OCEANIC MAGMA CHAMBER PROCESSES: GABBRO FROM ODP HOLE 1105A OF THE SOUTHWEST INDIAN RIDGE

ODP Hole 1105A recovered 118 m of gabbros at a site ENE of Hole 735B near the Atlantis II Fracture Zone. The gabbros are composed of intercalated primitive olivine gabbro and evolved Fe-Ti oxide gabbro. The crystallization order for the evolved gabbro is Fe-Ti oxides, orthopyroxene, apatite, and biotite; orthopyroxene replaces olivine in a narrow middle interval. The gabbros are interpreted to have resulted from crystallization in a crystal mush. The cryptic variation in the olivine gabbros defines several lenses, 40 to 60 m thick, each characterized by a distinct convex zoning with a lower segment indicating upward reverse fractionation and an upper segment showing normal fractionation. The Fe-Ti oxide gabbros show cryptic variations independent of the host gabbros and reveal a systematic upward normal fractionation trend transgressing host gabbros. Forward fractional crystallization modeling closely matches the compositions of coexisting cumulus minerals and suggests co-saturation of olivine, plagioclase, and clinopyroxene from 1155 °C and the addition of Fe-Ti oxides from 1100 °C. The liquid line of descent is typical tholeiitic. The olivine gabbro can be accounted for by less than 65% fractionation with the residual 35% liquid saturated in Fe-Ti oxides. Modeling of the solid fractionation products shows that both the olivine and Fe-Ti oxide gabbros contain small amounts of trapped liquid (<5%). The implications are that the gabbros represent solidified crystal mush of a subaxial chamber. Compaction and upward melt migration appear to have been terminated with relatively large amounts of interstitial liquid remaining in the upper parts of the cumulate mush. This termination may have been caused by tectonic disturbances, uplift, and associated withdrawal of magma into the subaxial dike and sill system. Prolonged compaction and cooling of the trapped melt in the mush formed small, differentiated bodies and lenses by pressure release migration and crystallization along syntectonic channels. This phenomenon resulted in differentiation products along lateral and vertical channelways in the host gabbro that vary from olivine gabbro, to Fe-Ti oxide gabbro, gabbronorite, and apatite gabbros, and show large compositional variations independent of the host olivine gabbros.