Composition Of Lithospheric Mantle Research Articles

Thermal state of the Siberian craton marginal zone at the time of Mesozoic kimberlitic magmatism within the Kuoika field (Yakutian diamondiferous province)

Research subject. Clinopyroxene xenocrysts from the heavy-mineral concentrates of 14 kimberlite bodies (western part of the field: Obnazhennaya, Rubin, Seraya, Vodorazdelnaya, Vtorogodnitsa, Antoshka; central part of the field: Sliudyanka, Skiff II; western part of the field: Irina, Noyabrskaya, Vechernyaya, Lyusya, Dianga and Zhila 79) of the Kuoika field (Yakutian diamondiferous province, Siberian craton). Aim. To reconstruct the mantle paleogeotherms Under the six kimberlite pipes (Vodorazdelnaya, Obnazhennaya, Vtorogodnitsa, Sliudyanka, Dianga, and Zhila 79) using two independent approaches. Materials and methods. The chemical composition of clinopyroxene xenocrysts was investigated and last equilibrium temperatures and pressures were estimated using single-crystal thermobarometry. The geothermal line fitting to the P-T data set was performed using two methods. The first method is based on the D. Hasterok and D.S. Champan model, and the second - on the D. McKenzie model. Results. The value of lithosphere thickness falls within the margin of error for the two methods, being also comparable with earlier reconstructions for the Vtorogodnitsa, Dyanga, and Obnazhennaya pipes. The obtained results indicate that, during the Mesozoic kimberlitic magmatism, the lithosphere thickness beneath the Kuoika field was about 200 km. Conclusion. The determined peculiarities of the chemical composition of clinopyroxene xenocrysts indicate heterogeneity of the lithospheric mantle composition. Different depths of transported mantle material for kimberlite pipes of the Kuoika field, which are coeval and are located a few km apart, may be related to peculiarities of kimberlite magma ascent to the surface and the presence of intermediate magma chambers. The eastern part of the Kuoika field contains more garnet and garnet-spinel peridotites compared to the central and western parts, which may indirectly indicate a greater diamondiferous potential of the eastern block, where the diamondiferous Dianga pipe is located. The absence of diamonds in other discovered pipes of the Kuoika field may be connected with the metasomatic enrichment of the lithospheric mantle in the area of the “diamond window”, which is confirmed by a large number of high-temperature clinopyroxenes at these depths.

Supercontinent-paced magmatic destabilisation and recratonisation of the Yilgarn Craton

Knowledge of the evolution of ancient cratonic lithospheres underpins our understanding of Precambrian Earth. The Yilgarn Craton has exceptionally well-preserved Archean geology, with juvenile crust formation and major orogenesis concluding in the Neoarchean, and a stabilised upper-crustal architecture developing before 2.42 Ga. However, in an apparent dichotomy, geophysical models resolve lithospheric mantle composition outside the range of xenolith data from Archean regions, indicating the lithospheric mantle has since been extensively refertilised. Post-Archean igneous and sedimentary rocks record a prolonged lithospheric evolution that is not well resolved in datasets recording bulk crustal isotopic evolution. Reconciling these, we combine interpretation of geological and geophysical data to resolve two phases of lithosphere destabilisation driven by major magmatic events at ∼2.06 Ga and at ∼1.08 Ga. During destabilisation, sub-lithospheric and sub-crustal mantle fluxes caused extensive mantle refertilisation. For 200–400 Ma post-refertilisation, distributed sedimentary basins formed during recratonisation of the now denser lithosphere. The timing of these events suggests a relationship with the early stages of supercontinent assembly: Dominant downwelling beneath the assembling supercontinent sustains a sufficiently non-tensile tectonic setting to inhibit lithospheric thinning and breakup and enhances lateral flow of any upwelling mantle. This setting allows widespread intraplate refertilisation to occur while later the assembled supercontinent provides a stable setting allowing thermal re-equilibration and recratonisation to occur. In contrast, lithospheric refertilisation during supercontinent breakup will be more susceptible to density instabilities and recycling in later collisions. Consequently, we suggest that refertilisation of extant cratonic lithosphere may dominantly have occurred during the assembly of supercontinents.

Subduction-related mantle accretion and makeover revealed by mantle xenoliths at the Pacific margin of NE Eurasia

Subduction is thought to be a major factor in the origin and makeover of continental lithospheric mantle, but essential aspects of these processes continue to be debated. Here, we report textural, chemical and Re Os isotope data on mantle xenoliths from far eastern Russia located at different distances from the Asian Pacific coast to examine how the proximity to subduction zones affects lithospheric mantle compositions. The xenoliths come from the Khanka block close to the border with China and several sites in the Sikhote-Alin orogenic belt (SAOB) near the Japan Sea coast. The inboard Khanka xenoliths are spinel lherzolites with chemical compositions, suggesting that they are residues of low- to moderate-degrees of melt extraction from fertile mantle and essentially unaffected by later events for 1.9 Ga. Such rocks are also common among peridotites in the SAOB, located closer to the subduction margins, but the SAOB suite also includes: a) heterogeneous rocks with veins and patches of pyroxenites or hydrous minerals, and b) rocks enriched in iron (8.8–13.5 wt%), Al 2 O 3 and incompatible minor elements. The positive correlation between elevated 187 Os/ 188 Os ratios (>0.135) and FeO contents in some Fe-rich SAOB xenoliths is interpreted to indicate that the metasomatic melts were derived from the asthenosphere hybridized with recycled materials from the Pacific slab. The lack of correlation between Re -depletion Os model ages (T RD ) and depletion indices, as well as the similarity of Os isotopic compositions between the SAOB peridotites and abyssal peridotites are interpreted to show that the SAOB mantle possibly formed and accreted to Eurasia from the asthenosphere during eastward migration of the Paleo-Pacific subduction trench. Based these results, we infer that the SAOB lithospheric mantle experienced multi-stage metasomatism after its formation, the most significant of which is localized infiltration of subduction-related basaltic melts with recycled characteristics induced by the extension of Japan Sea and release of volatiles from the slab. By contrast, the inboard Khanka lithospheric mantle shows little, if any, evidence for metasomatism. Such contrasting metasomatic modification implies different degrees and styles of makeover processes at different distances relative to the subduction margins. • The Khanka mantle was little affected by metasomatism since it was formed at ~1.9 Ga. • The Sikhote-Alin lithospheric mantle was formed by subduction and accretion. • Mantle metasomatism beneath Sikhote-Alin was caused by melts with recycled signature. • The nature and degrees of metasomatism are related to the distance from subduction front.

MINERAL COMPOSITION AND PT PARAMETERS OF CRYSTALLIZATION OF MANTLE ROCKS UNDER KIMBERLITE FIELDS OF THE ANABAR REGION

The composition of barophilic minerals from mantle xenoliths and Cpx from concentrates of the Kuranakh, Luchakan, Dyuken, and Ary-Mastakh fields of the Anabar region has been studied. Under these fields, the lithospheric mantle compositions vary significantly. The PT parameters of crystallization were calculated using the composition of clinopyroxenes from xenoliths and heavy fraction of kimberlites. The lithospheric mantle rocks under the northern fields have higher values of Mg# of minerals and calculated mantle geotherm (35–48 mW/m2) compared to the parameters for the southern diamond fields. The pipes from the southwestern part of the Ary-Mastakh field are promising for the diamond potential, presenting Grt-bearing lherzolites and harzburgites with a high content of Cr2O3 (to 8.5 wt. %).

Desilicification Rims of Zircon Xenocrysts Record the Timing of Kimberlite Emplacement

AbstractKimberlites are deep and relatively rare mantle‐derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle‐derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U–Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70–2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb–Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology.

Formation mechanism of the North–South Gravity Lineament in eastern China

Gravity lineaments are well recognized along plate boundaries and in the interiors of oceanic plates. However, the formation mechanism of intracontinental gravity lineaments remains poorly understood. The North–South Gravity Lineament (NSGL), which extends >4000 km from Russia to South China, is characterized by an abrupt change in the observed Bouguer gravity anomaly, ranging from −100 mGal in the west to −40 mGal in the east over a distance of <100 km. Here we employ gravity modeling along a deep seismic sounding profile that transects the NSGL to delineate the relative contributions of the thickness and composition of the crustal and upper mantle and the stagnant slab in the mantle transition zone to the observed gravity anomaly, and to ultimately constrain the formation mechanism of the NSGL. Our results indicate that crustal thickness is the dominant factor in the formation of the NSGL, with lithospheric composition also playing a role. Combining our results with those of previous studies, lateral variations in the lithospheric mantle composition and crustal/lithospheric thickness across the NSGL can be attributed to lithospheric thinning and mantle replacement in the eastern part of the North China Craton during the late Mesozoic. This was triggered by the subduction of the Paleo-Pacific plate and subsequently led to the persistence of a stagnant slab and big mantle wedge. This study highlights the fundamental need for integrated geophysical–petrological analyses to better understand the surface response to deep tectonic processes.

Marie Byrd Land lithospheric mantle: a review of the xenolith record

Abstract The Marie Byrd Land (MBL) lithospheric mantle xenolith record comprises over 100 samples from a range of localities spanning both major crustal terranes that comprise MBL: Ross and Amundsen provinces. Coarse granular to porphyroclastic in texture, the xenoliths are predominantly Type I spinel-bearing lherzolites to harburgites, but include rare dunite and pyroxenite examples. Garnet is absent and no hydrous phases, such as amphibole or mica, have been reported to date, although traces of apatite may be present. Characterisation of the lithospheric mantle composition and its evolution however, is hampered by patchy and uneven geochemical analyses across the xenolith suite. Nonetheless, a picture emerges of a heterogeneous lithosphere beneath both Ross and Amundsen Provinces. Previously published and new data reported here are consistent with samples ranging from variably cryptically metasomatized residua from variable (10–25%) degrees of partial melt extraction to refertilized compositions. Limited isotopic data point to a complex history, providing evidence for both ancient Proterozoic lithospheric mantle and preservation of Ordovician events. The Sr–Nd–Pb composition of the sampled lithospheric mantle overlaps the common low-µ isotopic endmember identified in Cenozoic magmatism from MBL and the wider West Antarctic Rift System.

Ultramafic mantle xenoliths in the Late Cenozoic volcanic rocks of the Antarctic Peninsula and Jones Mountains, West Antarctica

Abstract Abundant mantle-derived ultramafic xenoliths occur in Cenozoic (7.7–1.5 Ma) mafic alkaline volcanic rocks along the former active margin of West Antarctica, that extends from the northern Antarctic Peninsula to Jones Mountains. The xenoliths are restricted to post-subduction volcanic rocks that were emplaced in fore-arc or back-arc positions relative to the Mesozoic–Cenozoic Antarctic Peninsula volcanic arc. The xenoliths are spinel-bearing, include harzburgites, lherzolites, wehrlites and pyroxenites, and provide the only direct evidence of the composition of the lithospheric mantle underlying most of the margin. The harzburgites may be residues of melt extraction from the upper mantle (in a mid-ocean ridge type setting), that accreted to form oceanic lithosphere, which was then subsequently tectonically emplaced along the active Gondwana margin. An exposed highly depleted dunite–serpentinite upper mantle complex on Gibbs Island, South Shetland Islands, supports this interpretation. In contrast, pyroxenites, wehrlites and lherzolites reflect percolation of mafic alkaline melts through the lithospheric mantle. Volatile and incompatible trace element compositions imply that these interacting melts were related to the post-subduction magmatism which hosts the xenoliths. The scattered distribution of such magmatism and the history of accretion suggest that the dominant composition of sub-Antarctic Peninsula lithospheric mantle is likely to be harzburgitic.

Oceanic lithosphere heterogeneity in the eastern Paleo-Tethys revealed by PGE and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite

Platinum group elements (PGE) and Re–Os isotopes of mantle peridotites in the Jinshajiang ophiolite (SW China) were investigated in this study, in order to constrain the evolution of the lithospheric mantle beneath the Jinshajiang-Ailaoshan Ocean, which was a branch of the eastern Paleo-Tethys. The Jinshajiang peridotites have whole-rock compositions (e.g., MgO = 32.7–38.1 wt.%; Al2O3 = 0.67–1.30 wt.%) and spinels with moderate Cr# values (0.4–0.6) similar to those of abyssal peridotites, which indicate moderate degrees of partial melting (15%–20%). These peridotites exhibit U-shaped chondrite-normalized REE patterns that could be caused by hydrothermal alteration or melt-rock interaction after mantle melting. In addition, Pd concentrations and (Pd/Ir)N ratios of the Jinshajiang peridotites increases with decreasing Al2O3 concentrations. These negative correlations cannot be explained by simple partial melting but record a melt-rock reaction event after mantle melting. This study therefore demonstrates the efficiency of PGE in detecting the melt-rock reaction process relative to whole-rock major and trace elements. The suprachondritic 187Os/188Os ratios (0.1272–0.1374) further indicate that the later percolating melt derived from a mantle domain with distinct 187Os-enriched isotopic compositions. In comparison with peridotites in the Ailaoshan ophiolite belt, which were not significantly affected by melt percolation, this study further highlights that the lithospheric mantle compositions beneath different segments of the same ocean basin are highly variable and might be controlled by distinct mantle processes in response to different rifting mechanisms.

The role of lithospheric heterogeneity on the composition of kimberlite magmas from a single field: The case of Kaavi-Kuopio, Finland

Kimberlites are complex, ‘hybrid’ igneous rocks because their parental magmas entrain abundant crust- and mantle-derived components that can be readily assimilated during ascent to surface. Recent studies of olivine zonation patterns have shown compositional relationships between xenocrystic cores and magmatic rims, suggesting that kimberlite melt compositions might be controlled by assimilation of mantle material during emplacement. However, the nature and extent to which this process, as well as assimilation of crustal material, influences melt compositions within single kimberlite fields remains unclear. To address this issue, we have conducted a comprehensive geochemical and petrographic investigation of kimberlites from eight pipes in the Kaavi-Kuopio field in Finland, which were emplaced on the southern margin of the Karelian craton during the Neoproterozoic (~550–600 Ma).While magmatic olivine rims are usually homogeneous in composition within and between kimberlites of a single cluster and field (e.g., Lac de Gras), the Kaavi-Kuopio kimberlites appear to represent a unique case where there are statistically significant differences between the average Mg# of olivine rims in different pipes (89.9 ± 0.2 to 88.5 ± 0.3). Importantly, the Mg# of magmatic olivine rims exhibit a strong correlation with the Mg# of their mantle-derived xenocrystic cores. Furthermore, the compositions of olivine cores and rims exhibit a robust relationship with those of magmatic spinel (e.g., Mg#, TiO2 contents). These geochemical variations also align with the mineralogy of the kimberlites: whereby abundances of phlogopite and oxides (e.g., spinel) are negatively correlated with olivine rim Mg#. The robust relationship between entrained and assimilated lithospheric mantle material (i.e. olivine cores) and magmatic components (i.e. olivine rims, spinel, and groundmass mineral abundance), combined with numerical modelling suggests that up to 10 wt% assimilation of lithospheric mantle material has modified the compositions of the Kaavi-Kuopio kimberlites. These new data are also consistent with significant variations in the lithospheric mantle composition of the Karelian craton beneath the closely spaced (<10 km) kimberlites. Finally, in addition to mantle assimilation, formation of Si-Fe-rich mica in some of the examined kimberlites might be linked to late-stage increases in oxygen fugacity potentially enhanced by crustal contamination. This study shows for the first time that variable assimilation of mantle and crustal material can generate significant variations in kimberlites derived from seemingly similar sources.

Initiation of the North China Craton destruction: Constraints from the diamond-bearing alkaline basalts from Lan'gan, China

The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646–0.70925, εNd(t) = −2.1 to −4.9, 206Pb/204Pb(t) = 17.14–18.12, 207Pb/204Pb(t) = 15.28–15.61, 208Pb/204Pb(t) = 37.82–38.67, and zircon εHf(t) = −17 to −21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.

Potential role of lithospheric mantle composition in the Wilson cycle: a North Atlantic perspective

AbstractAlthough the Wilson cycle is usually considered in terms of wide oceans floored with normal oceanic crust, numerous orogens result from the closure of embryonic oceans. We discuss how orogenic and post-orogenic processes may be controlled by the size/maturity of the inverted basin. We focus on the role of lithospheric mantle in controlling deformation and the magmatic budget. We describe the physical properties (composition, density, rheology) of three types of mantle: inherited, fertilized and depleted oceanic mantle. By comparing these, we highlight that fertilized mantle underlying embryonic oceans is mechanically weaker, less dense and more fertile than other types of mantle. We suggest that orogens resulting from the closure of a narrow, immature extensional system are essentially controlled by mechanical processes without significant thermal and lithological modification. The underlying mantle is fertile and thus has a high potential for magma generation during subsequent tectonic events. Conversely, the thermal state and lithology of orogens resulting from the closure of a wide, mature ocean are largely modified by subduction-related arc magmatism. The underlying mantle wedge is depleted, which may inhibit magma generation during post-orogenic extension. These end-member considerations are supported by observations derived from the Western Europe–North Atlantic region.

Geophysical‐petrological model of the crust and upper mantle in the India‐Eurasia collision zone

AbstractWe present a new crust and upper mantle cross section of the western India‐Eurasia collision zone by combining geological, geophysical, and petrological information within a self‐consistent thermodynamic framework. We characterize the upper mantle structure down to 410 km depth from the thermal, compositional, and seismological viewpoints along a profile crossing western Himalayan orogen and Tibetan Plateau, Tarim Basin, Tian Shan, and Junggar Basin, ending in the Chinese Altai Range. Our results show that the Moho deepens from the Himalayan foreland basin (~40 km depth) to the Kunlun Shan (~90 km depth), and it shallows to less than 50 km beneath the Tarim Basin. Crustal thickness between the Tian Shan and Altai mountains varies from ~66 km to ~62 km. The depth of the lithosphere‐asthenosphere boundary (LAB) increases from 230 km below the Himalayan foreland basin to 295 km below the Kunlun Shan. To NE the LAB shallows to ~230 km below the Tarim Basin and increases again to ~260 km below Tian Shan and Junggar region and to ~280 km below the Altai Range. Lateral variations of the seismic anomalies are compatible with variations in the lithospheric mantle composition retrieved from global petrological data. We also model a preexisting profile in the eastern India‐Eurasia collision zone and discuss the along‐strike variations of the lithospheric structure. We confirm the presence of a noticeable lithospheric mantle thinning below the Eastern Tibetan Plateau, with the LAB located at 140 km depth, and of mantle compositional differences between the Tibetan Plateau and the northern domains of Qilian Shan, Qaidam Basin, and North China.

From the North-Iberian Margin to the Alboran Basin: A lithosphere geo-transect across the Iberian Plate

A ~1000-km-long lithospheric transect running from the North-Iberian Margin to the Alboran Basin (W-Mediterranean) is investigated. The main goal is to image the changes in the crustal and upper mantle structure occurring in: i) the North-Iberian margin, whose deformation in Alpine times gave rise to the uplift of the Cantabrian Mountains related to Iberia–Eurasia incipient subduction; ii) the Spanish Meseta, characterized by the presence of Cenozoic basins on top of a Variscan basement with weak Alpine deformation in the Central System, and localized Neogene–Quaternary deep volcanism; and iii) the Betic–Alboran system related to Africa–Iberia collision and the roll-back of the Ligurian–Tethyan domain. The modeling approach, combines potential fields, elevation, thermal, seismic, and petrological data under a self-consistent scheme. The crustal structure is mainly constrained by seismic data whereas the upper mantle is constrained by tomographic models. The results highlight the lateral variations in the topography of the lithosphere–asthenosphere boundary (LAB), suggesting a strong lithospheric mantle strain below the Cantabrian and Betic mountain belts. The LAB depth ranges from 180km beneath the Cantabrian Mountains to 135–110km beneath Iberia Meseta deepening again to values of 160km beneath the Betic Cordillera. The Central System, with a mean elevation of 1300m, has a negligible signature on the LAB depth. We have considered four lithospheric mantle compositions: a predominantly average Phanerozoic in the continental mainland, two more fertile compositions in the Alboran Sea and in the Calatrava Volcanic Province, and a hydrated uppermost mantle in the North-Iberian Margin. These compositional differences allowed us to reproduce the main trends of the geophysical observables as well as the inferred P- and S-wave seismic velocities from tomography models and seismic experiments available in the study transect. The high mean topography of Iberia can be partly consistent with a low-velocity/high-temperature/low-density layer in the sublithospheric mantle.

Thermal and petrophysical characterization of the lithospheric mantle along the northeastern Iberia geo-transect

We present a new model on the present-day lithospheric structure along a 1100km transect crossing the NE-Iberian Peninsula, the Western Mediterranean basin and the Algeria margin and ends at the Tell–Atlas Mountains. The modeling is based on an integrated geophysical-petrological methodology combining elevation, gravity, geoid, surface heat flow, seismic and geochemical data. Unlike previous models proposed for the region where the density of the lithospheric mantle is only temperature-dependent, the applied methodology allows inferring seismic velocities and density in the mantle down to 410km depth from its chemical composition through self-consistent thermodynamic calculations. We have considered five lithospheric mantle compositions including predominantly average Phanerozoic and lherzolitic Proterozoic in the continental mainland, and more fertile PUM (primitive upper mantle) compositions in the Western Mediterranean basin. Mantle petrology affects the resulting density distribution and LAB (lithosphere–asthenosphere boundary) geometry and allows a direct comparison with tomography models and seismic data. Measured low Pn velocities in the Western Mediterranean basin can be explained by either serpentinization and/or seismic anisotropy and only partly by transient thermal effects. The obtained lithospheric structure is compatible with P- and S-wave tomography models.

Geochemical and Sr–Nd–Pb isotopic compositions of lithospheric mantle: Spinel lherzolites in alkaline basalts from the northwestern Ethiopian plateau

The geochemical characteristics of mantle xenoliths (spinel lherzolites) in Quaternary alkaline basalts from the northwestern Ethiopian plateau provide new insight into the conditions of melting associated with the Afar plume. The major element compositions of the spinel lherzolites are in the range of those of the primitive mantle. The high modal content of the clinopyroxene (14–23%) indicate that the mantle xenoliths are fertile spinel lherzolites that have experienced insignificant partial melting. However, enrichment in highly incompatible elements (LREE, Ba, Pb, Th, and U) and the absence of any typical metasomatic minerals indicate that the spinel lherzolites underwent an event of later cryptic metasomatic enrichment induced by plume–related, hydrous fluid–rich silicate melts. The 87Sr/86Sr (0.70184–0.70324), 143Nd/144Nd (0.51313–0.51338), and 206Pb/ 204Pb (17.17–19.06) values of the clinopyroxene separate from the spinel lherzolites display systematic variations between those of depleted MORB mantle (DMM) and enriched compositions interpreted as related to the Afar mantle plume. The mineral compositions yield an estimated temperature and pressure in the range of 921– 973 °C and 13–17 kbar, respectively, indicating that the mantle xenoliths originated at shallow depths of 40 to 55 km above the Afar mantle plume.

Along-arc variations in lithospheric mantle compositions in Kamchatka, Russia: First trace element data on mantle xenoliths from the Klyuchevskoy Group volcanoes

We provide results of a detailed study of the first peridotite xenoliths of proven mantle origin reported from Bezymyanny volcano in the Klyuchevskoy Group, northern Kamchatka arc. The xenoliths are coarse spinel harzburgites made up mainly of Mg-rich olivine as well as subhedral orthopyroxene (opx) and Cr-rich spinel, and also contain fine-grained interstitial pyroxenes, amphibole and feldspar. The samples are unique in preserving the evidence for both initial arc mantle substrate produced by high-degree melt extraction and subsequent enrichment events. We show that the textures, modal and major oxide compositions of the Bezymyanny xenoliths are generally similar to those of spinel harzburgite xenoliths from Avacha volcano in southern Kamchatka. However, coarse opx from the Bezymyanny harzburgites has higher abundances of light and medium rare earth elements and other highly incompatible elements than coarse opx from the Avacha harzburgites. We infer that (1) the sub-arc lithospheric mantle beneath both Avacha and Bezymyanny (and possibly between these volcanoes) consists predominantly of harzburgitic melting residues, which experienced metasomatism by slab-related fluids or low-fraction, fluid-rich melts and (2) the degrees of metasomatism are higher beneath Bezymyanny. By contrast, xenolith suites from Shiveluch and Kharchinsky volcanoes 50–100km north of the Klyuchevskoy Group include abundant cumulates and products of reaction of mantle rocks with silicate melts at high melt/rock ratios. The high melt flux through the lithospheric mantle beneath Shiveluch and Kharchinsky may be related to the asthenospheric flow around the northern edge of the sinking Pacific plate; lateral propagation of fluids in the mantle wedge south of the plate edge may contribute to metasomatism in the mantle lithosphere beneath the Klyuchevskoy Group volcanoes.

Lithospheric structure in the Baikal–central Mongolia region from integrated geophysical‐petrological inversion of surface‐wave data and topographic elevation

Recent advances in computational petrological modeling provide accurate methods for computing seismic velocities and density within the lithospheric and sub‐lithospheric mantle, given the bulk composition, temperature, and pressure within them. Here, we test an integrated geophysical‐petrological inversion of Rayleigh‐ and Love‐wave phase‐velocity curves for fine‐scale lithospheric structure. The main parameters of the grid‐search inversion are the lithospheric and crustal thicknesses, mantle composition, and bulk density and seismic velocities within the crust. Conductive lithospheric geotherms are computed using P‐T‐dependent thermal conductivity. Radial anisotropy and seismic attenuation have a substantial effect on the results and are modeled explicitly. Surface topography provides information on the integrated density of the crust, poorly constrained by surface waves alone. Investigating parameter inter‐dependencies, we show that accurate surface‐wave data and topography can constrain robust lithospheric models. We apply the inversion to central Mongolia, south of the Baikal Rift Zone, a key area of deformation in Asia with debated lithosphere‐asthenosphere structure and rifting mechanism, and detect an 80–90 km thick lithosphere with a dense, mafic lower crust and a relatively fertile mantle composition (Mg# &lt; 90.2). Published measurements on crustal and mantle Miocene and Pleistocene xenoliths are consistent with both the geotherms and the crustal and lithospheric mantle composition derived from our inversion. Topography can be fully accounted for by local isostasy, with no dynamic support required. The mantle structure constrained by the inversion indicates no major thermal anomalies in the shallow sub‐lithospheric mantle, consistent with passive rifting in the Baikal Rift Zone.

Geochemical syntheses among the cratonic, off-cratonic and orogenic garnet peridotites and their tectonic implications

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr2O3), Cr-rich diopside (0.5–3.5 wt% Cr2O3), Al-poor orthopyroxene (0–5 wt% Al2O3), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al2O3, Cr2O3, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.

Mineral resources and development in the Russian Arctic

The paper concerns issues of geology and metallogeny of the Russian Arctic, namely, (i) limits of the Russian oceanic Arctic in the context of the continental origin of territories under jurisdictional dispute; (ii) geology and tectonic history of the region; (iii) distribution of mineral deposits; (iv) outlook for diamond, PGE, Ni, rare metals, gold, and bauxite resources development. Advanced diamond exploration and development can be expected proceeding from geology of new potentially diamondiferous areas, the Phanerozoic history and composition of lithospheric mantle beneath the Siberian craton which were controlled by the Siberian superplume at the Permian–Triassic boundary, and from new exploration approaches adapted to the prospecting conditions of Arctic Siberia. According to the available knowledge of Ni and PGE mineralization in the Noril’sk region, it is reasonable to develop depleted ores and tailings (mining dumps at the Noril’sk and Talnakh deposits). However, the key solution consists in new large discoveries within the Dzheltula and Kharaelakh volcanic and plutonic complexes. Gold production enhancement may be associated with black shale-hosted Au-As mineralization in the northeastern Russian Arctic, but the problem is in the lack of efficient and environmentally safe dressing technologies for these ores. Most of rare metals in the area (Nb, Sc, Y, and other elements) are stored in the giant Tomtor field, which has a complex structure and history. A special technology designed for the Tomtor ores ensures more than 60% extraction of ore components. Good prospects for the bauxite potential are expected from the Timan district where bauxite may occur in Vendian and Early Carboniferous formations. It is suggested to include the development of the Russian Arctic as a priority target in the national economic strategy.