Microstructures in Hole 1274A peridotites, ODP Leg 209, Mid-Atlantic Ridge: Tracking the fate of melts percolating in peridotite as the lithosphere is intercepted

We report the microstructures of harzburgites and dunites from ODP Leg 209, Hole 1274A, 15 degrees 39'N on the Mid-Atlantic Ridge. A set of features in these peridotites is so unaffected by plastic flow that it must have formed very late by magmatic processes. We believe that the microstructures record the interaction between a peridotite and a percolating melt as the thermal boundary layer was progressively intercepted. The following chronology for the microstructures is derived: Group 1, resorption of orthopyroxene (opx) associated with olivine precipitation, enhanced by a migrating melt; Group 2, conversion of opx to clinopyroxene (cpx) by a percolating melt; Group 3, precipitation of cpx and spinel as intricate intergrowth (symplectite), associated with olivine dissolution, from an interstitial melt. For these reactions the ratio of melt generated to melt consumed is progressively decreasing, as expected during progressive freezing of asthenosphere. Pristine reaction microstructures analogous to Site 1274A are also found in the Lanzo Massif (Italy) and the Little Port Complex (Canada). For all three settings we infer (1) slow spreading conditions and (2) an ancient depletion event. We thus infer that preservation of the described microstructures is favored by slow spreading and a previous depletion. Slow spreading may be required since it avoids the tectonic overprint during ductile corner flow generally inferred for fast spreading ridges. Slow spreading does not tend, however, to generate highly depleted residues because a thick lithospheric lid limits the extent of melting. Residues will thus tend to be lherzolitic, not harzburgitic. However, lherzolites have a lower permeability for mafic melts than harzburgites. In order to generate the percolation- dominated microstructures of Site 1274A, a harzburgitic host rock appears favorable. This may now explain the need for an ancient depletion event since only then can harzburgitic residues occur at the top of the mantle column during slow spreading conditions.