Asthenosphere Research Articles

Rheological transitions in progressive melting of rock and their geological constraints from the Fuhu metatexite-diatexite profile in Guangdong Province, SE China

Integration of the published experimental data of partially melted amphibolite, orthogneiss, granite and aplite reveals the existence of three rapid strength drops of the rock-assemblage at melt fraction around 7%, 21% and 41% respectively. The first and the last drops are equivalent to the well-known ‘melt connectivity transition’ (MCT) and ‘solid to liquid transition’ (SLT). The drop at melt fraction around 21% is newly identified and termed as the ‘framework-melting transition’ (FMT). The FMT is defined as the transition from eutectic quartz-feldspars melting to refractory biotite melting. The three rheological transitions are well constrained by the geological data from the Fuhu profile in Guangdong Province, SE China. This profile is a 15m high cliff. Rocks exposed on the profile are zoned as mottled, stripped, narrow-banded and wide-banded migmatites and diatexitic granite from the top of the profile downwards. The leucosome proportions of the zones measured on the outcrops combined with the experimental data of the partially melted rocks have significant implications for understanding the role of rheological transitions during crustal melting.The MCT, which corresponds to the boundary between the mottled and the narrow-banded migmatites, suggests the segregation of melt within the partially melted system along with the compaction of matrix. Thus, the MCT is deemed to be the transition of the partially melted system from matrix-supported (melt stored in pore-space) to framework-supported (melt stored in foliation/bedding-space of protolith).The FMT corresponding to the boundary between the narrow- and the wide-banded migmatites denotes the inception of melting of the solid framework. Mechanically, the FMT can be regarded as the transition of the framework from compaction to fusion, and geochemically, the transition from eutectic melting of quartz-feldspars to that of refractory biotite. The increasing melt due to framework-melting after the FMT is basically limited within the solid framework until the melt fraction exceeding the SLT.The SLT is geologically constrained by the in-situ metatexite-diatexite transition that is also termed as Magma Interface (MI). The SLT (MI) represents the upper limit of a crustal magma layer. Because of the downward increasing temperature and melt fraction, there should be no consecutive layer of felsic rock existed in the space between the SLT (MI) and the mafic lower crust except the isolated rock-blocks within the magma layer.

Evolution of a protolunar disk in vapor/melt equilibrium

AbstractA model of the viscous evolution of a two‐phase, vapor/melt protolunar disk is described. Droplets condense from the vapor and “rain out,” forming a stratified structure with a midplane magma layer surrounded by a vapor reservoir. The magma layer is gravitationally unstable, but material interior to the Roche distance cannot fragment, and instead develops an effective viscosity. However, magma flowing across the Roche limit can fragment and accrete into moonlets, while magma spreading inward is accreted by Earth. As mass leaves the melt layer, it is replenished by vapor condensation, leading to a quasi steady state (QSS). The layer's mass is maintained at ~13% of a lunar mass and replaced every ~ 3.2 years. The vapor atmosphere steadily decreases, and once exhausted, the disk would cool below condensation temperature and spread as a time‐dependent disk until it too is exhausted. The timescale of the QSS is regulated by the disk's ability to radiate all the released latent heat plus viscous dissipation energy to space at the photospheric temperature, i.e., ~2000 K for silicon phase equilibrium. For the protolunar disk, latent heat dominates viscous heating, and the QSS for a ~2 lunar mass disk lasts for ~ 50 years. For comparison, a hypothetical water/steam disk orbiting an ice giant planet in which viscous heating dominates is also modeled. The photospheric temperature is closer to ~373 K, and the replacement time of the water layer is ~136 years. Finally, disks confined by external torques at either the planet‐disk boundary or at its outer edge are also briefly examined.

Evidence for a magma reservoir beneath the Taipei metropolis of Taiwan from both S-wave shadows and P-wave delays.

There are more than 7 million people living near the Tatun volcano group in northern Taiwan. For the safety of the Taipei metropolis, in particular, it has been debated for decades whether or not these volcanoes are active. Here I show evidence of a deep magma reservoir beneath the Taipei metropolis from both S-wave shadows and P-wave delays. The reservoir is probably composed of either a thin magma layer overlay or many molten sills within thick partially molten rocks. Assuming that 40% of the reservoir is partially molten, its total volume could be approximately 350 km3. The exact location and geometry of the magma reservoir will be obtained after dense seismic arrays are deployed in 2017–2020.

Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance

Magnetic anisotropy can provide important information about mineral fabrics, and thus magmatic processes, particularly when it is known how multiple mineral species contribute to the anisotropy. It may also affect the direction of induced or remanent magnetization, with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies. Here, we aim at describing the magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion and identifying their carriers. Anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the Bjerkreim Sokndal Layered Intrusion. The intrusion is characterized by magmatic layering and has a synform structure, with strong foliation on the limbs. Detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial, but the minimum susceptibility, and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering. The minimum susceptibility and anhysteretic remanence are associated with pyroxene (100) axes, and the maximum susceptibility and anhysteretic remanence are sub-parallel to the pyroxene [001] axes in layers MCU IVc and MCU IVe for which electron backscatter data are available. However, the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy. We propose that the magnetic anisotropy of magnetite-free specimens is carried by hemo-ilmenite exsolutions within pyroxene, in addition to pyroxene itself. When present, multi-domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence, due to shape-preferred orientation and distribution anisotropy. The orientation of the magnetic fabric appears independent of carrier, due to their common deformation history, but the degree of anisotropy is stronger for magnetite-bearing specimens. The results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection.

Evidence for underplating in the genesis of the Variscan synorogenic Beja Layered Gabbroic Sequence (Portugal) and related mesocratic rocks

The Beja Layered Gabbroic Sequence (LGS) is a mafic, layered synorogenic intrusion that was emplaced at the SW border of the Ossa Morena Zone (OMZ) at ca. 350Ma during the early stages of the Variscan oblique continental collision. The LGS represents the primitive member of the Beja Igneous Complex (BIC), which records part of an important Variscan magmatic event to the north of the SW Iberian suture that led to the formation of several igneous complexes. Although LGS primary magmatic features are well-preserved from post-crystallization tectono-metamorphic events, the magma chamber processes were influenced by the Variscan regional stress field which affected also the development of the magmatic layering and associated foliation.The baric evolution recorded in LGS (from 9 to 4kbar) is compatible with early crustal underplating of juvenile basaltic magma at the Moho. Mild lower crustal contamination resulted in Nb-Ta-Rb-Th depletion and higher REE fractionation compared to MORB. Contamination proceeded mainly by assimilation-fractional-crystallization (AFC) at final level of emplacement.The obtained results suggest that the spatially associated BIC mesocratic rocks are genetically related to LGS, both deriving from an equivalent mantle source. It is proposed that the underplating of basaltic magmas at the lower crust gave rise to a deep crustal hot zone. This allows BIC evolution to be explained as a single, long-lived magmatic event of progressive geochemical and lithological diversification due to the involvement of distinct crustal components. The most important components were incorporated at depth by the reworking of OMZ lower crustal rocks, with the involvement of middle crustal rocks later on, and of upper crustal contamination during the final stages of emplacement.

Recycled ejecta modulating Strombolian explosions

Two main end-members of eruptive regimes are identified from analyses of high-speed videos collected at Stromboli volcano (Italy), based on vent conditions: one where the vent is completely clogged by debris, and a second where the vent is open, without any cover. By detailing the vent processes for each regime, we provide the first account of how the presence of a cover affects eruptive dynamics compared to open-vent explosions. For clogged vents, explosion dynamics are controlled by the amount and grain size of the debris. Fine-grained covers are entirely removed by explosions, favouring the generation of fine ash plumes, while coarse-grained covers are only partially removed by the explosions, involving minor amounts of ash. In both fine- and coarse-grained cases, in-vent ground deformation of the debris reflect variations in the volumetric expansion of gas in the conduit, with rates of change of the deformation comparable to ground inflation related to pre-burst conduit pressurization. Eruptions involve the ejection of relatively slow and cold bombs and lapilli, and debris is observed to both fall back into the vent after each explosion and to gravitationally accumulate between explosions by rolling down the inner crater flanks to produce the cover itself. Part of this material may also contribute to the formation of a more degassed, crystallized and viscous magma layer at the top of the conduit. Conversely, open-vent explosions erupt with hotter pyroclasts, with higher exit velocity and with minor or no ash phase involved.

Seismic imaging across the Eastern Ghats Belt-Cuddapah Basin collisional zone, southern Indian Shield and possible geodynamic implications

The evolutionary nature of the area encompassing Cuddapah basin, Nellore Schist Belt and the Ongole domain of Eastern Ghats Belt (south India), which contains Paleo-Neoproterozoic imprints of thick sedimentation, intense magmatism, subduction/accretion and possible collision between eastern Dharwar craton and east Antarctica, is still being debated. An attempt has been made here to decipher the deep crustal structure and tectonics of this region, by reprocessing Deep Seismic Sounding data acquired earlier through the modeling of first arrival refraction and wide angle reflection travel times. The derived crustal seismic velocity structure reveals that the intracratonic Proterozoic Cuddapah basin containing only 4km thick sediment is bounded by two major faults. The Moho reaching fault detected on its eastern boundary, demarcates Cuddapah basin from the Nellore Schist Belt. Although a deep normal fault is mapped within the Nallamalai Fold Belt, but no thrusting signatures are apparent. Almost all the crustal layers beneath Nellore Schist Belt and the Ongole domain of Eastern Ghats Belt, show distinct eastward dipping trend conforming to an upthrusted feature, suggesting possible presence of a collisional suture. Besides, the areas lying east of Cuddapah Basin appear to be an accreted orogenic terrain, beneath which lower crust has upwarped substantially. The entire stretch of the studied region is underplated by unprecedently thick (∼20km) high velocity (7.0-7.4km/s) magma layer above the Moho, indicating strong crust-mantle thermal perturbation and massive subcrustal erosion. Further, an expression of a deep seated mantle thermal anomaly has also been found below the Parnapalle region of the SW Cuddapah basin beneath which deeper crustal layers have exhumed.

Magma plumbing system of the Aso-3 large pyroclastic eruption cycle at Aso volcano, Southwest Japan: Petrological constraint on the formation of a compositionally stratified magma chamber

Aso volcano has the largest caldera (18×25km in diameter) in the southwestern Japan Island Arc, and it formed as the result of four large (VEI=6–7) pyroclastic-eruption cycles. We study the penultimate large eruption cycle, the Aso-3 cycle, which occurred 123ka with an ejecta volume of more than 150km3. The processes in the pre-eruptive magma chamber and the magma genesis of the Aso-3 cycle were inferred from geological data, phenocryst chemistry, and whole-rock chemical and Sr-, Nd-, and Pb isotopic analyses of juvenile clasts. The geological and petrological data indicate that the pre-eruptive magma chamber was stratified compositionally into three layers: from top to bottom, silicic, intermediate, and mafic magma layers. The three magma layers had a uniform isotope composition, suggesting that all the magmas were generated from a single source. The silicic and intermediate magmas were not generated from the mafic magma by fractional crystallization. The silicic magma has higher Ni content (compatible element) than the mafic magma. This suggests that these magmas were produced by partial melting of the same mafic crust but with differing amounts of partial melting: the silicic magma was produced by a low degree of partial melting of the source rock without fractional crystallization, and the mafic magma was produced by a large degree of partial melting followed by fractional crystallization. The intermediate magma compositions plot on the tie line between the silicic magma and the melt of the mafic magma in variation diagrams, and the intermediate magma has phenocrysts whose compositions are identical with those in the silicic magma. This observation indicates that, before the Aso-3 eruption cycle, a two-layer stratified magma chamber of the silicic and mafic magmas was formed as a result of melting of the mafic crust, which was followed by formation of the intermediate layer as a result of interfacial mixing between the silicic magma and the melt of mafic magma. During the eruption the three-layer stratified magma chamber was tapped from the top to the bottom.

Reply to “Comment on Manuella et al. ‘The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm’ by Beccaluva et al. (2015)”

One of the most important pieces of background information left in our pen (Manuella et al. 2015) regards the circumstance that, during the last 25 years, international marine geology expeditions brought crucial advances in understanding the composition and tectonic evolution of present and fossil oceanic lithosphere (e.g., Pearce 2002; Dick et al. 2003; Boschi et al. 2006; Snow and Edmond 2007; Ildefonse et al. 2007; Miranda and Dilek 2010; Silantyev et al. 2011). In particular, we would draw attention to some fault-bounded abyssal highs, called oceanic core complexes (OCCs), located in the crest zone of (ultra) slow-spreading mid-ocean ridges. OCCs mostly consist of serpentinized mantle peridotites and gabbroic rocks exhumed to the ocean floor along systems of detachment faults, related to serpentinite diapirism. Most elevated blocks even reach the ocean surface to form non-volcanic ocean islands, as well as the St. Peter and St. Paul Rocks located near the axial zone of MAR in the equatorial region (e.g., Campos et al. 2010; Sharkov 2012). More in general, magmatic layers of the normal oceanic crust are very thin or even absent at OCCs sites, seismic profiles being compatible with a serpentinite layer overlying almost unaltered mantle ultramafics (e.g., Blackman et al. 2004a, b). In this respect, the concept of a “crust” had to be called into question, and hence, the Moho can be regarded as a serpentinization front (e.g., Minshull et al. 1998). Oxide-rich gabbros with sheared texture are considered obliged components of the gabbroic suite of present and fossil OCCs (e.g., Sharkov 2012). Veins of plagiogranites are also relatively common in these oceanic structures, intruding both gabbros and peridotite bodies. Oxide gabbros and plagiogranites from OCCs typically bear zircon as accessory phase (e.g., Aranovich et al. 2013). OCC basalts, Introduction

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48–56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼74 mm/yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼28–34 mm/yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10–30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10–25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC∼0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge faster than the crust. Active asthenospheric upwelling is one possible explanation for these conditions. The other possible interpretation is that lower crustal reflections are caused by magmatic (mafic/ultramafic) layering associated with accretion from a central mid-crustal magma chamber. Considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re-organizations associated with ridge jumps or rift propagation, we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle, and therefore to shearing in the ductile lower crust or, alternatively, that plate reorganization could produce magmatic pulses which may lead to mafic/ultramafic banding.

Building a 3D model of lithological contacts and near‐mine structures in the Kevitsa mining and exploration site, Northern Finland: constraints from 2D and 3D reflection seismic data

ABSTRACTThe Kevitsa mafic–ultramafic intrusion in Northern Finland hosts a large, disseminated nickel–copper sulphide ore body. The Kevitsa intrusion is an active mining and exploration site, for which we have built a 3D model of the main lithological contacts and near‐mine structures in the area. To build the 3D model, 2D and 3D reflection seismic data have been used together with borehole data and geological map of the area. The Kevitsa reflection seismic data reveal the internal architecture of the Kevitsa intrusion and the surrounding units. For example, the seismic data have uncovered a previously unknown, deeper continuation of the Kevitsa intrusion. Improved 3D knowledge of the basal contact of the intrusion provides an exploration target for contact‐type mineralization. Within the intrusion, a limited area of strong reflections is observed in the data. This has been associated with discontinuous, smaller‐scale magmatic layering that is thought to control the extent of the Kevitsa main mineralization. Thus, our 3D model of the extents of the internal reflectors can provide a framework for near‐mine and deep exploration of the main type of mineralization in the area. In addition to exploration, the original purpose of the 3D seismic survey was geotechnical planning of the Kevitsa open‐pit mine. Accordingly, the 3D seismic data were used to create a 3D model of the subsurface structures, with a focus on the vicinity of the mine. The interpreted structures reveal a complex pattern of fault and fracture zones, some of which will be important for slope stability and operational planning of the final stages of the mine.

Thermal origin of continental red beds in SE China: An experiment study

The origin of continental red beds in SE China is the result of high diagenetic temperatures, rather than an arid climate during their deposition. Here we present results from an experimental study where black mud was heated to demonstrate the formation of red beds. Diffuse reflectance spectroscopy (DRS) of heated samples enables determination of the relative proportion of goethite and hematite. Iron in black mud is predominantly in the form of goethite that has an initial dehydration temperature of ca.150°C. Increasing temperature or prolonged heating time is accompanied by decreasing goethite and organic content, increasing hematite and red colouration. Heat provided to subsiding red bed basins is supplied by cooling of an intracrustal granitic magma layer. The thermal model can explain vertical colour, temperature, redox and mineral zonation in red bed sequences, from red (hematite-bearing), through green-yellow (Cu, Zn, V sulphide mineralization) to grey-black (hydrocarbon, halite-bearing) sediments. The model can also be used to help prospect for hydrocarbon and halite deposits in the SE China red bed basins.

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

Imaging before an unimaginable eruption

Volcanology![Figure][1] Lake Toba, northern SumatraCREDIT: BARRY KUSUMA/GETTY IMAGES Volcanic super-eruptions eject thousands of times the volume of the largest documented eruptions over human history. Several “supervolcanoes” capable of this type of unimaginable devastation dot the surface of Earth today. Jaxybulatov et al. use seismic background noise to estimate the size and maturity of one of the world's largest volcanic reservoirs, the Toba caldera in northern Sumatra. Magma storage under the caldera occurs slowly over time, in the form of horizontal layers of magma injected into the crust. These magmatic sills are documented to 20 kilometers below the surface, but more sills could exist even deeper. The characterization of magma storage in large volcanic systems may help us to prepare for future volcanic super-eruptions. Science , this issue p. [617][2] [1]: pending:yes [2]: /lookup/volpage/346/617?iss=6209

On the evolution of the protolunar disc.

The structure and viscous evolution of a post-impact, protolunar disc is examined. The equations for a silicate disc in two-phase (vapour-liquid) equilibrium are employed to derive an analytical solution to vertical structure. Both a vertically mixed phase disc and a stratified disc, where a magma layer exists in the mid-plane surrounded by a vapour reservoir, are considered. The former largely reproduces the low gas mass fraction, x≪1, profiles of the disc described in earlier literature that proposed that the disc would hover on the brink of gravitational instability. In the latter, the vapour layer has x∼1 and is generally gravitationally stable, while the magma layer is vigorously unstable. The viscous evolution of the stratified model is then explored. Initially, the disc quickly settles to a quasi-steady state with a vapour reservoir containing the majority of the disc mass. The magma layer viscously spreads on a time scale of approximately 3-4 years, during which vapour continuously condenses into droplets that settle to the mid-plane, maintaining the magma surface density in spite of disc spreading. Material flowing inwards is accreted by the Earth; material flowing outwards past the Roche boundary can become incorporated into accreting moonlets. This evolution persists until the vapour reservoir is depleted in approximately 50-100 years, depending on its initial mass.

The intensity of the geomagnetic field from 2.4 Ga old Indian dykes

AbstractPrecambrian paleomagnetic records from dyke swarms provide a unique source of information regarding the Archean geomagnetic field and more specifically the average field strength produced by the early dynamo. We sampled 16 paleomagnetic sites from the Dharwar giant dyke swarm in southern India which was emplaced between 2.365 and 2.368 Ga. Despite taking great care in selecting locations exempt of any geological disturbance, only two of these sites provided primary directions with very steep inclinations and therefore were emplaced in close to a magnetic pole. Paleointensity experiments were conducted on a subset of samples from the dyke margins. The characteristic magnetization is carried by single domain magnetite grains with a very narrow range of unblocking temperatures inferred from the sharp decrease by at least 75% of their remanence above 520°C. The paleointensity results indicate an average low field of 9.2 ± 7 µT, consistent with reported values from Canadian dyke swarms for the same period. These results combined with the Thellier‐Thellier determinations obtained so far for the Precambrian suggest that a low field period prevailed from circa 2.3 to 1.8 Ga, while the preceding and following time intervals are characterized by significantly stronger paleointensities. Although this suite of episodes is not fully incompatible with previous models for the long‐term evolution of the geodynamo, it is tempting to make the link with the recent suggestion of an early dynamo sustained within a conductive magma layer at the base of the mantle from 3.5 to 2.5 Ga which progressively declined until convection became sufficiently efficient to reactivate a strong dynamo process within the Earth's liquid core.

Trace element compositions of apatite from the middle zone of the Panzhihua layered intrusion, SW China: Insights into the differentiation of a P- and Si-rich melt

Abstract The Panzhihua layered intrusion in the ~ 260 Ma Emeishan large igneous province is composed of melagabbro and Fe–Ti oxide ore bodies in the lower zone (LZ) and the lower part of the middle zone (MZa), and Fe–Ti oxide-poor leucogabbro in the upper part of the middle zone (MZb) and upper zone (UZ). Cumulus apatite grains occur in the ~ 500- to 600-m-thick MZb, which makes up 25–30% of the ~ 2-km-thick intrusion. Apatite grains from the MZb show two compositional reversals in the composition of Sr, which divide the MZb into three sub-units from the base upwards, MZb1, MZb2 and MZb3. There is 1–3 vol.% apatite in the MZb1 and MZb2 and 2–5 vol.% apatite in the MZb3. Both apatite and plagioclase have an overall trend of decreasing Sr in each sub-unit. Most apatite grains from the MZb1 and MZb2 have negative Eu anomalies (Eu/Eu* = 0.70–0.98) on chondrite-normalized REE plots and some at the top of the MZb2 have positive Eu anomalies (Eu/Eu* = 1.09–1.18), whereas all grains from the MZb3 have positive Eu anomalies (Eu/Eu* = 1.11–1.25). We consider that the Panzhihua intrusion formed due to immiscibility of ferrobasaltic magmas in a large convection cell at high temperatures. The immiscible Fe-rich melt tended to move towards the base of the chamber, whereas the Si-rich melt moved upwards due to density differences. Crystallization of Fe–Ti oxides from the Fe-rich melt at high temperatures may result in the enrichment of P in the residual magmas. The upward moving residual P-rich magmas may have mixed with Si-rich melt to form a P- and Si-rich melt in the upper part of the chamber, from which the MZb formed. Double-diffusive convection circulated in the P- and Si-rich melt to form stratified magma layers. Magma mixing between the stratified magma layers resulted in the compositional reversals of apatite along the boundaries. Negative Eu anomaly of apatite in the MZb1 and MZb2 is attributed to prior crystallization of plagioclase, whereas replenishment of a syenitic magma to the MZb3 may result in the positive Eu anomaly of apatite in the MZb3. The immiscibility model can explain why the apatite-rich MZb is above the major Fe–Ti oxide layers in the Panzhihua intrusion.

The Formation of Low-Volume, High-Tenor Magmatic PGE-Au Sulfide Mineralization in Closed Systems: Evidence from Precious and Base Metal Geochemistry of the Platinova Reef, Skaergaard Intrusion, East Greenland

The majority of magmatic platinum-group element (PGE) deposits in layered mafic/ultramafic intrusions are characterized by visible Cu-Ni Sulfide mineralization, often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination. The Platinova Reef, hosted by the 55 Ma Skaergaard intrusion, East Greenland, is an example of a rarer type of PGE deposit, typified by a Pd-Au-Cu-dominant, Ni-Pt-poor, Sulfide assemblage present in the upper parts of the host intrusion. Such deposits are generally considered to form through prolonged fractional crystallization of the magma, with magnetite crystallization playing a role in late-stage S saturation. In the case of the Skargaard intrusion, the crystallization of the cumulate rocks, and the generation of the mineralization hosted within them, occurred within a closed system, from a single, homogenized batch of magma. The Platinova Reef is characterized by offsets in the stratigraphic position of individual metal peaks; an anomalously low Sulfide content ( 1 ppm), the Intermediate zone (Pd 0.1–1 ppm), the Au zone (Au >1 ppm), and the upper Cu zone (Cu >200 ppm with precious metals 3 to 10 4 ppm, low Cu/Pd ratios, and a remarkable upward decrease in Pd/Au ratio from 40 to 1, whereas the sulfides in the Cu zone are lower tenor (10 3 ppm Pd), higher volume, and have high Cu/Pd ratios and Pd/Au To explain these features, we present a multistage model of preconcentration, dissolution upgrading, and further metal enrichment. This involves initial S saturation of the Skaergaard magma at a late stage in the crystallization history. The Sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter, Fe-rich and S-undersaturated magma at the bottom of the chamber, producing a highly metal rich magma layer at the bottom of the chamber. This layer then became S saturated as a result of continued magnetite crystallization, with the tiny sufide droplets that segregated into this precious metal-enriched magma layer becoming highly enriched in PGE themselves. These were then trapped in situ by the crystallizing cumulate pile forming the Subzone, Pd zone and Intermediate zone. sulfides formed in the rest of the chamber, which was PGE depleted at this stage would have been of low tenor. These Sulfide droplets grew and settled, concentrating any remaining Au when they encounter the floor to form the Au zone and formed the low tenor sulfides of the Cu zone above this. We propose that the Platinova Reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through Sulfide dissolution upgrading in the latter stage of crystallization of a closed system. The offsets in individual metal peaks mineralization demonstrate the relative partitioning of Pd(+Pt) > Au > Cu into a Sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low-volume magmatic Sulfide melts. This feature can lead to the characteristic stratigraphic offsets in peak metal abundances. The role of magnetite crystallization appears to be an important control on S saturation in such systems, and thus may be an important indicator of the location of mineralized horizons. In addition, in such deposits, indicators of crustal contamination by external S and multiple injections of magma, common features of more Ni rich PGE reefs, are less likely to be present.

Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

We estimated time scales of magma-mixing processes just prior to the 2011 sub-Plinian eruptions of Shinmoedake volcano to investigate the mechanisms of the triggering processes of these eruptions. The sequence of these eruptions serves as an ideal example to investigate eruption mechanisms because the available geophysical and petrological observations can be combined for interpretation of magmatic processes. The eruptive products were mainly phenocryst-rich (28 vol%) andesitic pumice (SiO2 57 wt%) with a small amount of more silicic pumice (SiO2 62–63 wt%) and banded pumice. These pumices were formed by mixing of low-temperature mushy silicic magma (dacite) and high-temperature mafic magma (basalt or basaltic andesite). We calculated the time scales on the basis of zoning analysis of magnetite phenocrysts and diffusion calculations, and we compared the derived time scales with those of volcanic inflation/deflation observations. The magnetite data revealed that a significant mixing process (mixing I) occurred 0.4 to 3 days before the eruptions (pre-eruptive mixing) and likely triggered the eruptions. This mixing process was not accompanied by significant crustal deformation, indicating that the process was not accompanied by a significant change in volume of the magma chamber. We propose magmatic overturn or melt accumulation within the magma chamber as a possible process. A subordinate mixing process (mixing II) also occurred only several hours before the eruptions, likely during magma ascent (syn-eruptive mixing). However, we interpret mafic injection to have begun more than several tens of days prior to mixing I, likely occurring with the beginning of the inflation (December 2009). The injection did not instantaneously cause an eruption but could have resulted in stable stratified magma layers to form a hybrid andesitic magma (mobile layer). This hybrid andesite then formed the main eruptive component of the 2011 eruptions of Shinmoedake.

The Earth Expansion Generated the Asthenosphere and Its Formation Time

The Earth Expansion Generated the Asthenosphere and Its Formation Time