Compaction of crystal mush bodies aids extraction and accumulation of reactive melts throughout the ultraslow-spread oceanic crust at the Atlantis Bank OCC (SWIR): insights from grain-size variations in olivine gabbro.

Abstract:

Slow- to ultraslow- spreading oceanic crust is constructed by a complex magma injections history; their lower portions are believed to be composed mainly of ephemeral crystal mush reservoirs. Its formation can involve reactive migration of melts, and requires at least partial extraction of melts from the crystal mushes. The processes aiding melt migration, collection and extraction remain poorly documented. Here, we investigate these processes providing geochemical and microstructural constraints from olivine gabbros sampled in the plutonic section of the Atlantis Bank OCC - IODP Hole U1473A.

U1473A olivine gabbros display intense grain size variability throughout the Hole from fine- (FG) to coarse-grained (CG) intervals with commonly irregular contacts - one contact every ~4 m. Microstructures of CG minerals indicate that they deformed at magmatic conditions, and Ol and Pl show resorbed grain boundaries against Cpx and FG. FG are undeformed and show granular textures. Mineral compositions record a progressive chemical evolution from more primitive cores of CG minerals to more evolved compositions of their relative rims, the latter being similar to the unzoned FG minerals. Significant enrichments in the most incompatible elements are widespread at rims of CG Pl and Cpx and in FG minerals, highlighting that magma differentiation is associated with melt-mineral interactions during melt migration in the crystal mush.

Bended CG Pl associated with the weak foliation and lack of lineation in Crystallographic Preferred Orientation (CPO) of CG Pl suggest that CG intervals were deformed under coaxial compression of the crystal mush, which likely aided melt migration and extraction in melt-rich zones. These zones ultimately formed the FG intervals. CPO of FG Pl show weak lineation and foliation as result of further deformation localized in the melt-rich zones, involving a non-coaxial component.

We document microstructural evidences of compaction of a precursor Ol+Pl crystal mush, and mineral compositions consistent with subsequent reactive migration, melt extraction and accumulation in melt-rich zones at different depths in Hole U1473A, as temperature decreases. This contribution provides substantial constraints on a process that has the potential to collect melts likely contributing to the erupted MORBs.