Melt migration and mechanical state in the lower crust of oceanic ridges |
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| Authors: | A Nicolas & A Poliakov |
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| Institution: | ISTEEM-CNRS-UniversitéMontpellier 2, place E. Bataillon, 34095 Montpellier cedex 05, France |
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| Abstract: | Below the melt lens of fast spreading ridges, a low seismic velocity zone has been identified. From the study of ophiolite gabbros, in particular in Oman, this domain has been interpreted as a large magma chamber filled by a melt-poor mush where granular flow controlled by pressure solution-crystallization predominates over plastic flow. Melt migration through the mush is difficult to study in the field because the large magmatic flow taking place in this magma chamber has erased nearly all traces of migration paths. It is, however, still possible to identify sills and former dikes, now largely transposed into the layering. Physical traces for porous flow are rare, but petrological and geochemical evidence suggests that it also contributed to melt migration. Finally, in the lower gabbro horizons large magmatic folds and brecciated zones may bear evidence for magmatic intrusions. The combination of diking, porous flow and large-scale intrusions to carry melt through the magma chamber may be explained by the granular behaviour of the medium. It is suggested that the melt film present between grains and clots of grains reduces the large cohesive forces which characterize a solid, plastic, medium. Melt migration through the mush may thus depend on the size of cohesive clots, evolving through time and space, from porous flow to diking and melt intrusions for increasing larger clots. This process is illustrated by a physical experiment on pressurized air circulation through a granular medium. |
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