Compositions of dikes and lavas from the Pito Deep Rift: Implications for crustal accretion at superfast spreading centers

The northwest trending walls of the Pito Deep Rift (PDR), a tectonic window in the southeast Pacific, expose in situ oceanic crust generated ∼3 Ma at the superfast spreading southern East Pacific Rise (SEPR). Whole rock analyses were performed on over 200 samples of dikes and lavas recovered from two ∼8 km2 study areas. Most of the PDR samples are incompatible-element- depleted normal mid-ocean ridge basalts (NMORB; (La/Sm)N < 1.0) that show typical tholeiitic fractionation trends. Correlated variations in Pb isotope ratios, rare earth element patterns, and ratios of incompatible elements (e.g., (Ce/Yb)N) are best explained by mixing curves between two enriched and one depleted mantle sources. Pb isotope compositions of most PDR NMORB are offset from SEPR data toward higher values of 207Pb/ 204Pb, suggesting that an enriched component of the mantle was present in this region in the past ~3 Ma but is not evident today. Overall, the PDR crust is highly variable in composition over long and short spatial scales, demonstrating that chemically distinct lavas and dikes can be emplaced within the same segment over short timescales. However, the limited spatial distribution of high 206Pb/204Pb samples and the occurrence of relatively homogeneous MgO compositions (ranging <2.5 wt %) within a few of the individual dive transects (over distances of ∼1 km) suggests that the mantle source composition evolved and magmatic temperatures persisted over timescales of tens of thousands of years. The high degree of chemical variability between pairs of adjacent dikes is interpreted as evidence for along-axis transport of magma from chemically distinct portions of the melt lens. Our findings suggest that lateral dike propagation occurs to a significant degree at superfast spreading centers

Affleurements des roches profondes de la croûte océanique et du manteau sur le mur sud de la fracture Kane (Atlantique central) : observations par submersible = Deep layers of mantle and oceanic crust exposed along the southern wall of the Kane Fracture Zone : submersible observations

The aim of the 20 Nautile dives of the recent Kanaut cruise was the study of the Kane Fracture Zone from its intersection with the Mid-Atlantic Ridge up to 80 km to the west. The dives have been conducted along four massifs located along the southern wall of the fracture valley. The flanks of the massifs consist of slightly tilted peridotites, gabbros and basalts, which have been exposed along normal and strike-slip major faults. No dike complex similar to that observed along the ZF Vema was observed. The observed sections show pervasive cataclastic deformation and greenschist-facies metamorphic overprin

Observation of sections of oceanic crust and mantle cropping out on the southern wall of Kane FZ (N. Atlantic)

The objective of the 20 Nautile dives of the recent Kanaut cruise was to study the southern wall of the Kane Fracture Zone from its eastern intersection with the Mid-Atlantic Ridge (MAR) to 5 Myr in age. The geological mapping shows four successive massifs, wrench faulted and slightly tilted. The transform-facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites, gabbros and basalts. The entire crustal exposure is cataclased and metamorphosed to greenschist facies