The Maqsad area in the Oman ophiolite exposes a >300 m thick dunitic mantle-crust transition zone (DTZ) that developed above a mantle diapir. The Maqsad DTZ is primarily made of “pure” dunites ...(olivine with scattered chromite and chromite seams) and “impregnated” dunites, which exhibit a significant lithological variability, including various kinds of clinopyroxene-, plagioclase-, orthopyroxene-, amphibole (hornblende/pargasite)-bearing dunites. These minerals are interstitial between olivine grains and their variable abundance and distribution suggest that they crystallized from a percolating melt. Generally studied through in-situ mineral characterization, the whole rock composition of dunites is poorly documented. This study reports on whole rock and minerals major and trace element contents on 79 pure to variably impregnated dunites collected systematically along cross sections from the base to the top of the DTZ. In spite of its high degree of depletion, the olivine matrix is selectively enriched in the most incompatible trace elements such as LREE, HFSE, Th, U, Rb and Ba. These data support the view that this enrichment has been acquired early in the magmatic evolution of the DTZ, during the dunitization process itself. The dissolution of orthopyroxene from mantle harzburgites enhanced by the involvement of hydrothermal fluids produced low amounts of melts enriched in silica and in some trace elements that re-equilibrated with the olivine matrix. This pristine signature of the DTZ dunite was eventually variably altered by percolation of melts with a Mid-Ocean Ridge Basalt (MORB) affinity but displaying a wide spectrum of composition attributable to evolution by fractional crystallization and hybridization with the silica enriched, hydrated melts. The olivine matrix has been partially or fully re-equilibrated with these melts, smoothing the early strong concave-upward REE pattern in dunite. The chemical variability in the interstitial minerals bears witness of the percolation of MORB, issued from the mantle decompression melting, variably hybridized with melt batches produced within the DTZ by melt-rock reaction and poorly homogenized before reaching the lower crust. Our results lead to the conclusion that pure and impregnated dunites are end-members that recorded different stages of the same initial igneous processes: pure dunites are residues left after extraction of a percolating melt while impregnated dunites correspond to a stage frozen before complete melt extraction. Therefore dunites trace elements contents allow deciphering the multi-stage processes that led to their formation at the mantle-crust transition zone.
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.
•The dunitic transition zone in the Oman ophiolite shows vertical chemical evolutions.•Syn-magmatic faults cut across the DTZ and influence chemical variations.•Melt-fluid-rock reactions within the DTZ are strongly controlled by faults.
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•The Moroccan oceanic arc shows episodic magmatism with a 40–50 Myr tempo.•Episodic magmatism appears to be driven by with tectonic shortening, crustal thickening and delamination.
We ...present an integrated study combining detailed field, geochronological and geochemical data of a Neoproterozoic intra-oceanic arc systems exposed in the Pan-African belt of the Moroccan Anti-Atlas. The arc rock units exposed in Bou Azzer and Sirwa inliers consist of a tectonic patchwork of back-arc ophiolitic sequences to the north thrusted onto accreted arc complexes to the south. Arc complexes are composed of amphibolite, granodioritic and granitic gneisses intruded by various undeformed hydrous ultramafic (hornblendite), mafic (hornblende-gabbro, diorite) and felsic (granodiorite, tonalite, granite) arc lithologies. We show that these complexes are remnants of a long-lived (120 Myr) Neoproterozoic oceanic arc, punctuated by three successive magmatic episodes (760–730 Ma, 710–690 Ma, 660–640 Ma respectively) interspersed with periods of magmatic quiescence. The typical geochemical arc signature and positive ƐNdt values for the igneous rocks emplaced during each magmatic episode (medians at +7.1, +5.4 and +5.7, from older to younger) attest that their parental magmas derived from a depleted mantle source without substantial assimilation by the WAC older crustal basement. Trace-element geochemistry, i.e. Sr/Y, La/Yb, of intermediate to felsic arc rocks produced during each magmatic pulse suggests that the arc crust was thickened (>30–35 km) over a short time period between the first and second magmatic episodes (730–710 Ma) which coincides with an important regional shortening event. Soft-docking of the oceanic arc on a buoyant transitional margin is invoked to explain tectonic inversion in overriding plate, leading to shortening and related thickening of the arc crust. Concomitant magmatic shutdown resulting from a reorganization of subduction dynamics (i.e. change in slab geometry, flip in subduction polarity). A non-tectonic critical thickening of the arc crust is invoked to explain the second magmatic shutdown (680–660 Ma), by freezing the subarc mantle influx. This lull period is followed by a third magmatic episode which is likely triggered by delamination of the dense lower crust and reactivation of subarc mantle flow. This is supported by the bimodal chemical signature of evolved magmatic products, suggesting two distinct sources partial melts from the foundered lower crust and new magmatic products which differentiated from a post-delamination thinned crust.
The Earth’s continental crust represents the outermost envelope of the solid Earth, controlling exchanges within the geosphere and reflecting geodynamics processes. One of the fundamental issues of ...Earth Science aims to determine crustal thickness in past geodynamic environments in order to discuss the evolution of certain geodynamic processes through time. Despite presenting a continuing challenge, the evolution of crustal thickness during the last 3 billion years can be investigated using indirect clues yielded by the chemical signature of mafic magmas and associated ferromagnesian minerals (pyroxene, amphibole). Here, we present a new statistical assessment of a global database of magmatic and mineral chemical information. Analysis reveals the increasing occurrence of high-temperature pyroxenes and amphiboles growing in Ca-rich, Fe-poor magma since ~1 Ga, which contrasts with lower temperature conditions of minerals crystallization throughout the Meso- and Palaeoproterozoic times. This is interpreted to reflect temporal changes in the control of Earth’s crust on mantle-derived magma composition, related to changes in lithospheric thickness and mantle secular cooling. We propose that thick existing crust is associated with deeper, hotter magmatic reservoirs, potentially elucidating the mineral chemistry and the contrasting iron content between primary and derivative mafic magmas. Based on both the chemical and mineral information of mafic magma, an integrated approach provides qualitative estimates of past crustal thickness and associated magmatic systems. Our findings indicate that the Proterozoic was characterized by thicker crustal sections (>40–50 km) relative to the Phanerozoic and Archean (<35 km). This period of crustal thickening appears at the confluence of major changes on Earth, marked by the onset of mantle cooling and Plate Tectonics and the assembly of Columbia, the first supercontinent.
Obducted ophiolites expose oceanic lithosphere slivers in suture zones. The few previous studies focusing specifically on the serpentinization of the ultramafic part of ophiolites mostly concluded ...that serpentinization was driven by seawater. Recent developments in B isotope geochemistry have enabled the tracking of fluid source(s) in serpentinites, suggesting δ11B values above +10‰ mostly reflect seawater hydration and δ11B values below +10‰ mostly reflect hydration by subducted crust-derived metamorphic fluids. In this study, we further investigate the fluid sources recorded by ophiolitic serpentinites by presenting B isotopic geochemistry for seventeen samples from well-characterized ophiolites in Guatemala, Iran, and Brazil. The samples from Guatemala and Iran are from fossil Supra Subduction Zones (SSZ), while samples from Brazil represent a fossil Ocean-Continent Transition (OCT). Most of the samples display the pseudomorphic mesh matrix and bastite microtexture typical of serpentinites. Nine of the samples are either totally (both microtextures) or partially (one microtexture only) out of isotopic equilibrium, with δ11B variations of up to 18‰ within individual samples/textures. Among the samples with a narrow δ11B range (i.e., at isotopic equilibrium), those from a fossil OCT ophiolite have δ11B ranging from +3.8 to +23.2‰, which is broadly consistent with serpentinization by seawater. In contrast, serpentinites from fossil SSZ ophiolite have δ11B straddling 0‰, ranging from −7.7 to +13.5‰. These data indicate that seawater alone cannot be responsible for serpentinization of these ultramafic rocks, but rather either subducted crust-derived metamorphic fluids, or a mixture of seawater and subducted crust-derived metamorphic fluids. This interpretation agrees with the recent suggestions that SSZ ophiolites possibly represent fossil forearc lithosphere.
•Serpentinites in SSZ obducted ophiolites have discriminating B isotopic signatures.•Serpentinizing fluid is a mixture of seawater and subducted crust-derived fluids.•Mixing mostly occurred prior to serpentinization.
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•Bou Azzer ophiolite was studied using rock magnetism, petrography and geochemistry.•Unique massive magnetite veins result from iron leaching in the serpentinite.•Advanced Cr-spinel ...alteration despite low temperature (<350°C).•Hot acidic and Cl-rich fluid is involved.•Continental or abyssal black-smoker type hydrothermalism are proposed.
If magnetite is a common serpentinization product, centimetric, massive and pure magnetite veins are rarely observed in serpentinites. Unique example, in the Aït Ahmane ultramafic unit (Bou Azzer Neoproterozoic ophiolite, Anti-Atlas, Morocco) allows to assess the hydrothermal processes that prevailed at the ending Precambrian. In this study, rock magnetism, petrography, mineral and bulk chemistry are combined to assess iron behavior in these meta-ultramafics, in order to constrain the serpentinites alteration and magnetite veins formation processes.
Very high Cr#, low Mg#, high Fe3+# and low Ti content of Cr-spinels cores reflect a supra-subduction zone origin for the Aït Ahmane serpentinites precursor. Typical lizardite/chrysotile pseudomorphic texture in fresh serpentinites reveals an initial oceanic-like serpentinization, involving temperature <350°C while the abundance of magnetite (up to 10.14wt%) in these unaltered serpentinites attests of a relatively high serpentinization temperature >200°C. Magnetic measurements reveal a lower magnetite content in hydrothermalized serpentinites hosting the magnetite veins, with lowest values (down to 0.58wt%) for bleached serpentinites constituting the wall rock of the veins. These magnetic data are consistent with bulk rock chemistry showing a lower total iron content in hydrothermalized serpentinites. Hysteresis parameters and thermomagnetic measurements denote a magnetic grains size that increases with the alteration, associated with the emergence of a new magnetic phase (Cr-magnetite) produced by Cr-spinels alteration. A new proxy, based on thermomagnetic measurements, the CrM/M ratio, provides a quantification of its contribution to the magnetic susceptibility. Mineral chemistry allowed to identify the Cr-spinels alteration sequence and reveals an important chlorine enrichment in serpentine phases from hydrothermalized serpentinites.
These results suggest that a Cl-rich acidic hydrothermal event involving temperatures below 350°C produced an intense magnetite leaching in the host serpentinite and an advanced Cr-spinels alteration to ferritchromite and Cr-magnetite. Iron provided by this leaching have led to the formation of unique magnetite veins in the Aït Ahmane ultramafic unit. Two different settings are proposed for the hydrothermal event: (1) a continental hydrothermal system as advanced for the Co-Ni-As ores in the Bou Azzer inlier or (2) an oceanic black smoker type hydrothermal vent field on the Neoproterozoic seafloor.
The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose ...crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.
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•Magmatic age of the Bou Azzer ophiolite (Moroccan Anti-Atlas) is 759 ± 2 Ma.•Sirwa and Bou Azzer ophiolites originate from a same intra-oceanic back-arc system.•Both ophiolites recorded multiple arc-back-arc evolution stages.
To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and 87Sr/86Sr ratio of waters and shells from four locations: two ...coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous 87Sr/86Sr ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 ± 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (~25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable 87Sr/86Sr ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous 87Sr/86Sr ratio typical of OOW except close to SGD. In Oualidia, 87Sr/86Sr ratios decrease by 1.2 × 10−3 from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose 87Sr/86Sr ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low 87Sr/86Sr ratios correspond to high Sr concentrations (up to 150 μmol·l−1) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar 87Sr/86Sr gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal 87Sr/86Sr ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.
•Homogeneous 87Sr/86Sr ratios similar to the oceanic value in open shelf contexts•Noticeable heterogeneity of 87Sr/86Sr ratios in lagoons and epeiric seas•Importance of submarine groundwater discharges which may locally supply 10 to 60% of Sr in lagoons•Oceanic Sr inputs and 87Sr/86Sr homogenization controlled by the coastal morphology
Spinel is a ubiquitous mineral in mafic/ultramafic rocks. Spinel cores chemistry is extensively used as a petrogenetic proxy while their alteration phases, ferritchromite, and Cr‐magnetite, are used ...as metamorphic grade indicators. However, the magnetic properties and composition of these phases are still ill‐defined and no consensus exists concerning the metamorphic conditions involved in their formation. Here, we use the magnetic properties of these Cr‐spinel alteration phases, via field‐dependent parameters and observations with a magnetic microscope coupled with mineral chemistry and Mössbauer spectroscopy, to better constrain their composition. We identify Cr‐magnetite by a Curie point of ca. 520°C. We show that it is characterized by an n between 0.1 and 0.2 in the Fe‐Cr spinel formula Fe2+(Fe1−nCrn)2O4, which corresponds to 6–13 wt.% of Cr2O3. The abundance of Cr‐magnetite indicates a strong alteration of Cr‐spinels that could reflect a significant hydrothermal activity rather than a high metamorphism grade. Normalized variation curves of the magnetic susceptibility during heating allow a relative quantification of the contributions of different magnetic phases to the magnetic susceptibility. This highlights a link between ferritchromite destabilization into maghemite at ca. 130°C followed by the destabilization of this maghemite starting at 300°C. We identify specific covariation trends between these two magnetic species characterizing different alteration processes. This study opens the door to magnetic monitoring of the Cr‐spinel alteration state in mafic and ultramafic rocks. It constitutes a new, fast, and weakly destructive way to study the petrological history of both terrestrial and extraterrestrial rocks.
Plain Language Summary
Spinels are a common accessory mineral in Earth's mantle rocks, basaltic lavas and more generally, in mafic/ultramafic rocks from other telluric bodies. Because they are often (partially) preserved even in the case of highly altered rocks like serpentinites, and their chemistry is dependent on magmatic and metamorphic processes, they have been used for decades to investigate the history of their host rock. In this study, we focus on the magnetic and chemical characterization of the phases that compose spinel alteration rims, namely the ferritchromite and the Cr‐magnetite (Chromium‐rich magnetite). Coupling the magnetism and mineral chemistry methods allowed us to better constrain the magnetic behavior of these mineral phases, hence defining clear magnetic signatures for them and precise their chemical composition fields. We show that magnetic measurements can be used to finely monitor the spinels alteration state by quantifying the relative contributions of their different alteration products to the magnetization of a given rock sample. This constitutes a new way to study the metamorphic and/or hydrothermal history of both terrestrial and extraterrestrial rocks.
Key Points
Main spinel alteration products can be easily identified via thermomagnetic measurements
We redefine Cr‐magnetite as a Fe‐Cr spinel Fe2+(Fe1−nCrn)2O4, with 0.1 < n < 0.2 (6–13 wt.% of Cr2O3) and with a Tc at ca. 500–530°C
This study opens the door to magnetic monitoring of the Cr‐spinel alteration state in mafic and ultramafic rocks
The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a series of (~750 Ma) ophiolitic units which, when properly characterized, could provide ...important informations on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation. We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather high MgO concentrations (42.28 to 45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively, indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites whole-rock REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction, coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together, these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent transition (OCT) accreted on the eastern border of the Amazonian Craton.
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•The Araguaia Belt hosts a series of Neoproterozoic ophiolitic units.•Serpentinites derived from a depleted (f ~14 to 24%) protolith.•Mafic magmas are N- and E-MORB and do not show any subduction signature.•The Araguaia Belt is a fossil Neoproterozoic hyper-extended margin.
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