Accumulations of melt at the base of young oceanic crust

Expedition 335 summary

“…Chilled margins against the underlying dike screens precludes segregating a crystal residue that subsides to form the lower crust as in the gabbro glacier model, and its overall fractionated composition requires that crystals have been segregated elsewhere. This implies that sills or other bodies containing cumulate materials must exist deeper in the crust and/or below the crust/mantle boundary, consistent with recent models based on lower crustal sections of ophiolites (e.g., Boudier et al, 1996;Kelemen et al, 1997;MacLeod and Yaouancq, 2000) and some marine geophysical experiments (Crawford and Webb, 2002;Dunn et al, 2001;Garmany, 1989;Nedimovic et al, 2005;Canales et al, 2009). However, the gabbro glacier mode of accretion or a combination of the gabbro glacier and sheeted sills models cannot yet be rejected, as fractionated gabbros in the dikegabbro transition zone are not unexpected, and the predicted region of cumulate rocks could still exist just below the present maximum depth of Hole 1256D.…”

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“…Chilled margins against the underlying dike screens precludes segregating a crystal residue that subsides to form the lower crust as in the gabbro glacier model, and its overall fractionated composition requires that crystals have been segregated elsewhere. This implies that sills or other bodies containing cumulate materials must exist deeper in the crust and/or below the crust/mantle boundary, consistent with recent models based on lower crustal sections of ophiolites (e.g., Boudier et al, 1996;Kelemen et al, 1997;MacLeod and Yaouancq, 2000) and some marine geophysical experiments (Crawford and Webb, 2002;Dunn et al, 2001;Garmany, 1989;Nedimovic et al, 2005;Canales et al, 2009). However, the gabbro glacier mode of accretion or a combination of the gabbro glacier and sheeted sills models cannot yet be rejected, as fractionated gabbros in the dike-gabbro transition zone are not unexpected, and the predicted region of cumulate rocks could still exist just below the present maximum depth of Hole 1256D.…”

Shear and compressional wave structure of the East Pacific Rise, 9°–10°N

“…A strong axial magma chamber reflector, associated with the top of a low-velocity zone, is present along the rise axis [Derrick et al, 1987;Kent et al, 1993aKent et al, , 1993b. Off-axis the abyssal hill morphology in the southwestern portion of our study area is interrupted by three depressions, interpreted to be relict overlap basins [Carbotte and Macdonald, 1992 The primary purpose of our 1993 experiment, however, was to map the extent of off-axis melt accumulation at the base of the crust [Garmany, 1989;Garmany et al, 1994]. The large number of OBS receivers also allowed us to study the regional compressional and shear velocity structure of seismic layer 2B and layer 3, which is the focus of this paper.…”