn and Casey (1993) and Smith (1994). Loss on ignition (LOI) was determined from the total weight change of the sample powder by heating in a Lindberg Model 51440 furnace at 1000°C for 30 min. After determination of LOI, 0.2000 ± 0.0002 g of the ashed (“ignited”) sample powder was mixed with 1.0000 ± 0.0002 g of lithium metaborate (Aldrich, 99.9% trace metal grade) in a high purity graphite fusion crucible (SCP Science) for the fusion process. The fusion process was conducted in a Lindberg Model 51440 furnace at 1125°C for 15 min. After 15 min, the molten bead was immediately poured into 100 mL of 1.5 N HNO3 (made from double-distilled HNO3) for dissolution in a 150 mL Teflon beaker on a hot plate with magmatic stirring. After the molten bead completely dissolved, the solution was passed through Whatman No. 40 ashless filter paper to filter out any carbon residue. The filtered solutions were then further diluted with 1.5 N HNO3 (made from double-distilled HNO3) to a dilution factor of 1:5000 for major element analysis with ICP-AES. The measured elemental abundances were calibrated against five international reference standards (AGV-1, BHVO-2, JGb-1, BIR-1, and W-2) and a LiBO2 flux blank. Relative errors (precision and accuracy) monitored by repeated analysis of international reference standard BCR-2 are generally <1% to 5% for analyzed elements (Table T1).

For trace element (Li, Be, B, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Tb, Gd, Dy, Ho, Er, Yb, Lu, Hf, Ta, Pb, Th, and U) analysis, powdered samples (without “ignition”) were digested with mixed acids in a clean room based on a slightly modified method described by Gao et al. (2009). All acids used (HNO3, HCl, and HF) were double-distilled, and ultrapure 18.2 MΩ Milli-Q water was used. Precisely weighed samples (100 mg) were placed into Savillex PFA beakers, and then 1 mL of 16 N HNO3 and 2 mL of HF were added. The samples were then dried on a hot plate at about 150°C to evaporate the SiF4. Then 3 mL of 12 N HCl, 1 mL of 16 N HNO3, and 4 mL of 24 N HF were added to the beakers. The capped beakers were heated on a hot plate at about 180°C for at least 24 h. The samples were then dried to incipient dryness and refluxed with 4 mL of 8 N HCl. The samples were repeatedly fluxed twice with 2 mL of 16 N HNO3 to get rid of HF and HCl. After adding 4 mL of 8 N HNO3, the capped beakers were placed on a hot plate at temperatures about 100°C for 5–12 h so that the samples redissolved. After transferring the sample solutions into acid-cleaned low-density polyethylene bottles, a known amount of internal standard solution was added and then diluted with Milli-Q H2O to a dilution factor of 1:1000. The resulting solutions contain 2% HNO3 and a nominal internal standard concentration of 10 ppb. The internal standards used were Rh, In, Tm, Re, Bi, and enriched isotopes 6Li, 61Ni, 84Sr, and 145Nd, the mass of which spaces through the entire mass spectrum of all the 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 (Gao et al., 2009, Eggins et al., 1997). The samples were then analyzed with a Varian 810 ICP-MS at the University of Houston.

For ICP-MS analysis, the data reduction was performed offline, for which both drift and oxide interference were corrected by external and internal standards (Gao et al., 2009). Analytical precision and accuracy monitored by repeated analysis of international reference standard JGB-1, which was processed along with the samples, are typically better than 5% (Table T1).