Variability of volcanic constructions along the Reykjanes Ridge: Observations from downhole imaging at IODP395C/395 basement sites

International Ocean Discovery Program Expeditions 384, 395C and 395 investigated ocean crust formation at the Reykjanes Ridge, the variable influence of the nearby Iceland plume, the origin of V-shape ridges and troughs marking the flanks of the ocean ridge, and the alteration of basaltic crust with time. These Expeditions collected cores from a transect of five drill sites along a plate-spreading flowline spanning seafloor ages from 2.8 to 32 Ma. Combined, over 400 m of oceanic basalt core was recovered, and downhole logging collected physical property measurements in the crust, and resistivity and ultrasound images of the boreholes. These datasets provide a unique record of volcanic and tectonic characteristics of the uppermost basaltic crust, and of the progressive basalt alteration. Here we use downhole logging images along with observations from the recovered cores to characterize the lava morphology and quantify flow types in basement holes, and to investigate the fracturing and alteration of the basalts. This analysis complements the observations from the cores especially where basalt recovery was low. Data from the different sites along the flowline allows us to analyze how these physical characteristics vary with age, and to compare the flows emplaced at V-shaped ridges with those emplaced in the troughs. We estimate the fluid circulation paths from downhole images and compare with the basalt alteration observed from the cores.