Elevated Mantle Temperatures Research Articles

Generation of Archean TTGs via sluggish subduction

Abstract The trondhjemite-tonalite-granodiorite (TTG) suite of rocks prominent in Earth’s Archean continents is thought to form by melting of hydrated basalt, but the specific tectonic settings of formation are unclear. Models for TTG genesis range from melting of downgoing mafic crust during subduction into a hotter mantle to melting at the base of a thick crustal plateau; while neither uniquely defines a global tectonic regime, the former is consistent with mobile lid tectonics and the latter a stagnant lid. One major problem for a subduction model is slabs sinking too quickly and steeply in a hotter mantle to melt downgoing crust. I show, however, that grain size reduction in the lithosphere leads to relatively strong plate boundaries on the early Earth, which slow slab sinking. During this “sluggish subduction,” sinking plates can heat up enough to melt when the mantle temperature is ≳1600 °C. Crustal melting via sluggish subduction can thus explain TTG formation during the Archean due to elevated mantle temperatures and the paucity of TTG production since due to mantle cooling.

Flat subduction in the Early Earth: The key role of discrete eclogitization kinetics

Flat (shallow) subduction is mainly proposed as an Archean-Paleoproterozoic process, contributing to the growth of continental lithosphere. However, numerical models of Precambrian subduction commonly show steep subduction. Here we investigate the effects of lithology-dependent eclogitization (minor for gabbroic lower crust, strong for basaltic upper crust) of oceanic crust during subduction. We performed 2D petrological-thermomechanical modeling of subduction at potential mantle temperatures up to 250 °C warmer than present, using a numerical approach that accounts for buoyancy effects from both mantle depletion and eclogitization. The modeling reveals that increases in mantle potential temperature and the related thickness of oceanic crust and depleted mantle may induce transition to the flat subduction regime at ΔT ≥ 150 °C but only when a delayed eclogitization of gabbroic compared to basaltic crust is taken into account. Otherwise, subduction operates in the steep slab regime. Flat Precambrian subduction models show episodic bimodal magmatism within arcs located > 400 km from the trench, caused by short-lived (1–4 Myr) subduction transients consisting of three consecutive stages: (1) twisting downward of the eclogitized slab portion, (2) subvertical descent and break-off of this twisted portion, (3) rising of the remnant non-eclogitised slab tip and continuation of flat subduction. Limited formation of TTG-series granitoids is caused by partial melting of the subducting basaltic layer but becomes less pronounced at ΔT = 250 °C. Flat subduction produces an overthickened, layered keel-like continental mantle root, with an intermediate layer of subducted oceanic crust. However, construction of long-lived mantle keels by this mechanism might be limited by peeling off of the gradually eclogitizing oceanic crust and the underlaying mantle when plate convergence ends. At elevated mantle temperatures flat subduction produces voluminous hydration of the mantle wedge, forming a subcrustal serpentinite mélange layer that becomes a potential source of fluids in the subsequent evolution of the overlying continental crust.

Hot, Wide, Continental Back-arcs Explain Earth’s Enigmatic mid-Proterozoic Magmatic and Metamorphic Record

Higher than average thermobaric ratios (temperature/pressure) of metamorphic rocks and abundant ‘dry’ ferroan magmatism including massif anorthosite suites are two enigmatic features of the mid-Proterozoic (1.85–0.85 Ga) that have unclear origins. It has been proposed that elevated mantle temperatures due to insulation under the Columbia supercontinent, and/or to plate slowdown, combined with thin lithosphere, led to high continental geothermal gradients, high-temperature metamorphism, and an increase in dry, ferroan magmatism. Geodynamic modelling predicts that continental subduction zones at mid-Proterozoic mantle potential temperatures (80–150 °C hotter than at present) would exhibit key differences to the Phanerozoic, critically, extensive slab rollback combined with greater volumes of decompression melting of the asthenosphere would lead to wide regions of back-arc magmatism. We posit that these hot, wide continental back-arcs can effectively explain the abundance of ferroan magmatism, anorthosite suites, and high T/P metamorphism. Our model negates the need for extra mantle heating from supercontinental insulation or plate slowdown and shows that the tectonic regime of the mid-Proterozoic was a transitional phase between those of the Archean (likely comprising peel-back tectonics and episodic subduction) and the Phanerozoic (comprising deep continental subduction), and which could have resulted solely from secular cooling of the mantle.

Long‐Distance Asthenospheric Transport of Plume‐Influenced Mantle From Afar to Anatolia

AbstractThe origin of widespread volcanism far from plate boundaries and mantle plumes remains a fundamental unsolved question. An example of this puzzle is the Anatolian region, where abundant intraplate volcanism has occurred since 10 Ma, but a nearby underlying plume structure in the deep mantle is lacking. We employed a combination of seismic and geochemical data to link intraplate volcanism in Anatolia to a trail of magmatic centers leading back to East Africa and its mantle plume, consistent with northward asthenospheric transport over a ∼2,500 km distance. Joint modeling of seismic imaging and petrological data indicates that the east Anatolian mantle potential temperature is higher than the ambient mantle (∼1,420°C). Based on multiple seismic tomography models, the Anatolian upper mantle is likely connected to East Africa by an asthenospheric channel with low seismic velocities. Along the channel, isotopic signatures among volcanoes are consistent with a common mantle source, and petrological data demonstrate similar elevated mantle temperatures, consistent with little cooling in the channel during the long‐distance transport. Horizontal asthenospheric pressure gradients originating from mantle plume upwelling beneath East Africa provide a mechanism for high lateral transport rates that match the relatively constant mantle potential temperatures along the channel. Rapid long‐distance asthenospheric flow helps explain the widespread occurrence of global intraplate magmatism in regions far from deeply‐rooted mantle plumes throughout Earth history.

Temporal evolution of mantle temperature and lithospheric thickness beneath the ∼1.1 Ga Midcontinent Rift, North America: Implications for rapid motion of Laurentia

The production of the magma volumes necessary for the formation of continental flood basalts is thought to result from extension-related decompression melting of thermo-chemically anomalous mantle. The relative importance of elevated mantle temperature or decompression associated with lithospheric thinning remains difficult to constrain because the temporal relationship between the initiation of extension and continental flood basalt is often ambiguous. The ∼1.1 Ga Midcontinent Rift (MCR) in North America provides an opportunity to probe these magma generation factors because the mantle thermo-chemical anomaly (Keweenaw plume) thought responsible for the continental flood basalts of the Keweenaw Large Igneous Province (LIP) intersected an existing rift. The role of elevated mantle temperatures in Keweenaw LIP magmatism remains ambiguous because a temporal decrease in MgO content in primitive lavas may reflect decreasing melting depths with time due to progressive lithospheric thinning, a temporal decrease in mantle temperature, or both. Furthermore, paleomagnetic studies show that Laurentia was moving rapidly during Keweenaw LIP magmatism, raising the question of how the plume remained connected to the MCR during 25 Myr of volcanism. Using samples from the most complete volcanic section preserved at Mamainse Point, Ontario, Canada, we constrain mantle potential temperature and lithospheric thickness by combining three major element thermobarometry models with forward modeling of rare earth element compositions. We find that the earliest stage of Keweenaw LIP magmatism (Early phase; 1109-1104 Ma) was governed by initially deeper melting (>85-100 km thick lithosphere) and high mantle potential temperatures (>1480-1630°C) related to the plume. Potential temperatures decreased to near-ambient conditions (∼1400-1445°C) during the latter portion of the Early phase, with these lower temperatures persisting throughout the Latent (1104-1098 Ma) and Main (1098-1090 Ma) phases. Major element-derived pressures of melt equilibration place broad constraints on lithospheric thickness, which decreased from ∼60-110 km at the onset of magmatism to 45-65 km by the Main phase. The decrease in primitive MgO contents thus reflects thinning lithosphere and decreasing mantle potential temperatures. We argue that the temporal decrease in mantle potential temperature reflects the severing of the connection between the MCR and Keweenaw plume after the Early phase of magmatism by the rapid plate motion. Our constraints on mantle potential temperature resolve the relationship between the MCR, the plume, Keweenaw LIP magmatism, and rapid plate motion, and indicate that an alternative mechanism, such as enhanced small-scale convection due to rapid plate motion and plume-lithosphere interaction, was required to generate the voluminous Main magmatic phase.

Age, geochemistry, and origin of the mid-Proterozoic Häme mafic dyke swarm, southern Finland

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low- Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent.

Dynamic slab segmentation due to brittle-ductile damage in the outer rise.

Subduction is the major plate driving force, and the strength of the subducting plate controls many aspects of the thermochemical evolution of Earth. Each subducting plate experiences intense normal faulting1-9 during bending that accommodates the transition from horizontal to downwards motion at the outer rise at trenches. Here we investigate the consequences of this bending-induced plate damage using numerical subduction models in which both brittle and ductile deformation, including grain damage, are tracked and coupled self-consistently. Pervasive slab weakening and pronounced segmentation can occur at the outer-rise region owing to the strong feedback between brittle and ductile damage localization. This slab-damage phenomenon explains the subduction dichotomy of strong plates and weak slabs10, the development of large-offset normal faults6,7 near trenches, the occurrence of segmented seismic velocity anomalies11 and distinct interfaces imaged within subducted slabs12,13, and the appearance of deep, localized intraplate areas of reduced effective viscosity14 observed at trenches. Furthermore, brittle-viscously damaged slabs show a tendency for detachment at elevated mantle temperatures. Given Earth's planetary cooling history15, this implies that intermittent subduction with frequent slab break-off episodes16 may have been characteristic for Earth until more recent times than previously suggested17.

Warm and Slightly Reduced Mantle Under the Off-Rift Snæfellsnes Volcanic Zone, Iceland

AbstractOlivine (Fo75-91) with spinel inclusions (Cr# 10–61) in basaltic lavas/tephras from the off-rift Snæfellsnes Volcanic Zone in Iceland record the chemistry, temperature and oxygen fugacity of fractionating magmas. After a detailed assessment of equilibrium conditions, crystallization temperatures and oxygen fugacity can be calculated from the composition of homogeneous Cr-spinel and Al-chromite inclusions in olivine phenocrysts. Geologically meaningful results can occasionally be obtained when homogenous spinel is enclosed in mildly zoned olivine and KDMg-Fe [(Mg/Fe)olivine/(Mg/Fe2+)spinel] is within the range for homogenous spinel in homogeneous olivine (3.5–4.3 for our samples). Spinel in normal zoned Fo84.7–90.9 olivine records the primitive stages of magma fractionation and has crystallized from clinopyroxene-free primitive melts, probably at Moho depth and/or below. Discrepancies between Tol-liq (Mg-Fe2+ diffusion sensitive) and TAL (diffusion insensitive) suggest that some primitive olivines experienced magma mixing, completely overprinting their Fo content. Consequentially, Tol-liq in primitive olivines occasionally records residence rather than crystallization conditions. Temperature (1187–1317°C) gradually decreases across normal zoned Fo84.7–90.9 olivine and controls fO2 (Δlog fO2 (QFM) −0.6 ± 0.2). Recharge-related primitive Fo83.8–86.8 mantles of reverse zoned olivine contain the most primitive Cr-spinel linked to crustal magma storage zones. These spinels are mostly antecrysts with high Cr# (41.1–47.9) similar to spinel in normal zoned olivines that were captured by olivine and equilibrated in terms of Mg-Fe2+. A rare olivine macrocryst crystallized alongside clinopyroxene (wehrlite) and includes abundant homogeneous Al-rich Cr-spinels. These are unique because they appear to record closed-system fractional crystallization rather than magma mixing and because they show that Cr-poor, Al-rich spinel crystallized alongside clinopyroxene. The macrocryst olivine–spinel pairs record lower crustal crystal mush conditions with fO2 around the QFM buffer and Tol-liq of ∼1200°C, similar to recharge-related mantles of reverse zoned olivine. More evolved compositions occur in the cores of reverse zoned olivine (Fo75-85) that contain Cr-spinel, Fe-spinel and Al-magnetite. Contrary to spinel in more primitive olivine, these compositions are diverse and follow increasing 100Fe3+/(Cr+Al+Fe3+) of 12.3 to 54.8 and TiO2 (3.3 to 14.7 wt %) at decreasing Mg# (57.4 to 24.1) and Cr# (30.4 to 9.9) and rapidly increasing oxygen fugacities (Δlog fO2 (QFM) +0.2 to +2.0) over only a limited temperature decrease (Tol-liq: 1190 to 1145°C). These compositions span the ‘spinel gap’ and are extremely rare globally. Their preservation is probably related to high-temperature crystallization followed by rapid cooling. These compositions occur at two of the four investigated volcanic centres (Búðahraun and Berserkjahraun) and indicate a strong influence of crustal magmatic processes on crystal composition and fO2, which is absent in the other two locations (Ólafsvíkurenni and Nykurhraun). Spinel and olivine compositions support the tectonically controlled decompression melting of a fertile peridotitic source at elevated mantle temperatures relative to MORB and more reducing conditions than other off-rift magmatism in Iceland.

Cyclic tectono-magmatic evolution of TTG source regions in plume-lid tectonics

The appearance of the earliest felsic crust can be estimated by dating zircons and rocks of tonalite-trondhjemite-granodiorite (TTG) composition. However, the necessary geodynamic processes that form the basis for metamorphism and differentiation as well as the role of emerging TTG crust on evolving crustal dynamics is still poorly understood. To investigate the formation of felsic crust with TTG composition, we conduct a detailed analysis of a series of previously published 3D high-resolution magmatic-thermomechanical models at elevated mantle temperature corresponding to Archean conditions.In these models we observed two distinct phases during coupled cyclic tectonomagmatic crust-mantle evolution: a long quiet growth phase followed by a short rapid overturn phase. Results of the detailed model analysis presented here suggest that(1) Low- and medium-pressure TTGs are formed at the base of the crust during both growth and overturn phase. The formation of low- and medium-pressure TTGs is linked with Moho depth. The ratio of low- to medium-pressure TTGs changes with crustal growth or thinning and gives an approximation for crustal thickness. (2) To form high-pressure TTGs an entirely different mechanism is required, as hydrated basaltic rocks need to be buried below the crust. Direct partial melting of cold eclogitic drips can be excluded as a valid mechanism due to their low temperatures and rapid sinking into the deep mantle. Rather we suggest delamination (peeling-off) or subduction as the main process for some high-pressure TTG production.

Global receiver function observations of the X-discontinuity reveal recycled basalt beneath hotspots

The distribution of chemical heterogeneity beneath hotspots provides important constraints on the source of magmatism and mantle convection. Chemical heterogeneities in the upper mantle produce discontinuities that can be interrogated seismically. One such discontinuity, the X-discontinuity (230-350 km depth), is observed intermittently across the globe, with multiple possible causal mechanisms. However, to better understand the cause of the X-discontinuity and its relationship to mantle upwellings, we require global short wavelength observations, previously unresolved by reflected phases with broad upper mantle Fresnel zones. Discontinuities resulting from seismically observable impedance contrasts can be targeted using high frequency P wave receiver functions (RFs) possessing short wavelength lateral sensitivity (∼100 km) to structures almost directly beneath the station. Thus, below well-instrumented hotspots we can investigate the properties of the X-discontinuity and constrain the causal mechanisms in these locations.Using P-to-s converted teleseismic phases between 30-90∘, we stack RFs in the depth and time-slowness domains at 28 hotspots and 6 cratons globally. This reveals 15 robust X-discontinuity observations beneath hotspots between 244-344 km depth. A further 10 potential observations are present beneath hotspots, and 6 null observations beneath cratons. Two causal mechanisms remain possible at the elevated mantle temperatures expected beneath hotspots. The coesite-stishovite phase transition occurs in eclogite and/or carbonated silicate melt may form under certain conditions. For either mechanism to provide seismically observable impedance contrasts, the upper mantle must be locally enriched in basalt. Assessing the X-discontinuity global distribution reveals a correlation with the Large Low Velocity Provinces, suggesting that mantle plumes may entrain recycled basalt from the lowermost mantle.

Evolution of seaward-dipping reflectors at the onset of oceanic crust formation at volcanic passive margins: Insights from the South Atlantic

Seaward-dipping reflectors (SDRs) have long been recognized as a ubiquitous feature of volcanic passive margins, yet their evolution is much debated, and even the subject of the nature of the underlying crust is contentious. This uncertainty significantly restricts our understanding of continental breakup and ocean basin–forming processes. Using high-fidelity reflection data from offshore Argentina, we observe that the crust containing the SDRs has similarities to oceanic crust, albeit with a larger proportion of extrusive volcanics, variably interbedded with sediments. Densities derived from gravity modeling are compatible with the presence of magmatic crust beneath the outer SDRs. When these SDR packages are restored to synemplacement geometry we observe that they thicken into the basin axis with a nonfaulted, diffuse termination, which we associate with dikes intruding into initially horizontal volcanics. Our model for SDR formation invokes progressive rotation of these horizontal volcanics by subsidence driven by isostasy in the center of the evolving SDR depocenter as continental lithosphere is replaced by more dense oceanic lithosphere. The entire system records the migration of >10-km-thick new magmatic crust away from a rapidly subsiding but subaerial incipient spreading center at rates typical of slow oceanic spreading processes. Our model for new magmatic crust can explain SDR formation on magma-rich margins globally, but the estimated crustal thickness requires elevated mantle temperatures for their formation.

Crustal structure and origin of the Eggvin Bank west of Jan Mayen, NE Atlantic

AbstractThe Eggvin Bank, located between the Jan Mayen Island and Greenland, is an unusually shallow area containing several submarine volcanic peaks, confined by two transforms on the Northern Kolbeinsey Ridge (NKR). We represent P and S wave velocity models for the Eggvin Bank based on an Ocean Bottom Seismometer profile collected in 2011, showing igneous crustal thickness variations from 8 km to 13 km. A 2–5 km increase is associated with two separate 20–30 km wide segments under the main seamounts. The oceanic crust has three layers: upper crust (L2A: 2.8–4.8 km/s), middle crust (L2B: 5.5–6.5 km/s), and lower crust (L3: 6.7–7.35 km/s). Both the thick layer 2(A/B) and the high ratio of layer 2(A/B) thickness to total crustal thickness indicate that secondary, intraplate magmatism built the seamounts of the Eggvin Bank. The seamount in the north where the crust is thickest has a flat top indicating subaerial exposure but is deeper than those with rounded tops in the south and is therefore probably older. Comparing lower crustal seismic velocity with crustal thickness also indicates that the degree of mantle melting may be higher in the north than in the south. An enriched mantle source presently feeds the NKR magmatism and probably influenced the Eggvin Bank development also at earlier times. To what extent the Eggvin Bank has been influenced by the Iceland plume is uncertain, both an enriched mantle component and elevated mantle temperature may have played a role at different times and locations.

Magmatic development of the outer Vøring margin from seismic data

AbstractThe Vøring Plateau off mid‐Norway is a volcanic passive margin, located north of the East Jan Mayen Fracture Zone (EJMFZ). Large volumes of magmatic rocks were emplaced during Early Eocene margin formation. In 2003, an ocean bottom seismometer survey was acquired over the margin. One profile crosses from the Vøring Plateau to the Vøring Spur, a bathymetric high north of the EJMFZ. The P wave data were ray traced into a 2‐D crustal velocity model. The velocity structure of the Vøring Spur indicates up to 15 km igneous crustal thickness. Magmatic processes can be estimated by comparing seismic velocity (VP) with igneous thickness (H). This and two other profiles show a positive H‐VP correlation at the Vøring Plateau, consistent with elevated mantle temperature at breakup. However, during the first 2 Ma magma production was augmented by a secondary process, possibly small‐scale convection. From ∼51.5 Ma excess melting may be caused by elevated mantle temperature alone. Seismic stratigraphy around the Vøring Spur shows that it was created by at least two uplift events, with the main episode close to the Miocene/Pliocene boundary. Low H‐VP correlation of the spur is consistent with renewed igneous growth by constant, moderate‐degree mantle melting, not related to the breakup magmatism. The admittance function between bathymetry and free‐air gravity shows that the high is near local isostatic equilibrium, precluding that compressional flexure at the EJMFZ uplifted the high. We find a proposed Eocene triple junction model for the margin to be inconsistent with observations.

The impact of Rayleigh number on assessing the significance of supercontinent insulation

Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures, thus multiple interpretations of the concept of continental insulation exist. Although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones, there are differing views on what role continental insulation plays in the production of elevated mantle temperatures. Here we investigate how the heating mode of the mantle can change the influence of the “thermal blanket” effect. We present 2‒D and 3‒D Cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates. The evolution of mantle thermal structure is examined after continental accretion at subduction zones (e.g., the formation of Pangea) for a variety of different mantle‒heating modes. Our results show that in low‒Rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases, when compared to models with higher convective vigor. Broad, hot upper mantle features generated in low‒Rayleigh number models (due, in part, to the thermal blanket effect) are absent at higher Rayleigh numbers. We find that subcontinental heating in a high‒Rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin, rather than the influence of continental insulation. In our models featuring Earth‒like convective vigor, we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation.

Elevated mantle temperature beneath East Africa

Research Article| January 01, 2012 Elevated mantle temperature beneath East Africa Tyrone O. Rooney; Tyrone O. Rooney * 1Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA *E-mail: rooneyt@msu.edu. Search for other works by this author on: GSW Google Scholar Claude Herzberg; Claude Herzberg 2Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08855-1179, USA Search for other works by this author on: GSW Google Scholar Ian D. Bastow Ian D. Bastow 3Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (1): 27–30. https://doi.org/10.1130/G32382.1 Article history received: 22 Apr 2011 rev-recd: 28 Jul 2011 accepted: 18 Aug 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Tyrone O. Rooney, Claude Herzberg, Ian D. Bastow; Elevated mantle temperature beneath East Africa. Geology 2012;; 40 (1): 27–30. doi: https://doi.org/10.1130/G32382.1 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 SocietyGeology Search Advanced Search Abstract The causes of magmatism at magmatic rifted margins and large igneous provinces (LIPs) are uncertain because the condition of the mantle that underlay them during formation can no longer be directly observed. Therefore, whether the mantle was characterized by elevated potential temperatures (TP), small-scale convection, or anomalously fertile composition is debated. East Africa is an ideal area in which to address this problem because it contains both the young African-Arabian LIP and the tectonically and magmatically active East African Rift system. Here we present mantle TP estimates for 53 primitive magmas from throughout the region to reveal that thermal anomalies currently peak in Djibouti (140 °C above ambient upper mantle). Slightly warmer conditions accompanied the Oligocene African-Arabian LIP, when the TP anomaly was 170 °C. These values are toward the low end of the global temperature range of LIPs, despite the markedly slow seismic velocity mantle that underlies the region. Mantle seismic velocity anomalies in East Africa cannot, therefore, as is often assumed, be attributed simply to elevated mantle temperatures. We conclude that CO2-assisted melt production in the African superplume contributes to the markedly slow seismic velocities below East Africa. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variability in the crustal structure of the West Indian Continental Margin in the Northern Arabian Sea

ABSTRACT In this case study of the West Indian Continental Margin we present an interpretation of a volcanic margin structure based on regional mapping of high quality 2D seismic data in conjunction with regional satellite derived gravity data and selected subsidence analyses. The area shows many classic characteristics of a volcanic-type margin. Volcanic facies identified and mapped along the margin include seaward-dipping reflectors (SDRs), sub-aerial seamounts, clinoform packages interpreted as lava and volcaniclastic delta systems and thick, seismically layered sequences interpreted as volcanically derived sediment deposited in both fluvial and marine environments. The results show major variation in the overall thickness and style of volcanism across the margin both in dip and strike directions which may be related to variability in influence of the Deccan Plume in addition to localization along structural features inherited from older tectonic events. Our interpretation of rapid lateral variation in the thickness of extrusive volcanism has important implications for the distribution, preservation and hydrocarbon potential of the pre-rift sequence across the margin. The interpreted crustal structure also has a major impact on predictions of the historical and present-day heat-flow into the post-rift section. Our interpretation of the timing and distribution of volcanism is consistent with the presence of a broad region of elevated mantle potential temperatures at the time of the final break-up event on the West Indian Continental Margin, commonly attributed to the Deccan/Reunion Plume. Pre-existing structural heterogeneities appear to have played an important part in controlling the distribution of volcanism. Interpreted tectonic subsidence, based on backstripping analysis of the post-break-up interval, is also shown to be consistent with post-break-up thermal subsidence in combination with dynamic support associated with the elevated mantle temperatures into the Early Eocene.

Geochronological framework of Mesozoic volcanic rocks in the Great Xing’an Range, NE China, and their geodynamic implications

Mesozoic volcanic rocks are widespread throughout the Great Xing’an Range, NE China. However, precise data constraining the exact eruption ages are limited, especially for those from the southern Great Xing’an Range, which severely hampers our understanding of the petrogenesis and geodynamics of these rocks. In this paper, we report precise in situ LA-ICPMS zircon U–Pb age measurements for these volcanic rocks. Volcanic rocks in the southern Great Xing’an Range were divided into four units from bottom to top, namely, the Manketouebo, Manitu, Baiyingaolao and Meiletu formations. The previous studies suggested that these volcanic rocks were mainly formed in the Late Jurassic. Our data demonstrate that the Manketouebo formation erupted during Late Jurassic to Early Cretaceous time, whereas the other formations are all of Cretaceous age. The southern Great Xing’an Range age dataset, along with recently obtained precise ages for volcanic rocks from the northern Great Xing’an Range indicate that Mesozoic volcanism throughout the Great Xing’an Range commenced in Late Jurassic, but peaked during the Cretaceous. They formed under an extensional tectonic setting which resulted from closure of the Mongol–Okhotsk Ocean and subsequent orogenic collapse. The globally elevated mantle temperature in Cretaceous may provided thermal contributions to the generation of the volcanisms.

Tectonic events, continental intraplate volcanism, and mantle plume activity in northern Arabia: Constraints from geochemistry and Ar‐Ar dating of Syrian lavas

New 40Ar/39Ar ages combined with chemical and Sr, Nd, and Pb isotope data for volcanic rocks from Syria along with published data of Syrian and Arabian lavas constrain the spatiotemporal evolution of volcanism, melting regime, and magmatic sources contributing to the volcanic activity in northern Arabia. Several volcanic phases occurred in different parts of Syria in the last 20 Ma that partly correlate with different tectonic events like displacements along the Dead Sea Fault system or slab break‐off beneath the Bitlis suture zone, although the large volume of magmas and their composition suggest that hot mantle material caused volcanism. Low Ce/Pb (<20), Nb/Th (<10), and Sr, Nd, and Pb isotope variations of Syrian lavas indicate the role of crustal contamination in magma genesis, and contamination of magmas with up to 30% of continental crustal material can explain their 87Sr/86Sr. Fractionation‐corrected major element compositions and REE ratios of uncontaminated lavas suggest a pressure‐controlled melting regime in western Arabia that varies from shallow and high‐degree melt formation in the south to increasingly deeper regions and lower extents of the beginning melting process northward. Temperature estimates of calculated primary, crustally uncontaminated Arabian lavas indicate their formation at elevated mantle temperatures (Texcess ∼ 100–200°C) being characteristic for their generation in a plume mantle region. The Sr, Nd, and Pb isotope systematic of crustally uncontaminated Syrian lavas reveal a sublithospheric and a mantle plume source involvement in their formation, whereas a (hydrous) lithospheric origin of lavas can be excluded on the basis of negative correlations between Ba/La and K/La. The characteristically high 206Pb/204Pb (∼19.5) of the mantle plume source can be explained by material entrainment associated with the Afar mantle plume. The Syrian volcanic rocks are generally younger than lavas from the southern Afro‐Arabian region, indicating a northward progression of the commencing volcanism since the arrival of the Afar mantle plume beneath Ethiopia/Djibouti some 30 Ma ago. The distribution of crustally uncontaminated high 206Pb/204Pb lavas in Arabia indicates a spatial influence of the Afar plume of ∼2600 km in northward direction with an estimated flow velocity of plume material on the order of 22 cm/a.

Crustal structure of the Hatton and the conjugate east Greenland rifted volcanic continental margins, NE Atlantic

We show new crustal models of the Hatton continental margin in the NE Atlantic using wide‐angle arrivals from 89 four‐component ocean bottom seismometers deployed along a 450 km dip and a 100 km strike profile. We interpret prominent asymmetry between the Hatton and the conjugate Greenland margins as caused by asymmetry in the initial continental stretching and thinning, as ubiquitously observed on “nonvolcanic” margins elsewhere. This stretched continental terrain was intruded and flooded by voluminous igneous activity which accompanied continental breakup. The velocity structure of the Hatton flank of the rift has a narrow continent‐ocean transition (COT) only ∼40 km wide, with high velocities (6.9–7.3 km/s) in the lower crust intermediate between those of the continental Hatton Bank on one side and the oldest oceanic crust on the other. The high velocities are interpreted as due to intrusion of igneous sills which accompanied the extrusion of flood basalts at the time of continental breakup. The variation of thickness (h) and P wave velocities (vp) of the igneous section of the COT and the adjacent oceanic crust are consistent with melt formation from a mantle plume with a temperature ∼120–130°C above normal at breakup, followed by a decrease of ∼70–80°C over the first 10 Ma of seafloor spreading. The h‐vp systematics are consistent with the dominant control on melt production being elevated mantle temperatures, with no requirement for either significant active small‐scale mantle convection under the rift or of the presence of significant volumes of volatiles or fertile mantle.

Crustal structure of the SW-Moroccan margin from wide-angle and reflection seismic data (the DAKHLA experiment) Part A: Wide-angle seismic models

A total 1500 km of seismic reflection and wide-angle profiles were acquired off the southern Moroccan margin during the DAKHLA cruise, a joint project of Ifremer, the Universities of Brest, El Jadida and Lisbon and Total. The shots along two profiles parallel to the margin and two profiles perpendicular to the margin were also recorded by ocean bottom seismometers (OBS). The profiles perpendicular to the margin were additionally extended on land using 14 stations on the northern profile and 11 stations on the southern profile. Modelling of the reflection and wide-angle seismic data reveals a 10 km deep sedimentary basin including two high velocity carbonate layers. Lateral crustal thinning is observed from a 27 km thick crystalline continental crust to a 7 km thick oceanic crust occurring over less than 100 km. The crystalline continental crust can be divided into two distinct layers of 12 and 15 km thickness. The oceanic crust east of the magnetic anomaly M25 displays higher velocities in layer 3 than west of the magnetic anomaly. The change in seismic velocity suggests a possible link to changes in accretionary processes of the oceanic crust. Some regions show seismic velocities between 6.8 and 7.4 km/s which could be explained by slightly elevated mantle temperatures during accretion of the crust.