Why does mantle melt

However, when it comes to rocks, we run into a problem. The Earth actually isn't really hot enough to melt mantle rocks, which are the source of basalt at the mid-ocean ridges, hotspots and subduction zones.

So, if it is cooler as you head up, why does this peridotite melt to form basalt? Well, that is where you need to stop thinking about how to heat the rock to melting but rather how to change the rock's melting point solidus. Think about our ice analogy. During the winter, there are a lot of times where you'd like to get rid of that. So, what do you do? One solution is to get that ice to melt at a lower temperature by disrupting the bonding between the H2O molecules -- thus, halting the formation of rigid ice.

For a rock, water behaves as its salt. This requires decreasing lithospheric thickness, as in the case of mid oceanic ridges at divergent margins. Notice how as the lithosphere thins, the geotherm changes in response.

If the lithosphere thins sufficiently, the geotherm can cross the solidus and partial melting occurs. The field here on the diagram represents melting that occurs in this melt triangle. This lava, erupting in the Lau Basin, was formed by decreasing pressure on the mantle. Here is the global distribution of Divergent Margins. The third way to melt the mantle is by adding water, as in the case of a subduction zone at a convergent margin. Consider a simple cross section of a subduction zone.

The plate on the left is subducting beneath the plate on the right. The subducted plate releases water as it gets deeper in the earth. Water interacts with the peridotite in the mantle, changing its melting point. This is represented on the diagram by changing the dry peridotite solidus to a wet peridotite solidus.

Wet peridotite melts at much lower temperatures than dry peridotite, allowing a normal geotherm to intersect it. This causes melting in the mantle wedge above the subducted plates. Mt st helens is an example of a volcano fed by magma generated in a subduction zone. Here is the global distribution of convergent margins. The black boxes highlight the locations of olivine grains, and the dark pits in the olivines are actual measurements for the water content of the olivine.

The peridotite is the super fine-grained matrix. Image is courtesy of Emily Sarafian. Skip to main content. Figure 6a shows how residual modes scaled to a total mass of 1 — F produced by polybaric near-fractional melting vary with the extent of melting. The opx — cpx melting relationship Fig. Figure 6 and equation 1 also suggest that if the modes of a suite of residual peridotites are known, the initial melting pressure P o and mean melting pressure P M may be evaluated by the following empirical equations: 0.

Although a rigorous test requires accurate knowledge of the actual melting pressures, an estimate can be made by comparing abyssal peridotites with the model melting residues Figs 1 and 2. As the peridotites define trends on SiO 2 —MgO and FeO—MgO diagrams that converge toward the fertile model source composition Figs 2 and 4 , the task is to rotate the data array relative to the source composition on the two diagrams to match model melting residues.

The explicit assumptions are 1 polybaric near-fractional melting, and 2 initial melting pressures P o lying in the range of 20—30 kbar. Having removed the excess olivine that exists in abyssal peridotites, we can now examine the mineralogical systematics preserved in these residues. Changes in Na 2 O and FeO in the melt cannot occur alone, but must be accompanied by changes of other components when a phase crystallizes or melts.

Google Scholar Crossref. Search ADS. The effect of pressure-induced solid—solid phase transitions on decompression melting of the mantle.

Determining the composition of high-pressure mantle melts using diamond aggregates. Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations. Simple magma supply geometry inferred beneath a segment of the Mid-Atlantic Ridge. Chemistry of seamounts near the East-Pacific Rise: implications for the geometry of sub-axial mantle flow.

Uranium—thorium disequilibria and partitioning on melting of garnet peridotite. Implications of melting for stabilization of the lithosphere and heat loss in the Archaean. Thermal effects of seafloor spreading and the nature of the oceanic crust. Variation with spreading rate of oceanic crustal thickness and geochemistry. High- and low-temperature garnet peridotite xenoliths and their possible relation to the lithosphere—asthenosphere boundary beneath southern Africa.

Effects of pressure on the melting and polymorphism of enstatite, MgSiO 3. Plastic deformation and magmatic impregnation in serpentinized ultramafic rocks from the Garrett transform fault East Pacific Rise. Constraints on the melt production rate beneath the mid-ocean ridges based on passive flow models. Textural studies of garnet lherzolites: evidence of exsolution origin from high-temperature harzburgites.

Partial melting in the Josephine Peridotite; I, The effect on mineral composition and its consequence for geobarometry and geothermometry. Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. Mineralogic studies of the residues of mantle melting: abyssal and alpine-type peridotites. Late-stage melt evolution and transport in the shallow mantle beneath the East Pacific Rise.

Chemical trends in abyssal peridotites: refertilization of depleted oceanic mantle. Anhydrous partial melting of peridotite from 8 to 35 kb and the petrogenesis of MORB.

Anhydrous partial melting of a fertile and depleted peridotite from 2 to 30 kb and application to basalt petrogenesis. Geophysical constraints on mantle flow and melt generation beneath mid-ocean ridges. American Geophysical Union Monograph. Crustal thickness and the average depth and degree of melting in fractional melting models of passive flow beneath mid-ocean ridges.

Petrology of mantle derived ultramafics from the Owen Fracture zone, Northwest Indian Ocean: implications for the nature of the oceanic upper mantle.

Partial melting of dry peridotites at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamonds.

Experimental constraints on the dynamics of the partially molten upper mantle: deformation in diffusion creep regime. Dynamics of mantle flow and melting at a ridge-centered hotspot: Iceland and the Mid-Atlantic Ridge. Proceeding of 10th Lunar Planetary Science Conference.

Geochimica et Cosmochimica Acta Supplement. Anhydrous melting of peridotite at 0—15 kb pressure and the genesis of tholeiitic basalts. Open system melting and the temporal and spatial variation of peridotite and basalt compositions at the Atlantis II F. Melting in the oceanic upper mantle: an ion microprobe study of diopside in abyssal peridotites.

Focused melt flow and localized deformation in the upper mantle; juxtaposition of replacive dunite and ductile shear zones in the Josephine Peridotite, SW Oregon. Reaction between ultramafic rock and fractionating basaltic magma, II. Experiments on flow focusing in soluble porous media with applications to melt extraction from the mantle. Extraction of mid-ocean-ridge basalt from the upwelling mantle by focused flow of melt in dunite channels.

Corrections and further discussion of the primary magmas of mid-ocean ridge basalts, 1 and 2. Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness. Local versus global variation in ocean ridge basaltic composition: A reply. Petrological systematics of mid-ocean ridge basalts: constraints on melt generation beneath ocean ridges. Experimental partial melting of garnet lherzolite at 4 GPa: constraints on initial melting in plume environments. Compositional controls on the partitioning of U, Th, Ba, Pb, Sr, and Zr between clinopyroxene and haplobasaltic melts: implications for uranium series disequilibria in basalts.

Geochemical aspects of permeability controlled partial melting and fractional crystallization. The volume and composition of melt generated by extension of the lithosphere. Peridotite composition from the North Atlantic: regional and tectonic variations and implications for partial melting. Trace-element partitioning between garnet peridotite minerals and water-rich vapor: experimental data from 5 to 30 Kbar.

Partial melting of a lithologically heterogeneous mantle: inferences from crystallisation histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone. A melt extraction model based on structural studies in mantle peridotites.

An empirical method for calculating melt compositions produced beneath mid-ocean ridges: application for axis and off-axis seamounts melting. In-situ densities of silicate melts and minerals as a function of temperature, pressure, and composition.

Trace element evidence from seamounts for recycled oceanic crust in the eastern equatorial Pacific mantle. Spreading rate dependence of the extent of mantle melting beneath ocean ridges. Basaltic liquids and harzburgitic residues in the Garrett transform: a case study at fast-spreading ridges.

The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. The spreading rate dependence of three-dimensional spreading center structure. Phipps Morgan. Three-dimensional flow and temperature perturbations due to a transform offset: effect on oceanic crustal and upper mantle structure. Ultramafic and mafic dredge samples from the equatorial Mid-Atlantic Ridge and fracture zones.

Disequilibrium partial melting model and its implications for trace element fractionations during mantle melting. The generation of mid-ocean ridge basalts from the Hf and Nd isotope perspective. The hafnium paradox and the role of garnet in the source of mid-ocean ridge basalts. Fluxes and excess temperatures of mantle plumes inferred from their interaction with migrating mid-ocean ridges.

A self-consistent model of melting, magma migration and buoyancy-driven circulation beneath mid-ocean ridges. Geochemical constraints on initial and final depth of melting beneath mid-ocean ridges. Simple 2-D models for melt extraction at mid-ocean ridges and island arcs.

Spontaneous small-scale melt segregation in partial melts undergoing deformation. A phase diagram for mid-ocean ridge basalts: preliminary results and implications for petrogenesis. Melt segregation from partially molten source region, the importance of melt density and source region size. A mass proportion method for calculating melting reactions and application to melting of model upper mantle lherzolite.

Thermodynamics of multiple component systems containing several solid solutions. Mineralogical Society of America, Reviews in Mineralogy. Table A1: Expressions of the bulk distribution coefficients for mNB Open in new tab.

Table B1: Comparison of the preferred source with other commonly used fertile mantle compositions. SiO 2 Table E1: Summary of data and model results. Vulcan FZ. A II FZ. Bullard FZ. Bouvet FZ. Whole data set. E Wall. Orcadas FZ. W Wall. Average modes in abyssal peridotites vol. Issue Section:. Download all slides. View Metrics.