Melt Transport Research Articles

Formation of a high‐temperature metamorphic complex due to pervasive melt migration in the hot crust

Abstract Detailed reviews of metamorphic thermal structures and spatial distributions of granitic rocks in the high‐T Higo metamorphic complex (HMC), central Kyushu, Japan, and Ryoke metamorphic complex in the Yanai district (RMC‐Y), southwest Japan, reveal that the high‐grade metamorphic rocks were associated with migmatites or syn‐metamorphic granitoids. The close association of these rocks suggests that transport and solidification of granitic melts are appreciable heat sources. Partial melting of the pelitic host resists heating of the crust due to its endothermic reaction (i.e. absorption of latent heat). However, the effects of slower thermal evolution produced by partial melting are limited by the boundary conditions assumed for the HMC and RMC‐Y. New thermal numerical modeling reveals that the volume fraction of the solidified melt products and the duration of melt migration required for crustal heating to a certain temperature decrease with increasing rates of melt migration. The model predicts that the melt migration rate during formation of the HMC is about three to 10 times higher than that of the RMC‐Y, and the corresponding duration of melt migration is about one order of magnitude shorter. The model described here also explains the apparently contradictory observations made in different structural units of the RMC‐Y: the metamorphic pressure–temperature (P‐T) ratio of the southern unit is higher than those of the central and northern units, even though the volume fraction of granitic rocks in the southern unit is larger. The model suggests that the melt migration rate in the northern and central units was faster than that in the southern unit. Overall, the model implies that variations of metamorphic field P‐T arrays and the spatial distributions of granitic rocks in high‐T metamorphic complexes may be explained simply by variations in the rate and duration of melt migration in the hot crust.

Olivine-hosted melt inclusions and melting processes beneath the FAMOUS zone (Mid-Atlantic Ridge)

Major- and trace-element abundances have been obtained for olivine-hosted melt inclusions from a picritic basalt (ARP73–10–03) dredged in the FAMOUS locality along the Northern Mid-Atlantic Ridge. Isolated in homogeneous, highly primitive olivine phenocrysts (Fo 87.2–91.3), melt inclusions provide important information about the processes of melt generation beneath mid-ocean ridges. They contain near-primary magmas that display a large range of major- and trace-element compositions and form a primitive continuation of the FAMOUS whole rock suites. Both slightly depleted and enriched melt inclusions can be distinguished on the basis of their trace-element compositions, with (La/Sm) N ratios ranging from 0.58 to 1.52. Melt inclusions display chemical trends on both major- and trace-element variation diagrams, indicating that they constitute a suite of genetically-related, highly primitive magmas. The evolution of the melt inclusions is inconsistent with a model of mantle-melt chemical exchange during reactive transport, suggesting that the trapped melts are able to preserve pristine information about the melting process and/or source composition. Comparison of these trends with predicted curves for models of peridotite melting indicates that compositional variations are best reproduced by polybaric partial melting of a relatively homogeneous mantle source and subsequent mixing in various proportions of the melt batches produced at different degrees of melting and/or in different parts of the melting system. These observations require that transport of melts from the melting region to the site of olivine crystallization occurs without significant chemical exchange with the surrounding mantle.

Numerical method for hydrodynamic transport of inhomogeneous polymer melts

We introduce a mesoscale method for simulating hydrodynamic transport and self-assembly of inhomogeneous polymer melts in pressure driven and drag induced flows. This method extends dynamic self-consistent field theory (DSCFT) into the hydrodynamic regime where bulk material transport and viscoelastic effects play a significant role. The method combines four distinct components as a single coupled system, including (1) non-equilibrium self-consistent field theory describing block copolymer self-assembly, (2) multi-fluid Navier–Stokes type hydrodynamics for tracking material transport, (3) constitutive equations modeling viscoelastic phase separation, and (4) rigid wall fields which represent moving channel boundaries, machine components, and nano-particulate fillers. We also present an efficient, pseudo-spectral implementation for this set of coupled equations which enables practical application of the model in periodic domains. We validate the model by reproducing well known phenomena including equilibrium diblock meso-phases, analytic Stokes flows, and viscoelastic phase separation of glassy/elastic polymer melts. We also demonstrate the stability and accuracy of the numerical implementation by examining its convergence under grid-size refinement.

Deformation and melt transport in a highly depleted peridotite massif from the Canadian Cordillera: Implications to seismic anisotropy above subduction zones

Seismic anisotropy in subduction zones results from a combination of various processes. Although it depends primarily on the orientation of olivine in response to flow, the presence of water and melt in the wedge may modify the deformation of olivine. The melt distribution also influences anisotropy. Direct observations of the deformation and melt-rock interactions in a strongly depleted spinel-harzburgite massif from the Cache Creek terrane in the Canadian Cordillera allow evaluating the relative contribution of each process. Structural mapping shows that this massif has recorded high-temperature, low-stress deformation, high degrees of partial melting, and synkinematic melt-rock interaction at shallow depths (< 70 km) in the mantle, probably above an oblique subduction. Deformation, marked by shallow-dipping lineations and steep foliations, controlled melt distribution: reactive dunites and pyroxenite dykes are dominantly parallel to the foliation. Analysis of olivine crystal preferred orientations (CPO) indicates deformation by dislocation creep with dominant [100] glide. Glide planes are however different in harzburgites and dunites, suggesting that higher melt contents may favor glide on (001) relative to (010). Seismic properties, calculated by considering explicitly the large-scale structure of the massif, the olivine and pyroxene CPO, and possible melt distributions, show that the strain-induced olivine CPO results in up to 5% P- and S-wave anisotropy with fast seismic directions parallel to the lineation. Synkinematic melt transport by diffuse porous flow leading to melt pockets or dykes aligned in the foliation may significantly enhance this anisotropy, in particular for S-waves. In contrast, focused melt flow is not recorded by seismic anisotropy, unless associated with very high instantaneous melt fractions. Orientation of pyroxenite dykes suggests that the present orientation of the structures is representative of the pre-obduction situation, implying trench-parallel fast polarizations and high delay times as observed above the Kurils, Ryukyu, Taiwan, and Tonga subductions.

The Aguablanca Ni–(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex

The Aguablanca Ni–(Cu) sulfide deposit is hosted by a breccia pipe within a gabbro–diorite pluton. The deposit probably formed due to the disruption of a partially crystallized layered mafic complex at about 12–19 km depth and the subsequent emplacement of melts and breccias at shallow levels (<2 km). The ore-hosting breccias are interpreted as fragments of an ultramafic cumulate, which were transported to the near surface along with a molten sulfide melt. Phlogopite Ar–Ar ages are 341–332 Ma in the breccia pipe, and 338–334 Ma in the layered mafic complex, and are similar to recently reported U–Pb ages of the host Aguablanca Stock and other nearby calc-alkaline metaluminous intrusions (ca. 350–330 Ma). Ore deposition resulted from the combination of two critical factors, the emplacement of a layered mafic complex deep in the continental crust and the development of small dilational structures along transcrustal strike-slip faults that triggered the forceful intrusion of magmas to shallow levels. The emplacement of basaltic magmas in the lower middle crust was accompanied by major interaction with the host rocks, immiscibility of a sulfide melt, and the formation of a magma chamber with ultramafic cumulates and sulfide melt at the bottom and a vertically zoned mafic to intermediate magmas above. Dismembered bodies of mafic/ultramafic rocks thought to be parts of the complex crop out about 50 km southwest of the deposit in a tectonically uplifted block (Cortegana Igneous Complex, Aracena Massif). Reactivation of Variscan structures that merged at the depth of the mafic complex led to sequential extraction of melts, cumulates, and sulfide magma. Lithogeochemistry and Sr and Nd isotope data of the Aguablanca Stock reflect the mixing from two distinct reservoirs, i.e., an evolved siliciclastic middle-upper continental crust and a primitive tholeiitic melt. Crustal contamination in the deep magma chamber was so intense that orthopyroxene replaced olivine as the main mineral phase controlling the early fractional crystallization of the melt. Geochemical evidence includes enrichment in SiO2 and incompatible elements, and Sr and Nd isotope compositions (87Sr/86Sri 0.708–0.710; 143Nd/144Ndi 0.512–0.513). However, rocks of the Cortegana Igneous Complex have low initial 87Sr/86Sr and high initial 143Nd/144Nd values suggesting contamination by lower crustal rocks. Comparison of the geochemical and geological features of igneous rocks in the Aguablanca deposit and the Cortegana Igneous Complex indicates that, although probably part of the same magmatic system, they are rather different and the rocks of the Cortegana Igneous Complex were not the direct source of the Aguablanca deposit. Crust–magma interaction was a complex process, and the generation of orebodies was controlled by local but highly variable factors. The model for the formation of the Aguablanca deposit presented in this study implies that dense sulfide melts can effectively travel long distances through the continental crust and that dilational zones within compressional belts can effectively focus such melt transport into shallow environments.

Diffusive fractionation of U-series radionuclides during mantle melting and shallow-level melt–cumulate interaction

U-series radioactive disequilibria in basaltic lavas have been used to infer many important aspects of melt generation and extraction processes in Earth’s mantle and crust, including the porosity of the melting zone, the solid mantle upwelling rate, and the melt transport rate. Most of these inferences have been based on simplified theoretical treatments of the fractionation process, which assume equilibrium partitioning of U-series nuclides among minerals and melt. We have developed a numerical model in which solid-state diffusion controls the exchange of U-series nuclides among multiple minerals and melt. First the initial steady-state distribution of nuclides among the phases, which represents a balance between diffusive fluxes and radioactive production and decay, is calculated. Next, partial melting begins, or a foreign melt is introduced into the system, and nuclides are again redistributed among the phases via diffusion. U-series nuclides can be separated during this stage due to differences in their diffusivity; radium in particular, and possibly protactinium as well, can be strongly fractionated from slower-diffusing thorium and uranium. We show that two distinct processes are not required for the generation of 226Ra and 230Th excesses in mid-ocean ridge basalts, as has been argued previously; instead the observed negative correlations of the ( 226Ra/ 230Th) activity ratio with ( 230Th/ 238U) and with the extent of trace element enrichment may result from diffusive fractionation of Ra from Th during partial melting of the mantle. Alternatively, the ( 226Ra/ 230Th) disequilibrium in mid-ocean ridge basalts may result from diffusive fractionation during shallow-level interaction of mantle melts with gabbroic cumulates, and we show that the results of the interaction have a weak dependence on the age of the cumulate if both plagioclase and clinopyroxene are present.

Geochemical stratigraphy near the core-mantle boundary: Evidence from Hawaii drilling project results

The Hawaii Scientific Drilling Project (HSDP) sampled a 3 km section of basalt lava from the Mauna Kea volcano, covering ages of 200–600 ka. When combined with surface and dredge samples, there is a 600 ky record of the lava output from Mauna Kea as well as a 200 ky record from Mauna Loa. The isotopic stratigraphy of the core samples (Gcubed Theme) reflects the geochemical structure of the Hawaiian plume, given a model for the sampling of the plume by melting and melt transport (Bryce et al., 2005). The data show that there is radial geochemical zoning of the plume in terms of He, Pb, Nd, Sr and Hf isotopes. Data from other volcanoes indicate there is also heterogeneity along the axis of the plume, as well as asymmetry (Loa-Kea dichotomy). To first order, the radial geochemical structure of the plume represents the vertical structure at the thermal boundary layer from which the plume originates. Numerical models of plumes show that this vertical structure, which corresponds to potential temperature, is likely to be preserved during passage of the plume through the mantle. In the case of Hawaii, all of the lavas are derived from melting of mantle that originates from within 20–50 km of the base of the mantle, so the inferred radial geochemical structure maps to stratigraphy at the base of the mantle. HSDP data indicate that the high He/He anomaly (R/ Ra > 16) is restricted to the innermost core of the plume and is much larger in amplitude and smaller in diameter than the Nd, Sr or Hf anomalies. The He-3 anomaly must have an origin different from that of other isotopes; the anomalous mantle is restricted to the lowermost 10–20 km of the plume source. This observation accords with along-ridge variations of Nd, Sr, and He near Iceland. The HSDP data are consistent with two models for the configuration of the plume source. If the plume originates from the top of a dense layer separating the main mantle from the outer core, then the high-He signal must be attributed to the dense layer and distinguishes it from the main lower mantle. The dense layer is not significantly different from the rest of the lower mantle in terms of Nd, Sr, or Hf isotopes. If the plume originates directly from the CMB, then probably the He signal comes from the core. Models for core formation can accommodate this possibility. The dense layer may have distinctive He because it receives He from the outer core, or because it has retained primordial He. Hawaii and Iceland data imply that the lower mantle also has large regions with elevated He/He, but these typically have R/Ra 6 16.

Melt extraction in the Earth's mantle: Constraints from U–Th–Pa–Ra studies in oceanic basalts

U-series studies in oceanic basalts are critical for understanding melting and melt extraction in the Earth's mantle. The combined results of a detailed geochemical study of melting and melt extraction at Theistareykir, northern Iceland, provide a strong case for melt extraction via channeled melt flow at an active spreading ridge. It has often been argued, however, that widely used melting and melt extraction models, which simulate channeled melt extraction (i.e. fractional and/or dynamic melting), can only partially explain the global U-series systematics in oceanic basalts. As a consequence, more complicated models have been invoked, which suggest different styles of melt extraction at different depths/pressures in the mantle, so-called “two-porosity models”. Alternatively, diffusion-controlled mechanisms have been proposed. Here we show that U–Th–Pa–Ra systematics in oceanic basalts can indeed be explained by models where melt transport occurs without chemical equilibrium between melt and solid when variations in all three critical melting parameters (residual porosity, upwelling rate of the solid mantle and melt velocity) are taken into account. Melting at ridges requires systematic variation of at least two critical melting parameters, most likely upwelling and melt extraction rate. Melts generated with increasing lateral distance to the ridge axis are generated with slower upwelling rates and are extracted with lower velocities than melts created closer to the ridge axis. Melting at ocean islands, on the other hand, can successfully be explained by variations in upwelling rate only. Global U-series systematics in OIB originate from superimposed global variations in upwelling velocity due to different buoyancy fluxes and from local variation in upwelling velocity as a function of radial distance to the plume center. The model proposed here is consistent with other geochemical data for oceanic basalts and strongly supports melt extraction via high-porosity channels as a general means of melt extraction from the Earth's upper mantle.

Remelting of recently depleted mantle within the Hawaiian plume inferred from the 226Ra– 230Th– 238U disequilibria of Puʻu ʻŌʻō eruption lavas

Lavas from the Puʻu ʻŌʻō eruption of Kīlauea Volcano are thought to partially bypass the shallow magma reservoir beneath the volcano's summit, and thus, provide a relatively direct “window” to the mantle. Here we use high-precision U-series isotope measurements of Puʻu ʻŌʻō lavas (1985–2001) to investigate the timing and mechanism of melt transport within the Hawaiian mantle plume. The lavas display small, but significant, temporal decreases in their 230Th– 238U (∼2.5% to 1.4% excess 230Th) and 226Ra– 230Th (∼14% to 12% excess 226Ra) disequilibria. These trends correlate systematically with larger decreases in abundance ratios of trace elements that are highly versus moderately incompatible during partial melting of the mantle. These ratios vary from a maximum of ∼23% for Ba / Yb to a minimum of ∼4% for Nd / Sm. Modeling of these geochemical signatures suggests that Puʻu ʻŌʻō lavas are increasingly derived from a mantle source that was recently depleted by prior melt extraction within the Hawaiian plume. The timing of this depletion must be longer than the ∼20-year duration of the eruption but less than several half-lives of 226Ra (< 8 kyr ago). A single magmatic process – the transfer of melt from pores in steady-state equilibrium with the residual mantle into chemically isolated channels – ultimately seems to control the rapid fluctuation in lava chemistry at Puʻu ʻŌʻō. Specifically, this mode of melt extraction (if it occurs frequently) would create numerous patches of recently depleted mantle within Kīlauea's source region that may subsequently remelt. When melt is extracted into channels to supply a given eruption, such as Puʻu ʻŌʻō, it must be drained from an ever increasing volume in order to sustain the flow of melt to the surface. As the eruption continues, the volcano might tap melt from more distal areas and tend to encounter a greater number of patches of melt from recently depleted mantle. The source region of Puʻu ʻŌʻō lavas within the Hawaiian plume is also thought to contain long-lived, small-scale compositional heterogeneities based on temporal variations of ratios of highly incompatible trace elements (Ba / Th or Th / U) and 87Sr / 86Sr. This melt-transport mechanism would allow Kīlauea to sample chemically and isotopically heterogeneous pockets of melt from “fresh” mantle that was never melted beneath Hawaiʻi (in addition to melt from recently depleted mantle), and thus, efficiently transmit the geochemical signatures of these compositional heterogeneities from the volcano's source region to the surface.

Mechanisms and timescales of felsic magma segregation, ascent and emplacement in the Himalaya

Abstract We combine field, petrological, geochemical and experimental observations to evaluate the timescales of compaction-driven and shear-assisted melt extraction and ascent in the Himalaya. The results show that melt migration via compaction and channelling is inescapable and operates on timescales of less than 1 million years and possibly as short as 0.1 million years. Field and petrological data show that such a fast and efficient melt transfer results from a combination of favourable factors, including: (1) low but constant melt viscosity (10 4.5 Pa s) during extraction and ascent; (2) grain size coarsening of the source rocks in response to prolonged heating prior to melting; and (3) high source fertility and thus high melt fraction, owing to elevated modal amounts of muscovite in leucogranite sources. All three factors dramatically increase source permeability. Calculations show that shear-assisted melt extraction had a time interval recurrence in the range 10 000–100 000 years (10–100 ka), leading to sill thicknesses of 1–30 m. Yet melts falling at the low end of the viscosity range when coupled to high shear velocities may lead to veins several hundred metres thick. The deepest structural levels (e.g. central Zanskar Range) show that in-situ melts formed where pure shear compaction was greatest and where simple shear was also operative. Magma extracted from migmatite leucosomes was injected along planes of weakness parallel to the ductile shear fabric, probably by some form of hydraulic fracturing crack propagation mechanism. Large High Himalayan leucogranite (HHL) bodies (e.g. c. 5 km thick sills at Manaslu, Makalu and northern Bhutan) may thus represent inflated laccoliths assembled via dykes that tapped a 100–300 m melt layer produced by compaction of the Greater Himalayan Series (GHS). Thermal simulations show that such melt layers may have incubation times of several million years. Although transport time for magmas associated with the HHL is short, the time for assembly may take several million years for the largest HHL, as geochronological data indicate (up to 5 million years for Manaslu, Shisha Pangma). Transport of leucogranite melt from mid-crustal levels towards the surface was concomitant with active low-angle normal faulting along the South Tibetan Detachment (STD) normal fault, a structure that effectively formed the lid to the extrusion of a partially molten layer of mid-crustal rocks (channel flow). Rapid cooling of the granites emplaced at the top of the GHS implies rapid extrusion and lateral flow of GHS rocks beneath the STD during the period c. 20–17 Ma. Weakening of the crust by partial melting is thus likely to be pulsatory in time, and future thermomechanical models should incorporate such aspects to model tectonic evolution of hot orogens.

Partial melting processes above subducting plates: Constraints from 231Pa– 235U disequilibria

The processes involved in the formation and transport of partial melts above subducting plates remain poorly constrained relative to those at mid-ocean ridges. In particular, 238U– 230Th– 226Ra disequilibria, that might normally be used to constrain melting dynamics, tend to be swamped by the effects of fluid addition from the down-going plate. The 231Pa– 235U system provides an exciting exception to this because the highly incompatible nature of Pa means that fractionation and in-growth during partial melting overwrite the effects of fluid U addition. We present 231Pa– 235U data on 50 well-characterised lavas from seven subduction zones in order to examine partial melting processes. Measured ( 231Pa/ 235U) ratios are all >1 and 15% are >2. Overall ( 231Pa/ 235U) shows broad positive correlations with ( 230Th/ 238U) and La/Yb and negative trends against Ba/Th and ( 226Ra/ 230Th). These systematics can differ from arc to arc but suggest that ( 231Pa/ 235U) tends to be higher in sediment-rich arc lavas where the effects of fluid addition are muted and there is less of a 231Pa deficit for melting to overprint. We have explored the effects of decompression melting, frictional drag dynamic melting with and without ageing subsequent to fluid U addition to the wedge as well as flux melting models. Globally, average ( 231Pa/ 235U) appears to correlate negatively with convergence rate and so in the numerical models we use the local subduction rate for the rate of matrix flow through the melting zone. Using this assumption and reasonable values for other parameters, the melting models can simulate the overall range of ( 231Pa/ 235U) and some of the data trends. However, it is clear that local variations in some parameters, especially source composition and extent of melting, exert a major influence on 231Pa– 235U disequilibria. Some data, which lie at a high angle to the modelled trends, may be explained by mixing between small degree hydrous melts formed near the slab and larger degree, decompression melts produced at shallow depth.

Viscous Energy Dissipation and Strain Partitioning in Partially Molten Rocks

We develop a steady-state fluid-mechanical analysis describing the effect of strain partitioning on viscous energy dissipation. As observed in experimental studies of shear deformation of partially molten rocks, strain partitions when melt segregates because viscosity is reduced in regions of elevated melt fraction. The equations derived here are based on parameters measured in experiments, describing the evolution of melt distribution and rheological properties. We find that the dissipation depends strongly on the configuration of the melt-rich network of shear zones, including the average angle, volume fraction of melt and amplification of strain rate in the melt-rich bands. Minima in energy dissipation as a function of band angle develop, corresponding to configurations of melt networks that minimize the difference in mean stress between the band and the non-band regions. We propose that the organization of band networks occurs by the interplay between strain localization and viscosity variations associated with melt segregation. The band networks maintain a steady-state angle during shear by continuously pumping melt through the network. The development of strain partitioning in melt-rich networks will modify the energetics of melting and melt transport by efficiently extracting melt and reducing effective viscosity.

Analysis of Pipe Flow during Transportation of Magnesium Alloy Melt

Magnesium alloys have been well known as active metals. Thus, magnesium alloys in molten state must be handled with extreme care during melting and casting. In this study, water model experiments and numerical analyses were carried out to optimize a pipe flow that can transport magnesium alloys in molten state safely from melting to casting furnace. Especially, during a transportation of molten magnesium alloys, a flow pattern in a pipe becomes important, because the interface between air and melt can be the source of the metal oxidation, and therefore, an air/melt interface area must be minimized. For these purposes, two vessels connected with a long pipe having two elbows with different diameters and radii of the curvature were simulated as melting and casting furnace for magnesium alloys. Optimized conditions with minimized air/melt interface area for the melt transportation were discussed in several pipe configurations.

Crystal fractionation in the friction melts of seismic faults (Alpine Fault, New Zealand)

Compositional variations are documented in friction melts along the Hari Hari section of the Alpine Fault, New Zealand, with multiple stages of melt injection into quartzo-feldspathic schists. Intermediate to felsic melts were heterogeneous in composition, but all fractions show a common trend, with a tendency for the younger melt layers and glasses to be more alkali − (Na + K) and Si-enriched, while being depleted in mafic (Fe + Mg + Mn) components. These changes are attributed primarily to crystal fractionation of the melt during transport. Farther traveled molten layers were on the whole less viscous, mostly due to a higher melt-to-clast ratio; however, compositional change, together with a decrease in volatile content, produced a progressively more viscous liquid melt with time. The glass phase is interpreted as a remnant of this high viscosity felsic residual melt that was preserved during final quenching. Following initial failure, the formation of largely phyllosilicate-derived, volatile-rich, lower viscosity melt corresponds with a phase of fault weakening. Subsequent rapid crystal fractionation during melt transport, the loss of volatiles and freezing of residual melt contributed to the strengthening of the fault during seismic slip.

Elemental dispersion and stable isotope fractionation during reactive fluid-flow and fluid immiscibility in the Bufa del Diente aureole, NE-Mexico: evidence from radiographies and Li, B, Sr, Nd, and Pb isotope systematics

The 31.6±0.3 Ma old Bufa del Diente alkali-syenite (NE Mexico) intruded a sequence of Cretaceous limestones with intercalated sub-horizontal chert layers. The cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow-level (<1 kbar) contact-metamorphic aureole. Fluid-driven reactions between chert and marble wallrock, and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid-driven reaction. Aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers. Wollastonite formation occurred at the expense of limestone and chert and generated CO2. This CO2-induced fluid unmixing into an aqueous brine and a low-density CO2-rich fluid, which was lost to the overlying marble where it oxidized organic matter and caused δ13C and δ18O shifts in a zone some 5–10 cm wide. After wollastonite formation, the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar, quartz, aegirine-augite, eudialyte, zircon, and apatite. Aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross-fractures and migrated along the inner and outer borders of the wollastonite margins. Chemical dispersion patterns of U, Al, Na + K, P, S, Fe, and REE across the chert-to-marble boundary and its metasomatic rims are shown by autoradiography and neutron-induced radiography. Scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale. Isotopic data for Pb and Sr across a simple metachert-marble boundary and for Pb, Sr, Nd, B, and Li across a metachert-pegmatite-marble sequence demonstrate the following: (1) The Pb and Sr isotopic signature of early fluids was buffered by the carbonate wallrock. Only late fluids, shielded from wallrock interaction by a wollastonite mantle, variably preserved a memory of their initial magmatic signature. (2) Since the Nd isotope signature of marble and chert is bound to calcite and clay minerals, systematic shifts to unradiogenic Nd in marble reflect loss of carbonate-bound Nd as the wollastonite margin is approached. Nd in the wollastonite margin is dominated by Nd originally bound to clay minerals. The later emplacement of the pegmatite, which carried the Nd isotope signature of its alkali-syenite source, had little effect on the Nd isotopic composition of the wollastonite rim. (3) Although the Li and B isotopic compositions reflect the alkali-syenite source, they are also affected by isotopic fractionation and partitioning between melt, fluid, and solids.

U series disequilibria: Insights into mantle melting and the timescales of magma differentiation

Several U series nuclides have half‐lives (230Th, 76 kyr; 231Pa, 33 kyr; and 226Ra, 1.6 kyr) comparable to timescales of magmatic processes. We review the basic principles of extracting time information from U series nuclides and summarize variations in (230Th/238U), (226Ra/230Th), and (231Pa/235U) observed in magmas from mid‐ocean ridges, within‐plate settings, and subduction zones to contrast melt generation processes in different tectonic settings. U series disequilibria on melt and crystal phases of igneous rocks can provide temporal information on different stages in the magmatic history (melting duration, melt transport rates, magmatic crustal residence times, and timing of crystal growth) and potentially provide clues about the nature and mineralogy of mantle sources, mantle upwelling rates and porosity, fluid influences, and mechanisms of melt generation and transport. The subject is beginning to take a genuinely integrated approach to developing physically realistic quantitative models that offer increasingly exciting opportunities in the study of magmatic processes.

THE ROLE OF DUNITES FOR EXTRACTION OF MELTS FROM THE MANTLE TO THE OCEANIC CRUST

THE ROLE OF DUNITES FOR EXTRACTION OF MELTS FROM THE MANTLE TO THE OCEANIC CRUST

Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia

Research Article| July 01, 2005 Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia Roland Maas; Roland Maas 1School of Earth Sciences, University of Melbourne, Parkville, Victoria 3010, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Search for other works by this author on: GSW Google Scholar Maya B. Kamenetsky; Maya B. Kamenetsky 2Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Search for other works by this author on: GSW Google Scholar Alexander V. Sobolev; Alexander V. Sobolev 3Max-Planck Institut für Chemie, Mainz 55020, Germany, and Vernadsky Institute of Geochemistry, Moscow 117975, Russia Search for other works by this author on: GSW Google Scholar Vadim S. Kamenetsky; Vadim S. Kamenetsky 4Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Search for other works by this author on: GSW Google Scholar Nikolai V. Sobolev Nikolai V. Sobolev 5Institute of Mineralogy and Petrography, Novosibirsk 630090, Russia Search for other works by this author on: GSW Google Scholar Author and Article Information Roland Maas 1School of Earth Sciences, University of Melbourne, Parkville, Victoria 3010, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Maya B. Kamenetsky 2Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Alexander V. Sobolev 3Max-Planck Institut für Chemie, Mainz 55020, Germany, and Vernadsky Institute of Geochemistry, Moscow 117975, Russia Vadim S. Kamenetsky 4Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia, and Max-Planck Institut für Chemie, Mainz 55020, Germany Nikolai V. Sobolev 5Institute of Mineralogy and Petrography, Novosibirsk 630090, Russia Publisher: Geological Society of America Received: 03 Oct 2004 Revision Received: 24 Jan 2005 Accepted: 27 Jan 2005 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (7): 549–552. https://doi.org/10.1130/G21257.1 Article history Received: 03 Oct 2004 Revision Received: 24 Jan 2005 Accepted: 27 Jan 2005 First Online: 02 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 Roland Maas, Maya B. Kamenetsky, Alexander V. Sobolev, Vadim S. Kamenetsky, Nikolai V. Sobolev; Sr, Nd, and Pb isotope evidence for a mantle origin of alkali chlorides and carbonates in the Udachnaya kimberlite, Siberia. Geology 2005;; 33 (7): 549–552. doi: https://doi.org/10.1130/G21257.1 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 kimberlite rocks of the Udachnaya-East pipe (Siberia) are uniquely fresh and contain very high abundances of primary volatiles (Cl, CO2, S). Alkali elements and chlorine are extremely abundant in the reconstructed kimberlite melt compositions, and this enrichment is very important for our understanding of deep-mantle melting and melt transport. Here we present new isotopic data that confirm a mantle origin for these kimberlitic chlorides and carbonates, and constrain the kimberlite emplacement age as ca. 347 Ma. The initial Nd and Pb isotope ratios in a large salt aggregate, in a Cl-S–enriched water leachate of the groundmass, and in the silicate fraction of the groundmass are very similar ( ε Nd = +3 to +4, 206Pb/204Pb = 18.6, 207Pb/204Pb = 15.53), implying a comagmatic origin of the chlorides and carbonates and the silicates. Combined Sr, Nd, and Pb isotope data are used to rule out any significant contributions to the kimberlite chlorine budget from crustal sources, such as the Cambrian evaporite sequences of the Siberian platform. Our data support the interpretation that exsolved Na-K chloride and Na-K-Ca carbonate formed directly from original uncontaminated kimberlite magma. High Cl abundances in kimberlites suggest the presence of a Cl-rich reservoir in the deep sublithospheric mantle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

上部マントルにおけるマグマの形成過程：マントル物質科学における近年の展開

Recent developments in studies on the generation of magmas in the upper mantle are reviewed in this paper. This review focuses on mid-ocean ridges, where melting processes are believed to be simpler than those in other tectonic environments, thereby facilitating extensive application of clear-cut models. There are, however, still fundamental unsettled issues in magma generation beneath mid-ocean ridges. They are : (1) depth of initial melting, (2) depth of final melting, (3) style of melt transportation, (4) initial source composition, and, (5) heterogeneity. An attempt is made to clarify these points on the basis of petrologic and geochemical studies on abyssal peridotites and mid-ocean ridge basalts (MORB). A change in the style of melt transport from near fractional (chemical isolation) to near batch (permeable flow) melting is necessary to reconcile the observed behavior of major and trace elements in abyssal peridotites, suggesting the importance of resolving the melting history for each peridotite that has kept its own identity throughout the melting processes.

Low‐P–high‐T metamorphism and the role of heat transport by melt migration in the Higo Metamorphic Complex, Kyushu, Japan

AbstractThis paper characterizes the metamorphic thermal structure of the Higo Metamorphic Complex (HMC) and presents the results of a numerical simulation of a geotherm with melt migration and solidification. Reconstruction of the geological and metamorphic structure shows that the HMC initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels. Subsequently, this initial thermal structure has been collapsed by E–W and NNE–SSW trending high‐angle faults. Pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field P–T array that may be divided into two segments: the array at apparent higher structural levels has a low‐dP/dT slope, whereas that at apparent lower structural levels has a high‐dP/dT slope. This composite array cannot be explained by heat conduction in subsolidus rocks alone. Migmatite is exposed pervasively at apparent lower structural levels, but large syn‐metamorphic plutons are absent at the levels exposed in the HMC. Transport and solidification of melt within migmatite is a potential mechanism to generate the composite array. Thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth. The model also predicts that strata beneath the crossing point will consist of high‐grade solid metamorphic rocks and solidified melt products, such as migmatite. This prediction is consistent with the observation that migmatite was associated with the very high‐dP/dT slope. The melt migration model is able to generate the very high‐dP/dT segment due to the high rate of heat transfer by advection.