Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone
Résumé
On Earth, most of the critical processes happen at the frontiers between envelopes and especially at the Moho between the mantle and the crust. Beneath oceanic spreading centers, the dunitic transition zone (DTZ) appears as a major interface between the upwelling and partially molten peridotitic mantle and the accreting gabbroic lower crust. Better constraints on the processes taking part in the DTZ allows improved understanding of the interactions between silicate melts and hydrated fluids, which act competitively to generate the petrological Moho. Here we combine mineral and whole rock major and trace element data with a structural approach along three cross-sections up to 300 m thick above the fossil Maqsad mantle diapir (Oman ophiolite) in order to understand the vertical organization of the DTZ with depth. Our results highlight that most of the faults or fractures cross-cutting the DTZ were ridge-related and active at an early, high temperature magmatic stage. Chemical variations along the cross-sections define trends with a characteristic vertical scale of few tens of meters. There is a clear correlation between the chemical variation pattern and the distribution of fault zones, not only for fluid-mobile elements but also for immobile elements such as REE and HFSE. Faults, despite displaying very limited displacements, enhanced both melt migration and extraction up to the crust and deep hydrothermal fluids introduction down to the Moho level. We propose that these faults are a vector for upwelling melt modification by hybridization, with hydrothermal fluids and/or silicic hydrous melts, and crystallization. Infiltration of these melts or fluids in the country rock governs part of the gradational evolutions recorded in composition of both the olivine matrix and interstitial phases away from faults. Finally, these faults likely control the thermal structure of the mantle-crust transition as evidenced by the spatial distribution of the crystallization products from percolating melts, organizing the transition zone into pure dunites to impregnated dunites horizons. In this context, the DTZ appears as a reactive interface that developed by the combination of three primary processes: tectonics, magmatism and deep, high temperature hydrothermal circulations. Accordingly, these features fundamentally contribute to the variable petrological and geochemical organization of the DTZ and possibly of the lower crust below oceanic spreading centers, and may be a clue to interpret part the heterogeneity observed in MORB signatures worldwide.
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