IODP Expeditions 309 and 312 drill an intact section of upper oceanic basement into gabbros

The Integrated Ocean Drilling Program's (IODP) Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program's (ODP) Leg 206, now penetrates to >1500 mbsf and >1250 m sub-basement. The first gabbroic rocks were encountered at 1407 mbsf. Below this, the hole penetrates 3c100 m into a complex zone of fractionated gabbros intruded into contact metamorphosed dikes

Petrophysical properties of the root zone of sheeted dikes in the ocean crust: A case study from Hole ODP/IODP 1256D, Eastern Equatorial Pacific

ODP (Ocean Drilling Program)/IODP (Integrated Ocean Drilling Program) Site 1256 is located on the Cocos Plate in the Eastern Equatorial Pacific Ocean, in a 15. Ma old oceanic lithosphere formed at the EPR during a period of superfast spreading (> 200 mm/yr). ODP/IODP Hole 1256D reached for the first time the contact between sheeted dikes and underlying gabbros. It consequently offers a unique opportunity to study in situ, in present-day oceanic crust, the root zone of the sheeted dike complex. This root zone is a thin, 100. m thick boundary layer between the magmatic system (i.e., the axial melt lens, . 1100°C), and the overlying high temperature hydrothermal system (≤ 450 °C). The understanding of interactions within this boundary layer is critical to that of crustal processes along mid-ocean ridges.This work focuses on the petrophysical characterization of the root zone of the sheeted dike complex in order to further constrain the hydrothermal circulation system in the vicinity of the axial melt lens, as recorded in non-granoblastic dikes, granoblastic dikes, and varitextured gabbros. The petrophysical properties were determined from sample measurements in the laboratory and were compared to in situ downhole geophysical probing. The porosity structure is bipolar, depending on lithology, resulting in a layered system. Non-granoblastic dikes are generally altered in the greenschist facies ( >250°C) with relatively high and interconnected (cementation index m . 1.72, electrical tortuosity τ 28.3) porosity (1.5%). In contrast, gabbros are retrogressively metamorphosed in the amphibolite ( >450°C) and greenschist facies, with lower porosity (1.3%) that involves numerous fissures and cracks, resulting in a more connected medium (m 1.58, τ 11.8) than non-granoblastic dikes. These cracks are more abundant but also tend to close with increasing depth as indicated in downhole geophysical data. Porosity and alteration, as viewed from surface electrical conductivity, appear to be directly correlated. © 2010 Elsevier B.V

Comparison of alternative methodologies for identifying and characterizing preferential flow paths in heterogeneous aquifers

International audienceOne of the main difficulties encountered when characterizing the hydrodynamic properties of a fractured aquifer is to identify the preferential flow paths within it. Different methods may be applied to determine the variability of the permeability at the borehole scale and to image the structure of the main flow zones between boreholes. In this paper, we compare the information obtained from different measurement techniques performed in a set of three 100 m depth wells (well-to-well spacing: 5­10 m) in a fractured crystalline rock setting. Geophysical logging and borehole-wall imaging are used to identify open and closed fractures intersecting the boreholes and their orientation. The comparison with flowmeter and single packer tests shows that few of the fractures interpreted as open from geophysical logs are significantly transmissive. Cross-borehole connectivity is first investigated from single packer tests with pressure monitoring in adjacent boreholes. To determine fracture zone connectivity, we propose a methodology simply based on the variation with packer depth of the ratio of the drawdown in the observation well and the drawdown in the pumping well. The results are compared to the analysis of cross-borehole flowmeter tests. We show that both methods provide consistent results with a similar level of information on connectivity

Superfast spreading rate crust 2 and 3

IODP publications, Scientific Prospectu