The seismic attenuation structure of a fast-spreading mid-ocean ridge.

The two-dimensional P-wave attenuation structure of the axial crust of the East Pacific Rise was obtained from an inversion of waveform spectra collected during an active-source seismic tomography experiment. The structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis. The near-axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation. High attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers (less than 1 kilometer beneath the axial magma lens) to the base of the crust. The levels of attenuation in this deeper region require at most only a small fraction of partial melt.

Continuous mantle melt supply beneath an overlapping spreading center on the East Pacific Rise.

Tomographic images of upper mantle velocity structure beneath an overlapping spreading center (OSC) on the East Pacific Rise indicate that this ridge axis discontinuity is underlain by a continuous region of low P-wave velocities. The anomalous structure can be explained by an approximately 16-kilometer-wide region of high temperatures and melt fractions of a few percent by volume. Our results show that OSCs are not necessarily associated with a discontinuity in melt supply and that both OSC limbs are supplied with melt from a mantle source located beneath the OSC. We conclude that tectonic segmentation of the ridge by OSCs is not the direct result of magmatic segmentation at mantle depths.

The quantum event of oceanic crustal accretion: impacts of diking at mid-ocean ridges.

Seafloor diking-eruptive events represent the irreducible, quantum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy's real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subsea-floor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically active portions of our planet, other hydrothermally active planets may harbor similar life forms.