Plume Head Research Articles

Zircon Xenocrysts From Easter Island (Rapa Nui) Reveal Hotspot Activity Since the Middle Jurassic

AbstractWe report the finding of mantle‐derived zircon grains retrieved from red soils, regoliths, and beach sands from Easter Island, that are much older than the island volcanism (0–2.5 Ma) and its underlying lithosphere (Pliocene, 3–4.8 Ma). A large population of 0–165 Ma old zircons have coherent oxygen (δ18O 3.8–5.9‰) and hafnium (εHf(t)+3.5–+12.5) mantle isotopic signatures. These results are consistent with the crystallization of zircon from plume‐related melts. In addition, a chemically diverse population with ages from the Paleozoic to the Archean was found. These older populations are enigmatic but they could represent remnants of ancient subducted sediments. Meanwhile, the ∼0–165 Ma population is interpreted as plume‐derived, suggesting that the hotspot started at least ∼165 Ma ago. A spike of ∼164–160 Ma zircons could represent a Large Igneous Province (LIP) stage upon the first arrival of the plume. We use plate reconstructions to show that such a LIP would have formed on the Phoenix Plate and would have subducted below the Antarctic Peninsula around 100–105 Ma. There, LIP subduction would offer a solution for the enigmatic Palmer Land deformation event, previously proposed to result from a collision with an unknown indenter. The here‐reported “ghost” of a prolonged hotspot activity suggests that fragments of the Easter plume and of the ambient sub‐lithospheric mantle stored and re‐sampled zircon xenocrysts due to convective (re)circulation at the scale of the plume head. Our study demonstrates how zircon geochronology and geochemistry provide novel insights into global‐scale geodynamics, offering new perspectives on the dynamics of mantle plumes and hotspot activity.

Anatomy of the Emeishan Mantle Plume Head: Insights From New Geochronologic, Geochemical, and Geologic Data

AbstractThe link between mantle plumes and the formation of large igneous provinces (LIPs) is well established although the anatomy of these remains equivocal. Recent experimental studies and geophysical data suggest that the mantle plume head is more likely to be irregular and asymmetric, rather than an axisymmetric flattened disk. The Emeishan large igneous province (ELIP) provides a unique opportunity to test this hypothesis. According to robust petrographic, geochronologic, and geochemical evidence from the late Permian basalts in the Sichuan Basin, and in conjunction with a comprehensive compilation of geologic maps and published geochemical data from the ELIP, we identified several giant radial “fingering” structures. Based on the shallow mantle source from the center to margin in the ELIP and relief of the lithosphere‐asthenosphere boundary, we propose a new mantle plume model to explain the evolution of the Emeishan plume periphery, where narrow finger‐like protrusions and plumelets developed outwards from the main body of the plume to the edges of the flattened plume head. Dragged fingers might have been torn apart into some plumelets, which dispersed and were trapped beneath the thinnest lithosphere relief, and eventually erupted to form small‐scale flood basalt in the Outer Zone of the ELIP.

Translithospheric magma plumbing system fossilized in the Emeishan large igneous province

Lighting up the magma plumbing system beneath a large igneous province (LIP) is challenging because the complex magma migration paths are often covered by flood basalts and sediments. Here, we present a three-dimensional seismic image of the Permian Emeishan LIP in Southwest China, constructed by joint inversion of Rayleigh wave dispersions and receiver functions. The results outline a cylindrical, high-velocity anomaly extending to ~135 km depth below the inner zone of this continental LIP. The geometry and magnitude of the high-velocity anomaly suggest that it represents culminated crystallized materials of primary magmas, thereby mirroring a magma plumbing system fossilized in the lithosphere. Furthermore, our geodynamic models illustrate that the nearly vertical plumbing system was controlled by slow plate motion during the magma emplacement. The plume head beneath a nearly static plate has higher thermal buoyancy and thus promotes more intensive magma emplacement. This phenomenon may apply to other LIPs throughout Earth’s history

Dynamics of odor-source localization: Insights from real-time odor plume recordings and head-motion tracking in freely moving mice.

Animals navigating turbulent odor plumes exhibit a rich variety of behaviors, and employ efficient strategies to locate odor sources. A growing body of literature has started to probe this complex task of localizing airborne odor sources in walking mammals to further our understanding of neural encoding and decoding of naturalistic sensory stimuli. However, correlating the intermittent olfactory information with behavior has remained a long-standing challenge due to the stochastic nature of the odor stimulus. We recently reported a method to record real-time olfactory information available to freely moving mice during odor-guided navigation, hence overcoming that challenge. Here we combine our odor-recording method with head-motion tracking to establish correlations between plume encounters and head movements. We show that mice exhibit robust head-pitch motions in the 5-14Hz range during an odor-guided navigation task, and that these head motions are modulated by plume encounters. Furthermore, mice reduce their angles with respect to the source upon plume contact. Head motions may thus be an important part of the sensorimotor behavioral repertoire during naturalistic odor-source localization.

Intraplate alkali basalts related to end-Cretaceous Deccan magmatism: implications for tectonomagmatic processes

This study aims to investigate the earliest imprint of Deccan rift magmatism as preserved in alkali basalts from the northwestern Indian shield. The alkali basalts are petrographically classified as nephelinites and basanites. They are silica undersaturated and their high Mg numbers, CaO/Al 2 O 3 , Cr and Ni indicate their primitive character. Geochemically, they are similar to global ocean island basalts; their bulk rock trace elements and Sr–Nd–Hf isotopic signatures suggest their derivation from a garnet-bearing peridotite field. However, their elevated values of Sr/Sm, Sm/Hf, Zr/Hf, Nb/Ta; positive Ba, Sr and negative Zr, Hf spikes suggest that the magma source represents a mixture of garnet peridotites and carbonated melts. Estimated primary melt compositions closely follow the trajectory defined by the high-pressure experimental partial melting trend of a low-carbonated peridotite source. The melting environment approximates to a high mantle potential temperature. A low 87 Sr/ 86 Sr ratio and a negative correlation between 176 Hf/ 177 Hf and 143 Nd/ 144 Nd of the alkali basalts suggest that the mantle source is a mixture of a depleted Indian MORB-type mantle and an enriched mantle type 2. We correlate this event with the melting of the leading edge of the Réunion plume head during Gondwana break-up in a relatively short span of the Cretaceous/Paleogene boundary.

Prolonged Multi‐Phase Magmatism Due To Plume‐Lithosphere Interaction as Applied to the High Arctic Large Igneous Province

AbstractThe widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view that large igneous provinces (LIPs) originate from melting in plume heads. Hence, the suggested plume‐related origin and classification of HALIP as a LIP have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with lithosphere of variable thickness, that is, a basin‐to‐craton setting applicable to the Arctic. Models reveal that melt migration introduces significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30–40 Myr, because of the dynamic feedback between migrating melt and local lithosphere thinning. For HALIP, plume material deflected from underneath the Greenland craton can re‐activate melting zones below the previously plume‐influenced Sverdrup Basin after a melt‐free period of about 10–15 Myr, even though the plume is already ∼500 km away. Hence, actively melting zones do not necessarily represent the location of the deeper plume stem at a given time, especially for secondary pulses. Additional processes such as (minor) plume flux variations or local lithospheric extension may alter the timing and volume of HALIP pulses, but are to first order not required to reproduce the long‐lived and multi‐pulse magmatism of HALIP. Since melting zones are always plume‐fed, we would expect HALIP magmatism to exhibit plume‐related trace element signatures throughout time, potentially shifting from mostly tholeiitic toward more alkalic compositions.

Modeling Lobe‐And‐Cleft Instabilities on a River Plume

AbstractThe lobe‐and‐cleft instability is a widely recognized mechanism leading to along‐front structure on density current fronts. Early studies based on laboratory and numerical simulations suggested that the lobe‐and‐cleft instability is due to convective instability in the nose of gravity currents traveling over a nonslip boundary. Horner‐Devine and Chickadel (2017, https://doi.org/10.1002/2017gl072997) reported the presence of lobe‐and‐cleft instabilities at the Merrimack River, which are generated at the river front in the absence of a no‐slip boundary. Hence, the observed lobe‐and‐cleft instabilities must be due to other mechanisms. In this study, we carried out non‐hydrostatic large eddy simulations of a riverine outflow into an idealized 3D domain. With a fine grid resolution of 0.15 × 0.31 m in two horizontal directions and about 0.125 m in the vertical direction, the model reproduced the lobe‐and‐cleft feature, with the magnitude and size of lobes consistent with the field observation. The model results revealed that instabilities start from the primary Kelvin‐Helmholtz instability, followed by the secondary instability through stretching and tilting, generating counter‐rotating streamwise vortices in the plume and at the plume head. The upwelling associated with streamwise vortex cells brings a slower flow to the plume surface, resulting in lobe‐and‐cleft patterns at the front and positive and negative vertical vorticity at the plume surface. The model also predicted a lobe width of about two to three times the plume thickness, consistent with the field observation and the lobe/cleft spacing associated with pairs of counter‐rotating streamwise vortices. Modeled turbulent dissipation rate shows a trend of exponential decay from 10−4 to 10−3 m2/s3 at the frontal head to 10−7 to 10−6 m2/s3 behind the front, similar to the findings in the previous field studies.

Heng-o Corona, Venus: Dyke swarms record evolution of its underlying mantle plume

Coronae are enigmatic, quasi-circular/elliptical tectono-magmatic features, common across the surface of Venus ranging in size from 60 to 2600 km This study undertook a detailed mapping (1:500,000 scale) of Heng-o Corona, the second largest on Venus. 24,600 graben-fissure lineaments were mapped, interpreted to be underlain by mafic dykes, and grouped into radiating, circumferential and linear swarms. A series of 5 nearly-concentric rings were identified from unflooded or partially flooded circumferential swarms. The largest of these has a diameter of 1970 km, which almost doubles Heng-o's known size (previously 1060 km based on its topographic annulus). This new diameter also narrows the gap between it and Artemis Corona, the largest on Venus, to only 630 km. Three major radiating graben swarms were identified, extending >700–1000 km from their sources within the central annulus. From cross-cutting relationships, we interpreted that the major radiating swarms formed first, upon initial plume arrival and flattening against the base of the lithosphere. The circumferential swarms followed, starting with the innermost ring, migrating with the leading edge of the plume head as it spread outward against the base of the lithosphere. The distinctive annular ridge/trough system (at about 500 km radius) is associated with the transition between plume head arrival and spreading. The centers of most of the radiating and circumferential swarms are aligned along a NNW-SSE trend that is interpreted to represent an underlying zone of lithospheric weakness. Additional graben-fissure systems in the study area are not associated with Heng-o Corona; some are linked with other magmatic centers nearly up to 1000 km outside the study area.

The Zircon U-Pb Age, Hf Isotopes, and Lithogeochemistry of Ore-Bearing Rocks from the Archean Hongtoushan Volcanogenic Massive Sulfide Deposit in the North China Craton: Implications for Tectonic Setting

Volcanogenic massive sulfide (VMS) deposits are globally significant sources of metals. The Hongtoushan VMS deposit is the only large Archean Cu-Zn VMS deposit in the North China Craton, carrying substantial economic value. Significant deformation and metamorphism have made the tectonic setting of the Hongtoushan VMS deposit the subject of extensive debate. This study investigates the petrogenesis and chronology of the ore-bearing host rocks from the Hongtoushan Cu-Zn VMS deposit in the North China Craton. By utilizing whole-rock geochemical analyses and zircon dating, this research sheds light on the origin and evolution of the ore-bearing rocks within the deposit. The whole-rock geochemical analysis data indicate that the Hongtoushan ore-bearing rock series is mainly composed of amphibole plagioclase gneiss (basalt protolith) and biotite plagioclase gneiss (andesite and rhyolite protolith), suggesting a complete volcanic cycle from basic to medium-acidic volcanic rocks. The amphibole plagioclase gneiss has slight LREE enrichment patterns with unremarkable depletions of Nb, Ta, and Ti and belongs to contaminated ocean plateau basalt (OPB) in terms of composition, which is generally interpreted as being generated from the mantle plume head. Meanwhile, the biotite plagioclase gneiss has relatively steep LREE enrichment distribution patterns with remarkable negative Ta, Nb, and Ti anomalies and a wide range of Zr/Y ratios, indicating their classification as FI- and FII-type felsic rocks; they were likely formed through the fractional crystallization of basic magma combined with crustal assimilation. When combined with the zircon dating results, the ore-bearing host rocks of the Hongtoushan VMS deposit were generated via a continuous magmatic evolution process. The zircon dating of the host rocks indicates a formation age of between 2609 and 2503 Ma, with metamorphic events between 2540 and 2466 Ma, which is consistent with the 2.5 Ga-related global mantle plume event. Further research shows that the ore-bearing host rocks are more likely to have been formed in a mantle plume-related stretching environment, possibly a margin rift.

Experimental and numerical investigation of the effect of deep-sea mining vehicles on the discharge plumes

During mining activities, deep-sea mining vehicles (DSMVs) are highly susceptible to causing massive disturbance to the seafloor sediment, resulting in the formation of plumes due to underlying turbulence and currents. To gain a better understanding of the dispersion mechanism of sediment plumes, both experimental and numerical methods were employed. The numerical model was primarily used to characterize the solidity and liquidity of the sediment plume through volume of fluid and discrete phase model methods, respectively. The experimental data were validated against the numerical results. The plume distribution was studied in physical experiments for three different DSMV parameters. The study findings indicate that the discharge of the plume in the near field occurs in three stages due to a combination of plume release inertial forces, negative buoyancy in the water column, and wall restoring forces. Additionally, the increase in the travel velocity of the DSMV reduces the propagation of the plume in the direction of discharge and instead increases its lateral spread across the bottom surface. As the size of the DSMV decreases in three dimensions, changes in the vertical vortex structure become dominant in the plume distribution. This leads to a reduction in the length of the plume head and a faster sinking of the plume. When the wake Froude number Frw is between 0.7 and 6.8, representing the wake turbulence effect of DSMV on plume discharge, the diffusion width of the plume on the bottom surface is linearly related to Frw.

Far-travelled 3700 km lateral magma propagation just below the surface of Venus

The Great Dyke of Atla Regio (GDAR) is traced for ~3700 km on Venus, as a surface graben (narrow trough) interpreted to overlie a continuous laterally-emplaced underlying mafic dyke (vertical magma-filled crack). The GDAR belongs to a giant radiating dyke swarm associated with Ozza Mons (volcano), Atla Regio plume, and was fed from a magma reservoir ~600 km south of the Ozza Mons centre. A 50-degree counter-clockwise swing of the GDAR at 1200 km from the centre is consistent with a 1200 km radius for the underlying Ozza Mons plume head, and a stress link to the 10,000 km long Parga Chasmata rift system. Our discovery of the GDAR, should spur the search for additional long continuous single dykes on Venus (and Earth), with implications for estimating plume head size, locating buffered magma reservoirs, mapping regional stress variation at a geological instant, and revealing relative ages (through cross-cutting relationships) over regional-scale distances.

PLUME-GENERATED 90° STRESS CHANGE LINKED TO TRANSITION FROM RADIATING TO CIRCUMFERENTIAL DOLERITE DIKE SWARMS OF THE SIBERIAN TRAPS LARGE IGNEOUS PROVINCE AND TO EMPLACEMENT OF THE NORILSK-TALNAKH ORE DEPOSITS

Abstract A 90° change in stress orientation has been previously proposed as the trigger for the final emplacement of the world-class Norilsk-Talnakh magmatic sulfide mineralization via the migration of accumulated sulfide melts from elsewhere within the plumbing system of the Siberian Traps large igneous province (LIP). We propose that this stress change does not require and was not triggered by a distal change in plate boundary stresses, but instead can be explained both temporally and spatially by stress changes recorded in the dike swarm patterns of the Siberian Traps LIP, namely the transition from a giant radiating dike swarm (associated with mantle plume uplift) to a giant circumferential swarm (linked to flattening of the plume head). The mantle plume stress-related changes recorded by these dike swarms, rather than distal plate boundary stress changes, were therefore most likely the trigger for the emplacement of the Norilsk-Talnakh mineralization. Other LIPs that have both giant radiating and circumferential dike swarms most likely reflect similar major and rapid changes in stress orientation, indicating that mantle plume-induced stress changes revealed by dike swarms should be considered an additional tool in magmatic sulfide exploration.

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Dynamics of odor-source localization: Insights from real-time odor plume recordings and head-motion tracking in freely moving mice.

Animals navigating turbulent odor plumes exhibit a rich variety of behaviors, and employ efficient strategies to locate odor sources. A growing body of literature has started to probe this complex task of localizing airborne odor sources in walking mammals to further our understanding of neural encoding and decoding of naturalistic sensory stimuli. However, correlating the intermittent olfactory information with behavior has remained a long-standing challenge due to the stochastic nature of the odor stimulus. We recently reported a method to record real-time olfactory information available to freely moving mice during odor-guided navigation, hence overcoming that challenge. Here we combine our odor-recording method with head-motion tracking to establish correlations between plume encounters and head movements. We show that mice exhibit robust head-pitch motions in the 5-14Hz range during an odor-guided navigation task, and that these head motions are modulated by plume encounters. Furthermore, mice reduce their angles with respect to the source upon plume contact. Head motions may thus be an important part of the sensorimotor behavioral repertoire during naturalistic odor-source localization.

MODELS FOR THE FORMATION OF POLYPHASE GABBRO-MONZODIORITE MASSIFS OF THE WESTERN SANGILEN IN THE COLLISIONAL AND TRANSTENSIONAL-SHEAR SETTINGS

A model for the formation of intrusions of the collision stage of 525–490 Ma and a model of magmatism of the transtensional shear stage of 465–440 Ma within the Mugur-Chinchilig and Erzin-Naryn blocks of Western Sangilen (Tuva) have been developed to describe the process of crust-mantle interaction. Model experiments confirm petrological data on the presence of multi-level chambers during the formation of the Pravotarlashkinsky and Bashkymugur massifs. The proposed model describes the migration of mantle magmas above the head of the mantle plume at the collision stage and assumes the rise of magmas along a permeable tectonic zone in the mantle lithosphere and crust at the transtensional-shear stage. The modeling results allow us to establish that material from the magma chamber can reach depths of the upper crust in the volume ratio of gabbroids to diorites from 1 : 2 to 3 : 4 and additionally introduce about 5 % of the volume fraction of lower crustal material.The physical parameters of the primary magmas (viscosity, solidus and liquidus temperatures, degree of melting depending on temperature and composition, change in density) were calculated taking into account the real geochemical characteristics of igneous rocks from the polyphase massifs of Western Sangilen.

Dyke swarm history in Belet-ili and Gaia (Gaya) Coronae Region (13°–26°E, 0°–10°N), Central Eistla Regio, Venus

Eistla Regio, Venus, is a locus of major plume-generated magmatism and tectonics. We focus on the central portion, termed Central Eistla Regio. More than 19,000 extensional lineaments (grabens, fissures and fractures) were mapped in the area 13°-26°E, 0°-10°N, at 1:500,000 scale using full resolution Magellan SAR (radar) images. These structures are interpreted to represent the surface expression of underlying mafic dykes and have been grouped, on the basis of trend, into radiating, circumferential and linear dyke swarms. Thirty-two swarms are identified and linked to 20 magmatic centres, most of which are located within or near two coronae, Belet-ili and Gaia (Gaya) coronae, while others are linked to magmatic centres outside the study area. These magmatic centres are interpreted to be linked to underlying mantle plumes, and the sizes of the underlying flattened plume heads are estimated from the interpreted dyke swarms: for radiating swarms, the radius of the transition from proximal radiating to more distal linear pattern, and for circumferential dykes, the radial distance to the outer boundary of the circumferential pattern. Plume heads for the largest centres, Belet-ili and Gaia coronae are estimated at about 230 km and 275 km in radius, respectively. These sizes are less than the 1200 km radii predicted for a plume arriving from the deep mantle, and would suggest an origin at a shallower level (likely a mid-mantle boundary, i.e. between upper and lower mantle). The NW alignment of Belet-ili and Gaia corona and other magmatic centres in our study area potentially represents structural control by a rift zone, which can be traced further north to Anala Mons (one of two main plume centres of Central Eistla Regio).

The Corrugated Hills Continental Flood Basalt: a c. 2.17 Ga flood basalt province related to breakup of the Superior craton

Abstract This study documents a major continental flood basalt province in the central Labrador Trough that produced voluminous mafic magmatism along the palaeo-rifted margin of the Superior craton. This area, representing a preserved magma volume of approximately 25 000 km 3 , is characterized by classic ‘traps’ topography with columnar-jointed flows and sills separated by thin clastic sedimentary layers. A gabbroic sill near the bottom of the sequence yielded a U–Pb zircon age of 2166 ± 4 Ma, and another near the middle of the sequence yielded a 2171 ± 3 Ma U–Pb zircon age. These ages overlap in time with published ages of mafic dyke swarms that radiate outwards into the adjacent Superior craton, namely the Biscotasing (2172–2167 Ma) and Payne River (2170–2160 Ma) dykes, as well as the Otish sills (2172–2162 Ma) and Tasiataq (2170 Ma) dyke. Our data suggest that these central Labrador magmas, named the Corrugated Hills Continental Flood Basalt, were emplaced near or above a mantle plume head, leading to thermal uplift and then rifting of the Superior craton margin. Field relationships suggest that these magmas were erupted through thinned Archean crust as a rift-related continental flood basalt suite. The Corrugated Hills Continental Flood Basalt province represents remnants of one of the oldest continental flood basalt provinces on Earth.

Vapor-induced flow and its impact on powder entrainment in laser powder bed fusion

A 2D axisymmetric transient Thermal-Fluid-Evaporation model coupled with melt pool dynamics and gas kinetics is developed to study the formation mechanisms of vapor-induced flow and the resulting powder entrainment in powder bed fusion using laser beam (PBF-LB) for 316 L powders. The interactions between keyhole formation inside the melt pool, vapor plume flow, and vapor-induced shielding gas flow are investigated. Vapor plume flow results in powder spattering with much higher speed, while vapor-induced gas flow significantly contributes to powder denudation with lower speed. It is also reported that powder spattering is stronger in 1 atm argon than that in 1 atm helium because the drag force for spattering is 2.72 times larger in 1 atm argon, but powder denudation becomes greater in 1 atm helium as the ratio of drag force for denudation in 1 atm argon to that in 1 atm helium is only 0.582. Furthermore, the vapor plume results in more spatters with the decrease of ambient pressure from 1 atm to 0.05 atm in argon because the plume is diluted faster with a twofold wider plume head and the two times higher peak velocity as a result of the pressure drop-induced significant reduction of viscosity restriction. A larger divergency angle in 0.05 atm argon is also recorded at the same time for the weaker restriction and faster dilution. In combination with in-situ observations, the proposed model provides insights into the vapor-induced flow, and its impact on powder entrainment under different gas types and ambient pressures.

Dyke swarms record the plume stage evolution of the Atla Regio superplume on Venus

Atla Regio, Venus, is interpreted as a young major mantle plume centre, and we address whether it is at plume head or plume tail stage. Our approach uses graben-fissure lineaments, interpreted as the surface expression of dykes. Mapping > 40,000 such lineaments reveals giant radiating dyke swarms associated with major volcanic centres of Maat (>1500 km dyke swarm radius), Ozza (>2000 km), Ongwuti (>1100 km) and Unnamed montes (>1100 km), indicating that each is due to plume head magmatism rather than plume tail magmatism (maximum swarm length ~ 100 km). The size of an underlying flattened plume head is estimated by the radius where the swarm transitions from a radiating to linear pattern. All four centres and their plume heads group within the 1200 km radius of the Ozza Mons plume head, consistent with a single event. Atla Regio is at the plume head stage with coeval triple-junction rifting, which on Earth would typically precede attempted continental breakup.

Magnetotelluric Imaging of the Lithospheric Structure of the Southern Oklahoma Aulacogen: Evidence for Long‐Term Weakening Caused by Rifting

AbstractMagnetotelluric data were used to study the lithosphere structure of the Southern Oklahoma Aulacogen (SOA). Inversion of the data revealed two low resistivity anomalies beneath the SOA. The first is located in the depth range 0–90 km in the crust and upper lithospheric mantle. The second extends from a depth 100 km to the base of the lithospheric mantle and extends away from the SOA to the ends of the profile. The cause of low resistivity anomalies is discussed in relation to the tectonic evolution of the region and recent laboratory experiments on rock conductivity. The first anomaly is attributed to the combination of (a) water present in mantle minerals and (b) the formation of hydrous mineral phases by interactions between a plume and the lithosphere during rifting. Grain size reduction and fabric alignment from deformation during the Ancestral Rocky Mountain (ARM) orogeny may have also contributed to the low resistivity. This enrichment phase may have mechanically weakened the lithosphere and allowed deformation to occur during the ARM orogeny. The low resistivity of the deeper anomaly is attributed to a fluorine‐enriched phlogopite layer that is also coincident with an observed seismic mid‐lithosphere discontinuity (MLD). A lithosphere keel of mantle minerals enriched in water underlies this layer and may have formed by accretion of the plume head to the lower lithosphere after rifting, which also rethickened the lithosphere to its present‐day depths. The MLD may then reflect a melt layer along a paleo lithosphere‐asthenosphere boundary entombed during the accretion.