Melt-rich Layer Research Articles

Melt-Rock Reaction in the Lower Oceanic Crust and the Influence on the Evolution of Mid-Ocean Ridge Basalts at the Central Indian Ridge (7°50′–8°30′S)

Abstract Various crustal processes shape both the lower oceanic crust and mid-ocean ridge basalts (MORBs). To better understand how these crustal processes influence MORB compositions, we conducted comprehensive petrographic and geochemical investigations on gabbroic rocks and erupted lavas dredged from a segment of the Central Indian Ridge (CIR) spanning from 7°50′S to 8°30′S. The petrographic and geochemical analyses of the gabbroic rocks revealed evidence of melt-rock reaction through reactive porous flow in olivine gabbro and gabbro. This process resulted in distinctive features in clinopyroxene, including disequilibrium textures with a troctolite/anorthosite matrix, complex variations in Mg#-Cr-Ti [Mg# = molar Mg/(Mg + Fe2+)] relationships, and considerable enrichment and fractionation of incompatible trace elements. A significant finding of our study is the close resemblance of trace element ratios in MORB and olivine-hosted melt inclusions to those of melts in equilibrium with clinopyroxene from olivine gabbro and gabbro with Sr anomaly (Sr/Sr* = SrN/sqrt[PrN*NdN]; N refers to chondrite-normalized values) greater than ~0.7. This observation strongly indicates that the composition of MORB is influenced by the melt-rock reaction taking place in the lower oceanic crust. Furthermore, our findings suggest that evolved melts in equilibrium with clinopyroxene having Sr/Sr* values lower than ~0.7 are less likely to erupt onto the seafloor and are instead trapped within the lower oceanic crust. Oxide gabbronorite is characterized by coarse-granular, pegmatitic textures and exhibits mineralogically and chemically more evolved characteristics compared to olivine gabbro and gabbro. It is inferred that the oxide gabbronorite formed through the in situ freezing of highly evolved melts within a melt-rich layer. Finally, we present a comprehensive model for melt evolution in the lower oceanic crust at the 7°50′S–8°30′S CIR by integrating all petrological and geochemical data obtained from gabbroic rocks, MORB, and olivine-hosted melt inclusions. This holistic model contributes to a better understanding of the intricate processes governing MORB composition in the context of the lower oceanic crust dynamics at slow-spreading ridges.

Crystallization of a hydrous magma ocean in the shallow lower mantle

The solidification of a deep magma ocean occurred early in Earth's history. Although the initial amount of H2O in Earth's magma ocean is predicted to be low (e.g., <3000 ppm), as an incompatible element it becomes highly enriched (e.g. >10 wt%) in the final few percent of crystallization. In order to understand how a hydrous magma ocean would crystallize at the top of the lower mantle, we determined liquidus phase relations in the MgO-FeO-CaO-Al2O3-SiO2-H2O system at 24 GPa. We find that the bridgmanite (brg) + stishovite (st) + melt and bridgmanite (brg) + ferropericlase (fp) + melt cotectic boundary curves trend to Mg-rich melt compositions with decreasing temperature and extend to very high H2O contents (∼80 mol% H2O). The brg+st+melt curve is a subtraction curve at < ∼18 mol% H2O and a reaction curve at higher H2O contents, whereas the brg+fp+melt is a subtraction curve throughout its length. The density of melts along the two cotectics leads to neutral buoyancy with respect to shallow lower mantle and transition zone minerals at H2O contents up to ∼25 mol%. A transient melt-rich layer can form at the top of the lower mantle during late-stage crystallization in a mushy magma ocean when melt percolation dominates. When crystallization exceeds ∼98%, hydrous melts (>25 mol% H2O) become buoyant and can percolate into and hydrate the mantle transition zone.

Sublithosphere Mantle Crystallization and Immiscible Sulfide Melt Segregation in Continental Basalt Magmatism: Evidence from Clinopyroxene Megacrysts in the Cenozoic Basalts of Eastern China

AbstractThis study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to &amp;gt;10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts.

Estimating melt fraction in silicic systems using Bayesian inversion of magnetotelluric data

The location, volume and physical states of magma reservoirs are primary controls on the eruptive behavior of volcanic systems. Fundamental to understanding and monitoring these systems is the ability to identify reservoir size and physical properties, in particular melt fraction which plays an important role in the rheology and stability of a magmatic system. Large silicic volcanic eruptions in the geological record suggest that extensive pockets of melt-rich silicic magma must exist in the subsurface but such melt pockets have not been detected by geophysics. This has led to the question of whether the reservoirs that feed large volcanic eruptions are only melt-rich for a short time and thus would only be detected by geophysics shortly prior to an eruption. Magnetotelluric data measure the electrical resistivity of the subsurface and are sensitive to subsurface fluids and partial melts making it a powerful tool for imaging subvolcanic magma reservoirs. This study examines the ability for magnetotelluric data to accurately estimate melt fraction using both stochastic Bayesian inversion and deterministic regularized inversion. Results from synthetic modelling indicate that magnetotelluric data are best able to predict the melt fraction for the thick melt-rich layer using both inversion methods, though both methods under-estimate the true amount of melt. In addition, magnetotelluric data can accurately detect changes in melt fraction from crystal-rich mush (0.1 melt fraction) to melt-rich magma (0.9 melt fraction) for thick layers. Thickness is a key parameter which provides a method to assess the total volume of melt present, but it is difficult to estimate using smooth regularized inversions. Fixed-dimension Bayesian inversions provide estimates of layer thickness and their uncertainties and it is shown that estimates of total volume of melt are more accurate for both thin (0.2 km) and thick (1 km) layers than individual estimates of melt fraction or thickness alone. The melt fraction of the thin melt-rich layer is under-estimated using both the deterministic inversion method and the most probable solution of the Bayesian inversion. The Bayesian distribution of solutions has long tails which includes the true solution and better represents the uncertainty of the modelled parameters. As we consider past large melt-rich caldera eruptions at Taupo (New Zealand), Toba (Indonesia), Yellowstone and Long Valley (USA), our analysis suggests that thin melt-rich, potentially eruptible zones involved in large (e.g. >500 km3) eruptions could be systematically misinterpreted as thicker non-eruptible crystal-rich mush zones. Testing the sensitivity of MT to detect melt accumulation in active magmatic systems has important implications for volcanic hazard assessment and the use of Bayesian inversions allows for a better understanding of the range of possible interpretations.

A Turbulent Magmatic Density Current and the Origin of the Anastomosing UG-1 Chromitites at Dwars River in the Bushveld Complex

AbstractThe origin of the enigmatic UG-1 chromitites at Dwars River in the eastern limb of the Bushveld Complex has been vigorously debated. The UG-1 chromitites form an anastomosing network of multiple layers that are hosted in poikilitic anorthosites and their formation has previously been explained by depositional, erosional, and intrusive processes. We propose that the UG-1 chromitites formed in response to the emplacement of a turbulent magmatic density current into the developing chamber. We use theoretical constraints in fluid mechanics to describe the evolution of the current and in this context, we provide an explanation for the enigmatic igneous features that are preserved at Dwars River. The current was emplaced as an instantaneous single pulse (fixed volume) of dense plagioclase-charged magma (i.e. a plagioclase slurry) that turbulently propagated along the chamber floor. Settling of the initial cargo of entrained plagioclase laths resulted in the formation of a thick sequence of feldspathic mush. The remaining melt turbulently outruns on the chamber floor ahead of the feldspathic mush. This led to the density current becoming stratified into two layers: (1) a basal granular layer, and (2) an upper melt-rich layer. Both layers evolved in a viscous-dominated regime from a Newtonian to a non-Newtonian behaviour (i.e. a power law fluid). In the current’s propagation stage, the resident magma in the chamber was entrained and efficiently admixed into the melt-dominated upper layer, producing a superheated chromite-saturated hybrid melt. Chromite precipitation from this hybrid melt led to the formation of a series of chromite-laden slurries that flowed in the turbulent current and were split (‘bifurcated’) as they passed around rising buoyant plagioclase diapirs that were developing in the back of the current. This led to the anastomosing and bifurcating nature of the UG-1 chromitites. The chromite slurries merged in the tail of the current and eventually back-injected into the deposited plagioclase cumulates on the chamber floor—forming the main ∼2-m thick UG-1 chromitite layer. The cooling and crystallization of the propagating upper layer of the current led to an increase in its viscosity that slowed its velocity until it eventually stagnated on the chamber floor. The mechanical sorting of crystals in plume structures became dominant at the stagnation stage. We argue that spatial variations in crystal packing controlled the heterogeneous distribution of brittle and ductile deformation features that are observed at Dwars River. In situ crystallization of trapped pore melts led to the nucleation of large orthopyroxene and plagioclase oikocrysts that cemented the anorthosite and the chromitite layers, respectively. The emplacement of magmatic density currents and intra-chamber magma mixing may be pertinent to the development of stratiform chromitites in incrementally constructed magma chambers.

Slab-parallel advection versus Rayleigh-Taylor instabilities in melt-rich layers in subduction zones: A criticality analysis

We show slab-parallel advection and Rayleigh-Taylor instability (RTI) as two competing gravity-driven flow mechanisms in the melt-rich layer atop a subducting slab. Scaled laboratory model results, supported by CFD simulations, indicate a transition of the RTI to advection mechanism at a threshold slab-dip angle (α) between 20° and 30°. The advection mechanism results in updip flow, forming a single plume at the upper edge of the melt-rich layer. Based on lubrication approximation, we present an analytical solution of the linear stability problem to perform a criticality analysis of α for the transition between RTI and advection, as observed in the experiments. The solution indicates this transition to occur at α⁎ = 28°, in favorable agreement with the experimental values. The article finally highlights the implications of this criticality analysis in interpreting the spatial distributions of subduction volcanism.

The Lithospheric Thickness Control on the Compositional Variation of Continental Intraplate Basalts: A Demonstration Using the Cenozoic Basalts and Clinopyroxene Megacrysts From Eastern China

AbstractStudies on intraplate ocean island basalts have demonstrated the control of lithosphere thickness on the extent of melting and pressure of melt extraction (i.e., the lid effect). However, whether lithosphere thickness also controls the composition of within‐continent basalts remains unclear. Here, we test this hypothesis by studying the Cenozoic basalts contaitning clinopyroxene megacrysts from 10 localities throughout eastern continental China with a north‐south spatial coverage in excess of 2,500 km. Indeed, the geochemical parameters (e.g., abundances and ratios of major and trace elements) correlate well with the depth of the lithosphere‐asthenosphere boundary (LAB) calculated using the clinopyroxene barometry, showing significant lithospheric thickness control on basalt compositions. These observations offer further evidence for melt pooling (a melt rich layer) close beneath the LAB as a “stable magma reservoir” for crystallizing compositionally uniform clinopyroxene megacrysts to be carried by subsequent pulses of melt transport and eruption.

Sensitivity of glacial isostatic adjustment to a partially molten layer at 410 km depth

We present a sensitivity analysis aimed at testing whether observables related to glacial isostatic adjustment can support or refute the occurrence of a low viscosity melt-rich layer (MRL) above the mantle transition zone, as required by the 'transition-zone water-filter' model (Bercovici & Karato 2003). In total, 1600 model runs were performed sampling a range of MRL thicknesses (1, 10 and 20 km) and viscosities (10 15 -10 19 Pa·s), plausible viscosity values in the upper and lower mantle regions and four distinct ice histories. To determine decay time constraints, we consider relative sea level (RSL) data from two sites [Angerman River (AR), Sweden and Richmond Gulf (RG), Canada] and use a new method of observational sea level data correction. Comparing model output of postglacial decay times and J 2 to observational constraints, we find numerous possible solutions, largely as a result of parameter trade-off. The investigated observables are sensitive to the existence of an MRL and reasonable variations in its thickness and viscosity. The magnitude and nature of this sensitivity varies between the two data types as well as the adopted background viscosity structure. Decay time results from either considered location do not strictly support or exclude MRL existence. However, both locations offer MRL viscosity requirements for given thicknesses, with AR being more restrictive. RG constraints allow MRL viscosities as low as 10 16 Pa·s (10 km) and 10 17 Pa·s (20 km). AR results narrow these permitted viscosity ranges to 10 18 Pa·s or greater for both 10 and 20 km MRL thicknesses. In the case of a 1 km thick MRL, AR constraints permit the viscosity to be as low as 10 17 Pa·s, whereas those of RG permit any MRL viscosity. The decay time observations are satisfied by only a small subset of 'background' mantle viscosities (regardless of the MRL properties), none of which support a spherically symmetric solution of Earth viscosity. Finally, comparing model output to the observed J 2 value did not provide any constraints on MRL properties. However, our results show that this observable has a strong preference for viscosity values in the lower mantle that are equal to or greater than 10 22 Pa·s.

Melt redistribution by pulsed compaction within UltraLow Velocity Zones

This article investigates the melt distribution and resultant seismic signature within UltraLow Velocity Zones (ULVZs) forced by pulsed compaction at the mantle–ULVZ interface. Transient flow in the ambient mantle causes periodic compaction in the ULVZ matrix. For a neutrally buoyant melt, an initially uniform melt distribution is modified by the formation of a thin, decompacting, melt-rich layer near the top and a wide, melt-poor, compacting layer near the bottom. Such a structure is reflected in large reductions in S and P wave velocities near the top and smaller reductions near the bottom of the ULVZ. A dense melt pools near the bottom of the ULVZ, leading to larger reductions in seismic wave speed near the bottom. The magnitude of melt segregation in the decompaction layer is controlled by the viscosity of the ULVZ matrix in a nonlinear fashion. At high ULVZ viscosities, the compaction length becomes substantially larger than the dimension of thin ULVZs, leading to a reduction in the magnitude of melt segregation in the decompaction layer. In a ULVZ of matrix viscosity 1020Pas containing an average melt volume fraction of 0.05, formation of decompacting, melt-rich layers reduce the S and P wave velocities by 25% and 8%, respectively. Vertical variation in seismic velocity reduction within the ULVZ column is a consequence of melt redistribution by compaction, rather than variation of melt microstructure within the ULVZ.

Nickel and helium evidence for melt above the core–mantle boundary

High (3)He/(4)He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth; if so, it is now either a primitive refugium at the core-mantle boundary or is distributed throughout the lower mantle. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high (3)He/(4)He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary.

Dynamics and structure of a stirred partially molten ultralow-velocity zone

The seismic properties of thin (5–40 km thickness) patches exhibiting low seismic velocities—termed ultralow-velocity zones or ULVZ—just above the core–mantle boundary (CMB) might be explained by the presence of partially molten rock, where a liquid phase occupies interstices within a skeletal network of solid grains. However, a key problem with this explanation is that in the absence of improbably strong surface tension effects, partial melt is expected to drain by percolation over geological time scales, to form a dense, melt-rich layer at the CMB with physical properties that are inconsistent with seismic and other geophysical constraints. Here we consider whether stirring within ULVZ, driven by viscous coupling to convective motions in the overlying mantle, can inhibit the production of such stratification and maintain a partially molten region with a structure and constitution comparable to what is inferred seismically. We use two-dimensional numerical simulations of the response of a melt–solid mixture to stirring imposed from above and scaling analysis to identify conditions leading to melt separation, retention and drainage over a broad range of parameters. We find that melt migration at plausible ULVZ conditions is governed predominantly by dynamic pressure gradients arising from the viscous deformation related to mantle stirring, rather than by the buoyancy effects driving melt percolation. In particular, dense melt that would otherwise drain downward and accumulate at the CMB is expected to remain in suspension as a result of the stirring driven within ULVZ. In addition, our model predicts that partially molten ULVZ patches will be characterized by a positive gradient in seismic shear velocity (i.e., increasing with depth), consistent with seismic inferences, and may persist in this state over geological time scales.

Some basic concepts and problems on the petrogenesis of intra-plate ocean island basalts

Basaltic magmatism that builds intra-plate ocean islands is often considered to be genetically associated with “hotspots” or “mantle plumes”. While there have been many discussions on why ocean island basalts (OIB) are geochemically highly enriched as an integral part of the mantle plume hypothesis, our current understanding on the origin of OIB source material remains unsatisfactory, and some prevailing ideas need revision. One of the most popular views states that OIB source material is recycled oceanic crust (ROC). Among many problems with the ROC model, the ocean crust is simply too depleted (e.g., [La/Sm]PM <1) to be source material for highly enriched (e.g., [La/Sm]PM ≫ 1) OIB. Another popular view states that the enriched component of OIB comes from recycled continental crust (RCC, i.e.; terrigenous sediments). While both CC and OIB are enriched in many incompatible elements (e.g., both have [La/Sm]PM ≫1), the CC has characteristic enrichment in Pb and deletion in Nb, Ta, P and Ti. Such signature is too strong to be eliminated such that CC is unsuitable as source material for OIB. Plate tectonics and mantle circulation permit the presence of ROC and RCC materials in mantle source regions of basalts, but they must be volumetrically insignificant in contributing to basalt magmatism. The observation that OIB are not only enriched in incompatible elements, but also enriched in the progressively more incompatible elements indicates that the enriched component of OIB is of magmatic origin and most likely associated with low-degree melt metasomatism. H2O and CO2 rich incipient melt may form in the seismic low velocity zone (LVZ). This melt will rise because of buoyancy and concentrate into a melt rich layer atop the LVZ to metasomatize the growing lithosphere, forming the metasomatic vein lithologies. Erupted OIB melts may have three components: (1) fertile OIB source material from depth that is dominant, (2) the melt layer, and (3) assimilation of the metasomatic vein lithologies formed earlier in the growing/grown lithosphere. It is probable that the fertile source material from depth may be (or contain) recycled ancient metasomatized deep portions of oceanic lithosphere. In any attempt to explain the origin of mantle isotopic end-members as revealed from global OIB data, we must (1) remember our original assumptions that the primitive mantle (PM) soon after the core separation was compositionally uniform/homogeneous with the core playing a limited or no role in causing mantle isotopic heterogeneity; (2) not use OIB isotopes to conclude about the nature and compositions of ultimate source materials without understanding geochemical consequences of subduction zone metamorphism; and (3) ensure that models and hypotheses are consistent with the basic petrology and major/trace element geochemistry.

Melt migration in plume–ridge systems

We assess the potential for melt migration, separate from solid flow, to accommodate the transport of plume-signature material from off-axis mantle plumes to nearby mid-ocean ridges. We use a boundary-element method to estimate the solid pressures induced by buoyant plume flow, and find that the solid pressure gradients are small compared to melt buoyancy, suggesting that melt streamlines in mantle plumes are essentially vertical. We combine our plume pressure solutions with analytical pressure fields for ridge corner flow and find that the combined plume–ridge pressure field is not sufficient to drive porous flow in the upper mantle over the distances (hundreds of km) observed in many natural systems. We also examine melt transport via porous flow in a melt-rich layer at the base of the lithosphere for plume–ridge systems, and find that melts can traverse plume–ridge offsets of several hundred kilometers in a few hundred thousand years, or less. Our results suggest that plume signatures observed in ridge basalts can be explained by lateral migration of plume melts in a sub-lithospheric channel augmented by solid flow pressure gradients. We apply our models to the Galápagos plume–ridge system and find that melt migration, as opposed to solid flow, provides a means to explain many aspects of the observed chemical anomalies on the Galápagos Spreading Center, including the position of the maximum anomalies due north of the archipelago, the symmetric pattern, and the gradual along-axis gradient.