Mantle Xenoliths Research Articles

Nature and evolution of lithospheric mantle beneath the Northeast China: Constraints from the Houtuancun mantle xenoliths

Mantle xenoliths entrained in a newly discovered Cenozoic basalt at Houtuancun within the Yilan-Yitong fault zone, situated beneath the Xing'an Mongolian Orogenic Belt in Northeast China, provide a unique opportunity to constrain the nature and evolution of the lithospheric mantle beneath this region. The mantle xenoliths are predominantly spinel lherzolite, with rare occurrences of harzburgite, wehrlite, and websterite. The Houtuancun spinel lherzolite and harzburgite are characterized by high forsterite contents in olivines (89.3–91.1), and their clinopyroxene rare earth element patterns range from slightly light rare earth element (LREE)-depleted to LREE-enriched, reflecting variable degrees of partial melt extraction (up to 20%) overprinted by later metasomatism. Despite this, the Fe isotopic composition of the Houtuancun lherzolite and harzburgite exhibits little variation (δ57/54Fe = −0.05–0.09‰), with an average of 0.02 ± 0.09‰ (2SD, n = 9), close to the suggested δ57/54Fe value of 0.04 ± 0.04‰ for the upper mantle, and there is no significant inter-mineral disequilibria. In contrast, the wehrlite has a slightly lower Mg# [=100 ╳ Mg2+/(Mg2+ + Fe2+)] (88.4) and δ57/54Fe (−0.05‰) compared to the spinel lherzolite and harzburgite, reflecting kinetic iron fractionation during melt-rock interaction. The websterite exhibits the lowest MgO (20.6–24.8 wt%), highest Al2O3 content (5.8–7.3 wt%), and the highest δ57/54Fe (0.25–0.28‰) among the studied xenoliths, suggesting an episode of subduction-related silicate melt circulation through the upper mantle. The results demonstrate compositional heterogeneity of the mantle beneath the Xing'an Mongolian Orogenic Belt in Northeast China. The observed heterogeneity may result from a combination of varying degrees of melt extraction of the subcontinental lithospheric mantle and metasomatism by melts related to the subduction of the (Paleo)-Pacific plate.

Fossil metasomatized and newly-accreted fertile lithospheric mantle volumes beneath the Bakony-Balaton Highland Volcanic Field (central Carpathian-Pannonian region)

This study focuses on upper mantle xenoliths from Sabar-hill (Bakony-Balaton Highland Volcanic Field), a newly discovered locality of mantle xenoliths, situated in the central Carpathian-Pannonian region (CPR). The investigated seven peridotite and one pyroxenite xenoliths record two episodes in the geochemical evolution of the local subcontinental lithospheric mantle. The moderately deformed Group I xenoliths reveal modal contents (≥9 vol% orthopyroxene), major element characteristics (depletion in Al2O3 and CaO in orthopyroxenes) and trace element features (e.g., enrichment in U, Pb and Sr in clinopyroxenes) typical in supra-subduction zones (i.e., mantle wedge) xenoliths. All these suggest that the studied xenoliths were formed by silica-rich fluid/melt - peridotitic wall rock interactions. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of an oceanic slab during the Mesozoic-Paleogene. This may have happened far from the current position of Sabar-hill, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The less deformed, dominantly protogranular and porphyroclastic Group II xenoliths are depleted in light rare earth elements (LREE). In contrast, they are enriched in clinopyroxene (≥11 vol%), incompatible major elements (e.g., Al and Na) and heavy rare earth elements (HREE), by a minimum of four times compared to primitive mantle. All these suggest that Group II xenoliths represent a geochemically fertile mantle partition and the degree of partial melting affected this mantle section was limited (≤7% based on trace elements). Group II xenoliths have higher equilibrium temperatures (1088–1168 °C) compared to Group I xenoliths (1018–1089 °C). The lack of petrographic and geochemical evidence pointing to metasomatism-related annealing in Group II xenoliths suggests that they are derived from a greater depth. It follows that Group II xenoliths could represent prior asthenosphere volumes which recently accreted to the lithosphere. The lithospherization of the upper part of the asthenosphere may have initiated after the tectonic inversion of the region, when compressional tectonic regime beneath the CPR gradually overtook the former extensional one in the last ∼8 Ma. The high bulk structural hydroxyl content in both Group I (22–1699 ppm) and II (55–320 ppm) xenoliths is likely linked to the subduction events that took place in the CPR from the Mesozoic onwards. These events transported significant amounts of fluid (H2O) back to the upper mantle, affecting both the mantle wedge and the asthenosphere.

Carbon cycling during the India-Asia collision revealed by δ26Mg−δ66Zn−δ98Mo evidence from ultrapotassic volcanoes in NW Tibet

India-Asia continental collision−induced volcanic gas emissions are thought to have played an important role in driving Cenozoic atmospheric CO2 variations, yet the details of how the deep carbon cycle may influence volcanic CO2 degassing are not understood. We present a novel study employing Mg-Zn-Mo isotopic compositions of Cenozoic ultrapotassic lavas from NW Tibet. The negative Mg-Zn isotope correlation (δ26Mg = −0.39‰ to −0.19‰; δ66Zn = +0.27‰ to +0.36‰), bolstered by petrographic analysis of mantle-derived xenoliths from these lavas, demonstrates that the ultrapotassic magmas originated from a lithospheric mantle source that had been enriched by recycled carbonate-bearing sediments rich in calcite and dolomite. Very low δ98Mo values (−0.78‰ to 0‰) relative to the average continental crust (δ98Mo = +0.10‰ to +0.35‰) further indicate that the sedimentary components were derived from the subducted Indian continental crust after its dehydration. Monte Carlo modeling estimates that the input flux of carbon (elemental C) from such sediments into the lithospheric mantle is ∼5.6 Mt/yr, with a predicted CO2 emission rate of ∼15.5 Mt/yr. We suggest that the still ongoing subduction of the Indian tectonic plate has played a crucial role in introducing substantial quantities of carbonate-rich sediments into the Tibetan lithospheric mantle, leading to the sequestration of large amounts of CO2 via carbonatite metasomatism. Hence, partial melting of such a carbon-rich mantle reservoir in an orogenic setting provides the positive feedback mechanism that can explain the high flux of volcanic CO2 during India-Asia collision. These findings not only highlight the importance of continental subduction, sediment recycling, and mantle metasomatism by carbon-rich melts/fluids in the generation of Tibetan ultrapotassic volcanism, but they also show how the deep carbon cycle influences volcanic CO2 degassing.

Geochemistry of forty‐one eclogitic and pyroxenitic mantle xenoliths from the Central Slave Craton, Canada (Ekati Diamond Mine)

AbstractThis article describes a novel dataset on non‐diamondiferous eclogite and garnet pyroxenite xenoliths from four kimberlite pipes of the Ekati Diamond Mine (Central Slave Craton, Canada). Xenoliths brought to the surface by kimberlite eruptions are direct sources of information on the composition and evolution of the Earth's mantle. Eclogite and garnet pyroxenite xenoliths, specifically, are testimony of subduction into, and metasomatism of, the mantle beneath cratons. Furthermore, these rocks are major hosts for diamond and thus an important part of the deep carbon cycle. The sample suite consists of 41 small xenoliths (2–5 cm) recovered from drill cores. The dataset includes major and trace element concentrations for garnet, clinopyroxene and ilmenite, as well as stable oxygen isotope compositions of garnets. Strontium and neodymium isotopic compositions are reported for garnet and clinopyroxene for four samples which were large enough to allow for analysis. Overall, this dataset significantly expands and complements existing datasets on diamondiferous and non‐diamondiferous xenoliths from the Slave Craton in Canada, furthering our understanding of the composition of the Slave subcratonic lithosphere. The dataset includes several samples with rare mineral assemblages, including an olivine‐bearing eclogite as well as ilmenite and apatite‐bearing garnet‐pyroxenites, and thus provides data shedding light on rarely reported compositional nuances in xenolith suites found in kimberlites.

The Volcanism of the Meseta del Lago Buenos Aires, Patagonia: the Transition from Subduction to Slab Window

Abstract The Meseta del Lago Buenos Aires (MLBA) in southern Patagonia, a volcanic plateau formed from ~12 Ma to present, provides an opportunity to investigate the temporal evolution in volcanism as this region transitions from the subduction of the Nazca plate to the formation of the slab window produced by the collision of the Chile Ridge and the Andean subduction zone. Here, we report new major, minor, and trace element contents, as well as Sr, Nd, Pb, and Hf isotopes of the MLBA lavas. Three distinct geochemical endmembers can be distinguished in the MLBA basalts: a subduction-influenced endmember, a transitional component similar to the South Atlantic enriched mid-ocean ridge basalts, and an enriched component akin to the EM1 mantle composition. Lavas older than ~1.5 Ma define a compositional continuum between the subduction-influenced and transitional endmembers; this trend is also present in many other southern Patagonian plateaus regardless of their distance to the trench, eruption age, and the composition of the continental blocks where they are located. In contrast, MLBA basalts younger than ~1.5 Ma uniquely define a transition into the EM1 mantle component at the time when this region was affected by the slab window. The estimated pressures and temperatures of mantle-melt equilibration for the MLBA basalts indicates an increase in both parameters after the formation of the slab window that roughly correlate with the changes in lava composition. The basalts’ composition from all southern Patagonia plateaus points to the presence of the South Atlantic mid-ocean ridge basalt mantle influenced by the Discovery, Shona, and Bouvet hotspots rather than the sub-slab mantle, as represented by the Chile Ridge basalts. This observation challenges the hypothesis that the sub-slab mantle within the slab window has had an important role in the composition of the erupted lavas. Instead, it suggests the presence of a South Atlantic mantle beneath southern Patagonia either within the mantle wedge, consistent with a long-lasting South Atlantic convection cell beneath South America, or in the subcontinental lithospheric mantle metasomatized before or just after the opening of the South Atlantic basin, as demonstrated by the composition of southern Patagonia mantle xenoliths. Although it is difficult to precisely distinguish the contributions of the asthenosphere from that of the metasomatized subcontinental lithospheric mantle beneath this region, our work suggests significant contributions from the latter in the composition of the MLBA lavas.

In-situ trace element and Sr isotope in mantle xenoliths from the West Qinling orogen: Evidence for carbonate metasomatism

Subduction process plays an important role in triggering chemical metasomatism of the lithospheric mantle beneath orogenic belts. The lithospheric mantle beneath the West Qinling orogen is extensively heterogeneous, but the lithospheric modification process and related metasomatic agents remain poorly constrained. In this paper, we report in-situ elemental and isotopic compositions of minerals in mantle xenoliths from the West Qinling orogen. The mantle xenoliths are lherzolites and can be categorized into two types. Lherzolite with refractory compositions represents the residue of lithospheric mantle that have undergone high degrees of melt extraction. In contrast, lherzolites with fertile compositions contain clinopyroxenes variably enriched in incompatible elements and radiogenic Sr isotopes. These fertile lherzolites represent lithospheric mantle modified by metasomatic process in different degrees. High (La/Yb)N, Ca/Al, Nb/Ta, Nd/Yb, 87Sr/86Sr ratios and low Ti/Eu ratios of the clinopyroxenes suggest the metasomatic agent was dominated by carbonate-rich melts. This inference is supported by the identification of carbonate inclusion, reaction structures, and clinopyroxene chemical zoning in the lherzolites. Considering the tectonic background of the West Qinling orogen, the orogenic lithospheric mantle would have been metasomatized by carbonate-rich melts derived from partial melting of subducted carbonate-bearing Paleo-Tethyan oceanic slab. In this regard, petrological observations, along with in-situ elemental and Sr isotopic compositions of clinopyroxenes in mantle xenoliths can be powerful means to identify carbonate metasomatism and constrain the evolution of the lithospheric mantle beneath orogenic belts.

Hydrous veined mantle lithosphere and implications for the source of Zealandia intraplate magmas

Although amphibole has long been predicted to be a major component in the mantle source of intraplate volcanic rocks in New Zealand, the physical evidence for this phase, despite inspection of hundreds of peridotite mantle xenoliths from across the country, is limited to a handful of samples in which it is mostly a sparse component. Here, we present a suite of ultra-refractory peridotite xenoliths in an ultramafic lamprophyre from the intraplate Alpine Dike Swarm, New Zealand, that contain abundant MARID-like veins of mica (phlogopite), amphibole (pargasite) and clinopyroxene, with minor to trace rutile, ilmenite, carbonate and apatite. A key feature of inspecting these formerly refractory peridotites (olivine Mg# commonly >92) is that the chemistry of the precipitated metasomatic agent is less diluted than in cases where an invading melt reacts with fertile lithosphere. The bulk compositions of the precipitated veins vary between sub-alkaline basalt and alkaline basanite, with the peridotite wallrock hosting the basanite veins commonly enriched in Fe ± K, Ti and Al. Amphibole in all veins has high Nb, and the 24 Ma age-corrected 87Sr/86Sr values for diopside and amphibole (0.7027 to 0.7031) are indicative of derivation from a depleted mantle free of crustal material, and thus not related to subduction and arc magmatism. The Sr isotope ratios also overlap with the composition of many primitive intraplate magmas across Zealandia. Melt modelling indicates that partial melting of the veins could generate magmas with geochemical aspects of the known intraplate rocks. These rare veined peridotite occurrences therefore confirm that the Zealandia mantle lithosphere contains fusible zones that, if melted, could have played an important role in the source of intraplate basaltic magma.

Melt inclusions in spinel from a composite mantle xenolith

Composite mantle xenoliths from the Cima Volcanic Field (CA, USA) contain a variety of melt (now glassy) inclusions hosted within mantle phases. The compositions and textures of these melt inclusions have the potential to constrain their trapping processes, melt sources, and the rates of ascent of their parent xenoliths. Here we focus on unusual spinel-hosted melt inclusions from one composite xenolith, reporting glass and daughter mineral compositions and textures and attempting to reconstruct inclusion bulk compositions. The xenolith contains spinel-hosted melt inclusions in its harzburgite, olivine-websterite and lherzolite layers; there are none in its orthopyroxenite layer.The glass compositions and reconstructed bulk compositions of the partly-crystallized inclusions correspond to alkaline intermediate melts, mostly trachyandesites. Such melts are most likely to be generated and trapped by vapor-undersaturated phlogopite or amphibole dehydration melting to an assemblage of liquid + spinel + olivine ± pyroxenes. We modeled the near-liquidus phase relations of the inclusion bulk compositions and noted the closest approach of each inclusion to simultaneous saturation with spinel and either phlogopite or amphibole, resulting in estimated trapping pressures of ∼0.5–1.5 GPa and temperatures of ∼1000–1100 °C. The large size of the hosting spinel grains suggests a slow process associated with these breakdown reactions, probably thinning of the lithosphere and steepening of the geotherm during regional extension.A linear correlation between the vesicle area and inclusion area (as proxies for volume) suggests an in-situ exsolution process from melts of relatively uniform volatile initial contents, consistent with trapping of vapor-undersaturated melts that later exsolve vapor during cooling and daughter crystal growth. A negative correlation between the glass content in melt inclusions and the size of the inclusion itself suggests a control on the degree of crystallinity with the size. There appears to be a two-stage cooling history captured by the inclusions, forming first prismatic daughter crystals and large round vesicles at the wall of the inclusion, followed by quenching to form a mat of fine crystallites and small vesicles in most inclusions. We connect the final quench to rapid ascent of the xenolith in its host melt, which also triggered partial breakdown of remaining amphibole to fine glassy symplectites.

Sub-arc Mantle Heterogeneity of the Northern Luzon Volcanic Arc: Mineral and Whole Rock Compositional Variability in Mantle Xenoliths from Lutao Island

Abstract Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo92.5–88.9), orthopyroxene (Mg# = 94.6–89.2), and clinopyroxene (Wo49.1–38.1En57.0–45.4Fs3.0–11.0) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# < 45) and low-Al (Cr# > 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95–2.07 wt %; Mg# = 88.5–90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (87Sr/86Sri = 0.704400–0.707908; εNd(t) = 0.0 − +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 °C with the majority of temperature estimates >1000 °C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded ΔFMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1–2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya <1000 °C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (<6 Ma) between the Luzon arc and the Eurasian margin.

Clinopyroxenite Xenoliths Record Magma Transport and Crystallization in the Middle and Upper Crust: A Case Study from the Rockeskyllerkopf Volcanic Complex, West Eifel, Germany

Abstract Clinopyroxenite xenoliths comprising cumulus clinopyroxene ± amphibole together with intercumulus phlogopite ± olivine ± apatite ± titanite form a large part of the xenolith load in the oldest deposits of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field. The xenoliths also contain xenocrysts of olivine and clinopyroxene derived from mantle peridotite and clinopyroxene from lower crustal granulite. The clinopyroxenite xenoliths are divided into five groups on the basis of their modal mineralogy and mineral compositions. Groups 1 to 4 define a continuous compositional trend indicative of fractionation of a mafic alkaline magma. Group 5 xenoliths are compositionally distinct and have been tentatively linked to high pressure crystallization of phonolitic magma within the RVC system. Thermobarometry of the group 1 to 4 xenoliths indicates that they crystallized between 1 and 4 kilobars, equivalent to a depth of 4 to 14 km. Group 1 to 3 xenoliths all crystallized at between 1050°C and 1150°C, whereas the amphibole-rich group 4 xenoliths give temperature estimates of ~900°C. The clinopyroxenites share a common parent magma with clinopyroxene–phlogopite veins found in subcontinental lithospheric mantle xenoliths. However, the vein forming mama was richer in incompatible elements, in particular Zr and Hf and is interpreted to be an early formed batch of magma with the clinopyroxenites crystallizing from magma derived from the same mantle, which had been depleted by the earlier phase of melting. Intrusion of magma began around 155 ky prior to the eruption of the RVC. Fe–Mg interdiffusion profiles in zoned clinopyroxene show that the magma that formed the xenoliths was present in the crust for up to 28 ky prior to the eruption. However, most samples give interdiffusion times between 1.5 and 9.9 ky. Based on xenocryst residence times and the calculated P–T conditions for clinopyroxene, there were at least seven separate batches of magma emplaced below Rockeskyllerkopf, probably as sills.

Seismic properties of mantle metasomatism from mantle xenoliths beneath the North Tanzania Divergence, East African Rift

We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6–80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80–90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040–1200°C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2–4 % to 28–37 % for Vp and by 2–3 % to 25–29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %–CO2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between −9 % and −15 %) and vein-bearing peridotites (depth 40–90 km) corresponds to 12–25 % of crystallized veins or 8–15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD.

Mantle xenoliths from Komsomolskaya kimberlite pipe, Yakutia: Multistage metasomatism

Minerals from > 200 mantle xenoliths from Komsomolskaya kimberlite pipe were studied by electron microprobe and LA-ICP-MS. They are metasomatised garnet and spinel peridotites containing phlogopite, amphibole and ilmenite with garnets (up to 12.5 wt% Cr2O3) and clinopyroxenes (up to 5 wt% Na2O) or rarer Fe-pyroxenites and A, B, C eclogites.Thermobarometry indicates that the lithospheric mantle beneath the Komsomolskaya pipe is layered. Heated porphyroclastic, deformed peridotites at the lithosphere base (7–6 GPa) are enriched in Fe. The cold group at 6.0–5.5 GPa (34 mW/m2) are depleted peridotites with sub-Ca garnets. Cpx-fertilized varieties belong to the middle part of the mantle section. Amphiboles range from Cr-hornblendes to edenites (2–6 GPa), showing K-Ti enrichment. Picroilmenites yield two pressure intervals from 6.5 to 5.0 GPa and from 5.0 to 4.0 GPa, forming two differentiation branches. Eclogites mainly occur in the lower part of the section with a peak at pressures of 4–6 GPa.Trace elements of melts that formed harzburgitic garnets-pyroxenes refer to oceanic MORB like melt interaction with peridotites. The sub-calcic S-type garnets are similar to subduction-related melts (S-type REE) with troughs in HFSE. Adakite-like hybrid metasomatism formed Na, Al-rich pyroxenes with peaks in Sr and HFSE. K-bearing pyroxenes and amphiboles refer to shoshonitic metasomatism. Trace elements for Cpx of re-fertilized mantle peridotites reveal high LREE, Nb-Ta troughs and peaks in Zr, Th, Sr, U. They are reacted to carbonatite –alkaline melts. Protokimberlite (essentially carbonatitic) interaction produced HFSE-enrichment. Type B eclogites show more subduction-related features with HFSE troughs while type A eclogites are closer to hybrid and peridotitic signatures. We suggest six types of major metasomatic agents. The 40Ar/39Ar ages of phlogopites vary in the 440–690 Ma range, with some at 1.6 Ga, suggesting multistage metasomatism.

High P‐T Sound Velocities of Amphiboles: Implications for Low‐Velocity Anomalies in Metasomatized Upper Mantle

AbstractMetasomatized mantle xenoliths containing hydrous minerals, such as amphiboles, serpentine, and phlogopite, likely represent the potential mineralogical compositions of the metasomatized upper mantle, where low seismic velocities are commonly observed. This study presents the first experimentally determined single‐crystal elasticity model of an Fe‐free near Ca, Mg‐endmember amphibole tremolite at high pressure and/or temperature conditions (maximum pressure 7.3(1) GPa, maximum temperature 700 K) using Brillouin spectroscopy. We found that sound velocities of amphiboles strongly depend on the Fe content. We then calculated the sound velocities of 441 hydrous‐mineral‐bearing mantle xenoliths collected around the globe, and quantitatively evaluated the roles that amphiboles, phlogopite and serpentine played in producing the low velocity anomalies in the metasomatized upper mantle.

Depth of the lithospheric mantle discontinuities beneath the northwest flank of Oku Volcanic Complex, Cameroon Volcanic Line: constraints from mantle xenolith and teleseismic data

Depth of the lithospheric mantle discontinuities beneath the northwest flank of Oku Volcanic Complex, Cameroon Volcanic Line: constraints from mantle xenolith and teleseismic data

Alkaline Silicate Metasomatism Recorded through Fe-Ti-Rich Mantle Xenoliths from the Calatrava Volcanic Field (Spain)

Much of the lithospheric subcontinental mantle (SCLM) sampled in the Calatrava Volcanic Field (CVF) shows refertilization by alkaline metasomatic agents. The Cerro Pelado and El Palo ultramafic xenolith suites record the best evidence of this type of metasomatism in this volcanic field. Several groups of peridotite (lherzolite, wehrlite, and dunite) and pyroxenite (clinopyroxenite and websterite) xenoliths have been distinguished. Despite having scarce phlogopites and amphiboles as modal metasomatic phases, all studied xenoliths present a variable cryptic metasomatism, highlighted by the strong Fe-Ti enrichment and fractionated REE patterns in the most evolved wehrlite and pyroxenite varieties. They show a common trend of an Fe-Ti-Ca increase, whereas the pyroxenites are more depleted in Fe compared to the lherzolites and wehrlites. Trace-element (REE and multi-trace) patterns are roughly similar among them, suggesting different interactions and refertilization degrees by alkaline silicate melts. The same Sr–Nd isotopic EAR composition, combined with trace-element chemistry of metasomatic xenolith phases and phenocrysts from the Calatrava volcanics, highlights the main role of this magmatism in percolation processes beneath Central Iberia. These mantle xenoliths also show variable amounts of interstitial glass that originated by in situ partial melting, favored by the enriched chemical nature of cryptically metasomatized clinopyroxene during their volcanic transport. This alkaline-refertilized mantle type represents the main domain within the SCLM beneath Central Iberia, as was also recorded in other Western European Cenozoic volcanic fields.

High-precision analysis of carbon isotopic composition for individual CO2 inclusions via Raman spectroscopy: Addressing issues arising from the laser-heating effects

The carbon isotopic composition of carbon dioxide fluid inclusions trapped within minerals reflects the origin and evolution of the CO2-bearing fluids and melts. Recently, we developed a highly accurate method using Raman spectroscopy to determine the carbon isotopic composition of CO2 fluid inclusions, by simultaneously collecting 13CO2 and hot bands at a controlled temperature, and using the peak height ratio of 13CO2 and one of hot bands near 1410 cm−1 (HR) to determine δ13C value. This microanalytical technique enables precise measurements of the δ13C values of different CO2 fluid inclusions within zoned mineral crystals to reveal the multiple-stages evolution of CO2-bearing fluids and melts. However, when studying fluid inclusions hosted within high laser heating coefficient minerals such as olivine and pyroxene in mantle xenoliths, local temperature near the inclusions will be higher than the temperature controlled by heating-cooling stage. To address this issue, we examined the effects of laser heating on CO2 isotopic standard samples sealed in the silica capillary, and developed a novel procedure to determine the δ13C values of natural fluid inclusions by using two CO2 standard samples with known carbon isotopic compositions. The proposed workflow successfully determines the δ13C values of natural CO2 inclusions, with an average deviation of 0.58 ‰ between the measurements and the model.

Mineral Chemistry of Olivine, Oxy-Spinel, and Clinopyroxene in Lavas and Xenoliths from the Canary, Azores, and Cape Verde Islands (Macaronesia, North Atlantic Ocean): New Data and Comparisons with the Literature

An electron microprobe study was carried out on olivine, clinopyroxene, and oxy-spinel occurring in basalts and dunite xenoliths from the archipelagos of the Azores, the Canary Islands, and Cape Verde. By comparing our results with previously published data from the volcanic islands of Macaronesia, we confirmed the validity of the compositions of olivine, clinopyroxene, and oxy-spinel as geochemical tracers. The origin of olivine, i.e., crystallized in the lithospheric mantle or in volcanic rocks, was successfully discriminated. Olivine from Lanzarote dunite xenoliths, which represent fragments of the mantle transported to the surface by host magmas, exhibited higher Fo% values (Fo91.02 to Fo91.94) and a different distribution of minor elements Ca, Ni, and Mn (CaO up to 0.42 wt%, NiO 0.07–0.41 wt%, MnO 0.06–0.3 wt%) when compared with olivine occurring as phenocrysts in basaltic lavas from the Macaronesian islands. The highly variable forsterite contents (Fo75.1 to Fo94.4) in olivine from gabbro and peridotite xenoliths found across the islands of Macaronesia were attributed to fractional crystallization that started in a deep magma reservoir, suggesting that these xenoliths represent cumulate rocks and not mantle fragments. Alternatively, these xenoliths may have been affected by the interaction with metasomatic fluids. The composition of clinopyroxene phenocrysts was used to decipher formation conditions under extensional tectonics. Their composition suggests that the host lavas have an alkaline to calc-alkaline signature. Furthermore, clinopyroxene euhedral shapes and compositions suggest an origin by fractional crystallization in a closed magmatic system. The composition alone of oxy-spinel from Macaronesian basalts and xenoliths was not sufficient to draw conclusions about the geodynamic environment where they were formed. Nevertheless, the relationship between oxy-spinel and olivine crystallized in equilibrium was successfully used as oxybarometers and geothermometers. The oxy-spinel–olivine pairs show evidence that the basaltic lavas were crystallized from melts with higher oxygen fugacity and different cooling histories than those of the mantle xenoliths, as the latter crystallized and re-equilibrated much slower than the basalts.

A revision of the Ni-in-garnet geothermometer with special regard to its pressure dependence

We have explored the effect of temperature (T) and pressure (P) on Ni partitioning between garnet and olivine in well-equilibrated mantle xenoliths from on-craton, marginal-craton and off-craton settings and in high-P–T experiments at natural Ni abundance. Contrary to previous evaluations, the xenolith and experimental data indicate that the P effect is not negligible, consistent with the significant volume change of the garnet–olivine Mg-Ni exchange reaction. The recognition of a P effect satisfactorily resolves the discrepancies observed using previous calibrations of the Ni-in-Grt geothermometer and provides a solution to the long-standing controversy as to which Ni-in-Grt geothermometer is best applied to natural chromian pyrope compositions. A recalibrated, P-dependent Ni-in-garnet geothermometer reproduces the pyroxene T estimates for the xenoliths and the T conditions of the experiments with a standard error of estimate of 44 °C. The P dependence (ca. 40 °C/GPa) is comparable to that of the garnet–olivine Fe–Mg exchange geothermometer. A small tendency to overestimate at T < 900 °C relative to two-pyroxene thermometry is observed, which is unrelated to garnet compositional parameters. A set of simplified, geotherm-referenced calibrations permit traditional use of the Ni-in-garnet geothermometer as a single-mineral method, provided the local geotherm is known or can be estimated or inferred.

Two styles of melt-peridotite interactions in the upper mantle revealed by mantle xenoliths from northeastern China

To better constrain the mechanisms of interactions between different types of melts and mantle peridotites in the upper mantle of the big mantle wedge, we studied petrology, mineral chemistry, and temperatures recorded by major element and rare earth element (REE) for mantle xenoliths from the Wangqing area in the eastern part of northeastern China, and compared these data with those for xenoliths from the adjacent Jiaohe and Shuangliao areas. The Wangqing xenoliths, including olivine clinopyroxenite, wehrlite, orthopyroxenite, and websterite, generally contain clinopyroxene that is surrounding or replacing olivine. Mineral compositions of the xenoliths from northeastern China vary along melt–peridotite reaction trends, where the Wangqing xenoliths have lower mineral Mg#’s (77–85 in olivine and 81–87 in clinopyroxene) than the Jiaohe and Shuangliao peridotite and pyroxenite xenoliths. In addition, clinopyroxenes in the Wangqing samples have moderately elevated rare earth element contents and (La/Yb)N and 87Sr/86Sr ratios (0.7035–0.7046), within the range of those for clinopyroxenes in the Jiaohe and Shuangliao samples. The REE temperatures of the Wangqing samples (1024–1253 °C) are higher than those of the Jiaohe and Shuangliao samples (844–1120 °C). Based on the differences in mineral chemistry and temperature, we conclude that the Jiaohe and Shuangliao peridotites and pyroxenites were mainly derived from lithospheric mantle accreted from carbonated regions of the asthenosphere, whereas the Wangqing wehrlites and pyroxenites were formed by reaction of lithospheric mantle peridotites with hot carbonated silicate melts. These results provide evidence for two different types interactions between of carbonate-related melts and mantle peridotite, which are important for understanding the chemical evolution of the upper mantle in the big mantle wedge of northeastern Asia.

Origin and evolution of lithospheric mantle beneath the juvenile Arabian–Nubian Shield: Evidence from the Natash peridotite xenoliths, Southern Eastern Desert, Egypt

Compared with the Archean cratonic mantle, much less is known about the genesis and composition of the subcontinental lithospheric mantle (SCLM) beneath the juvenile terranes. In this study, we present petrology and mineral chemistry analyses of peridotite xenoliths carried by Cretaceous volcanics from the Natash area in the western Arabian–Nubian Shield (ANS). Combined with mantle xenoliths from the eastern ANS, we aim to constrain the nature, origin, and evolution of SCLM beneath the juvenile terrane. The Natash peridotite xenoliths, including well-preserved clinopyroxene (Cpx) and spinel (Sp), are categorized into two groups. Group A peridotites display exceptionally high Mg# (molar100 × Mg/(Mg + Fetotal)) in Cpx (95.6–96.7) and Cr# (molar100 × Cr/(Cr + Al)) in Sp (45.5–51.7), signifying a high degree of melting and refractory nature. Cpx in Group A demonstrates low Ti/Eu (532–1785) and high (La/Yb)N (5.34–26.29) ratios, indicative of carbonatitic metasomatism. Comparatively, Group B peridotites exhibit relatively high Mg# in Cpx (93.8–95.5) and Cr# in Sp (15.9–26.2), indicating a low degree of melting and fertile nature. Cpx trace element patterns subdivide Group B into three sub-groups. Cpx in Group B1 are characterized by high Ti/Eu (3934–5622) but low (La/Yb)N (0.91–2.98), implying silicate melt metasomatism. Both Groups B2 and B3 Cpx show steep rare earth element (REE) patterns, negative anomalies in high field-strength elements, and high (La/Yb)N (3.62–7.57). However, Cpx in B3 exhibit higher Ti/Eu ratios and water-soluble element contents than those in B2 peridotites. In addition, the spoon-shaped REE patterns are prominently unique to Cpx in Group B3. Hence, it is inferred that Group B2 were influenced by carbonatitic melts, whereas the evolved small-volume CO2-H2O-rich fluids infiltrated into Group B3 samples. Furthermore, the high Sr contents (71–404 ppm) but low 87Sr/86Sr (0.7003–0.7034) ratios of Cpx in Natash peridotites suggest that the various metasomatic agents all originated from the asthenosphere. Collectively, the Natash peridotite xenoliths exhibit similar nature and metasomatism processes with those from Cenozoic volcanics in the eastern ANS, and they all show affinity to continental peridotites. The pMELTS simulation suggests that the refractory peridotites have a high melting pressure (typically >3 GPa), while the fertile samples register a low melting pressure. Consequently, we infer that the SCLM beneath the juvenile ANS was formed through the cooling of upwelling asthenosphere, incorporating the ancient mantle fragments, and subsequently underwent asthenosphere-derived carbonatitic and silicate melt/fluid metasomatisms.