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

Abstract

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 centres, 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 *Manuscript EPSL-D-18-00894R3 Click here to view linked References 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.

Multi-scale development of a stratiform chromite ore body at the base of the dunitic mantle-crust transition zone (Maqsad diapir, Oman ophiolite): The role of repeated melt and fluid influxes

“…7a-c). This is reminiscent of the trends observed globally all across the DTZ (Abily and Ceuleneer, 2013;Rospabé et al, 2018Rospabé et al, , 2019. Parallel trends are observed in the composition of silicate inclusions in chromite, as in the TiO 2 content of amphibole and alkalis of mica (Fig.…”

“…incongruent dissolution of the mantle harzburgites' orthopyroxene), melt percolation associated with chemical re-equilibration with the surrounding olivine matrix, refertilization by variably evolved melt batches, compaction, syn-magmatic faulting, later low temperature alteration, etc. (Rospabé et al, 2018(Rospabé et al, , 2019.…”

“…recorded by the troctolite and ol-gabbro cumulates in the mantle section - Benoit et al, 1996;Ceuleneer et al, 1996;Python et al, 2003), and a hydrated component coming from above, probably seawaterderived. The volatile-, Si-, Na-and trace elements-rich hybrid lithospheric melt formed during or/and contributed to the harzburgite dunitization process (Rospabé et al, 2017(Rospabé et al, , 2018(Rospabé et al, , 2019.…”

“…It probably reflects the involvement of several melt injections and of melt fractions of contrasted composition. Additionally, it was shown in the Maqsad area that fault zones developed early, during oceanic spreading and crystallization of the crust, and were probably the main vectors for the introduction of the hydrated component in the lower oceanic crust and at Moho level (Abily et al, 2011;Rospabé et al, 2019). In this way, magmatic breccias we observe within the stratiform chromitite may represent the nucleation state of such syn-magmatic faults, developed within a still unconsolidated mush and responsible of fluids and/or melt pulses that hybridized with magmatic mush containing interstitial MORB.…”

Hydrated Peridotite–Basaltic Melt Interaction Part II: Fast Assimilation of Serpentinized Mantle by Basaltic Magma

“…Additionally, an initial stage of the basaltic melt reaction with dehydrated serpentinite may result in formation of chromite-bearing harzburgite and dunite. The first direct evidence that the initial stage really happens at the 0.2 GPa mantle-crust transition zone is occurrence of chromite-hosted silica-rich inclusions of the Oman ophiolite chromitite ore bodies (Rospabé et al, 2019b).…”