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doi:10.2204/iodp.proc.309312.203.2009 Data report: whole-rock major and trace elements and mineral compositions of the sheeted dike–gabbro transition in ODP Hole 1256D1Shusaku Yamazaki,2 Natsuki Neo,2 and Sumio Miyashita2AbstractWe analyzed whole-rock major element, trace element, and mineral compositions of basalt and gabbro samples recovered from Ocean Drilling Program (ODP) Hole 1256D during Integrated Ocean Drilling Program Expedition 312. Hole 1256D, located at ODP Site 1256 in the eastern equatorial Pacific, is drilled into ~15 Ma oceanic crust at the East Pacific Rise, which was formed during a period of superfast spreading (up to 220 mm/y full rate). The analyzed samples were collected from the upper dikes (1255–1348.3 meters below seafloor [mbsf]), granoblastic dikes (1348.3–1406.6 mbsf), Gabbro 1 (1406.6–1458.9 mbsf), the upper dike screen (1458.9–1483.1 mbsf), Gabbro 2 (1483.1–1495 mbsf), and the lower dike screen of uncertain origin (1495–1507.1 mbsf). Whole-rock compositions were measured for 19 basalts and 13 gabbros by X-ray fluorescence and inductively coupled plasma–mass spectrometry. Mineral compositions were analyzed for 18 basalts and 24 gabbros by electron probe X-ray microanalysis. Results show the geochemical characteristics of each lithology and the downhole variations from the sheeted dike complex to gabbros in a superfast spreading oceanic crust. The upper dikes, granoblastic dikes, and upper dike screen have similar normal mid-ocean-ridge basalt (N-MORB)-like trace element patterns. However, the plagioclase MgO contents of the granoblastic dikes and upper dike screen are notably lower than those of the sheeted dikes. The lower dike screen has a unique P-, Zr-, Hf-, and light rare earth element–depleted character. On the other hand, clinopyroxene and plagioclase compositions resemble those of the granoblastic dikes and the upper dike screen. Gabbros 1 and 2 have almost the same N-MORB–normalized trace element patterns, though their La/Sm ratios differ. Clinopyroxene TiO2 and the zoning of olivine Fo content also differ between Gabbros 1 and 2. Downhole variations show that Gabbro 1 decreases in whole-rock TiO2, P2O5, and Zr content and increases in pyroxene Mg# (100 × Mg/[Mg + Fe]), except for the most differentiated sample in the middle part of Gabbro 1. |
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