Sublithospheric Mantle Research Articles

ГАББРОИДЫ И МОНЦОНИТОИДЫ ТИГРИНОГО РУДНОГО УЗЛА (СИХОТЭ-АЛИНЬ): СОСТАВ, ВОЗРАСТ, ОБСТАНОВКИ ФОРМИРОВАНИЯ И СВЯЗЬ С ОРУДЕНЕНИЕМ

The article presents the results of the study of mineralogical and petrographic characteristics, composition and age of gabbroids and monzonitoids of the Burelomny Creek massif located in the Central Sikhote-Alin and spatially associated with the Tigriny rare-metal-granite intrusion, and Sn-W deposit. It is shown that the gabbroids and monzonitoids of the massif correspond to the high-potassium units of the normal and moderately alkaline series, and have low concentrations of titanium, high concentrations of phosphorus, moderate concentrations of HFSE and REE elements. The geochemical characteristics of the rocks indicate that their source combines lithospheric and sublithospheric mantle substrates and that they formed in the setting of a transform continental margin. The age of the gabbroids, determined by the Ar-Ar dating of a biotite monofraction, is 101+1.5 Ma and is synchronous with the "peak" of basite and granitoid magmatism in the Sikhote-Alin. A significant difference in age (more than 20 Ma) and discrepancy in geochemical specialization indicate that the Tigriny massif rare-metal granites, and associated mineralization have no genetic relationship with gabbroids and monzonitoids of the Burelomny Creek massif.

Mantellic degassing of helium in an extensional active tectonic setting at the front of a magmatic arc (central Mexico)

Abstract The physicochemical and isotopic characteristics of groundwater and dissolved gas of central Mexico provide valuable information about the geologic and tectonic context of the area. Low–high-enthalpy manifestations (up to 98 °C in springs and more than 100 °C in geothermal wells) are distributed within the San Juan del Río, Querétaro, and Celaya hydrologic basins, located at the boundary between the current Mexican magmatic arc and an extensional continental area with intraplate volcanism called Mesa Central Province. Groundwaters in the study area represent a mixture between the cold water end-member with a Ca2+-Mg2+-HCO3-composition and a hydrothermal end-member enriched in Na+, K+, SO42−, and Cl-. Cold and hot groundwaters δ2H and δ18O plot along the same evaporation lines and do not exhibit a magmatic input. Dissolved and free gas do not show a typical volcanic composition signature. He and Ne isotope composition provide evidence of an important contribution of non-atmospheric noble gases. Although helium composition mainly has a crustal origin (21–83%), the mantellic contribution (1–39%) is higher than expected for an area lacking recent volcanism. A volatilerich magma aging at depth was discarded as the source of this mantellic helium signature but points out a recent mantellic contribution. Thus, we propose that mantellic helium comes from the sublithospheric mantle into the shallow crust through the highly permeable tectonic boundaries between the geologic provinces, namely the N−S Taxco−San Miguel de Allende and Chapala-Tula fault systems. Mantellic helium flow rates through these fault systems were estimated to have values ranging from 0.1 m/yr to 2.9 m/yr. This He flux range implies that aside from subduction, mantle volatile degassing enhanced by crustal fault systems is the main degassing process in the region studied.

Subduction-related oxidation of the sublithospheric mantle evidenced by ferropericlase and magnesiowüstite diamond inclusions

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth’s lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93. Here we use Synchrotron Mössbauer Source (SMS) spectroscopy and single-crystal X-ray diffraction to determine the iron oxidation state and structure of two magnesiowüstite and three ferropericlase inclusions in diamonds from São Luiz, Brazil. Inclusion Mg#s vary between 16.1 and 84.5. Ferropericlase inclusions contain no ferric iron within the detection limit of SMS, while both magnesiowüstite inclusions show the presence of monocrystalline magnesioferrite ((Mg,Fe)Fe3+2O4) with an estimated 47–53 wt% Fe2O3. We argue that the wide range of Fe concentrations observed in (Mg,Fe)O inclusions in diamonds and the appearance of magnesioferrite result from oxidation of ferropericlase triggered by the introduction of subducted material into sublithospheric mantle.

Lithospheric S-velocity structure of the on-shore Potiguar Basin, NE Brazil: High heat-flow in an aborted rift

The lithospheric structure of the on-shore Potiguar Basin has been investigated through velocity-depth profiles developed from the joint inversion of receiver functions and surface-wave dispersion at 16 seismic stations in and around the basin. The Potiguar Basin is an aborted rift basin that formed during the opening of the South Atlantic Ocean in the Lower Cretaceous, and is characterized by an unusual surface heat-flow with values as high as 101 mW/m2. Our results reveal: (i) A relatively thin crust of ∼30 km below the on-shore Potiguar Basin and a relatively thicker crust of ∼32 km around the basin; (ii) the existence of an anomalous uppermost mantle of ∼4.3 km/s at 30–40 km depth under most seismic stations; and (iii) the presence of a negative velocity gradient centered at ∼125 km depth, which probably represents a shallow Lithosphere Asthenosphere Boundary (LAB). We argue that the anomalous uppermost mantle is associated with magmatic intrusions just below the Moho, deeper than previously postulated from independent heat-flow studies, and that those intrusions result from heating by an active, hot sublithospheric mantle under the basin that keeps the lithosphere thin. We further argue that heating from the magmatic intrusions, along with direct heating from the sublithospheric mantle, may explain the unusually elevated heat flow observed at the surface.

Heterogeneous seismic anisotropy beneath Madeira and Canary archipelagos revealed by local and teleseismic shear wave splitting

SUMMARYMid-plate upward mantle flow is a key component of global mantle convection, but its patterns are poorly constrained. Seismic anisotropy is the most direct way to infer mantle flow as well as melt distribution, yet the convection patterns associated with plume-like mantle upwelling are understudied due to limited seismic data coverage. Here, we investigate seismic anisotropy beneath the Madeira and Canary hotspots using a dense set of shear wave splitting observations and combining teleseismic and local events recorded by three-component broad-band and short-period seismic stations. Using a total of 26 stations in the Madeira archipelago and 43 stations around the Canary Islands, we obtain 655 high-quality measurements that reveal heterogeneous flow patterns. Although local event results are sparse around most islands, we can observe a small average of S-wave splitting times of 0.16 ± 0.01 s, which significantly increase with source depth beneath El Hierro (>20 km) and Tenerife (>38 km) up to 0.58 ± 0.01 and 0.47 ± 0.05 s. This suggests an influence of melt pocket orientation in magma reservoirs developed at uppermost-mantle depths. Likewise, anisotropy increases significantly beneath the islands with shield stage volcanism (up to 9.81 ± 1.78 per cent at El Hierro, western Canaries, against values up to 1.76 ± 0.73 per cent at Lanzarote, eastern Canaries). On average, teleseismic SKS-wave splitting delay times are large (2.19 ± 0.05 s), indicating sublithospheric mantle flow as the primary source for anisotropy in the region. In the Canaries, the western islands show significantly smaller average SKS delay times (1.93 ± 0.07 s) than the eastern ones (2.25 ± 0.11 s), which could be explained by destructive interference above the mantle upwelling. Despite complex patterns of fast polarization directions throughout both regions, some azimuthal pattern across close stations can be observed and related to present-day mantle flow and anisotropy frozen in the lithosphere since before 60 Ma. Additionally, we infer that the current presence of a mantle plume beneath the archipelagos leads to the associated complex, small-scale heterogeneous anisotropy observations.

Trace element and Sr-Nd-Hf-Pb isotopic constraints on the composition and evolution of eastern Anatolian sub-lithospheric mantle

Quaternary Elazığ mafic alkaline volcanism is part of the anorogenic volcanic system of the circum-Mediterranean region, and it provides crucial insights into the chemical nature of the mantle source domains beneath eastern Turkey. Elemental and isotope geochemistry reveals that these mafic lithologies are mainly free of crustal contamination , reflecting geochemical signatures inherited from partial melting of their mantle sources. The trace element and Sr-Nd-Hf-Pb isotope systematics of the most primitive members of this volcanism are consistent with a predominant contribution from a sub-lithospheric component which compositionally resembles component C, believed to be common to global OIBs and MORBs . However, the total range of the isotope data also requires the involvement of two additional mantle components into their genesis: a depleted asthenospheric component (DM) and an enriched component (EM-type) representing metasomatized lithospheric mantle. We propose that the recycled oceanic lithologies with C-like signatures are dispersed in the asthenospheric mantle beneath eastern Anatolia and have undergone mixing with the depleted mantle matrix. When considering these with the melting models, it appears that the garnet-facies partial melts of the C-DM mixture have experienced a small degree of interaction with the melts from an SCLM domain modified by slab-derived melts and/or fluids. The metasomatized SCLM domain may be intrinsic to the eastern Anatolian lithosphere or occur as pods embedded in the asthenosphere . The chemical characteristics of the Elazığ Volcanics may confirm the wide-range dispersal of the common component in the sub-lithospheric mantle beneath the region extending from western Europe to eastern Turkey. However, the common component resides beneath Anatolia appear to have distinct composition (i.e., Pb isotope systematics) than the ubiquitous component shared mostly by European intraplate volcanics (i.e., LVC/EAR). • The sub-lithospheric mantle beneath eastern Turkey is heterogeneous. • A recycled C-like component is dispersed in the sub-lithospheric mantle. • The C-like component mixed with the depleted asthenospheric mantle matrix • Elazığ Volcanics represent melts predominantly derived from a C-DM mixture.

Geochemistry of dolerite intrusions of the southeastern Main Karoo Basin, South Africa: Magma evolution, evidence for thermogenic gas sequestration, and potential implications for the early Toarcian Oceanic Anoxic Event

The igneous intrusions of the Karoo Large Igneous Province (LIP) provide critical information on the magma plumbing system, which served as source rock maturation agent for organic-rich sedimentary rocks of the Main Karoo Basin as well as pathway and barrier to subsurface fluid flow. Here, we present results from the research borehole KWV-01 drilled in the Eastern Cape Province of South Africa. The intersected succession, spanning the Dwyka to the lower Beaufort Group, provides representative samples of the Karoo LIP magma plumbing system.Using whole-rock and Sr-Nd isotope analyses, our data suggest that primary melts were derived from a sub-lithospheric mantle source and acquired a subduction signature through interaction with a metasomatised lithospheric mantle. Three magma types identified within this study show a geochemical overlap with previously reported data from the Golden Valley Sill Complex, ca. 220 km to the NW of the analysed borehole. The results of MELTS and EC-RAFC modelling on the three magma types reveal that their compositional differences are related to variable degrees of fractional crystallisation and contamination by lower crustal rocks and/or the sedimentary host rocks of the Main Karoo Basin.Our results indicate that at least one of the sills acted as fractured reservoir, thereby providing evidence that igneous intrusions are important for CO2 sequestration. We quantified the implications of this process for the Karoo LIP-related Toarcian Oceanic Anoxic Event using the thermogenic gas budget produced by state-of-the-art computer simulations and tested various end-member scenarios for partial thermogenic gas sequestration. In agreement with recent studies, we show that all the flat parts of the Karoo sills acted as sequestration reservoirs. Our results have general implications for LIP volcanism in organic-rich basins, suggesting that a good first-order approximation of thermogenic gas emission is half the amount mobilised in the contact aureoles of LIP plumbing systems.

The Kalba batholith dyke swarms (Eastern Kazakhstan): Mafic magmas effect on granite formation

The Kalba granitoid batholith formed in Early Permian is one of the largest in the western part of the Central Asian Orogenic Belt. Batholith granites are intruded by mafic and intemediate dykes forming several swarms. Previously it was believed that the dykes were Mesozoic and had no relationship with the formation of the batholith. Our geochronological studies allow to determine the age of the dykes at 279 Ma (also Early Permian). Dyke rocks are represented by dolerites, variolites, monzodiorites, and plagioclase porphyrites. The petrographic study of plagioclase porphyrites allow to find the sign of magma mixing. The another indicator of magma mixing are the contacts between mafic and felsic rocks in largest dykes of complex structure. A model for the interaction of mafic magmas with crustal substrates is proposed. Mafic magmas intruded from sublithospheric mantle, provoked a partial melting and then interacted with granitic melts at the basement level of granite batholith. Dyke swarms were formed after the cooling and consolidation of main volume of Kalba batholith by inrtusion of mafic and hybride magmas from the basement level of granite batholith. Chemical diversity of the mafic dykes was caused mainly by the differentiation of parental mafic magmas and also a lesser extent by their interaction with the granitoid magmas. Comparison with data on coeval magmatism in neighboring regions suggests that the Kalba batholith may be included in areal of Early Permian Tarim Large Igneous Province. • The dyke swarms cut the granites of the Kalba batholith in Eastern Kazakhstan. • The dykes and the Kalba batholith granites are Early Permian. • Dyke rocks composition was caused mainly by the differentiation of mafic magmas. • Parent mafic magmas formed in lithospheric mantle and provoked melting in crust. • Geodynamic setting was a post-orogenic extension with effect of Tarim mantle plume.

Sublithospheric melt input in cratonic lamproites

AbstractCratonic lamproites are diamondiferous ultrapotassic rocks that are emplaced through thick continental lithosphere and thought to derive from melting of metasomatized (i.e., geochemically enriched) regions of the subcontinental lithospheric mantle (SCLM). We explored the alternative hypothesis that melts sourced from sublithospheric (i.e., convective mantle) sources dominate the genesis of cratonic lamproites. Supporting evidence includes a robust linear correlation between the Mg/Fe ratios of xenocrystic and magmatic olivine in lamproites worldwide, overlapping the trend observed for kimberlite olivine. This indicates that, similar to kimberlites, primary lamproite melts originate from broadly similar sublithospheric mantle sources before assimilating SCLM material of variable composition. The lamproites are also characterized by a direct correlation between olivine Mg/Fe ratio and bulk-lamproite K2O/Al2O3, an index of potassium enrichment in the melt that is independent of mantle-xenocryst entrainment and magmatic differentiation. Quantitative modeling indicates that this correlation results from the interaction between carbonate-bearing sublithospheric melt and phlogopite-rich wall rocks in the SCLM. Our data show that cratonic lamproites and kimberlites have similar mantle sources in the convective mantle, with lamproites acquiring their peculiar enrichment in K by interaction with metasomatized SCLM en route to the surface. Modification of sublithospheric melts during transit through the continental lithosphere might represent a common process for the genesis of alkaline mafic/ultramafic magmas.

Coupled influence of tectonics, climate, and surface processes on landscape evolution in southwestern North America

The Cenozoic landscape evolution in southwestern North America is ascribed to crustal isostasy, dynamic topography, or lithosphere tectonics, but their relative contributions remain controversial. Here we reconstruct landscape history since the late Eocene by investigating the interplay between mantle convection, lithosphere dynamics, climate, and surface processes using fully coupled four-dimensional numerical models. Our quantified depth-dependent strain rate and stress history within the lithosphere, under the influence of gravitational collapse and sub-lithospheric mantle flow, show that high gravitational potential energy of a mountain chain relative to a lower Colorado Plateau can explain extension directions and stress magnitudes in the belt of metamorphic core complexes during topographic collapse. Profound lithospheric weakening through heating and partial melting, following slab rollback, promoted this extensional collapse. Landscape evolution guided northeast drainage onto the Colorado Plateau during the late Eocene-late Oligocene, south-southwest drainage reversal during the late Oligocene-middle Miocene, and southwest drainage following the late Miocene.

Multi-thermobarometry and microstructures reveal ultra-high temperature metamorphism in the Grenvillian Oaxacan Complex, Southern Mexico

ABSTRACT The Oaxacan Complex represents the largest outcrop of the Mesoproterozoic microcontinent Oaxaquia. It experienced a major tectonometamorphic event, locally known as Zapotecan Orogeny (ca. 1004–980 Ma), during the latest stages of the amalgamation of the supercontinent Rodinia. It is typically interpreted that the syn-tectonic metamorphism occurred at granulite facies conditions (<800°C and ≈0.7 GPa). However, the mineralogical record and recent classic geothermobarometry suggest that the Oaxacan Complex reached peak ultra-high-temperature metamorphism (UHTM) but no conclusive results have been obtained. In this work, we present a combination of microstructural observations, mineral chemistry, phase equilibria thermobarometry, and trace element thermometry to document the processes associated with the granulite petrogenesis and calculate the peak metamorphic conditions. Petrographic observations show that the peak syn-tectonic metamorphism was contemporaneous to partial melting. This is revealed by the presence of euhedral and subhedral plagioclase and feldspar growing with quartz, melt relics included in garnets, films, or veinlets surrounding peak phases, among others. Exsolved clinopyroxene on orthopyroxene, exsolved rutile needles in garnet, ubiquitous mesoperthites, and F- and Ti-rich biotite grains indicate UHTM. The combination of phase equilibria modelling in two metapelites and one garnet-charnockite, along with Zr-in-Rutile and Ti-in-Quartz thermometry, confirms UHT conditions for the Zapotecan Orogeny (T = 895–916°C and P = 0.83–0.99 GPa). Preconditioning of the crust during the earlier Olmecan tectonothermal event (ca. 1100 Ma) and the ascent of the sub-lithospheric mantle follwed by the Zapotecan crustal thickening might account for the UHTM recorded in the Oaxacan rocks. The recognition of syn-tectonic UHTM in the Oaxacan Complex supports palaeographic reconstructions that place Mexican-Colombian terranes entrapped among Amazonia and Baltica during their collision as part of the amalgamation of Rodinia. Our novel results provide new avenues for petrological research for a better understanding of Oaxaquia and its role in the final configuration of Rodinia.

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths, raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle. They are the major source of information on the state of stress,pressure and temperature in the deep mantle. They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface. Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano. The Tello is the continental sector of the Cameroon Line, located in the South East of the town of Ngaoundéré at 75 km approximately between (N7° 13’, N7° 14’) and (E13° 40’ and E13° 60’). Minerals’ composition of the xenoliths is ~64% olivine, ~24% orthopyroxene, ~11% clinopyroxene and ~1% spinel. Significant variation in (Cr/Cr + Al) of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle. Their mineral chemistries suggest equilibrium condition of 830° - 925° and 1.4 GPa-2.3 GPa. These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel. The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively. The majority of samples caracterise the lithospheric mantle.

Volatile-bearing Partial Melts in the Lithospheric and Sub-Lithospheric Mantle on Earth and Other Rocky Planets

Research Article| May 01, 2022 Volatile-bearing Partial Melts in the Lithospheric and Sub-Lithospheric Mantle on Earth and Other Rocky Planets Rajdeep Dasgupta; Rajdeep Dasgupta Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA rajdeep.dasgupta@rice.edu Search for other works by this author on: GSW Google Scholar Proteek Chowdhury; Proteek Chowdhury Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USACurrent address: Department of Earth and Planetary Sciences, University of California at Riverside, 900 University Avenue, Riverside, CA 92521, USA proteekc@ucr.edu Search for other works by this author on: GSW Google Scholar James Eguchi; James Eguchi Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USACurrent address: Department of Earth and Planetary Sciences, University of California at Riverside, 900 University Avenue, Riverside, CA 92521, USA james.eguchi@ucr.edu Search for other works by this author on: GSW Google Scholar Chenguang Sun; Chenguang Sun Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USADepartment of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA csun@jsg.utexas.edu Search for other works by this author on: GSW Google Scholar Sriparna Saha Sriparna Saha Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USACurrent address: School of Teacher Education and School of Earth and Environment, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand sriparna.saha@pg.canterbury.ac.nz Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2022) 87 (1): 575–606. https://doi.org/10.2138/rmg.2022.87.12 Article history first online: 01 May 2022 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Rajdeep Dasgupta, Proteek Chowdhury, James Eguchi, Chenguang Sun, Sriparna Saha; Volatile-bearing Partial Melts in the Lithospheric and Sub-Lithospheric Mantle on Earth and Other Rocky Planets. Reviews in Mineralogy and Geochemistry 2022;; 87 (1): 575–606. doi: https://doi.org/10.2138/rmg.2022.87.12 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Generation and extraction of partial melts from the interior of rocky planets are essential steps for supplying heat and life-essential nutrients to the surface environments. Mantle-derived partial melts dissolve key volatile elements such as carbon, hydrogen, and sulfur and when brought to the surface or near-surface conditions, liberate them as gases such as carbon dioxide (CO2), water vapor (H2O), and sulfur dioxide (SO2) or hydrogen sulfide (H2S). These gases influence the composition of the atmosphere–ocean systems, silicate-weathering feedback, (Walker et al. 1981) and/or ocean chemistry, thereby affecting the climate on... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An apatite trace element and Sr-Nd isotope geochemical study of syenites and carbonatite, exemplified by the Epembe alkaline‑carbonatite complex, Namibia

The Epembe Alkaline Carbonatite Complex (EACC) in northwestern Namibia was emplaced along a fault zone into medium- to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC), and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Nepheline syenite with minor syenite constitute the main lithologies, cross-cut by a calcite‑carbonatite dyke. Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were studied using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189–44,100 ppm) with light rare-earth element (LREE) enrichment (LaN/YbN = 4–91) relative to heavy (H) REE consistent with a magmatic origin. Negative Eu anomalies (Eu/Eu* = 0.4–0.9) in syenite apatite are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite and carbonatite-hosted apatite is also commonly homogeneous in CL, while core-rim zoning and patchy textures are observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (LaN/YbN = 24–9) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by rim-ward increase in REE concentrations, which can be attributed to mineral fractionation. Patchy apatite is depleted in Na, Y and REE, particularly the LREE (LaN/YbN = 4–19) relative to other nepheline syenite apatite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the alteration fluids. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. An LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages are 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs <4.0, 2 SE). The Sr and Nd isotopic composition of apatite in syenite (87Sr/86Sr(i) = 0.7035–0.7048; ƐNd(t) = +2.5 to +3.2), nepheline syenites (87Sr/86Sr(i) = 0.7031–0.7037; ƐNd(t) = +1.5 to +4.4) and carbonatite (87Sr/86Sr(i) = 0.7031–0.7033; ƐNd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous source, located in the sub-lithospheric mantle.

Zinc isotopic systematics of the Mt. Baekdu and Jeju Island intraplate basalts in Korea, and implications for mantle source lithologies

Zinc isotope data for late Cenozoic intraplate basalts from Mt. Baekdu (North Korea) and Jeju Island (South Korea) were determined to characterize their mantle source lithologies. The average δ66Zn values (relative to JMC-Lyon) are 0.34‰ ± 0.08‰ (2SD; N = 13) for the Mt. Baekdu basalts and 0.37‰ ± 0.08‰ (2SD; N = 12) for the Jeju basalts, which are higher than values for refractory peridotites (0.18‰ ± 0.06‰) from the lithospheric mantle beneath the North China Craton. Given that Zn isotopic fractionation is limited during lithospheric processes, such as crustal contamination, magmatic differentiation, degassing, or thermal/chemical diffusion, the elevated δ66Zn values indicate the incorporation of recycled carbonates into the sub-lithospheric mantle source. Plots of Zn/Fe versus δ66Zn or δ26Mg for the Korean basalts can be explained by mixing of at least three source lithologies: (1) carbonated peridotite; (2) recycled siliciclastic sediments; and (3) eclogite. Strontium–Nd–Pb isotopic systematics show that the mantle source follows the DMM-EM1 array for the Mt. Baekdu basalts and a DMM-EM2 array for the Jeju basalts. Both the enriched components have isotopically light Zn and heavy Mg, which are typical of recycled siliciclastic sediments. Combined with the Pb isotope and trace element data, the Zn isotopic compositions suggest that the EM1 component could be ancient K-hollandite-bearing siliciclastic sediments and the EM2 component could be recently recycled clay-rich pelagic sediments.

Towards composition of carbonatite melts in peridotitic mantle

It is generally accepted that carbonatite metasomatism in the subcontinental lithospheric mantle (SCLM) inevitably causes wehrlitization of the primary lherzolite substrate. However, the K-rich carbonatite inclusions in kimberlitic diamonds containing orthopyroxene indicate that this is not always the case. In the present study, we equilibrated natural garnet lherzolite with carbonate melts containing 33–38 wt% K2O with various Ca# = 10, 20, 30, and 40 at 6 GPa and 1200–1500°C, where Ca# = 100⋅Ca/(Ca+Mg+Fe). The original ratio of peridotite to carbonate was 58 to 42 by weight. In the studied temperature range, the melt retains essentially carbonate composition with silica content increasing from 1 to 11–12 wt%. The melt with Ca# 10 alters lherzolite to harzburgite, replacing clinopyroxene by orthopyroxene and decreasing CaO content in garnet below 4 wt%. The melts with Ca# 20-30 also consume clinopyroxene; although CaO content in garnet remains in the range of lherzolitic compositions. The melt with Ca# 40 yields wehrlitization, consuming orthopyroxene, increasing clinopyroxene fraction, and increasing CaO content in garnet above 6 wt%. After the interaction, the Ca# of the melt changes as follows 10 → 16–28, 20 → 20–33, 30 → 27–34, and 40 → 30–34. The olivine + orthopyroxene + clinopyroxene + garnet assemblage was found in equilibrium with carbonatite melt with Ca# 34 at 1200°C and Ca# 30 at 1400°C. Thus, K-rich (26–35 wt% K2O) carbonatite melts with Ca# = 30-34 can appear in equilibrium with garnet lherzolite, while the melts with Ca# < 30 and > 34 can be in equilibrium with harzburgite and wehrlite, respectively, at 6 GPa and 1200–1400°C. Considering that Ca-Mg-Fe carbonates do not melt at the geothermal conditions of the SCLM, while sodic, dolomitic melt causes wehrlitization, high-Mg (Ca# < 35) K-rich dolomitic melt is the only possible carbonatite fluids that are thermodynamically stable in equilibrium with garnet harzburgites and lherzolites in the SCLM at a depth of about 200 km. At higher temperatures corresponding to the underlying asthenosphere, the high alkalinity ceases to be a requirement for the stability of the carbonate melt. Nevertheless, the regularities established here for the K-rich melts remain valid for less alkaline (4–15 wt% Na2O+K2O) primary kimberlite (i.e., mantle carbonatite) melts in the sublithospheric mantle.

Sublithosphere Mantle Crystallization and Immiscible Sulfide Melt Segregation in Continental Basalt Magmatism: Evidence from Clinopyroxene Megacrysts in the Cenozoic Basalts of Eastern China

AbstractThis study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to &amp;gt;10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts.

Crustal and upper mantle density structure below the Indian Ocean Geoid Low based on 3-D constrained potential field modelling: Inferences on causative sources

The Indian Ocean Geoid Low (IOGL) in the Central Indian Ocean is the largest geoid depression on the globe. Spectral decomposition of geoid anomalies reveals that a significant fraction (up to ~90%) of the IOGL is due to mass anomalies lying within the lower mantle (~ > 700 km). The crustal density structure and Moho depth variations obtained from 3-D gravity inversion along with the upper mantle density structure derived from SL2013sv tomographic model are used to compute the total geoid anomaly up to 700 km depth and compare it with degree-10 residual geoid anomaly map. Comparison of these two maps necessitated the presence of relatively shallow low- density sources within the upper mantle that were not resolved in the 3-D modelling. The presence of two anomalous sources: i) low S-wave velocities at the base of the LAB (4.4 km/s) and ii) an anomalous source of Vs = −1.0% at a depth of 320–400 km within the sub-lithospheric mantle observed in the tomographic model accounted for this difference. We model the geometry of these low-density anomalous bodies using 2-D joint gravity-geoid modelling, which reveals the total contribution of density structure up to the upper mantle depths (700 km) accounts for only 10% of the IOGL whereas the remaining part is primarily caused by mass anomalies in the lower mantle probably related to high-density subducted slabs or plume sources at the core-mantle boundary as proposed by earlier studies.

Evolution of mafic lavas in Central Anatolia: Mantle source domains

Abstract We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas, Develidağ, Erciyes, and Erkilet volcanic complexes in central Turkey and Tendürek in eastern Turkey to evaluate the mantle sources for volcanism in the context of the geodynamic evolution of the Anatolian microplate. Early Miocene through Quaternary volcanism in Western Anatolia and latest Miocene through Quaternary activity in Central Anatolia were dominated by contributions from two distinct source regions: heterogeneous metasomatized or subduction-modified lithosphere, and roughly homogeneous sublithospheric ambient upper mantle; we model the source contributions through mixing between three end members. The sublithospheric mantle source plots close to the Northern Hemisphere reference line (NHRL) with radiogenic 206Pb/204Pb of ∼19.15, while the other contributions plot substantially above the NHRL in Pb isotope space. The lithospheric source is heterogeneous, resulting from variable pollution by subduction-related processes likely including direct incorporation of sediment and/or mélange; its range in radiogenic isotopes is defined by regional oceanic sediment and ultrapotassic melts of the subcontinental lithospheric mantle. The geochemical impact of this contribution is disproportionately large, given that subduction-modified lithosphere and/or ocean sediment dominates the Pb isotope signatures of mafic Anatolian lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked crustal shortening, took place throughout the region until ca. 16–17 Ma, after which time broad delamination of the thickened lower crust and/or the Tethyan slab beneath Central Anatolia allowed for sediment and/or mélange and slab-derived fluids to be released into the overlying evolving modified mantle. Aggregation of melts derived from both mantle and lithospheric domains was made possible by upwelling of warm asthenospheric material moving around and through the complexly torn younger Aegean-Cyprean slab that dips steeply to the north beneath southern Anatolia.

New geochemical and geochronological data on the Cenozoic Veneto Volcanic Province: Geodynamic inferences

The Veneto Volcanic Province (VVP; NE Italy) is an intraplate magmatic area whose activity occurred intermittently in the Cenozoic, generating five districts (Val d'Adige, Lessini Mts., Marostica Hills, Berici Hills, and Euganean Hills). This intraplate magmatism was concomitant to the collision of the European plate and Adria microplate and the orogenesis of the neighboring Alpine belt. Different geodynamic models suggested relationships between VVP and the coexisting subduction processes. To give new insights on this on-going debate, this work provides new petrographic and geochemical data, including Sr-Nd-Pb isotopic and 40Ar/39Ar and KAr geochronological analyses for lavas sampled in the Lessini Mts. and Val d'Adige, which are the oldest VVP magmatic districts, as well as Marostica Hills and Berici Hills, which are the least investigated districts. The trace element distribution indicates that VVP melts were variously affected by metasomatic enrichments stabilized as phlogopite and/or amphibole. The Sr-Nd-Pb isotope ratios conform to the dominant features of sub-lithospheric mantle components widespread at regional scale throughout the whole European and Mediterranean area. Within this framework, the geochronological data indicate that the oldest magmatic episodes occurred in the Eocene (45–42 Ma) in the Lessini Mts. and Val d'Adige, simultaneously with the orogenic magmatism of Adamello intrusive complex along the Giudicarie Fault, a portion of the Periadriatic lineament which is an important Alpine suture. In our geodynamic reconstruction we propose that the Giudicarie Fault is the superficial expression of a slab tear, responsible for the uprising of an asthenospheric poloidal mantle flow, which induced both the Adamello and VVP magmatism. Subsequently, VVP activity migrated eastwards following the general mantle flow as indicated by minor volcanic pulses in the Euganean and Marostica Hills during Oligocene and Miocene.