Mantle Units Research Articles

Assessment of heavy metal pollution in soil-parent material relationship across ecosystems.

The works of assessing the pollution posed by metals in agricultural areas in developing countries are limited. This study aims to assess metal concentrations and pollution indices of parent materials and soils representing the mantle and oceanic crust units of the ophiolite in the Eastern Mediterranean region, specifically in Kahramanmaraş Province. A total of 88 samples, comprising 44 soil (0-30cm) and 44 parent material (90 + cm), were collected from the study area. Arsenic (As), mercury (Hg), selenium (Se), uranium (U), molybdenum (Mo), tin (Sn), and cesium (Cs) concentrations were analyzed in these samples, along with the reference metal, iron (Fe). Pollution levels were assessed using enrichment factor (EF) and contamination factor (CF) calculations. Results showed that elemental concentrations (Hg, Se, U, Mo, Sn, and Cs) in soils from the mantle and oceanic crust generally reflected those of the parent material. However, the average As concentration in soils from oceanic crust and mantle units was notably elevated, showing a 3 to fourfold increase compared to the parent material. Based on pollution index values, soils from these units demonstrated a moderate level of enrichment (2 < EF < 5) for As, while other elements (Hg, Se, U, Mo, Sn, and Cs) fell into the low enrichment class (EF < 2). Furthermore, the CF index indicated significant contamination (3 < CF < 6) for As. These findings suggest As contamination in soils from different units of the ophiolite (mantle and oceanic crust), potentially resulting from agricultural chemicals like pesticides and fertilizers.

Ice Content of Mantling Materials in Deuteronilus Mensae, Mars

AbstractLayers of ice and dust (i.e., “mantle”) are found throughout the mid‐latitudes of Mars, where they drape and mute topography and record the recent climate history. Many of the youngest Late Amazonian “latitude‐dependent mantles” have been characterized, but the full range of mantle characteristics, including variations in ice content and age, has not been well documented. To advance our understanding of these ice‐dust units, we use SHAllow RADar (SHARAD) radar sounding data to constrain the subsurface physical properties of a widespread Middle Amazonian mantle unit within Deuteronilus Mensae. This region hosts a high density of glacial landforms, which allows assessment of the stratigraphic relationship between mantle deposits and glacial ice. SHARAD reflectors at the base of the mantle units are used to determine moderate dielectric constants (∼5.75) and high loss tangents (∼0.038). These radar properties imply relatively limited ice content and high‐loss mantling materials with possible enhanced roughness or subsurface scattering. Further, we interpret deeper reflectors that are continuous with glacier basal reflectors and that suggest materials with lower dielectric constants (∼3.2–4.5) and loss tangents (0.0071–0.0120) to be due to extensions of stagnant, possibly lithic‐rich, glacier ice that are buried by a mantle layer. This study demonstrates the high variability in the origins and evolution of mid‐latitude mantling units on Mars. While the most recent latitude‐dependent mantling deposits may still be ice‐rich, older deposits, such as those preserved in Deuteronilus Mensae and elsewhere in the mid‐latitudes of Mars, may be largely desiccated or contain a more heterogeneous and limited distribution of ice.

Platinum Group Elements (PGE) Geochemistry and Mineralogy of Low Economic Potential (Rh-Pt-Pd)-Rich Chromitites from Ophiolite Complexes

This contribution provides an overview of platinum group elements (PGE) distribution and mineralogy in ophiolitic chromitites, which are unusually enriched in the low melting-point Rh, Pt and Pd (PPGE) compared with most chromite deposits associated with ophiolites, which are dominated by the refractory Os, Ir and Ru (IPGE). The PPGE-rich chromitites examined in this paper have a PPGE/IPGE ratio equal to or higher than 1 and represent about 7% of the ophiolitic chromitite population. These chromitites occur in the mantle unit, in the mantle-transition zone (MTZ), as well as in the supra-Moho cumulate sequence of ophiolite complexes. The age of their host ophiolites varies from Proterozoic to Eocene and, based on their composition, the chromitites can be classified into Cr-rich and Al-rich categories. Mineralogical assemblages observed in this investigation suggest that the PPGE enrichment was achieved in the magmatic stage thanks to the formation of an immiscible sulfide liquid segregating during or immediately after chromite precipitation. The sulfide liquid collected the available chalcophile PPGE that precipitated as specific phases together with Ni-Cu-Fe sulfides in the host chromitite and the silicate matrix. After their magmatic precipitation, the PPGM and associated sulfides were altered during low-temperature serpentinization and hydrothermal processes. Therefore, the original high-temperature assemblage underwent desulfurization, generating awaruite and alloys characterized by variable Pt-Pd-Rh-Cu-Ni-Fe assemblages. The occurrence of secondary PPGM containing Sb, As, Bi, Te, Sn, Hg, Pb and Au suggests that these elements might have been originally present in the differentiating magmatic sulfide liquid or, alternatively, they were introduced by an external source transported by hydrothermal and hydrous fluids during the low-temperature evolution of the host ophiolite. Although the PGE content may be as high as 81,867 ppb, as was found in one sample from Shetland chromite deposits, the ophiolitic chromitites are not presently considered as a potential resource because of the following circumstances: (1) enrichment of PPGE in podiform chromitites is a local event that occurs randomly in ophiolite sequences, (2) ore deposits are small and characterized by uneven distribution and high discontinuity, (3) physical characters of the mineralization only allow poor recovery of the precious metals mainly due to the minute grain size, and (4) for these reasons, the PPGE reserves in ophiolitic chromitites cannot compete, at the moment, with those in chromite deposits of the Bushveld type that will supply world demands for centuries using current mining techniques.

Archean versus Phanerozoic oceanic crust formation and tectonics: Ophiolites through time

• Global compilation of the salient features of ophiolites. • Lithological differences between Archean and Proterozoic/Phanerozoic ophiolites. • Subduction-dominant Archean oceanic crust with accretionary cycle plate tectonics. • For Proterozoic and Phanerozoic a combination of both accretionary cycle and Wilson cycles. A global compilation of structural, lithological, and geochemical data on a selection of Archean, Proterozoic and Phanerozoic magmatic complexes, interpreted as ophiolites, is presented. Ophiolites, based on Phanerozoic examples, can be classified into subduction-related and subduction unrelated categories. These categories can be further subdivided into several subtypes depending on the proximity to subduction zones, and on sequential development from rifting – drifting to seafloor spreading for the subduction-unrelated category. From bottom to top ophiolites exhibit a magmatic sequence of ultramafic rocks (upper mantle units), gabbros (layered and isotropic), basaltic dikes and lavas, as well as boninites and felsic dikes and lavas in the subduction-related types. Archean greenstone belts show large variations in their construction, but Eoarchean examples display identical structure and lithology to the Phanerozoic ophiolites, attesting to the operation of seafloor spreading and subduction zone processes in the early Earth's history. Lithological differences between the Archean and Proterozoic/Phanerozoic ophiolites are demonstrated in the common occurrence of komatiites and felsic rocks, and scarcity of sheeted dike complexes in the former, compared to the opposite situation in those ophiolites that are younger than ca. 2 Ga. Geochemically there is a concomitant decrease in the content of incompatible elements (e.g., Sr, Zr, Y, Nb) and an increase in the content of compatible elements (e.g., Mg, Cr, Ni). In terms of tectonic environment analyzes, the Archean ophiolites are more abundantly subduction-related than those of Proterozoic and Phanerozoic ages. The subduction-dominant Archean oceanic crust (ophiolites) was characterized by accretionary cycle plate tectonics, whereas for those of Proterozoic and Phanerozoic ages, a combination of both accretionary cycle and Wilson cycles plate tectonic processes was operative.

PLATINUM METAL MINERALIZATION OF THE SOUTH URALS MAGMATIC COMPLEXES: GEOLOGICAL AND GEODYNAMIC CHARACTERISTICS OF FORMATIONS, PROBLEMS OF THEIR GENESIS, AND PROSPECTS

In the South Urals, we have identified and investigated two platinum-bearing formations – ophiolite chromitebearing complexes, and the Khudolaz differentiated mafic-ultramafic complex with sulfide Cu-Ni mineralization. The ophiolite chromite-bearing complexes include fragments of the upper mantle and lower crust of the Paleouralian Ocean, which were induced by collision onto the edge of the East European platform. The origin of the Khudolaz complex is related a mantle plume activity. Here, we review and compare the main features of platinum-metal mineralization (PMM) in these two formations.The article presents the results of mineralogical and geochemical studies of PMM associated with chromite and sulfide Cu-Ni ores. In association with chromitites, two types of PMM are distinguished: (1) predominating refractory platinoids in chromitites of the mantle unit of the section, and (2) predominating platinum and palladium in chromitites of the transitional wehrlite-clinopyroxenite complex. Compositions of platinum group minerals (PGM) and relations between their elements and host minerals suggest that the minerals of the ophiolite chromite-bearing complexes are of a restite origin, while the Khudolaz complex results from a combination of magmatic processes and solid-phase redistribution of material. Palladium (michenerite, froodite, merenskyite, borovskite, sudburyite) and platinum (sperrylite, moncheite) minerals are found in magmatic sulfide ores of the Khudolaz complex, which were subjected to hydrothermal metasomatization. Texture observations using electron microscope and optical (reflected light) images, as well as LA ICP MS analyses of sulfides suggest late- and post-magmatic crystallization of PMM in three phases: (1) immiscible metalloid or highly fractionated residual sulfide melts trapped in sulfides; (2) segregation of isomorphic impurities of platinum group elements (PGE) and chalcogenide elements from sulfide solid solutions; and (3) interaction of hydrothermal fluids with soluble sulfides.Prospective for PMM are extended bodies of disseminated chromitites in marginal dunites of the Kraka and Nurali massifs, and wehrlite-clinopyroxenite complexes of the same massifs containing PGE (above 500 ppb). In the Khudolaz complex, promising PMM bodies are low-metasomatized parts of sulfide ore bodies (1 ppm of ΣPGE and above) located in the largest massifs, Severny Buskun and Zapadny Karasaz. Exocontact zones of these intrusions are also promising for PMM.

Focal Mechanisms of Intraslab Earthquakes: Insights From Pseudotachylytes in Mantle Units

AbstractDevastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear‐heating event. The well‐preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal ferromagnetic phase in these rocks is a Ti‐poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10−6 [SI] volume, with P′ ranging from 1.05 to 2.5) to reconstruct the co‐seismic deformation parameters, that is, fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included mostly normal mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite‐hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution.

Slab-controlled progressive evolution of the Kudi back-arc ophiolite in response to the rollback of the Proto-Tethys oceanic slab, in Western Kunlun, NW Tibetan Plateau

Slab-derived components can significantly diversify subduction-related magmatism, for example, fore-arc ophiolites evolving from normal mid-ocean ridge basalts to fore-arc basalts and then to boninites. Here we show a progressive evolution of back-arc ophiolites controlled by slab-derived components but different from fore-arc ophiolites through studying the Kudi back-arc ophiolite in Western Kunlun, NW Tibetan Plateau. Our results reveal that the Kudi ophiolite was formed at 516–512 Ma in a back-arc setting in response to the rollback of the Proto-Tethys oceanic slab. The back-arc setting is supported by geochemistry of basalts (e.g., low Th/Nb and V/Ti ratios), low melting pressure (<10 kbar) revealed by amphibole geobarometer and low δ11B values (−19.1 to −8.1‰) of the mantle units. In such a scenario, the early-stage depleted Kudi basalts are characterized by depletions in light rare earth elements and enrichments in fluid-mobile elements, probably resulting from decompression melting of the primary back-arc mantle slightly modified by slab-derived fluids. Subsequently, partial melting of the back-arc mantle enriched by progressive addition of slab-derived sediments resulted in the formation of residual harzburgites and corresponding primary magmas that evolved into dunites, gabbros, quartz diorites and enriched pillow lavas. Additionally, both the residual harzburgites and cumulated dunites experienced minor impregnation of melts, without significant interstitial melt-crystal reactions. Compared with the early-stage depleted Kudi basalts, the late-stage enriched pillow lavas are characterized by enrichments in light rare earth elements and depletions in high field strength elements, typical arc affinities, confirming the addition of slab-derived sediments. Compared with typical fore-arc ophiolitic series, it is possible that there is a progressive evolution in back-arc basins in which a “new” enriched mantle source (corresponding arc basalts) modified by slab-derived sediments displaces an “old” depleted mantle source (corresponding basalts characterized by enrichments in fluid-mobile elements) modified by slab-derived fluids.

RomUkrSeis: Seismic model of the crust and upper mantle across the Eastern Carpathians – From the Apuseni Mountains to the Ukrainian Shield

RomUkrSeis is a controlled source wide-angle reflection and refraction (WARR) profile acquired in August 2014. It is 675 km long, running roughly SW-NE from the Apuseni Mountains in Romania and the Transylvanian Basin, crossing the arc of the Eastern Carpathian orogen and terminating in the East European Craton (EEC) in SW Ukraine. Well-constrained 2-D ray-tracing P- and partly S-wave velocity models have been constructed along the profile from 348 single-component seismic recorders and eleven shot points. The Eastern Carpathian arc formed in the Cenozoic and have obscured the pre-existing Teisseyre-Tornquist Zone (TTZ), which is a transition zone between the Precambrian EEC and continental terranes accreted to it from the southwest in the Palaeozoic. The TTZ is characterised by low-velocity through its entire crust (6.0–6.3 km/s) and a considerable width (~140 km). It is interpreted as EEC crust stretched during rifting and continental margin formation in the Neoproterozoic and early Palaeozoic. The crust of the TTZ has a “trough in trough” structure wherein an upper body of ~40 km width comprising Outer Carpathian (Vp 4.9 km/s) and Late Palaeozoic-Mesozoic (Vp 5.4 km/s) units to 15 km depth lies above a wider, deeper one of inferred Neoproterozoic-early Palaeozoic strata. The crust of the Transylvanian Basin and Apuseni Mountains is relatively thin (~32 km). A high-velocity body at 4–12 km depth in this area is interpreted as a rootless fragment of an ophiolite complex exposed at the surface in this area. The lower crust beneath the Transylvanian Basin displays higher velocities than adjacent segments. Moho topography is strongly differentiated along the profile, varying from 32 to 50 km. The Moho shape, especially in the area between the Inner and Outer Carpathians, suggests a NE dip and, hence, thrusting of the Tisza-Dacia lowermost crustal and upper mantle units under the TTZ domain which, in turn, could be thrust under the cratonic (EEC) block.

The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.

A three-dimensional lithospheric-scale thermal model of Germany

Abstract. We present a 3-D lithospheric-scale model covering the area of Germany that images the regional characteristics of the structural configuration and of the thermal field. The structural model resolves major sedimentary, crustal and lithospheric mantle units integrated from previous studies of the Central European Basin System, the Upper Rhine Graben and the Molasse Basin, together with published geological and geophysical data. A combined workflow consisting of 3-D structural, gravity and thermal modelling is applied to derive the 3-D thermal configuration. The modelled temperature distribution is highly variable in response to an imposed heterogeneous distribution of thermal properties assigned to the different units. First order variations in the temperature field are mainly attributed to the thermal blanketing effect from the sedimentary cover, the variability in the amount of radiogenic heat produced within the different crystalline crust compartments and the implemented topology of the thermal Lithosphere-Asthenosphere Boundary.

Petrogenetic evolution of Cryogenian Halaban ophiolite, Saudi Arabia: A fragment of fore-arc oceanic lithosphere mantle

In this study, Neoproterozoic Halaban ophiolite situated in the eastern part of the Arabian shield, Saudi Arabia, was investigated. The Halaban ophiolite represents a section of a dismembered ophiolite, tectonically emplaced over a volcano-sedimentary succession of island arc assemblages. It is variably, altered, deformed and metamorphosed to serpentinized rocks. It includes serpentinized peridotite and metagabbro with rare metabasalt, chromitite pods, listvenite and rodingite. The contact between the lower mantle unit (serpentinized peridotite) and the upper crustal unit (metagabbro) is originally magmatic, although it is disrupted by tectonism. Nevertheless, because of the high degree of alteration and metamorphism, few fresh relicts of primary minerals (chromian spinel, olivine and pyroxenes) were recorded. The mineral assemblages in the Halaban ophiolite are comparable to greenschist, to lower amphibolite facies metamorphism. The fresh olivine relics have high Fo and Ni content, while the fresh chromian spinel cores have high Cr# accompanied by a low TiO2. The preserved texture (mesh and bastite), fresh relict minerals, and the whole-rock compositions (Mg# = molar Mg/ (Mg + Fe2+) = 0.91–0.93 with low CaO and Al2O3 content) suggest that the protolith is mainly harzburgite with minor dunite. The chemical composition of Halaban metagabbro is indicative of a tholeiitic affinity. In addition, the very low modal abundance of clinopyroxene and the high Ni, Cr and Co content of the serpentinized peridotite, and high-Cr# (av. 0.66) with low-Ti (mostly ≤0.1 wt%) of chromian spinel, suggest significant partial melting. The serpentinized peridotite (harzburgite and dunite) undergone different degrees of partial melting, ranging from approximately 32% to 37% for harzburgite and approximately 37% to 42% for dunite. The variation of the degree of partial melting between harzburgite and dunite is indicative of mantle heterogeneity beneath the Arabian Shield. The general petrological characteristics of the Halaban ophiolite are substantially similar to those formed by melt-peridotites interaction and subsequent melt mixing in a fore arc setting within a supra-subduction zone system. Halaban chloritite is formed as a secondary product due to the metasomatic interaction of the ophiolitic ultramafic rocks. The content of Cr, Co and Ni of the chloritite sample was consistently close to that of the serpentinites.

Petrogenesis and Tectonic Significance of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, China

AbstractThe Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo‐Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ comprises mainly clinopyroxene (cpx)‐rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), yielding Re‐depletion model ages (TRD) ages from 1.8 to 1.7 Ga, indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8‐1.7Ga; whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes. Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting of a primitive mantle source, and they were subsequently modified by melt‐rock interactions in a mid‐oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx‐harzburgites, which later interacted with mid‐oceanic ridge basalt– or island‐arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo‐Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Probing supraglacial debris on Mars 2: Crater morphology

Lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF) on Mars, interpreted to be debris-covered glaciers, possess craters with a suite of distinct interior landforms (called “ring-mold craters”) that have been attributed to the presence of glacial ice at depth or surface modification processes. We tested current hypotheses for the formation of ring-mold craters by conducting a comprehensive analysis of the size and morphology of 16,457 impact craters ≥125 m in diameter formed within glacial deposits in Deuteronilus Mensae. Two major groups, bowl-shaped craters and ring-mold craters, are found, with at least nine distinct crater types. While there is statistical difference in median diameters between these crater types, this difference is relatively small and is within the estimated uncertainty in diameter measurements and may be related to more enhanced erosion of the rims of ring-mold craters. Clear degradation sequences are observed, supporting a role for post-impact modification in producing at least some of the diversity in crater landforms. The spatial density of ring-mold craters is also directly correlated with the development of LDA, LVF, and CCF surface textures. Flow lineations cross-cut two ring-mold crater types but they maintain their circular planforms in some cases, suggesting that the craters initially formed completely within mantling layers deposited after glacial flow had ceased. We also find analogous craters in non-glacial units; glacial ice is therefore not required to form the observed morphologic diversity. Our observations are most consistent with formation of crater landforms by emplacement and modification of at least two depositional episodes of icy dust (i.e., “mantle”). This mantle was initially tens of meters in thickness to support crater formation, and has experienced much downwasting and erosion since emplacement. Derived crater retention ages of 460 Ma for LDA, LVF, and CCF features in the region therefore reflect deposition of mantle units and only give a very minimum age for the formation of LDA, LVF, and CCF.

Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.

Petrological and Re-Os isotopic constraints on the origin and tectonic setting of the Cuobuzha peridotite, Yarlung Zangbo suture zone, southwest Tibet, China

The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the Yarlung Zangbo suture zone in southwest Tibet comprises mainly clinopyroxene (cpx)-rich and depleted harzburgites. Spinels in the cpx-harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx-harzburgites have suprachondritic 187 Os/ 188 Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx-harzburgites formed from low-degree partial melting of a primitive mantle source, and they were subsequently modified by melt-rock interactions in a mid-oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx-harzburgites, which later interacted with mid-oceanic ridge basalt– or island-arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo-Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Petrological and Re‐Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north (Dupuis et al., 2005; Yang et al., 2011). In the western part of the YZSZ, the Northern and the Southern sub‐belts form two sub‐parallel zones of mafic – ultramafic rock assemblages with overlapping crystallization ages (Xiong et al., 2011; Hebért et al., 2012; Liu et al., 2015). The upper mantle section of the Cuobuzha ophiolite in the northern sub‐belt of the Yarlung–Zangbo Suture Zone (YZSZ) in SW Tibet comprises mainly clinopyroxene (cpx)–rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). The cpx‐harzburgites plot in a Re vs. Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes (Uysal et al., 2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐ocean ridge type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting (∼5%) of a primitive mantle source, and they were subsequently modified by melt–rock interactions in a mid‐ocean ridge environment. The depleted harzburgites, on the other hand, were produced by re‐melting of the cpx‐harzburgites, which later interacted with MORB‐ or island arc tholeiite (IAT)‐like melts (Fig. 1) possibly in a trench‐distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Subduction initiation for the formation of high-Cr chromitites in the Kop ophiolite, NE Turkey

The Kop ophiolite in NE Turkey is a forearc fragment of Neo-Tethys ocean, consisting mainly of a paleo-Moho transition zone (MTZ) and a harzburgitic upper mantle unit. Locally, the Kop MTZ contains cumulate dunites and high-Cr chromitites (Cr# up to ca. 79), which are cut by pyroxenites. Dunites and chromitites in the MTZ have REE concentrations that are 1–2 orders of magnitude lower than those of chondrite; they are either depleted in LREE or have concave REE shapes. The LREE depleted patterns are interpreted to reflect production of cumulate rocks by magmas derived from a depleted mantle, the concave patterns the modification of these rocks by LREE-enriched fluids. Clinopyroxenes from pyroxenites are diopsidic and characterized by high Mg#s (ca. 92–96) and high CaO contents (ca. 24–25wt.%); their Al2O3 contents (1.0–3.0wt.%) fall between those of clinopyroxenes in N-MORB and komatiite/boninite, suggesting that the parental melts originated from more refractory mantle than abyssal lherzolites. However, these clinopyroxenes display LREE depleted patterns consistent with those of clinopyroxenes in abyssal lherzolites, indicating their genetic connection with decompression melting of asthenosphere. The cross-cutting relationship between pyroxenite veins and chromitiferous rocks suggests that depleted mantle remained beneath the proto-forearc after chromitite formation; it had not been significantly modified by slab-derived components and continued interacting with the upwelling asthenosphere until pyroxenite crystallization. This study provides a temporal constraint on the formation of high-Cr chromitites; they possibly began to be produced during the transition between early and late proto-forearc spreading, during which subduction dehydration had not well developed.

Optical space weathering on Vesta: Radiative-transfer models and Dawn observations

Exposure to ion and micrometeoroid bombardment in the space environment causes physical and chemical changes in the surface of an airless planetary body. These changes, called space weathering, can strongly influence a surface’s optical characteristics, and hence complicate interpretation of composition from reflectance spectroscopy. Prior work using data from the Dawn spacecraft (Pieters, C.M. et al. [2012]. Nature 491, 79–82) found that accumulation of nanophase metallic iron (npFe0), which is a key space-weathering product on the Moon, does not appear to be important on Vesta, and instead regolith evolution is dominated by mixing with carbonaceous chondrite (CC) material delivered by impacts.In order to gain further insight into the nature of space weathering on Vesta, we constructed model reflectance spectra using Hapke’s radiative-transfer theory and used them as an aid to understanding multispectral observations obtained by Dawn’s Framing Cameras (FC). The model spectra, for a howardite mineral assemblage, include both the effects of npFe0 and that of a mixed CC component. We found that a plot of the 438-nm/555-nm ratio vs. the 555-nm reflectance for the model spectra helps to separate the effects of lunar-style space weathering (LSSW) from those of CC-mixing. We then constructed ratio–reflectance pixel scatterplots using FC images for four areas of contrasting composition: a eucritic area at Vibidia crater, a diogenitic area near Antonia crater, olivine-bearing material within Bellicia crater, and a light mantle unit (referred to as an “orange patch” in some previous studies, based on steep spectral slope in the visible) northeast of Oppia crater. In these four cases the observed spectral trends are those expected from CC-mixing, with no evidence for weathering dominated by production of npFe0. In order to survey a wider range of surfaces, we also defined a spectral parameter that is a function of the change in 438-nm/555-nm ratio and the 555-nm reflectance between fresh and mature surfaces, permitting the spectral change to be classified as LSSW-like or CC-mixing-like. When applied to 21 fresh and mature FC spectral pairs, it was found that none have changes consistent with LSSW.We discuss Vesta’s lack of LSSW in relation to the possible agents of space weathering, the effects of physical and compositional differences among asteroid surfaces, and the possible role of magnetic shielding from the solar wind.

Lower Cretaceous Xigaze ophiolites formed in the Gangdese forearc: Evidence from paleomagnetism, sediment provenance, and stratigraphy

The India–Asia suture zone of southern Tibet exposes Lower Cretaceous Xigaze ophiolites and radiolarian cherts, and time-equivalent Asian-derived clastic forearc sedimentary rocks (Xigaze Group). These ophiolites have been interpreted to have formed in the forearc of the north-dipping subduction zone below Tibet that produced the Gangdese magmatic arc around 15–20°N, or in the forearc of a sub-equatorial intra-oceanic subduction zone. To better constrain the latitude of the ophiolites, we carried out an integrated paleomagnetic, geochronologic and stratigraphical study on epi-ophiolitic radiolarites (Chongdui and Bainang sections), and Xigaze Group turbiditic sandstones unconformably overlying the ophiolite's mantle units (Sangsang section). Detrital zircon U–Pb geochronology of tuffaceous layers from the Chongdui section and sandstones of the Xigaze Group at the Sangsang section provides maximum depositional ages of 116.5±3.1 Ma and 128.8±3.4 Ma, respectively, for the Chongdui section and an Asian provenance signature for the Xigaze Group. Paleomagnetic analyses, integrated with rock magnetic experiments, indicate significant compaction-related inclination ‘shallowing’ of the remanence within the studied rocks. Two independent methods are applied for the inclination shallowing correction of the paleomagnetic directions from the Sangsang section, yielding consistent mean paleolatitudes of 16.2°N [13°N, 20.9°N] and 16.8°N [11.1°N, 23.3°N], respectively. These results are indistinguishable from recent paleolatitude estimates for the Gangdese arc in southern Tibet. Radiolarites from the Chongdui and Bainang sections yield low paleomagnetic inclinations that would suggest a sub-equatorial paleolatitude, but the distribution of the paleomagnetic directions in these rocks strongly suggests a low inclination bias by compaction. Our data indicate that spreading of the Xigaze ophiolite occurred in the Gangdese forearc, and formed the basement of the forearc strata.

The effects of partial melting, melt–mantle interaction and fractionation on ophiolite generation: Constraints from the late Cretaceous Pozantı-Karsantı ophiolite, southern Turkey

The Pozantı-Karsantı ophiolite consists of a mantle unit and overlying crustal rocks. The mantle peridotite is composed of harzburgitic to dunitic rocks that are depleted in Al2O3 and CaO compared to primitive mantle. The low whole-rock Al and Ca values are consistent with the high Cr# [= 100 × Cr/(Cr + Al)] values of spinel that range from 44 to 70. These spinels generally have low TiO2 (< 0.06 wt.%) concentrations, although spinels with high-Cr compositions in certain samples exhibit enrichments of up to 0.16 wt.% TiO2. The concentrations of chondrite-normalized rare earth elements (REEs) indicate depletion toward heavy to middle REEs. However, the entire mantle peridotite samples exhibit marked enrichment in light REEs and large-ion lithophile elements (LILEs) compared to middle REEs. The heavy REE patterns of some peridotite samples are comparable to the calculated melting curves representing various degrees of melting and are modeled by ~ 24% to 30% melting in the spinel stability field. However, some samples are more depleted in the middle REEs than heavy REEs. The patterns of heavy to middle REEs do not follow the melting lines produced by various degrees of melting in the spinel stability field. The heavy REE composition of these peridotite samples suggests that partial melting began in the garnet stability field and continued into the spinel stability field; they represent ~ 22% to 26% melting under various pressure conditions. High anorthite content of plagioclase as well as Nb-depleted composition of mafic cumulates and isotropic gabbros indicate that the crustal rocks of the Pozantı-Karsantı ophiolite formed from a melt that has been produced by melting of depleted source. This melt is thought to have enriched in light REE and fluid mobile elements that were released from the subducting slab during the subduction of Neotethyan ocean. The whole-rock light REE and LILE enrichments and the higher TiO2 concentrations of the high-Cr# spinels in some mantle peridotite samples cannot be explained by simple melt extraction in a mid-ocean ridge (MOR) environment and require melting and enrichment processes in a supra-subduction zone tectonic setting. However, these features of the peridotites can be explained by the interaction of light REEs and TiO2-rich melts and fluids with peridotites that were depleted during melting events in an MOR environment. This interaction, took place in a supra-subduction zone environment, may have increased the light REE concentrations in the peridotites and caused the re-equilibration of the Ti-depleted spinel to produce Ti-richer spinel and therefore explains the geochemical signatures of the Pozantı-Karsantı peridotites.