Ascending Mantle Plume Research Articles

Contribution of recycled oceanic crust to the extremely light molybdenum isotopic compositions of mid-ocean ridge basalts from the South China Sea

The contribution of recycled oceanic crust to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Nd–Hf isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the South China Sea (SCS). The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). The ESB MORBs display extremely light Mo isotopic compositions (δ98/95Mo = −0.80‰ to −0.24‰) with NdHf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show higher δ98/95Mo values (−0.44‰ to +0.05‰) and depleted NdHf isotopic compositions. The δ98/95Mo values of the SCS MORBs are negatively correlated with Ce/Mo ratios and positively correlated with εHf values, suggesting the incorporation of dehydrated slab components into the source of the SCS MORBs. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone, as suggested by mantle tomography. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. This study, therefore, provides compelling evidence for the involvement of recycled oceanic slabs in generating MORB and the influence of an ascending mantle plume on seafloor spreading in a continental marginal sea. Mantle recycling processes in marginal sea basins exhibit distinct characteristics compared to those occurring in open oceans.

On the Origin of Seismic Anisotropy in the Shallow Crust of the Northern Volcanic Zone, Iceland

AbstractThe Icelandic crust is a product of its unique tectonic setting, where the interaction of an ascending mantle plume and the Mid‐Atlantic Ridge has caused elevated mantle melting, with the melt accreted and cooled in the crust to form an oceanic plateau. We investigate the strength and orientation of seismic anisotropy in the upper crust of the Northern Volcanic Zone using local earthquake shear‐wave splitting, with a view to understanding how the contemporary stress field may influence sub‐wavelength structure and processes. This is achieved using a data set comprising 50,000 earthquakes located in the top 10 km of the crust, recorded by up to 70 stations over a 9 year period. We find that anisotropy is largely confined to the top 3–4 km of the crust, with an average delay time of 0.10 ± 0.05 s, and an average orientation of the fast axis of anisotropy of N014°E ± 27°, which is perpendicular to the spreading direction of the Eurasian and North American plates (N106°E). These results are consistent with the presence of rift‐parallel cracks that gradually close with depth, the preferential opening of which is controlled by the regional stress field. Lateral variations in the strength of shear wave anisotropy (SWA) reveal that regions with the highest concentrations of earthquakes have the highest SWA values (∼10%), which reflects the presence of significant brittle deformation. Disruption of the orientation of the fast axis of anisotropy around Askja volcano can be related to local stress changes caused by underlying magmatic processes.

Zircon megacrysts from Devonian kimberlites of the Azov Domain, Eastern part of the Ukrainian Shield: Implications for the origin and evolution of kimberlite melts

Zircon megacrysts are commonly found in kimberlites and, together with olivine, low-Cr garnet, pyroxene, phlogopite, and ilmenite megacrysts, they constitute a mineral assemblage known as the “low-Cr suite”. The generally close similarity of ages and similar isotope geochemical characteristics of megacrysts and matrix minerals in the host kimberlites support a cognate origin. However, alteration rims commonly develop on zircon and ilmenite megacrysts, providing evidence for a lack of chemical equilibrium between the megacrysts and kimberlitic melts. Here, we report results of a detailed geochronological and geochemical study of zircon megacrysts found in the Middle Devonian Novolaspa kimberlite pipe and dyke located in the Azov Domain of the Ukrainian Shield. The concordia age of zircons is 397.0 ± 2.0 Ma, and it is 14 m.y. older than the age of kimberlite emplacement as defined by a Rb-Sr isochron on phlogopite. The average εHf(397) value for unaltered zircon megacrysts is 6.8 ± 0.14, with the alteration rims having similar Hf isotope systematics. These hafnium isotope data indicate a moderately depleted mantle source for zircon. Unaltered megacrystic zircons have low abundances of trace elements and fractionated REE, with pronounced positive Ce/Ce* anomalies and almost no Eu/Eu* anomalies. In contrast, alteration rims have very high and variable concentrations of trace elements, indicating a reaction between zircon and kimberlite melt. The melt or fluid responsible for zircon and ilmenite megacryst formation, in contrast to kimberlitic melt, was poor in incompatible trace elements, except for the HFSE (Zr, Hf, Nb, Ta, and Ti). The oxygen fugacity during crystallization of the megacryst suite was close to the FMQ buffer.Azov zircon megacrysts do not demonstrate close geochronological and isotope-geochemical similarities with their host kimberlites. They are cognate in the broad sense of being related to the same plume event, but their direct affinity is not clearly defined. The megacryst suite may have crystallized from the earliest melts/fluids that separated from the ascending mantle plume, whereas kimberlite magmas were emplaced 14 m.y. after this event.

Geodynamic experiments suggest that mantle plume caused Late Permian Emeishan Large Igneous Province in Southern China

ABSTRACT Stratigraphic, petrological and geophysical studies suggest that the Late Permian (~ 260 Ma) Emeishan Large Igneous Province in southern China may be formed by mantle plume activity. However, the plume impingement hypothesis remains controversial since interpretations based on volcano-stratigraphic analyses around plume induced domal uplift/inner zone suggest that the volcanism occurred under submarine environment rather than elevated sub-aerial (above sea level) conditions, usually associated with the dynamic topography effects of the ascending mantle plumes. Here, 2-D numerical and 3-D scaled laboratory (analogue) plume experiments are used to explore the coupled dynamics of plume-mantle-lithosphere interaction and their evolution of surface topography characteristics. Experimental results show that the initial (plume incubation) phase is characterized by rapid, transient domal uplift above the plume axis, subsequently, as plume head flattens, there is short wavelength topographic variation (ie. subsidence and uplift occurs synchronously) due to the shear stress imposed onto the base of the lithosphere and loss of gravitational potential energy. The surface depressions predicted by the plume models, next to the plume axial/inner/uplift zone, may explain the deposition of submarine volcanics at Lake Erhai, Dali in the western side and Xiluo and Daqiao in the eastern side, which may resolve the plume controversy for the formation of Emeishan Large Igneous Province. Notably, while experimental results from these two different techniques show some differences, (e.g much bigger plume head for the laboratory experiment), the overall characteristics of the predictions have robust similarities. For instance, the extension above the plume axis may explain the enigmatic cause of the Panxi rift system, in the middle of the inner zone where giant dyke swarms radiate from, and mafic magma underplatings in the lower crust has been described by seismological studies.

On the Problem of Distinction between Recent Geodynamics of Central and East Asia

Abstract—The Central Asian and East Asian transitional zones are distinguished in Central and East Asia south of the Late Precambrian–Paleozoic fold belts of the Southern Russia and Kazakhstan and east of the Siberian Platform which are parts of the North Eurasian lithospheric plate. These transitional zones separate the plate from the Indian, Australian, and Pacific plates. The Central Asian zone is presently dominated by tectonic transpression accompanied by the thrusts and strike-slip deformations extensively developed under the influence of the Hindustan indenter pressure. The East Asian zone is mainly controlled by transtension with extensive shear strain processes associated with the deep submergence and, apparently, gradual disintegration and disappearance of the Pacific slab at a depth. This regime can also be connected with the ascending mantle plumes beneath Transbaikalia and North Mongolia. The Cenozoic rift structures and large depressions prevail in the East Asian transition zone. The difference in the geodynamic regimes in the transitional zones is demonstrated by seismic data and by the analysis of geophysical fields and anomalies in their deep structure. The boundary between the zones approximately coincides with the extended submeridional 102°–104° E geodivider which is identified based on numerous geological and tectonic manifestations in the North, Central, and South Asia. In the central part, the geodivider is supported by the data on recent crustal block structure, seismicity, active faults, and deep structure anomalies.

Thermal imprints of late Permian Emeishan basalt effusion: Evidence from zircon fission-track thermochronology

Late Permian Emeishan large igneous province (LIP) developed in response to an ascending mantle plume beneath the southwestern Yangtze Craton, China. Mantle plume models predict two types of thermal effect on the lithosphere: (1) high lithospheric heat flow anomalies due to the ascending magma, and (2) heating of underlying country rocks by erupted lava flow. Evidence for these two effects of the Emeishan LIP may be documented in pre-LIP strata. As such, we determined the maximum temperature of sedimentary rocks deposited before and after the LIP using zircon fission-track (ZFT) thermochronology, which is sensitive to temperatures of 150–250 °C. ZFT ages of post-LIP sedimentary rocks are variable and overlap with depositional ages, reflecting post-LIP burial heating should be lower than the ZFT closure temperature. Pre-LIP samples yield scattered ZFT ages younger than their depositional ages, and contain an older age component that increases with depth. This indicates that the upper part of the pre-LIP strata should have experienced higher temperature than lower ones. Such a heating pattern cannot be explained by pre- or post-LIP burial heating, which predict higher temperatures and thus younger ages for stratigraphically lower samples. We propose that the downward increasing age pattern is evidence for massive top-down lava heating. Forward thermal modelling suggests that a >1.5-km-thick effusive basalts is required to reset the ZFT ages of rocks that are ∼3 km beneath the LIP. Our results support the existence of massive effusive basalts in the intermediate zone of the Late Permian Emeishan LIP.

An appraisal of geochemical signatures of komatiites from the greenstone belts of Dharwar Craton, India: Implications for temporal transition and Archean upper mantle hydration

Archean ultramafic–mafic magmatism associated with dynamic mantle melting processes represents an integral part of crustal evolution and continental growth. Archean mantle plumes and their high temperature derivatives in komatiitic magmas provide a cornerstone in understanding the thermal and chemical evolution of the early Earth and its mantle. In this study, we evaluate the geochemical characteristics of the temporally distinct Sargur Group and Dharwar Supergroup greenstone belts of Dharwar Craton in southern India. The komatiites associated with these greenstone belts are characterized by Al‐depleted and Al‐undepleted compositions. Their MgO, Ni, Co, and Cr concentrations reflect primitive, undifferentiated nature of precursor melts, whereas the HFSE and REE chemistry corroborates high‐degree (30–50%) partial melting of mantle with prominent role of majorite garnet and perovskite in the generation of komatiitic melts. The Al‐depleted komatiites were formed at a greater depth (~13 GPa) by equilibrium melting leaving a garnet‐bearing residue. The resultant komatiitic melt retained buoyancy under extreme pressure and high temperature conditions, segregated and accumulated in the ambient peridotite mantle source and subsequently escaped with progressive decrease in pressure in the ascending plume. The Al‐undepleted and relatively rare Al‐enriched komatiites were derived by fractional melting at intermediate to shallow depths after the discharge of a large proportion of melt ensued by the collapse and entrance of residual garnet into the melt phase. The Mesoarchean–Neoarchean Dharwar komatiites provide evidence for heterogeneous, hydrated Archean upper mantle trapped by ascending mantle plumes. The non‐plume, arc‐related signatures invoke contributions from (a) sub‐cratonic lithospheric mantle roots influenced by ancient subduction events and (b) hydrated Archean upper mantle. The Sargur Group and Dharwar Supergroup komatiites record a distinct temporal transition and geochemical heterogeneity in the Archean mantle beneath the Dharwar Craton and can be attributed to Archean upper mantle hydration by flat slab subduction of >3.3 Ga oceanic crust at shallow level with vestiges of primordial oceanic crust forming deep cratonic mantle roots beneath the Dharwar Craton. The temporal transition towards mantle hydration beneath Dharwar Craton during the Mesoarchean can be correlated with the onset of subduction‐driven plate tectonics and ocean–crust–mantle interactions.

Specifics of the Neoarchean Plume–Lithospheric Processes in the Kola–Norwegian Province of the Fennoscandian Shield: II. Petrology and Geodynamic Nature of Komatiite–Tholeiite Association

Numerical modeling of the generation and evolution of parental melts of the komatiite–tholeiite association of the Uraguba structure was carried out using previously obtained geochemical and isotope data. It was established that komatiite, komatiite and tholeiite basalts depleted in LREE and having eNd(Т = 2.79) = +2.9…+3.2 were generated by equilibrium partial melting (F > 15%) of a depleted source (garnet-bearing Ol0.63 + Opx0.22 + Cpx0.06 + Grt0.09 mantle peridotite) at 4–8 GPa, while the genesis of primary melts of LREE-enriched komatiites (LaN/SmN ~ 1.2–1.6) with eNd(Т = 2.79) = +2.5…+2.2 was related to the equilibrium partial melting (F > 20%) of an “enriched mantle peridotite” (EM–Ol0.60 + Opx0.20 + Cpx0.08 + Grt0.12) at pressure of 2.5–4 GPa. Coexistence in space and time of two types of melting products of mantle peridotites formed at different depths is explained by melting of different parts of adiabatically ascending mantle plume.

Petrogenesis and metallogenesis of the Wajilitag and Puchang Fe-Ti oxide-rich intrusive complexes, northwestern Tarim Large Igneous Province

The Wajilitag and Puchang intrusive complexes of the Tarim large igneous province (TLIP) are associated with significant resources of Fe–Ti oxide ores. These two mafic–ultramafic intrusions show differences in lithology and mineral chemistry. Clinopyroxenite and melagabbro are the dominant rock types in the Wajilitag complex, whereas the Puchang complex is generally gabbroic and anorthositic in composition with minor plagioclase–bearing clinopyroxenites in the marginal zone. Disseminated Fe–Ti oxide ores are found in the Wajilitag complex and closely associated with clinopyroxenites, whereas the Puchang complex hosts massive to disseminated Fe–Ti oxide ores mainly within its gabbroic rocks. The Wajilitag intrusive rocks are characterized by a restricted range of initial 87Sr/86Sr ratios (0.7038–0.7048) and positive εNd(t) (+0.04 − +3.01), indicating insignificant involvement of continental crustal contamination. The slightly higher initial 87Sr/86Sr ratios (0.7039–0.7059) and lower εNd(t) values (−1.05 − +2.35) of the Puchang intrusive rocks also suggest that the isotopic characteristics was controlled primarily by their mantle source, rather than by crustal contamination. Both complexes have Sr–Nd isotopic compositions close the neighboring kimberlitic rocks and their hosted mantle xenoliths in the TLIP. This indicates that the ferrobasaltic parental magmas were most probably originated from partial melting of a metasomatized subcontinental lithospheric mantle, modified recently with subduction–related materials by the impingement of the ascending mantle plume. The recycled subduction–related materials preserved in the lithospheric mantle could play a key role in the formation of the parental Fe–rich magmas in the context of an overall LIP system. The distinct variations in mineral assemblage for each complex and modeled results indicated that the Wajilitag and Puchang complexes experienced different crystallization path. Fe–Ti oxides in Wajilitag joined the liquidus earlier in the crystallization sequence, especially relative to the crystallization of plagioclase. This is attributed to the relatively high TFeO, TiO2 and initial H2O contents of the parental magma. In combination with definitive field and petrological evidence, the enrichment of highly incompatible elements (e.g., Zr, Hf, Nb and Ta) and the depletion of rare earth elements in the Fe–Ti oxide ores is consistent with the plausible interpretation that the ores could be formed by fractional crystallization and crystal accumulation of the Fe–Ti oxide crystals from the ferrobasaltic parental magmas. A considerable amount of the Fe–Ti oxides in the Puchang has transported and sunk from higher up in the chamber to the underlying unconsolidated silicate crystal pile. The highly dense Fe–Ti oxide crystal slurries further tended to effective accumulate Fe–Ti oxides to form high–grade Fe–Ti oxide ore bodies, and subsequent rapid collapse and intrusive into lower lithologies within the complex under a H2O–rich environment during the late–stage of magmatic differentiation. The development of massive Fe–Ti oxide ores in the case of the Puchang, could plausibly result from a combination of the protracted differentiation history of a Fe highly enriched parental magma and the later addition of external H2O from the country rocks (e.g., carbonates) to the slowly cooling magma chamber.

Physicochemical parameters of deep-seated mantle plumes

Thermodynamic analysis of experimental data has demonstrated that FeO activity in silicate melts identical in composition to natural magmas can be described by the regular-solution model, which takes into account interactions of all cations with Si and interaction of Ca with Al. Using this model, we propose an oxygen barometer for spinel + magma phase association. In contrast to the earlier proposed methods for estimation of oxygen chemical potential, this barometer can work in the PJ-domain close to the liquidus of magmatic process. The new oxygen barometer has been applied to magmas related to mantle plume activity, including Siberian meimechites, Hawaiian picrites, and picrites from the Emeishan large igneous province (LIP) and Greenland. We have shown that most magmas related to the activity of deep-seated mantle plumes are characterized by a higher relative chemical potential of oxygen than magmas of mid-ocean ridges. Thermodynamically calculated stability fields of rocks with different carbon-containing phases show that under PJ-conditions of the lower mantle, the ascending mantle plumes are characterized by relatively high oxygen fugacity. Formation of diamond in the lower mantle requires more oxidizing conditions as compared with the major part of this geosphere, where the presence of Fe-Ni alloy is predicted. We have put forward a hypothesis that the main reason for the oxygen fugacity increase in particular domains of the lower mantle is a shift of redox equilibria toward a decrease in the amount of Fe-Ni alloy, up to its disappearance, with temperature growth.

The composition of mantle plumes and the deep Earth

Determining the composition and geochemical diversity of Earth's deep mantle and subsequent ascending mantle plumes is vital so that we can better understand how the Earth's primitive mantle reservoirs initially formed and how they have evolved over the last 4.6 billion years. Further data on the composition of mantle plumes, which generate voluminous eruptions on the planet's surface, are also essential to fully understand the evolution of the Earth's hydrosphere and atmosphere with links to surface environmental changes that may have led to mass extinction events. Here we present new major and trace element and Sr–Nd–Pb–Hf isotope data on basalts from Curacao, part of the Caribbean large igneous province. From these and literature data, we calculate combined major and trace element compositions for the mantle plumes that generated the Caribbean and Ontong Java large igneous provinces and use mass balance to determine the composition of the Earth's lower mantle. Incompatible element and isotope results indicate that mantle plumes have broadly distinctive depleted and enriched compositions that, in addition to the numerous mantle reservoirs already proposed in the literature, represent large planetary-scale geochemical heterogeneity in the Earth's deep mantle that are similar to non-chondritic Bulk Silicate Earth compositions.

Geochemical and Sr–Nd isotopic constraints on the mantle source of Neoproterozoic mafic dikes of the rifted eastern Laurentian margin, north-central Appalachians, USA

Abundant and widely distributed unmetamorphosed mafic dikes intrude Mesoproterozoic rocks of the New Jersey Highlands. The age of the dikes is imprecisely known but interpreted to fall between 615 and 576 Ma, which is consistent with the range of ages of mafic dikes from Labrador and Nova Scotia south to Pennsylvania that were emplaced along the rifted eastern Laurentian margin. New Jersey Highlands dikes are a few cm to 18 m wide and have lengths of as much as several km. They have sharp, largely discordant contacts against enclosing Mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors. Columnar joints are present locally and suggest emplacement at a shallow crustal level. Geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high TiO 2 , P 2 O 5 , Zr, Nb, Y, and La/Yb, and low MgO, Cr, and Ni. TiO 2 contents define high-Ti and low-Ti dikes that differ in high field strength elements (HFSE) and light rare earth elements (LREE) but overlap in abundances of most other elements. Dike magma evolved in an ascending mantle plume of OIB-like asthenosphere from enriched higher TiO 2 compositions to more depleted lower TiO 2 compositions. Subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation. Sr–Nd isotope values of both dike compositions overlap and are characterized by ε Nd (T) of + 1.5 to + 3.8 and initial 87 Sr/ 86 Sr ranging from 0.7032 to 0.7077. Higher Sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous, high 87 Sr/ 86 Sr lithospheric mantle having EMI or EMII-like geochemical characteristics. Dikes form tabular structures that have long segments striking an average of N44°E and short segments striking about east–west. Their regional geometries form right-stepping, rhomb-shaped patterns due to emplacement into rift-related dilational fractures likely formed through a combination of southeast-directed extension and strike-slip shear stresses. Geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event. They, along with other mafic dikes in the north-central Appalachians, were emplaced in a within-plate tectonic setting along the rifted margin of eastern Laurentia, prior to opening of the Iapetus Ocean. • Mantle sources of Neoproterozoic mafic dikes were modeled from Sr–Nd isotopes and geochemistry. • Dikes formed from partial melting of a garnet-free, enriched OIB-like source. • Geochemical similarity to other regional Neoproterozoic dikes along the eastern Laurentian margin • Dikes emplaced from single magmatic event during rifting that formed the Iapetus Ocean

Magmatic sources of dikes and veins in the Moncha Tundra Massif, Baltic Shield: Isotopic-geochronologic and geochemical evidence

The dike-vein complex of the Moncha Tundra Massif comprises dolerites, gabbro-pegmatites, and aplites. The dolerite dikes are classified into three groups: high-Ti ferrodolerites, ferrodolerites, low-Ti and low-Fe gabbro-dolerites. The U-Pb age of the ferrodolerites is 2505 ± 8 Ma, and the amphibole-plagioclase metagabbroids hosting a ferrodolerite dike are dated at 2516 ± 12 Ma. Data on the U-Pb isotopic system of zircon from the gabbro-pegmatites and titanite from the aplites indicate that the late magmatic evolution of the Moncha Tundra Massif proceeded at 2445 ± 1.7 Ma, and the youngest magmatic events in the massif related to the Svecofennian orogeny occurred at 1900 ± 9 Ma. The data obtained on the Sm-Nd and Rb-Sr isotopic systems and the distribution of trace elements and REE in rocks of the dike-vein complex of the massifs provide insight into the composition of the sources from which the parental magmas were derived. The high-Ti ferrodolerites were melted out of a deep-sitting plume source that contained an asthenospheric component. The ferrodolerites were derived from a mantle MORB-type source that contained a crustal component. The parental melts of the gabbro-dolerites were melted out of the lithospheric mantle depleted in incompatible elements after Archean crust-forming processes above an ascending mantle plume, with the participation of a crustal component. The gabbro-dolerites and the rocks of the layered complex of the Moncha Tundra Massif exhibit similar geochemical characteristics, which suggest that their parental melts could be derived from similar sources but with more clearly pronounced crustal contamination of the parental melts of the rocks of the massif itself. The geochemical traits of the gabbro-pegmatites are thought to be explained not only by the enrichment of the residual magmas in trace elements and a contribution of a crustal component but also by the uneven effect of sublithospheric mantle sources. The aplites were derived from a sialic crustal source.

New petrological data on the volcanic rocks of the Chichinautzin region: The sources of the magmatic melts and the origin of the Trans-Mexican volcanic belt

New petrographic, isotopic-geochemical, and mineralogical data are presented for the volcanic rocks of the Chichinautzin region of the Trans-Mexican volcanic belt (TMVB). The geological setting and the peculiarities of the composition of the volcanic rocks from different regions of the belt are compared to the plume-related volcanic rocks from the areas of the Gulf of California, Central America, and the Galapagos hot spot. It was concluded that the composition of the intraplate rocks from the western and eastern parts of the TMVB was subjected to the Californian and Galapagos plumes, respectively. In its turn, the ascending mantle plumes provoke melting of the subcontinental lithospheric mantle related to the formation of islandarc rocks. The model of the consecutive propagating rifting in the eastward direction suggested by some researchers (Marquez et al., 1999; Verma, 2001) instead of the subduction hypothesis is in agreement with the geological and geophysical data and the isotopic-geochemical peculiarities of the volcanic rocks within the TMVB.

Denudation history of Eastern Indian peninsula from apatite fission track analysis: Linking possible plume-related uplift and the sedimentary record

The Late Archaean-Early Proterozoic (~ 2.5 Ga) Eastern Dharwar Craton (EDC), eastern India, which underlies much of the drainage basins of the Krishna and Godavari rivers, has undergone an erosionally controlled evolution since the Pan African event (~ 500–550 Ma). This evolution has been responsible for the sedimentation and overall development of the Krishna–Godavari (KG) sedimentary basin. In order to reconstruct the denudation history of the EDC, which forms the hinterland to the petroliferous KG basin, we report apatite fission track (AFT) data from 41 samples. Thermal history modelling suggests that little denudation occurred before Mid-Cretaceous time. However, during the Late Cretaceous, accelerated cooling commenced from a temperature range of 60–75 °C. Assuming a low palaeogeothermal gradient of 10 °C/km, similar to that of the present day, cooling of the EDC translates to ~ 0.5–2 km of denudation during the Late Cretaceous and a total of ~ 4 km since that time. The denudation history is closely related to the sedimentary record in the KG basin. The episode of accelerated cooling in the Late Cretaceous is interpreted as a geomorphic response to uplift of the Indian peninsula possibly resulting from an ascending mantle plume, which produced the voluminous Deccan Traps at the K–T boundary. The denudation history coupled with previous palaeocurrent studies in the KG basin suggest that the Krishna–Godavari drainage system was established in Late Cretaceous time. • AFT study from Dharwar craton suggests little denudation before Mid-Cretaceous. • During the Late Cretaceous, accelerated cooling occurred. • Cooling of the EDC translates to ~0.5–2 km of denudation in the Late Cretaceous. • The cooling is due to the dynamic uplift of Indian peninsula by a mantle plume. • Krishna–Godavari drainage system was established in the Late Cretaceous.

Likely “mantle plume” activity in the Skellefte district, Northern Sweden. A reexamination of mafic/ultramafic magmatic activity: Its possible association with VMS and gold mineralization

Most attention has been given to the geology of the extensive VMS and subordinate precious metals mineralization in the Skellefte district. Less attention has been given to indications of deep-seated origins of felsic and mafic/ultramafic volcanic rocks; of VMS and precious metals mineralizing fluids; and the primary origins of these metals. A holistic view of the significance of mafic/ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented. These subjects are discussed in this investigation.Primitive mantle normalized spider diagrams of rare-earth-elements (REE) distinguish two groups of mafic/ultramafic volcanic rocks, each with distinct geochemical characteristics: a mid-ocean-ridge “MORB”-type, and a geochemically unusual and problematic calc–alkaline–basalt “CAB”-type which is the main subject of this investigation. The “MORB”-type mafic volcanic rocks are mostly older than the Skellefte Group felsic volcanic rocks hosting the VMS deposits, whereas the more primitive “CAB”-type mafic/ultramafic volcanic rocks are mostly younger.A common source for these “CAB”-type, mafic-(MgO wt.%<14%) and ultramafic-(MgO wt.% >14%) volcanic rocks is suggested by their similar and distinctive geochemical features. These are near-chondritic (Al-undepleted) Al2O3/TiO2 ratios; moderate to strong high-field-strength-element (HFSE) depletion; light-rare-earth-element (LREE) enrichment and moderate heavy-rare-earth-element (HREE) depletion. They outcrop throughout an area of at least 100×100km. Gold mineralization is spatially associated with ultramafic volcanic rocks.Zr and Hf depletion has been shown to be associated with Al-depletion in mafic/ultramafic volcanic rocks elsewhere, and has been attributed to deep-seated partial melting in ascending mantle plumes. Zr and Hf depletion in “CAB”-type Al-undepleted mafic/ultramafic volcanic rocks is therefore unusual. The solution to this dilemma is suggested to be contamination of an Al-depleted mantle plume by felsic crustal rocks whereby Al-depleted ultramafic magmas become Al-undepleted. It will be argued that this model has the potential to explain previous observations of deep-seated origins; the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization; and even the primary origin of metals in VMS deposits.

Metasomatized lithospheric mantle beneath Turkana depression in southern Ethiopia (the East Africa Rift): geochemical and Sr–Nd–Pb isotopic characteristics

Mantle xenoliths entrained in Quaternary alkaline basalts from the Turkana Depression in southern Ethiopia (the East Africa Rift) were studied for their geochemical and Sr–Nd–Pb isotopic compositions to constrain the evolution of the lithosphere. The investigated mantle xenoliths are spinel lherzolites in composition with a protogranular texture. They can be classified into two types: anhydrous and hydrous spinel lherzolites; the latter group characterized by the occurrences of pargasite and phlogopite. The compositions of whole-rock basaltic component (CaO = 3.8–5.6 wt%, Al2O3 = 2.5–4.1 wt%, and MgO = 34.7–38.1 wt%), spinel (Cr# = 0.062–0.117, Al2O3 = 59.0–64.4 wt%) and clinopyroxene (Mg# = 88.4–91.7, Al2O3 = 5.2–6.7 wt%) indicate that the lherzolites are fertile and have not experienced significant partial melting. Both types are characterized by depleted 87Sr/86Sr (0.70180–0.70295) and high 143Nd/144Nd (0.51299–0.51348) with wide ranges of 206Pb/204Pb (17.86–19.68) isotopic compositions. The variations of geochemical and isotopic compositions can be explained by silicate metasomatism induced by different degree of magma infiltrations from ascending mantle plume. The thermobarometric estimations suggest that the spinel lherzolites were derived from depths of 50–70 km (15.6–22.2 kb) and entrained in the alkaline magma at 847–1,052°C. Most of the spinel lherzolites from this study record an elevated geotherm (60–90 mW/m2) that is related to the presence of rising mantle plume in an active tectonic setting. Sm–Nd isotopic systematic gives a mean TDM model age of 0.95 Ga, interpreted as the minimum depletion age of the subcontinental lithosphere beneath the region.

Geochemistry and genesis of metamorphosed volcanic rocks in the Kholodnikan greenstone belt, southern Aldan Shield

The Kholodnikan Complex consists of two units: lower volcanic and upper volcanic-sedimentary. The distributions of major and trace elements suggest that the protoliths of the lower unit were volcanics of the komatiite-tholeiite series (komatiite-basalt association) and those of the upper unit were volcanics of the calc-alkaline series (andesite-dacite-rhyolite association). The model assumed for the genesis of these associations involves two stages: (1) decompression-induced partial melting of the material of an ascending mantle plume with the derivation of melts of the komatiite-basalt association and (2) derivation of volcanic rocks of the andesite-dacite-rhyolite association via the partial melting of various rocks in the basement of the Aldan Shield under the effect of the heat of the ascending mantle plume. The magmatic protoliths of the Kholodnikan Complex were formed in the Paleoproterozoic at 2.41 Ga.

Redox potential of mantle magmatic systems

A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi2O6(Cpx) + 2Fe3O4(Spl) = 3CaMgSiO4(Ol) + 3Fe2SiO4(Ol) + O2. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere from ascending mantle plumes.

Early Archean sialic crust of the Siberian craton: Its composition and origin of magmatic protoliths

This study demonstrates that the base of the Archean deep-seated granulite complexes within the Siberian craton consists of a metabasite-enderbite association. The major and trace element distribution patterns revealed that the protoliths of this association are represented by calc-alkaline andesites and dacites, containing several minor sequences of komatiitic-tholeiitic volcanic rocks. The origin of the primary volcanic rocks of the metabasite-enderbite association is inferred on the basis of a model of mantle plume magmatism, which postulates that both andesitic and dacitic melts were derived from the primary basitic crust at the expense of heat generated by ascending mantle plumes. The formation of the protoliths of the Archen metabasite-enderbite association of the Siberian craton began at 3.4 Ga and continued until the late Archean.