Journal of Conference Abstracts

Structure of the Axial Magma Chamber at the Southern East Pacific Rise

Satish Singh (singh@esc.cam.ac.uk)

BIRPS, Bullard Labs, Madingley Road, Cambridge.

Recent seismic data from 14 degree south on the East Pacific Rise show that the axial magma chamber is continuous over tens of kilometres along the axis at about 1200 m beneath the seabed. The width of the magma chamber is less than a kilometer. The ridge here spreads superfast at 16 cm/yr.

I have analysed seismic data from a two-ship multi-channel seismic experiment using a waveform inversion technique. My result shows that the magma chamber is 50 m thick with 3.3-3.5 km/s P-wave velocity and 0.0-0.5 km/s S-wave velocity; the near-zero S-wave velocity suggests the presence of pure melt in the magma chamber. The magma chamber is sandwiched between a 60 m high velocity roof layer and an at least 150 m high velocity basal layer. The basal layer may result from crystal settling at the bottom of the melt lens through compaction and solidification, and may explain why Moho reflections are observed beneath or very close to the ridge axis. Furthermore, the presence of this high velocity basal layer does not require a thick low velocity mushy zone beneath the magma chamber. If the lower crust is formed by fractionation of melt in a 50 m thick magma chamber, then melt from the mantle should replenish the magma chamber every 10-30 years to maintain a steady-state magma chamber. The high velocity roof layer could be formed due to cooling from the top. It is overlain by a 200 m thick low velocity layer, which may be the result of hydrothermal fracturing and may represent the base of the hydrothermal circulation, and this hydrothermal circulation may be responsible for releasing heat from the magma chamber in 10-30 years.

Computer Simulations of Clay-Fluid Interactions Under Sedimentary Basin Conditions

N. T. Skipper¹, A. de Siqueira¹ & P. V. Coveney²

¹ Dept. Physics and Astronomy, UCL.

² Schlumberger Cambridge Research.

We will report on our Monte Carlo molecular modelling studies of smectite-water interactions under sedimentary basin conditions, at burial depths of up to 9km. We find that the density of interlayer pore water exceeds that of the bulk, except at very low porosities. Hydrated smectite is therefore enthalpically stable at burial depths of up to 1.5km. Below this depth the remaining fluid prefers the bulk phase. This is consistent with the observation that diagenetic release of interlayer water and conversion of smectite to illit occurs at burial depths greater than 1.5km.