Study Of Mantle Xenoliths Research Articles

Multi-scale and multi-modal imaging study of mantle xenoliths and petrological implications

Abstract The accurate textural characterization of mantle xenoliths is one of the fundamental steps to understanding the main processes occurring in the upper mantle, such as sub-solidus recrystallization, magmatic crystallization, and metasomatism. Texture, composition, and mineralogy reflect the temperature, pressure, stress conditions, melting, and/or contamination events undergone before and during the entrapment in the host magma. For these reasons, characterizing the three-dimensional (3D) texture of silicate, oxide, sulfide, and glass phases has great importance in the study of mantle xenoliths. We performed a multi-scale and multi-modal 3D textural analysis based on X-ray computed microtomography (µ-CT) data of three mantle xenoliths from different geodynamic settings (i.e., mobile belt zone, pericraton, oceanic hotspot). The samples were selected to represent different, variably complex internal structures composed of grains of different phases, fractures, voids, and fluid inclusions of different sizes. We used an approach structured in increasing steps of spatial and contrast resolution, starting with in-house X-ray µ-CT imaging (spatial resolution from 30 µm down to 6.25 µm) and moving to high-resolution synchrotron X-ray µ-CT at the micrometer scale. We performed a 3D characterization of mantle xenoliths, comparing the results with the analysis of conventional 2D images (thin sections) obtained by optical microscopy and simulating the random sectioning of several thin sections to estimate the probability of correct modal classification. The 3D models allow the extraction of textural information that cannot be quantified solely from thin sections: spinel layering, distribution of silicic glass, and related vesicles. Moreover, high-density volumes identified as sulfides were detected in two xenoliths, showing no relation with the spinel layering in one case and a preferential concentration along fractures in the other. Given the variety of textures and mineral assemblages of mantle xenoliths worldwide, the results are used to suggest experimental and analytical protocols for the characterization of these materials.

Architecture of the subcontinental lithospheric mantle of the Archean segment of the Fennoscandian Shield: Analysis of seismic data

Here, Karelian seismological network data were studied to reveal the architecture of the subcontinental lithospheric mantle (SCLM). The receiver functions method was used, which is based on the identification of converted waves from teleseismic earthquakes and their joint inversion into a velocity model. Original one-dimensional (1D) velocity models for the continental lithosphere of the central Karelian Craton (KC) and the western Belomorian Mobile Belt (BMB; both part of the Archean segment of the Fennoscandian Shield) were presented, analysed and interpreted in geological and petrological terms.The obtained results reveal that the SCLM is >200 km thick both in the KC and the BMB. The mantle in both structures was shown to contain a well-defined, contrasting boundary with the crust. However, the boundary between the SCLM and the asthenosphere was not identified in this study. The SCLM was found to be stratified and divided into upper, middle and lower layers in both structures. The boundary between the upper and middle layers was identified at a depth near the upper line of the garnet stability limit in mantle peridotites; the boundary between the middle and lower layers corresponds to the graphite/diamond equilibrium line.The SCLM of the BMB was shown to exhibit a distinct lowered velocity zone in the middle layer. The 1D velocity model of the central KC was found to be in good agreement with the evidence for the SCLM stratigraphy, based on the study of mantle xenoliths and xenocrysts from kimberlites and lamproites from the area.

Structure of the Earth's crust and upper mantle and problems of global geodynamics

This work presents findings of deep geological and geophysical studies, which have revealed new and often unexpected features of the structure of the Earth's crust and the upper mantle of the continents and oceans, not easily explainable by current geodynamic concepts. For example, deepwater drilling has shown the old age of the oceanic crust and its inhomogeneous composition. Seismic studies of long-range profiles with nuclear explosions and laboratory studies of mantle xenoliths at high pressures and temperatures have revealed that the advection of energy-intensive and geochemically active deep fluids is central to the formation of the upper mantle structure. A comprehensive analysis of these data provides a geodynamic model for the natural transformation of the upper mantle substance influenced by deep fluids and the formation of continents and oceans as a result of unevenly distributed degassing of the Earth. The global variability of the deep fluid advection intensity led to formation of two hemispheres: the Indo Atlantic Hemisphere where almost all continents are located and the Pacific Hemisphere that consists of oceans. This formation of the hemispheres can be explained by the specific features of the Earth's rotation in the Earth-Moon-Sun system. This work provides a review and analysis of these data and then offers a possible explanation based on an integrated geodynamic model.

Mantle Xenoliths from Huanul Volcano (Central-West Argentina): A Poorly Depleted Mantle Source under Southern Payenia

Huanul is a shield volcano with several lava flows hosting mantle xenoliths erupted during the Pleistocene (0.84 ± 0.05 Ma). It is located in the southern part of the Payenia Volcanic Province, which is among the largest Neogene-Quaternary volcanic provinces of South America. The volcanism here has been ascribed as the northernmost expression of the back-arc volcanism of the Andean Southern Volcanic Zone. We present the first petrographic and mineral chemistry study of mantle xenoliths collected from Huanul lavas with the aim of reconstructing directly the mantle source of the Payenia Volcanic Province. Xenoliths are commonly small (<5 cm in radius) but scarcely crossed by basaltic veins. All xenoliths have a fertile lherzolitic modal composition and are equilibrated in the spinel-facies. Most of them exhibit an almost primitive-mantle geochemical affinity, characterized by slightly depleted clinopyroxene REE patterns reproducible by partial melting degrees between 0 and 4% of a PM source. Geothermobarometric P-T estimates of clinopyroxene-orthopyroxene couples form a linear trend between 10 and 24 kbar with constant increase of T from 814 to 1170 °C along a 50–60 mW/m2 geotherm. Evidences of interaction with the host basalts occur as spongy textures in clinopyroxene and reacted spinel, which tend to became more restitic in composition and show chromatographic or complete overprinting of the trace element compositions. The presence of plagioclase and calculated P-T values constrain this melt/rock reaction process between 6 and 14 kbar, during magma ascent, and fit the mantle adiabat model. Calculated melts in equilibrium with the primary clinopyroxenes do not fit the composition of the host basalt and, together with the geothermobarometric estimations, point to an asthenospheric mantle source for the magmatism in southern Payenia. The PM geochemical affinity of the xenoliths of Huanul is an extremely rare finding in the South America lithospheric mantle, which is commonly extensively refertilized by subduction-derived melts.

A review of the composition and chemistry of peridotite mantle xenoliths in volcanic rocks from Antarctica and their relevance to petrological and geophysical models for the lithospheric mantle

Abstract This chapter reviews the geochemistry and petrology of mantle peridotite xenoliths from across Antarctica, including parameters that are of most relevance to geophysical studies. This Memoir is the first time such a complete overview of the chemistry of Antarctic mantle xenoliths has been available and Antarctica should no longer be the ignored continent in studies of mantle xenoliths in volcanic rocks. Xenoliths indicate that the chemistry, heat flow and water content of the Antarctic lithospheric mantle varies regionally at scales of one to thousands of kilometres. The prevalence of variability in xenoliths suggests that the Antarctic mantle is ubiquitously heterogeneous. This has important, yet unquantified, implications for interpreting geophysical data and for reference Earth models used in Antarctic geophysical studies. Information about and interpretations of Antarctic mantle xenoliths can be linked to studies from once adjacent continental blocks in Africa, India, Australia, New Zealand and South America. Together, this can improve understanding of the mantle contribution to glacial isostatic adjustment and geodynamic models to show how the Antarctic mantle fits with adjacent continents in the puzzle of lithospheric blocks. Numerous, fundamental and important research questions remain unanswered making further study of the Antarctic mantle an exciting prospect for future research.

Interaction of a Spreading Mantle Plume Head and the Ancient Lithosphere: Studying Mantle Xenoliths in Basalts and Lamprophyre Diatremes of Western Syria

Abstract —Our study of mantle xenoliths in the Cretaceous lamprophyre diatremes and late Cenozoic plateau basalts of western Syria has shown that the ancient lower crust that existed in the Cretaceous and was composed of garnet granulites and eclogite-like rocks was replaced by mantle peridotites in the late Cenozoic. We conclude that the heads of the local (secondary) plumes of the present-day Afro-Arabian thermochemical mantle plume responsible for the regional basaltic magmatism reached the basement of the ancient upper sialic crust, where they spread, leading to a displacement of the mafic lower crust.

Isopycnicity of cratonic mantle restricted to kimberlite provinces

The isopycnicity hypothesis states that the lithospheric mantle of ancient platforms has a unique composition such that high density due to low lithosphere temperature is nearly compensated by low-density composition of old cratonic mantle. This hypothesis is supported by petrological studies of mantle xenoliths hosted in kimberlite magmas. However, the representativeness of the kimberlite sampling may be questioned, given that any type of magmatism is atypical for stable regions. We use EGM2008 gravity data to examine the density structure of the Siberian lithospheric mantle, which we compare with independent constraints based on free-board analysis. We find that in the Siberian craton, geochemically studied kimberlite-hosted xenoliths sample exclusively those parts of the mantle where the isopycnic condition is satisfied, while the pristine lithospheric mantle, which has not been affected by magmatism, has a significantly lower density than required by isopycnicity. This discovery allows us to conclude that our knowledge on the composition of cratonic mantle is incomplete and that it is biased by kimberlite sampling which provides a deceptive basis for the isopycnicity hypothesis.

Diamondiferous Paleoproterozoic mantle roots beneath Arctic Canada: A study of mantle xenoliths from Parry Peninsula and Central Victoria Island

While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes.

Lithospheric layering beneath the contiguous United States constrained by S-to-P receiver functions

To image upper mantle seismic discontinuities beneath the contiguous United States, a total of 284,121 S-to-P receiver functions (SRFs) recorded by 3,594 broadband seismic stations in the EarthScope Transportable Array and other permanent and temporary deployments are stacked in circular bins of 2° in radius. A robust negative arrival, representing a sharp discontinuity of wave speed reduction with depth, is visible in virtually all the stacked traces in the depth range of 30–110 km. Beneath the western U.S., the mean depth of this discontinuity is 69±17 km, and beneath the eastern U.S., it is 76±5 km, both are comparable to the depth of the tomographically-determined lithosphere–asthenosphere boundary (LAB). In contrast, the depth of the discontinuity beneath the stable cratonic region of the central U.S. is 87±6 km, which is significantly shallower than the ∼250 km LAB depth determined by seismic tomography. Based on the amplitude of the corresponding arrival in the SRFs and findings from previous seismic tomography and mantle xenolith studies, this discontinuity beneath the central U.S. is interpreted as the top of an intra-lithospheric low wave speed, probably phlogopite-rich layer. The observations provide new constraints on a number of regional scale tectonic processes, such as lithospheric stretching in the Texas–Louisiana Gulf Coastal Plain and the Basin and Range Province, and possible lithospheric basal erosion beneath the northeastern U.S.

Diffusion-driven magnesium and iron isotope fractionation at a gabbro-granite boundary

Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie Orogen, China. Chilled margin and granitic veins at the gabbro side and gabbro xenoliths in the granite indicate the two intrusions were emplaced simultaneously. The δ26Mg decreases from −0.28 ± 0.04‰ to −0.63 ± 0.08‰ and δ56Fe increases from −0.07 ± 0.03‰ to +0.25 ± 0.03‰ along a ∼16 cm traverse from the contact to the granite. Concentrations of major elements such as Al, Na, Ti and most trace elements also systematically change with distance to the contact. All the observations suggest that weathering, magma mixing, fluid exsolution, fractional crystallization and thermal diffusion are not the major processes responsible for the observed elemental and isotopic variations. Rather, the negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg# reflect Mg-Fe inter-diffusion driven isotope fractionation, with Mg diffusing from the chilled gabbro into the granitic melt and Fe oppositely. The diffusion modeling yields a characteristic diffusive transport distance of ∼6 cm. Consequently, the diffusion duration, during which the granite may have maintained a molten state, can be constrained to ∼2 My. The cooling rate of the granite is calculated to be 52–107 °C/My. Our study suggests diffusion profiles can be a powerful geospeedometry. The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale, with implications for Mg and Fe isotope study of mantle xenoliths, mafic dikes, and inter-bedded lavas.

Sub-continental lithospheric mantle deformation in the Yerer-Tullu Wellel Volcanotectonic Lineament: A study of peridotite xenoliths

Volumetrically, the lithospheric mantle comprises the bulk of the continental lithosphere, yet the mechanisms by which the lithospheric mantle is deformed during rifting are unresolved. Stretching and thermo-mechanical erosion are often cited mechanisms for facilitating lithospheric deformation during continental rift development; however, the infiltration of melt into the lithosphere during rift development also results in chemical alteration of the lithospheric mantle. The purpose of this study is to test the potential mechanisms by which the continental lithospheric mantle is chemically altered during rift development. Here we present a study of mantle xenoliths derived from the lithospheric mantle in Ethiopia, which has been deformed during rifting. The data collected suggest that the lithospheric mantle beneath this region exhibits evidence of focused magma-lithosphere interaction, resulting in four distinct types of peridotite xenoliths: a) deformed xenoliths representing the ancestral lithospheric mantle; b) granular xenoliths representing overprinted lithospheric mantle; c) replacement dunite xenoliths that are evidence of pervasive melt-lithosphere interaction; and d) cumulate xenoliths representing remnants of a metasomatic agent. The deformed xenoliths have a high Mg# (>89) and exhibit little, if any, interaction with melt. The remaining xenolith groups exhibit lower Mg# (<89), suggestive of magma-lithosphere interaction. The high Ni content in olivine and depleted incompatible elements concentrations in orthopyroxene of the granular xenoliths are inconsistent with simple metasomatic enrichment. Furthermore, the presence of dunite containing olivine with low Ca and Sc, and spinel with a Cr# of ~60 is suggestive of a replacement dunite. These data support a melt channeling model over a pervasive chromatographic metasomatism event. Given that the xenoliths examined in this this study are derived from a lithospheric shear zone, we conclude that melt channeling is likely facilitated by the focusing of melts along topography at the lithosphere-asthenosphere boundary, and shear-induced porosity.

A new geophysical model of the Serbian part of the East Vardar ophiolite: Implications for its geodynamic evolution

The geotectonic setting and emplacement style of ophiolites are very significant for an understanding of the geological evolution of any region. In this context, it is essential to know the true tectonic relationship between ophiolites and adjacent units as accurately as possible. This study focuses on the Balkan sector, where Mesozoic convergence/collision processes left behind several NNW–SSE stretching ophiolite belts. The easternmost belt, known as the East Vardar Zone (EVZ), differs from other Balkan ophiolites (i.e. West Vardar) in having pronounced supra-subduction affinity as well as in lacking large peridotitic masses, metamorphic sole assemblages, and clear evidence of a westward obduction. This study was aimed at unraveling the true structural position of the EVZ ophiolites and the surrounding units. We applied 2D modeling of gravity and magnetic data and pseudo 3D modeling of the two largest exposures of the East Vardar Zone (EVZ) ophiolites. The methodology included: qualitative (remote sensing and field observation data as well as mathematical transformations of geophysical data), and quantitative analysis (2D modeling of both, aeromagnetic and gravity data along six cross-sections and the creation of pseudo 3D models). The obtained results invariably suggested that the EVZ ophiolites are dipping to the east beneath the Serbo-Macedonian Massif (SMM). The buried ophiolite bodies display steep contacts in shallow levels, but the contacts become much less steep with depth. The models are considered robust for the first 2–3km, however, there are indications that the EVZ ophiolites exist at much greater depths, as well. Our geophysical–geological model implies that the mode of emplacement of the EVZ ophiolites was related to accretion/underthrusting and it was strikingly different from the emplacement of the other Balkan ophiolites that were uniformly obducted toward the west. This scenario is supported by the independent evidence provided by mantle xenolith studies and studies of lamproitic lavas, which indicated that at least parts of the mantle underneath the Tethyan Mesozoic suture are compositionally more similar to oceanic supra-subduction mantle.

Depleted and enriched matter in the upper mantle of Spitsbergen: Evidence from the study of mantle xenoliths

Depleted and enriched matter in the upper mantle of Spitsbergen: Evidence from the study of mantle xenoliths

Mantle-derived indicator mineral compositions as applied to diamond exploration

ABSTRACT Diamond exploration and mantle studies have pushed geochemical methods to very high resolution, which is useful both in evaluating kimberlites and in describing the subcontinental lithosphere. Developing these methods initiated with direct comparisons of distinctive indicator minerals from kimberlite and sediment concentrates to unusual compositions of the same minerals included in diamond. More recently, single grain thermobarometer methods have been developed based on simplifications of equilibrium phase studies of mantle xenoliths and high pressure experiments. These single grain methods have enhanced evaluation of indicator mineral concentrates for diamond potential by greatly increasing the proportion of each suite that provides useful data on mantle conditions. Concurrently, these methods have broadened studies of the subcontinental lithosphere by permitting detailed evaluations beyond the occurrence of kimberlites with well studied mantle xenoliths. In this review, simple comparison of diamond inclusion compositions for Cr-pyropes, eclogitic garnets, chromite, chrome diopside and enstatite is followed by a review of the concepts behind equilibrium phase thermobarometry, an underlying foundation of increasingly widely applied single grain thermobarometry methods. Descriptions of such single grain methods based on peridotitic chrome diopside, pyrope garnet saturated with chromium and Ni in Cr-pyrope are followed by brief examples of the power of these methods to describe the subcontinental lithospheric mantle.

About this title ‐ Petrological Evolution of the European Lithospheric Mantle

Several different databases and models have been developed over many years of petrological study carried out by several European and non-European groups on mantle xenoliths, peridotite massifs, ophiolites and mafic magmas spanning in age from Archaean to Recent times. This volume aims to bring together these different approaches and to integrate the geochemical perceptions of the European upper mantle. The papers include regional petrological studies of the European lithospheric mantle, from Spain to the Pannonian Basin, from Corsica and Serbia as far north as Svalbard. Six contributions are based on studies of mantle xenoliths, while the remaining three deal with ophiolitic and peridotitic complexes. A further article provides an update on the textural classification of mantle rocks using a computer-aided approach and there is an introductory overview.

Ancient metasomatism recorded by ultra-depleted garnet inclusions in diamonds from DeBeers Pool, South Africa

Peridotitic garnet inclusions in diamonds from the DeBeers Pool mines represent samples from an extremely depleted mantle source. A large number of garnet inclusions have CaO contents of less than 1.8 wt.% and olivine inclusions are characterized by very high Mg-numbers (average: 94.6). These observations suggest that initial melt depletion in the protolith for the cratonic root beneath Kimberley (Kaapvaal Craton) exceeded orthopyroxene exhaustion (i.e. ≥ 50% melt extraction) and hence went farther than typically observed for cratonic peridotites worldwide. This signature of extreme depletion is not readily apparent from the study of mantle xenoliths from the same area and strongly underlines the unique role that diamond may play as a time capsule. Analyses of rare earth and other trace elements show that DeBeers Pool garnet inclusions, despite their ultra-depleted major element composition, were affected by metasomatic enrichment processes similar to those recorded by inclusions in diamonds worldwide. The DeBeers Pool garnet inclusions have a wide range of REE concentrations with variable sinuosity of their chondrite normalized abundance patterns. Sinuosity of garnet REE N patterns generally decreases with increasing fertility (i.e. higher Ca Ti, Fe, Y, and Zr); however, a distinction between the REE N signatures of harzburgitic and lherzolitic garnets is not possible. The REE signatures indicate that the source rocks of these garnets were affected by two phases of metasomatism. LREE–MREE enrichment is preserved by all garnets and indicates interaction with a fluid of highly fractionated character. The variable LREE–MREE concentrations of the garnet inclusions result from progressive evolution of the fluid to higher LREE/HREE ratios during continued equilibration with or precipitation of garnet. This metasomatic fluid was pervasive and affected the entire depth section of the lithosphere represented by the DeBeers Pool inclusions. A second phase of enrichment by HREE- and other HFSE-carrying melts, that also transported major elements (Ca and Fe), affected only a portion of the diamond source region and was restricted to comparatively shallower lithosphere at depths < 180 km. Diamond precipitation was likely associated with both styles of metasomatic re-enrichment. The similarity of lherzolitic and harzburgitic REE N patterns and their similarly variable REE concentrations suggests diamonds of both parageneses were formed concurrently during both phases of metasomatic re-enrichment.

Intraplate continental deformation: Influence of a heat-producing layer in the lithospheric mantle

Deformation of the continental lithosphere is traditionally modeled assuming that heat production in the subcontinental lithospheric mantle is negligible. Although this may be appropriate for highly depleted Archean lithosphere, studies of mantle xenoliths and lithospheric-derived mafic rocks suggest that heat productions of up to 0.4 μW/m 3 may be attained in the mantle of Phanerozoic and Neoproterozoic regions due to modal metasomatism. To investigate the effect of a local enrichment in heat-producing elements within the lithospheric mantle on the continental deformation, we performed a series of 2D thermo-mechanical models simulating the deformation of a lithospheric plate 1000 km long, which contains in its central part a 200 km wide and 20 km-thick mantle layer with a heat production of 0.05–0.25 μW/m 3 between 40 and 80 km depth, leading to a local enhancement of the surface heat flow by 1–5 mW/m 2. Compression models show that a 20 km-thick layer with a heat production as low as 0.05 μW/m 3 within the shallow lithospheric mantle leads to strain localization in both the enriched mantle and the overlying crust. Strain localization depends exponentially on the temperature increase in the lithosphere section, which is controlled by the volume of metasomatized mantle and the intensity of the metasomatic enrichment in heat-producing elements. A heat production of 0.25 μW/m 3 between 60 and 80 km depth results, for instance, in strains within and above the metasomatized mantle 5 times higher than in the surrounding lithosphere. Strain localization also depends on the location of the heat-producing domain and on the plate rheological structure; deeper heat-producing domains and weaker plates leading to stronger localization. These results provide an explanation for strain localization in intraplate environments, leading to the formation of orogenic belts situated hundreds to thousands of kilometers away from known plate boundaries, as observed in the Neoproterozoic belts of Gondwana and in the present-day deformation of the Asian plate.

Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths

Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H 2O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H 2O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.

Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations

Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms and rheology may vary over short spatial (shear zone) and temporal (earthquake cycle) scales.

High-field-strength elements in carbonatitic rocks: Geochemistry, crystal chemistry and significance for constraining the sources of carbonatites

Carbonatites and related rocks exhibit a significant variation in their content of high-field-strength elements (HFSE), i.e. Ti, Nb, Ta, Zr and Hf. The average abundances of these elements (calculated for 119 localities worldwide) decrease from phoscorites and silicocarbonatites to calciocarbonatites to magnesiocarbonatites. The analytical data currently available for ferrocarbonatites are insufficient to establish if this trend persists through the entire carbonatitic series. In comparison with the primitive mantle, the average carbonatite has significantly higher Nb/Ta and Zr/Hf ratios, and lower Zr/Nb and Zr/Ta ratios (35, 60, 1 and 29, respectively). Alkali-ultramafic rocks associated with carbonatites exhibit, on average, higher Zr/Nb ratios and no (or little) depletion in heavy HFSE (Ta and Hf). Implications of these data for constraining the source(s) of intracratonic carbonatites are discussed using the Kola Alkaline Province (NW Russia and Finland) as an example. The bulk of incompatible HFSE (> 85%) is estimated to reside in Ti, Nb and Zr oxide minerals (in particular, perovskite, pyrochlore, ilmenite, baddeleyite and zirconolite) and zircon, whereas a significant proportion of Ti is contained in magnetite and ferromagnesian silicates. The lowest recorded Nb/Ta and Zr/Hf ratios in the primary HFSE oxides and titanite (but not zircon) are significantly below the primitive-mantle values, which makes these phases an effective vehicle of heavy-HFSE fractionation at crustal levels. The HFSE geochemistry of carbonatites en masse is not consistent with their derivation from a hypothetical carbonate-rich alkali-ultramafic (e.g., nephelinitic) magma by either liquid immiscibility or crystal fractionation. The bulk of carbonatites are interpreted to originate by low-degree partial melting of a metasomatized HFSE 5+-enriched mantle source with elevated Nb/Ta and Zr/Hf ratios. The trace-element signature of carbonatites cannot be adequately explained by clinopyroxene-controlled decoupling of HFSE 5+ and HFSE 4+ during partial melting, and requires the presence of amphibole and, possibly, refractory phlogopite in their mantle source. The HFSE geochemistry, field relations and relative timing of Kola carbonatites and associated silicate rocks are consistent with their derivation by low-degree melting of a vertically heterogeneous amphibole-bearing mantle reservoir, followed by differentiation of the primitive magmas. The previously established volume, tectonic and geochemical constraints suggest that the metasomatic enrichment of the mantle may have resulted from reaction of garnet lherzolite with carbonate melts of asthenospheric provenance, which is in agreement with the published experimental and mantle-xenolith studies. There is no evidence for the presence of refractory rutile (i.e., subducted crust) in the postulated mantle source.