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Continuous tow investigations have shown that the present vent field inventory along fast to intermediate spreading ridges may be underestimated by at least 3–6 times, while the limited towed line investigations of venting sites along slow to ultra-slow spreading ridges make it impossible to determine their distribution. The Chinese Dayang cruise has conducted detailed towed line surveys of hydrothermal activity on segment 27 of the ultra-slow spreading southwest Indian ridge in 2015. The results have identified as many as 9 hydrothermal fields along 85-km-long segment, including one confirmed hydrothermal field, three inferred hydrothermal fields and five suspected fields. Hydrothermal activities are not only limited along-axis but also found approximately 10 km away from the axis. These vent fields are likely powered by a seismically identified axial magma chamber, including melt migration along normal faults to flank areas. The calculated hydrothermal activity frequency on segment 27 is approximately 3.6–8 times higher than that calculated from the Interridge database, suggesting that careful system exploration can reveal more hydrothermal activities even on ultra-slow spreading ridges effected by hotspot.

Bulletin of Volcanology, 74 (7), ISSN:0258-8900, ISSN:1432-0819

Melting experiments on a gneissic protolith using the diamond trap technique produced an accumulation of crystals of biotite in the boundary of the diamond trap. Several hypotheses can be inferred to explain this feature, but textural evidence and calculations of the settling and drag velocities of the crystals suggest a model of extraction of melt due to a pressure gradient inside the capsule as the most likely origin of this biotite layer. This biotite layer and the trapped melt resemble the melanosome and leucosome, respectively, of stromatic metatexite migmatites. The melt extraction model developed is similar to the “filter pressing” model for migmatite formation. The diamond trap technique is a suitable method to develop migmatites experimentally.

The conditions and the mechanism of glass melt migration in wide melting tanks for sheet glass are investigated for different types of melting zone charts: symmetric and asymmetric arrangement of batch heaps on the glass melt surface.

Pl-peridotites dredged along the Romanche Fracture Zone (FZ) are peculiar rocks. These peridotites derived from weakly deformed upper mantle rocks widespread percolated by melts with gabbroic composition. Mantle peridotite reacted with hot melt percolating through it and trapped melt which crystallised in pockets and in localised conduits at higher levels in the lithosphere. Reaction and crystallisation caused the peculiar microstructures, modal and chemical compositional variations, both in the bulk rocks and in the residual mineral phases. Electron and ion microprobe analyses suggest that percolating melts, characterised by different geochemical signatures, interacted, at deep levels in the lithosphere, with the Romanche peridotites mostly modifying the chemical compositions of Spl and Cpx. These new data show that impregnated mantle peridotites were emplaced all along the Romanche FZ, also in the western Romanche ridge transform intersection, were only fertile Pl-free peridotites were previously described. The cold equatorial upper mantle belt seems to influence strongly the dynamic of melting, melt migration, extraction and its final emplacement on the seafloor. Melts produced in this cold oceanic lithosphere are not able to segregate and join the overlying oceanic crust, but freeze and remain trapped at depth.

Determining the flow of magma and solid mantle beneath mid-ocean ridges is crucial for understanding the dynamics of plate spreading and the formation of new oceanic crust. Theoretical models suggest a range of possible flow regimes—from passive, plate-driven flows1,2 to ‘active’, buoyantly driven solid convection3,4,5,6,7—and have spurred an ambitious field programme to attempt to distinguish these flow fields using geophysical techniques8. Models that explore the geochemical consequences of melt transport9, however, suggest that these different flow fields can also have distinctive geochemical signatures. Here we compare model predictions to the chemistry of well located and closely sampled basalts from across the ridge-crest of the fast-spreading East Pacific Rise at 12° N (refs 10,11,12). These data show features that are not explained by traditional geochemical models of ocean-ridge magma generation, yet are consistent with the geochemical consequences of the new transport models that have passive mantle flow and convergent lateral melt migration. These results are also consistent with those of the seismic MELT experiment8, but add new information about the relative flow of melt and solid in the mantle which is probably unmeasurable by geophysical techniques.

The structure of columnar-jointed lava flows and intrusions has fascinated people for centuries and numerous hypotheses on the mechanisms of formation of columnar jointing have been proposed. In cross-section, weakly developed semicircular internal structures are a near ubiquitous feature of basalt columns. Here we propose a melt-migration model, driven by crystallization and a coeval specific volume decrease inside cooling and solidifying columns, which can explain the observed macroscopic features in columnar-jointed basalts. We study basalts from Hrepphólar (Iceland), combining macroscopic observations, detailed petrography, thermodynamic and rheological modelling of crystallization sequences, and Anisotropy of Magnetic Susceptibility (AMS) of late crystallizing phases (that is, titanomagnetite). These are all consistent with our proposed model, which also suggests that melt-migration features are more likely to develop in certain evolved basaltic lava flows (with early saturation of titanomagnetite), and that the redistribution of melt within individual columns can modify cooling processes.

THE segregation of melts from the Earth's upper mantle into the crust is an important process in the chemical evolution of the crust–mantle system. The processes of melt formation and migration in the upper mantle are inadequately understood, but some important characteristics of these processes can be inferred from upper-mantle rocks exposed at the Earth's surface. The Horoman peridotite body in northern Japan is a layered upper-mantle rock. The major-element compositions of the layers are consistent with their formation as residues from varying extents of melting; however, abundances of rare-earth elements (REE) require additional processes to have occurred1, such as post-melting enrichment (metasomatism) resulting from reaction with a migrating fluid phase. We report here that chondrite-normalized REE patterns in clinopyroxenes show abrupt changes in slope, which vary with stratigraphic position and rock type. These data can be modelled by chromatographic fractionation as melts migrated through and interacted with peridotite, creating compositional heterogeneities in the upper mantle. In the Horoman peridotite these heterogeneities occur on a scale length of tens of metres.

We have derived the thermodynamic functions describing the change in the energy of a system on penetration of a liquid into three-phase sintered compositions, and we offer a prediction with respect to the structures which will develop as a result, in addition to examining the moving forces of this process.

The petrological characteristics of axial basalts along the global mid-ocean ridge system shed light on the processes of magma supply involved in the formation of ocean crust. Several models of magma supply1–4 predict regular patterns of chemical variation along the strike of mid-ocean ridge axes. Here we present chemical data from closely spaced samples of axial basalts from the Mid-Atlantic Ridge, between the Rio Grande and Moore fracture zones at ∼26° S. This segment of the ridge is far from active hotspots and erupts normal mid-ocean-ridge basalt. The samples show regular patterns of chemical variation along the ridge axis, indicating systematic and correlated differences in depth and extent of partial melting and extent of shallow fractionation. These patterns support the existence of a deep central supply system but preclude along-axis melt migration and the presence of large well-mixed chambers below this segment of the southern Mid-Atlantic Ridge.

Most of the phenomena studied by geologists and geochemists are due, at least in part, to the ability of fluids to migrate within the Earth's interior. Three processes are believed to operate in melt migration: diapirism (large buoyant masses of partially molten rock, rising by viscously deforming the surrounding solid rock), magmafracturing (the rapid migration of melt along macroscopic fissures in cold rock) and two-phase flow in porous media (the microscopic percolation of buoyant melt in a partially molten rock, and an essential precursor to the other two processes). As a recent paper by McKenzie shows, it is in understanding the third of these processes that the most significant recent developments have occurred.