Hot Mantle Research Articles

Boninites and Adakites from the Northern Termination of the Tonga Trench: Implications for Adakite Petrogenesis

Adakitic rocks were recovered by dredging from the northern termination of the Tonga Trench during the 1984 voyage of the R.V. Natsushima and the 1996 voyage of the R.V. Melville. These contain magmatic zircons that have been dated at 2� 5M a by U^Pb methods, indicating that they are contemporaneous with boninite magmatism previously described from this area. This is the first time adakites and high-Ca boninites have been reported from the same active tectonic setting. The Tonga adakites are classified as high-SiO2 adakites, and are compositionally consistent with an origin as partial melts of subducted Pacific oceanic crust and sediment. Geochemical modelling suggests that the adakites are not involved in the petrogenesis of the Tongan high-Ca boninites. However, the recovery of adakites and boninites from the termination of the northern Tonga Trench suggests that both magma types are related to the unique tectonic setting of this region, where a transition from steep subduction to a transform fault plate boundary has created a slab window with an associated slab edge. Boninites are generated as a result of hot Samoan plume mantle moving through the slab window and subsequently being fluxed by H2O-rich fluids from the subducting Pacific oceanic crust. The Tonga adakites, in contrast, result from the direct melting of the slab edge as a result of the juxtaposition of the subducting slab against hot mantle derived from the Samoan plume.

Plate tectonics on the early Earth: Limitations imposed by strength and buoyancy of subducted lithosphere

The tectonic style and viability of modern plate tectonics in the early Earth is still debated. Field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the Archean continue to accumulate. Here, we present the first numerical modeling results that address for a hotter Earth the viability of subduction, one of the main requirements for plate tectonics. A hotter mantle has mainly two effects: 1) viscosity is lower, and 2) more melt is produced, which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer. Although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation, our modeling results show that eclogitization significantly relaxes this limitation for a developed, ongoing subduction process. Furthermore, the lower viscosity leads to more frequent slab breakoff, and sometimes to crustal separation from the mantle lithosphere. Unlike earlier propositions, not compositional buoyancy considerations, but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter Earth. These results suggest a new explanation for the absence of ultrahigh-pressure metamorphism (UHPM) and blueschists in most of the Precambrian: early slabs were not too buoyant, but too weak to provide a mechanism for UHPM and exhumation.

Wide angle converted shear wave analysis of a North Atlantic volcanic rifted continental margin: constraint on sub-basalt lithology

Continental break-up between Greenland and Europe in the early Tertiary was accompanied by the eruption of > 1 million km3 of basalt flows generated by decompression melting of the underlying, abnormally hot mantle of the proto-Iceland plume (White and McKenzie, 1989). Extrusion occurred extremely rapidly in two phases, an early pre-breakup phase between 62-58 Ma and a main syn-breakup phase from 56 to ~54 Ma (Saunders et al., 1997) with a high rate of magmatism continuing to the present time in Iceland. Lava produced during continental break-up flowed up to 150 km away from the rift across sedimentary basins while intrusion into the lower crust of large volumes of melt caused permanent crustal thickening and uplift across the continent-ocean transition (White et al., 1987). A profile across the NW European volcanic rifted continental margin close to the Faroe Islands was studied as part of a joint UK research council and industry sponsored project: integrated Seismic Imaging and Modelling of Margins (iSIMM, White et al., 2002).

Time variability in Cenozoic reconstructions of mantle heat flow: Plate tectonic cycles and implications for Earth's thermal evolution

The thermal evolution of Earth is governed by the rate of secular cooling and the amount of radiogenic heating. If mantle heat sources are known, surface heat flow at different times may be used to deduce the efficiency of convective cooling and ultimately the temporal character of plate tectonics. We estimate global heat flow from 65 Ma to the present using seafloor age reconstructions and a modified half-space cooling model, and we find that heat flow has decreased by approximately 0.15% every million years during the Cenozoic. By examining geometric trends in plate reconstructions since 120 Ma, we show that the reduction in heat flow is due to a decrease in the area of ridge-proximal oceanic crust. Even accounting for uncertainties in plate reconstructions, the rate of heat flow decrease is an order of magnitude faster than estimates based on smooth, parameterized cooling models. This implies that heat flow experiences short-term fluctuations associated with plate tectonic cyclicity. Continental separation does not appear to directly control convective wavelengths, but rather indirectly affects how oceanic plate systems adjust to accommodate global heat transport. Given that today's heat flow may be unusually low, secular cooling rates estimated from present-day values will tend to underestimate the average cooling rate. Thus, a mechanism that causes less efficient tectonic heat transport at higher temperatures may be required to prevent an unreasonably hot mantle in the recent past.

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume

Geophysical data have led to the interpretation that Beta Regio, a 2000 × 25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain.

Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

Two geochemical proxies are particularly important for the identification and classification of oceanic basalts: the Th–Nb proxy for crustal input and hence for demonstrating an oceanic, non-subduction setting; and the Ti–Yb proxy for melting depth and hence for indicating mantle temperature and thickness of the conductive lithosphere. For the Th–Nb proxy, a Th/Yb–Nb/Yb projection demonstrates that almost all oceanic basalts lie within a diagonal MORB–OIB array with a principal axis of dispersion along the array. However, basalts erupted at continental margins and in subduction zones are commonly displaced above the MORB–OIB array and/or belong to suites with principal dispersion axes which are oblique to the array. Modelling of magma–crust interaction quantifies the sensitivity of the Th–Nb proxy to process and to magma and crustal compositions. For the Ti–Yb proxy, the equivalent Ti/Yb–Nb/Yb projection features a discriminant boundary between low Ti/Yb MORB and high Ti/Yb OIB that runs almost parallel to the Nb/Yb axis, reflecting the fact that OIB originate by melting beneath thicker lithosphere and hence by less melting and with residual garnet. In the case of volcanic-rifted margins and oceanic plume–ridge interactions (PRI), where hot mantle flows toward progressively thinner lithosphere (often becoming more depleted in the process), basalts follow diagonal trends from the OIB to the MORB field. Modelling of mantle melting quantifies the sensitivity of the Ti–Nb proxy to mantle potential temperature and lithospheric thickness and hence defines the petrogenetic basis by which magmas plot in the OIB or MORB fields. Oceanic plateau basalts lie mostly in the centre of the MORB part of that field, reflecting a high degree of melting of fertile mantle. Application of the proxies to some examples of MORB ophiolites helps them to be further classified as C (contaminated)-MORB, N (normal)-MORB, E (enriched)-MORB and P (plume)-MORB ophiolites, which may add a useful dimension to ophiolite classification. In the Archean, the hotter magmas, higher crustal geotherms and higher Th contents of contaminants all result in widespread crustal input that is easy to detect geochemically with the Th–Nb proxy. Application of this proxy to Archean greenstones demonstrates that almost all exhibit a crustal component even when reputedly oceanic. This indicates, either that some interpretations need to be re-examined or that intra-oceanic crustal input is important in the Archean making the proxy less effective in distinguishing oceanic from continental settings. The Ti–Yb proxy is not effective for fingerprinting Archean settings because higher mantle potential temperatures mean that lithospheric thickness is no longer the critical variable in determining the presence or absence of residual garnet.

A Developing Plate Boundary: Tianshan-Baikal Active Tectonic Belt

It is obvious that a great amount of strong earthquakes in central Asia occurred densely along Tianshan-Baikal belt; however, none of huge continuous fault has been revealed on the surface along the belt. It caused academic argument related to their relationship. The authors analyzed its seismic activity in terms of seismicity in time-space domain, zoning feature, stress field, and recurrence period of earthquakes. In addition, we studied the Bouguer gravity and isostatic anomaly, and calculated the distribution of density and shear wave velocity in the upper mantle. It is found that there is a gradient belt of density and velocity along NE direction at the depths of 70–250 km, which is actually a Cenozoic boundary of cold mantle and hot mantle, and has an overpass relation with the structure existing in the upper lithosphere. It is inferred that this gradient belt in the upper mantle played an important role in controlling present tectonic movement and stress field in Eurasia continent. As far as the earthquake nature in this belt is concerned, it is neither an interplate earthquake nor an intraplate one in the traditional sense. Such kind of earthquake caused by overpass structure actually belongs to the structural earthquake, and is of interplate within a continent. In this paper, the overpass relation and mirror relation in time-space domain between deep and shallow structures are discussed briefly. It is also noted that the overpass structures shown in the areas of both north-south seismic belt of China and eastern seismic belt of Iran are related to a special structure in the upper mantle. We conclude that the Tianshan-Baikal active tectonic belt is a developing boundary of plates, a typical tectonic within a continent, which separates Eurasian plate into two parts, i.e., the stable Russian-Siberia subplate to the north and the active China-Southeast Asia subplate to the south.

Breakup and early seafloor spreading between India and Antarctica

SUMMARY We present a tectonic interpretation of the breakup and early seafloor spreading between India and Antarctica based on improved coverage of potential field and seismic data off the east Antarctic margin between the Gunnerus Ridge and the Bruce Rise. We have identified a series of ENE trending Mesozoic magnetic anomalies from chron M9o (∼130.2 Ma) to M2o (∼124.1 Ma) in the Enderby Basin, and M9o to M4o (∼126.7 Ma) in the Princess Elizabeth Trough and Davis Sea Basin, indicating that India–Antarctica and India–Australia breakups were roughly contemporaneous. We present evidence for an abandoned spreading centre south of the Elan Bank microcontinent; the estimated timing of its extinction corresponds to the early surface expression of the Kerguelen Plume at the Southern Kerguelen Plateau around 120 Ma. We observe an increase in spreading rate from west to east, between chron M9 and M4 (38–54 mm yr–1), along the Antarctic margin and suggest the tectono-magmatic segmentation of oceanic crust has been influenced by inherited crustal structure, the kinematics of Gondwanaland breakup and the proximity to the Kerguelen hotspot. A high-amplitude, E–W oriented magnetic lineation named the Mac Robertson Coast Anomaly (MCA), coinciding with a landwards step-down in basement observed in seismic reflection data, is tentatively interpreted as the boundary between continental/transitional zone and oceanic crust. The exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust, which typically occurs above anomalously hot mantle. Together with the existence of the MCA zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring/steady state magmatic production of the Kerguelen LIP. An alternative model suggests that the northward ridge jump was limited to the Elan Bank region, whereas seafloor spreading continued in the West Enderby Basin and its Sri Lankan conjugate margin. In this case, the MCA magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 Ma.

Anomalous deepening of a belt of intraslab earthquakes in the Pacific slab crust under Kanto, central Japan: Possible anomalous thermal shielding, dehydration reactions, and seismicity caused by shallower cold slab material

A belt of intraslab seismicity in the Pacific slab crust parallel to iso‐depth contours of the plate interface has been found beneath Hokkaido and Tohoku. Hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso‐depth contours beneath Kanto, deepening toward the north from ∼100 km to ∼140 km depth. The depth limit of the contact zone with the overlying Philippine Sea slab is located close to and parallel to this obliquely oriented seismic belt, suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab. The contact with this cold slab hinders the heating of the Pacific slab crust by hot mantle wedge, which would cause delay of eclogite‐forming phase transformations and hence deepening of the seismic belt there. The depth limit of the subducting low‐velocity crust also deepens toward the north, supporting this idea.

Uppermost mantle structure and its relation with seismic activity in the central Tien Shan

Using arrival data from GHENGIS and Kyrgyz Network (KNET) stations, we determined the uppermost mantle structure of the central Tien Shan from Pn tomography. Pn velocities in the north‐central Tien Shan are slower than normal for stable lithosphere. They are very slow in the south‐central Tien Shan, where a hot mantle previously has been postulated. The Pn velocity variation correlates well with seismicity. Most earthquakes are concentrated in the north‐central Tien Shan, but almost no earthquakes in the south‐central Tien Shan, even though both geologic and GPS observations show active convergence there. This correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle. Specifically, we propose that the very slow Pn velocity beneath the south‐central Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has conducted well up into the crust, inducing a ductile rheology that prevents significant seismicity.

Deep structure of the Japan subduction zone

We determined 3-D P-wave velocity structure down to 700 km depth under the Japan Islands using a large number of arrival time data from local and teleseismic events simultaneously. We collected 207,000 arrival times from 7743 shallow and deep earthquakes occurred in and around Japan and 34,148 data from 333 teleseismic events, which were recorded by over 1000 seismic stations on the Japan Islands. Our tomographic model revealed some new features. The Philippine Sea slab is found to subduct down to 500 km depth under southwest Japan though the seismicity within the slab ends at 150–200 km depth. Significant low-velocity anomalies are found to exist in the deep portion of the mantle wedge above the Pacific slab, which may be caused by the deep dehydration process of the slab. Slow anomalies are detected in the mantle beneath the Pacific slab, which may be caused by a mantle plume or upwelling of hot mantle materials associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.

Testing the plume theory

The physics of low Reynolds number plumes is well understood, and this allows a number of testable predictions to be made about mantle plumes. Mantle plumes are predicted to originate from the core–mantle boundary and consist of a large head, ∼ 1000 km in diameter followed by a narrower tail. When the head reaches the top of its ascent it flattens to form a disk with a diameter from 2000 to 2500 km. The prediction that plumes originate from the core–mantle boundary has recently been confirmed by seismic studies, using a new finite frequency technique, which has successfully traced plumes to the core–mantle boundary. The prediction that plume heads should form flattened disks 2000 to 2500 km across is confirmed by the length of thickened oceanic crust that developed during the early stages of the opening of the North Atlantic above the Iceland plume head. This resulted in hot mantle from the plume head being drawn into the spreading centre to produce 2400 km of thickened oceanic crust on both sides of the Atlantic, as predicted. Initial eruption from a plume head should be preceded by ∼ 1000 m of domal uplift. Uplift prior to volcanism has been documented for a number of flood basalt provinces, the best example being Emeishan in China where the shape and magnitude of the observed uplifted dome agrees closely with that predicted from laboratory and numerical modelling. High-temperature picrites are expected to dominate the first eruptive products of a new plume and should be more abundant near the centre of the volcanic province. This distribution of picrites is observed in the Karoo and Emeishan. Picrites are also found in the Iceland–North Atlantic Igneous Province, Reunion–Deccan, Parana–Etendeka–Tristan da Cunha, Siberian Traps (Meymechites), Caribbean–Colombian and Hawaii large igneous provinces (LIPs). The excellent agreement between the predictions of the mantle plume hypothesis and observations provides strong support for the veracity of the hypothesis. Confirmation that plume heads have a diameter of 1000 km in the upper mantle requires that plumes originate near the core–mantle boundary. As a consequence, plume tails sample a thermal boundary layer near the core–mantle boundary and plume heads are a mixture of this material and lower mantle that becomes entrained into the head. The melting products of plume tails and heads can therefore be used to deduce the composition of the material in the thermal boundary layer near the core and the lower mantle respectively. Furthermore, because the high-temperature melting products of plumes can be recognised throughout the geological record they can be used to document the variations in the chemistry of the mantle in the boundary layer near the core with time.

Continental magmatism, volatile recycling, and a heterogeneous mantle caused by lithospheric gravitational instabilities

Removal of the lower lithosphere (mantle lithosphere with or without portions of the crust) through ductile gravitational instabilities can produce magma under continents. Using numerical experiments approximating the rheology of continental crust and lithosphere and underlying asthenosphere and using phase equilibria from the literature, I investigate the topographic and magmatic results of lithospheric gravitational instabilities, along with the fate of the sinking material. Lithospheric removal, commonly referred to as delamination regardless of the mechanism, may allow asthenospheric material to rise and to melt adiabatically, and this asthenosphere can conductively heat portions of the lithosphere previously at lower temperatures. The size and rheology of the sinking material greatly influence the resulting surface topography as well as whether or not melting occurs. The sinking material may devolatilize as it reaches higher temperatures and pressures, just as a subducting slab does, triggering further melting. Gravitational instabilities are possible causes of nonmagmatic basins, continental magmatism of varying volume and composition in the absence of subduction, areas of high heat flow and uplift, and creation of an upper mantle heterogeneous in major and trace elements and volatiles. Magmatism can be simultaneous with topographic subsidence and possibly with subsequent uplift. In those cases that produce magma, melting can occur over a depth range from ∼30 to 200 km, though anomalously hot mantle is required to reach the volume of flood basalts.

Regional uplift associated with continental large igneous provinces: The roles of mantle plumes and the lithosphere

The timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes. Here we review geological indicators of surface uplift associated with five continent-based magmatic provinces: Emeishan Traps (260 million years ago: Ma), Siberian Traps (251 Ma), Deccan Traps (65 Ma), North Atlantic (Phase 1, 61 Ma and Phase 2, 55 Ma), and Yellowstone (16 Ma to recent). All five magmatic provinces were associated with surface uplift. Surface uplift can be measured directly from sedimentary indicators of sea-level in the North Atlantic and from geomorpholocial indicators of relative uplift and tilting in Yellowstone. In the other provinces, surface uplift is inferred from the record of erosion. In the Deccan, North Atlantic and Emeishan provinces, transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma. Transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation. Observations of the spatial and temporal relationships between surface uplift, rifting and magmatism are also important in constraining models of LIP formation. Onset of surface uplift preceded magmatism in all five of the provinces. Biostratigraphic constraints on timing of uplift and erosion are best for the North Atlantic and Emeishan Provinces, where the time interval between significant uplift and first magmatism is less than 1 million years and 2.5 million years respectively. Rifting post-dates the earliest magmatism in the case of the North Atlantic Phase 1 and possibly in the case of Siberia. The relative age of onset of offshore rifting is not well constrained for the Deccan and the importance of rifting in controlling magmatism is disputed in the Emeishan and Yellowstone Provinces. In these examples, rifting is not a requirement for onset of LIP magmatism but melting rates are significantly increased when rifting occurs. Models that attempt to explain emplacement of these five LIPs without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift, rifting and magmatism. For example, small-scale convection related to craton or rift boundaries (edge-driven convection) cannot easily explain widespread (1000 km scale) transient surface uplift (Emeishan, Deccan, North Atlantic), and upper mantle convection initiated by differential incubation beneath cratons (the hotcell model) is at odds with rapid onset of surface uplift (Emeishan, North Atlantic). The start-up plume concept is still the most parsimonious way of explaining the observations presented here. However, observations of surface uplift cannot directly constrain the depth of origin of the hot mantle in a plume head. The short time interval between onset of transient surface uplift and magmatism in the North Atlantic and Emeishan means that the associated starting plume heads were probably not large (∼ 1000 km diameter) roughly spherical diapirs and are likely to have formed narrow (∼ 100 km radius) upwelling jets, with hot mantle then spreading rapidly outward within the asthenosphere. In cases where rifting post-dates magmatism (N Atlantic Phase 1) or where the degree of lithospheric extension may not have been great (Siberia), a secondary mechanism of lithospheric thinning, such as gravitational instability or delamination of the lower lithosphere, may be required to allow hot mantle to decompress sufficiently to explain the observed volume of magma with a shallow melting geochemical signature. Any such additional thinning mechanisms are probably a direct consequence of plume head emplacement.

Late Cretaceous-Early Cenozoic volcanism of Southern Mongolia: A trace of the South Khangai mantle hot spot

The Gobi-Tien Shan volcanic area (in Southern Mongolia) is part of the South Khangai volcanic region (SKVR). The formation of its lava fields was related to three stages of volcanic activity: the Late Cretaceous (88–71 Myr), Paleocene-Early Eocene (62–47 Myr), and Early Oligocene (37–30 Myr). Volcanic occurrences of different age are represented by trachybasalt, trachyandesitobasalt, basanite, and melanephelinite with similar geochemical characteristics, which are also close to the geochemical characteristics of OIB basalt. The isotope composition (Sr, Nd) of the rocks indicates that the magma sources were formed as a result of mixing of a moderately depleted PREMA mantle and an EM-I mantle enriched in neodymium.The patterns of migration of volcanic centers of different ages over the area of interest have been studied. The earliest (Late Cretaceous) volcanic occurrences were concentrated mainly in the south of the area, the Paleocene-Early Eocene eruptions took place at the center of the area, and the Early Oligocene volcanism occurred in the northern area. The observed migration of the volcanic activity centers is related to lithospheric plate motions relative to a localized source of hot mantle (the South Khangai mantle hot spot), which controlled volcanic activity within SKVR. In the lithospheric structure of this region, local asthenospheric high, reaching a depth of ∼50 km, correspond to this hot spot.

The circum-Mediterranean anorogenic Cenozoic igneous province

During the Cenozoic widespread anorogenic magmatism, unrelated to recent supra-subduction zone modification of its mantle source, developed within the Mediterranean and surrounding regions; this is referred to collectively as the CiMACI (Circum-Mediterranean Anorogenic Cenozoic Igneous) province. On the basis of a comprehensive review of published and new major and trace element and Sr–Nd–Pb isotopic data (more than 7800 samples) for the magmatic rocks, a common sub-lithospheric mantle source component is identified for most of the region. This has geochemical affinities to the source of HIMU oceanic island basalts and to the European Asthenospheric Reservoir (EAR) and the Low Velocity Component (LVC) of previous workers; we refer to this as the Common Mantle Reservoir (CMR). Global and local seismic tomography studies of the mantle beneath the CiMACI province have revealed a range of P- and S-wave velocity anomalies, some of which have been related to the presence of mantle plumes. Detailed local tomography experiments in the Massif Central of France and the Eifel region of central Germany suggest that, locally, there are diapiric upwellings rooted within the upper mantle which induce adiabatic decompression melting and magma generation. These velocity anomalies can be interpreted as evidence of mantle temperatures up to 150 °C hotter than the ambient mantle. However, the seismic attenuation could also be attributable to the presence of fluid or partial melt and significant thermal anomalies are not necessary to explain the petrogenesis of the magmas. A model is proposed in which the geochemical and isotopic characteristics of the sub-lithospheric mantle beneath the CiMACI province reflect the introduction of recycled crustal components (derived from both oceanic and continental lithosphere) into the ambient depleted upper mantle. This sub-lithospheric mantle is subsequently partially melted in a variety of geodynamic settings related to lithospheric extension, continental collision and orogenic collapse, and contemporaneous subduction, slab roll-back and slab-window formation. On the basis of this in-depth geochemical and petrological study of the CiMACI province, we consider that there is no need to invoke the involvement of anomalously hot mantle (i.e., the presence of a single or multiple deep mantle plumes) in the petrogenesis of the magmas. If, however, we adopt a more permissive definition of “mantle plume”, allowing it to encompass passive, diapiric upwellings of the upper mantle, then we can relate the CiMACI province magmatism to multiple upper mantle plumes upwelling at various times during the Cenozoic. To avoid confusion we recommend that such upper mantle plumes are referred to as diapiric instabilities.

Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume

Research Article| December 01, 2007 Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume Xuan-Ce Wang; Xuan-Ce Wang 1Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Search for other works by this author on: GSW Google Scholar Xian-Hua Li; Xian-Hua Li 2Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Search for other works by this author on: GSW Google Scholar Wu-Xian Li; Wu-Xian Li 2Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Search for other works by this author on: GSW Google Scholar Zheng-Xiang Li Zheng-Xiang Li 3Institute of Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Xuan-Ce Wang 1Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Xian-Hua Li 2Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Wu-Xian Li 2Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Zheng-Xiang Li 3Institute of Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia Publisher: Geological Society of America Received: 12 Mar 2007 Revision Received: 22 Jul 2007 Accepted: 30 Jul 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2007) 35 (12): 1103–1106. https://doi.org/10.1130/G23878A.1 Article history Received: 12 Mar 2007 Revision Received: 22 Jul 2007 Accepted: 30 Jul 2007 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 Xuan-Ce Wang, Xian-Hua Li, Wu-Xian Li, Zheng-Xiang Li; Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume. Geology 2007;; 35 (12): 1103–1106. doi: https://doi.org/10.1130/G23878A.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 Mantle plume or superplume activities have repeatedly been invoked as a cause for the breakup of the Neoproterozoic supercontinent Rodinia, with supportive evidence including radiating dike swarms, globally synchronous anorogenic igneous activity, large-scale lithospheric doming and unroofing, and geochemical signatures similar to recent plume-related magmatism. However, high-temperature magmas such as picrite or komatiite, a requisite product of mantle plume activities, have not previously been identified for those events. We present here geochronological and geochemical data from a suite of pillow lavas in central South China. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from an evolved member of andesitic composition within the suite indicates that the lavas were erupted at 823 ± 6 Ma. All but a few highly evolved, crust-contaminated basaltic rocks are characteristically high in MgO (10.2%–17.5%), Ni (183–661 ppm), and Cr (677–1672 ppm), but low in TiO2 (0.5%–0.7%), Al2O3 (10.6%–12.7%), and FeOT (total Fe as FeO) (7.4%–10.5%), typical of komatiitic basalts. Our geochemical modeling, which removes the effect of olivine crystallization, suggests that their primary magma has typical komatiitic compositions with MgO ≈ 20%, FeOT ≈ 11%, SiO2 ≈ 47%, TiO2 ≈ 0.48%, Al2O3 ≈ 10%, Ni ≈ 860 ppm, and Cr ≈ 1780 ppm. Such a high MgO content in the primary melts implies a melt temperature of >1500 °C, suggesting that the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature (Tp) 260 ± 50 °C higher than the ambient mid-oceanic-ridge basalt (MORB)–source mantle, similar to that of modern mantle plumes. The Yiyang komatiitic basalts are thus the first solid petrological evidence for the proposed ca. 825 Ma mantle plume that played a key role in the breakup of the supercontinent Rodinia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology, Geochronology, and Geodynamics of the Khan Bogd alkali granite pluton in southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Eastern Wrangellia – A New Ni-Cu-PGE Metallogenic Terrane in North America

Triassic ultramafic-mafic intrusive complexes along the eastern margin of ``Wrangellia' adjacent the Denali fault from east-central Alaska to northern British Columbia constitutes a newly recognized Ni-Cu-PGE metallogenic terrane that can be traced for at least 600 km. These sill- like intrusive centres acted as subvolcanic magma chambers that fed the extensive thick, overlying basalts and locally picritic pyroclastcs of the Nikolai Group in an oceanic plateau flood basalt setting. Confinement of the olivine-rich ultramafic sills, Ni-Cu-PGE mineralization, and more primitive coeval basalts and picrites exclusively to the eastern portion of Wrangellia is believed to be the product of melts forming in closer proximity to the hotter mantle plume tail or ``axial jet' relative to the cooler more distal portions. The recent discovery of proximal, picritic volcanics with some of the larger mineralized ultramafic bodies in Alaska finally concludes the past paradox oftholeiitic parental magmas and primitive Ni-Cu-PGE ores in this terrane. The unprecedented levels of platinum group elements, the vast extent of this mineralized terrane, the temporal association with Siberian Trap magmatism, and the accompanying Noril'sk mineralizing event, suggest that mantle plume activity during the Permo-Triassic was unique with respect to the Phanerozoic record.

New structural and geochemical observations from the Pacific‐Antarctic Ridge between 52°45′S and 41°15′S

We investigated the morphology and structure of the Pacific‐Antarctic Ridge between 52°45′S and 41°15′S during the Pacantarctic2 cruise using multibeam echosounder together with gravity measurements and dredges. Analysis of the bathymetric, gravity and geochemical data reveal three ridge segments separated by overlapping spreading centers south of the Menard transform fault (MTF) and five segments north of it. Calculation of the cross‐sectional area allows quantification of the variation in size of the axial bathymetric high. Together with the calculation of the mantle Bouguer anomaly, these data provide information about variations in the temperature of the underlying mantle or in crustal thickness. Areas with hotter mantle are found north and south of the MTF. Geochemical analyses of samples dredged during the survey show a correlation of high cross‐sectional area values and negative mantle Bouguer anomalies in the middle of segments with relatively less depleted basalts.