Melt Migration Research Articles

Fluid-fluxed partial melting of the Buncheon granitic gneiss in the Yeongnam Massif, Korea: Protracted (ca. 1.86–1.84 Ga) reworking of the Paleoproterozoic Korean arc

The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn-kinematic wet melting in the presence of ca. 0.9–1.4 wt% H2O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H2O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton.

Crust-scale reactive porous flow revealed by the brown amphibole in the IODP hole U1473A gabbros, Southwest Indian Ridge

Mid-ocean ridge magma reservoirs are mush-dominated systems, where reactive porous flow is a dominant post-cumulus process. However, the length scale of this process is poorly constrained, especially in ultraslow-spreading ridges. Here, we seek to address this question by presenting a comprehensive brown amphibole dataset of gabbros in IODP Hole U1473A drilled on the Atlantis Bank ocean core complex, Southwest Indian Ridge (SWIR). The results suggest a magmatic origin of the brown amphiboles. Intra-grain chemical variations and compositional zoning of these amphibole grains suggest they were crystallized in a closed system. Nevertheless, geochemical modelings indicate the remarkable Zr/Nd ratio variations in brown amphibole can only be explained by assimilation-fractional crystallization processes triggered by the reactive porous flow. Thus, the brown amphibole in olivine gabbros was crystallized from trapped melts residual after earlier melt-mush interaction. In this light, the reactive porous flow should have ceased prior to the brown amphibole precipitation. This, in turn, suggests the stratigraphic variations in amphibole trace elements reflect the final-stage reactive melt migration. The stratigraphic variations in amphibole trace elements reflect a major episode of reactive melt migration through an ∼800-m section upwardly. This implies that the porosity of the entire gabbroic section remained connected, providing a minimum estimate of the size of the melt-present zones in the lower oceanic crust during the spreading of the 57-60°E magmatic segment of SWIR. Our study reveals that large-scale reactive porous flow can occur in the magmatic segments of ultraslow-spreading ridges, and has important implications for the thermal and rheological evolution of the lower oceanic crust.

Early Paleozoic crustal anatexis during Wuyi-Yunkai orogenesis: Insights from zircon of Fuhuling migmatites in the Yunkai region, South China

Abstract Crustal anatexis is an important process in the tectonic evolution of many orogenic systems. In the Wuyi-Yunkai orogen in the South China block, the duration of partial melting and its relationship with orogenesis are poorly constrained. In this study, we present a multifaceted approach to determine the timing of anatexis and unravel the petrogenesis of Fuhuling migmatites in the Yunkai region of the southwestern South China block. Geochemical analyses indicate that the migmatites have (meta-)sedimentary protoliths. The absence of anhydrous peritectic minerals but the presence of microstructural and outcrop-scale indicators of partial melting suggest that the Fuhuling migmatites experienced fluid-present melting. Complex zoning and variable trace element concentrations in newly formed zircons in migmatites reflect their evolutionary histories during partial melting. Anatectic melt was present at Fuhuling in the Yunkai region from ca. 449–427 Ma during early Paleozoic Wuyi-Yunkai orogenesis. The wide variety of morphologies observed in the Fuhuling migmatites implies that migmatites in the Yunkai region experienced incipient partial melting, melt segregation, and melt migration. Combining new and previous results, we argue that the Yunkai region experienced two stages of crustal anatexis during the early Paleozoic, which may have been triggered by crustal thickening followed by rapid exhumation and orogenic collapse during the intra-plate Wuyi-Yunkai orogeny in the South China block.

Continental crustal growth and differentiation in intraoceanic arcs: Insights from plagiogranites in the forearc ophiolite of the southeastern Central Asian Orogenic Belt, China

Oceanic plagiogranites in subduction initiation zones record key information of continental crustal growth and differentiation. However, their petrogenesis remains controversial, due to the lack of complete subduction initiation records in most of ophiolites. In this study, we report a newly recognized plagiogranite suite (including intrusive plagiogranite porphyries and extrusive rhyolites) from the Diyanmiao forearc ophiolite in Inner Mongolia, southeastern section of the Centra Asian Orogenic Belt, where a relatively complete forearc magmatic succession is preserved. In the Diyanmiao suite, the plagiogranite porphyries, occur as small irregular dykes or pods intruding pillowed transitional lavas, and rhyolite lavas within or overlying the upper boninitic lavas, from bottom to top. Zircon UPb dating of two plagiogranite porphyry samples and one rhyolite sample yields mean ages of 322.5 ± 3.6, 319.9 ± 1.9, and 319.6 ± 2.6 Ma respectively. Similar to other oceanic plagiogranites, the plagiogranite porphyries and rhyolites in our study exhibit high SiO2 and Na2O contents and low K2O. They have chondrite-normalized flat to slightly depleted rare earth element patterns, and significantly negative Nb, Ta, and Ti anomalies similar to arc magmatic rocks. They also display high positive εHf(t) values (+12.5 − +18.7) and εNd(t) values (+6.9 − +10.0), comparable to the values of their host rocks. Petrographic characteristics and Rhyolite-MELTS modeling suggest that the plagiogranite porphyries and rhyolites were generated by high degree of crystal fractionation from parent tholeiitic basalt magmas. Our results suggest that melt migration triggered by fractional crystallization of mafic magmas played a vital role in the growth of continental crust in intraoceanic arcs in the southeastern Paleo-Asian Ocean.

The Origin and Evolution of DMM-Like Lithospheric Mantle Beneath Continents: Mantle Xenoliths from the Oku Volcanic Group in the Cameroon Volcanic Line, West Africa

Abstract The lithospheric mantle as sampled by peridotite xenoliths in some continental settings resembles the source of mid-ocean ridge basalts (MORB). Whether this resemblance is a primary feature or the result of post-formation secondary processes remains controversial. Here, the age, origin and thermochemical evolution of fertile continental mantle are constrained based on the chemical composition of minerals in spinel-facies lherzolite and websterite xenoliths from the Wum maar and Befang cinder cone of the Oku Volcanic Group (Cameroon Volcanic Line, West Africa), combined with in-situ Sr isotope compositions of clinopyroxene and fabric investigation by Electron Backscatter Diffraction (EBSD). The majority of lherzolites (here assigned to Group I) consist of minerals with fertile composition (olivine Fo89, Al-rich pyroxenes, spinel Cr# 0.08–0.10). Clinopyroxene is LREE-depleted and has depleted 87Sr/86Sr (0.7017–0.7020). Crystal-preferred orientation determined by EBSD reveals that clinopyroxene, and sporadically both clino- and orthopyroxene, post-date the olivine framework. Subordinate Group II lherzolites also contain secondary clinopyroxene which is LREE-enriched and has higher 87Sr/86Sr (0.7033). In contrast, the scarce lherzolites of Group III are more refractory: they contain 72–78 vol.% olivine, Al-poor pyroxenes, and spinel with Cr# 0.18. Clinopyroxene (87Sr/86Sr 0.7021) is texturally coeval with olivine and orthopyroxene. Few lherzolites contain amphibole (87Sr/86Sr 0.7031) which post-dates the nominally anhydrous minerals. Most of the websterites (Group A) are aluminous (spinel Cr# 0.04–0.06) with LREE-depleted clinopyroxene having depleted 87Sr/86Sr ratios (0.7017–0.7020) similar to Group I lherzolites. Chemical characteristics of minerals coupled with the crystal-preferred orientation data suggests that Group I lherzolites originated in the spinel stability field by reactive intergranular percolation of an incompatible element-depleted MORB-like melt. Group A websterites likely formed as cumulates from that melt. The Group II lherzolites supposedly occur close to lithosphere-asthenosphere boundary and record interaction with lavas of the Cameroon Volcanic Line, whereas Group III lherzolites occur in the shallow part of the mantle profile and represent the protolith from which the Group I lherzolites were formed. Local crystallization of amphibole and concomitant recrystallization of the host lherzolite were driven by supply of water in an event post-dating the formation of LREE-depleted rejuvenated rocks. Migration of alkaline melts of the CVL apparently did not significantly affect the mineral and chemical composition of the lithospheric mantle, which allowed Group I lherzolites and Group A websterites to retain very low 87Rb/86Sr (average 0.002) and depleted 87Sr/86Sr ratios in clinopyroxene. This not only indicates their formation in the Paleoproterozoic (~2.0–2.25 Ga), possibly during the Eburnean orogeny at the margin of the Congo craton, but also indicates surprisingly little influence of the regionally recognized Pan-African event.

Magma storage and transfer in the Villarrica volcanic chain, South Chile: MT insights into volcano-tectonic interactions

The active volcanic arc of the Andes is controlled by subduction of the Nazca and Antarctic Plates beneath the South American Plate. The Southern Volcanic Zone is the segment with the most active volcanoes in the Andes and the one where volcano-tectonic interactions are more evident. Magmatic fluids are there channeled through the brittle crust by faults and fractures, becoming an important region for understanding the mechanisms associated with melt migration and storage in subduction zones. The Liquiñe–Ofqui Fault System and Andean Transverse Faults, control the overall architecture of the volcanic arc and play an important role on magmatic transport and compositional partitioning. Evidence of this is the oblique chain composed of Villarrica, Quetrupillán and Lanín stratovolcanoes and a number of minor eruptive centers, where a wide range of magma compositions have been erupted with basalts and basaltic andesites dominating the suite. We used long–period and broad–band magnetotelluric stations deployed surrounding of Villarrica volcano to investigate the implications of crustal faulting and volcanism and their consequence on crustal reservoirs. Inversion of the data was used to generate three–dimensional electrical resistivity models. The resistivity distribution shows the upper crust as highly resistive, but below and east of Villarrica volcano, the model suggests the presence of a magmatic reservoir at shallow crustal levels (between 1.5 and 3 km b.s.l.), possibly corresponding to a transient magma storage. Meanwhile, the middle crust contains several intermediate to low conductive features interpreted as fluid pathways and/or melt storage regions, respectively, revealing the important role of fault systems. The lower crust also contains low resistivity zones indicating the presence of partial melt and/or fluids, associated with deep reservoirs (8–20 km), with a significant proportion of them likely non eruptible parts. This would suggest that melt is accumulated as highly crystallized mush or disconnected melt pockets and highlight the complex vertical extent of the structurally-controlled plumbing systems even in thin crust settings.

Rheological behavior of amphibolite facies migmatites during orogenesis: A case study from the North Qinling Belt, China

The rheological behavior of amphibolite-facies migmatites are key to understanding the evolution of middle–lower crust during orogenesis. The amphibolite-facies metamorphosed migmatites from the North Qinling Terrane are investigated to elucidate high temperature melt-present deformation and shear localization. The metatexite (plagioclase amphibolite and biotite plagiogneiss), diatexites, and deformed residuum (mylonitic amphibolite and layered amphibolite) are well-preserved in outcrops. The SW dipping and E-W striking foliation, and ptygmatic, asymmetrical folds indicate compressional deformation with top-to-the-WNW shearing. Zircon UPb geochronology indicates that crustal anatexis mainly occurred at ca. 400–386 Ma, associated with the collision between the North and South China Blocks. The occurrence of peritectic amphibole, the estimated anatectic conditions of 2.32–4.29 kbar and 595–691 °C and variation of mineral major compositions jointly demonstrate incongruent water-present melting reactions, which involved biotite- or amphibole-breakdown, while the deformation temperature is constrained at 605–644 °C. In the plagioclase amphibolite, the amphibole is dominated by (100)[001] dislocation slip, while plagioclase doesn't display a clear fabric. In the biotite plagiogneiss, plagioclase is dominated by strain-induced (010)[100] slip and quartz shows prism <a > slip. Rheological processes (brittle fracturing, melt(fluid)-assisted partial mylonitization and layering process) indicate that melt migration has an effect on the residuum. The formation of amphibole type-II fabric is controlled by cataclastic flow and rigid rotation during progressive deformation process and dissolution-precipitation processes lead to further weakening and strain localization.

High‐Temperature Fracture Growth by Constrained Sintering of Jadeite and Quartz Aggregates

AbstractFractures control heat and mass transfer and rheology in a wide range of subsurface regimes, ranging from low‐temperature diagenetic environments to high‐temperature metamorphic and magmatic systems. To investigate processes of opening‐mode fracture growth at high homologous temperature, we conducted constrained high‐temperature sintering experiments of thin layers of porous jadeite and quartz aggregate attached to a non‐sintering mullite substrate. Samples were heated stepwise at a low rate in a muffle furnace from 25°C to 1,000°C under ambient air pressure and examined for changes in mineral composition, texture, porosity, and fracture morphology using powder‐X‐ray diffraction, macro‐photography, reflected incident light microscopy, and secondary electron microscopy. Mineral reactions in the jadeite and quartz sample layer include jadeite and quartz reacting to albite at 600°C and the formation of nepheline and orthoclase at 900°C and 1,000°C. Opening‐mode fractures are first observed at 850°C coincident with the first presence of a melt phase with low aperture‐to‐length ratios similar to elastic‐brittle fractures. At 900°C, melt becomes increasingly abundant, and fractures obtain ductile morphology with high aperture‐to length ratios and blunted tips resulting from fracture growth by growth and coalescence of larger pores at the expense of smaller pores. In the absence of an externally applied mechanical load, we conclude that fracture growth is driven primarily by sintering stress resulting from differential contraction between sample layer and substrate associated with high‐temperature mineral reactions, melt formation and redistribution, and changes in pore structure. Similar fracture processes may be relevant to the segregation and migration of melt in magmatic systems.

Crustal Anatexis and Initiation of the Continental‐Scale Chongshan Strike‐Slip Shear Zone on the Southeastern Tibetan Plateau

AbstractContinental‐scale strike‐slip shear zones often record significant tectono‐magmatism and dynamic deformation processes of the crustal lithosphere. However, the genetic relationships and timing among the anatexis, deformation, and initial shearing along a strike‐slip shear zone have not been well defined. Here, we carried out detailed field, microstructural, zircon U–Pb geochronology, geochemical and EBSD texture analyses of leucogranites and migmatites in the Chongshan shear zone (CS‐SZ). The results indicate that most migmatites and leucogranites exhibit strong shear deformation and well‐developed high‐temperature mylonitic microstructures. The quartz aggregated from foliated leucogranites developed dominant high‐temperature prism &lt;c&gt; and prism &lt;a&gt; slip systems. The pre‐ and syn‐kinematic crustal anatexis and localized weak zone mainly occurred from 35 to 29 Ma along the CS‐SZ, which is closely related to the post‐collisional extension and collapse of overthickened crust. Biotite dehydration melting formed pre‐ and syn‐kinematic melts and leucogranites that further experienced fractional crystallization of plagioclase and K‐feldspar during melting and subsequent melt migration and emplacement upward along the tectonic weak zone. The thinning and weakening of lithospheric crust further facilitated the initial and formation strike‐slip displacement along the CS‐SZ, which occurred from 29 to 20 Ma or much later to 18 Ma. Finally, we propose that crustal anatexis and upward migrating melts play a key role in controlling the thermal state and rheological strength of the crust, resulting in nucleation and initiation of the localized deep‐seated shear zone that accommodates significant displacement for the India–Asia continental forward collision and intracontinental deformation.

Transport of melt, pressure and heat through a magma mush

Prior to intrusion, magma migrates through the crustal plumbing system that likely contains layers or columns of crystal mush. To better understand the behavior of the crustal magmatic system during magmatic unrest, it is important to examine the process of melt migration within the crystal mush and the associated evolution in pressure and temperature. In this study I use an analytical model to explore the characteristics of transport of melt, pressure, and heat through an idealized crystal mush layer/column under uniaxial strain condition. The model invokes a thermo-poro-viscoelastic rheology and uses a frequency-domain method to explore two scenarios of magmatic unrest: harmonic perturbation of fluid pressure, and step-rise in fluid pressure at a source location. Several factors influence the transport of melt, pressure and heat, including the thermal-mechanical coupling arising from the mush rheology, the advection of heat by melt flows, the competition between thermal diffusivity and poroelastic diffusivity, and the viscoelastic relaxation of the crystalline framework. One key finding is the development of transport asymmetry: when a background temperature gradient exists, the transport properties become different for propagation along the background thermal gradient and propagation against the background thermal gradient. Analysis on an endmember case shows that the transport asymmetry is associated to the competition between the diffusion and advection of pore pressure, which determines a Peclet number that depends on the temperature difference across the mush and the thermal expansion coefficients. Because the temperature in magma mushes in the crust likely increase with depth, the observed propagation asymmetry suggests some intrinsic difference between a bottom-up vs. a top-down triggering mechanism for magmatic unrest. The results from this study highlight the importance for further exploration for a more complete description of the transport properties in the crystal mush.

Melt migration in rubble-pile planetesimals: Implications for the formation of primitive achondrites

Primitive achondrites represent residual mantle material of planetesimals from which up to 20% partial melts were extracted. Melting experiments on chondritic compositions suggest that melts produced by ≲20% partial melting are rich in silica and alkali elements. Such melts are highly viscous (≳103Pa⋅s), and percolation models predict that they would only migrate negligible distances over timescales of 1–10Myr. After these timescales, a planetesimal would either be melted into a magma ocean by radiogenic heating from Al26, if it formed early; or it would cool below solidus, if it formed relatively late. However, melt migration is also controlled by permeability, which could be high for aggregates of rock boulders (compared to those of mineral grains). Specifically, the theory of planet formation suggests that collisions occurred frequently between planetesimals in the early solar system. These collisions may have shattered the planetesimals into fragments with sizes of meters to tens of meters, which would have accreted gravitationally into one or more daughter bodies. We develop a model to investigate melt migration in “rubble-pile” planetesimals; in particular, the melt exchange between partially molten rock boulders and the void space between them. The results suggest that, with typical properties of primitive achondrite materials at the conditions of low-degree partial melting, melts may have been squeezed into the voids between boulders, and migrated rapidly through these channels. Therefore, primitive achondrites may record melt migration in rubble-pile bodies reaccreted from fragments of partially molten planetesimals.

The peridotite-pyroxenite sequence of Rocca d'Argimonia (Ivrea-Verbano Zone, Italy): Evidence for reactive melt flow and slow cooling in the lowermost continental crust

The present study investigates the origin of a ∼ 400 m thick peridotite-pyroxenite sequence, locally known as Rocca d'Argimonia, which is encased within the lowest levels of the ∼8 km thick Mafic Complex, a gabbro-dioritic batholith from the lower continental crust section of the Ivrea-Verbano Zone (Southern Alps). The broad purpose of this work is to elucidate the evolution of mantle-derived magmas emplaced in the lowermost continental crust. We also wish to provide new geochronological constraints on the intrusion and the cooling evolution of the Lower Mafic Complex, which are still mostly unknown.Zircon grains separated from two gabbronorites enclosing the peridotite-pyroxenite sequence document a whole reset of the original UPb isotopic system at 286 ± 2 Ma. A Rocca d'Argimonia peridotite provided a consistent age of 296 ± 19 Ma, based on a LuHf clinopyroxene-amphibole alignment (initial εHf = +7.9). These results are interpreted to date the early cooling evolution of the Lower Mafic Complex. The clinopyroxene-amphibole peridotite pair also gave a RbSr alignment corresponding to an age of 250 ± 16 Ma (initial 87Sr/86Sr = 0.7045), which is reconciled with the subsequent, slow cooling evolution of the Lower Mafic Complex.The Rocca d'Argimonia peridotites (dunites to harzburgites and lherzolites) have whole-rock εNd(286 Ma) and εHf(286 Ma) values ranging from +0.4 to −1.1 and from +8.8 to +3.4, respectively, and 87Sr/86Sr(286 Ma) ratios varying from 0.7048 to 0.7060. The peridotites are crosscut by gabbronorite dykes and associated with olivine-free orthopyroxene-dominated pyroxenites. The gabbronorite dykes and the pyroxenites share the Nd-Hf-Sr signature of the peridotites. The gabbronorites enclosing the peridotite-pyroxenite sequence differ in the relatively enriched Nd-Hf-Sr isotopic fingerprint (εNd(286 Ma) = −4.5 to −5.6, εHf(286 Ma) = +0.2 to −2.5, 87Sr/86Sr(286 Ma) = 0.7073 to 0.7086). Remarkably, pyroxenes from the pyroxenites typically contain higher Cr2O3 than the peridotite counterparts (e.g., 0.23–0.37 wt% Cr2O3 in peridotite orthopyroxenes and 0.43–0.50 wt% Cr2O3 in pyroxenite orthopyroxenes).We propose that the peridotite-pyroxenite sequence formed by reactive melt percolation through an olivine-rich spinel-bearing matrix. In particular, the relatively high Cr2O3 content of the pyroxenitic pyroxenes is attributed to redistribution of Cr released by the breakdown of accessory spinel that was originally present in the olivine-dominated matrix, in response to a high melt/solid ratio. The melt residual after the crystallization of the pyroxenites continued to percolate the olivine-dominated matrix, thereby forming spinel-bearing lherzolites, harzburgites and dunites with the progression of the melt migration process. The chemically most primitive dunite records Nd-Hf-Sr isotopic disequilibrium conditions, thereby providing further information about the process of reactive melt flow. It is inferred that the original olivine-dominated matrix crystallized from a mantle magma having a slightly enriched Nd-Hf-Sr isotopic signature, whereas the percolating melts overall had a relatively depleted Nd-Hf-Sr isotopic fingerprint, despite being variably contaminated by the continental crust.

Reactive interaction between migmatite-related melt and mafic rocks: clues from the Variscan lower crust of Palmi (southwestern Calabria, Italy)

Abstract. In the Variscan lower–intermediate crust exposed in the Palmi area (southwestern Calabria, Italy), amphibolites occur as foliated, decimeter-thick layers within migmatitic paragneiss and as a decametric main body adjacent to the migmatites. The main body is mostly fine-grained and weakly to moderately foliated; unfoliated medium-grained portions rarely occur. Amphibolites are mainly composed of plagioclase (An80−91) frequently developing triple junctions, amphibole consisting of cummingtonite rimmed by hornblende and variable amounts of biotite. Minor quartz is present in amphibolite layers within paragneiss. Accessory allanite occurs in amphibolite layers within migmatites and in foliated, fine-grained portions from the main body. This study mainly aims to achieve information about the effects triggered by the migration of migmatite-related melts into associated mafic rocks and its role in the re-distribution of major and trace elements out of the anatectic source. On the basis of whole-rock major- and trace-element compositions, the protolith of amphibolite is recognized as of cumulus origin, likely derived from basic melt emplaced in the framework of the late-Variscan lithospheric extension. The rocks experienced high-temperature subsolidus re-equilibration (∼800 ∘C) in conjunction with the development of amphibole. The origin of amphibole is attributed to a coupled dissolution–precipitation process related to the reaction between migrating SiO2-rich hydrous melt and precursor orthopyroxene (± plagioclase). Reactive melt migration also caused the crystallization of biotite ± allanite ± quartz ± plagioclase. SiO2-rich hydrous melt had REE (rare earth element) compositions similar to late-Variscan peraluminous granites and could have been derived by partial melting of metasediments akin to neighboring migmatitic paragneisses. Both whole-rock and amphibole analyses reveal a decrease in Mg# (Mg/(Mg+Fe2+)) from amphibolite layers within paragneiss to fine- and medium-grained rocks of the main body. Hornblende shows an increase in SiO2 and a decrease in Al2O3 and K2O with increasing Mg#. Amphibolites interlayered with paragneiss have higher K2O, Rb, Ba, Th, U and Zr relative to those from the main body. Furthermore, amphibole from amphibolites interlayered with paragneiss is distinct for relatively high Rb, Ba, MREE (middle rare earth element) and HREE (heavy rare earth element) concentrations. Within the main body, foliated, fine-grained rock has both the whole rock and amphibole enriched in Rb and Ba and high Zr bulk-rock contents. Whole-rock and mineral chemistry heterogeneity most likely reflects (i) variation of the composition of the melt during its reactive migration, in response to dissolution of pre-existing minerals and crystallization of new phases, and (ii) variable modification of the original compositions. Foliated and fine-grained amphibolites record the strongest modification, thereby suggesting that they represent permeable pathway enabling effective interaction of the reacting melt with precursor minerals and nucleation of new mineral phases.

The Meaning of Pressure for Primary Magmas: New Insights From PRIMELT3‐P

AbstractThis paper reports new software, PRIMELT3‐P, for computing the pressure and temperature range over which a primary basaltic magma was formed by adiabatic decompression and fractional melting of fertile mantle peridotite. The underlying model was developed to explore the meaning of magmatic pressure and its connection to the physics of mantle melting, melt extraction, and melt migration. We present a comparison of the results of the batch melting model FractionatePT3 (Lee et al., 2009, https://doi.org/10.1016/j.epsl.2008.12.020) and the new fractional melting model PRIMELT3‐P, each applied to basalts from various localities, with a particular focus on Hawai'i and Iceland. The unexpected result is that pressures for batch melting and fractional melting are quite similar across a broad range of potential temperatures, agreeing to within ±0.29 GPa (1σ) when applied to magmas that pass tests for derivation from peridotite sources. Although samples from both Hawai'i and Iceland yield a range of final pressures of melting, the minimum final pressure in each location coincides with independent geophysical constraints—the depth to the lithosphere‐asthenosphere boundary beneath Hawai'i and to the Moho below Iceland. These results clarify the meaning of primary magma pressure, but the concept remains complex. We suggest that computed temperature and pressure arrays arising from heterogeneous peridotite‐source primary magmas may reflect melt transport and mixing in a thermally zoned mantle plume with a complex network of branching melt channels that nucleate over a range of depths.

Palaeoproterozoic structural evolution of polyphase migmatites in Olkiluoto, SW Finland

In migmatitic environments the behaviour of the system is controlled by the generation and amount of the anatectic melt. Accordingly, migmatites typically show a genetic linkage between the tectonic deformation and melt migration. We investigated this relationship in Olkiluoto (SW Finland) and identified four phases of ductile deformation, which are distinguished by the multiple folding phases, ductile shear events and cross-cutting features associated with pegmatitic leucosomes and/or a specific type of diatexitic migmatite with feldspar megacrysts. U–Pb LA-MC-ICPMS data on zircon cores and rims from migmatites and cross-cutting pegmatites indicate two distinct metamorphic events associated with melt generation and migration at 1.87–1.84 Ga and 1.82–1.78 Ga. These two migmatitic events suggest that the orogenic evolution of the area was long-lasting and characterized by slow cooling. The structural data and the age constraints presented in this paper support the idea of similar tectonic evolution and metamorphic environment in SW Finland and central E Sweden.

Seismic evidence for uniform crustal accretion along slow-spreading ridges in the equatorial Atlantic Ocean

The crustal accretion along mid-ocean ridges is known to be spreading-rate dependent. Along fast-spreading ridges, two-dimensional sheet-like mantle upwelling creates relatively uniform crust. In contrast, the crust formed along slow-spreading ridges shows large along-axis thickness variations with thicker crust at segment centres, which is hypothesised to be due a three-dimensional plume-like mantle upwelling or due to focused melt migration to segment centres. Using wide-angle seismic data acquired from the equatorial Atlantic Ocean, here we show that the crustal thickness is nearly uniform (~5.5 km) across five crustal segments for crust formed at the slow-spreading Mid-Atlantic Ridge with age varying from 8 to 70 Ma. The crustal velocities indicate that this crust is predominantly of magmatic origin. We suggest that this uniform magmatic crustal accretion is due to a two-dimensional sheet-like mantle upwelling facilitated by the long-offset transform faults in the equatorial Atlantic region and the presence of a high concentration of volatiles in the primitive melt in the mantle.

Supplementary Material for "Melt migration in rubble-pile planetesimals: Implications for the formation of primitive achondrites"

Supplementary Material for "Melt migration in rubble-pile planetesimals: Implications for the formation of primitive achondrites"

Trapped K‐feldspar phenocrysts as a signature of melt migration pathways within active high‐strain zones

AbstractMelt migration through high‐strain zones in the crust fundamentally influences their rheological behaviour and is important for the transfer of fluids to upper crustal regions. The inference of former melt‐present deformation, based on field observations, may be hampered if the high‐strain zone experience a low time‐integrated melt flux or high melt volume expulsion during deformation. In these cases, typical macro‐scale field evidence of former melt presence limits interpretations. In this contribution, we investigate igneous field evidence ranging from obvious to cryptic in the Gough Dam shear zone (central Australia), a 2‐ to 4‐km‐wide high‐strain zone shown to have acted as a significant melt pathway during the Alice Springs Orogeny. Within bands of the high‐strain zone, granitic lenses are easily discernible in the field and are inferred to have formed during melt present deformation. Related coarse K‐feldspar is observed in biotite‐rich (&gt;75 vol%) schist (glimmerite) as either isolated grains, forming trails (sub)parallel to the main foliation, or in aggregates with subordinate quartz. Detailed characterization of the granitic lenses shows that pockets of phenocrysts may be entrained in the shear zone. If melt expulsion and melt‐rock interaction is severe, isolated K‐feldspar grains in glimmerite may form. These grains exhibit (i) partially preserved crystal faces; (ii) a lack of internal grain deformation; (iii) reaction textures preferentially formed along the main crystallographic axes showing dissolution of K‐feldspar and precipitation of dominantly biotite; (iv) low‐strain domains between multiple K‐feldspar grains being inferred to enclose crystallized melt pockets, with some apparently isolated grains showing connectivity in three dimensions; and (v) a weak quartz and K‐feldspar crystallographic preferred orientation. These observations suggest an igneous phenocrystic origin for the isolated K‐feldspar grains hosted in glimmerite, which is consistent with the observed REE concentration patterns with positive Eu anomaly. We propose that the K‐feldspar phenocrysts are early‐formed crystals that were entrained into the glimmerite rocks as reactive melt migrated through the actively deformatting high‐strain zone. Previously entrained K‐feldspar phenocrysts were trapped during the collapse of the melt pathway when melt flux‐related fluid pressure waned while confining pressure and tectonic stress were still significant. The active deformation facilitated expulsion or loss of the melt phase but retainment and trapping of phenocrysts. Hence, the presence of isolated or ‘trains’ of K‐feldspar phenocrysts is a cryptic signature of syndeformational melt transfer. If melt transfer occurs in an open chemical system, phenocrysts will be entrained within the reaction product of melt‐rock interaction. We suggest that these so‐called trapped phenocrysts are a viable indicator of former syntectonic melt passage through rocks.

Deep long period seismicity preceding and during the 2021 Fagradalsfjall eruption, Iceland

We use a dense seismic network on the Reykjanes Peninsula, Iceland, to image a group of earthquakes at 10–12 km depth, 2 km north-east of 2021 Fagradalsfjall eruption site. These deep earthquakes have a lower frequency content compared to earthquakes located in the upper, brittle crust and are similar to deep long period (DLP) seismicity observed at other volcanoes in Iceland and around the world. We observed several swarms of DLP earthquakes between the start of the study period (June 2020) and the initiation of the 3-week-long dyke intrusion that preceded the eruption in March 2021. During the eruption, DLP earthquake swarms returned 1 km SW of their original location during periods when the discharge rate or fountaining style of the eruption changed. The DLP seismicity is therefore likely to be linked to the magma plumbing system beneath Fagradalsfjall. However, the DLP seismicity occurred ~ 5 km shallower than where petrological modelling places the near-Moho magma storage region in which the Fagradalsfjall lava was stored. We suggest that the DLP seismicity was triggered by the exsolution of CO2-rich fluids or the movement of magma at a barrier to the transport of melt in the lower crust. Increased flux through the magma plumbing system during the eruption likely adds to the complexity of the melt migration process, thus causing further DLP seismicity, despite a contemporaneous magma channel to the surface.

Recognition of melferite – A rock formed in syn-deformational high-strain melt-transfer zones through sub-solidus rocks: A review and synthesis of microstructural criteria

Melt transfer and migration occurs through both supra- and sub-solidus rocks. Mechanisms of melt transfer include dyking, mobile hydrofracturing and diffuse porous melt flow where melt flow may or may not be channelized via instabilities or into high-strain zones of active deformation. Here, we highlight the microstructural- and outcrop-scale signatures of syn -deformational melt-migration pathways through high-strain zones that cut sub-solidus rocks. High-strain zones with high proportions (>10%) of macroscopic, internally undeformed, felsic or leucocratic material are readily interpreted as important melt-migration pathways and are most common in supra-solidus host rocks. However, it is challenging to recognise high-strain melt-migration pathways through sub-solidus rocks; these pathways may lack noticeable felsic or leucocratic components at the outcrop scale and share many macroscopic features in common with ‘classic' sub-solidus mylonite, such that the two are generally conflated. We contrast field and microstructural characteristics of ‘classic' mylonite originating from solid-state deformation with those of high-strain zones that also cut sub-solidus rocks yet have microstructural indicators of the former presence of melt. We compile several features allowing one to distinguish solid-state from melt-present deformation in high-strain zones that cut sub-solidus rocks. Our aim is to encourage geologists to assess such high-strain zones on a case-by-case basis, in view of sub-solidus (i.e., mylonitic) versus melt-present deformation. Such assessment is crucial as (1) rocks deformed in the presence of melt, even small percentages of melt, are orders of magnitude weaker than their solid-state equivalents, (2) melt-rock interaction in such zones may result in metasomatism, and (3) such zones may sustain long-lived melt migration and ascent enabling chemical differentiation at a crustal scale. With this contribution we aim to increase the ease of recognising this important subset of melt-migration pathways by assisting in clarity of description and interpretation of high-strain rocks. • Briefly review mylonite, and the field and solid-state deformation characteristics considered typical • Review high-strain zones recognized as syn-deformational melt-transfer zones through sub-solidus rocks • Focus on microstructural features of high-strain zones atypical for mylonites but typical for former melt-presence