Mantle-crust Transition Research Articles

The Sources of Ore-Forming Materials and Fluids for the Jinqingding Gold Deposit in the Mouping–Rushan Metallogenic Belt, Jiaodong Peninsula: Evidence from S-H-O Isotopes and Trace Elements in Pyrite

The Jinqingding gold deposit, characterized as an extra-large quartz-vein-type deposit, is located in the middle of the Mouping–Rushan metallogenic belt in the Jiaodong Peninsula, and there is still controversy over its sources of ore-forming materials and fluids. This paper divides the mineralization of Jinqinding gold deposits into four stages, based on a field geological investigation and indoor petrographic observations: (1) coarse-grained pyrite–quartz stage, (2) quartz–fine-grained pyrite stage, (3) quartz–polymetallic sulfide stage, and (4) quartz–carbonate stage. The quartz fluid inclusions showed δD values of −96.0 to −81.8‰ and δOV-SMOW values of 0.70 to 6.32‰, indicating that the ore-forming fluids were mainly magmatic water, with some metamorphic water and atmospheric precipitation. The in situ δ34S values in different subzones of the pyrites of the Jinqingding gold deposit range from 6.69 to 10.86‰. The δ34S value range of the Jinqingding gold deposit is basically consistent with the contemporaneous intermediate–basic dikes in the region, suggesting a shared material source. In situ LA-ICP-MS geochemical analyses of the pyrites show large variations of Co/Ni ratios (0.21 to 99.5), which suggest a hydrothermal origin for the gold deposit. We infer that the ore-forming fluid of the Jinqingding gold deposit originated from the magma from the upper mantle and the mantle–crust transition zone.

New geochemical and age constraints ( 40 Ar/ 39 Ar and U–Pb) on forearc intrusive rocks from the New Caledonia Ophiolite (SW Pacific): diversity of melts generated at hot subduction inception

The New Caledonia Ophiolite is cross-cut by coarse- to medium-grained pyroxenite and hornblende gabbro/diorite dykes intruded between 55.5 and 50 Ma (U–Pb zircon and 40 Ar/ 39 Ar hornblende), whereas the finer grained dolerites of tholeiitic affinity are younger (50–47 Ma). The production of hornblende gabbros/diorites was modelled by moderate degrees (20–40%) of partial melting of the high-temperature amphibolites of the metamorphic sole. The end-member compositions (hornblendites and anorthosites) resulted from solid-state phase segregation of crystal mushes within tectonically active magmatic conduits. Cascade reactions of the slab melts with mantle wedge peridotites successively formed clinoenstatite-bearing boninite magmas, which fed gabbronorite cumulate lenses at the mantle–crust transition; in turn, the clinoenstatite-bearing boninite melts reacted with peridotites to form websterites. The youngest magmas (of tholeiitic affinity) appeared c. 6 Myr after the inception of subduction when the cooler subducting slab plunged more steeply. Incipient slab retreat allowed corner flow, triggering low-pressure hydrous melting of the uplifted asthenosphere. The early stages of forearc magmatism were closely associated with transcurrent shear zones, which recorded oblique subduction inception. The early Eocene tectonic and magmatic features of the New Caledonia Ophiolite provide evidence for a north- or NE-dipping hot (forced) subduction zone in the SW Pacific, notably distinct from the slightly younger west-dipping Izu-Bonin–Marianna cold (spontaneous) subduction system. Supplementary material: Field pictures, additional diagrams, GPS location of dated samples, whole-rock geochemical and isotope data, Ar/Ar data, U–Pb zircon data, and distribution coefficients ( K d ) of trace elements used for modelling are available at https://doi.org/10.6084/m9.figshare.c.6949265 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Geochemical variability among stratiform chromitites and ultramafic rocks from Western Makran, South Iran

The geochemical compositions of minerals from the Moho transition zone of ophiolites potentially can help to understand the magmatic evolution of the ophiolites, and subsequent mantle-melt interactions. The Jurassic-Late Cretaceous Makran ophiolite of south Iran comprises one of the most extensive tracts of oceanic crust which were scraped off and preserved in an accretionary complex. The Makran ophiolite records traces of MORB-OIB-type magmatism during the Jurassic, but mostly supra-subduction zone magmatic activity during the Late Cretaceous. Despite a few geochemical studies on the crustal rocks, the nature and geochemical signatures of the mantle rocks from this ophiolite remain controversial. The Sorkhband mantle-crust transition zone underlying crustal cumulates in the western Makran consists of stratiform chromitites, harzburgites, chromite-rich dunites and dunites, with crosscutting dikes of olivine websterite and olivine clinopyroxenite. Major- and trace-element compositions of clinopyroxene grains in olivine websterite and clinopyroxenite dikes indicate crystallization from melts similar to boninites and low-Ti fore-arc basalts. Spinel compositions in olivine websterite and clinopyroxenite dikes suggest crystal fractionation from boninitic or high- Mg# magmas have played a major role in the genesis these rocks.We propose a two-stage model for the formation of the Sorkhband dunites including (1) supra-subduction zone-related melt infiltrates through harzburgites in the mantle-crust transition zone to dissolve peridotite orthopyroxene and leave dunites with high forsterite-NiO olivines, and (2) boninitic melts accumulate and react with surrounding harzburgites to crystallize cumulate dunites with low-Mg# olivine and high-Ti spinels. We conclude that there have been temporal changes in the composition of mantle melts in the fore-arc mantle section of the Makran ophiolite during the initial subduction of the Neotethyan ocean beneath the Lut block during the Late Cretaceous.

Experimental diopsidite: Implications for natural diopsidite genesis through fluid-melt-mantle peridotite reaction

Occurrences of diopsidites, rocks made predominantly of gem-like diopside whose composition precludes a purely igneous origin, allow tracking the pathway of high-temperature fluids and fluid-saturated melts in the oceanic lithosphere in vicinity of the mantle-crust transition zone (the Moho) and the adjacent mantle in an oceanic setting. We have experimentally explored the origin of the mantle diopsidites by reacting serpentinite with synthetic haplobasaltic glass (corresponding to anorthite-diopside eutectic at 0.1 MPa pressure) at 900 and 1250 °C, and 0.2 GPa pressure for 120 h. At 900 °C, no reaction is observed in the sample; in contrast, in the experimental runs heated to 1250 °C, we distinguish two mineral associations (1) early Al-poor diopside [Mg# = Mg/(Mg + Fe) = 99 ± 1; Al2O3 = 1.9 ± 1.6 wt%] with the diopside-hosted forsterite (Fo99.2 ± 0.1) inclusions and (2) late Al-enriched diopside (Mg# = 98 ± 1; Al2O3 = 3.7 ± 2.8 wt%). Our experiments confirm that mantle diopsidites can be produced at ≥ 1100 °C in response to partial melting of hydrated peridotite (serpentinite) in the presence of haplobasaltic melt and aqueous fluid at the conditions typical of the mantle-crust transition zone and the shallow mantle beneath oceanic spreading ridges.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Composition gradients in silicate inclusions in chromites from the dunitic mantle-crust transition (Oman ophiolite) reveal high temperature fluid- melt-rock interaction controlled by faulting

The transition between the mantle section and the oceanic crust in the Maqsad area (Oman ophiolite) is mainly made of variably impregnated dunites locally associated with chromitite ore bodies. There, the dunitic transition zone (DTZ) developed above a mantle diapir that fed with MORB the former oceanic spreading centre. However, orthopyroxene and amphibole impregnations in dunites from the DTZ are witnesses of a hydrated magmatism that looks restricted to this interface. The main other piece of evidence is the nature of silicate minerals included in chromite grains scattered in dunite (e.g., amphibole, orthopyroxene, mica), which are mostly issued from a hydrated and silica-rich melt or fluid. Here, we report on a study of such inclusions along a section sampled in detail in the Maqsad DTZ. It brings critical information on the processes involved in the fluid-melt-peridotite reaction below oceanic spreading centres, complementary to the one provided by the interstitial silicates forming the matrix of the dunite. We first show that both the nature and the composition of the inclusions are well-correlated to those of the impregnations in the host dunites, then that the chemical evolution along the cross-section for all materials correlate to the presence of faults that developed at an early, syn-magmatic stage. This confirms that the early tectonics in the deep oceanic lithosphere primarily controls the fluid-melt-rock reactions and can condition chemical cycling, including for halogens (Cl, F), in oceanic spreading centre setting

Formation process of dunites and chromitites in Orhaneli and Harmancık ophiolites (NW Turkey): Evidence from in-situ Li isotopes and trace elements in olivine

Trace elements and Li isotopic compositions of olivine from the mantle-crust transition zone of the Bursa ophiolites (including Orhaneli ophiolite and Harmancık ophiolite) in NW Turkey were measured to constrain the genesis of these dunites and chromitites. A cumulate origin for dunite can be ruled out due to the depletion of incompatible trace elements (Zr, Ti, and heavy rare earth elements) in olivine, instead the chemical signatures point to a replacive origin via melt-rock interaction. The olivine grains in the dunites have lower MnO (0.06–0.15 wt%), Co (106–137 ppm), and higher NiO (0.23–0.44 wt%) concentrations than olivine phenocrysts in MORB, suggesting these transition-zone dunites have equilibrated with extremely depleted melts. Additionally, the relatively small δ7Li variations of olivine (average δ7Li +4.8 to +8.7‰) of the Orhaneli suite indicate the Li isotopic compositions of melts percolating through these dunites are relatively homogeneous. However, the large δ7Li variations of olivine (−2.5 to +20.3‰) in Harmancık dunites can be explained by incomplete diffusive equilibration with melts percolating through these dunites, suggesting infiltration happened not long before obduction of the ophiolite. Olivine in chromitites has higher Fo (92.6–94.7) than coexisting dunites, likely induced by subsolidus Mg-Fe exchange between olivine and chromite. The higher chromite contents of the chromitites can also explain the lower concentrations of Sc, V, Co and Zn in coexisting olivine grains. Mixing of depleted mantle-derived melts and boninitic magmas is suggested to induce a compositional shift from the olivine-chromite cotectic line to the liquidus field of chromite, causing the precipitation of chromite and formation of chromitite layers in the dunites. The heavy Li isotopic compositions (+6 to +11‰) of olivine in chromitites compared to MORB, together with the estimated compositions of parental magmas (Al2O3: 9.8–11.4 wt%; TiO2: 0.22–0.38 wt%) for the chromitites, indicate an arc-like geochemical affinity, hence a subduction-related setting in which these mantle-crust transition zones formed

Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth’s mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth’s surface.

Precious metals in magmatic Fe-Ni-Cu sulfides from the Potosí chromitite deposit, eastern Cuba

The Moa-Baracoa ophiolite in eastern Cuba is one of the few known ophiolites that display sulfide mineralization attributable to a magmatic origin in association with podiform-chromite ores hosted in the mantle-crust transition. These sulfide ores chiefly consist of Fe-Ni-Cu sulfides, namely pyrrhotite, pentlandite, chalcopyrite and cubanite partly altered to valleriite. The sulfide mineralization is located along the contact between the podiform-like chromite ores and intruding pegmatitic gabroic dykes. The detailed mineralogical study of the sulfide mineralization coupled with the first ever laser ablation ICP-MS analysis reveals that this sulfide mineralization show contents of the precious metals (Os, Ir, Ru, Pt, Re, Au, Ag) and other (semi)-metals (Co, Ni, Cu, Se, Te, Bi, Pb, As Sb) comparable to those sulfides from the magmatic sulfide deposits associated with mafic complexes hosted in the continental crust. The results obtained from this study confirm that Fe-Ni-Cu sulfides at Potosí are magmatic in origin, and very likely derived from the solidification of droplets of sulfide melt segregated by immiscibility from the intruding mafic melts once they interacted with the pre-existing chromitite at the mantle-crust transition zone of the ophiolite. The immiscibility of sulfide melt was achieved as a result of a progressive increase of fS2, very likely triggered by a set of circumstances, including the progressive fractionation of the intruding mafic melt leading to increase of aSiO2 and accumulation of volatiles as well as the crystallization of oxides. Two main generations of pentlandite were observed. One generation is primary in origin and it was locally exsolved along with pyrrhotite from monosulfide solid solution (MSS) during low-temperature cooling. The second type of pentlandite resulted from the reaction of MSS with coexisting droplets of Cu-and Ni-rich sulfide melt. LA-ICP-MS analysis reveals that most precious metals (Ru, Os, Ir, Re, Au, Ag) were concentrated along with the base-metal sulfides (BMS), although their distribution among the different BMS (pyrrhotite, pentlandite, chalcopyrite and cubanite) does not strictly follow the expected distribution according to the known melt-solid and solid-solid partition coefficients. Unlike the other analyzed PGEs, Pt was not preferentially concentrated in BMS but as discrete micrometer-sized sperrylite grains. The crystallization of sperrylite took place before and contemporaneous to sulfide segregation, and Pt-As nanoparticles probably played an important role in the Pt uptake as nucleation seeds for the formation of micron-sized sperrylite grains. These observations highlight the open-system nature of the ore forming system as well as the important role of arsenic in concentrating PGE in high-temperature silicate and sulfide melts.

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

A stratiform chromite ore body crops out in the lower part of the dunitic mantle-crust transition zone (DTZ) that developed at the top of a mantle diapir in the Maqsad area in the Oman ophiolite. It is made of layers ranging in thickness from a few mm to a maximum of 3 m, and in modal composition from massive to antinodular and disseminated ore. The ore body is about 50 m thick and its lateral extent does not exceed several hundred meters. The layering dips gently to the southeast, parallel to that of the overlying gabbroic cumulates. The chromite composition is typical of a MORB kindred - moderate XCr (100 × Cr/(Cr + Al) atomic ratio), ranging from 48 to 60, and relatively high TiO2 content, ranging from about 0.3 to 0.5 wt% -, a characteristic shared by most lithologies issued from the igneous activity of the Maqsad diapir. The silicate matrix is essentially made of slightly serpentinized olivine with minor clinopyroxene and rare pargasitic amphibole, orthopyroxene and garnet. This strongly contrasts with the nature of the mineral inclusions mostly made of the assemblage amphibole-orthopyroxene-mica, enclosed in the chromite grains and represented in abundance all along the ore body whatever the ore grade. The inclusions demonstrate the involvement of a silica- and water-rich melt and/or fluid, in addition to MORB, in the early stages of chromite crystallization. The chemical composition of chromite, silicate matrix, together with the one of silicate inclusions display well-defined evolutions vertically along the stratiform chromitite. At the scale of the ore body, the compositional trends are independent of the ore concentration but the major kinks in these trends are well-correlated with levels of magmatic breccias. This shows that abrupt chemical changes can be attributed to sudden melt ± fluids injection events followed mainly by melt-fluid-rock interaction and in a lesser extent by quieter evolution by fractional crystallization. At the thin section scale, second order chemical variations, essentially in the Mg# (100 × Mg/(Mg + Fe2+) atomic ratio) of chromite and Fo of olivine, are clearly attributable to re-equilibration between these two solid phases, possibly in the presence of an interstitial melt/fluid.

Eruptive Activity on the Western Flank of Piton de la Fournaise (La Réunion Island, Indian Ocean): Insights on Magma Transfer, Storage and Evolution at an Oceanic Volcanic Island

AbstractPetrological and geochemical (major element, trace element, Sr–Nd isotope) data for recent (<5 kyr old) basalts that sporadically erupt on the western flank of Piton de la Fournaise (PdF), one of the most active volcanoes on Earth, allow the tracking of magma transfer and evolution from mantle to crustal depths. In the western peripheral area of PdF we document the broadly synchronous eruptions of (1) primitive olivine and olivine–clinopyroxene transitional basalts with tholeiitic affinity that are closely associated in space with (2) transitional olivine basalts with alkaline affinity, and (3) hybrid lavas, intermediate between the ‘alkaline’ and the ‘tholeiitic’ end-members. The composition of the latter overlaps with that of the lavas frequently erupted from the conduit system feeding the main summit cone. AlphaMELTS modelling, and fluid inclusion and clinopyroxene barometry, constrain the conditions of magma storage at 10–30 km, and the ascent of magma from the upper mantle to the shallow crustal plumbing system. Variable degrees of mantle melting, together with minor source heterogeneity and contamination with cumulate-derived partial melts, contribute to the diversity of PdF magmas. However, all these processes do not represent the dominant factors that produce the large variability we found in major element composition. Indeed, the composition of basalts erupted from PdF peripheral centers is strongly controlled by polybaric olivine–clinopyroxene fractionation at pressures higher than 3 kbar. Crystal textures and geochemical modelling suggest that fast magma ascent is critical to prevent clinopyroxene dissolution. Conversely, long-lasting magma stagnation promotes pyroxene resorption and magma differentiation. ‘Central’ eruptions occurring close to the PdF summit cone emit variably more evolved melts, which result from olivine–clinopyroxene–plagioclase differentiation at intermediate–shallow pressure (<3 kbar and in most cases <1 kbar). Deep and extensive magma mixing before injection into the crustal magma conduit system, located below the summit region, results in the apparent homogeneity of basalts erupted from the central area. As regards ‘peripheral’ eruptions, deep-seated stagnation of basaltic melts and differentiation at the mantle–crust transition zone (c. 4 kbar) produces a range of magma compositions. We demonstrate that rapid magma ascent from deep-seated reservoirs can bypass the central plumbing system. The eruptions of these magmas both in the central area and on the densely populated flanks have major consequences in terms of volcanic hazard at PdF.

High-temperature chromium isotope fractionation and its implications: Constraints from the Kızıldağ ophiolite, SE Turkey

Chromium isotope data were obtained from olivine, orthopyroxene and chromite separates of the Kızıldağ ophiolite, SE Turkey, to investigate the effects of high temperature magma processes on Cr isotope fractionation. Harzburgite in the Kızıldağ ophiolite has the δ53Cr values of −0.14 to −0.12‰ in chromite, and −0.08 to −0.01‰ in bulk rocks. These Cr isotope fractionations could be driven by partial melting and metasomatism. The dunite and chromitite samples from the mantle-crust transition zone of the Kızıldağ ophiolite are cumulates, and their chromite and olivine have δ53Cr values of −0.29 to −0.06‰ and −0.11 to 0.41‰, respectively. The δ53Cr values of chromite are negatively correlated with the chemical indices of fractional crystallization (e.g., Fe# of chromite), suggesting that Cr isotopes were fractionated during fractional crystallization. In the chromitite, the degree of Cr isotope fractionation increases with fractional crystallization, with 0.23‰ Cr isotope fractionation occurring at mineral scale. During fractional crystallization, solid phases preferentially incorporate Cr, particularly heavy Cr isotopes, driving the depletion of Cr and enrichment of light Cr isotopes in the evolved melts. The δ53Cr values of olivine are higher than the coexisting chromite, which could be explained by the fact that the olivine grains were probably formed earlier than the chromite. The Cr isotopic features of chromite in the podiform chromitite (−0.22 to −0.04‰) from the mantle sequence are consistent with the modelled fractional crystallization trend, confirming the magmatic origin of the podiform chromitite. Therefore, a significant Cr isotope fractionation at chromite from the Kızıldağ ophiolite could be induced by the high-temperature fractional crystallization.

Initial subduction of Neo-Tethyan ocean: Geochemical records in chromite and mineral inclusions in the Pozantı-Karsantı ophiolite, southern Turkey

Chromitites in the Pozantı-Karsantı ophiolite in Turkey mainly occur as podiform chromitites within mantle harzburgite and stratiform-like chromitites in mantle-crust transition zone. Chromites in chromitites have varing Cr# from 62.8 to 80.3 and can be divided into two types, namely; intermediate (Cr#: 62.8 – 69.2) and high-Cr (Cr#: 73.9 – 80.3) types. Major elements of the high-Cr chromitite have an affinity with boninite, whereas the intermediate chromitite shows transitional features between MORB and boninite. The compositional differences in clinopyroxene inclusions between intermediate- and high-Cr chromitite, coupled with the relatively high trace element contents (e.g. V, Ga) in the high-Cr chromitite, indicate distinctive parental magmas. Trace elemental profile analysis of a nodular chromite grain in one nodular chromitite sample PK14-41 demonstrates significant but non-systematic variations from the core to the rim, which also confirmed the compositional heterogeneity of the parental magmas. The presence of primary hydrous mineral inclusions such as amphibole in chromite, together with Ca-rich minerals (e.g. calcite), reflect the water-rich and Ca-rich characteristics of the parental magma. The higher fO2 of high-Cr chromitite evidenced by the lower V/Mn values may be due to more oxidized fluids released from downgoing crustal materials. Thus, we conclude that the parental magmas of the Pozantı-Karsantı chromitite were derived from a proto-forearc mantle and evolved to higher fO2 with the subduction initiation at that time, but were water- and Ca-rich in general.

Melt hybridization and metasomatism triggered by syn-magmatic faults within the Oman ophiolite: A clue to understand the genesis of the dunitic mantle-crust transition zone

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.

Melt-dunite interactions at 0.5 and 0.7 GPa: experimental constraints on the origin of olivine-rich troctolites

Olivine-rich troctolites and dunites are diffuse lithologies at crust-mantle transition of the oceanic lithosphere. Disequilibrium textures coupled to mineral compositions indicate that melt-rock reactions play an important role in their origin. The prevailing view is that olivine-rich troctolites are related to extensive melt impregnation of a precursor mantle dunite.In order to provide experimental constraints, we performed reactive dissolution and crystallization experiments by juxtaposing three variably evolved MORB-type glasses with a melt-bearing dunite at 1300 °C and then cooling to 1150 °C at constant pressure (0.5 and 0.7 GPa). Additionally, an isothermal experiment (0.7 GPa, 1250 °C) provides a snapshot of olivine-melt reaction after the high-temperature step.Runs result in glass-bearing gabbro overlain by olivine-rich troctolite (at 0.5 GPa) or dunite (at 0.7 GPa) showing disequilibrium textures comparable with natural occurrences typically related to melt-rock reaction, e.g. embayed and resorbed subhedral olivine with lobate contacts against plagioclase and clinopyroxene, often occurring as large poikiloblasts including rounded olivines. Modal abundance of interstitial phases and mineral chemistry are strongly controlled by the extent of reacting melt infiltrated into the dunite matrix, i.e. the melt/olivine ratio. We found that higher pressure further limits olivine dissolution and results in a lower high-T porosity, decreasing the final abundance of interstitial phases.Olivine composition is mostly buffered by the starting San Carlos olivine, resulting in high XMg (0.88–0.90). NiO content decreases at increasing melt/olivine ratio, matching the composition of olivine in natural samples. Remarkably, at very low melt/olivine ratio, NiO can exceed the starting San Carlos value as a result of the increase of Ni olivine-melt partition coefficient during cooling after olivine assimilation in the reacted melt. This might potentially produce high-Ni, at high XMg, magmatic olivine.Melt composition affects the chemistry of interstitial minerals that show large compositional variability (anorthite in plagioclase, TiO2 in clinopyroxene) as a result of local equilibrium driven by trapped melt. Remarkably, mineral co-variation trends (e.g. anorthite in plagioclase vs. olivine XMg) match those of some natural olivine-rich troctolites occurrences.Experimental results corroborate and constrain the lead role of melt-rock reactions in the origin of olivine-rich rocks at mantle crust transition in the oceanic lithosphere.

Mineralogy and geochemistry of peridotites and chromitites in the Aladag Ophiolite (southern Turkey): melt evolution of the Cretaceous Neotethyan mantle

The Aladag Ophiolite (also known as the Pozanti-Karsanti Ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel harzburgites, dunites and chromitites. The harzburgites display a depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides, which crystallized from S-saturated melts during melt–rock interactions. Their spoon-shaped rare earth element patterns indicate metasomatism by fluids, derived from a subducting slab, which were enriched in light rare earth elements. The chromitites in both the mantle and mantle–crust transition zone rock units show similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized platinum group element patterns characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. The parental melts of the Aladag Ophiolite chromitites were S-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircon and needle-shaped clinopyroxene exsolutions in chromite (chr) grains suggests that their formation might have originally involved high-temperature and high-pressure conditions within the mantle transition zone before their participation in further melting and the formation of oceanic crust at shallow depths in a forearc tectonic setting during the initiation of intra-oceanic subduction within the Inner Tauride seaway. Supplementary material: Sampling and analytical methods, representative microprobe analyses and whole-rock major and trace element compositions are available at https://doi.org/10.6084/m9.figshare.c.4153631 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Early-middle Triassic basic magmatism and metamorphism of ultramafic-mafic complexes of the Ust’-Belaya terrane (central Chukotka, NE Russia): 40Ar/39Ar ages, petrological and geochemical data, geodynamic interpretations

ABSTRACTThe Ust’-Belaya terrane (Chukotka, NE Russia), belonging to the West Koryak fold belt, is made of mantle and crustal ultramafic-mafic complexes which originated during several discrete episodes of magmatic activity in the Neoproterozoic, the late Neoproterozoic-Cambrian and the early-middle Triassic and accreted to the Asian continental margin in the early Cretaceous. This paper focuses on the latest magmatic episode expressed in intrusion of microgabbro dikes and nearly coeval metamorphism superimposed on both ancient complexes and the dikes. We present original 40Ar/39Ar ages, mineral and bulk-rock chemistry of the microgabbro dikes, metamorphosed dike and vein cutting ultramafic-mafic complexes. Dike microgabbros resemble differentiated arc-tholeiitic magmas originated in a subduction setting. Differentiated magmas intruded into much older spinel peridotites and dunites located at mid-crustal levels. Intrusion and crystallization of these magmas was followed by down-going movement of spinel peridotites and rocks of the mantle-crust transition zone towards a mantle wedge where they were metamorphosed at high-P conditions. This metamorphism resulted in transformation of microgabbro to garnet amphibolite, diorite to albite-zoisite-paragonite-pargasite rock and spinel peridotites to metaperidotites. P-T parameters of this metamorphism reconstructed based on mineral assemblage of garnet amphibolite correspond to those of epidote amphibolite – amphibolite – amphibole eclogite facies transition. The peculiar zoisite (clinozoisite)-paragonite mineral assemblage typical of metamorphosed vein rock indicates high-P metamorphic conditions of epidote-amphibolite facies.

Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

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.

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

Determining which petrological processes build the mantle-crust dunitic transition zone (DTZ) in oceanic spreading settings has a direct impact on our understanding of thermal and chemical transfers on Earth. We report on understated but widespread mineral assemblages present in the DTZ at the top of a mantle diapir (Oman ophiolite), including pargasite, grossular, and pyroxenes of peculiar composition. These minerals are present interstitially between olivines and as inclusions in the disseminated chromite grains, indicating that they are early, high-temperature features. They call for hybridization between the mid-oceanic ridge basalt melts that fed the crustal section and supercritical water saturated with silica. Our synoptic survey (∼300 samples collected along 11 cross sections) demonstrates that the DTZ was pervasively infiltrated by such hybrid melts and that the abundance of their crystallization products increases upsection, likely in response to increasing supply of water and decreasing temperature. This indicates that water is involved in the reaction leading to the transformation of mantle harzburgite into dunite in the DTZ. On the basis of field evidence, a hydrothermal origin of the water is a reasonable hypothesis.

Cogenetic origin of mafic microgranular enclaves in calc-alkaline granitoids: The Permian plutons in the northern North China Block

Mafic microgranular (microgranitoid) enclaves (MMEs) with fine-to medium-grain–size igneous microstructures are abundant in Permian granitoid plutons in the northern North China Block (NCB). They are mainly dioritic in composition with SiO 2 contents from 51.0 wt% to 62.7 wt% and are contiguous with their host granitoids (SiO 2 = 60.4–68.4 wt%). Their main mineral assemblage of plagioclase (oligoclase and andesine), hornblende, biotite, K-feldspar, and quartz is similar to that of their host granitoids but with different mineral proportions. Zircon laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb dating, hornblende-plagioclase thermobarometry, and Ti-in-zircon thermometry results show that the crystallization ages and pressure-temperature ( P-T ) conditions of the MMEs in each Permian granitoid pluton are very similar to those of their host granitoids, indicating simultaneous crystallization at the middle to upper crustal levels. Petrographical, geochemical, and Sr-Nd-Hf isotopic data show that the source areas of the Permian granitoid plutons in the northern NCB are temporally and spatially different and magma mixing was likely involved during formation of some granitoid plutons. However, the whole-rock Sr-Nd and zircon Hf isotopic compositions of the MMEs in each pluton are very similar to those of their host granitoids, indicating complete isotopic equilibration between the host granitoids and their enclaves. Combined with similar mineral assemblage and mineral chemistry between the MMEs and their host granitoids and absence of coeval mantle-derived rocks in or nearby the plutons, we confirm that the MMEs were crystallized from a coeval magma that gave rise to the host granitoids, and not by mixing and/or mingling between mantle-derived mafic and crustal-derived felsic magmas or by synplutonic injections of mafic magma into the host granitoid magma. We conclude that magma isotopic equilibration between host granitoids and their enclaves was achieved prior to emplacement and the magma mixing process during formation of some Permian granitoid plutons occurred mainly in the melting, assimilation, storage, and homogenization (MASH) zone in the lowermost crust or mantle-crust transition, not in the magma conduits or chambers at middle to upper crustal levels. A two-stage model that includes rapid cooling within the cogenetic host granitoid magma operating at pluton margins to form solid to sub-solid dioritic rocks and crystal accumulations of the host granitoid magma at the bottom of the pluton to form cumulates, and dioritic rocks and cumulates then fragmentated and interacted with progressive evolved host granitoid magma to change their shapes and orientations is proposed for origin of the MMEs in the Permian granitoid plutons in the northern NCB. This model may be more widely applicable to the generation of many MMEs in granitic rocks, especially where no direct evidence exists for the presence of mafic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves. Our new results on origin of the enclaves in the granitoid plutons in the northern NCB show that some MMEs previously used as evidence for magma mixing and/or mingling are “autoliths (cognate xenoliths)” crystallized from the same magma as their host granitoids. There is an essential need for caution in using the MMEs as evidence for crustal-mantle interaction or mixing and/or mingling between crustal-derived felsic and mantle-derived mafic magmas during formation of granitic rocks, especially where no independent evidence is found for such a process. The role of MMEs in evaluating the mixing and/or mingling processes between crustal-derived felsic and mantle-derived mafic magmas has been much more likely overestimated by some previous studies and should be reconsidered. Instead, the MMEs in this origin can provide valuable information on emplacement mechanism and incremental growth of granitoid plutons in continental crust.