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doi:10.2204/iodp.proc.335.201.2015

Introduction

Oceanic crust formed at fast-spreading rates exhibits relatively uniform seismic stratigraphy (e.g., Canales et al., 2003) and is regarded as layered and relatively homogeneous. This contrasts with the oceanic crust generated at slow-spreading ridges that is characterized by considerable heterogeneity, and where crustal accretion is partly controlled by tectonic rather than solely by magmatic processes (e.g., Cannat et al., 1995; Dick et al., 2008). In spite of the global importance of fast-spreading oceanic crust, up to now only one penetration of intact fast-spreading oceanic crust from the pillow basalts down to the plutonic layer exists: it was established by the IODP (Integrated Ocean Drilling Program) multiexpedition mission “Superfast Crust” at Site 1256. It is located in the eastern equatorial Pacific on 15 Ma oceanic crust of the Cocos plate formed at the East Pacific Rise (EPR) under superfast spreading conditions (220 mm/y, full spreading rate; Wilson, 1996). Hole 1256D, initiated during Ocean Drilling Program (ODP) Leg 206 and continued during IODP Expeditions 309, 312, and 335, penetrated the entire upper oceanic crust, passing through a ~250 m thick sediment sequence, a ~800 m thick lava series, and a relatively thin ~350 m thick sheeted dike complex before finally extending ~100 m into the uppermost gabbros, which consist of two apparently intrusive bodies (e.g., Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Teagle, Ildefonse, Blum, and the Expedition 335 Scientists, 2012). Hole 1256D penetrated for the first time the sheeted dike rooting zone, thus representing our only reference section for the important dike–gabbro transition of typical fast-spreading oceanic crust. This transition is a very complex zone, where magmatic, metamorphic, and hydrothermal processes all occur in close proximity: below, the axial melt lens (AML; e.g., Detrick et al., 1987) filled with a basaltic magma at a temperature of ~1200°C, and above, the seawater hydrothermal cells operate at temperatures of ~400°C (based on the measured exit temperatures at EPR vents; e.g., Von Damm et al., 2003). Theoretical models predict that the heat of the AML is exchanged across a thin (<100 m), hot (>650°C), impermeable conductive boundary layer (CBL) sandwiched between the AML and the sheeted dikes (e.g., Lister, 1974).

Thanks to IODP Expeditions 312 and 335, now the first in situ samples from such a CBL zone are available: a ~70 m thick zone of “granoblastic dikes,” representing high-grade metamorphosed, previously hydrothermally altered basalts of the sheeted dike complex, in which gabbros were intruded, showing a marked heterogeneity in lithofacies. These gabbros are interpreted as frozen remnants of a deeper AML, not yet reached by drilling (e.g., France et al., 2009; Koepke et al., 2011). The recovered samples document a wide spectrum of magmatic/metamorphic rocks ranging from basaltic to evolved compositions and the intimate coupling between temporally and spatially intercalated magmatic, hydrothermal and (prograde and retrograde) metamorphic processes. Here we present detailed microanalytical results obtained with electron microprobe on six granoblastic dike samples recovered during Expedition 335. Four of them are identical to samples from which shipboard thin sections were prepared (Table T1), which are petrographically characterized in Teagle, Ildefonse, Blum, and the Expedition 335 Scientists (2012). Special focus of this investigation was to estimate the temperature of the granoblastic overprint by applying geothermometers based on pyroxene and amphibole compositions and to characterize the composition of plagioclase phenocrysts observed in the granoblastic dikes and interpreted as relics from the primary magmatic stage that survived the granoblastic overprint (Teagle, Ildefonse, Blum, and the Expedition 335 Scientists, 2012). Detailed reports on the geology and characterization of Site 1256 including maps are given in Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists (2006) and Teagle, Ildefonse, Blum, and the Expedition 335 Scientists (2012).

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