Moho Transition Research Articles

Petrogenesis of the chromitite body from the Cerro Colorado ophiolite, Paraguaná Peninsula, Venezuela

Ultramafic-mafic rocks of ophiolitic affinity crop out along the Venezuelan Caribbean region. They have been interpreted as remnants of the oceanic lithosphere of the Caribbean volcanic arc (135-70 Ma) as well as relicts of proto-Caribbean oceanic lithosphere (Upper Jurassic-Lower Cretaceous) related to Pangea’s break-up. The Cerro Colorado ophiolite, located in the Paraguaná Peninsula, together with the case of the Cordillera de la Costa in north-central Venezuela, are a unique case of these Venezuelan ophiolites containing chromitite bodies. However, the petrogenesis of such a mafic-ultramafic complex and associated chromite ore remains are unknown to date. To advance our understanding of chromite ores in the Caribbean region, the genesis of the Cerro Colorado chromitite body is challenged. The Cerro Colorado chromitite body is characterized by a low-Cr content [Cr# =Cr/ Cr+Al= 0.44-0.60] and a distribution of trace elements in chromite as is typical of high-Al chromitites found in the shallower portions of the petrological Moho Transition Zone of Mesozoic ophiolites. The calculated melts in equilibrium with chromite forming this high-Al chromitite body are back-arc basin basalts. These melts were extracted after ~20 % partial melting of moderately depleted peridotites, which resulted in the precipitation of high-Al chromitite relatively impoverished in PGE (≤ 100 ppb total PGE). A comparison of the geochemical signatures of minor and trace elements in chromite and bulk-PGE contents of the Cerro Colorado chromitite with those of other known chromitites in the peri-Caribbean ophiolites show certain similitude with those high-Al described in the Moa-Baracoa ophiolite in eastern Cuba. The obtained results allow us to suggest that the ultramafic rocks of the Cerro Colorado and the chromitite body associated with it are closely related to the formation of a back-arc basin developed between ca. 125-120 Ma in the rear of the Great Antilles Arc.

SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites

AbstractOne of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a supra‐subduction zone (SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range (MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing (Moseley and Abbotts 1979, Dilek and Furnes, 2011; Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well‐developed ophiolite, known as the Masirah ophiolite (Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains (Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of mid‐Cretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc‐alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back‐arc basin model for this ophiolite (e.g. Tamura and Arai, 2006; Godard et al., 2008; Rollinson and Adetunji, 2015).The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantle‐normalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment.Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean (MOR) or suprasubduccion zone (SSZ) (Arai and Matsukage, 1998; Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean.Coexisting high‐ and low‐Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low‐Cr# chromitites at the Moho transition zone, possibly in a mid‐ocean‐ridge setting. The second chromitite‐forming event involves higher degree partial melting, which produced high‐Cr# discordant chromitite in the upper mantle, possibly in a supra‐subduction zone setting.Assemblages of mono‐ and poly‐phase silicate inclusions (including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High‐T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.

Sulfide Aggregation in Ophiolitic Dunite Channels Explains Os‐Isotope Mismatch between Oceanic Crust and Mantle

AbstractThe osmium‐isotope mismatch commonly reported between mid‐ocean‐ridge basalts (MORBs) and residual mantle might reflect evolution of the MORB Re‐Os system after extraction from the asthenosphere, or preferential contribution of radiogenic Os components from mantle. However, in a MOR system, the role of dunite melt channels from the upper mantle and Moho transition zone in regulating isotopic systems between mantle and crust has rarely been evaluated. We report new Re‐Os isotopic compositions of base‐metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/ (Cr3++Al3+) ≤ 0.66] (products of interaction between MORB‐like melts and upper‐mantle harzburgites) from the Zedang ophiolite (South Tibet). Re‐Os isotopic compositions of low‐Cr# dunites from the Oman ophiolite are also shown for comparison. Mineralogical evidence suggests that the Zedang sulfides were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios (0.1191‐0.1702) and low 187Re/188Os (<0.22) of the sulfides suggest that the chromite acted as a sink for Os‐bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub‐PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os‐rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os‐poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Oman ophiolite and present‐day mid‐ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os‐bearing sulfides with chromite, leaving high Re/Os components into the residual melts. Once such channel systems were built up at the crust‐mantle transition zone, the newly incoming MOR magmas would preferentially melt and dissolve the volumetrically abundant radiogenic BMS and retain Os‐rich nanoparticles in the channels, further amplifying the Os‐isotope mismatch between oceanic crust and mantle. This study sheds new light on the multistage evolution and small‐scale behaviors of chalcophile and siderophile elements (e.g., Re‐Os) and their isotopes (e.g., 187Re‐187Os) with sulfides and chromites in a silicate‐dominated melt plumbing system beneath mid‐ocean ridges.

Compositional Variations and Tectonic Settings of Podiform Chromitites Associated with Ultramafic Rocks of the Pan‐African Proterozoic Ophiolites from South Eastern Desert, Egypt

AbstractThe Precambrian podiform chromitites associated with ophiolites are abundant in Pan‐African belt in central Eastern Desert (CED) and south Eastern Desert (SED), Egypt and range from 690 to 890 Ma in age. The studied chromitites associated with Neoproterozoic ophiolites are distributed in southern Eastern Desert, Egypt in Baranis ‐ Shalaten sheet and occur as lenticular bodies with variable dimensions in ultramafic component (serpentinites). We present geochemical and mineralogical data from three areas of ophiolites and associated chromitites namely Gebel Abu Dahr (D), Gebel Arais (A) and Gebel Anbat in the Wadi Hodein area (H) (Fig. 1). The paper studies the compositional variations and tectonic settings of podiform chromitites associated with ultramafic rocks, in addition to the alteration process of chromite during metamorphism.The ophiolite in the present areas is composed of the ultramafic rocks (mainly serpentinites) with minor relics of fresh dunite and harzburgite. All these rocks are affected by metamorphism and subsequent retrograde during subduction and exhumation.Six samples selected from the serpentinites geochemically analyzed for major, trace and some REE elements and the geochemical results reflect that harzburgite and dunite compositions are typical of depleted mantle peridotite. Microprobe analyses and SIMS investigations were carried out for three massive podiform chromitite ore bodies and disseminated chromites in serpentinites (1215 spot probe analyses), and silicate minerals in serpentinite rocks such as serpentine and olivine (102 spots). Serpentine minerals are mainly antigorite with some chrysotile in serpentinite rocks and in chromitites, mainly filling cross‐cutting veins.In this study, we consider that the alteration occurred in two stages: during the first one chromite reacted with olivine and water to form Cr‐ and Fe‐rich, porous chromite and chlorite; during the second event magnetite filled the pores, created in the porous chromite and defused into this chromite to form homogeneous magnetite.According to this, the composition of chromite is a key factor controlling the metamorphic reaction between olivine and chromite because if the primary chromite is very poor in Al, the chlorite‐forming reaction hardly takes place. In this case, during the second event, the addition of magnetite only contributes to create a magnetite corona around the former chromite grains without any diffusion at the chromite‐magnetite boundary as suggested by Gerbilla et al. (2012).Barnes (2000) studied the chromite in komatiites and modification during green schist to mid amphibolite facies metamorphism. He suggested that the chromite cores continually equilibrated with magnetite rims document metamorphic grade conditions. Barnes (2000) suggested that the relative proportions of Cr3+, Al3+ and Fe3+ of chromite are not affected by metamorphism up to lower temperature amphibolite facies implying restricted mobility of these elements occurred under lower amphibolite facies. So, the chromite in lower temperature amphibolite facies preserves its primary igneous chemistry and can be used to estimate the metamorphic grade. Sack and Ghiorso (1991) and Barnes (2000) suggested that all chromite cores are equilibrated at temperature below ∼ 500–550°C corresponding to lowest amphibolite facies metamorphism and reflect magmatic composition not influenced by metamorphism. In this study, there is no alteration but only nearly pure magnetite deposition and development with restricted Cr‐solubility at < 500°C in the chromite rims on crystal boundaries and within fractures as shown in Fig 2a, b. Also magnetite alters later to hematite.The podiform chromitites are common in the Moho transition zone (MTZ) to the mantle section of ophiolites or harzburgite dominant peridotite massifs (e.g., Arai, 1997; Miura et al., 2012). They have been interpreted as a product of peridotite / melt reaction and subsequent melt mixing within the MTZ to the upper mantle; they are basically magmatic cumulates that formed at the upper mantle level (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). They are thus a good marker of peridotite / melt reaction (e.g., Arai, 1997). The Pan‐African podiform chromitites may have formed in the same way as the Phanerozoic, namely by melt‐ harzburgite reaction and subsequent melt mixing. The podiform chromatites and disseminated chromites are high‐Cr chromites and have range in Cr#(Cr/Cr+Al) atomic ratio from 0.75 to 0.95 and low Ti with boninitic affinity (Fig. 3a), indicating an island arc setting in supra ‐ subduction zone setting. The present massive chromitites and disseminated chromites in serpentinites fall in the field of chromites de Bou Azer, chromites de Cordoba, Argentinia in the Cr# versus Mg# diagram (Fig. 3b, c) (Gervilla et al., 2012)The studied chromatites contain some grains of platinum‐group minerals (PGM) such as sulfides (Os‐rich laurite) and Os–Ir alloy as shown in Fig. 4 and as reported in South Eastern Desert by Ahmed (2007).

Geochronology and Geochemistry of Gabbros from Moa‐Baracoa Ophiolitic Massif, Eastern Cuba: Implication for Early Cretaceous SSZ Magmatism

AbstractA series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.

Textural Insights on Significance of Ophiolitic Chromitites, with Special Reference to Oman

AbstractBased on the textural and crystallographic study of four chromitite sites in the Oman ophiolite, we show that chromite crystallized in situ at Moho transition zone, where exposed as podiform chromitite deposits.Crystallization operated either by crystal fractionation in a mini‐magma chamber (Tuf dyke), or more commonly by melt‐rock reaction of a hydrated Cr‐rich melt and enclosing dunite. Oxidizing conditions at Moho level triggered the crystallization of chromite at the expense of the corroded olivine network. High‐temperature mantle flow (1100–1200 °C), recorded in the joined reacting olivine aggregates, constrains the timing of chromitite formation.Models of genesis of chromitite deposits must account for a hydrous component initiating partial melting of refractory peridotite, and the revealed occurrence of ultra‐high‐pressure cratonic phases included in some chromite crystals of some ophiolitic chromitites.

Magma water content of Pico Volcano (Azores Islands, Portugal): a clinopyroxene perspective

Clinopyroxenes from the Pico Volcano (Pico Island, Azores Archipelago) have been used as a proxy to define the water content of primitive magmas and the volcanological history of the erupted rocks. This very young volcano (53 ± 5 ka) is at a primordial stage of its evolution in comparison with the other volcanoes of the Azores. Clinopyroxenes from Pico Volcano underwent important dehydration processes and after annealing experiments under H2 gas flux, a pre-eruptive H2O content between 93 and 182 ppm was recovered. A moderately high cooling rate for the cpx-host lavas expressed by the clinopyroxene closure temperature (Tc = 755–928 °C ± 20 °C) correlates with the dehydration, suggesting that this process may have occurred during magma ponding at the Moho Transition Zone (17.3–17.7 km) and/or after the eruption. By applying an IVAl-dependent partition coefficient to the measured H amount in clinopyroxene, the pre-eruptive water content of the parental magma was calculated to vary between 0.71 and 1.20 (average of 1.0) wt%. Clinopyroxene geobarometry performed by combining X-ray diffraction with mineral chemistry points to a general crystallisation from the mantle lithosphere (~ 8–9 kbar) to the oceanic mantle/crust boundary (~ 4–5 kbar). The similar major and trace chemistry, water content and Fe3+/Fetot ratio of clinopyroxene, suggest similar conditions of oxygen fugacity, water content and fractional crystallisation of the magma from which clinopyroxene cores crystallised during the Pico Volcano central eruptions from 40 ka to historical times.

Fe-Ti-Zr metasomatism in the oceanic mantle due to extreme differentiation of tholeiitic melts (Moa-Baracoa ophiolite, Cuba)

Ti-rich amphibole, Mg-rich ilmenite, baddeleyite, zirconolite, srilankite, and zircon are important high-field-strength elements (HFSE) bearing phases in the Potosí chromitite bodies located in the Moho Transition Zone of the Cretaceous Moa-Baracoa suprasubduction zone ophiolite (eastern Cuba). Such HFSE-bearing phases were found in the interaction zone between gabbroic intrusions and chromitite pods. In addition to HFSE-bearing minerals, the studied samples are composed of Fe3+ and Ti-rich chromite, olivine (Fo86-90), clinopyroxene (En44-49), plagioclase (An51-56), orthopyroxene (En84-94), F-rich apatite, and Fe-Cu-Ni sulfides. The studied ilmenite hosting Zr oxides (baddeleyite, zirconolite, and srilankite) contains up to 13 wt.% MgO. The Potosí zirconolite is the first record of this mineral in ophiolitic chromitites and non-metamorphic ophiolite units, and it has relatively high REE contents (up to 10 wt.% of REE2O3) and the highest concentrations in Y2O3 (up to 11 wt.%) reported so far in zirconolite from terrestrial occurrences. Zircon is observed forming coronas surrounding ilmenite grains in contact with silicate minerals, and is characterized by very low U and Pb contents. The zircons formed after high temperature Zr diffusion in ilmenite (exsolution) and a subsequent reaction along grain boundaries following crystallization. Finally, U-Pb dating of baddeleyite exsolutions within ilmenite yielded an average age of 134.4 ± 14 Ma, which provides the first ever dating for a metasomatic event in Potosí that matches well (within uncertainty) the formation age of the oceanic crust of the eastern Cuba ophiolite. We propose that the occurrence of HFSE- and REE-bearing minerals in the Potosí chromite deposit is the result of a two stage process: first, water-rich and HFSE-rich residual melts are produced by intercumulus crystal fractionation after an evolved MORB (BABB)-like melt; and secondly, these residual melt fractions escaped the solidifying mush and extensively reacted and metasomatized the surrounding chromitites, crystallizing HFSE- and REE-bearing minerals and Fe-Cu-Ni sulfides.

The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

AbstractThe Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the Attic‐Cycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modeling, U–Pb geochronology and whole rock geochemistry, resulting in a tectonothermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by U‐Pb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a suprasubduction zone setting, comparable to the “East‐Vardar Ophiolites,” and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at P‐T conditions of ~8.5 kbar 850–600 °C, downstructural section to greenschist‐facies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a U‐Pb zircon protolith age of ~190 Ma and a high‐grade metamorphic‐anatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myr after it formed during initiation of a NE dipping intraoceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in high‐pressure metamorphism of the Cycladic continental margin ~25 Myr later.

Unravelling the magma feeding system of a young basaltic oceanic volcano

A multidisciplinary approach combining petrological, geochemical, and fluid-inclusion studies with seismic monitoring data was used to build a model of the magma feeding system of Pico volcano (Azores islands, North Atlantic Ocean). We explore how magma has ascended to the surface in the last 10 ka and how this ascent is associated with the selective activation of the three tectonic systems intersecting the volcano. The deepest and most important ponding level for all ascending magmas is located at 17.3–17.7 km and corresponds to the Moho Transition Zone (MTZ), which marks the transition from mantle rocks to ultramafic cumulates. At shallower depth ascending magmas carry >30 vol% of clinopyroxene and olivine. Each magma ascent followed a distinct path and ponded often for a limited period. Ponding levels common to all feeding systems are present at 16.3–16.7 km, 12.1–14.5 km, 9.4–9.8 km, and 7.7–8.1 km. These depths mark important discontinuities where magmas formed stacked sills and evolved through fractional crystallisation. Dense and un-decrepitated fluid inclusions show rapid ascent from the MTZ along the Lomba do Fogo-São João fault (N150° system) and along the N120° regional transtensive system, despite multiple intrusions. Magma ponding at 5.6–6.8 km occurs where the N150° and N60° tectonic systems intersect each other. Here magma evolves towards plagioclase-rich and is only erupted at the summit crater and subterminal vents. This region is the source of the frequent microseismicity recorded at 4 to 7 km beneath the southern flank of Pico volcano, which might be associated with the early stages of formation of a more complex magma reservoir. The local and regional tectonics are of paramount importance in the activation of the different magma feeding systems over time. This new information is fundamental to improve the knowledge on the future eruptive behaviour of Pico volcano and can have significant implications on the mitigation of volcanic risk. This multidisciplinary approach can be applied not only to other volcanoes of the Azores but also to poorly monitored oceanic volcanoes, where magma ascent strongly depends on the activation of tectonic systems.

Sulfide in dunite channels reflects long-distance reactive migration of mid-ocean-ridge melts from mantle source to crust: A Re-Os isotopic perspective

The Earth's major heat and mass fluxes are output from the mantle to surface via mid-ocean-ridge-basalt (MORB) magmatism. The variations of MORBs are determined both by the heterogeneity of mantle source and the melt migration processes from mantle source to seafloor. However, before the MORB magmas enter crust, the role of the major melt migration passageways within the mantle (dunite channel systems) in regulating the compositions of MORBs has been rarely evaluated, especially from the perspective of Re-Os isotopes. Here, we report new Re-Os isotopic compositions of base-metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/(Cr3++Al3+) ≤0.66] (products of interaction between MORB-like melts and upper-mantle harzburgites) from the Zedang ophiolite (South Tibet). Rhenium-Os isotopic compositions of low-Cr# dunites from the Samail and Wadi Tayin massifs (typical MOR segments) of the Oman ophiolite are also shown for comparison to reveal the melt plumbing processes at mantle depths beneath mid-ocean ridges. Mineralogical evidence suggests that the Zedang sulfides and desulfurized alloy portions were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios (0.1191-0.1702) and low 187Re/188Os (<0.22) of the sulfides suggest that the chromite acted as a sink for Os-bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub-PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os-rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os-poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Samail and Wadi Tayin massifs of the Oman ophiolite and present-day mid-ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os-bearing sulfides with chromite, leaving high Re/Os components into the residual MOR melts. Once such channel systems were built up at the Moho transition zone, the newly incoming MOR magmas would preferentially dissolve the volumetrically abundant radiogenic BMS and retain Os-rich alloys/sulfides in the channels, further amplifying the Os-isotope mismatch observed between global oceanic crust (more radiogenic) and lithospheric mantle (less radiogenic).This study reveals that the long-distance reactive migration of melts from mantle source to at least Moho depths can result in strong Re-Os fractionation and isotopic mismatch between primary MORBs and mantle residues. It also shows that, unlike other lithophile tracers (e.g., Nd-Hf isotopes) whose signatures are almost completely transferred from mantle source to crust, the sub-crustal melt plumbing processes beneath mid-ocean ridges can be finely constrained by the multistage evolution of chalcophile and siderophile elements (e.g., Re-Os) and their isotopes (e.g., 187Re-187Os) with sulfides and chromites. Our study supports that the MORB compositions are actually regulated by their reactive migration processes occurred between melt generation at depth and eruption on seafloor.

Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano

Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O-undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H2O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume.

Melt-rock interaction and fractional crystallization in the Moho transition Zone: Evidence from the cretaceous Naga Hills Ophiolite, North-East India

The Moho Transition Zone (MTZ) within the suboceanic lithosphere records an episode of multiple magmatic exchanges between the underlying mantle and the overlying oceanic crust. This study investigated a fossilized MTZ from the Naga Hills Ophiolite (NHO) by characterizing its lithology and describing the petrogenesis of various rock types. Like the other well-studied MTZs around the world, the NHO MTZ has an association of dunite, wehrlite, clinopyroxenite and gabbroic rocks. The dunite formed by crystal fractionation from an olivine-normative basaltic melt that successively crystallized olivine-gabbro, gabbro and oxide-gabbro. Our study of the wehrlite provides evidence of melt-mantle interactions that lead to the formation of replacive dunite, later impregnated with clinopyroxene saturated melt. The melt percolated through the replacive dunite and re-equilibrated with the olivine to some extent. The clinopyroxenite formed simultaneously with the wehrlite as a crystal mush in pockets from clinopyroxene saturated melt. In summary, the major and trace element mineral chemistry of the NHO MTZ rocks documents their evolution through melt-rock interaction and fractional crystallization. Clinopyroxene played an important role in the evolution of NHO MTZ, particularly the formation of wehrlite and clinopyroxenite. There is no direct evidence for the involvement of water in the formation of the MTZ rocks. However, the presence of primary hydrous phases in the evolved rock types and the signatures of early crystallization of clinopyroxene prior to plagioclase in the initial cumulates, discernible from a positive Sr anomaly, suggest the hydrous nature of the parent melt. Therefore, the NHO possibly represents a fragment of oceanic lithosphere that had its origin above a subduction zone and was later incorporated within a mountain belt during terminal closure of the Neo-Tethys Ocean.

Chlorine-rich amphibole in deep layered gabbros as evidence for brine/rock interaction in the lower oceanic crust: A case study from the Wadi Wariyah, Samail Ophiolite, Sultanate of Oman

Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

Polymineralic inclusions in mantle chromitites from the Oman ophiolite indicate a highly magnesian parental melt

Polymineralic inclusions interpreted as melt inclusions in chromite from the dunitic Moho Transition Zone in the Maqsad area of the Oman ophiolite have been analysed and compositions integrated using a rastering technique on the scanning electron microscope. The inclusions now comprise a range of inter-grown hydrous phases including pargasite, aspidolite, phlogopite and chlorite, indicating that the parental melts were hydrous. Average inclusion compositions for seven samples contain between 23.1 and 26.8 wt% MgO and 1.7–3.6 wt% FeO. Compositions were corrected to allow for the low FeO concentrations using coexisting olivine compositions. These suggest that the primary melt has between 20 and 22 wt% MgO and 7–9.7 wt% FeO and has an affinity with boninitic melts, although the melts have a higher Ti content than most boninites. Average rare earth element concentrations suggest that the melts were derived from a REE depleted mantle source although fluid-mobile trace elements indicate a more enriched source. Given the hydrous nature of the inclusions this enrichment could be fluid driven.An estimate of the melt temperature can be made from the results of homogenisation experiments on these inclusions and suggests ~1300 °C, which implies for a harzburgite solidus, relatively shallow melting at depths of <50 km and is consistent with a boninitic origin.The current “basaltic” nature of the chromite host to highly magnesian melt inclusions suggests that the dunitic Moho Transition Zone operated as a reaction filter in which magnesian melts were transformed into basalts by the removal of high magnesian olivines, particularly in areas where the Moho Transition Zone is unusually thick. We propose therefore that podiform mantle chromitites, even those with an apparent MORB-like chemical signature, have crystallised from a highly magnesian parental melt. The data presented here strongly support the view that this took place in a subduction initiation setting.

The Calzadilla Ophiolite (SW Iberia) and the Ediacaran fore-arc evolution of the African margin of Gondwana

In the Ossa Morena Domain of SW Iberia, the Calzadilla Ophiolite is formed by an ensemble of ultramafic and mafic rocks that seem to represent a fragment of an oceanic Moho transition zone. The ophiolite consists of a main sheet with a minimum thickness of c. 1000m and several ultramafic slices that appear imbricated with Ediacaran siliciclastic rocks of the so called Serie Negra. The mafic rocks of the Calzadilla Ophiolite show an extreme depletion in some HFSE such as Nb, Zr, Th, Hf and Ta, low TiO2 contents and high MgO, which allow identifying them as magmatic types of boninitic affinity, such as those frequently associated with fore-arc settings. U-Pb zircon dating of the gabbroic rocks suggests that the igneous protoliths crystallised at c. 600Ma and were extracted directly from the mantle, according to the juvenile isotopic sources revealed by the Hf isotopic composition of the dated zircons. At c. 540Ma the U-Pb isotopic system was affected by a partial resetting event with moderate generation of new zircon. According to these data, it is considered that the Calzadilla Ophiolite was formed in a supra-subduction zone setting, very likely in a fore-arc domain, during a roll-back episode affecting the peri-Gondwanan subductive slab. The fore-arc domain was probably proximal to the West Africa Craton. It is very likely that the ductile deformation, metamorphism and tectonic imbrication of the Calzadilla Ophiolite and the Serie Negra occurred at c. 540Ma, during an increase in the subduction rate and a significant decrease of the subduction angle of the oceanic slab. The information provided by the Calzadilla Ophiolite allows to better constrain the geodynamic evolution of the African margin of Gondwana.

Report on the ICDP workshop DIVE (Drilling the Ivrea–Verbano zonE)

Abstract. The Ivrea–Verbano Zone is the most complete, time-integrated crust–upper mantle archive in the world. It is a unique target for assembling data on the deep crust and the Moho transition zone and testing several hypotheses of formation, evolution, and modification of the continental crust through space and time across the Earth. The ICDP workshop Drilling the Ivrea–Verbano zonE (DIVE), held in Baveno, Italy, from 1 to 5 May 2017, focused on the scientific objectives and the technical aspects of drilling and sampling in the Ivrea–Verbano Zone at depth. A total of 47 participants from 9 countries with a wide variety of scientific and/or drilling expertise attended the meeting. Discussion on the proposed targets sharpened the main research lines and led to working groups and the necessary technical details to compile the full drilling proposal. The participants of the workshop concluded that four drilling operations in the Val Sesia and Val d'Ossola crustal sections represent the scientifically most promising solution to achieve the major goals within DIVE to unravel the physico-chemical properties and architecture of the lower continental crust towards the crust–mantle (Moho) transition zone.

Lithospheric architecture of the South-Western Alps revealed by multiparameter teleseismic full-waveform inversion

The Western Alps, although being intensively investigated, remains elusive when it comes to determining its lithospheric structure. New inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the Alps. This situation led to the deployment of the CIFALPS temporary experiment, conducted to address the lack of seismological data amenable to high-resolution seismic imaging of the crust and the upper mantle. We perform a 3-D isotropic full-waveform inversion (FWI) of nine teleseismic events recorded by the CIFALPS experiment to infer 3-D models of both density and P- and S-wave velocities of the Alpine lithosphere. Here, by FWI is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0.2 Hz. We show that the application of the FWI at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere. In the shallowest part of the crust, we retrieve the shape of the fast/dense Ivrea body anomaly and detect the low velocities of the Po and SE France sedimentary basins. The geometry of the Ivrea body as revealed by our density model is consistent with the Bouguer anomaly. A sharp Moho transition is followed from the external part (30 km depth) to the internal part of the Alps (70–80 km depth), giving clear evidence of a continental subduction event during the formation of the Alpine Belt. A low-velocity zone in the lower lithosphere of the S-wave velocity model supports the hypothesis of a slab detachment in the western part of the Alps that is followed by asthenospheric upwelling. The application of FWI to teleseismic data helps to fill the gap of resolution between traditional imaging techniques, and enables integrated interpretations of both upper and lower lithospheric structures.

Comparison of Different‐sized Chromite Mineralizations in the Yarlung‐Zangbo Ophiolite Belt, Southern Tibet

Podiform chromitites are characteristically occurred in ophiolites (e.g., Thayer, 1964; Dickey, 1975). However, the metallogenic processes for podiform chromitites are still unclear. Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers (Dickey, 1975; Boudier and Coleman, 1981), but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites (Dick, 1977; Dick and Bullen, 1984). Recent studies by the majority of authors have suggested that melt‐rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites (Zhou and Robinson, 1994; Zhou et al., 1996, 2005, 2014; Robinson, 2008; Page and Barnes, 2009; Uysal et al., 2009, 2012; González‐Jiménez et al., 2011, 2015). Based on the occurrence of some ultrahigh pressure minerals (e.g. diamond and coesite) in chromitites, it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling (Yang et al., 2007; Yamamoto et al., 2013). Although geat progresses have been made towards understanding the genesis of podiform chromitites, some fundamental issues in remain unanswered. For examples, what are the major controls on the size of chromitites? And why some ophiolites contain large podiform chromitite bodies, whereas most ophiolitic massifs are essentially chromitite‐barren?The Yarlung‐Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world. It contains fresh peridotites as well as different‐sided podiform chromitites. The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China. In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small‐scale chromitite bodies. Such characteristics make the Yarlung‐Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites.The Luobusha chromitites are large lens and enclosed in dunite. In contrast, the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes. The closely spatial association of the chromitites and dunite envelopes, together with their textural features, support a petrogenetic model that the chromitites from the Luobusha, Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite. In terms of chemical composition of chromite, there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo. Chromite from the Luobusha chromitites has high Cr# (71–82), whereas Chromite in the Dazhuqu chromitites show relatively low Cr# (16–63), and chromite in the Dongbo chromitites includes low Cr# (11–47) and high Cr# (70–81) types. For the Dongbo and Dazhuqu massifs, linear trends of Cr# with MgO, FeOt, Ni, Ga, V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites, suggesting that the melt‐rock reaction may provide major budget of Cr for the chromitites. The similar compositions at a given Cr# in chromite from these rocks also demonstrate that the chromitites may have been formed by in‐situ crystallization of chromite under low melt/rock ratio. In contrast, the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites, implying that the formation of the chromitite bodies requires a continual replenishment of Cr‐rich melts from deeper mantle. Fractionation and accumulation of chromite from a large volume of Cr‐rich melt may play an important role on the formation of the Luobusha chromitites. MORB‐normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr‐rich boninitic melt at surpra‐subduction zone (SSZ) setting. However, the Dongbo and Dazhuqu chromitites have formed originally from a MORB‐affinity melt at a mid‐ocean ridge (MOR) environment.In summary, the Luobusha chromitites crystallized from a Cr‐rich melt in a dynamic conduit, where fractional crystallization and crystal settling play a key role in formation of the large chromitites. In contrast, the small‐scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts. Podiform chromitites can be formed in MOR environment, whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

Mylonites in ophiolite of Mirdita (Albania): Oceanic detachment shear zone

The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a common horizontal Jurassic–Lower Cretaceous sedimentary cover showing that they were not deeply and intrinsically affected by later Alpine thrusting. The western massifs of Mirdita, first oceanic core complex (OCC) and detachment shear zone described in ophiolites, compare with OCCs in slow-spreading ridges and provide continuous exposure of the deep internal structure of this system, revealing its kinematics, thanks to detailed structural mapping in peridotites and gabbros. The Mirdita detachments root in the Moho transition zone (MTZ), a weak zone at the top of asthenospheric mantle, where basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ∼400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho beneath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, despite being deeply affected by hydrothermal alteration. From there, the partially molten MTZ was detached as a shear zone, mixing with lower gabbros. The OCC emerged, migrating upsection and eastward over 5 km. Finally, the OCC front is observed in hornblende-rich syntectonic mylonites derived from upper gabbros and from the overlying former lid. Serpentinization is static within these mylonites. A low-temperature detachment fault is expressed as a sheared antigoritic melange at the margin of the mylonitic shear zone. Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin has been preserved below the OCC rooting. The dominant asthenospheric flow direction trends parallel to the ridge axis. This mantle flow rotates over 200 m into the low-temperature mylonitic detachments, where OCC motion turns transversal to the ridge. Crystal preferred orientation measurements on six samples point to brown hornblende crystal growth during mylonitic flow and illustrate the change of olivine intra-crystalline slip system in mylonites compared to porphyroclastic harzburgite.