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doi:10.2204/iodp.proc.309312.204.2009

Introduction

Hole 1256D is in 15 Ma oceanic crust of the Cocos plate, which formed during an episode of superfast (~220 mm/y full rate; Wilson, 1996) spreading of the East Pacific Rise (Fig. F1). Following drilling operations during Ocean Drilling Program (ODP) Leg 206 (Wilson, Teagle, Acton, et al., 2003) and Integrated Ocean Drilling Program (IODP) Expedition 309/312 (see the “Expedition 309/312 summary” chapter), Hole 1256D now penetrates 1507.1 meters below seafloor (mbsf) and provides the first complete sampling of intact upper oceanic crust down to gabbros.

Four major lithologic zones were distinguished in Hole 1256D: lavas, transition zone, sheeted dike complex, and plutonic section (Fig. F2). The upper part of Hole 1256D consists of a thick sequence of lavas (lava pond, ~250–350.3 mbsf and inflated flows, 350.3–533.9 mbsf) and sheet and massive flows (533.9–1004.2 mbsf) separated by a 60 m thick transition zone (1004.2–1060.9 mbsf) from the underlying thin sheeted dike complex (1060.9–1406.6 mbsf) (upper dikes, 1060.9–1348.3 mbsf and granoblastic dikes, 1348.3–1406.6 mbsf). The first gabbroic rocks were encountered at 1406.0 mbsf, and these intrusive gabbros extend to 1458.9 mbsf. Beneath these gabbros, a 24.2 m thick dike screen appears between 1458.9 and 1403.1 mbsf, which separates the upper gabbros from a second intrusive gabbro body between 1483.1 and 1495 mbsf. These lower gabbros are underlain by a further dike screen, and the lowermost rocks recovered from Hole 1256D are from a late-stage, crosscutting basaltic dike (1483.1–1507.1 mbsf) (see the “Expedition 309/312 summary” chapter).

The samples analyzed in this study are part of the Expedition 309/312 sample pool. This was a collaborative effort by shipboard scientists to ensure that a representative suite of samples were comprehensively chemically characterized for their major and trace element compositions. All samples were prepared for analysis at the National Oceanography Centre, University of Southampton, United Kingdom. Samples were cleaned by sawing and grinding to remove drilling contamination before multiple ultrasonic washes with millipore (18.2 MΩ) water to remove contamination. Samples were reduced to a coarse sand grain size using a flypress and reduced to powder by grinding in a Cr-steel shatterbox. Sample powders were then split into 0.5 to 20 g aliquots and distributed for chemical and isotopic analysis by scientists from the shipboard science party.

The pool samples of Expedition 309/312 provide a representative sample suite from the sheet and massive flows down to the late dike unit of the present bottom of hole. During the IODP Expedition 309/312, major and several trace element abundances were analyzed aboard ship by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (see the “Expedition 309/312 summary” chapter). In this study, 159 samples from the Expedition 309/312 sample pool were analyzed at Niigata University, Japan, by X-ray fluorescence (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS). We report our analytical procedures and our estimates of the precision and accuracy of the analyses. Our new analyses are then compared to the shipboard data, though it should be noted that the pool samples and shipboard samples are not identical and were generally selected from a different positions. Finally, we briefly describe geochemical features of these analyses.

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