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

A dynamic model for generating small-scale heterogeneities in ocean floor basalts

Small-scale distribution of basaltic compositions along the mid-ocean ridge crests suggests that the magmatic processes are sensitive to spatial and temporal fluctuations related to proximity of ridge hot spots, changes in spreading rate, thermal boundary effects associated with ridge offsets, off-axis volcanoes and ridge propagators, and variations in magma supply through time. A detailed sampling of the East Pacific Rise (EPR) axis, near 13 degrees N, revealed that complex basalt compositional variations unrelated to morphology and structure occur both on the ridge axis and on nearby off-axis seamounts. This small-scale heterogeneity is attributed to successive magmatic cycles separated by periods of quiescence (amagmatic phases) where each cycle involves several melting stages of a composite mantle source, interrupted by extraction and a rapid migration of the melt toward the upper levels of the lithosphere. For a given composition of the mantle source, this process will produce primitive melts which are progressively depleted in incompatible elements. The final contrast in composition between the various extruded magmas depends on the opposing effects of mixing and fractionation during transport toward the ocean floor. A multistage melt extraction model for trace elements, based on nonmodal near-fractional melting (1% increments) with three steps of accumulation (<2.5-5%) and extraction above the melting region, gives reliable results for less than 8-13% total melting of a spinel-lherzolite (olivine 49-55%, orthopyroxene 25-29%, clinopyroxene 18-21%, spinel 1-2%). This partial melting model is only partly constrained on the basis of available chemical and physical data, as well as laboratory experiments, and it has several implications for the dynamics of the upper mantle and the lower crust which are not taken into account by present-day physical models. The major consequences of this model are the introduction of discontinuities in the melting regime and the cyclicity of magma production

Annotated record of the detailed examination of Mn deposits from the 1993 R/V Sonne SO-80a expedition in the Pacific Ocean

SONNE 80a was a multidisciplinary expedition to map and sample intraplate volcanic activity associated with the proposed Easter "hotline" and its relation to the Orongo Transform Fault. The main objectives of the cruise were: 1) to study the area between Easter island, the Orongo Transform Fault and the East Pacific Rise (EPR), in particular to examine the relationship between near-spreading axis and true intraplate volcanic processes. 2) to study the Orongo Transform Fault (probably a "leaky transform") which forms the southern boundary of the of the Easter microplate, to determine to what extent it is involved in the Easter "Hotline" volcanism. 3) to examine the proposed extension of the Easter "hotline" to the west of the EPR in the direction of Henderson, Ducie and Crough Seamounts

(Table 2, page 96) Observation of manganese deposits on basaltic rocks from the Rift Valley in the Atlantic Ocean near 36°N

Various types of rocks consisting of plagioclase basalts, olivine basalts, pyroxene basalts, picritic basalts and volcanoclastics were found in the Rift Valley between 36°52'N and 36°47'N in the Atlantic Ocean. Palagonite and manganese are the main coating materials encountered on the chilled margins of the pillow-lava fragments. Palagonite is often associated with montmorillonite and chlorite. The thickness of palagonite coating the chilled margins increases in the fragments found from the innerfloor axis towards the walls of the Rift Valley. There is a linear correlation between the thickness of palagonite and that of the manganese encrustation. Considering a rate of manganese and palagonite accumulation of about 3my/10**3 year it is speculated that the age of a hill located near the axis of the inner floor near 36°50'N is between 3,000 and 12,000 years. The relative age of the inner floor near the intersection of the Rift Valley walls is less than 120,000 years. The specimens having a thicker manganese and palagonite crust located at 4-6 km away from the inner floor axis are 130,000–482,000 years old

Basement Rocks from the East Pacific Rise Near 9°N Compared With Other Ocean-Floor Volcanic Provinces

The East Pacific Rise (EPR) has an "S" shape extending from the Gulf of California to the southwestern Pacific Ocean where it joins the Mid-Indian Oceanic Ridge in the Tasman Sea region. The length of the EPR is about 17,000 km ; its width is about 2000 km-comparable to that of the Mid-Atlantic Ridge (MAR). [NOT CONTROLLED OCR