Melt Transport Processes Research Articles

Double Power-Law in Leucosome Width Distribution: Implications for Recognizing Melt Movement in Migmatites

Leucosome sizes in migmatites has been shown to follow power-law distribution, which is indicative of self-organized criticality governing accumulation and transport of partial melts in anatexis. In our measurements of leucosome widths in the Olkiluoto migmatite in western Finland, we found double power-law distributions in addition to single power-law distributions. We divided the double power-law behaviors into those with positive and negative kinks separating the two power-law regimes. We propose that double power-law distributions in migmatites result from impediments on the bottom-up self-organized criticality that melt transport processes commonly display. These impediments can be caused by some leucosomes solidifying before others, in which case the leucosomes were effectively formed in two distinct events or phases. We also consider the emergence of double power-laws as a result of melt accumulation in and sudden melt removal from conduits of a specific size.

Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrence of pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphere boundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We present coupled microstructural and geochemical study of profiles across various types of pyroxenite-peridotite layering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltration and mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionally interleaved with dunite, crosscut by centimeter- to decimeter-wide pyroxenite layers. The peridotites have a porphyroclastic texture and show a penetrative tectonic foliation subparallel to the pyroxenite layering. Peridotite-pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites and pyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine [100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation. This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinel lherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomatic imprint that decreases with distance from the pyroxenite layers, whereas the strength of the olivine CPO decreases from the country peridotites towards the pyroxenite layers. The parallelism between pyroxenite layers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPO and geochemical changes, are consistent with syn- to late-kinematic emplacement of the pyroxenites. These observations point to a strong interplay between deformation and melt transport processes in the mantle, characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processes due to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacement of the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationship between plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associated with pervasive deformation of the peridotite, but rather accommodated by deformation localized in discrete shear zones, not sampled in the present study.

Effects of the magnetic field on direct contact melting transport processes during rotation

Contact melting heat transfer occurs via relative motion between the heating source and a phase change material (PCM) during melting in various applications. In this study, we investigated the physics of the close contact melting process generated by rotation and when subjected to an applied magnetic field. We transformed the physical model comprising the three-dimensional mass, momentum, and energy equations of the liquid melt layer in the cylindrical coordinate system, including the effects of the Lorentz forces and coupled with an interfacial energy jump condition, into a set of nonlinear similarity equations. Various characteristic dimensionless variables were identified, including an external force parameter σ, which defines the relationship between the external load on the PCM and the centrifugal force due to rotation, and a magnetic field parameter M. Numerical results were obtained and we systematically studied and interpreted the effects of various dimensionless variables on the contact melting and heat transfer processes during rotation, including the structures of the flow and thermal fields, melt layer thickness, and the melting and heat transfer rates. In particular, our results demonstrate that the melting and heat transfer rates increase while the liquid melt film becomes thinner as the external force parameter σ increases. By contrast, an increase in the magnetic field parameter M decreases the melting and heat transfer rates, while yielding relatively thicker melt layers.

A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations

Systematic variations in mineralogy and chemical composition across dunite-harzburgite (DH) and dunite-harzburgite-lherzolite (DHL) sequences in the mantle sections of ophiolites have been widely observed. The compositional variations are due to melt-rock reactions as basaltic melts travel through mantle peridotite, and may be key attributes to understanding melting and melt transport processes in the mantle. In order to better understand melt-rock reactions in the mantle, we conducted laboratory dissolution experiments by juxtaposing a spinel lherzolite against an alkali basalt or a mid-ocean ridge basalt. The charges were run at 1 GPa and either 1,300°C or 1,320°C for 8–28 h. Afterward, the charges were slowly cooled to 1,200°C and 1 GPa, which was maintained for at least 24 h to promote in situ crystallization of interstitial melts. Cooling allowed for better characterization of the mineralogy and mineral compositional trends produced and observed from melt-rock reactions. Dissolution of lherzolite in basaltic melts with cooling results in a clinopyroxene-bearing DHL sequence, in contrast to sequences observed in previously reported isothermal-isobaric dissolution experiments, but similar to those observed in the mantle sections of ophiolites. Compositional variations in minerals in the experimental charges follow similar melt-rock trends suggested by the field observations, including traverses across DH and DHL sequences from mantle sections of ophiolites as well as clinopyroxene and olivine from clinopyroxenite, dunite, and wehrlite dikes and xenoliths. These chemical variations are controlled by the composition of reacting melt, mineralogy and composition of host peridotite, and grain-scale processes that occur at various stages of melt-peridotite reaction. We suggest that laboratory dissolution experiments are a robust model to natural melt-rock reaction processes and that clinopyroxene in replacive dunites in the mantle sections of ophiolites is genetically linked to clinopyroxene in cumulate dunite and pyroxenites through melt transport and melt-rock reaction processes in the mantle.

Perspective on the Genesis of E-MORB from Chemical and Isotopic Heterogeneity at 9–10°N East Pacific Rise

The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U–Th–Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9–10°N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large 230Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226Ra excesses at the expense of diminished 230Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9–10°N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9–10°N EPR, the asymmetric, off-axis distribution of E-MORB at 9–10°N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (∼0–3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9–10°N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce.

The 226Ra enrichment in oceanic basalts: Evidence for melt‐cumulate diffusive interaction processes within the oceanic lithosphere

U‐Th decay chains in young volcanic rocks have become a unique geochemical tool with the potential to constrain the physical processes involved in the generation and transport of magmas. Among the different isotopes of the decay series, the measured disequilibrium between 226Ra and 230Th isotopes in basaltic melts has been a crucial player in the interpretation of the U‐series data as a whole. However, the key assumption proposed in all previous models, that the origin of 226Ra‐230Th disequilibrium in oceanic basalts is only produced during mantle melting processes, has remained largely unchallenged. Here we present an alternative process for the origin of the 226Ra‐230Th disequilibrium in oceanic basalts. We argue that diffusive interaction of magmas with cumulates formed in the oceanic crust beneath mid ocean ridges or oceanic islands may be responsible for the observed 226Ra‐230Th disequilibrium in oceanic basalts. Hence the inferences drawn from (226Ra)/(230Th) activity ratios about mantle melting and melt transport processes beneath mid‐ocean ridges and ocean islands are at best equivocal.

Extreme chemical variability as a consequence of channelized melt transport

The spatial and temporal variability of chemical signals in lavas, residues, and melt inclusions provides important constraints on source heterogeneity, the efficiency of convection, and melt transport processes in the mantle. The past decade has seen an impressive increase in the number, precision, and spatial resolution of chemical analyses. However, as resolution has increased, the picture of variation that emerges has become increasingly difficult to understand. For example, mid‐ocean ridge basalts can display large variations in trace element concentration on scales from 1000 km of ridge to melt inclusions in 500 micron crystals. These observations suggest that melt transport processes do not readily homogenize partially molten regions. While some of the observed variability is due to source variations, a large proportion could be the consequence of magma transport in channelized systems. We present results of numerical models that calculate the trace element signatures of high‐porosity dissolution channels produced by reactive fluid flow. These models were originally developed to explain the organization of melt transport networks, based on observations of “replacive dunites” found in ophiolites. Channelized flow can produce orders of magnitude variation in the concentrations of highly incompatible elements, even for idealized systems with a homogeneous source, constant bulk partition coefficients, and equilibrium transport. Most importantly, the full range of variability may be found in each channel because channelization can transpose the chemical variability produced by melting throughout the melting column into horizontal variability across the width of the channel. The centers of channels contain trace element–enriched melts from depth, while the edges of the channels transport highly depleted melts extracted from the interchannel regions at shallower levels. As dunite channels may be spaced on scales of 1–100 m in the mantle, this mechanism allows highly variable melt compositions to be delivered to the Moho on small length scales. The chemical variation produced in the models is consistent with that seen in melt inclusion suites, lavas, and residual mantle peridotites dredged from the ridges and sampled in ophiolites.

Seismological evidence for three-dimensional melt migration beneath the East Pacific Rise

The extent to which crustal processes along mid-ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known. Models of mantle upwelling vary from two-dimensional, passive flow1 to three-dimensional, diapiric flow2,3,4. Similarly, beneath a ridge segment bounded by tectonic offsets, crustal magma chambers may be replenished continuously along the ridge5,6,7 or at a central injection zone2,3,4 from which magma migrates towards the segment's ends. Here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the East Pacific Rise. The anisotropy is consistent with two-dimensional mantle flow diverging from the rise, whereas the anomalous isotropic structure requires a three-dimensional but continuous distribution of melt near the crust–mantle interface. Our results indicate that crustal magma chambers are replenished at closely spaced intervals along-axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle.