Crustal accretion at slow-spreading ridges is controlled by relatively low magma supply rates. This results in periods of amagmatic spreading and pronounced lateral and vertical variations in crustal architecture and thickness. Magma migration in slow-spreading crust will consequently be influenced by tectonic processes that will lead to the emplacement of dikes and sills and small ephemeral chambers at all crustal levels. Despite this, the internal evolution of such magma reservoirs will fundamentally be controlled by the same processes that control magma evolution at fast-spreading centers. Cooling and crystallization will proceed by the formation of crystal mush zones along chamber margins and floors. Slow spreading and low magma supply will result in less frequent chamber tapping and replenishment and restrict magma mixing compared to chambers at fast-spreading centers. The restricted flux of magma at the same time will provide conditions for efficient fractionation processes. Crystal mush in slow-spreading chambers will solidify by compaction and vertical and horizontal melt migration partially in response to deformation processes affecting the mush zone. Compaction and plastic flow in the crystal mush will result in or intensify preexisting modal layering and lamination. The differentiation of magma at slow-spreading ridges may further be affected by tectonic uplift and withdrawal of magma from the melt lens and cessation or reduction in compaction. Trapped interstitial liquid of crystal mushes may crystallize in the host rock to form intercalated, small differentiated bodies and lenses as a result of predominantly lateral migration and syn- and post-tectonic melt-channeled movement into uplift- and deformation-induced pressure release zones.
The gabbro and ferrogabbro complex recovered from Hole 1105A near the Atlantis II Fracture Zone, Southwest Indian Ridge, records fractionation and melt migration processes in chambers at a slow-spreading regime (Shipboard Scientific Party, 1999). In this contribution, the petrological results of a study of gabbroic cumulates from the 150-m, nearly continuous drill core that sampled parts of a small crustal magma chamber are presented. The results confirm the observations from Hole 735B and show that trapped interstitial melt migrates and differentiates largely without reequilibrating with the primitive olivine gabbro host.