Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

Mantle process beneath Philippine Sea back-arc spreading ridges: A synthesis of peridotite petrology and tectonics

Abstract In order to obtain a general view of the mantle process beneath a back‐arc basin spreading ridge, the diversity of peridotite petrology and tectonic occurrences in two back‐arc basin spreading ridges from the Philippine Sea were examined: the Parece Vela Rift and the Mariana Trough. The Parece Vela Basin spreading ridge (Parece Vela Rift) was a physically fast/intermediate‐spreading ridge, although many tectono‐magmatic features resemble those of slow‐ to ultraslow‐spreading ridges. Two unusual features of the Parece Vela Rift further demonstrate the uniqueness of the ridge: full‐axial development of oceanic core complexes and exposure of mantle peridotite at segment midpoints. The Parece Vela Rift yields a lithological assemblage of residual but still fertile lherzolite/harzburgite, plagioclase‐bearing harzburgite and dunite; similar assemblages are reported from the equatorial Mid‐Atlantic Ridge at the Romanche Fracture Zone and the ultraslow‐spreading ridges from the Indian and Arctic Oceans. The tectono‐magmatic characteristics of the Parece Vela Rift suggest that diffuse porous melt flow and pervasive melt–mantle interaction were the important mantle processes there. Globally, this ‘porous melt flow‐type’ mantle process is likely to occur beneath a segment midpoint of the ridge having a thick lithosphere, typically an ultraslow‐spreading ridge. In contrast, the Mariana Trough is a typical slow‐spreading ridge, exposing mantle peridotite at segment ends. The Mariana Trough yields a lithological assemblage of residual harzburgite and veined harzburgite, a common assemblage among the global abyssal peridotite suite. The tectono‐magmatic characteristics of the Mariana Trough suggest that channeled melt/fluid flow and limited melt–mantle interaction are the important mantle processes there, because of the colder wall‐rock peridotite in the segment end. This ‘channeled melt flow‐type’ mantle process is likely to occur in the shallow lithospheric mantle at the segment ends of any spreading ridges.     

Mantle flow patterns at the Neyriz Paleo-spreading center, Iran

The mantle peridotites of Neyriz record two successive episodes of plastic deformations; the first one related to the igneous accretion of the lithosphere and the second one developed during the first stage of the emplacement of the peridotites. These two events have been distinguished on the basis of microstructural criteria. The diapiric pattern, particularly relevant to the mantle process beneath spreading ridges, features vertical flow lines and elliptic flow plane trajectories in a pipe and extends along the ridge axis about 5 km. These structures rotate to horizontal and diverge in every direction in a narrow transition zone, a few hundred meters thick, below the Moho discontinuity. Such a diapiric pattern has been recognized in a few places along the Neyriz paleo-ridge. A large amount of magma passed through these mantle diapirs that were probably the main zones feeding the overlying magma chamber. The most common pattern features very regular structures over several kilometers along the strike of the paleo-ridge: the flow plane dips away from the ridge axis, and the flow line is parallel to the spreading direction. This flow pattern is frozen during the gradual accretion of the lithospheric mantle away from the ridge in a steady-state spreading regime. A shear-sense inversion at just below the Moho is commonly observed, pointing to forced asthenospheric flow. The reconstructed orientation of the Neyriz paleo-spreading center is 105°, compatible with the geometry and orientation of harzburgite foliations and lineations and sheeted dikes.     

Experimental Studies of Shear Deformation of Mantle Materials: Towards Structural Geology of the Mantle

—A brief outline is given on experimental studies carried out in the Minnesota Mineral and Rock Physics Laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues, using a simple shear deformation geometry. A simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures, and thus provides a unique opportunity to investigate the "structural geology of the mantle." We have developed a simple shear deformation technique for use at high pressures and temperatures (pressure up to 16 GPa and temperature up to 2000 K) in both gas-medium and solid-medium apparati. This technique has been applied to the following mineral systems (i) olivine aggregates, (ii) olivine basaltic melt, (iii) CaTiO₃ perovskite aggregates. The results have provided important data with which to understand the dynamics of the earth’s mantle, including the geometry of mantle convection, mechanisms of melt distribution and migration beneath mid-ocean ridges, and the mechanisms of shear localization. Limitations of laboratory studies and future directions are also discussed.     

Simple 2-D models for melt extraction at mid-ocean ridges and island arcs

A general set of 2-D equations for the conservation of mass and momentum of a two-phase system of melt in a deformable matrix is used to derive analytic solutions for the corner flow of a constant porosity melt-saturated porous medium. This solution is used to model the melt extraction processes at mid-ocean ridges and island arcs. The models indicate that flow of melt is controlled by pressure gradients induced by the Laplacian of the matrix velocity field and by the dimensionless percolation velocity which measures the relative contributions of buoyancy-driven flow to advection by the matrix. The models can account for many features of ridge and arc volcanism. Matrix corner flow at ridges causes melt to be drawn to the ridge axis enabling the extraction of small melt fractions from a wide melting zone while showing a narrow zone of volcanism at the surface. At subduction zones melts do not percolate vertically but are drawn to the junction of the upper plate and subducting slab by corner flow in the mantle wedge. For subduction zones, if the dimensionless percolation velocity is below a critical value, slab-derived fluids will be carried down by the matrix and cannot interact with the mantle wedge. The geochemistry of island arcs will be controlled by the geometry of melt streamlines. This model is consistent with geophysical and geochemical data from the Aleutian arc.     

岩浆与构造作用对洋壳厚度的影响——以西北印度洋为例

洋中脊及邻区洋盆的洋壳厚度能很好地反映区域岩浆补给特征,对于研究洋中脊内部及周缘岩浆活动和构造演化过程具有很好的指示意义.西北印度洋中脊作为典型的慢速扩张洋中脊,其扩张过程与周缘构造活动具有很强的时空关系.本文利用剩余地幔布格重力异常反演了西北印度洋洋壳厚度,由此分析区域内洋壳厚度分布和岩浆补给特征.研究发现,西北印度洋洋壳平均厚度为7.8 km,受区域构造背景影响厚度变化较大.根据洋壳厚度的统计学分布特征,将区域内洋壳分为三种类型:薄洋壳(小于4.5 km)、正常洋壳(4.5~6.5 km)和厚洋壳(大于6.5 km),根据西北印度洋中脊周缘(~40 Ma内)洋壳厚度变化特征可将洋中脊划分为5段,发现洋中脊洋壳厚度受区域构造活动和地幔温度所控制,其中薄洋壳主要受转换断层影响造成区域洋壳厚度减薄,而厚洋壳主要受地幔温度和地幔柱作用影响,并在S4洋中脊段显示出较强的热点与洋中脊相互作用,同时微陆块的裂解和漂移也可能是导致洋壳厚度差异的原因之一.     

Deformation, static recrystallization, and reactive melt transport in shallow subcontinental mantle xenoliths (Tok Cenozoic volcanic field, SE Siberia)

Partial melting and reactive melt transport may change the composition, microstructures, and physical properties of mantle rocks. Here we explore the relations between deformation and reactive melt transport through detailed microstructural analysis and crystallographic orientation measurements in spinel peridotite xenoliths that sample the shallow lithospheric mantle beneath the southeastern rim of the Siberian craton. These xenoliths have coarse-grained, annealed microstructures and show petrographic and chemical evidence for variable degrees of reaction with silicate melts and fluids, notably Fe-enrichment and crystallization of metasomatic clinopyroxene (cpx). Olivine crystal preferred orientations (CPO) range from strong to weak. [010]-fiber patterns, characterized by a point concentration of [010] normal to the foliation and by dispersion of [100] in the foliation plane with a weak maximum parallel to the lineation, predominate relative to the [100]-fiber patterns usually observed in lithospheric mantle xenoliths and peridotite massifs. Variations in olivine CPO patterns or intensity are not correlated with modal and chemical compositions. This, together with the analysis of microstructures, suggests that reactive melt percolation postdated both deformation and static recrystallization. Preferential crystallization of metasomatic cpx along (010) olivine grain boundaries points to an influence of the preexisting deformation fabrics on melt transport, with higher permeability along the foliation. Similarity between orthopyroxene (opx) and cpx CPO suggests that cpx orientations may be inherited from those of opx during melt-rock reaction. As observed in previous studies, reactive melt transport does not weaken olivine CPO and seismic anisotropy in the upper mantle, except in melt accumulation domains. In contrast, recovery and selective grain growth during static recrystallization may lead to development of [010]-fiber olivine CPO and, if foliations are horizontal, result in apparent isotropy for vertically propagating SKS waves, but strong anisotropy for horizontally propagating surface waves.     

洋中脊构造及地震调查现状

介绍了洋中脊的全球分布和构造特征,对全球主要的、不同扩张速率的洋中脊进行了分类和列表描述;对洋中脊的构造特征,如地形特征、地壳厚度与扩张速率的关系及扩张轴下的岩浆房的特征、洋中脊与地幔柱的相互作用进行了阐述。回顾了海底地震仪在洋中脊构造调查中的应用及取得的主要成果。简要介绍了我国将用海底地震仪开展洋中脊构造调查的技术路线。     

Mantle flow and melting beneath oceanic ridge–ridge–ridge triple junctions

Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This study investigates how triple junction (TJ) geometry may influence mantle dynamics. An earlier study [Georgen, J.E., Lin, J., 2002. Three-dimensional passive flow and temperature structure beneath oceanic ridge-ridge-ridge triple junctions. Earth Planet. Sci. Lett. 204, 115–132.] suggested that the effects of a ridge–ridge–ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity model that focuses on the slowest-diverging ridge of a triple junction with geometry similar to the Rodrigues TJ. This spreading axis may be considered to be analogous to the Southwest Indian Ridge. Within 100 km of the TJ, temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by ~ 40 °C and 1 km, respectively. We also investigate the effects of differential motion of the TJ with respect to the underlying mantle, by imposing bottom model boundary conditions replicating (a) absolute plate motion and (b) a three-dimensional solution for plate-driven and density-driven asthenospheric flow in the African region. Neither of these basal boundary conditions significantly affects the model solutions, suggesting that the system is dominated by the divergence of the surface places. Finally, we explore how varying spreading rate magnitudes affects TJ geodynamics. When ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to the Azores TJ), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~ 150 °C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the TJ. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of TJ geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.     

Petrologic consequences of rift propagation on oceanic spreading ridges

The production of anomalously differentiated lava compositions at several mid-ocean spreading centers can be attributed to magmatic processes associated with propagating rifts. The degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers. Low supply rates and moderate cooling rates allow advanced degrees of closed-system fractionation to occur. High supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma. Propagating rift tips are a special class of ridge-transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge. This balance is affected by the spreading rate, the propagation rate of the rift, the length of the bounding transform and proximity to hotspots. Maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance. Rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion. Geochemical anomalies may persist, though changing in degree and extent, for several million years on ridge segments that stop propagating. The concept of “magnetic telechemistry” is generally supported by our study, but in the vicinity of hotspots, magnetic anomaly amplitude may be controlled more by bathymetric and/or thickened magnetic layer effects than by geochemistry.     

Volcanoes and regional geophysical trends

Active volcanoes occur in at least two fundamentally different tectonic settings. Taphrogenic volcanoes are aligned along the mid-ocean fracture system which is characterized by a broad ridge of rugged relief, «tensional» horizontal stress components perpendicular to the ridge, shallow earthquakes in a zone vertically beneath the ridge crest, thin to normal occanic crust and low to normal seismic velocities in the uppermost mantle, positive free-air gravity anomalies, and high heat flow. Orogenic volcanoes are aligned along the mobile Pacific rim and Indonesian archipelago which are characterized by double or single arcuate ridges with a deep oceanic trench on the convex side, compressional horizontal stress components perpendicular to the arcs, shallow to deep earthquakes in a zone dipping from the trench to beneath the volcanoes and beyond, transitional crustal thicknesses and seismic velocities, parallel belts of negative and positive free-air gravity anomalies from the trench to the volcanic arc, and low heat flow from the trench. The diverse nature of most geophysical lineaments associated with volcanism suggests that magma generation is independent of these phenomena. The remaining correlation of shallow earthquakes with taphrogenic volcanocs and intermediate depth earthquakes with orogenic volcanoes suggests that active fracture systems reaching these depths can tap latent magma sources. Seismic evidence for a low velocity layer beginning 100 km beneath continents and 60 km beneath oceans gives independent support to this hypothesis.     

The ‘small-world’ nature of fracture/conduit networks: Possible implications for disequilibrium transport of magmas beneath mid-ocean ridges

In this contribution we show that natural fracture/conduit networks can be studied by using a new method based on Graph Theory. A number of natural networks at different length scales (from the meter to the millimeter) are analysed and results show that they have typical attributes of ‘small-world’ networks, a special class of networks characterized by high global and local transport efficiency. To our knowledge, this topological feature of natural fracture networks is recognized here for the first time. By starting from results on natural fracture/conduit networks, the possible implications are discussed by focusing on disequilibrium transport of magmas in the upper mantle beneath mid-ocean ridges. Results indicate that the ‘small-world’ topology of natural fracture/conduit networks is an important characteristic to ensure disequilibrium delivery of melts through the upper mantle, thus offering a good explanation of geochemical features of magmas. The remarkable point here is that the modelling of melt migration has been constrained by using real fracture network systems. The results presented in this work may contribute to a better understanding of melt migration in fracture network systems and of the way geochemical features of magmas may be influenced by their transport history.     

Water and partial melting of Earth’s mantle

Water plays a crucial role in the melting of Earth’s mantle. Mantle magmatisms mostly occur at plate boundaries (including subduction zones and mid-ocean ridges) and in some intraplate regions with thermal anomaly. At oceanic subduction zones, water released by the subducted slab may induce melting of the overlying mantle wedge or even the slab itself, giving rise to arc magmatism, or may evolve into a supercritical fluid. The physicochemical conditions for the formation of slab melt and supercritical fluid are still under debate. At mid-ocean ridges and intraplate hot zones, water and CO₂ cause melting of the upwelling mantle to occur at greater depths and in greater extents. Low degree melting of the mantle may occur at boundaries between Earth’s internal spheres, including the lithosphere-asthenosphere boundary (LAB), the upper mantletransition zone boundary, and the transition zone-lower mantle boundary, usually attributed to contrasting water storage capacity across the boundary. The origin for the stimulating effect of water on melting lies in that water as an incompatible component has a strong tendency to be enriched in the melt (i.e., with a mineral-melt partition coefficient much smaller than unity), thereby lowering the Gibbs free energy of the melt. The partitioning of water between melt and mantle minerals such as olivine, pyroxenes and garnet has been investigated extensively, but the effects of hydration on the density and transport properties of silicate melts require further assessments by experimental and computational approaches.     

Electrical Structure in Marine Tectonic Settings

This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations, (2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle. The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab) are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent. Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity patterns in the depth range of 60–200 km.     

Rates of hydrothermal cooling of new oceanic upper crust derived from lithium-geospeedometry

Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (> 450 °C hr^− 1) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column.     

Magma migration beneath an ocean ridge

Basaltic volcanism which forms the oceanic crust at mid-ocean ridges is the result of pressure release melting associated with ascending mantle convection. We present a model that gives the distribution of melting beneath the ridge and the subsequent migration of magma through the asthenosphere. In order to produce the degree of partial melting associated with the basaltic rocks making up the ocean crust, melting must extend to a depth of at least 70 km. Small degrees of partial melting are expected to result in an interconnected permeability along grain intersections. Due to the differential buoyancy of the magma relative to the residual solid the magma will be rapidly driven upwards. Solid-state creep allows the solid matrix to collapse as the magma migrates upwards and the lithostatic pressure in the matrix is nearly equal to the fluid pressure in the magma. The percentage partial melt present is only slightly greater than that necessary for the development of interconnected permeability and is much less than the degree of partial melting. The first partial melt fraction produced at the greatest depths migrates upwards and mixes with the later partial melt fractions produced at shallower depths. The uniformity of this mixing will have a profound effect on the chemistry of the basalts of the oceanic crust.     

Control on seafloor spreading geometries by stress- and strain-induced lithospheric weakening

Seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults. Few regions exhibit different patterns, such as the East Pacific Rise (EPR), which additionally displays overlapping spreading centres (OSCs) and microplates. We introduce a dynamical model using two independent, scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes, suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins. The elastic modulus that is affected by the damage determines the type of localized deformation. Damage reducing the bulk modulus tends to result in tensile fractures, while a reduction in shear modulus leads to shear fractures. The damage source determines the fracture orientation. Material weakening in tension results in fractures perpendicular to the most tensile principal stress, while shear weakening results in two conjugate fractures at 45° relative to the applied stress. Strain or energy-dependent damage results in propagating, localized fractures. Stress-dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones. Starting from small perturbations with reduced elastic moduli as nucleation points, all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli. Orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress. The orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones. OSCs form when the shear damage leading to transform faults is suppressed, while microplate formation requires additional reduction of the shear modulus by tensile energy. Oblique spreading at 45°, recently discovered along ultraslow spreading ridges, occurs when both moduli are weakened by shear energy. A parameter regime exists in which ridge-transform patterns develop at low applied tension, while microplates form at higher stresses. These results indicate that at least three different micromechanical processes operate with different evolution rates. OSCs and oblique spreading require different material properties.     

Subduction influence on magma supply at the East Scotia Ridge

Despite a spreading rate of 65–70 km Ma⁻¹, the East Scotia Ridge has, along most of its length, a form typically associated with slower rates of sea floor spreading. This may be a consequence of cooler than normal mantle upwelling, which could be a feature of back-arc spreading. At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at or near the boundary between the South American and Scotia Sea plates and propagating southwards, supplanting older segments. The most prominent of these, north of 56°30′S, has been propagating at a rate of approximately 60 km Ma⁻¹ for at least 1 Ma, and displays a morphology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations indicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographic and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea show little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American plate is tearing to the east, perhaps allowing mantle flow around the end of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed. A southward mantle flow may have existed beneath the axis of the East Scotia Ridge throughout its history.     

Heat and mass transfer in a thermochemical plume under an oceanic plate far from the mid-ocean ridge axis

Heat and mass transfer processes in the conduit of a thermochemical plume located beneath an oceanic plate far from a mid-ocean ridge (MOR) proceed under conditions of horizontal convective flows penetrating the plume conduit. In the region of a mantle flow approaching the plume conduit (in the frontal part of the conduit), the mantle material heats and melts. The melt moves through the plume conduit at the average velocity of flow v and is crystallized on the opposite side of the conduit. The heat and the chemical dope transferred by the conduit to the mantle flow are carried away by crystallized mantle material at the velocity v. The main equations of heat and mass transfer are obtained for a thermochemical plume interacting with a horizontal convective mantle flow. The joint multiparameter problem of heat and mass transfer is solved for a thermochemical plume located far from an MOR axis. The dope concentration at the base of the plume is found as a function of the Lewis number. The Lewis numbers and, accordingly, the diffusion coefficients of the chemical dope in the plume conduit far from the MOR axis are determined.