Accretion, structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations

Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43190/1/11001_2005_Article_BF01204282.pd

International Ocean Discovery Program Expedition 360 Preliminary Report: Southwest Indian Ridge Lower Crust and Moho the nature of the lower crust and Moho at slower spreading ridges (SloMo Leg 1)

International Ocean Discovery Program (IODP) Expedition 360 was the first leg of Phase I of the SloMo (shorthand for "The nature of the lower crust and Moho at slower spreading ridges") Project, a multiphase drilling program that proposes to drill through the outermost of the global seismic velocity discontinuities, the Mohorovičić seismic discontinuity (Moho). The Moho corresponds to a compressional wave velocity increase, typically at ∼7 km beneath the oceans, and has generally been regarded as the boundary between crust and mantle. An alternative model, that the Moho is a hydration front in the mantle, has recently gained credence upon the discovery of abundant partially serpentinized peridotite on the seafloor and on the walls of fracture zones, such as at Atlantis Bank, an 11-13 My old elevated oceanic core complex massif adjacent to the Atlantis II Transform on the Southwest Indian Ridge. Hole U1473A was drilled on the summit of Atlantis Bank during IODP Expedition 360, 1-2 km away from two previous Ocean Drilling Program (ODP) holes: Hole 735B (drilled during ODP Leg 118 in 1987 and ODP Leg 176 in 1997) and Hole 1105A (drilled during ODP Leg 179 in 1998). A mantle peridotite/gabbro contact has been traced by dredging and diving along the transform wall for 40 km. The contact is located at ∼4200 m depth at the drill sites but shoals considerably 20 km to the south, where it was observed in outcrop at 2563 m depth. Moho reflections have, however, been found at ∼5-6 km beneath Atlantis Bank and <4 km beneath the transform wall, leading to the suggestion that the seismic discontinuity may not represent the crust/mantle boundary but rather an alteration (serpentinization) front. This then raises the interesting possibility that a whole new planetary biosphere may thrive due to methanogenesis associated with serpentinization. The SloMo Project seeks to test these two hypotheses at Atlantis Bank and evaluate carbon sequestration in the lower crust and uppermost mantle. A primary objective of SloMo Leg 1 was to explore the lateral variability of the stratigraphy established in Hole 735B. Comparison of Hole U1473A with Holes 735B and 1105A allows us to demonstrate a continuity of process and complex interplay of magmatic accretion and steady-state detachment faulting over a time period of ∼128 ky. Preliminary assessment indicates that these sections of lower crust are constructed by repeated cycles of intrusion, represented in Hole U1473A by approximately three upwardly differentiated hundreds of meter-scale bodies of olivine gabbro broadly similar to those encountered in the deeper parts of Hole 735B. Specific aims of Expedition 360 focused on gaining an understanding of how magmatism and tectonism interact in accommodating seafloor spreading, how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere

Chemical analysis of sediments from the Galapagos Spreading Center (DSDP Leg 54, site 424)

The deposits from the DSDP Site 424 hole are divided into two major categories according to the relative proportions of the green clay-rich fractions and the presence of Fe-Mn concretions. The Fe-Mn concretions are associated with interlayers of pelagic foraminiferal nannofossil ooze from the Upper Pleistocene. This carbonate ooze is rather enriched in Mn by comparison with the green clay-rich fractions. Representative carbonate ooze samples have been chemically analysed by Cambon P. and Etoubleau J. from COB-CNEXO (Ifremer) using X-ray fluorescence