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Data report: microstructure of chilled margins in the sheeted dike complex of IODP Hole 1256D: Expedition 309/312

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Proceedings of the IODP

Integrated Ocean Drilling Program

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      Abstract

      Chilled margins were recovered from the sheeted dike complex (SDC) of superfast (>200 mm/y)-spreading East Pacific Rise–spread crust during drilling of Integrated Ocean Drilling Program Hole 1256D on the Cocos Plate. The chilled margins contain stretched spherules, oriented plagioclase laths, grain-size segregation, and color banding. These rheomorphs locally crosscut veins but are elsewhere crosscut by veins. Electron microprobe investigations found that the chilled margins comprise dispersed micrometer-scale minerals and veins including chlorite, actinolite, quartz, anhydrite, sphene, calcite, sphalerite, K-feldspar (adularia and/or orthoclase), magnetite, pyrite, and chalcopyrite. Though many of these phases are present throughout the SDC, anhydrite and calcite have not been previously recognized >100 m below the SDC–lava transition zone, and, with one exception, K-feldspar has not been previously identified in Hole 1256D core. Microstructures include quartz clasts surrounded by anhydrite, K-feldspar veins and clasts cut or surrounded by chilled margin material, and lenses of ductily deformed sphene. Some of the crosscutting relationships and distribution of mineral phases could be explained by hydrothermal alteration that occurred roughly simultaneously with dike intrusion.

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        Drilling to gabbro in intact ocean crust.

        Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.
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          Quartz-cemented breccias from the Mid-Atlantic Ridge: Samples of a high-salinity hydrothermal upflow zone

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            Author and article information

            Journal
            10.2204/iodp.proc.309312.2006
            Proceedings of the IODP
            Integrated Ocean Drilling Program
            1930-1014
            09 July 2009
            10.2204/iodp.proc.309312.205.2009

            This work is licensed under a Creative Commons Attribution 4.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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            Self URI (journal page): http://publications.iodp.org/

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