Mantle Coupling Research Articles

Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source

Research Article| April 01, 2007 Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source J. Brendan Murphy; J. Brendan Murphy 1Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Search for other works by this author on: GSW Google Scholar Jaroslav Dostal Jaroslav Dostal 2Department of Geology, St. Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. Brendan Murphy 1Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada Jaroslav Dostal 2Department of Geology, St. Mary's University, Halifax, Nova Scotia B3H 3C3, Canada Publisher: Geological Society of America Received: 07 Jun 2006 Revision Received: 23 Nov 2006 Accepted: 27 Nov 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (4): 335–338. https://doi.org/10.1130/G23072A.1 Article history Received: 07 Jun 2006 Revision Received: 23 Nov 2006 Accepted: 27 Nov 2006 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 J. Brendan Murphy, Jaroslav Dostal; Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source. Geology 2007;; 35 (4): 335–338. doi: https://doi.org/10.1130/G23072A.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 Antigonish Highlands, Nova Scotia, are within the Avalon terrane of the northern Appalachians and contain four distinct episodes of rift-related magmatism: Neoproterozoic, Cambrian, Middle Ordovician, and Late Devonian. All four magmatic suites are composed of basalts and subordinate crustally derived felsic rocks. The mafic rocks of these suites, which do not appear to be significantly contaminated by continental crust, display similar geochemical and Sm-Nd isotopic characteristics consistent with an enriched mantle source that was metasomatically enriched prior to the oldest rifting event, probably between 0.8 and 1.1 Ga. These data also imply that the Avalonian crust and its subcontinental lithospheric mantle remained coupled during four magmatic events. Because the earliest phases of magmatism occurred when Avalonia was located along the Gondwanan margin, and the latest phases after Avalonia accreted to Laurentia, the coupling of crust and mantle in the Antigonish Highlands suggests that the migration of Avalonia did not completely detach its lithospheric mantle from its crustal cover. This study demonstrates the importance of comparing magmatism of different ages in the same terrane. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Constraining the extent of crust–mantle coupling in central Asia using GPS, geologic, and shear wave splitting data

We have obtained constraints on mechanical crust–mantle coupling for Tibet and Yunnan/Indo China, by comparing the observed surface deformation field inferred from GPS and Quaternary fault slip rate data, with the mantle deformation field inferred from several SKS shear wave splitting data sets. We first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction, ϕ, which is assumed to be parallel to the horizontal projection of the direction of maximum shear. For asthenospheric flow, we solved for a best-fitting uniform sub-asthenospheric velocity model for Eurasia. The fit, however, was not satisfactory (RMS misfit Δ ϕ = 20°). Solving for separate uniform flow fields in each region improves the fit (Δ ϕ = 15° for Tibet, Δ ϕ = 11° for Yunnan), although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely. We then considered lithospheric models. For Tibet, vertically coherent deformation (i.e., maximum shear direction from surface deformation is parallel to ϕ) yields an improved match (Δ ϕ = 11°) for left-lateral shear. Both the goodness of fit and the dominance of left-lateral surface faulting in Tibet, argue for a lithospheric source of anisotropy. The misfit for Yunnan is large for either right- (Δ ϕ = 53°) or left-lateral (Δ ϕ = 49°) shear, which argues for complete crust–mantle decoupling in Yunnan. We show that the fast polarization directions throughout both the Tibet and Yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath Tibet, but a complete decoupling between crust and mantle beneath Yunnan crust. This dynamic model predicts left-lateral maximum shear directions within the mantle that align with fast polarization directions in both regions (Δ ϕ = 9°). These maximum shear directions within the mantle align with the left-lateral maximum shear directions in the crustal deformation field in Tibet, but are not in Yunnan. Our results have the following implications. First, the coherence between crust and mantle deformation in Tibet implies strong crust–mantle mechanical coupling, since this is the only way that crustal buoyancy forces, required to account for the surface deformation field, can be transmitted into the mantle. This behavior is consistent with a uniform-strength lithosphere or strong crust, but not with a substantially weaker crust. For example, it is inconsistent with the popular “jelly-sandwich” rheology, and thus precludes behavior such as large-scale lower crustal flow in Tibet. The crust–mantle coherence is also incompatible with mantle delamination. Second, crust–mantle decoupling within the Yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions; crustal buoyancy forces are not transmitted into the mantle beneath Yunnan. Moreover, the dynamic model for the mantle shows that the Yunnan crust is moving south with respect to the mantle at rates as high as ∼30 mm/yr. Third, there is a fundamental rheological lithospheric transition between Tibet and Yunnan that may provide a key to understanding this significant orogen.

The free precession and libration of Mercury

An analysis based on the direct torque equations including tidal dissipation and a viscous core–mantle coupling is used to determine the damping time scales of O ( 10 5 ) years for free precession of the spin about the Cassini state and free libration in longitude for Mercury. The core–mantle coupling dominates the damping over the tides by one to two orders of magnitude for the plausible parameters chosen. The short damping times compared with the age of the Solar System means we must find recent or on-going excitation mechanisms if such free motions are found by the current radar experiments or the future measurement by the MESSENGER and BepiColombo spacecraft that will orbit Mercury. We also show that the average precession rate is increased by about 30% over that obtained from the traditional precession constant because of a spin–orbit resonance induced contribution by the C 22 term in the expansion of the gravitational field. The C 22 contribution also causes the path of the spin during the precession to be slightly elliptical with a variation in the precession rate that is a maximum when the obliquity is a minimum. An observable free precession will compromise the determination of obliquity of the Cassini state and hence of C / M M R 2 for Mercury, but a detected free libration will not compromise the determination of the forced libration amplitude and thus the verification of a liquid core.

The combined influences of variable thermal conductivity, temperature- and pressure-dependent viscosity and core–mantle coupling on thermal evolution

Most convection studies of thermal history have not considered explicitly the thermal interaction between the mantle flow and the core. We have investigated the influences of variable thermal conductivity and variable viscosity (temperature- and pressure-dependent) on the boundary layer and thermal characteristics of the D ″ layer, and the evolution of the thermo-mechanical profiles of horizontally averaged viscosity and thermal conductivity. Viscosity contrast due to temperature dependence of up to 30,000 has been considered. Our results show clearly that variable thermal conductivity, though small in magnitude as compared to variations in the viscosity, does exert a significant delaying influence on mantle cooling, thereby keeping the Urey ratio low, reducing the growth of the bottom thermal boundary layer, and changing the viscosity profiles over time. A higher temperature at the core–mantle boundary increases the overall time-dependent behavior of the thermal boundary layers. Enhanced radiative conductivity results in faster cooling, opposite to the effect of the phonon conductivity component and a superadiabatic temperature gradient in the deep lower mantle. Finally, the initial value of the core–mantle boundary temperature can be inferred to wield a strong influence on the subsequent mantle thermal evolution in this model with both variable thermal conductivity and viscosity. We may conjecture that other rheological and conductivity complexities, such as grain-size dependence of mantle properties, would also have an impact on the current state of the mantle resulting from the primordial thermal condition.

VLBI observations of free core nutations and viscosity at the top of the core

Very long baseline interferometry (VLBI) nutation measurement series, both in excess of 23 years length, from Goddard Space Flight Center (GSFC) and the United States Naval Observatory (USNO), have been analyzed for free core nutation resonances. VLBI nutation observations can only be made when the radio sources being used are visible, rendering the data sequence inherently non-equispaced. This poses the problem of the approach to be taken with unevenly spaced sampling. Both the conventional Discrete Fourier Transform (DFT) and the Fast Fourier Transform Algorithm (FFT) for its computation strictly require a fixed sampling interval. Our approach is to find the Discrete Fourier Transform of the non-equispaced record by minimizing an objective function which weights the error between the DFT representation and the measured values in inverse proportion to the square of their standard errors. The resulting conditional equations have a coefficient matrix of Toeplitz form but the recursive Levinson algorithm has been found inadequate for their solution, even when implemented in double quad precision. Instead, we employ the Singular Value Decomposition technique to solve the least squares problem of fitting the Discrete Fourier Transform to the non-equispaced VLBI nutation observations. A novel feature of our procedure is to use the Parseval relation to determine the number of singular values of the coefficient matrix to be eliminated. We report the observation for the first time of the prograde mode predicted by Jiang [Jiang, X., 1993. Wobble–nutation modes of the earth, Ph.D. thesis, York University, Toronto, Canada]. The long series of observations allow the determination of the time evolution of the free core nutations. We observe both the prograde and the retrograde modes to be in free decay. In addition to providing measures of the viscosity just below the core–mantle boundary (CMB), the free decays suggest impulsive excitations rather than continuous excitation by electromagnetic core–mantle coupling or the atmosphere. The average recovered viscosity at the top of the core is of the order 615 Pa s in contrast to the value of 8 × 1 0 − 3 Pa s found by Gans [Gans, R., 1972. Viscosity of the Earth’s core, J. Geophys. Res. 77, 360–366] from the extrapolation of laboratory measurements, and commonly used by dynamo theorists.

Subduction and terrane collision stabilize the western Kaapvaal craton tectosphere 2.9 billion years ago

Integrative models of crust and mantle structure, age, and growth of the oldest continental nuclei—the Archean cratons—are critical to understanding the processes that stabilize continental lithosphere. For the Kaapvaal craton of southern Africa, conflicting ages of stabilization have been derived from studies of its crust and underlying mantle. New U–Pb zircon geochronological data from the western Kaapvaal craton reveal that two older (3.7 to 3.1 billion year old) continental masses, the Kimberley and Witwatersrand blocks, were juxtaposed by a significantly younger, previously unresolved episode of subduction and terrane collision between 2.93 and 2.88 billion years ago. Geological evidence indicates that convergence was accommodated by subduction beneath the Kimberley block, culminating in collisional suturing in the vicinity of the present-day Colesberg magnetic lineament. The timing of these convergent margin processes is further shown to correlate with the strong peak in Re–Os age distributions of Kimberley block mantle peridotites, eclogites, and eclogite-hosted diamonds. These data thus support the petrogenetic coupling of continental crust and lithospheric mantle through a model of continental arc magmatism, subduction zone mantle wedge processing and terminal collisional advective thickening to form Archean continental tectosphere.

Subduction and terrane collision stabilize the western Kaapvaal craton tectosphere 2.9 billion years ago

Integrative models of crust and mantle structure, age, and growth of the oldest continental nuclei—the Archean cratons—are critical to understanding the processes that stabilize continental lithosphere. For the Kaapvaal craton of southern Africa, conflicting ages of stabilization have been derived from studies of its crust and underlying mantle. New U–Pb zircon geochronological data from the western Kaapvaal craton reveal that two older (3.7 to 3.1 billion year old) continental masses, the Kimberley and Witwatersrand blocks, were juxtaposed by a significantly younger, previously unresolved episode of subduction and terrane collision between 2.93 and 2.88 billion years ago. Geological evidence indicates that convergence was accommodated by subduction beneath the Kimberley block, culminating in collisional suturing in the vicinity of the present-day Colesberg magnetic lineament. The timing of these convergent margin processes is further shown to correlate with the strong peak in Re–Os age distributions of Kimberley block mantle peridotites, eclogites, and eclogite-hosted diamonds. These data thus support the petrogenetic coupling of continental crust and lithospheric mantle through a model of continental arc magmatism, subduction zone mantle wedge processing and terminal collisional advective thickening to form Archean continental tectosphere.

Syn-convergent exhumation and lateral extrusion in continental collision zones—insights from three-dimensional numerical models

Three-dimensional thermomechanical finite element models are used to simulate the evolution of collisional orogens of finite axial extent. Special focus is on the spatial and temporal variations of stress and strain in the resulting orogen leading to lateral extrusion and syn-convergent exhumation. Besides the collision zone, the model incorporates an area, which is not constrained by a convergent boundary condition and, thus, allows orogenic material to flow out laterally. Altogether, 17 different model scenarios were tested. Starting from a model with ‘standard’ material parameters and boundary conditions, several variations concerning crustal temperature, rheology, convergence rates, crust–mantle coupling as well as the geodynamic setting of the evolving orogen are examined. Modelling results indicate that orogen-parallel extension is intimately related to continental collision and occurs even during the early stages of convergence. Unless a rigid block adjoins and prevents lateral extrusion, it typically reaches 20–40% of the amount of convergence. The models show substantial spatial and temporal variations of stress and strain in the collision zone and its surrounding. In particular, the near-surface stress field in the orogen's interior completely reorganises during convergence. At later convergence stages, the uppermost crust is subject to normal faulting, while contemporaneously at deeper crustal levels compressive stress fields prevail. The spatial relation of these different stress regimes in the orogenic crust strongly favours syn-convergent exhumation by tectonic denudation. Finally, a semiquantitative comparison between the modelling results and three modern collision zones in the Alpine–Himalayan belt addresses the distribution of different tectonic styles, the respective amounts of lateral extrusion as well as seismological observations.

The electrical structure of the Slave craton

The Slave craton in northwestern Canada, a relatively small Archean craton (600×400 km), is ideal as a natural laboratory for investigating the formation and evolution of Mesoarchean and Neoarchean sub-continental lithospheric mantle (SCLM). Excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available. Over the last 5 years deep-probing electromagnetic surveys were conducted on the Slave, using the natural-source magnetotelluric (MT) technique, as part of a variety of programs to study the craton and determine its regional-scale electrical structure. Two of the four types of surveys involved novel MT data acquisition; one through frozen lakes along ice roads during winter, and the second using ocean-bottom MT instrumentation deployed from float planes. The primary initial objective of the MT surveys was to determine the geometry of the topography of the lithosphere–asthenosphere boundary (LAB) across the Slave craton. However, the MT responses revealed, completely serendipitously, a remarkable anomaly in electrical conductivity in the SCLM of the central Slave craton. This Central Slave Mantle Conductor (CSMC) anomaly is modelled as a localized region of low resistivity (10–15 Ω m) beginning at depths of ∼80–120 km and striking NE–SW. Where precisely located, it is spatially coincident with the Eocene-aged kimberlite field in the central part of the craton (the so-called “Corridor of Hope”), and also with a geochemically defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced during early tectonism. The CSMC lies wholly within the NE–SW striking central zone defined by Grütter et al. [Grütter, H.S., Apter, D.B., Kong, J., 1999. Crust–mantle coupling; evidence from mantle-derived xenocrystic garnets. Contributed paper at: The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume, 1, 307–313] on the basis of garnet geochemistry (G10 vs. G9) populations. Deep-probing MT data from the lake bottom instruments infer that the conductor has a total depth-integrated conductivity (conductance) of the order of 2000 Siemens, which, given an internal resistivity of 10–15 Ω m, implies a thickness of 20–30 km. Below the CSMC the electrical resistivity of the lithosphere increases by a factor of 3–5 to values of around 50 Ω m. This change occurs at depths consistent with the graphite–diamond transition, which is taken as consistent with a carbon interpretation for the CSMC. Preliminary three-dimensional MT modelling supports the NE–SW striking geometry for the conductor, and also suggests a NW dip. This geometry is taken as implying that the tectonic processes that emplaced this geophysical–geochemical body are likely related to the subduction of a craton of unknown provenance from the SE (present-day coordinates) during 2630–2620 Ma. It suggests that the lithospheric stacking model of Helmstaedt and Schulze [Helmstaedt, H.H., Schulze, D.J., 1989. Southern African kimberlites and their mantle sample: implications for Archean tectonics and lithosphere evolution. In Ross, J. (Ed.), Kimberlites and Related Rocks, Vol. 1: Their Composition, Occurrence, Origin, and Emplacement. Geological Society of Australia Special Publication, vol. 14, 358–368] is likely correct for the formation of the Slave's current SCLM.

Lithosphere development in the Slave craton: a linked crustal and mantle perspective

The late tectonic evolution of the Slave craton involves extensive magmatism, deformation, and high temperature-low pressure (HT-LP) metamorphism. We argue that the nature of these tectonic events is difficult to reconcile with early, pre-2.7 Ga development and preservation of a thick tectosphere, and suggest that crust–mantle coupling and stabilization occurred only late in the orogenic development of the craton. The extent and repetitiveness of the tectonic reworking documented within the Mesoarchean basement complex of the western Slave, together with the development of large-volume, extensional mafic magmatism at 2.7 Ga within the basement complex argue against preservation of a widespread, thick, cool Mesoarchean tectosphere beneath the western Slave craton prior to Neoarchean tectonism. Broad-scale geological and geophysical features of the Slave craton, including orientation of an early F1 fold belt, distribution of ca. 2.63–2.62 Ga plutonic rocks, and the distribution of geochemical, petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an NE–SW structural grain to the craton. These trends are oblique to the earlier, ca. 2.7 Ga north–south trending boundary between Mesoarchean and Neoarchean crustal domains, and are interpreted to represent a younger structural feature imposed during northwest or southeast-vergent tectonism at ca. 2.64–2.61 Ga. Extensive plutonism, in part mantle-derived, crustal melting and associated HT-LP metamorphism argue for widespread mantle heat input to the crust, a feature most consistent with thin (<100 km) lithosphere at that time. We propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the D1 structural grain, producing the early 2.63–2.62 Ga arc-like plutonic rocks. Subsequent collision (external to the present craton boundaries) possibly accompanied by partial delamination of some of the underthrust lithosphere, produced widespread deformation (D2) and granite plutonism throughout the province at 2.6–2.58 Ga. An implication of this model is that diamond formation in the Slave should be Neoarchean in age.

Core–mantle coupling including a viscoelastic inner core: an application to the axial rotation associated with the Quaternary glacial cycles

The mass redistribution associated with the Quaternary glacial cycles, with a cycle of approximately 90 kiloyears glaciation and 10 kiloyears deglaciation phases, causes differential rotations of the outer and inner cores relative to the mantle. The differential rotations are examined for an earth model composed of incompressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. A quasi-rigid rotation is assumed for both cores, and viscous and electromagnetic torques at the core–mantle boundaries (CMB) and inner core boundaries (ICB) are taken into account. The effect of the inner core viscosity is negligibly small, but the lower mantle viscosity ( η l) influences significantly on the predictions. In a weak frictional coupling case, the differential rotation of both cores amounts to 5×10 −10 rad s −1 (∼0.9° per year) at the end of the deglaciation. Its magnitude gradually increases with increasing lower mantle viscosity, and generally approaches to a constant value at η l∼10 22 Pa s. The predictions are sensitive to the frictional torques at the CMB and ICB. If the strength of the poloidal field at the ICB ( B p) is larger than the half at the CMB, then both cores rotate at a similar rate. The critical poloidal strength ( B pc), of course, depends on an uncertain value of the conductivity at the top of the inner core. In the case of B p< B pc, significant differential rotation between the outer and inner cores (<3×10 −10 rad s −1) is predicted for a plausible range of the torque at the CMB. Moreover, the predictions at the present-day indicate a westward drift for the outer core and eastward drift for the inner core in some cases with B p< B pc and 3×10 21 Pa s ≤η l ≤6×10 21 Pa s. The magnitude of the outer and inner cores is, however, less than 5×10 −11 and 1×10 −10 rad s −1, respectively. The response to rapid potential variations, corresponding to a pulse-like sea-level change such as meltwater pulse observed at 14 kiloyears BP, is dominantly elastic, and takes a maximum value of ∼10 −10 rad s −1 (∼0.2° per year). The acceleration for this response, however, reaches ∼10 −20 rad s −2, significantly larger than the maximum estimate of ∼10 −21 rad s −2 for an average potential variation model. Time series of the predicted differential rotation may be grouped into three distinctive types from the degree of the coupling at the CMB. In a relatively weak coupling case, the predictions are approximately similar to an average sea-level change of the Quaternary glacial cycles and the spectrum evaluated from the time series has a sharp peak at the period of about 100 kiloyears. In a strong coupling case, the dominant signals are due to the pulse-like potential variations, resulting in a white spectrum.

Possible influences of core processes on the Earth's rotation and the gravity field

In this work, we review the processes in the Earth's core that influence the Earth's rotation on the decadal time scale. While core–mantle coupling is likely to be responsible for the decadal length-of-day variations, this hypothesis is controversial with respect to polar-motion variations. The electromagnetic-coupling torques are strongly dependent on the assumed electrical conductivity of the lower mantle, while the topographic torques are influenced by the topography of the core–mantle boundary (CMB). Because no comprehensive theoretical framework for determining the topographic and material parameters of the CMB region is currently available, the modeled results about the coupling torques can only be verified by their consistency with the observed variations of the Earth's rotation and the geomagnetic field. A second path of investigation is to consider the relative angular momentum of the core. Recently, the axial angular-momentum balance has been found to coincide with observed variations in the geomagnetic field and the length of day. However, with respect to polar-motion variations, the angular-momentum balance is not yet closed. We also discuss the role of an irregular motion of the figure axis of the oblate inner core with respect to the outer core and mantle in the excitation of polar motion. In particular, we assume that the associated changes of the Earth's inertia tensor cause the observed decadal variations in polar motion. From this assumption, we can derive the temporal variation of the orientation of the figure axis of the inner core from polar-motion data. Finally, we calculate the gravity variations caused by this relative inner-core motion and compare them with the accuracy of current and planned satellite missions.

Lorentz force effects in magneto-turbulence

We experimentally characterize magnetic field fluctuations in a strongly turbulent flow of liquid sodium in the presence of a large externally applied field. We reach high interaction parameter (up to N=17) for moderate magnetic Reynolds number (up to Re m=18), a previously unexplored parameter range for liquid metal flows. As the interaction parameter (i.e. the ratio of Lorentz to inertial forces) is increased, the system passes through distinct regimes, which we classify. We find that for certain ranges of the applied magnetic field, particularly at high values, the induced magnetic field exhibits large, coherent oscillations. Spatial structure in these induced field oscillations suggests the formation of non-axisymmetric vortices that precess at a fraction of the impeller rotation rate. We also investigate the effect of rough versus smooth boundaries and relate these results to topographic core–mantle coupling in the Earth.

Heat transfer and interfacial temperature of two‐layered convection: Implications for the D″‐mantle coupling

We performed laboratory measurements of the heat flux and the interfacial temperature of two‐layered thermal convection. Experiments show that the scaling law of the heat flux (Nu‐Ra relation) can be classified by the buoyancy number B. The heat flux under B &gt; 1 follows the well‐known Nu‐Ra relation. However, under B &lt; 1, undulations develop at the interface, and heat flux is systematically enhanced. The interfacial temperature is determined only by the ratio of physical properties between the upper and lower layers. Our results suggest that the existence of less viscous and slightly denser D″ than the overlying mantle would enhance the heat transfer from the core to the mantle.

The origin of the Cretaceous gabbros in the Fujian coastal region of SE China: implications for deformation-accompanied magmatism

The Cretaceous gabbros in the Fujian coastal region, southeastern China, experienced different magmatic and tectonic processes associated with deformation of the Changle-Nanao shear zone. Group 1 gabbros (115 Ma) show marked LILE (e.g., Sr, Ba and K) and LREE enrichment, and HFSE (e.g., Ta, Nb, Zr and Hf) depletion, strongly suggesting an island-arc affinity. The parental magma of group 1 gabbros, arguably derived from an arc-related source mantle modified by previous subduction, rose into and interacted with the coexisting syntectonic granitic magma generated during the main stage of shear zone deformation, and produced the gabbro–diorite–granite complexes. Group 2 and 3 gabbros (106–95 Ma) possibly formed by coupling of the subduction-modified source mantle with part of undepleted mantle (e.g., Nb enrichment) in an extensional environment at the end of deformation. Their parental magmas did not undergo significant magma mixing but produced a cumulate nature. All the three groups of gabbros were contaminated by crustal material during the processes of intrusion and emplacement.

Petrogenesis of tholeiite associations in Kudi ophiolite (western Kunlun Mountains, northwestern China): implications for the evolution of back-arc basins

The Kudi ophiolite in the western Kunlun Mountains comprises harzburgites, dunites, cumulate dunites, cumulate pyroxenites and gabbros, diabase dikes, and pillow and massive lavas, and are fragments of a supra-subduction zone (SSZ) ophiolite from the Early Paleozoic. The extrusive rocks can be classified into three groups of tholeiites: back-arc basin (BAB) tholeiites, low-Ti island arc tholeiites (IAT), and LREE-enriched IAT, as shown by their distinctive geochemical characteristics. The SSZ-type mantle peridotites, the cumulate complex with arc tholeiite affinity, and BABB-type diabase dikes and basalts constitute an upper mantle and crustal section of a back-arc basin formed by coupling of MORB-type mantle upwelling with fluid efflux from slab devolatilization. The low-Ti IAT are characterized by low Ti and HFSE, and slightly U-shaped or LREE-depleted chondrite-normalized REE patterns, and represent melts derived from a depleted mantle source region (extraction of BABB magma) modified compositionally by fluids and/or melts from the subducting lithospheric slab during propagation and extension of the back-arc basin. We interpret the LREE-enriched IAT as products of closure of the back-arc basin because an interaction between the parental magma of this IAT and the mantle peridotites (formerly the upper mantle of the basin) in a newly formed mantle wedge had occurred.

Determining the magnetic field in the core-mantle boundary zone by non-harmonic downward continuation

Summary The length of day and the geomagnetic field are clearly correlated over decadal periods (10–100 yr). Provided the electrical conductivity of the lower mantle is sufficiently high, a considerable part of this correlation can be explained by electromagnetic core–mantle coupling. Investigating the associated core–mantle coupling torque and fluid velocity fields near the core surface, as well as the interpretation of the observed time lag between length of day and geomagnetic field variations requires the calculation of the temporally variable magnetic field near both sides of and on the core–mantle boundary by solving the magnetic induction equation. Such a solution presents a downward field continuation that has a non-harmonic character if the electrical conductivity is accounted for. In this paper the Earth's mantle is assumed to be a two-layer spherical shell, whose inner layer is electrically conductive. We only consider the poloidal part of the magnetic field with boundary values that are conventionally given by a spherical harmonic expansion of the observed geomagnetic potential field at the Earth's surface. Thus, we are concerned with a one-side-data supported problem, analogous to the inverse heat conduction problem (or sideways heat equation problem), well-known as an ill-posed inverse boundary value problem for a parabolic partial differential equation (diffusion equation). We develop a several-step solution procedure for this inverse problem in its integral form and use a special regularization method for the final solution. The capabilities of this downward continuation method (which includes varying the mantle conductivity model, quality of data approximation in the regularization and two different depths) are presented and discussed in comparison with the perturbation approach and the usual harmonic downward continuation. The data series used are the single magnetic field components (Gauss coefficients) of the spherical harmonic field expansion (Bloxham & Jackson 1992, J. geophys. Res., 97, 19 537–19 563) beginning from the year 1840 to 1990. In addition, to investigate the spectral effects (changing amplitudes, phase shifting), an artificial data series is used. The main result is the downward continuation of the radial component of a global (8,8) field onto the core–mantle boundary, produced using the different methods and for the two epochs, 1910 and 1960. Comparing the results with the perturbation solution reveals temporally and locally variable differences up to the order of 5000 nT, while the difference to the harmonic downward continuation amounts to 15000 nT.

Mantle structure and rifting processes in the Baikal–Mongolia region: geophysical data and evidence from xenoliths in volcanic rocks

The origin of the Baikal rift zone (BRZ) has been debated between the advocates of passive and active rifting since the 1970s. A re-assessment of the relevant geological and geophysical data from Russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many “active rifting” models. Results of a large number of early and recent studies favour the role of far-field forces in the opening and development of the BRZ. This study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern Siberia and central Mongolia. These xenoliths are direct samples of the upper mantle in the vicinity of the BRZ. Of particular importance are suites of garnet-bearing xenoliths that have been used to construct P– T- composition lithospheric cross-sections in the region for the depth range of 35–80 km. Xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient Siberian platform (sampled by kimberlites) and beneath younger mobile belts south of the platform. The uppermost mantle in southern Siberia and central Mongolia is much hotter at similar levels than the mantle in the Siberian craton and also has significantly higher contents of ‘basaltic’ major elements (Ca, Al, Na) and iron, higher Fe/Si and Fe/Mg. The combination of the moderately high geothermal gradient and the fertile compositions in the off-cratonic mantle appears to be a determining factor controlling differences in sub-Moho seismic velocities relative to the Siberian craton. Chemical and isotopic compositions of the off-cratonic xenoliths indicate small-scale and regional mantle heterogeneities attributed to various partial melting and enrichment events, consistent with long-term evolution in the lithospheric mantle. Age estimates of mantle events based on Os–Sr–Nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long-term crust–mantle coupling. The ratios of 143/144Nd in many LREE-depleted xenoliths are higher than those in MORB or OIB source regions and are not consistent with a recent origin from asthenospheric mantle. Mantle xenoliths nearest to the rift basins (30–50 km south of southern Lake Baikal) show no unequivocal evidence for strong heating, unusual stress and deformation, solid state flow, magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the Moho. Comparisons between xenoliths from older and younger volcanic rocks east of Lake Baikal, together with observations on phase transformations and mineral zoning in individual xenoliths, have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small-scale ascent of deep mantle material. Overall, the petrographic, P– T, chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies, which found no evidence for a large-scale asthenospheric upwarp beneath the rift, and lend support to passive rifting mechanism for the BRZ.

Formation of Archean continental lithospheric roots: The role of mantle plumes

Deep lithospheric roots, commonly extending to the diamond stability field, are distinctive characteristics of Archean cratons, but their origin remains controversial. The 2.7 Ga Abitibi greenstone belt initially developed in an intraoceanic setting, and provides unique insights into the formation of Archean continental lithospheric mantle. Data provided by seismic transects, young U-Pb isotopic ages in the exposed deeper crust of the Kapuskasing uplift, and anomalous Ar-Ar isotopic ages from lode gold deposits point to a late coupling of the diamondiferous Abitibi lithospheric mantle to arc crust. These constraints and a lack of Archean crust-forming events following ca. 2650 Ma indicate that the thick Abitibi cratonic root formed by the diapiric ascent of buoyant residue from which mantle plume– derived komatiitic liquids had been extracted. Although substantial variation may have existed in the development of Archean cratonic roots worldwide, the evidence from the Abitibi belt indicates that all such roots were initiated with the coupling of plume-melt residue to greenstone-belt crust.

Phase relations for high frequency core–mantle coupling and the Earth’s axial angular momentum budget

Although it has long been thought that dynamical core–mantle coupling is necessary for understanding decadal variations in the length of day (LOD), the physics of the coupling mechanism are very poorly constrained. There several hypotheses — electromagnetic, topographic, gravitational, viscous — but it is notoriously difficult to distinguish between them on observational grounds. There is both an expectation [Geophys. J. Int. 138 (1999) 679] and some evidence [J. Geophys. Res. 100 (1995) 8233; Geophys. Res. Lett. 24 (1997a) 1799] that there is significant core–mantle coupling on much faster timescales, down to subannual periods. At such frequencies, the core would no longer be the dominant driving force for LOD, but would instead interact with both the mantle and external reservoirs of angular momentum such as the atmosphere and ocean. On these much faster timescales, it is possible that some information that is obscured on decadal timescales becomes available: in particular, any characteristic timescales at very short periods may become observable. Here, I examine the angular momentum budget of the Earth on subannual to annual timescales for any additional implied constraints on the physics of angular momentum exchange between the core and the mantle. I find that the discrepancy between atmospheric angular momentum and mantle angular momentum on timescales where the ocean is thought to be unimportant has a distinctive signature which is difficult to reproduce using simple models of core–mantle coupling, although an ad hoc “frictional” model does moderately well. Rather than indicating simplicity, this may instead be symptomatic of great complexity in the physics of the core–mantle boundary region.