Vein Formation Mechanisms in the Sheeted Dike Complex from Hole 504B

Abstract

Diabases were recovered during Legs 137 and 140 at Hole 504B from depths between 1621.5 and 2000.4 meters below seafloor in the lower sheeted dike complex. The samples contain multiple generations of millimetric to centimetric veins. The orientation of the measured veins suggests that two main vein sets exist: one characterized by shallow dipping and the other by random trend. Thermal contraction during rock cooling is considered the main mechanism responsible for fracture formation. Vein infill is related to the circulation of hydrothermal fluids near the spreading axis. Some veins are surrounded by millimeter-sized alteration halos due to fluid percolation from the fractures through the host rock. Vein-filling minerals are essentially amphibole, chlorite, and zeolites. Amphibole composition is controlled by the microstructural site of the rock. Actinolite is the main amphibole occurring in the veins and also in the groundmass away from the halos. In the alteration halos, amphibole shows composition of actinolitic hornblende and Mg-hornblende. Late-stage tension gashes and interstitial spaces in some amphibole-bearing veins are filled with zeolites, suggesting that the veins likely suffered multiple opening stages that record the cooling history of the circulating fluids. Evidence of deformation recorded by the recovered samples seems to be restricted to veins that clearly represent elements of weakness of the rock. On the basis of vein geometry and microstructure we infer structural inteΦretations for the formation mechanism and for deformation of veins.

Hydrothermal Alteration of a Section of Upper Oceanic Crust in the Eastern Equatorial Pacific: A Synthesis of Results from Site 504 (DSDP Legs 69, 70, and 83, and ODP Legs 111, 137, 140, and 148)

“…Amphiboles in the lower dikes range from actinolite to magnesiohornblende and to edenitic hornblende (Figs. 13, 14;Alt et al, 1995;Laverne et al, 1995;Tartarotti et al, 1995;Vanko et al, this volume). A limited data set suggested that the Ti content of amphibole ranged to higher values and that magnesiohornblende was more common in the lower dikes than the upper dikes (Alt et al, 1995, this volume).…”

“…These include the presence of secondary calcic plagioclase and clinopyroxene, increasing amphibole abundance, the common presence of magnesiohornblende, more common ilmenite exsolution in titanomagnetite, the presence of anhydrite locally within the rocks rather than in veins, generally increasing intensity of recrystallization, and losses of metals and sulfur from the lower dikes. The petrography of the lower dikes is described in detail elsewhere (Dick, Erzinger, Stokking, et al, 1992;Alt, Kinoshita, Stokking, et al, 1993;Alt et al, 1995, this volume;Laverne et al, 1995;Tartarotti et al, 1995;Vanko et al, this volume), and is briefly summarized here.…”

“…Rare vein types include the following: one vein of quartz + chlorite + actinolite, anhydrite cutting a quartz + epidote vein, and rare crosscutting veins of late prehnite and laumontite (Alt, Kinoshita, Stokking, et al, 1993). Vein amphiboles range in composition from actinolite to magnesiohornblende, in some cases with actinolite at the center and hornblende along the edges Laverne et al, 1995;Tartarotti et al, 1995;Vanko et al, this volume). In general, amphiboles are more actinolitic in veins than in the adjacent alteration halo, where actinolitic hornblende and magnesiohornblende are more common.…”

“…The sequence and conditions of alteration in the lower dikes of Hole 504B have been divided into five stages on the basis of secondary mineral relationships in veins and host rocks, and from mineral chemical and isotopic data (Alt et al, 1995, this volume;Laverne et al, 1995;Tartarotti et al, 1995;Vanko et al, this volume).…”

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

“…The development of remotely operated vehicles and manned submersibles have recently permitted to obtain plentiful structural data sets, provided that clearly visible planar surfaces were exposed on the seafloor, such as dike contacts, escarpments, faults or other damage zones [e.g., Choukroune et al, 1984;Lagabrielle et al, 1996;Lawrence et al, 1998;Karson et al, 2002;Karson, 2007, 2009]. Finescale structural data from drilling the volcanic section of the oceanic crust have been obtained only in a few places, i.e., at DSDP/ODP Hole 504B which has penetrated the lavas and dikes of the uppermost crust created at the intermediate spreading Costa Rica Rift [Agar, 1990;Alt et al, 1993;Agar and Marton, 1995;Allerton et al, 1995;Dilek et al, 1996aDilek et al, , 1996bHarper and Tartarotti, 1996;Tartarotti et al, 1995Tartarotti et al, , 1996Tartarotti et al, , 1998], and at the present study ODP/IODP Site 1256 [Wilson et al, 2003;Tartarotti et al, 2006aTartarotti et al, , 2006bTeagle et al, 2006]. Difficulties in obtaining a 3-D structural architecture of the upper oceanic crust are mostly due to the fact that structural and other geometric data derive from one-dimensional samples and need to be reoriented to the geographic coordinates, especially for gaining insights into the regional stress field [e.g., MacLeod et al, 1994; Haggas et al, 2001;Tartarotti et al, 2006a; E. Fontana et al, Depth shifting and orientation of core data using a core-log integration approach: A case study from ODP-IODP Hole 1256D, manuscript in preparation, 2009a; E. Fontana et al, Structural and tectonic characterization of upper oceanic crust formed at a superfast spreading ridge using core and log data (ODP-IODP Site 1256, equatorial Pacific), manuscript in preparation, 2009b].…”