Oceanic Lithospheric Mantle Research Articles

Pn Wave Velocity and Anisotropy in the Northeastern South China Sea and Adjacent Region

AbstractUsing seismic data from Chinese and ISC stations between 1980~2004, we inverted Pn wave velocity structure and anisotropy of the northeastern South China Sea and adjacent region. The velocity variations at uppermost mantle reflect the features of regional geology and tectonics. Southeastern China has fast velocities, correspondent to the lithospheric mantle in tectonically stable regions. Slow velocities appear near the Binhai fault zone along the coast of southeastern China, which indicates a possible penetration of the fault zone into the uppermost mantle. Similar to southeastern China, fast velocities throughout the northern South China Sea and Taiwan Strait reveal a property of the lithospheric mantle in continental margin, while fast velocities in the Xisha Trough reflect a southward extension of continental shelf and a mantle upwarp produced by Cenozoic rifting. There is no evidence of large‐scale mantle heat flows in the northern South China Sea. However, a very slow velocity anomaly is observed in the eastern sub‐basin of the South China Sea, where is an extinct mantle upwelling center with high heat flows, indicating a thinning or removal of lithospheric mantle. Along the eastern Taiwan, Luzon and northern Philippine, slow velocities are closely related to seismic and volcanic activity and magmatism in the arc zone, while fast velocities in the eastern South China Sea and western Philippine Sea reflect the character of oceanic lithospheric mantle. Pn velocity anisotropy also reveals the stress state in regional tectonics and the history of deformation of lithospheric mantle. Southeastern China has small anisotropy in response to less deformation there. Anisotropy is observed in the northern South China Sea, with fast directions of anisotropy aligned with shallow structures in crust, which reflect the evidence of the Mesozoic‐Cenozoic rifting and shear deformation in lithospheric mantle. Strong anisotropy is also found along the Ryukyu‐Taiwan‐Luzon arc zone, with fast directions parallel to trenches, which indicates that strong deformation of lithospheric mantle in the leading edge of the Philippine Sea plate and Eurasian continent. The change of the fast directions of anisotropy near the eastern Taiwan is probably caused by a conversion of collision mechanisms between the Eurasian continent and the Philippine Sea plate, and the tear of the lithosphere.

Fluid-mobile element budgets in serpentinized oceanic lithospheric mantle: Insights from B, As, Li, Pb, PGEs and Os isotopes in the Feather River Ophiolite, California

Serpentinized oceanic lithosphere may be an important source for boron and other fluid-mobile elements that are anomalously enriched in arc volcanic rocks. However, the integrated water/rock ratios associated with different styles of serpentinization may be variable. For example, large water/rock ratios are involved in the serpentinization of abyssal peridotites exhumed to the seafloor, whereas much lower water/rock ratios are likely to dictate serpentinization along deep faults and fractures. To address how fluid-mobile element enrichments vary with serpentinization at different settings, we investigated serpentinized harzburgites from the Feather River Ophiolite ( FRO) in northern California. Major and trace element systematics indicate that serpentinization of the FRO ultramafics involved seawater. However, FRO serpentinites have unradiogenic Os isotopic compositions and near-chondritic platinum group element relative abundances, contrasting with serpentinized abyssal peridotites, which have radiogenic Os isotopic compositions and disturbed platinum group element systematics. These observations indicate that the integrated water/rock ratio involved in FRO serpentinization was smaller than that involved in abyssal peridotite serpentinization. B concentrations in the FRO (5–15 ppm), while substantially higher than primitive mantle (< 0.1 ppm), are still lower than in abyssal peridotites (10–170 ppm). These low values are not due to metamorphic loss as there is no petrographic evidence for prograde metamorphism (the serpentine minerals are low temperature forms like chrysotile and lizardite) and there is no consistency between observed fluid-mobile element (B, As, Pb, and Li) contents and depletions predicted from metamorphic dehydration models. Low B and fluid-mobile element contents in the FRO may thus be an intrinsic feature of low water/rock ratio serpentinization. Such values may be more representative of serpentinized oceanic lithospheric mantle rather than abyssal peridotites, which sample only the top veneer of the lithosphere.

Quantifying trace element disequilibria in mantle xenoliths and abyssal peridotites

We apply a tool based on element distribution between orthopyroxene and clinopyroxene for quantifying rare earth element (REE) disequilibria in ultramafic rocks in the subsolidus state. We present case studies of the REE contents of mineral cores in mantle xenoliths and abyssal peridotites using in situ analytical tools. Even when only mineral cores are measured (to avoid enriched rims), equilibrium is not always achieved on the mineral scale. Mineral cores in mantle xenoliths are closer to equilibrium than those in abyssal peridotites even though mantle xenoliths are known to be light REE-contaminated from the host lava. In the case of the abyssal peridotites, 13 out of 14 are out of equilibrium with the least metasomatized most in disequilibrium and the most metasomatized closest to equilibrium. We discuss hypotheses for these observations, but regardless of what caused the disequilibria, this tool allows one to “see through” the effects of secondary processes, such as infiltration by fluid inclusions via cracks and diffusive exchange between minerals and melts/fluids along grain boundaries. The ease of making in situ REE measurements makes this tool formidable in identifying different generations of clinopyroxenes in ultramafic lithologies. Such data will complement the interpretation of isotopic and petrographic studies of continental and oceanic lithospheric mantle.

The origin of enriched mantle beneath São Miguel, Azores

Lavas from the island of São Miguel, Azores Archipelago have long been known to display large radiogenic isotopic variability, that ranges from “depleted” isotopic signatures (e.g. high ε Nd ∼ +5) in the west, typical of many ocean island basalts, to more “enriched” compositions (e.g. low ε Nd ∼ +1) in the east. Here, we further characterise the geochemistry of lavas from this remarkable locality, focussing on the nature and origin of the enriched source. Our new isotope data define a striking, linear array in Nd and Hf isotope space that points towards an unusual, enriched composition below the mantle array. This distinctive Hf–Nd isotope signature is associated with elevated values of all three radiogenic Pb isotope ratios. Although the enriched component has certain geochemical similarities to both terrigenous sediments and some samples of the continental mantle lithosphere, such comparisons do not stand closer examination. In the absence of a clear, modern analogue we explore the isotope evolution of some simple, model melt compositions to investigate plausible means of producing an appropriate enriched component. Nd–Hf isotope characteristics provide the tightest constraints and can be reproduced by an ancient (∼3 Ga), modest-degree melt (∼2%) from a garnet peridotite source. Currently, modest-degree melts from garnet-bearing sources are found forming some major oceanic islands. Subduction, isolation and later mixing of small amounts (<5%) of such basaltic material with more ubiquitous ambient mantle can account for the isotopic characteristics of the enriched São Miguel source. Yet the incompatible element ratios of the enriched São Miguel lavas do not show “recycled” signatures of near-surface alteration nor subduction zone dehydration. Thus, we infer that the enriched component was originally under-plated basalt, intruded into oceanic mantle lithosphere rather than forming the island edifice itself. Since the extreme isotope compositions of São Miguel reflect unextraordinary, albeit ancient, magmatic fractionation, the general rarity of such signatures indicates the efficiency of mantle processes in homogenising or hiding similar sources.

Geochemical investigation of serpentinized oceanic lithospheric mantle in the Feather River Ophiolite, California: Implications for the recycling rate of water by subduction

The petrology and geochemistry of serpentinized harzburgites within the Feather River Ophiolite in northern California were investigated to constrain the origin of serpentinization. Trace-element systematics indicate that serpentinization was associated almost solely with relatively low temperature hydrothermal addition of seawater and not with the addition of metamorphic fluids associated with subduction or tectonic obduction. Major element systematics show almost negligible disturbance, suggesting low water/rock ratios rather than the very high water/rock ratios characteristic of serpentinization of exhumed mantle at the seafloor such as seen in abyssal peridotites. These observations were taken to indicate that serpentinization occurred by the addition of seawater to ultramafic protoliths while they were structurally within the oceanic lithosphere prior to tectonic accretion. Major and trace element systematics were used with simple melting models and assumptions about the thermal state of the mantle to put rough constraints on the paleo-depths at which the ultramafic protoliths were the last melted. Assuming that these peridotites were formed beneath a passive upwelling system, these paleo-melting depths were assumed to equate with paleo-serpentinization depths. For a model mantle potential temperature of 1350 °C, the paleo-serpentinization depth was estimated to be at least ∼ 40 km based on the minimum melting degree observed in the Feather River Ophiolite peridotites. This depth constraint on serpentinization was then combined with model estimates of the lateral extent of serpentinization assuming that serpentinization is associated with infiltration of water through faults and fractures. If these observations can be generalized, we estimate that recycling rate of serpentine-bound water in subduction zones is roughly one order of magnitude higher than that associated with mineralogically bound water in subducting oceanic crust and sediments. After accounting for serpentine recycling, it is shown that the recycling rate of water via subduction is roughly equal to the global volcanic dewatering rate, suggesting that the water budget of the ocean and Earth's interior may presently be close to steady state.

Porphyry Cu–Au and associated polymetallic Fe–Cu–Au deposits in the Beiya Area, western Yunnan Province, south China

The Alkaline porphyries in the Beiya area are located east of the Jinshajiang suture, as part of a Cenozoic alkali-rich porphyry belt in western Yunnan. The main rock types include quartz-albite porphyry, quartz-K-feldspar porphyry and biotite–K-feldspar porphyry. These porphyries are characterised by high alkalinity [(K 2O + Na 2O)% > 10%], high silica (SiO 2% > 65%), high Sr (> 400 ppm) and 87Sr/ 86Sr (> 0.706)] ratio and were intruded at 65.5 Ma, between 25.5 to 32.5 Ma, and about 3.8 Ma, respectively. There are five main types of mineral deposits in the Beiya area: (1) porphyry Cu–Au deposits, (2) magmatic Fe–Au deposits, (3) sedimentary polymetallic deposits, (4) polymetallic skarn deposits, and (5) palaeoplacers associated with karsts. The porphyry Cu–Au and polymetallic skarn deposits are associated with quartz–albite porphyry bodies. The Fe–Au and polymetallic sedimentary deposits are part of an ore-forming system that produced considerable Au in the Beiya area, and are characterised by low concentrations of La, Ti, and Co, and high concentrations of Y, Yb, and Sc. The Cenozoic porphyries in western Yunnan display increased alkalinity away from the Triassic Jinshajiang suture. Distribution of both the porphyries and sedimentary deposits in the Beiya area are interpreted to be related to partial melting in a disjointed region between upper mantle lithosphere of the Yangtze Plate and Gondwana continent, and lie within a shear zone between buried Palaeo-Tethyan oceanic lithosphere and upper mantle lithosphere, caused by the subduction and collision of India and Asia.

Lithospheric contributions to high-MgO basanites from the Cumbre Vieja Volcano, La Palma, Canary Islands and evidence for temporal variation in plume influence

New geochemical and isotopic data are presented from the oldest part of the Cumbre Vieja volcano, La Palma (Canary Islands), located near the assumed emergence of the Canary mantle plume. The volcanics comprise a suite dominated by basanite flows with subordinate amounts of phono-tephrite, tephri-phonolite and phonolite flows and intrusives. Two compositionally different basanite groups have been identified, both with HIMU (high-μ)-type incompatible trace element characteristics: Primitive high-MgO basanites (10.7–12.1% MgO), found only at the base of a stratigraphic profile near Fuencaliente on the south coast, and intermediate-MgO basanites (6.0–7.3% MgO), exposed in the upper part of the profile and widespread on the east coast of La Palma. The high-MgO basanites are interpreted as near-primary mantle melts (primary composition 14–15% MgO) derived by progressive melting (2.9% to 4.5%) of a common lithospheric mantle source. Model calculations indicate that it is not possible to generate the intermediate-MgO basanites from the high-MgO group by crystal fractionation of observed phenocrysts. Relative to intermediate-MgO basanites, the high-MgO flows have lower concentrations of LIL and HFS elements, except for Ti, which is markedly enriched in the primitive rocks (3.7–4.7% TiO 2 vs 3.4–3.9% TiO 2). Fuencaliente volcanics display limited temporal isotopic variations suggested to be a result of mixing of melts originating from the rising plume and the metazomatized lithospheric mantle. 87Sr / 86Sr and 143Nd / 144Nd ratios range 0.70305–0.70311 and 0.51285–0.51291, respectively, while the corresponding ranges in Pb-isotope ratios are 206Pb / 204Pb = 19.46–19.64, 207Pb / 204Pb = 15.55–15.61, and 208Pb / 204Pb = 39.16–39.53. The overall variation of the Cumbre Vieja isotopic data can be accounted for by mixtures of three mantle components in the proportions 72–79% plume source (LVC = low velocity component), 9–16% depleted mantle (DM) and up to 12% enriched mantle (EMI). Negative Δ7 / 4 Pb (− 0.6 to − 5.4) in the Cumbre Vieja volcanics suggest derivation from a young HIMU mantle source. The relative abundance of plume source material increase in younger rocks in the Fuencaliente section, suggesting waning plume–lithosphere interaction during the emplacement of this part of the Cumbre Vieja volcano. The high-MgO volcanics define regular and systematic geochemical trends, interpreted as partial melting trends, when plotted against abundances of highly incompatible elements (P, Ce). Evaluation of minor and trace element variation in consecutive melts suggests control by residual amphibole, phlogopite, garnet and a Ti-bearing phase, possibly ilmenite. The melting mode changed gradually, allowing increasing input from residual phlogopite during partial melting. The residual mineralogy constrains the source region of the high-MgO basanites to the lowermost oceanic lithospheric mantle, presumably around 100 km depths.

Hf-Nd-Sr isotope systematics of garnet pyroxenites from Salt Lake Crater, Oahu, Hawaii: Evidence for a depleted component in Hawaiian volcanism

We present the first comprehensive major, trace element and Hf, Nd and Sr isotope investigation of clinopyroxene and garnet mineral separates from a set of garnet clinopyroxenite xenoliths from the Salt Lake Crater, Oahu, Hawaii. These xenoliths occur in the posterosional Honolulu Volcanics Series lavas and represent some of the deepest samples from the oceanic mantle lithosphere. Our study shows that the Salt Lake Crater pyroxenites represent high pressure (>20 kb) accumulates from melts similar (but not identical) to the erupted Honolulu Volcanics, and unlike MORB or E-MORB-type melts. All clinopyroxene-garnet mineral pairs in these xenoliths show, within error, zero-age Lu-Hf and Sm-Nd isotope systematics. These pyroxenites have relatively radiogenic Hf isotope compositions (for a given Nd) and define a distinct steep slope (3.3) in ε Hf-ε Nd isotope space, similar to the Honolulu Volcanics but unlike other ocean island basalts (OIB). These compositions require an end-member component that falls above the OIB array in Nd-Hf space. This component is different than present-day MORB-mantle and it is best explained by an old depleted oceanic lithosphere. We suggest that this depleted component most likely represents a recycled depleted lithosphere that is intrinsic to the Hawaiian plume. In this respect, the Hawaiian plume is sampling both the enriched portion of a subducted oceanic crust (basalt and sediments) as well as the depleted lithospheric portion of it. This suggests that, at least for Hawaii, the whole subducted oceanic slab package has retained its integrity during subduction and subsequent mixing and storage in the mantle, probably in the order of a billion years, and that the plume is sampling the full range of these compositions.

100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits

Mineral deposit types commonly have a distinctive temporal distribution with peaks at specific periods of Earth history. Deposits of less redox-sensitive metals, such as gold, show long-term temporal patterns that relate to first-order changes in an evolving Earth, as a result of progressively declining heat production and attendant changes in global tectonic processes. Despite abundant evidence for plate tectonics in the early Precambrian, it is evident that plume events were more abundant in a hotter Earth. Episodic growth of juvenile continental crust appears to have been related to short-lived (<100 m.y.) catastrophic mantle-plume events and formation of supercontinents, whereas shielding mantle-plume events correlated with their breakup. Different mineral deposit types are associated with this cycle of supercontinent formation and breakup. Broadly synchronous with juvenile continental crust formation was the development of subcontinental lithospheric mantle, which evolved due to progressively declining heat flow and decreasing plume activity. Archean subcontinental lithospheric mantle has a distinct mineralogical composition and is buoyant, whereas later lithosphere was progressively more dense. Changes in the buoyancy of both oceanic lithosphere and subcontinental lithospheric mantle led to evolution of tectonic scenarios in which buoyant, roughly equidimensional, early Precambrian cratons were rimmed by Proterozoic or Phanerozoic linear elongate belts of neutral to negative buoyancy. Orogenic gold deposits, which formed over at least 3.4 b.y., had the highest preservation potential of any gold deposit type. The pattern of formation and preservation, from episodic to more cyclic, broadly mirrors that of crustal growth. Early Precambrian (mostly ca. 2.7 and 2.0–1.8 Ga) deposits, protected from uplift and erosion in the centers of buoyant cratons, are rare between ca. 1.7 Ga and 600 Ma due to the change to more modern-style plate tectonic processes, with nonpreservation of deposits of this age due to uplift and erosion of more vulnerable orogenic belts. Volcanic-hosted massive sulfide (VHMS) deposits were accreted into the convergent margin terranes in which orogenic gold deposits were forming. Their temporal distribution, from strongly episodic to more cyclic peaks, also supports a model of selective preservation. The first appearance of iron-oxide copper-gold (IOCG) deposits at ~2.55 Ga closely follows development of early Precambrian subcontinental lithosphere mantle. Their genesis involved melting of metasomatized subcontinental lithosphere mantle, so they could not form until such metasomatized mantle evolved below cratons with buoyant lithosphere. Giant Precambrian paleoplacer gold deposits probably formed by effective fluvial sorting under extreme climatic conditions but were largely preserved due to early buoyant subcontinental lithospheric mantle below hosting foreland basins. Unequivocal intrusion-related gold deposits are related to complex felsic intrusions with a mixed mantle-crustal signature, which intruded deformed shelf sedimentary sequences close to but outside craton margins. Given that post-Paleoproterozoic uplift and erosion is likely in vulnerable orogenic belts with negatively buoyant lithosphere, this deposit type is likely to be rare in Paleozoic and older terranes. Gold-bearing deposit types thus display distinctive temporal distributions related to change from a more buoyant plate tectonic style in the early hotter Earth to a modern plate tectonic style typical of the Phanerozoic. Late Archean formation of buoyant subcontinental lithospheric mantle was particularly important in the anomalous preservation of some earlier formed deposit types located inboard of craton margins and in providing critical conditions for the formation of others. Development of negatively buoyant subcontinental lithospheric mantle can explain the lack of preservation of some deposit types that formed in the later Proterozoic. A single fundamental concept of coupled episodic crustal growth and preservation in the Archean and Paleoproterozoic, evolving to decoupled episodes of growth and preservation from the Mesoproterozoic onward, can thus explain the temporal distribution of a number of gold-bearing mineral deposit types.

A source for Icelandic magmas in remelted Iapetus crust

The geochemistry and large melt volume in the Iceland region, along with the paucity of evidence for high, plume-like temperatures in the mantle source, are consistent with a source in the extensive remelting of subducted Iapetus crust. This may have been trapped in the Laurasian continental mantle lithosphere during continental collision in the Caledonian orogeny at ∼420–410 Ma, and recycled locally back into the asthenosphere beneath the mid-Atlantic ridge by lithospheric delamination when the north Atlantic opened. Fractional remelting of abyssal gabbro can explain the major-, trace- and rare-earth-element compositions, and the isotopic characteristics of primitive Icelandic tholeiite. An enriched component already present in the recycled crustal section in the form of enriched mid-ocean-ridge basalt, alkalic olivine basalt and/or related differentiates could contribute to the diversity of Icelandic basalts. Compositions ranging from ferrobasalt to olivine tholeiite are produced by various degrees of partial melting in eclogite, and the crystallization of ferrobasalt as oxide gabbro, i.e., containing the magmatic Fe–Ti oxide minerals, ilmenite and magnetite, may explain the anomalously high density of the Icelandic lower crust. The very high 3He/ 4He ratios observed in some Icelandic basalts may derive from old helium preserved in U+Th-poor residual Caledonian oceanic mantle lithosphere or olivine-rich cumulates in the crustal section. The persistence of anomalous volcanism at the mid-Atlantic ridge in the neighborhood of Iceland suggests that in the presence of lateral ridge migration, the shallow fertility anomaly must be oriented transverse to the mid-Atlantic ridge. The Greenland–Iceland–Faeroe ridge is co-linear with the western frontal thrust of the Caledonian collision zone, which may thus be associated with the fertility source. The fertile material beneath the Iceland region must lie at a steep angle or be thickened by deformation or imbrication to supply the large volumes of basalt required to build the thick crust there. “Hot spot” volcanism and large-igneous-province emplacement often occurs within or near to old suture zones and similar processes may thus explain anomalous magmatism elsewhere that is traditionally attributed to plumes.

The Evolution of the Upper Mantle beneath the Canary Islands: Information from Trace Elements and Sr isotope Ratios in Minerals in Mantle Xenoliths

AbstractLaser ablation microprobe data are presented for olivine, orthopyroxene and clinopyroxene in spinel harzburgite and lherzolite xenoliths from La Palma, Hierro, and Lanzarote, and new whole-rock trace-element data for xenoliths from Hierro and Lanzarote. The xenoliths show evidence of strong major, trace element and Sr isotope depletion (87Sr/86Sr ≤ 0·7027 in clinopyroxene in the most refractory harzburgites) overprinted by metasomatism. The low Sr isotope ratios are not compatible with the former suggestion of a mantle plume in the area during opening of the Atlantic Ocean. Estimates suggest that the composition of the original oceanic lithospheric mantle beneath the Canary Islands corresponds to the residues after 25–30% fractional melting of primordial mantle material; it is thus significantly more refractory than ‘normal’ mid-ocean ridge basalt (MORB) mantle. The trace element compositions and Sr isotopic ratios of the minerals least affected by metasomatization indicate that the upper mantle beneath the Canary Islands originally formed as highly refractory oceanic lithosphere during the opening of the Atlantic Ocean in the area. During the Canarian intraplate event the upper mantle was metasomatized; the metasomatic processes include cryptic metasomatism, resetting of the Sr–Nd isotopic ratios to values within the range of Canary Islands basalts, formation of minor amounts of phlogopite, and melt–wall-rock reactions. The upper mantle beneath Tenerife and La Palma is strongly metasomatized by carbonatitic or carbonaceous melts highly enriched in light rare earth elements (REE) relative to heavy REE, and depleted in Zr–Hf and Ti relative to REE. In the lithospheric mantle beneath Hierro and Lanzarote, metasomatism has been relatively weak, and appears to be caused by high-Si melts producing concave-upwards trace element patterns in clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-rich harzburgites/lherzolites, dunites, wehrlites and clinopyroxenites formed from mildly alkaline basaltic melts (similar to those that dominate the exposed parts of the islands), and appear to be mainly restricted to magma conduits; the alkali basalt melts have caused only local metasomatism in the mantle wall-rocks of such conduits. The various metasomatic fluids formed as the results of immiscible separations, melt–wall-rock reactions and chromatographic fractionation either from a CO2-rich basaltic primary melt, or, alternatively, from a basaltic and a siliceous carbonatite or carbonaceous silicate melt.

The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite

Structural and fabric analysis of the well-exposed Hilti mantle section, Oman ophiolite, suggests that shear zone development, which may have resulted from oceanic plate fragmentation, was influenced by pre-existing mantle fabric present at the paleo-ridge. Detailed structural mapping in the mantle section revealed a gently undulating structure with an east–west flow direction. A NW–SE strike-slip shear zone cuts across this horizontal structure. The crystal preferred orientation (CPO) of olivine within the foliation is dominated by (010) axial patterns rather than more commonly observed (010)[100] patterns, suggesting that the horizontal flow close to the Moho involved non-coaxial flow. Olivine CPO within the shear zone formed at low temperature is characterized by (001)[100] patterns and a sinistral sense of shear. The olivine CPO becomes weaker with progressive mylonitization and accompanying grain size reduction, and ultimately develops into an ultra-mylonite with a random CPO pattern. The olivine [010]-axis is consistently sub-vertical, even where the horizontal foliation has been rotated to a sub-vertical orientation within the shear zone. These observations suggest that the primary mechanical anisotropy (mantle fabric) has been readily transformed into a secondary structure (shear zone) with minimum modification. This occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling. We propose that primary olivine CPO fabrics may play a significant role in the subsequent structural development of the mantle. Thus, the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center.

ALPINE-APENNINE OPHIOLITIC PERIDOTITES: NEW CONCEPTS ON THEIR COMPOSITION AND EVOLUTION

Ophiolites exposed in the Alpine-Apennine mountain range represent the oceanic lithosphere of the Ligurian Tethys, a small oceanic basin separating the Europe and Adria plates in Mesozoic times. Most of the peridotites represent former subcontinental mantle which was: (i) isolated from the convective mantle at different times (from Proterozoic to Permian); (ii) accreted to the thermal lithosphere, where it cooled along a conductive geothermal gradient under spinel peridotite facies conditions. Our investigations reveal important records of melt/peridotite interaction and melt impregnation (i.e. formation of plagioclase peridotites), which were related to asthenosphere/lithosphere interaction occurred during lithospheric extension leading to rifting and drifting of the Jurassic Ligurian Tethys. The early asthenosphere/ lithosphere interaction was caused by the reactive percolation of asthenospheric melts, which induced significant depletion, refertilization and heating of the lithospheric mantle peridotites. The plagioclase peridotites of the Alpine-Apennine ophiolites mostly derive from melt impregnation, whereas part of the depleted spinel peridotites result from reactive percolation of depleted MORB-type melts, rather than being solely refractory residua after nearfractional melting. The presence of large areas of impregnated peridotites indicates that significant volumes of melts were trapped in the lithospheric mantle. Subsequently, the asthenospheric melts reached the surface, both intruding as MORB gabbroic bodies or extruding as MORB lava flows. These peridotites record two magmatic cycles: (1) An early magmatic, non-volcanic stage: the early diffuse porous flow percolation and impregnation by single melt increments, focused percolation in dunite channels and intrusion of MORB-type melts; (2) A late magmatic, volcanic stage: the late intrusion and extrusion of magmas deriving from aggregated MORB liquids. The sequence of periods characterized by absence (non-volcanic or a-magmatic or magma-starved stages) and presence (volcanic or magmatic stages) of volcanism represents one of the most peculiar feature of slow and very-slow spreading ridges. Melt stagnation in the oceanic lithospheric mantle has been proposed as the dominant mechanism of peridotite impregnation and plagioclase peridotite formation along slow spreading systems. It could be that amagmatic periods of (ultra-) slow spreading ridges are characterized by melt stagnation in the thermal lithosphere, leading to plagioclase peridotite formation. Our results evidence the great variability in terms of melt composition and regime of melt percolation during the rift evolution. They provide, moreover, a mechanism to explain non-volcanic and volcanic stages during the rift evolution of the Ligurian Tethys and might be equally applicable to modern slow spreading ridges, which are characterized by variable magmatic (volcanic) and amagmatic (non-volcanic) stages.

The Dzela complex, Polar Urals, Russia: a Neoproterozoic island arc

Abstract The Neoproterozoic Dzela complex in the southern part of the Polar Ural Mountains occurs within the Uralian fault zone. It represents residual oceanic lithospheric mantle, partial melt derived from it, and seafloor basalts. In low strain domains, a depleted mantle residue of ultramafic rocks (pyroxenite, lherzolite and dunite) and mafic rocks (gabbro, gabbro-norite and quartz gabbronorite) are preserved. Distinctive negative europium anomalies in the ultramafic rocks (and their metamorphosed equivalents) indicate their residual nature after basaltic melt extraction. Gabbroic rocks (and their metamorphosed equivalents) have elevated rare earth element (REE) concentrations, but similar overall REE patterns as the ultramafic rocks that host them, suggesting that the ultramafic rocks are the source of the gabbroic melts. Blueschist- and greenschist-facies metabasalts overlie the ultramafic and mafic rocks. Two groups of metabasalts are defined. One has NMORB-like REE patterns, but is uniformly more depleted than NMORB, and the other has slightly enriched (relative to NMORB) LREE/HREE patterns. The latter has high field-strength elements (HFSE) concentrations consistent with formation in an intraoceanic subduction zone setting. Dzela mafic rocks have been dated for the first time to 578 ± 8 Ma (2σ) (U-Pb zircon from quartz-bearing gabbro-norite), defining a Neoproterozoic crystallization age. The complex was probably accreted to the NE margin of Baltica ( sensu lato ) during the Timanian Orogeny. Locally reset U-Pb zircon ages of about 350 Ma probably record Uralian orogenesis and recrystallization associated with high-pressure/low-temperature metamorphism. The Dzela complex provides further evidence that ocean-continent collision was the driving mechanism for Timanian Orogeny.

Age, geochemistry and tectonic setting of Buqingshan ophiolites, North Qinghai-Tibet Plateau, China

The Buqingshan ophiolite complex is a sector of the A'nyemaqen ophiolite belt in the East Kunlun southern marginal suture zone in the northern part of the Qinghai-Tibet Plateau. The WNW-trending ophiolite complex consists of metaperidotite, gabbro, diabase, pillow basalt, massive basalt and pelagic sedimentary rocks including radiolarian chert. We have determined that Early Paleozoic and Early Carboniferous–Early Permian ophiolites are present in this ophiolite complex that were previously thought of Permian–Middle Triassic age. A zircon U–Pb age of 467.2±0.9 Ma for gabbro, and a Rb–Sr isochron age of 481±130 Ma for diabase and gabbro have been obtained from the Early Paleozoic ophiolite slice, and Middle–Late Ordovician acritarchs have been found in the mélange matrix that envelopes the sliver. We interpret the first two ages to represent the formation age of the ophiolite. Granodiorite–tonalite plutons with a zircon U–Pb age of 402±24 Ma intrude the slice. Early Carboniferous–Early Permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice. Geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle. Most mafic rocks in the Early Paleozoic ophiolite are of N-MORB type with small amount of T-MORB, and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination. The basalts in the Early Carboniferous–Early Permian ophiolite have similar characteristics to the mafic rocks and also show DUPAL anomaly and a mixed source of DMM and EM II. The original formation environments of the two-stage ophiolites are both mid-ocean ridges. These lines of evidence suggest that a mature Early Paleozoic Kunlun–Qilian–Qinling ocean basin and a mature Paleotethyan ocean basin once existed in the study area.

Peridotitic diamonds from the Slave and the Kaapvaal cratons—similarities and differences based on a preliminary data set

A comparison of the diamond productions from Panda (Ekati Mine) and Snap Lake with those from southern Africa shows significant differences: diamonds from the Slave typically are un-resorbed octahedrals or macles, often with opaque coats, and yellow colours are very rare. Diamonds from the Kaapvaal are dominated by resorbed, dodecahedral shapes, coats are absent and yellow colours are common. The first two features suggest exposure to oxidizing fluids/melts during mantle storage and/or transport to the Earth's surface, for the Kaapvaal diamond population. Comparing peridotitic inclusions in diamonds from the central and southern Slave (Panda, DO27 and Snap Lake kimberlites) and the Kaapvaal indicates that the diamondiferous mantle lithosphere beneath the Slave is chemically less depleted. Most notable are the almost complete absence of garnet inclusions derived from low-Ca harzburgites and a generally lower Mg-number of Slave inclusions. Geothermobarometric calculations suggest that Slave diamonds originally formed at very similar thermal conditions as observed beneath the Kaapvaal (geothermal gradients corresponding to 40–42 mW/m 2 surface heat flow), but the diamond source regions subsequently cooled by about 100–150 °C to fall on a 37–38 mW/m 2 (surface heat flow) conductive geotherm, as is evidenced from touching (re-equilibrated) inclusions in diamonds, and from xenocrysts and xenoliths. In the Kaapvaal, a similar thermal evolution has previously been recognized for diamonds from the De Beers Pool kimberlites. In part very low aggregation levels of nitrogen impurities in Slave diamonds imply that cooling occurred soon after diamond formation. This may relate elevated temperatures during diamond formation to short-lived magmatic perturbations. Generally high Cr-contents of pyrope garnets (inside and outside of diamonds) indicate that the mantle lithosphere beneath the Slave originally formed as a residue of melt extraction at relatively low pressures (within the stability field of spinelperidotites), possibly during the extraction of oceanic crust. After emplacement of this depleted, oceanic mantle lithosphere into the Slave lithosphere during a subduction event, secondary metasomatic enrichment occurred leading to strong re-enrichment of the deeper (>140 km) lithosphere. Because of the extent of this event and the occurrence of lower mantle diamonds, this may be related to an upwelling plume, but it may equally just reflect a long term evolution with lower mantle diamonds being transported upwards in the course of “normal” mantle convection.

Microscopic deformation of the Neoarchean oceanic lithospheric mantle: evidence from the Zunhua Neoarchean ophiolitic mélange, North China Craton *

Abstract The microstructures of the mantle tectonites in the Zunhua Neoarchean ophiolitic melange are mainly coarse-mosaic structures with locally interstitial impregnated melts. Olivine and orthopyroxene occur as residual porphyroclastic blasts, dynamic recrystallization neoblasts or elongated to be tabular. The podiform chromitites are mostly strongly deformed with the development of pull-apart structure. These microstructures are typical high-temperature plastic deformation in oceanic upper mantle resulting from ocean-floor spreading. Besides the high-temperature plastic deformation, undeformed magmatic intrusions such as undeformed podiform chromitite. dunite and pyroxenite intrusions are also preserved in the mantle tectonite. Structures of high-temperature plastic deformation and intensive magmatic activity prove that the Zunhua ophiolite was formed under fast spreading oceanic ridge, similar to the Oman ophiolite. And the microstructures of country rocks, such as quartz ribbon, core and mantle stru...

Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)

Abundances of major and trace elements in whole rocks and minerals in lherzolites and harzburgites from the northern Oman ophiolite are used to understand the mantle processes creating compositional variation in oceanic lithospheric mantle. Detailed mapping shows that lherzolites occur near the base of a mantle section in the northern Fizh block. Geochemical analyses identify two types of basal lherzolite. The first type (Type I lherzolite) displays porphyroclastic microstructure and occurs sporadically in the basal mylonite zone. Whole rock and clinopyroxene are highly depleted in incompatible elements such as Na, Ti, Zr, and rare earth elements (REE). The chondrite‐normalized patterns of Type I lherzolites show steep slopes from heavy REE (HREE) to light REE (LREE) that are ascribed to melt extraction, up to 12–18%, from a source containing a small amount of garnet. The chondrite‐normalized patterns have slight enrichment in LREE relative to the patterns expected for residues of partial melting thereby indicating reaction with a LREE‐enriched melt or fluid at a low melt/rock ratio. The second type (Type II lherzolite) shows mylonitic microstructure and only occurs at the contact between the mantle section and the metamorphic sole. Abundances of incompatible elements in whole rocks and clinopyroxenes are greater than those of Type I lherzolites, and clinopyroxenes in Type II lherzolites have high Na2O contents (&gt;1 wt.%). To a first approximation, the high Na content of clinopyroxenes and whole rocks and the LREE‐depleted, chondrite‐normalized whole rock REE patterns are consistent with Type II lherzolite being in equilibrium with a mid‐ocean ridge basalt (MORB)‐type melt at relatively high pressure (&gt;2 GPa). However, the flatness of chondrite‐normalized patterns for middle and heavy REE are inconsistent with residual garnet peridotite. The characteristics of Type II lherzolites are better explained by a mixing process in which residual peridotite was refertilized by addition of a LREE‐depleted melt. The large compositional gradient near the basal thrust in the northern Fizh block may have recorded a transient state in which the degree of partial melting was progressively decreased as a result of reducing mantle temperature and upwelling rate. This scenario is consistent with the inferred failing ridge associated with a transform zone in the western side of the northern Fizh block proposed by Nicolas et al. [2000]. In the detachment stage of the Oman ophiolite, a small amount of ascending melt may have crystallized near the basal part of mantle section thereby forming Type II lherzolites. Basal lherzolites and their spatial chemical variations in the northern Fizh block may provide a key for understanding the processes of ridge segmentation and detachment at fast spreading ridges.

Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins

Abstract Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene ± spinel and precipitated orthopyroxene + plagioclase ± olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene ± plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.

Microscopic deformation of the Neoarchean oceanic lithospheric mantle; evidence from the zunhua Neoarchean ophiolitic melange, North china Craton

Microscopic deformation of the Neoarchean oceanic lithospheric mantle; evidence from the zunhua Neoarchean ophiolitic melange, North china Craton