Caractéristiques pétrographique, géochimique et structurale de la section crustale profonde de l'ophiolite d'Oman : implications pour la genèse des magmas et le fonctionnement des chambres magmatiques à l'aplomb d'un centre d'expansion océanique

Ce travail repose sur (1) une étude de terrain des cumulats lités de l'ophiolite d'Oman, notamment dans plusieurs massifs où cette unité restait largement inexplorée, (2) une étude pétrographique et géochimique (microsonde électronique et LA-ICP-MS) d'environ 700 échantillons et (3) une simulation en laboratoire de la cristallisation de magmas boninitiques en présence d'eau. La cinquantaine de faciès pétrographiques définie, gabbroïques et ultrabasiques, est indispensable pour décrire rigoureusement la croûte profonde omanaise. Cette variété lithologique est révélatrice de la très grande variabilité des paramètres intensifs (P, T°, PH2O ...) et extensifs (composition) lors de la cristallisation des magmas au sein des chambres magmatiques à l'aplomb d'un centre d'expansion océanique. L'orthopyroxène précoce dans les cumulats primitifs (XMg > 80 %) est beaucoup plus commun que ce qui était proposé jusqu'à présent, ce qui remet en cause certaines idées reçues concernant l'origine de l'ophiolite. Je démontre que cette variété lithologique, l'abondance d'orthopyroxène, et d'autres caractéristiques pétrographiques et géochimiques des cumulats d'Oman s'expliquent par le mélange, dans des proportions variables, de deux principaux magmas : un d'affinité tholéiitique issu de la fusion d'une source asthénosphérique " N-MORB ", et un d'affinité andésitique à boninitique issu de la fusion hydratée d'une source lithosphérique déprimée. Ces deux magmas ont circulé dans des chenaux mantelliques (" filons ") avant d'alimenter les chambres. Le rôle de " mélangeur " des chambres magmatiques est ainsi confirmé. La contribution relative de ces deux magmas évolue au cours de l'ascension de diapirs asthénosphériques dans la lithosphère. Des variations abruptes de la pression partielle d'eau sont également indispensables pour rendre compte de l'ensemble de mes observations. Je montre aussi que les failles syn-accrétions, dont l'importance en Oman restait largement sous-évaluée, sont un vecteur principal de la pénétration de fluides hydrothermaux jusque dans la croûte profonde en cours de cristallisation. Mon étude me permet également de mieux contraindre l'origine, encore très discutée, des intrusions " wehrlitiques " et de la zone de transition dunitique.My work consisted in (1) a fieldwork focused on Oman ophiolite layered cumulates, particularly in several massifs wherein this unit was largely unexplored, (2) a petrographic and geochemical study (electron microprobe and LA-ICP-MS) of about 700 samples and (3) crystallisation experiments on hydrous boninitic melts. The fifty petrographic facies defined, gabbroic and ultrabasic, are essential to scrupulously describe the Oman ophiolite lower crust. This lithological diversity is indicative of the high variability of intensive (P, T°, PH2O ...) and extensive (composition) parameters during magmas crystallisation within magma chambers at an oceanic spreading centre. Early orthopyroxene in primitive cumulates (XMg > 80 %) is more common that proposed by previous works and undermines some present-day ideas concerning the origin of the Oman ophiolite. I demonstrate that this lithological diversity, the orthopyroxene abundance and other petrographic and geochemical characteristics of Oman cumulates can be explained by the mixing in various proportions of two main magmas: one with tholeiitic affinity related to the partial melting of an asthenospheric source and one with andesitic to boninitic affinity related to the hydrous partial melting of a depleted lithospheric mantle source. These two magmas migrated in mantle channels ("dykes") before feeding magma chambers. The "shaker" function of magma chambers is proved in this way. The relative contribution of these two magmas evolved during the upwelling of asthenospheric diapirs within the lithosphere. Abrupt variations of the water partial pressure are also essential to explain all the observations. I evidence that syn-accretion faults, whose role in the building of the Oman ophiolite was largely underestimated, are a main vector for the penetration of hydrothermal fluids down to the crystallizing lower crust. My study also allows to better constraint the origin, still strongly discussed, of the "wehrlitic" intrusions and of the dunitic mantle/crust transition zone

Origin of the dunitic mantle-crust transition zone in the Oman ophiolite: The interplay between percolating magmas and high-temperature hydrous fluids

International audienc

The Dunitic mantle-crust transition zone in the Oman ophiolite : residue of melt-rock interaction, cumulates from high-MgO melts, or both?

The contact between mantle peridotite and gabbro from the lower oceanic crust is usually underlined by a horizon of dunite. The origin of this dunitic transition zone (DTZ) is still debated. It is viewed either as a pile of cumulus olivine from high-MgO melts, or as former mantle peridotite pervasively percolated by melts undersaturated with pyroxene (e.g., as mid-oceanic ridge basalts [MORBs] at low pressure), and transformed into dunite. We show that the two hypotheses are not mutually exclusive, although they do not account for the same parts of the DTZ. We determined a petrological profi le through a 330-m-thick DTZ that developed at the top of a mantle diapir in the Oman ophiolite. The lowermost 280 m have a reactional origin: olivine and Cr-spinel record the complex percolation and interaction history between mantle peridotite and MORB. In the uppermost 50 m, chemical trends become consistent with a cumulus origin of the dunite, olivine crystallization being a prelude to the crystallization of the overlying gabbros. The DTZ develops largely in response to melt-rock reaction, consistent with the "reactive fi lter" hypothesis, but the proportion of cumulate dunite is high enough to require parent melts with a signifi cantly higher Mg content than the most primitive MORB erupted on the seafl oor.4 page(s

Synmagmatic normal faulting in the lower oceanic crust : evidence from the Oman ophiolite

Structural and petrological study of the Oman ophiolite shows that normal faults at oceanic spreading centers can root in incompletely crystallized cumulates and form at the same time as melt is being supplied to the magma chamber. In the vicinity (≤200 m) of a ridge-facing normal fault located a couple of kilometers off a former axial mantle diapir, blocks of layered cumulates were tilted at high angles relative to the Moho. Block rotation was accommodated by a swarm of anastomosed fault planes connected to the main fault. These planes are underlain by screens of gabbroic micropegmatites (former melt layers) and by ptygmatic folds, pointing to viscous deformation of a compacting crystal mush. Flat-lying, undeformed cumulate layers from the same parent melts as the deformed ones settled directly over the tilted blocks. Water introduction into the melt is evidenced by the crystallization of anomalously high anorthite (An 90%-95%) cumulus plagioclase, regardless of the degree of evolution of the magma. Shearing continued at subsolidus temperatures along some, but not all, fault planes, attested to by the local development of plastic deformation structures. The subsolidus development of amphibolespinel coronas around olivine grains points to pervasive percolation of high-temperature hydrothermal fl uids through the cumulates. As the crust cooled further, water penetration progressively focused within the main fault zone, where the crust was fractured and eventually altered in greenschist facies conditions.4 page(s

Caractéristiques pétrographique, géochimique et structurale de la section crustale profonde de l'ophiolite d'Oman (implications pour la genèse des magmas et le fonctionnement des chambres magmatiques à l'aplomb d'un centre d'expansion océanique)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Anatomy of a chromitite dyke in the mantle/crust transition zone of the Oman ophiolite

International audienceThe igneous and mechanical processes controlling the formation of nodular chromite ore have been investigated through the study of a chromitite dyke emplaced in the uppermost part of the 330 m-thick dunitic mantle/crust transition zone that developed at the top of a mantle diapir in the Maqsad area of the Oman ophiolite. The dyke is parallel to the paleo-ridge axis, has a vertical extent of about 30 m and an average thickness of 2 m. It presents spectacular variations in ore texture, offering a unique opportunity to identify the zones of nodule nucleation in the upper parts of the dyke, growth in the intermediate parts and accumulation at the bottom.Nodules grew by progressive accretion of euhedral chromite grains, 100–200 μm in size, around a nucleus made essentially of olivine and plagioclase embedded in skeletal chromite. At a critical size of 2 to 3 cm, the nodules, still poorly consolidated, sunk, accumulated and compacted at the bottom of the dyke. The interstitial silicate matrix between the nodules is essentially troctolitic (high Mg (Fo~92) and high Ni (NiO ~0.35 wt%) olivine and calcic (An83 to An85) plagioclase with minor pargasite). At about mid-height, the dyke broadens significantly, reaching a width of 12 m, the center of this bulge being filled with smaller-sized nodules embedded in an anorthositic matrix. This feature is interpreted to represent a magma pocket where the melt and nodule nuclei accumulated before complete crystallization and cooling of the system. The alteration of the silicate matrix is less intense in this bulge than in the rest of the dyke.Silicate inclusions within chromite grains indicate that the parental melt of the chromite was hybrid between two endmembers: a common MORB-like melt and a silica-rich hydrous fluid or a water-rich trondhjemitic melt, possibly produced by low degree melting of hydrothermally altered gabbro and/or serpentinized peridotite from the country rocks. The Ti content in the chromite (average ~0.5 wt% TiO2) from the dyke is significantly higher than that of chromites emplaced at deeper levels in the mantle/crust dunitic transition zone (DTZ) and in the harzburgitic mantle from the Maqsad area. This points to the progressive evolution of the MORB component (product of decompression melting in the diapir) during its ascent from the mantle diapir to the base of the crust. Fractional crystallization occurred in a context of buffering of compatible element concentrations (Mg, Cr, Ni) around elevated, “primitive” values through exchanges between the percolating melt and the host harzburgite and dunite.The chemical composition of both chromite and silicates is constant (i.e. evenly scattered) from the bottom to the top of the dyke and does not mimic the evolution in the ore texture nor in the size and abundance of the nodules. This implies that the parental melt composition remained globally unchanged during the formation of the ore body arguing for open system conditions during nodule formation and accumulation. The only significant evolution is observed in the central bulge where the chromite Ti content is higher (average ~0.7 wt% TiO2 with spikes reaching several percent) confirming that this magma pocket was filled with more evolved melt.No bottom to top evolution in the XCr of chromite is observed within the dyke but individual nodules show a well-developed zoning in XCr ratio from their nucleus (XCr ~58) to their margins (XCr ~48). An increase in the TiO2 content of chromite toward the nodules' edges is not systematic and, when present, is quite moderate. In the largest nodules, inclusions of hydrated silicates are preferentially distributed in a circle, midway between the nodule's nucleus and its edge. This garland of inclusions coincides with a positive peak in the XCr profile, while the TiO2 profile is not affected. This implies that the zoning in XCr cannot be assigned to fractional crystallization alone. It is tentatively explained by a scenario where the nodules crossed a gradient in the proportion of the MORB vs. more reducing hydrous melt during their growth. The gradient could have been maintained by a density contrast between a buoyant hydrous silica-rich melt and denser MORB

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey : petrogenesis and relationship to mantle diapirism

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 x 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al₂O₃], low [TiO₂]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO₂ content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic-troctolitic-gabbroic cumulates making up the crustal section above the Maqsad diapir. We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.17 page(s

Ocean crust accretion along a high-temperature detachment fault in the Oman ophiolite: A structural and petrological study of the Bahla massif

International audienceThe Bahla massif exposes the lower crustal section of the Oman ophiolite located close to the thrust front of the Semail nappe. It is affected by intense faulting previously attributed to tectonic events that dismembered a classical ophiolitic sequence during or after the obduction. Here we show that most of this complexity is primary, inherited from syn-accretion tectonics. The crustal section is exposed in a 15 by 8 km tectonic enclave surrounded by mantle peridotite. Its northern boundary corresponds to a major, steeply dipping normal fault striking WNW-ESE, at low angle to the paleo-ridge axis. Movement along this fault was accommodated by intense plastic deformation of the crustal cumulates and adjacent mantle peridotites at temperature conditions ≥900 °C. The thickness of the deformed zone reaches several hundred meters. The flattening of the cumulate layering away from the fault is correlated to a decrease in the deformation intensity. Undeformed olivine-gabbro dykes cross-cut this “tectonic Moho” indicating that the tilting occurred before the end of the igneous activity. To the southwest, the crustal enclave is bounded by a NW-SE trending transtentional shear zone that was active in the amphibolite to greenschist facies and was intensely injected by syn- to post-kinematic gabbronorite and tonalite/trondhjemite dykes and plugs. The age of one felsic sample (95.214 ± 0.032 Ma, high-precision Usingle bondPb zircon dating) is within error of the age of intrusive felsic intrusions into the mantle and lowermost axial crust from the length of the Oman ophiolite, which slightly post-dates the mean crystallization age of the Semail crust (V1 magmatism; 96.1–95.6 Ma). Other contacts are low temperature features including cataclastic faults, serpentine‑carbonate breccias and flat-lying décollements.Parent melts of the Bahla crustal cumulates were more siliceous and hydrous, i.e. more andesitic, than typical mid-ocean ridge basalt (MORB) as deduced from the frequent occurrence of early crystallizing orthopyroxene (opx) and late crystallizing amphibole. Some facies such as cumulate harzburgite and opx-troctolite have not been documented elsewhere in the Oman ophiolite and may be specific to the tectonic context in which the frontal massifs accreted. The chemical composition of the lower crustal cumulates can be accounted for by the hybridization in various proportions between MORB and a primitive andesite from a depleted source whose origin can be looked for in melts from a nascent subduction zone or from high temperature hydrothermal processes.The structure of the Bahla lower crustal section is reminiscent of the plutonic growth faults documented along present-day slow-spreading centres in both mid-ocean ridge and back arc settings. The distinctive characteristics of the Moho and lower crustal section in the Bahla massif are tentatively related to their position at the leading edge of the ophiolite, i.e. closer to the Arabian continental margin at the time of accretion than the massifs from the internal part of the ophiolite that have a more continuous and less deformed lower crust. It indicates that the style of crustal accretion may have changed during the opening of the oceanic basin from which the Oman ophiolite issue

Heterogenous Clinical Landscape in a Consanguineous Malonic Aciduria Family

Malonic aciduria is an extremely rare inborn error of metabolism due to malonyl-CoA decarboxylase deficiency. This enzyme is encoded by the MLYCD (Malonyl-CoA Decarboxylase) gene, and the disease has an autosomal recessive inheritance. Malonic aciduria is characterized by systemic clinical involvement, including neurologic and digestive symptoms, metabolic acidosis, hypoglycemia, failure to thrive, seizures, developmental delay, and cardiomyopathy. We describe here two index cases belonging to the same family that, despite an identical genotype, present very different clinical pictures. The first case is a boy with neonatal metabolic symptoms, abnormal brain MRI, and dilated cardiomyopathy. The second case, the cousin of the first patient in a consanguineous family, showed later symptoms, mainly with developmental delay. Both patients showed high levels of malonylcarnitine on acylcarnitine profiles and malonic acid on urinary organic acid chromatographies. The same homozygous pathogenic variant was identified, c.346C > T; p. (Gln116*). We also provide a comprehensive literature review of reported cases. A review of the literature yielded 52 cases described since 1984. The most common signs were developmental delay and cardiomyopathy. Increased levels of malonic acid and malonylcarnitine were constant. Presentations ranged from neonatal death to patients surviving past adolescence. These two cases and reported patients in the literature highlight the inter- and intrafamilial variability of malonic aciduria

Next-Generation Molecular Investigations in Lysosomal Diseases: Clinical Integration of a Comprehensive Targeted Panel

Diagnosis of lysosomal disorders (LDs) may be hampered by their clinical heterogeneity, phenotypic overlap, and variable age at onset. Conventional biological diagnostic procedures are based on a series of sequential investigations and require multiple sampling. Early diagnosis may allow for timely treatment and prevent clinical complications. In order to improve LDs diagnosis, we developed a capture-based next generation sequencing (NGS) panel allowing the detection of single nucleotide variants (SNVs), small insertions and deletions, and copy number variants (CNVs) in 51 genes related to LDs. The design of the LD panel covered at least coding regions, promoter region, and flanking intronic sequences for 51 genes. The validation of this panel consisted in testing 21 well-characterized samples and evaluating analytical and diagnostic performance metrics. Bioinformatics pipelines have been validated for SNVs, indels and CNVs. The clinical output of this panel was tested in five novel cases. This capture-based NGS panel provides an average coverage depth of 474× which allows the detection of SNVs and CNVs in one comprehensive assay. All the targeted regions were covered above the minimum required depth of 30×. To illustrate the clinical utility, five novel cases have been sequenced using this panel and the identified variants have been confirmed using Sanger sequencing or quantitative multiplex PCR of short fluorescent fragments (QMPSF). The application of NGS as first-line approach to analyze suspected LD cases may speed up the identification of alterations in LD-associated genes. NGS approaches combined with bioinformatics analyses, are a useful and cost-effective tool for identifying the causative variations in LDs