Lisa A. Gilbert

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Porosity of the upper edifice of Axial Seamount

Seamounts are not solid basalt structures, but have relatively high porosities in their upper crustal sections. At Axial Seamount, off the coast of Oregon, United States, we used on-bottom gravity measurements with a Bell gravity meter within deep-sea submersible Alvin to determine a porosity of 31% for the uppermost ∼100 m of the edifice. The southwestern caldera wall has a porosity of 22% and the caldera floor has a slightly higher porosity of 33%. Seafloor observations and models indicate that these high porosities result from large-scale structural features such as lava tubes and cracks, large lava drain backs, or regions of open pillow basalts in the near subsurface. These high-porosity zones can affect the subsurface permeability, and models of hydrothermal upflow zones explain observed localized gravity anomalies. The variety of hydrothermal alteration, hydrothermally active areas, and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano.

Oceanic crustal velocities from laboratory and logging measurements of Integrated Ocean Drilling Program Hole 1256D

Drilling and logging of Integrated Ocean Drilling Program (IODP) Hole 1256D have provided a unique opportunity for systematically studying a fundamental problem in marine geophysics: What influences the seismic structure of oceanic crust, porosity or composition? Compressional wave velocities (Vp) logged in open hole or from regional refraction measurements integrate both the host rock and cracks in the crust. To determine the influence of cracks on Vp at several scales, we first need an accurate ground truth in the form of laboratory Vp on crack-free, or nearly crack-free samples. We measured Vp on 46 water-saturated samples at in situ pressures to determine the baseline velocities of the host rock. These new results match or exceed Vp logs throughout most of the hole, especially in the lower dikes and gabbros, where porosities are low. In contrast, samples measured at sea under ambient laboratory conditions, had consistently lower Vp than the Vp logs, even after correction to in situ pressures. Crack-free Vp calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are controlled by porosity. In particular, the models demonstrate significant large-scale porosity in the lavas, especially in the sections identified as fractured flows and breccias. However, crustal velocities in the lower dikes and gabbros are increasingly controlled by petrology as the layer 2-3 boundary is approached.

Depth-shifting cores incompletely recovered from the upper oceanic crust, IODP Hole 1256D

Seafloor drilling operations, especially those in crustal rocks, yield incomplete recovery of drilled sections, and depths of the recovered core pieces are assigned with some uncertainty. Here we present a new depth-shifting method that is simple and rapid, requires little subjective input, and is applicable to any core-log integration problem where sufficient comparable data have been collected in both the open hole and from the recovered core. Over the depth range for which both core and log data have been collected, an automatic algorithm selected the best new depth for each piece. The criteria for determining the best depth were as follows: (1) find new depths for as many pieces as possible, and (2) minimize the difference between core density and log density. In this study, depth-shifting is applied at Integrated Ocean Drilling Program (IODP) Hole 1256D, which is our first opportunity to study a section of intact, in situ upper ocean crust drilled down to gabbro. The new depths significantly improve the agreement between an independent data set and the logging record.