Diffusive reactive percolation and impregnation processes in the Oman peridotites: a multidisciplinary and multiscale investigation

The Oman ophiolite exposes a complete and preserved section of oceanic lithosphere with a layered structure. The ophiolite sequence is made up of several massifs, each exposing a complete stratigraphic sequence, offering the opportunity to study the genesis and structure of the lithosphere formed in an oceanic ridge environment (Nicolas & Boudier, 2015) as well as the processes that characterize the supra-subduction lithosphere (Agard et al., 2016; Ambrose et al., 2018). Despite its importance, the large-scale tectonic settings in which the Oman ophiolite formed, and its geodynamic evolution are widely debated. Particularly, only few studies have focused on the evolution of the uppermost lithospheric mantle with no petro-chemical detailed work available to date. The study presented in this Thesis aims at constraining the recrystallization and reactive melt percolation processes that contributed to shape and modify the Oman lithospheric mantle during its accretion into the lithosphere and evolution in the oceanic environment. The work has been developed through investigations on the Wadi Tayin Massif, located in the southern part of the ophiolite, where the magmatic features related to the oceanic accretion of the lithosphere in a ridge environment are best preserved and least overprinted by subduction related processes. I have used a multidisciplinary field, structural (EBSD) and geochemical approach (in-situ mineral major and trace element analyses) to provide: (1) a first chronology of the events recorded by the Wadi Tayin lithospheric mantle that can be summarized in four successive stages: (i) progressive cooling of the residual mantle at lithospheric environment, following mantle partial melting and incorporation in the Thermal Boundary layer (TBL); (ii) reactive porous melt percolation forming olivine-rich harzburgites; (iii) plagioclase-facies melt impregnation and origin of impregnated harzburgites and dunites; (iv) melt intrusion and formation of olivine-gabbro dikes; (2) A detailed documentation of (i) the reactive porous melt percolation and plagioclase-facies melt impregnation events involved in the evolution of the Oman mantle peridotites, and (ii) the correlation between the structural and geochemical variations induced by melt/rock interaction in a field-controlled petrological setting. Reactive porous melt flow: Reactive porous melt flow and melt/rock interaction are fundamental processes in the evolution of the oceanic lithospheric mantle. Melt migration through reactive porous flow in the Oman ophiolite has been extensively studied in the context of focused melt flow, leading to the formation of dunite channels and the Moho Transition Zone. On the other hand, diffuse melt percolation within the Oman uppermost mantle remains poorly constrained. I document microstructural and mineral modal changes that allow to reconstruct melt/rock interaction during the pervasive percolation of an olivine-saturated melt causing pyroxenes consumption and olivine crystallization in harzburgites. Newly formed olivine is interstitial, rims coarse orthopyroxene grains, and presents an axial-[010] Crystal Preferred Orientation (CPO), different from that of the residual olivine porphyroclasts, which displays orthorhombic CPOs. Notably, Oli crystals are fine-grained, indicating that nucleation was more efficient than crystal growth during their crystallization. Their formation most likely occurs in a low-temperature system in which nucleation is the dominant crystallization mechanism, suggesting that Oli crystallization occurred in a cooling environment, most likely within the Thermal Boundary Layer (TBL), hot enough to allow for diffuse melt percolation and melt-rock reaction. Moreover, the transition of the bulk olivine CPO from orthorhombic to axial-[010] is correlated to an increase of modal proportion of newly formed interstitial olivine and reflects the increasing melt/rock ratio integrated over time. Fe-Mg partitioning between the residual harzburgite minerals and the interstitial olivine indicates chemical equilibrium. However, variations in Mn contents among olivine support the reactive origin of interstitial crystals. Plagioclase-facies reactive melt percolation and impregnation: The oceanic lithosphere exposed in the Oman ophiolite records a complex history of melt migration and magmatic accretion, involving melts with various geochemical affinities. To date, several studies focused on documenting the magmatic records in the layered oceanic crust and the Moho-Transition Zone (mantle-crust boundary) within the Oman ophiolite, whereas detailed microstructural and geochemical investigations of melt migration and impregnation features in the Oman upper mantle harzburgite section are still very limited. I focus on plagioclase-bearing structures recorded in the Wadi Tayin mantle section, that allow to document the geochemical signature of melts percolating through the shallow lithospheric mantle and their relationships with the overlying MTZ and oceanic crust. Studied impregnation structures consist of various proportions of interstitial plagioclase (Plag), clinopyroxene (Cpx), and orthopyroxene (Opx) in a dunitic or harzburgitic host rock, forming impregnated dunites and harzburgites, respectively. The interstitial texture of Plag and Pxs, as well as high TiO2 contents in spinel of impregnated dunites and harzburgites, indicate subsequent melt/rock interaction of dunites and harzburgites, and the crystallisation of Plag and Pxs from a melt during impregnation of the host residual mantle. The crystallisation of interstitial Plag and Pxs at the expense of the porphyroclastic and granoblastic associations of the harzburgitic host rock point towards Plag and Pxs formation in a previously cooled systems. Furthermore, the transition of the bulk olivine CPO from orthorhombic to axial-[010], in impregnated harzburgites, unrelated to the impregnation event, suggests that (i) melt impregnation does not microstructurally affect the pre-existing harzburgite matrix, and (ii) the heterogeneity of olivine CPO was inherited from the previous history of reactive porous melt flow recorded by spinel harzburgite, before melt impregnation, thus definitely placing the impregnation event after the evolution of the lithospheric mantle under spinel-facies conditions. Magmatic clinopyroxene Na2O (wt%) contents and An (An= Ca/Ca+Na mol%) values in coexisting plagioclase are correlated and highlight a general depleted geochemical signature of all impregnating melts. Trace element compositions of computed melts in equilibrium with clinopyroxene from impregnated lithotypes show variable Light (L-) Rare Earth Elements (REE) depletion. Namely, some computed melts show L- to Heavy (H-) REE ratios similar to the Oman V1 (Geotimes) lava and the Oman lower crustal gabbros, whereas other computed melts exhibit very low L-/H-REE, evidencing a significant depletion in highly incompatible trace elements. Such impregnating melts characterized by a different geochemical signature are genetically correlated and their geochemical heterogeneity reflects different extents of melt/rock interaction between MORB-type melts and the host residual mantle, rather than the occurrence of unaggregated melts characterized by primary depletion in incompatible trace elements. This in turn indicates that melt-rock interaction is a powerful tool that can modify the melt geochemical signature, generating highly depleted melts. The observed mantle microstructural history points to a magmatic overprinting during the progressive cooling and lithospheric accretion of the Wadi Tayin upper mantle, in turn indicating a mantle high-temperature deformation history partially overprinted by a melt-assisted deformation event. Moreover, deciphering such reactive percolation and impregnation processes will shed light on the latest magmatic events recorded in the Wadi Tayin residual mantle and the role of migrating melts in the geochemical budget of the overlying Moho-Transition Zone and oceanic crust, thus adding an important piece to the complex accretional history of the Oman oceanic lithosphere.