Formation of the lower ocean crust and the crystallization of gabbroic cumulates at a very slowly spreading ridge

Ocean Drilling Program Hole 735B was extended to 1508 m below the sea floor during Leg 176, atop a shallow bank near Atlantis 11 Fracture Zone on the very slowly spreading Southwest Indian Ridge. All the drilling was in gabbro, and recovery averaged nearly 87%. The drill penetrated a series of stacked plutons consisting mostly of olivine gabbro, but some with troctolite. Each pluton is some 200-500 in thick, each has its own internally coherent stratigraphy, and each apparently represents an individual event of significant magma inflation and addition to the crust. The entire column was extensively deformed along inclined zones of distributed shear before it was completely frozen, this marking the onset of unroofing of the rocks and their ascent to high rift mountains. The deformation mobilized late-stage melts into flow patterns which led to concentration of ilmenite and magnetite in hundreds of seams of oxide gabbro along or near zones of strong crystal-plastic deformation, the highly differentiated melts overall being concentrated by buoyancy forces toward the top of the section, especially in one zone nearly 70 in thick. However, upward flow was ultimately blocked or deflected by zones of impermeable rock resulting either from downward freezing or grain-size reduction during shear. A melt tens probably did not form at the base of sheeted dikes, as it does at the East Pacific Rise. Despite this, the rocks are cumulates, and most are adcumulates, with very low residual melt porosities. Cumulate theory based on stratiform, layered intrusions does not entirely apply to these rocks. Instead, all gabbros, including the oxide gabbros, crystallized in a dense crystal mush in patterns dominated by fractures, channelized flow, and intergranular porous flow. Most gabbros are not layered; weak modal layering of uncertain origin is present in <2% of the rocks. Porosity reduction leading to formation of adcumulates at all stages was extremely efficient. This occurred in the course of synkinematic differentiation, also called differentiation by deformation, and involved compaction under conditions of lithostatic loading and shear, emplacement of crystal mushes along inclined, possibly curving faults, dissolution and reprecipitation of minerals along grain boundaries, and pressure solution, which was the final agent in porosity reduction. As the block was lifted from beneath the rift-valley floor, crystal-plastic deformation gave way to brittle fracture, and the now dominantly subsolidus metamorphism shifted from high-temperature assemblages characterized by amphibole near the top of the section, to low-temperature assemblages with smectite-chlorite and zeolite near the bottom. The rocks acquired their stable magnetization during formation of amphiboles and secondary magnetite during this metamorphism. The entire body of rock is reversely stably magnetized at a consistent inclination, and it is rotated to the south, perhaps along a curving detachment surface, away from the ridge segment where it formed, by about 20degrees. It has an intensity of magnetization sufficient to account for the magnetic anomaly observed over the site. Drilling did not reach ultramafic rock, but dredging indicates that peridotite is probably within a few hundred meters of the bottom of the hole. Seismic Moho. however. is placed at 5 km beneath the summit of Atlantis Bank. Much of the rock between Moho and the bottom of Hole 735B must therefore be partially serpentinized peridotite. (C) 2001 Elsevier Science B.V. All rights reserved.