Mantle Xenoliths Research Articles

Lithospheric structure beneath the boundary region of North China Craton and Xing Meng Orogenic Belt from S-receiver function analysis

The boundary region of the North China Craton (NCC) and Xing-Meng Orogenic Belt (XMOB), including the northern boundary of the NCC and the southern XMOB, is the ideal area to investigate the spatially uneven lithospheric deformation and associated tectonic evolution in the boundary region of the craton and orogen. The depths and structures of the lithosphere–asthenosphere boundary (LAB) and mid-lithosphere discontinuity (MLD) are the key information to describe the nature and deformation pattern of the lithosphere, and thus can provide basic constraints on the associated tectonic processes and mechanism of the lithospheric deformation. Based on waveform data collected from 226 broadband seismic stations, high resolution spatial variations in the LAB and MLD depths were obtained by using the wave equation migration method for the S-receiver functions. The migrated images show a lateral variation in the LAB depth from 100–120 km in the northern boundary of the NCC to 120–140 km in the southern XMOB. The imaged LAB within the NCC is much shallower than that revealed by mantle xenoliths enclosed in the nearby Early Paleozoic kimberlites (~180 km), suggesting that the cratonic lithosphere of the area may have undergone significant thinning during the Phanerozoic evolution. Furthermore, a coherent MLD is identified at depths of 70–90 km beneath the northeastern NCC, which corroborates the speculation that the MLD probably existed in the eastern NCC before lithospheric destruction in the Mesozoic. The spatial variations in the LAB and MLD depths probably reflect complex lithospheric deformation and thinning around the boundary region of NCC and XMOB. The properties, including the gradient thickness and S-wave velocity contrast, of the two discontinuities further indicate that lithospheric deformation and thinning might be related to partial melting induced by the Big Mantle Wedge associated with the subducting Paleo-Pacific plate and its stagnation during the Late Mesozoic.

Effects of melt-percolation, refertilization and deformation on upper mantle seismic anisotropy: constraints from peridotite xenoliths, Marie Byrd Land, West Antarctica

Abstract We report on the petrology, microstructure and seismic properties of 44 peridotite xenoliths extracted from the upper mantle beneath Marie Byrd Land (MBL), West Antarctica. The aim of this work is to understand how melt-rock reaction, refertilization, and deformation affected the seismic properties (velocities, anisotropy) of the West Antarctic upper mantle, in the context of MBL tectonic evolution and West Antarctic Rift System formation. Modal compositions, mineral major element compositions, microstructures, and crystallographic preferred orientations (CPOs) provide evidence for diachronous reactive melt percolation and refertilization. Olivine shows three main CPO patterns, the A-type, axial-[010], and axial-[100] texture types. Average seismic properties of the MBL mantle lithosphere are mainly controlled by the strength of olivine crystallographic texture. Reactive melt percolation and refertilization likely modified seismic velocities and anisotropy, as is suggested by a systematic decrease in maximum P-wave and S-wave anisotropies with increasing modal abundance of pyroxene. At larger spatial scales, the seismic properties of the MBL mantle xenoliths are dominated by the anisotropy resulting from the A-type olivine CPO. Variations between individual volcanic centres, however, attest to spatial variations in the mantle structure, potentially related to 3D deformation and the prolonged tectonic history of MBL.

Microstructural Analysis of a Mylonitic Mantle Xenolith Sheared at Laboratory-like Strain Rates from the Edge of the Wyoming Craton

Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep.

Three-stage modification of lithospheric mantle: Evidence from petrology, in-situ trace elements, and Sr isotopes of mantle xenoliths in the Cenozoic basalts, northeastern North China Craton

Abstract We present mineralogical and geochemical compositions of mantle xenoliths from two Cenozoic basalt localities of the northeastern North China Craton. These xenoliths include lherzolite, harzburgite, and websterite. They are generally fertile in major elements and different from the typical cratonic lithosphere, which is consistent with previous hypotheses regarding craton destruction. The ratios of 87Sr/86Sr and (La/Yb)N of clinopyroxenes (Cpx) in one lherzolite are relatively low in the core but high in the rim. The center of the Cpx grain has a high U concentration. Changes in trace elements and Sr isotopes indicate that later stage high 87Sr/86Sr melt metasomatism superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths are low in Ti/Eu but high in Ca/Al and light rare earth elements, which indicates carbonate melt metasomatism. 87Sr/86Sr is increased in the core and decreased in the rim of most Cpx grains, which reflects the superposition of two-stage metasomatism. The early agent should be high in 87Sr/86Sr, and the recent agent should be low in 87Sr/86Sr. The Cpxs in olivine websterite are low in 87Sr/86Sr (0.70220–0.70320), which reflects the recent metasomatism of asthenosphere-derived melt. Collectively, these observations reflect a three-stage modification of the lithospheric mantle. First-stage hydrous melt/fluid could come from the dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.

Heat Generation in Cratonic Mantle Roots—New Trace Element Constraints From Mantle Xenoliths and Implications for Cratonic Geotherms

AbstractHeat generation within the cratonic lithospheric mantle (CLM) is an important but poorly determined parameter for constructing cratonic geotherms. Direct measurement of heat producing element (HPE: K, U, and Th) concentrations in bulk‐rock samples of cratonic mantle roots—provided as xenoliths in volcanic rocks such as kimberlites—is mostly compromised by infiltration of host melt into the xenoliths, resulting in over‐estimates of heat production. Here we use in situ methods (laser ablation inductively coupled plasma mass spectrometry) on minerals from a variety of cratonic mantle peridotites to avoid host‐magma contamination enabling new, more accurate determinations of heat production for a wide spectrum of model mantle lithologies. The most melt‐depleted, least metasomatized peridotites indicate that heat generation in un‐metasomatized depleted cratonic lithospheric mantle is negligible, at ∼0.00004 µW/m3, 102 to 103 times less than models that use more enriched or metasomatized compositions. Refertilized cratonic peridotites, typical of many kimberlite hosted xenoliths, have more elevated, but still low heat generation, of 0.006 µW/m3. We propose that the heat generation of typical cratonic mantle peridotite lies between these two bounds, that is, between 0.00004 and 0.006 µW/m3. Both values produce lower estimates of lithospheric thickness, by ∼10 to up to 80 km, depending on model assumptions, than estimates using higher HPE concentrations measured on bulk‐rock xenolith material.

“Black” Pyroxenites in Mantle Xenoliths Found in Volcanics of Some Regions in the East Asian Margin. The Evolution and Petrogenesis. Part 2. The Petrologic-Geochemical Composition, toward a Petrogenetic Model

“Black” Pyroxenites in Mantle Xenoliths Found in Volcanics of Some Regions in the East Asian Margin. The Evolution and Petrogenesis. Part 2. The Petrologic-Geochemical Composition, toward a Petrogenetic Model

Siberia’s largest pulse of kimberlites: U-Pb geochronology of perovskite and rutile from the Obnazhennaya kimberlite and its xenoliths, Siberia craton

ABSTRACT Mantle xenoliths from the Middle-Late Jurassic Obnazhennaya kimberlite are often compared with those from the Udachnaya kimberlite (ca. 367 Ma) to inform the evolution of the Siberia craton. However, there are no direct constraints on the timing of the Obnazhennaya kimberlite eruption. Such uncertainty of the kimberlite age precludes a better understanding of the mantle xenoliths from the Obnazhennaya pipe, and thus also of the evolution of the Siberia craton. This paper reports U-Pb ages for both perovskite from the Obnazhennaya kimberlite and rutile in an Obnazhennaya eclogite xenolith. The fresh perovskite formed during the early stage of magmatic crystallization and yields a U-Pb age of 151.8 ± 2.5 Ma (2σ). Rutile in the eclogite xenolith yields an overlapping U-Pb age of 154.2 ± 1.9 Ma (2σ). Because rutile has a Pb closure temperature lower than the inferred residence temperature of the eclogite prior to eruption, the U-Pb isotope system in rutile was not closed until the host eclogite was entrained and delivered to the surface by the kimberlite and therefore records the timing of kimberlite eruption. These data provide the first direct constraints on the emplacement age of the Obnazhennaya kimberlite and add to the global ‘kimberlite bloom’ from ca. 250–50 Ma as well as to the largest pulse of kimberlite volcanism in Siberia from ca. 171–144 Ma. The timing of this Jurassic–Cretaceous pulse coincides with the closure of the Mongol–Okhotsk Ocean, but the depleted Sr-Nd isotopic characteristics of 171–144 Ma kimberlites are inconsistent with a subduction-driven model for their petrogenesis. Thus, the closure of the Mongol-Okhotsk Ocean may act as a trigger for the initiation of 171–144 Ma kimberlite emplacement of Siberia, but was not the source.

Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ({D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}}) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} = 0.63 ± 0.10 and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{cpx}/mathrm{melt}} = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}} function alone allows ~ 40% of the variation in MORB compositions. If we allow {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

Metasomatism of the off-cratonic lithospheric mantle beneath Hangay Dome, Mongolia: Constraints from trace-element modelling of lherzolite xenoliths

Interactions of hydrous fluid and melt with dry mantle rocks are known to result in metasomatic alteration of the lithospheric mantle. Here we investigate such interactions that occurred beneath the Hangay Dome, Mongolia, in 22 mantle xenoliths, which were recovered from Cenozoic basalts at the Tsagan, Zala, Horgo, and Shavaryn-Tsaram localities near the village of Tariat. The xenoliths are medium- to coarse-grained spinel lherzolites that exhibit variable degrees of reaction with silicate melt and fluid (indicated by their Fe- and LREE-enrichment and the presence of secondary clinopyroxene, amphibole, phlogopite, apatite, and sulfide). According to their normalized REE patterns and microstructures, the spinel lherzolites were divided into three groups. Group 1 lherzolites contain LREE-depleted clinopyroxene and whole-rock compositions, exhibit a greater number of preserved deformation textures, and are the least affected by metasomatism. These lherzolites are interpreted to represent the sub-continental lithosphere before the rejuvenation processes that occurred during the Cenozoic. Group 3 lherzolites are characterized by partial annealing of pre-existing textures, and LREE-enrichment in clinopyroxenes and whole-rock compositions compared to Group 1 lherzolites. Group 3 lherzolites are interpreted to be the result of the interaction of depleted lithospheric mantle with a basaltic melt during the Cenozoic. The lack of correlation between the intensity of metasomatism, the degree of annealing, and the calculated temperatures of the lherzolites suggests that the Cenozoic basaltic melt percolation postdates the static recrystallization. Group 2 lherzolites exhibit characteristics from both Groups 1 and 3.Numerical modelling of the interaction between depleted lherzolites and basaltic melts evidences that: (i) a single initial liquid may fractionate to produce a range of element patterns: the observed spectrum of REE compositions of the Tariat lherzolites cannot have resulted from simple mixing of basaltic melt with a depleted mantle rocks; instead, it can be explained by chromatographic fractionation during reactive porous melt flow; (ii) highly fractionated element patterns are derived from conventional initial melt compositions and do not imply the existence of exotic melts; and (iii) strong element fractionation can be produced along short distances even at the thin section and mineral scale; this opposes the view that long percolation distances are required to produce significant chromatographic effects: the clinopyroxene core-rim disequilibrium demonstrates that REE variations in clinopyroxene rims were acquired in response to interactions with a more evolved REE-rich melt.

Recycled crustal carbon in the depleted mantle source of El Hierro volcano, Canary Islands

The Canary Islands, in the eastern Atlantic, are among the most enigmatic Oceanic Island provinces on Earth, as the mantle source feeding its volcanism exhibits wide spatial heterogeneity and a multiplicity of sources. Multi-isotope whole-rock studies have long revealed the presence of a recycled oceanic crust/lithosphere component in the mantle source. However, noble gas systematics have been more challenging to interpret, and the available carbon isotope data is limited and cannot support/dismiss this interpretation. Here, we present the very first isotopic characterisation of CO2 and noble gases (He-Ne-Ar) in fluid inclusions (FI) in minerals hosted in mantle xenoliths from El Hierro, the youngest and westernmost island of the Canary volcanic archipelago. Six fresh xenoliths from El Julan cliff valley were analysed (3 spinel lherzolites and 3 spinel harzburgites). We find carbon isotopic compositions of CO2 in FI (δ13C) ranging from −2.38 to −1.23‰ in pyroxenes and from −0.19 to +0.96‰ in olivines. These unusually positive δ13C values, well above the typical mantle range (−8‰ < δ13C < −4‰), prove, for the first time, the presence of a recycled crustal carbon component in the local source mantle. We interpret this 13C-rich component as inherited from a mantle metasomatism event driven by fluids carrying carbon from C. In contrast, our El Hierro xenoliths identify a depleted mantle-like He signature, with an average Rc/Ra ratio (3He/4He normalised to air ratio and corrected for atmospheric contamination) of 7.45 ± 0.26 Ra. The involvement of depleted mantle-like fluids, variably admixed with air-derived components (possibly recycled via paleo-subduction event(s)), is corroborated by Ne-Ar isotopic compositions. The depleted mantle-like He signature suggests instead the involvement of a primordial He source in the local lithospheric mantle and indicates a marginal role played by past subduction events in modifying the local mantle He budget. When put in the context of previous 3He/4He measurements in FI and surface gases along the Canary archipelago, our results confirm an overall west-to-east decrease of Rc/Ra ratios, which may be interpreted as due to increasing contributions from the African sub-continental mantle, the addition of radiogenic 4He during magma migration in the oceanic crust (whose thickness increases eastward) and/or magma ageing.

The São Francisco cratonic root beneath the Neoproterozoic Brasilia belt (Brazil): Petrophysical data from kimberlite xenoliths

Petrostructural analysis of 31 mantle xenoliths from three kimberlitic pipes intruding the Neoproterozoic Brasilia belt close to the southwestern margin of the São Francisco craton (SFC) reveals microstructures and compositions similar to those observed in cratonic roots worldwide. (1) The spinel-peridotites sampling the upper section of the lithospheric mantle have dominantly refractory modal and mineral compositions, whereas garnet-peridotites sampling the deep lithospheric mantle have more fertile compositions, consistent with those observed in cratonic roots worldwide. (2) The spinel-peridotites present a variation in microstructure from coarse-granular to coarse-porphyroclastic, but similar olivine crystallographic preferred orientations (CPO). (3) The garnet-peridotites have fine-porphyroclastic microstructures. (4) Many coarse-porphyroclastic spinel-peridotites display Fe-enrichment in olivine and pyroxenes, often associated with Ti-enrichment in pyroxenes or spinel and occurrence of modal phlogopite. (5) Equilibrium temperatures and pressures of garnet-peridotites are consistent with a cratonic geotherm, but equilibrium conditions of spinel-peridotites require a warmer geotherm. We interpret these observations as indicating that the xenoliths sample the SFC mantle root, which extends beneath the Brasilia belt, but was modified by reactive transport of the magmas forming the Alto Parnaiba Igneous Province (APIP) between 120 and 90 Ma. The APIP magmatism resulted in heterogeneous modal metasomatism, Fe enrichment, development of coarse-porphyroclastic microstructures in spinel peridotites and fine-porphyroclastic microstructures in garnet-peridotites, and moderate heating of the cratonic mantle root. These changes may produce a decrease in seismic velocities explaining the local weak negative anomaly observed in the lithospheric mantle beneath the APIP, which contrasts with the positive velocity anomalies characterizing the SFC mantle root in P-wave tomography models. However, reactive magma transport did not erase the olivine CPO. Comparison of the average seismic properties of the xenoliths with seismological data implies dominantly subhorizontal fossil flow directions and a non-negligible contribution of the cratonic root to teleseismic S-waves splitting.

Origins of olivine in Earth’s youngest kimberlite: Igwisi Hills volcanoes, Tanzania craton

Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far. This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~ 100–145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160–180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed. Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas. Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO2–H2O-rich ultramafic melts.

Geochemical Characterization of Intraplate Magmatism from Quaternary Alkaline Volcanic Rocks on Jeju Island, South Korea

The major and trace elements of Quaternary alkaline volcanic rocks on Jeju Island were analyzed to determine their origin and formation mechanism. The samples included tephrite, trachybasalts, basaltic trachyandesites, tephriphonolites, trachytes, and mantle xenoliths in the host basalt. Although the samples exhibited diversity in SiO2 contents, the relations of Zr vs. Nb and La vs. Nb indicated that the rocks were formed from the fractional crystallization of a single parent magma with slight continental crustal contamination (r: 0–0.3 by AFC modeling), rather than by the mixing of different magma sources. The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, associated with the subduction of the Pacific Plate beneath the Eurasian Plate. Therefore, we inferred that two separate magmatic events occurred on Jeju Island: one associated with the subduction of the Pacific Plate beneath the Eurasian Plate (represented by xenoliths), and another associated with a divergent setting when intraplate magmatism occurred (represented by the host rocks). With AFC modeling, it can be proposed that the Jeju volcanic rocks were formed by the fractional crystallization of the upper mantle combined with assimilation of the continental crust. The xenoliths in this study had different geochemical patterns from previously reported xenoliths, warranting further investigations.

Heat flow constraints on the mafic character of Archean continental crust

The bulk composition of the continental crust can be constrained from its surface heat flow. Published values of heat flow and heat production from 17 areas ranging in age from 3.3 to 0.4 Ga are compared. In the Superior province, the ca. 2.7 Ga accreted belts have a heat flow of 41mWm−2, significantly higher than that in the older, ca. 3.0 Ga, craton core (32mWm−2) (Jaupart et al., 2014). The mean surface heat flow values are 40±1.2mWm−2 for the Archaean areas, and 53±1.9mWm−2 for the post-Archaean. Heat flow values into the base of the crust are constrained from P-T estimates in lithospheric mantle xenoliths, and subtracted from the surface heat flow to calculate the bulk crustal heat production, whereupon there is a strong correlation between the surface heat flow data and the calculated average bulk crustal heat production values. Average crustal heat production values are described in terms of two component mixtures of felsic crust with >57% SiO2, and mafic crust with <57% SiO2, which is suggested to be more robust than relying on an average seismic model, given the complexity of the crustal structure and how it varies between provinces, and sometimes within a single province. The proportions of the mafic crustal component required to account for the regional bulk heat production values in 5 areas older than 2.8 Ga range from 60–78% consistent with a fixed mafic proportion of ∼70%. This is higher than the proportion of lower, mafic crust in the present bulk crust of Rudnick and Gao (2014), which is 43%, and the estimated bulk crust MgO values change from 10–6.6% in areas older than 2.8 Ga, to ∼5% MgO for the younger areas. The bulk crust in the late Archaean was on average more mafic than the bulk crust at the present day, and it had a bulk crust Rb/Sr value of 0.12±0.04.

A review of mantle xenoliths in volcanic rocks from southern Victoria Land, Antarctica

Abstract Mantle xenoliths from southern Victoria Land have been collected and extensively studied for over a century. In this chapter, chemical and petrological data are, for the first time, comprehensively collated, and petrogenetic models for the regional mantle are reviewed and assessed. The most common lithologies are spinel lherzolite and harzburgite; plagioclase lherzolite also occurs, and pyroxenite xenoliths found across the province comprise &lt;20% of all mantle xenoliths. The lithospheric mantle in the region has Paleoproterozoic stabilization ages, although pockets of younger mantle may exist. This peridotite mantle comprises a HIMU (high 238 U/ 204 Pb = high μ)-component sensu stricto , has been variably carbonated and has undergone multiple melt-depletion events. Regional variations in a sedimentary (EMI: Enriched Mantle I) component to the west, and iron-rich components to the east, reflect a complex history of refertilization and metasomatism. The sources of these fluids are likely to have been oceanic crust subducted during c. 0.5 Ga and older events. Peridotites have been cross-cut by pyroxenite veins, probably in multiple episodes, with the geochemistry of some samples reflecting the involvement of an upper continental crust (EMII: Enriched Mantle II) component. Future research directions should apply advanced isotopic, noble gas and volatile techniques to better understand the upper mantle below this dynamic rifting environment.

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.

Confocal Raman spectroscopic study of melt inclusions in olivine of mantle xenoliths from the Bultfontein kimberlite pipe (Kimberley cluster, South Africa): Evidence for alkali‐rich carbonate melt in the mantle beneath Kaapvaal Craton

AbstractIdentifying the composition of primary/primitive mantle melts is crucial for understanding the mantle's evolution and mantle‐derived magmatism. Melt inclusions in mantle xenoliths provide key information about such melts. This study used confocal Raman spectroscopy to characterize mineral assemblage of unexposed crystallized secondary melt inclusions in the olivine from the xenoliths of the sheared garnet peridotites from the Bultfontein kimberlite pipe (Kimberley cluster, Kaapvaal Craton, South Africa). The studied peridotites originated from 112‐ to 146‐km mantle depths. In total, 16 minerals were identified among daughter crysatls, which include carbonates (calcite, magnesite, dolomite), alkali carbonates (eitelite, nyerereite, gregoryite/natrite, K–Na–Ca–carbonate (K,Na)2Ca(CO3)2 (K analogue of nyerereite), alkali carbonates with additional anions (northupite, bradleyite, burkeite), alkali sulfates (glauberite, thenardite, aphthitalite), apatite, tetraferriphlogopite, and magnetite. Several more daughter minerals gave distinct Raman spectra, but they were not determined due to the lack of similar spectra in the databases. Carbonates are predominant among the daughter minerals in the melt inclusions. Many daughter minerals are rich in alkalis. These facts indicate that melt(s), parental for the inclusions, is alkali‐rich carbonate in composition. Two possible models were suggested for the origin of these melt inclusions: (1) in situ fracturing of olivine and the mantle melt infiltration shortly before the sheared peridotites were entrained by the ascending kimberlitic magma; (2) infiltration of the transporting kimberlite melt into xenoliths during ascent. Both models imply that the alkali‐rich carbonate melt(s) that interacted with peridotites originated at a greater depth than the entrapment level of studied xenoliths (&gt;150 km), that is, at the base of the lithosphere or in the asthenosphere. This melt is genetically related to the kimberlite magmatism that formed the Bultfontein pipe and points at the alkali‐rich carbonate composition of primary kimberlite melt.

Transpressional deformation in the lithospheric mantle beneath the North Anatolian Fault Zone

We studied mantle xenoliths sampled from two late Miocene volcanic centers located immediately north of the North Anatolian Fault Zone (NAFZ) in western Turkey. The spinel lherzolite and harzburgite xenoliths equilibrated at temperatures ranging from 757 to 1019 °C, estimated from two-pyroxene geothermometry. Pressures estimated from the Cr-in-clinopyroxene geobarometer and pseudosection modelling range from 1.0 to 2.1 GPa, which correspond to depths of 38–75 km. We used X-ray computed tomography to quantify the three-dimensional shape preferred orientation of spinel grains. Spinel displays dominantly oblate fabric ellipsoids, consistent with flattening strain. Olivine crystallographic preferred orientation (CPO), determined by electron backscatter diffraction, exhibits chiefly axial-[010] and A-type patterns. The axial-[010] CPO pattern is consistent with the spinel fabric that shows flattening strain, and is primarily developed in xenoliths from the volcanic center nearer to the NAFZ. Obliquity of olivine CPO to the spinel lineation decreases with proximity to the NAFZ, suggesting an increase in shear strain. Our results favor an interpretation of transpressional deformation in the upper mantle below the NAFZ. We attribute this pattern to a restraining bend in the mantle section of the NAFZ.

“Black” Pyroxenites in Mantle Xenoliths Found in Volcanics of Some Regions in the East Asian Margin. The Evolution and Petrogenesis. Part 1. The Mineralogical Compositions, Conditions of Generation

“Black” Pyroxenites in Mantle Xenoliths Found in Volcanics of Some Regions in the East Asian Margin. The Evolution and Petrogenesis. Part 1. The Mineralogical Compositions, Conditions of Generation

Opposite Symmetry in the Lithospheric Structure of the Alboran and Algerian Basins and Their Margins (Western Mediterranean): Geodynamic Implications

AbstractThe geodynamic evolution of the Western Mediterranean for the past 35 My is a matter of debate. Present‐day structure and composition of the lithosphere and sublithospheric mantle may help in constraining the geodynamic evolution of the region. We use an integrated geophysical‐petrological modeling to derive and compare the present‐day thermal, density and compositional structure of the lithosphere and sublithospheric mantle along two NNW‐SSE oriented transects crossing the back‐arc Alboran and Algerian basins, from onshore Iberia to the northern Africa margin. The crust is constrained using available seismic data and geological cross‐sections, whereas seismic tomography and mantle xenoliths constrain the upper mantle structure and composition. Results show a thick crust (37 and 30 km) and a relative deep LAB (130 and 150 km) underneath the HP/LT metamorphic units of the Internal Betics and Greater Kabylies, respectively, which contrast with the 16 km thick magmatic crust of the Alboran Basin and the 10 km thick oceanic crust of the Algerian Basin. The sharp change in lithosphere thickness, from the orogenic wedge to the back‐arc basins, contrasts with the gentler lithosphere thickening toward the respective opposed margins. Our results confirm the presence of detached slabs ∼400 °C colder than upper mantle and a fertile composition than the continental lithospheric mantle beneath the External Betics and Saharan Atlas. Presence of detached quasi‐vertical sublithospheric slabs dipping toward the SSE in the Betics and toward the NNW in the Kabylies and the opposed symmetric lithospheric structure support an opposite dipping subduction and retreat of two adjacent segments of the Jurassic Ligurian‐Tethys realm.