Layer Of Partial Melt Research Articles

Granitoids indicating moderate to high heat production associated with oceanic subduction system in the NE Tibetan Plateau

Regional heat flow provides a direct surface indication of the thermal state and energy balance of the lithosphere. Heat flux in the Tibetan Plateau remains poorly studied due to the lack of adequate information on heat flow. In this study, we investigated granitoids from the Gonghe-Guide district located in the NE Tibetan Plateau, which have important significance as hot and dry rocks with potential for geothermal resources. Samples from granitoid rocks as well as felsic dikes were collected from this area, and their geochemical data were combined with regional geophysical and drilling core data to evaluate a high-heat-flow anomaly. Our data show that granitoid rocks, including granodiorite, monzogranite, and syenogranite, crystallized at ca. 253−240 Ma, while felsic dikes formed ca. 230 Ma. The former were emplaced during the oceanic subduction process, whereas the latter formed in the syncollisional stage associated with the closure of the paleo-Zongwulong Ocean. Geochemically, these granitoid rocks are metaluminous to slightly peraluminous, with an average differentiation index (DI) value of 80, and they are classified as weakly to moderately fractionated I-type granites. The felsic dikes are peraluminous with high DI values (average of 95), typical of highly fractionated I-type granites. In terms of their high K and especially U and Th abundances, the calculated radioactive heat generation produced by the granodiorite, monzogranite, syenogranite, and felsic dikes is 1.85, 2.91, 6.85, and 3.02 μW/m3, respectively. Their moderate to high heat production generates a local heat-flow anomaly of 14.8−22.2 mW/m2 above the regional value of subduction zones, accounting for 11%−18% of the total regional heat flow. In combination with regional magnetotelluric data, the high-heat-flow anomaly may be attributed to an additional heat-flow contribution from a partial melt layer beneath this region. Furthermore, there is a significant positive correlation between the radioactive heat generation and magmatic fractionation, indicating that the enrichments of heat-producing elements are controlled by fractional crystallization and subsequently source composition. The granitoid rocks with high heat generation correspond to the large-scale intermediate-felsic magmatism during the subduction stage. We propose a model wherein a continuous subduction process during the closure of the paleo-Zongwulong Ocean and protracted cooling and crystallization processes resulted in the enrichment in heat-producing elements.

3-D Inversion of Gravity Data of the Central and Eastern Gonghe Basin for Geothermal Exploration

The Gonghe Basin is one of the most important regions for the exploration and development of hot dry rock geothermal resources in China. However, there is still some controversy about the main heat source of hot dry rock geothermal resources in the Gonghe Basin. Combined with previous research results including three-dimensional magnetotelluric imaging and linear inversion of Rayleigh wave group and phase velocity result, we obtained a high-resolution underground spatial density distribution model of the Gonghe Basin based on satellite gravity data by using 3-D gravity focusing inversion method. According to the results, there are widely distributed low density anomalies relative to surrounding rock in the middle crust of the study area. The low-density layer is speculated to be a low-velocity, high-conductivity partial melting layer in the crust of the Gonghe Basin. The inversion result confirms for the first time the existence of a partial melt layer from the gravity point of view, and this high temperature melt layer may be the main heat source of the hot dry rock geothermal resources in the Gonghe Basin. It can provide a new basis for further research on the genesis of the hot dry rock geothermal system in the Gonghe Basin.

A Subsurface Magma Ocean on Io: Exploring the Steady State of Partially Molten Planetary Bodies

Intense tidal heating within Io produces active volcanism on the surface, and its internal structure has long been a subject of debate. A recent reanalysis of the Galileo magnetometer data suggested the presence of a high-melt-fraction layer with >50 km thickness in the subsurface region of Io. Whether this layer is a “magmatic sponge” with interconnected solid or a rheologically liquid “magma ocean” would alter the distribution of tidal heating and would also influence the interpretation of various observations. To this end, we explore the steady state of a magmatic sponge and estimate the amount of internal heating necessary to sustain such a layer with a high degree of melting. Our results show that the rate of tidal dissipation within Io is insufficient to sustain a partial-melt layer of ϕ > 0.2 for a wide range of parameters, suggesting that such a layer would swiftly separate into two phases. Unless melt and/or solid viscosities are at the higher end of the estimated range, a magmatic sponge would be unstable, and thus a high-melt-fraction layer suggested in Khurana et al. is likely to be a subsurface magma ocean.

Experimental Evidence Supporting an Overturned Iron‐Titanium‐Rich Melt Layer in the Deep Lunar Interior

AbstractDense Fe‐Ti‐rich cumulates, formed as the last dregs of the lunar magma ocean, are thought to have driven a large‐scale overturn of the lunar mantle over 4 Ga ago. Analysis of lunar seismic data has implied that some of the overturned bodies may have reached the lunar core‐mantle boundary and remained there until the present day as a partially molten layer. However, whether such a molten layer could be stable during >4 Ga of post‐magma‐ocean lunar history and explain lunar seismic observations remains poorly constrained. Here, we report the first sound velocity measurements on a Fe‐Ti‐rich lunar melt up to conditions of the lowermost lunar mantle. Our results suggest that a partial melt layer with at least 20% overturned Fe‐Ti‐rich melt can be trapped atop the lunar core‐mantle boundary until the present day, strongly influencing the thermochemical evolution of the lunar interior.

Magnetotelluric Constraints on the Temperature, Composition, Partial Melt Content, and Viscosity of the Upper Mantle Beneath Svalbard

AbstractLong‐period magnetotelluric (MT) data can be used to interpret upper mantle temperature, hydrogen content, and the presence of partial melt, all of which strongly influence mantle viscosity. We have collected the first long‐period MT data in Svalbard and have combined them with preexisting broadband MT data to produce a model of the electrical resistivity of Svalbard's upper mantle. Asthenospheric resistivities are low compared to stable continental settings but more comparable to young oceanic asthenosphere, suggesting that the physical state of Svalbard's upper mantle is controlled by its proximity to the Mid‐Atlantic Ridge. Interpretation of the MT model using a petrologically constrained genetic algorithm approach shows that partial melt is present in the uppermost asthenosphere beneath Svalbard. This is the first direct evidence of partial melt in Svalbard's asthenosphere from deep geophysical soundings. Viscosities calculated from the geophysical data show a low viscosity layer (~1018 Pa s) coincident with the partial melt layer, underlain by a higher viscosity layer (~1020 Pa s) extending to the transition zone. Viscosities calculated from glacial isostatic adjustment (GIA) data in Svalbard show a considerable range due mainly to uncertainties in past ice sheet models. Improved constraints on Svalbard's mantle viscosity from geophysical data may help to improve these GIA models.

Correlation of a 410-km Discontinuity Low Velocity Layer with Velocity Tomograms Beneath the Colorado Plateau Using the RISTRA Array

The transition zone water filter model (Nature 425(6953), 39–44; 2003) predicts that a hydrous partial melt layer is only actively produced in a region of upwelling mantle. We test the transition zone water filter model via stacking of P-to-S converted receiver functions by using the IRIS-PASSCAL RISTRA (Colorado Plateau/Rio Grande Rift Seismic Transect Experiment) array. Assuming the high velocity regions found at the northwest and southeast ends of the array at 350–440 km by teleseismic velocity tomograms e.g. Schmandt and Humphreys (Earth and Planetary Science Letters 297(3–4): 435–445; 2010) are cold and sinking vertically, the 410-km low velocity layer should be absent in these regions. The receiver function stacking profiles find the mean depths of the two primary discontinuities at 417 ± 7.1 km for the 410-km discontinuity and 667 ± 8.2 km for the 660-km discontinuity. The average arrival amplitudes with respect to Z component are 3.0% for the 410-km discontinuity, 2.8% for the 660-km discontinuity, and − 1.8% for the 410-km low velocity layer. The stacked Pds image show the 410-km low cabsent at ~ 350 to 390 km in the high velocity regions, but present in low velocity region. A correlation plot of sum of the 410-km low velocity arrival amplitudes and P-wave perturbation finds a positive linear relationship. Therefore, our findings provide seismic evidence for the transition zone water filter model at a small scale.

Electrical conductivity of the plagioclase–NaCl–water system and its implication for the high conductivity anomalies in the mid-lower crust of Tibet Plateau

In order to investigate the origin of the high conductivity anomalies geophysically observed in the mid-lower crust of Tibet Plateau, the electrical conductivity of plagioclase–NaCl–water system was measured at 1.2 GPa and 400–900 K. The relationship between electrical conductivity and temperature follows the Arrhenius law. The bulk conductivity increases with the fluid fraction and salinity, but is almost independent of temperature (activation enthalpy less than 0.1 eV). The conductivity of plagioclase–NaCl–water system is much lower than that of albite–NaCl–water system with similar fluid fraction and salinity, indicating a strong effect of the major mineral phase on the bulk conductivity of the brine-bearing system. The high conductivity anomalies of 10−1 and 100 S/m observed in the mid-lower crust of Tibet Plateau can be explained by the aqueous fluid with a volume fraction of 1 and 9%, respectively, if the fluid salinity is 25%. The anomaly value of 10−1 S/m can be explained by the aqueous fluid with a volume fraction of 6% if the salinity is 10%. In case of Southern Tibet where the heat flow is high, the model of a thin layer of brine-bearing aqueous fluid with a high salinity overlying a thick layer of partial melt is most likely to prevail.

Magmas trapped at the continental lithosphere–asthenosphere boundary

The lithosphere–asthenosphere boundary (LAB) beneath the continents is a key interface in plate tectonics, yet its nature remains elusive. A partial melt layer has been advocated to explain its geophysical characteristics. However, the main counter-argument is that such a layer cannot be stable as melts should rise through the lithosphere. Density measurements of volatile-containing alkali basalts taken as a proxy for LAB melts show that they are neutrally buoyant at the pressure (P)–temperature (T) conditions of the LAB under continents. Complementary X-ray diffraction and Raman data provide structural insights on melt compaction mechanisms. Basalts generated below the lithosphere may thus be gravitationally trapped and accumulate over time. Their presence provides answers to key questions on continental lithosphere geodynamics, and in particular on cratonic keels stability. This buoyancy trap would cease to exist at higher mantle T such as those relevant of the Archean, and as such, could be linked to the onset of plate tectonics.

The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary

The lithosphere-asthenosphere boundary (LAB) beneath ocean basins separates the upper thermal boundary layer of rigid, conductively cooling plates from the underlying ductile, convecting mantle. The origin of a seismic discontinuity associated with this interface, known as the Gutenberg discontinuity (G), remains enigmatic. High-frequency SS precursors sampling below the Pacific plate intermittently detect the G as a sharp, negative velocity contrast at 40- to 75-kilometer depth. These observations lie near the depth of the LAB in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base. I propose that the G reflectivity is regionally enhanced by dynamical processes that produce melt, including hot mantle upwellings, small-scale convection, and fluid release during subduction.

Processing the Moon's Core

![Figure][1] CREDIT: WEBER ET AL. The Apollo Passive Seismic Experiment measured lunar ground motion until the mid-1970s. Since then, various efforts have attempted to extract all possible information about the interior of the Moon from this unique data set. Recent computational and methodological advances suggest that reprocessing the aging seismic data may provide previously inaccessible information buried within. To that end, Weber et al. (p. [309][2], published online 6 January) reanalyzed the Apollo data using a method typically reserved for processing seismic array data on Earth. The data indicate that the lunar core, like Earth's, consists of a solid inner core and molten outer core. Unlike Earth, however, the Moon also has a thick partial melt layer resting above the outer core. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1199375

New geophysical insight into the origin of the Denali volcanic gap

SUMMARY Volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins. They are regions where the necessary conditions to produce melt may appear favourable, but where volcanoes are surprisingly absent from the surface. In this study, we present a new model that can explain the occurrence of such volcanic gaps. It is based on seismic imaging and geodynamic modelling of the Denali volcanic gap, a ∼400-km-wide region at the eastern end of the Alaska-Aleutian subduction zone. Here, the thick crust of the Pacific Plate and Yakutat terrane subduct at shallow angle beneath North America. A high-resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth, and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge. We interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate. In steadystate subduction models, melt accumulates at the apex of a vaulted mantle wedge, the ‘pinch zone’, from where it may break through the overlying lithosphere to the surface. Beneath the Denali volcanic gap, the pinch zone is absent (or greatly reduced) because shallow subduction of the Yakutat terrane progressively cools the system, and causes the slab to advance and replace the hot core of the mantle wedge. This regime can be seen as the opposite of subduction rollback. It prevents the formation of a pinch zone, reduces the length of the melting column and causes melt to pool at the base of the overriding plate, thus inhibiting magma generation and extraction.

Melting in the deep upper mantle oceanward of the Honshu slab

We examine the upper mantle and transition zone beneath the western Pacific using multiple ScS reverberations. A low-velocity zone (LVZ) is found above the 410-km discontinuity oceanward of the subducting Honshu slab at an average depth of 356 km, with a thickness that ranges from 50 to 75 km assuming the LVZ continues to the 410-km discontinuity, which is locally elevated. The low-velocity region is evident in previous tomographic studies, and our results suggest that the anomaly is best explained by a layer of partial melt. The layer may be entrained from above by subduction or produced in situ by the combined effects of water and temperature. A self-consistent model that explains local P-wave velocities (Obayashi, M., Sugioka, H., Yoshimitsu, J., Fukao, Y., 2006. High temperature anomalies oceanward of subducting slabs at the 410-km discontinuity. Earth and Planetary Science Letters 243, 149–158) and our observations calls for a maximum temperature anomaly of ∼150 °C and a resulting maximum olivine water content after melting of 0.100 wt. %.

Deep upper-mantle melting beneath the Tasman and Coral Seas detected with multiple ScS reverberations

Multiple ScS reverberations are used to search for mantle reflectors beneath the Tasman and Coral Seas with a hierarchical waveform-inversion/migration method. In addition to the major transition zone discontinuities, a low-velocity layer above the 410-km discontinuity is detected. The top of the low-velocity layer lies at an average depth of 352 km, indicating that the layer could be more than 70-km thick if it persists to the 410-km discontinuity, which occurs at an average depth of 420 km along paths containing the low-velocity layer. We attribute the low velocities to partial melt resulting from volatile-induced melting. The considerable thickness of the partial melt layer may require thin films of a hydrous melt with a zero-degree dihedral angle surrounding grains or the combined effect on melting of the addition of both water and carbon to the deep upper mantle via subduction. Although the depths of the transition zone discontinuities do not indicate that the transition zone itself is rich in water, the impedance contrasts do contain a subtle signature that could be related to transition zone water, namely a decrease in the impedance contrast across the 410-km discontinuity and a relatively strong 520-km discontinuity.

Fluid dynamics of local martian magma oceans

The evolution of a melt region produced by a large impact during Mars formation is addressed. While some impact induced melt is redistributed during crater excavation, sufficiently large impacts (much larger than basin forming impacts) generate an intact melt region which is retained beneath the excavation zone, i.e., a local magma ocean. Local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet, mechanism of crystallization, and melt region size. Within the uncertainties of various parameters, two scenarios are possible. For sufficiently weak rheology or large melt region size, evolution is characterized by rapid extrusion and formation of a global magma ocean. For sufficiently strong rheology or small melt region size, in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment. Subsequent to in situ crystallization, local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling. For large melt regions, lateral spreading occurs via plastic deformation and results in an asymmetric, global, partial melt layer. For small melt region size, viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer. A hypothesis for the origin of the hemispherical crustal dichotomy and Tharsis rise is suggested. The dichotomy is associated with a global partial melt layer produced by evolution of a large, local magma ocean. After dichotomy formation, evolution of a second, smaller, local magma ocean is related to Tharsis development.

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States.

The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.

Shape and intrusion style of the Coastal Batholith, Peru

The Coastal Batholith of Peru extends over 1600 km parallel to the coast along the Andean trend. Gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2.0–3.2 km, and a thick root 4–10 km wide to the west. Granitic material does not extend to depths greater than 3 km below sea level datum.This study supports recent gravity work which indicates plutons are commonly thin, 5 km or less in thickness. Detailed mapping in the Lima segment of the Coastal Batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression. However, early roof uplift also created some space. Stoping occurs but is not a major space maker. Floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites, tuffisites, microbreccia, faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the Coastal Batholith some space was probably made by a cantilever mechanism. In brief, space making processes involved early roof uplift and regional doming, then floor depression mainly by piston and probably subsidiary cantilever mechanisms and, finally, local stoping producing the cut-out rectilinear nature of the batholith.The Coastal Batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km. Floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt. This is an efficient space making process and is limited here to shallow levels of the upper crust only. The melts ascended to within 2 or 3 km of the surface, up dyke-like conduits then spread horizontally to form tabular plutons.

The Longevity of Lunar Volcanism: Implications of Thermal Evolution Calculations with 2D and 3D Mantle Convection Models

Lunar volcanism, as is generally accepted, started well before the emplacement of the mare fill at 3.1–3.9 Ga b.p. and likely extended well into the Eratosthenian (3.1–1.1 Ga b.p.). The early volcanism is relatively easily understood. The extension of the activity to, possibly, 1.5 Ga b.p., albeit at a decreasing rate, poses a problem since a relatively small body such as the Moon may be expected to cool rapidly and freeze a partially molten mantle layer quickly. We present thermal history models in which a region of partial melt forms in the mantle underneath the lithosphere almost immediately after the start of the model at depths between 300 and 700 km depending on the chosen initial depth of the magma ocean. To calculate the thermal histories we use axisymmetric 2D and fully 3D spherical shell convection codes with viscosity depending on the azimuthally averaged temperature. The thermal evolution of the Moon is found to be characterized by the growth of a massive 700- to 800-km-thick lithosphere while the lower mantle and core cool only by 100–200 K. The partial melt zone decreases in thickness with time from top to bottom until it vanishes at times between 3.4 and 2.2 Ga b.p. The maximum degree of partial melting is between roughly 10 and 20%. The partial melt layer is initially global and is disrupted by cold downwellings as cooling proceeds. The melt zone freezes from above due to the thickening of the lithosphere, which implies that the source region of volcanic rock proceeds to increasing depth with time. This corresponds nicely with the variation of titanium with age of lunar mare basalts. Mixing of the melt zone with the underlying mantle chemically rejuvenates the melt zone to some degree and, thus, the source region of the mare basalts.

Double-diffusive convection in partially molten silicate systems: Its role during magma production and in magma chambers

In this paper, two problems involving double-diffusive convection in partially molten, silicate systems are investigated mathematically. The first problem concerns the role of double-diffusive convection during magma production. The systems Di-An and Di-An-Ab are used as examples. The calculations show that, in a growing layer of partial melt, heated from below, the thickness of the layer at which convection initiates depends strongly upon the bulk composition. For example, assuming reasonable values of the physical parameters, in a Di-rich system, convection will occur when the layer is ∼ 800 m thick, whereas in an An-rich system convection, driven by the destabilizing compositional gradient, will begin in a layer only centimeters thick. A qualitative investigation of the Di-An-Ab system suggests similar results, depending upon whether the bulk composition lies in the diopside or the plagioclase field, respectively. Moreover, if one considers a two-component, one-dimensional diapir of Di-An, an examination of the phase diagram indicates that vigorous compositional convection will occur in the partially molten diapir as it ascends, regardless of whether the bulk composition is Di-rich or An-rich. These results suggest that during magma production, convective processes will tend to homogenize the melt before it segregates from the source zone; however, the vigor of mixing is dependent upon the bulk composition of the source, among several other factors. Models of melt segregation should be modified to include double-diffusive processes. The second problem concerns the structure of a porous boundary layer that forms as a result of side-wall crystallization in a convecting magma chamber. An examination of the steady-state boundary layer equations for flow through a porous medium shows that the boundary layer structure may be of two types. If the residual melt fraction, upon crystallization at the wall, has negative compositional buoyancy, or if the negative thermal buoyancy at the cold wall exceeds the positive compositional buoyancy of the residual melt, the flow across the entire boundary layer will be downward. If the residual melt fraction has negative compositional buoyancy, the magma chamber will become stratified as a result of accumulation of a layer of dense, cold liquid on the floor; whereas if the melt fraction has positive compositional buoyancy, the boundary layer fluid will tend to be remixed into the interior of the magma chamber. If, however, the positive compositional buoyancy exceeds the negative thermal buoyancy, counterflowing boundary layers will occur; and the compositionally buoyant liquid will tend to be fractionated towards the top of the magma chamber. An examination of the Di-An and Di-An-Ab systems suggests that both types of boundary layer structures may occur in magma chambers. An approximate calculation of the rate of fractionation suggests that, for a range of parameters that may be representative of basaltic magmas, the flow in the compositional boundary layer may transport ∼ 10 −3 km 3/yr of low-density liquid to the top of the magma chamber.