Mantle Plume Research Articles

Caught in the moment: interaction of immiscible carbonate and sulfide liquids in mafic silicate magma—insights from the Rudniy intrusion (NW Mongolia)

The Devonian Rudniy intrusion is a composite magmatic body comprising two gabbroid units. Located in the Tsagaan-Shuvuut ridge in NW Mongolia, it is the only one known to contain disseminated sulfide Ni-Cu-PGE minerals out of numerous gabbroid intrusions surrounding the Tuva depression. The ore occurs as disseminated sulfide globules made of pyrrhotite, pentlandite, chalcopyrite, and cubanite, confined to a narrow troctolitic layer at the margins of a melanogabbro, at the contact with a previously emplaced leucogabbro. Globules generally display mantle-dominated sulfur isotopic signatures but show variable metallogenic and mineralogical characteristics, as well as notably different sizes and morphologies reflecting variable cooling and crystallization regimes in different parts of the intrusion. Sulfides from the chilled margin of the melanogabbro are surrounded and intergrown with volatile-rich (i.e., CO2-, H2O-, F-, and Cl) phases such as calcite, chlorite, mica, amphibole, and apatite. Based on the mineralogical and textural relationships of volatile-rich phases with sulfides, we argue that this assemblage represents the product of the crystallization of volatile-rich carbonate melt immiscible with both silicate and sulfide liquids. We put forward the hypothesis that volatile-rich carbonate melt envelops sulfide droplets facilitating their transport in magmatic conduits and that this process may be more widespread than commonly thought. The smaller sulfide globules, which are interpreted to derive from the breakup of larger globules during transport and emplacement, do not display an association with volatile-rich phases, suggesting that the original carbonate melt could have been detached from them during the evolution of the magmatic system. Variable rates of crystallization may have been responsible for the observed disparities in the mineralogical and metallogenic characteristics of different sulfide globules entrained in the Rudniy intrusion.

Petrogenesis of the U-rich Permian Akkulen syenite intrusion, Tien Shan, Kyrgyzstan: insights into its magmatic evolution and geodynamic setting

ABSTRACT Permian alkaline granitoids are widely distributed throughout the Tien Shan Orogen and adjacent region of the southern Central Asian Orogenic Belt. However, their petrogenesis and related tectonic setting remain equivocal. A detailed mineralogical, U-Pb zircon dating, in situ elemental and Sr-Nd-Hf isotopic studies of the uranium-rich Akkulen syenite intrusion in the Northern Tien Shan of Kyrgyzstan were undertaken to better understand its magmatic processes and geodynamic evolution. Four independent oxygen barometers have shown that the oxygen fugacity of the Akkulen syenitic magma evolved from initial low fO2 (<NNO +1, where NNO is the nickel-nickel oxide buffer) to relatively high fO2 (>NNO +1) during magma ascent and cooling; Apatite geochemical characteristics also indicate that the magma is relatively high F (2.9–5.1 wt.%), with low Cl (0.053–0.13 wt.%) and anhydrous (<4 wt %) contents. According to whole-rock geochemical data, the syenite samples have abnormally high U (38.5–57.1 ppm) and Th (96.2–137 ppm) concentrations, we suggest that high F content and moderate oxygen fugacity are beneficial for uranium enrichment. Zircon grains from a syenite sample yielded a weighted 206Pb/238U age of 283.5 ± 2.6 Ma. The intrusion has an A-type granite affinity with high alkali (Na2O + K2O) contents and Fe/(Fe+Mg) ratios, high LREE/HREE (~5), and low Sr, Ba, and Eu contents. They have negative εNd(t) (−5.2 to −3.6) values with Mesoproterozoic two-stage model ages (T DM2 = 1.35–1.47 Ga), and variable εHf(t) values (−4.6 to +2.4) with Mesoproterozoic two-stage model ages (T DM2 = 1. 2–1.59 Ga), which suggest these rock derivations from the relatively old crustal with minor contribution from juvenile crustal melts. In comparison with regional available dataset, we propose that the alkaline Permian granitoids in Kyrgyz Northern Tien Shan may have been originated from large-scale partial melting of Mesoproterozoic metamorphic basement with minor juvenile crustal material in a post-extensional setting. In contrast, the Early Permian granitoid magmatism in the Tarim Craton may have been caused by the partial melting of Neoproterozoic basement rocks associated with Permian mantle plume activities.

Basalts of a Seamount on the Eastern Flank of the Charlie Gibbs Transform Fracture Zone, North Atlantic: Petrochemical and Isotopic Evidence of a Microplume-Affected Formation in the Axial Zone of the Mid-Atlantic Ridge

The composition of basalts dredged from the slopes of a single seamount (guyot), which is located south of the eastern end of the Charlie Gibbs Fracture Zone in the North Atlantic, is studied. Groups of high- and low-Ti rocks, which belong to oceanic tholeiites of the E-MORB and T-MORB types, respectively, are recognized. The first group of basalts have higher contents of TiO2, Na2O, K2O, P2O5, Fe2O3, and incompatible trace elements. The basalts of both groups are similar in the normalized lithophile element patterns indicating their melting from a similar mantle substrate. The difference in their composition is probably caused by melting of high-Ti volcanic rocks at a deeper intermediate level between spinel and garnet facies. Our studies give grounds to believe that the seamount also formed as a large neovolcanic rise 64–67 Ma ago under the influence of a microplume, which was a branch of the Milne deep mantle plume.

182W and 187Os constraints on the origin of siderophile isotopic heterogeneity in the mantle

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/RA). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary.

Mineralogy and geochemistry of trachytic rocks from the Lichi Volcanics, Eastern Himalaya: insights into the Kerguelen mantle plume activity in the Eastern Himalayan Region

Mineralogy and geochemistry of trachytic rocks from the Lichi Volcanics, Eastern Himalaya: insights into the Kerguelen mantle plume activity in the Eastern Himalayan Region

Hydrothermal activity and metallogenic environment revealed by seismic wave tomography in Yunnan Province, China

S-wave velocity structures are revealed using double-difference tomography, which shows large-scale low-velocity anomalies around the Xiaojiang and Lijiang−Xiaojinhe fault zones at 10−25 km depth. In contrast, an obvioushigh-speed anomaly exists in the center of Emeishan area, which is consistent with the EmeishanLarge Igneousprovince. Combined with the geological and geophysical data, the low-velocity anomaly might relate to the lower crustal flowof central Tibetan Plateau. The high-velocity anomaly associated with mantle plume in Emeishan would obstruct the crustal flow and limit it around the large deep faults. Meanwhile, the results also show that many mineral resources are mainly concentrated on the major faults and low-velocity zones, indicating that the lower crustal flow and the large deep faults provide conditions for mineral fluids and minerals deposit.

Deep Seismic Reflection Imaging of Mesozoic Kachchh Rift, NW India: Implications for Evolution

AbstractDeep crustal seismic reflection profiling is carried out for first time across various tectonic domains of seismically active Mesozoic Kachchh rift basin, formed during the breakup of Gondwanaland. The seismic data, processed using the common reflection surface stack approach, provided maiden images of the shallow and deep sub‐surface structures in the region. These images reveal a 15 km thick subhorizontal lower crustal reflection fabric and crustal‐scale domal‐type structure extending from the surface to the Moho. We interpret the earlier structure represent magmatic underplating and the latter as the Kachchh Mainland uplift. We find large variations in the thickness of sediments from 150 m to 6.5 km and crustal thickness from 45 to 35 km from north to south, with a Moho up warp of 4 km beneath the Kachchh Mainland fault. The Kachchh rift basin exhibits an unusually thick crust of 45 km, contrary to many rift basins. We interpret the syn‐rifting and the Reunion mantle plume activity, manifested as Deccan volcanics, are responsible for magmatic underplating and crustal thickening. Uplift in the region is multi‐genetic in origin. Present study illuminated new faults and nature of various other faults. Moderate to large earthquakes in the region are attributed to the regional and local stresses resulting from the plate boundary and heterogeneous crustal structure. Based on the mafic lower crust and distribution of aftershocks through the entire crust, we interpret the lower crust is brittle, contrary to most models of continental rheology.

Khangai Intramantle Plume (Mongolia): 3D Model, Impact on Cenozoic Tectonics and Comparative Analysis

The Khangai plume is located beneath Central and Eastern Mongolia and corresponds to the mantle volume with significantly reduced longitudinal wave (P) velocities. The plume was identified as a result of the analysis of the MITP08 volumetric model of variations in P wave velocities, expressed as deviations of these velocities from the mean values for the corresponding depths in percent. Above the plume, the lithospheric mantle is thinned to ~50 km. Particularly low velocities (up to –6%) were found in the sublithospheric mantle down to a depth of 400 km. The main body of the plume is located under the Khangai Highland and spreads north to the edge of the Siberian Platform. The Khentei branch of the plume is identified southeast of the Khentei Highlands. It is connected to the main body of the plume at depths of 800–1000 km. Branches of the plume and its Khentei branch spread to Transbaikalia. The size of the plume decreases with depth, and its deepest part (1250–1300 km) is located under the southern part of the Khangai Highland. On the Earth’s surface, the main body of the Khangai plume corresponds to a Cenozoic uplift up to 3500–4000 m high in the south of the Khangai Highland. From the southeast, the territory of the Khangai plume and its Khentei branch is limited by the Late Cenozoic troughs stretching along the southeastern border of Mongolia. On other sides, the Khangai uplift is limited by a C-shaped belt of depressions, consisting of the southeastern part of the Baikal rift zone, the Tunka and Tuva basins in the north, the Ubsunur Basin and the Great Lakes Basin in the west and the Valley of Lakes in the south. The depressions are filled with lacustrine and fluvial sediments from the Late Oligocene to the Pliocene. In the Quaternary, the Southern and Central basins of Baikal, formed no later than the Early Paleogene, became part of the Baikal rift, and other depressions were involved in the general uplift of the region. The structural paragenesis of the Khangai uplift and surrounding basins is due to the impact of the Khangai plume. Above the plume with its Khentei and Transbaikalian branches, the Cenozoic basaltic volcanism of the plume type occurred, in some places inheriting Cretaceous volcanic manifestations. Plume structural paragenesis is combined with structural paragenesis, derived from the interaction of plates and lithosphere blocks, which is expressed by active faults, but developed synchronously with plume paragenesis. The kinematics of active faults shows that the western and central parts of the region develop under conditions of transpression, and the northeastern part ‒ under conditions of extension and transtension. The Khangai plume is connected at depth with the Tibetan plume, located under the central and eastern parts of Tibet north of the Lhasa block. The Tibetan plume rises from depths of 1400–1600 km and is accompanied by thinning of the lithosphere and rise of the earth’s surface. The Khangai and Tibetan plumes represent a special category of plumes that rise from the upper part of the lower mantle and this differs from the upper mantle plumes and the African and Pacific superplumes, rising from the core-mantle boundary. A connection between the Khangai and Tibet plumes with branches of superplumes is possible, but their independent origin is also admitted.

Low–Ti Continental Tholeiite Origin of Magmas With Calc‐Alkaline Signature in Transcurrent Settings: The Mississippian Matachel Volcanic Field (SW Iberian Massif)

AbstractIn the Mississippian Matachel small volcanic field of the Ossa‐Morena Zone (southern Iberian Massif) outpoured basic‐intermediate lavas exhibit geochemical characteristics of Low‐Ti continental tholeiites and calc‐alkaline lavas. Low‐Ti continental tholeiites integrate two contrasting groups of rocks: basalts (Mg#: 54 to 70; Ti/Zr: 61–79; LaN/LuN: 1.6–2.9; εNdi: +4.0–+6.6; “Group #1”), and basalts and basaltic andesites (Mg#: 43 to 66; Ti/Zr: 36–58; LaN/LuN: 2.5–5.9; εNdi: −0.2–+3.5; “Group #2”). Primitive Group #1 tholeiitic magmas were generated by partial melting of a garnet‐free lherzolite from an enriched lithospheric mantle, near the lithosphere‐asthenosphere thermal boundary layer (with a very limited asthenosphere melting input). Progressive interaction of these magmas with crustal alkali igneous rocks resulted in the formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low‐Ti tholeiites. Further assimilation of amphibole‐rich calc‐alkaline metaigneous rocks might have originated the basalts and basaltic andesites with calc‐alkaline signature (Mg#: 33 to 56; Ti/Zr: 25–78; LaN/LuN: 2.0–5.6; εNdi: +2.8–+4.8). These exhibit a “Cordilleran‐type” evolutionary trend, though are unrelated to plate convergence. The magmas with calc‐alkaline signature attest to a closed‐system differentiation process controlled by the fractionation of plagioclase, clinopyroxene, magnetite and ilmenite. It is proposed that Mississippian lithospheric‐scale intra‐continental wrenching, unrelated to coeval mantle plume upwelling, reworked complexly docked mantle domains and triggered mantle melting. Enduring mid‐upper crustal processes (magma storage in mid‐crustal chambers and crustal assimilation) likely shaped the latest petrologic and geochemical aspects of the Matachel Low‐Ti tholeiites and related rocks with calc‐alkaline signature.

High-resolution lithospheric structure of continental China from joint inversion of surface wave and gravity data

The lithospheric structure of continental China has been previously determined by seismic travel time tomography, surface wave tomography, and joint inversion of body wave and surface wave data. However, due to the inherent limitations of seismic data, the lithospheric structure of continental China is still not well resolved in the shallow part and in some regions where the station coverage is relatively sparse. In this study, we aim at improving the lithospheric structure by joint inversion of seismic surface wave data and satellite gravity data to take advantage of the uniform distribution and complementary strength of the gravity data. The empirical relationship between velocity and density is used as a bridge for joint inversion of surface wave and gravity data. The joint inversion shear-wave velocity Vs (density) model, named as USTClitho1.0g, can fit both surface wave and gravity data well. This high-resolution Vs model can better fit the active airgun source seismic arrival times and better delineate some features in continental China, such as the velocity contrast across the north–south gravity lineament (NSGL), the lithosphere thinning in eastern China, the crustal footprint of the Hainan mantle plume, the likely magma chamber beneath volcanos in northeast China, the middle-lower crust low velocity layer beneath the Tibetan plateau, and the tearing of subducted Indian mantle lithosphere. Our joint inversion Vs model can provide a reference model for geosciences in continental China and surrounding areas.

The moon before mare

The crust of the Moon experienced a unique geodynamic evolution, beginning with its crystallization from a magma ocean, continuing through a period of heavy impact bombardment, and followed by extensive basaltic mare volcanism. All these events have left crucial records imprinted in the form of topographic features and gravity anomalies. Here, we invert gravity and topography data using a two-layer thin-shell loading model under the premise of pre-mare isostasy to investigate the global structure of the crust and solve for feldspathic crust and mare thickness, together with mare-induced flexure. The tectonic record and partially buried crater population are used to constrain the bulk of mare volcanism to have been emplaced on a 40 km elastic lithosphere, although mare within large impact basins may have formed on a thinner elastic lithosphere. The mare thickness and associated flexure are removed to calculate a map of the surface and crust of the Moon before mare volcanism. The pre-mare surface in the Oceanus Procellarum region is found to be ∼2 km lower than the surrounding nearside, and several possible explanations, including a giant impact, pore space annealing, isostatic adjustment, and crustal erosion induced by a mantle plume or thermal anomaly, are discussed. The pre-mare elevation map further sheds light on the ring structure of Imbrium, which is seen to resemble that of Orientale. Imbrium's outermost ring is observed to be at a larger radial distance to the northeast relative to the south, indicating that some level of lithospheric variability affected ring formation at the time of impact. The western part of Imbrium's ring within Oceanus Procellarum is not found in the pre-mare topography, implying that it either never formed or that some processes erased its signature from gravity and topography. The feldspathic, pre-mare, crust is found to be ∼7 km thinner within large nearside basins than in models not accounting for the high-density mare. The pre-fill floor of these basins was ∼6 km deeper than currently observed, and together with their updated crustal structure, these new insights have implications for impact simulations that try to reproduce the crustal structure of nearside mare basins.

Geochemical benchmarks in the Phanerozoic LIPs constrained from well-cores in the Deccan Volcanic Province, India

The ∼66 Ma Deccan flood basalt province is one of the largest in the World. Here, we present mineral chemistry, bulk-rock geochemistry and NdSr isotopes in the ∼1000 m thick basalt lava sequence extracted from the deep bore-hole well-cores KBH-8 and KBH-7 from the main Deccan Plateau in the southwestern Deccan Volcanic Province. On the basis of trace and rare earth element systematics, we have statistically grouped the accumulated lava into fifteen Eruptive Phases (EPs). The basalts show extreme variation in their initial 87Sr/86Sr (0.7038–0.7166) and εNd (+5.9 to −12.1) isotopic compositions consistent with the involvement of depleted and enriched mantle sources in their genesis. The lavas sourced from the enriched subcontinental lithospheric mantle has been categorized as EP4-type, and EP11-type represents the isotopically depleted lavas derived from a plume asthenospheric source. The remaining EPs resulted from mixing of the components from these two endmembers in some proportion. This essentially reflects upon source contamination rather than crustal assimilation enroute to eruption or emplacement. The unique geochemical and isotopic fingerprint of EP4 and EP11 is traceable in the Phanerozoic LIPs, as well. Therefore, they potentially qualify as “geochemical benchmarks”. The systematic “pinch and swell” pattern observed in the trace element and isotopic compositions throughout the depth profile cannot be reconciled with continuous outpouring of lava. Instead, the EPs erupted in discreet batches in a quick span of time from a periodically replenished tapped fractionated magma chamber fed by a mantle plume at the asthenosphere-lithosphere interface. The results of this study enunciate the critical role of subcontinental lithospheric mantle in the evolution of a continental flood basalt province, irrespective of space and time.

Petrological models of magma evolution on the formation of mafic-felsic igneous complexes of the Emeishan large igneous province, SW China

Many Late Permian world-class Fe-Ti oxide-bearing layered intrusions in the Pan-Xi region, SW China are commonly considered to be fractionation products of the Emeishan mantle plume-related high-Ti basaltic melts. These layered intrusions coexist spatially and temporally with A-type granitoids, which consist of syenites and granites and are further classified into isotopically depleted metaluminous to peralkaline and isotopically enriched peraluminous to metaluminous groups. The petrogenesis of these mafic-felsic igneous complexes has been controversial. This study investigates geochemical correlations of the microgabbros at the margin of the Baima layered intrusions and the intruding isotopically enriched granitoids. The amphiboles at both sides of the contact zone are calcic (CaO = 10.4 to 12.5 wt.%) and display obvious mixing trends. Thermodynamical modeling using the MAGEMin program indicates that fractionation of basaltic melts compositionally similar to the Baima microgabbros seems incapable to crystallize olivine compositions comparable to those of the layered intrusions, but can be reconciled by early separation of 7.5 to 10 wt.% Fe-Ti-rich melts. Continued differentiation of the basaltic melts may lead to formation of the isotopically depleted granitoids, among which perthite accumulation in water-rich environments could result in the formation of metaluminous granitoids with high Na, K and Al contents and positive Eu anomalies. The isotopically enriched granitoids, on the other hand, can be produced by anatexis of the juvenile mafic lower crust together with the Yangtze ancient lower crust. Our study therefore provides a panoramic view on the complicated petrogenesis of the igneous complexes related to Emeishan mantle plume impingement through thermodynamic modeling.

The Corrugated Hills Continental Flood Basalt: a c. 2.17 Ga flood basalt province related to breakup of the Superior craton

Abstract This study documents a major continental flood basalt province in the central Labrador Trough that produced voluminous mafic magmatism along the palaeo-rifted margin of the Superior craton. This area, representing a preserved magma volume of approximately 25 000 km 3 , is characterized by classic ‘traps’ topography with columnar-jointed flows and sills separated by thin clastic sedimentary layers. A gabbroic sill near the bottom of the sequence yielded a U–Pb zircon age of 2166 ± 4 Ma, and another near the middle of the sequence yielded a 2171 ± 3 Ma U–Pb zircon age. These ages overlap in time with published ages of mafic dyke swarms that radiate outwards into the adjacent Superior craton, namely the Biscotasing (2172–2167 Ma) and Payne River (2170–2160 Ma) dykes, as well as the Otish sills (2172–2162 Ma) and Tasiataq (2170 Ma) dyke. Our data suggest that these central Labrador magmas, named the Corrugated Hills Continental Flood Basalt, were emplaced near or above a mantle plume head, leading to thermal uplift and then rifting of the Superior craton margin. Field relationships suggest that these magmas were erupted through thinned Archean crust as a rift-related continental flood basalt suite. The Corrugated Hills Continental Flood Basalt province represents remnants of one of the oldest continental flood basalt provinces on Earth.

Micropore Structure of Deep Shales from the Wufeng–Longmaxi Formations, Southern Sichuan Basin, China: Insight into the Vertical Heterogeneity and Controlling Factors

The microscopic pore throat structure of shale reservoir rocks directly affects the reservoir seepage capacity. The occurrence and flow channels of shale gas are mainly micron–nanometer pore throats. Therefore, to clarify the microstructural characteristics and influencing factors of the deep organic-rich shales, a study is conducted on the marine shale from the Upper Silurian to Lower Ordovician Wufeng–Longmaxi Formation in the southern Sichuan Basin. Petrographic lithofacies division is carried out in combination with petro-mineralogical characteristics, and a high-resolution scanning electron microscope, low-temperature nitrogen and low-temperature carbon dioxide adsorption, and micron-computed tomography are used to characterize the mineral composition and pore structure qualitatively and quantitatively, upon which the influencing factors of the microstructure are further analyzed. The results show that with the increase in burial depth, the total organic carbon content and siliceous mineral content decrease in the Wufeng formation to Long-11 subsection deep shale, while clay mineral content increases, which corresponds to the change in sedimentary environment from anoxic to oxidizing environment. Unexpectedly, the total pore volume of deep shale does not decrease with the increase in burial depth but increases first and then decreases. Using total organic carbon (TOC), siliceous mineral content showed a good correlation with total pore volume and specific surface area, with correlation coefficients greater than 0.7, confirming the predominant role of these two factors in controlling the pore structure of deep shales. This is mainly because the Longmaxi shale is already in the late diagenetic stage, and organic matter pores are generated in large quantities. Clay minerals have a negative correlation with the total pore volume of shale, and the correlation coefficient is 0.7591. It could be that clay minerals are much more flexible and are easily deformed to block the pores under compaction. In addition, the longitudinal heterogeneity of the deep shale reservoir structure in southern Sichuan is also controlled by the thermal effect of the Emei mantle plume on hydrocarbon generation of organic matter and the development of natural microfractures promoted by multistage tectonic movement. Overall, the complex microstructure in the deep shales of the Longmaxi Formation in the southern Sichuan Basin is jointly controlled by multiple effects, and the results of this research provide strong support for the benefit development of deep shale gas in southern Sichuan Basin.

W. Jason Morgan (1935-2023).

Discoverer of plate tectonics and mantle plumes.

Petrogenesis and geochemistry of WNW-ESE to NW-SE trending doleritic dykes of the Paleoproterozoic Liptako basement (West African Craton, West Niger)

Doleritic dykes with regional orientations WNW-ESE to NW-SE, cut the Paleoproterozoic Birimian formations of the Liptako basement (Ouest-Niger). Mineralogical compositions studied from these doleritic dykes consist of augite (Wo21-40-En24-45-Fs13-54), pigeonite (Wo8-12-En45-59-Fs31-41), plagioclase, alkali feldspars, titano-magnetite, ilmenite, edenite, apatite, biotite and chlorite. The geochemical affinity of the doleritic dykes in the two trends is similar to oceanic plateau basalts (OPB), with high iron content relative to Mg. Negative Nb, Ti and Ta anomalies and the Y/Nb ratio (1.59–3.15) indicate a continental tholeiitic character for while light rare earth element enrichment, low to moderate Ti contents and the Th/Yb (0.28–0.59) and Nb/Yb (3.25–7.46) ratios suggest sources similar to E-MORB. Their Th/Nb (0.06–0.10), Th/Yb (0.28–0.59), TiO2/Yb (0.54–0.82) ratios suggest that the magma was generated by partial melting (5–17%) of a residual spinel-garnet lherzolite, that could have been part of a mantle plume. Relatively low Mg# (36–51) reflect some degrees of fractional crystallization, with some of this differentiation recorded between the margins and interiors of dykes. This differentiation was accompanied by unusually low degrees of crustal contamination, as indicated by relatively low Th/Ta ratios (1.18–2.03). This petrological and geochemical information provides new constraints on the origin of these WNW to NE trending mafic dykes of the West African Craton.

Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observations

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (&amp;lt; 14 s) and teleseismic surface waves at long periods (&amp;gt; 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

Highest terrestrial 3He/4He credibly from the core.

The observation that many lavas associated with mantle plumes have higher 3He/4He ratios than the upper convecting mantle underpins geophysical, geodynamic and geochemical models of Earth's deep interior. High 3He/4He ratios are thought to derive from the solar nebula or from solar-wind-irradiated material that became incorporated into Earth during early planetary accretion. Traditionally, this high-3He/4He component has been considered intrinsic to the mantle, having avoided outgassing caused by giant impacts and billions of years of mantle convection1-4. Here we report the highest magmatic 3He/4He ratio(67.2 ± 1.8 times the atmospheric ratio) yet measured in terrestrial igneous rocks, in olivines from Baffin Island lavas. We argue that the extremely high-3He/4He helium in these lavas might derive from Earth's core5-9. The viability of the core hypothesis relaxes the long-standing constraint-based on noble gases in lavas associated with mantle plumes globally-that volatile elements from the solar nebula have survived in the mantle since the early stages of accretion.

Petrogenesis of alkaline magmas across a continent to ocean transect, northern Ross Sea, Antarctica

The West Antarctic Rift System (WARS) is one of the world's largest active rift systems and alkaline magmatic provinces. The alkaline magmatism crosscuts oceanic and continental lithosphere but maintains generally similar Sr-Nd-Pb isotope signatures. Here we present new major-, trace- and highly siderophile-element (Os, Ir, Ru, Pt, Pd, Re) abundances and Sr-Nd-Pb-Os isotope data for a Miocene to recent WARS alkaline basalt suite from a continental-oceanic transect in the northwest Ross Sea. The samples are geochemically enriched similar to ocean island basalts, with oceanic lavas having higher absolute incompatible trace element abundances than continental lavas. The suite exhibits a large range in measured 187Re/188Os (1.7 to 1305) and age-corrected 187Os/188Os (0.1254 to 1.055) that has a positive, highly curved relationship with measured 87Sr/86Sr (0.70287–0.70331), but a negative, highly curved relationship with both measured 143Nd/144Nd (0.51280–0.51300) and 206Pb/204Pb (19.237–20.237). These data suggest that contamination by a mafic continental lithospheric component has occurred to varying degrees and exerts some control on the geochemistry of the continental lavas. Primitive lavas from the oceanic Adare Basin that have experienced negligible contamination are used to constrain the primary source of northwest Ross Sea alkaline lavas and to assess previously proposed hypotheses regarding the origin of magmatism. Explanations for the origin of WARS alkaline lavas range from plume magmatism to partial melting of continental lithospheric mantle metasomatized by subduction fluids prior to cessation of subduction of the Phoenix Plate underneath the Gondwana margin. Samples from the continental-oceanic transect, however, neither exhibit classic arc-lava geochemical signatures nor provide any evidence to support an active mantle plume source. Rather, they are geochemically enriched magmas generated through partial melting of previously subducted oceanic lithospheric mantle concomitant with continental lithospheric thinning during the Cenozoic.