Previous   |    Next

doi:10.2204/iodp.proc.344.205.2016

Materials and methods

Study sites

Detailed descriptions of the drilling results are given in Expedition 334 Scientists (2012a, 2012b) and the “Expedition 344 summary,” “Methods,” and “Input Site U1414” chapters [Harris et al., 2013a, 2013b, 2013c]). Site U1381, located on the incoming plate 50 km offshore Osa Peninsula and 43 km from Caño Island (Fig. F1B), was drilled to investigate the lithostratigraphy of the sedimentary sequence on top of the Cocos Ridge as well as the uppermost portions of the ridge (Expedition 334 Scientists, 2012a). Hole U1381A sediment (90.8 m thick) was divided into two lithostratigraphic units (Expedition 334 Scientists, 2012b; Fig. F1C) and overlies >69.6 m of pillow basalt. Hole U1381C sediment (100.55 m thick) was divided into four lithostratigraphic units (see the “Methods” chapter [Harris et al., 2013b]; Fig. F1D) and overlies 0.33 m of basaltic breccia. Site U1414, located ~1 km seaward of the deformation front offshore the Osa Peninsula (Fig. F1B), was drilled to investigate the lithostratigraphy and pore water of the sedimentary sequence on top of the oceanic basement and in the uppermost portions of the underlying igneous basement (see the “Input Site U1414” chapter [Harris et al., 2013c]). Hole U1414A sediment (375.25 m thick) was divided into three units and overlies 96.35 m of oceanic basement (Fig. F1E).

Analytical methods

Sample preparation

The igneous rocks recovered from Hole U1381A mainly consist of aphyric to highly plagioclase (and/or pyroxene) phyric basalt. Vesicles are commonly filled with authigenic minerals (Expedition 334 Scientists, 2012a, 2012b), reflecting certain extents of alteration. The basement rocks recovered from Hole U1381C are brecciated basaltic fragments consisting of sparsely to highly plagioclase-clinopyroxene phenocrysts, groundmass, and vesicles (3%). Overall alteration of the basaltic groundmass is slight to moderate (see the “Expedition 344 summary” and “Methods” chapters [Harris et al., 2013a, 2013b]). The basement rocks recovered from Hole U1414A are basaltic rocks consisting of sparsely to highly plagioclase and/or pyroxene phenocrysts, groundmass, and vesicles. Alteration is slight to moderate, mainly manifested in the partial replacement of groundmass by clay minerals such as smectite (see the “Expedition 344 summary” and “Input Site U1414” chapters [Harris et al., 2013a, 2013c]).

Samples were crushed into centimeter-sized chips with a hydraulic press. The freshest fragments of each sample were then picked under a binocular microscope. These fragments were leached in 4 N nitric acid for 2 h to remove surface contamination, crushed into 0.5–1 cm³ chips in a stainless steel mortar and pestle, rinsed in distilled water, and dried. The fragments were powdered in an alumina ceramic mill. Details of the sample preparation are the same as those described by Janney and Castillo (1996). The powdered samples were analyzed for major element, trace element, and Sr-Nd-Pb isotopic compositions.

Major and trace element analysis

The SiO₂ contents of the samples were analyzed by X-ray fluorescence (XRF) at the Number 4 Exploration Institute of Geology and Mineral Resources of Shandong Province (China). The other major element oxides and certain trace elements (Ba, Cu, Sr, V, Zn, and Cr) were determined by inductively coupled plasma–optical emission spectrometry (ICP-OES) at Key Laboratory of Marine Sedimentary and Environmental Geology, State Oceanic Administration (KLMSEG-SOA) (China). Concentrations of trace elements including high field strength elements (HFSE) (except for Zr), rare earth elements (REE), and other trace elements (i.e., Li, Be, Cr, Co, Ni, Ga, Rb, Mo, Cd, In, Cs, W, Tl, Bi, Sc, U, Th, and Pb) were measured by inductively coupled plasma–mass spectrometer (ICP-MS), also at KLMSEG-SOA. Samples were prepared by digesting 50 mg of powder with a HF:HNO₃ (2:1) mixture following the method described in Janney and Castillo (1996) with some modifications. Prior to ICP-OES analysis, the glass chips were ultrasonically washed twice in deionized water for 30 min and hand-picked under a binocular microscope. The selected chips were further ultrasonically washed in quartz-distilled water. For each sample, about 25 mg of clean glass chips was digested in an ultrapure 2:1 concentrated HF:HNO₃ solution in a Teflon beaker, and then the mixture was placed on a hot plate (~60°C) and dried under a heat lamp. About 2 mL of ultrapure 12 M HNO₃ was twice added to the digested sample and evaporated to dryness. After dryness, the digested sample was diluted 4000-fold with a 2% HNO₃ solution containing 1 ppb indium as an internal standard. Precisions are ±0.2%–2% for major elements at concentrations >1 wt% (SiO₂, Al₂O₃, and CaO) and about ±2%–5% for minor elements at concentrations <1.0 wt% (MnO, K₂O, TiO₂, and P₂O₅). Loss on ignition (LOI) was also measured. The precision of this method for trace elements is within 10%. The measured values for basement rocks from Holes U1381A, U1381C, and U1414A are presented in Tables T1 and T2. Accuracy was checked by measuring the United States Geological Survey chemical reference standard BHVO-2 with every batch of unknowns. The measured and recommended values for BHVO-2 for the present study are also presented in Tables T1 and T2.

Sr-Nd-Pb isotopic analysis

Sr and Nd isotopic analyses were carried out for three samples from Hole U1381A, one sample from Hole U1381C, and nine samples from Hole U1414A. Pb isotopic analysis was carried out for two samples from Hole U1381A, one sample from Hole U1381C, and eight samples from Hole U1414A. Prior to dissolution, rock powders to be analyzed for Sr isotopic ratios were subjected to a harsh multistep HCl-leaching procedure (e.g., Castillo et al., 1991) to mitigate the effects of seawater alteration on ⁸⁷Sr/⁸⁶Sr ratios. The Sr and Nd separation procedure is similar to that described by Janney and Castillo (1996). Powders were rinsed with 0.75 M HCl for about 10 min to remove possible contamination by seawater. REEs and Sr were separated on cation ion exchange resin using HCl as the eluent, and Nd was separated from other REEs using MCI GEL CK08P cation exchange resin in alpha hydroxyisobutyric acid (α-HIBA) medium. For Pb isotopic analysis, chips of the fresh basaltic breccias were leached in ultrapure warm 6 M nitric acid for 6 h in an ultrasonic bath to remove possible Pb contamination. The samples were then rinsed with quartz-distilled water, dried, and crushed in a tungsten carbide shatterbox. About 300–400 mg of powder was digested in ultrapure 2:1 concentrated HF:HNO₃ solution in a Teflon beaker and dissolved in 1 M HBr. Lead was separated using a standard anion exchange method in a HBr medium (Lugmair and Galer, 1992). Sr, Nd, and Pb isotopic ratios were measured on a high-resolution multicollector inductively coupled plasma–mass spectrometer at KLMSEG-SOA. ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and ⁸⁷Sr/⁸⁶Sr ratios to ⁸⁶Sr/⁸⁸Sr = 0.1194. During the analysis period, the National Bureau of Standards reference material (NBS 987) yielded an average value of ⁸⁷Sr/⁸⁶Sr = 0.710268 ± 12 (2σ) and the JNdi-1 standard gave an average value of ¹⁴³Nd/¹⁴⁴Nd = 0.512120 ± 8 (2σ). Procedural blanks were <200 pg for Sr and <50 pg for Nd. Pb standard NBS 981 was used to correct the measured isotopic ratios of samples for isotopic fractionation; the average correction is 0.1% per atomic mass unit. During the analysis, the NBS 981 standard yielded an average value of ²⁰⁶Pb/²⁰⁴Pb = 16.9377, ²⁰⁷Pb/²⁰⁴Pb = 15.4932, and ²⁰⁸Pb/²⁰⁴Pb = 36.7247. Initial Pb isotopic ratios were calculated from measured ratios in unleached samples using the U, Th, and Pb concentrations of the samples.

Top of page   |    Previous   |    Next