Hotspot Volcanoes Research Articles

Deep Low‐Frequency Earthquake Reveals Unsteady Fluid Flow Beneath Tengchong Volcano Field in Southeast Tibet

AbstractDeep low‐frequency earthquakes (DLFE) are observed beneath volcanoes worldwide but are limited to island arc volcanoes, hotspot volcanoes, and rift zones. Here we show DLFEs in the Tengchong Volcano Field, southeast Tibet, located ∼300 km from the Indo‐Burma volcanic arc, by analyzing a 12‐year continuous seismic data set. The earthquakes were at a depth of ∼12 km, near the sidewall of the magma body detected by the magnetotelluric survey. The features of isotropic focal mechanism, episodic occurrence, and possible non‐power‐law scaling, with no detectable geodetic deformation, as well as the petrological signatures of the Holocene eruption product, suggest that the earthquakes were likely associated with the weak intermittent magma flows near the magma body. This finding may demonstrate the existence of unsteady magmatic processes in the margin of the Indo‐Eurasia collision zone, which could indicate unneglectable volcanic hazards, underestimated geothermal resources, and mineralization processes in similar regions.

Seamount Subduction Dynamics and Long‐Term Evolution of the Franciscan Active Margin

AbstractThe Snow Mountain Volcanic Complex (SMVC; northern California, USA) is a well‐preserved example of a coherently‐exhumed subducted seamount. This study reappraises the genesis and evolution of this complex and surrounding units through detailed field, petro‐structural and geochronological analyses. This work demonstrates that the SMVC (a) erupted at ∼166 Ma as a hotspot volcano on the Farallon Plate, (b) entered the Franciscan subduction trench at ∼118 Ma, and (c) was subsequently subducted to a depth of ∼20 km (within the seismogenic zone), as shown by local blueschist‐facies assemblages formed at 0.6 GPa, 240°C. Transient subduction interfaces are preserved above, within, and below the SMVC, making it an exceptional target to study seamount subduction dynamics. Like other seamounts, the subduction‐related deformation was mainly accommodated along kilometer‐scale internal thrust zones lubricated by serpentinite/metasediments, and within centimeter‐thick crack‐seal veins recording pulsed fluid flow near peak metamorphism. No unequivocal proof of seismic activity was found. The integration of other seamounts (some potentially belonging to a former seamount chain) in the Franciscan Complex suggests that exhumed seamounts are more abundant than previously thought. Moreover, pressure‐temperature‐time estimates of subduction metamorphism for the surrounding units, combined with previous work constrain the thermal maturation of the subduction zone through time and the in‐sequence emplacement of the SMVC. Rapid changes in age of the subducted oceanic plate when subducted additionally hint to the subduction of large‐offset transform faults on the former Farallon plate. Such a process might have been linked to changes in accretion dynamics and magmatic flare‐ups in the arc.

Linking rates of slab sinking to long-term lower mantle flow and mixing

Numerical models of Earth's mantle dynamics that aim to comply with a variety of surface observations and/or modern mantle structure, still predict a widely varying vigour of mantle flow which governs the long-term evolution of mantle structure and mixing. A yet unexplored source of intrinsic information on mantle flow characteristics are the geologically reconstructed average slab sinking rates. Here we evaluate from numerical experiments how average slab sinking rates relate to the vigour of mantle convection and mixing. We use a simplified mantle convection model and show that long-term mantle flow velocity and mixing is strongly sensitive to slab sinking rates. Models tuned to match lower mantle average sinking rates of 10–15 mm/a, yield lower mantle convection rates of only several mm/a. Furthermore, they reveal large unmixed regions in the mid-mantle which preserve 25% of ‘primordial’ lower mantle material after 1000 Ma, which may explain geochemical observations from hotspot volcanoes.

Constraining the Lithospheric Discontinuity Structure Beneath Hawaiʻi Using Teleseismic Receiver Functions

AbstractThe Hawaiian Islands represent the youngest portion of the long‐lived Hawaiian‐Emperor Seamount chain sourced from hotspot volcanism on the northwesterly moving Pacific plate. Despite being one of the best studied hotspots on Earth, our understanding of the lithospheric discontinuity structure of Hawaiʻi is inhibited by a lack of a long‐term distributed broadband seismic network and lithospheric‐scale imaging of other oceanic hotspots. In this study, we analyze 3,530 teleseismic waveforms from 1,130 events recorded at 47 stations to compute P‐wave radial receiver functions. We then incorporate an elevation correction to adaptive common conversion point stacking to create a 3D discontinuity volume of the lithospheric structure associated with an ocean island hotspot. We find a positive amplitude conversion consistently at ∼12 km below sea level, likely representing the crust‐mantle transition below much of the island. However, this conversion occurs at only ∼6 km below Kīlauea, where local tomography images a zone of high P‐wave velocities (>7 km/s) within the crust and deformation of olivine phenocrysts within a magmatic reservoir is thought to occur. At this location, seismicity also extends vertically to 35 km depth before merging with an interpreted sill complex at ∼40 km. We interpret this anomalously shallow high amplitude discontinuity as an olivine‐dominated magmatic reservoir with an estimated maximum 5% melt over the resolution of our data at crustal depths. This represents the shallow seismic signature of a much more extensive magma system extending to depth. These images provide a better understanding of the magmatic structure associated with oceanic hotspot volcanoes.

An isotopically enriched mantle component in the source of Rodrigues, Réunion volcanic hotspot

The Mascarene Islands in the western Indian Ocean, encompassing La Réunion, Mauritius, and Rodrigues, are the recent (<10 Ma) surface expressions of the Réunion hotspot. Ocean island basalts (OIB) from these islands exhibit a remarkably homogeneous long-lived radiogenic isotopic composition, coinciding with the convergence field of many global OIB trends in the mantle array. Réunion plume-related OIB therefore provide one of Earth’s most pristine representations of this “focal zone” component, which may have a primordial heritage. Besides this signature, Mascarene lavas have been suggested to retain contributions from sources with distinct compositions, including: (1) Archaean-aged zircons assimilated from continental crust within the oceanic lithosphere by trachytic magmas from Mauritius; (2) more deeply recycled continental crust components preserved by elevated 87Sr/86Sr and 208Pb/206Pb in lavas from the Piton des Neiges volcano of Réunion; and (3) an isotopically depleted mantle component resulting from interaction with Central Indian Ridge material.In this study we use Sr-Nd-Pb isotope systematics, along with major and trace element compositions of basaltic lavas from all three Mascarene Islands to investigate the relationship of their sources to well-characterized mantle endmembers. Among the Mascarene Islands, Rodrigues lavas are the most enriched in highly incompatible elements, likely reflecting shallower and lower degrees of partial melting than Réunion or Mauritius. Combined Sr-Nd-Pb isotope compositions indicate that lavas from the Older Series of Mauritius resemble those from Réunion, whereas lavas from the Younger and Intermediate Series, together with Rodrigues, are consistent with contributions from an isotopically depleted component. In addition, the Pb isotopic compositions of Rodrigues samples require an additional contribution from a component with a long-term enrichment in its Th/U ratio. Based on isotope mixing models, direct assimilation of continental crust embedded within the oceanic lithosphere is unlikely to account for the Pb isotopic variation of Rodrigues. A metasomatized mantle component, previously envisaged as a “fossil” Réunion plume near the Central Indian Ridge, is partially able to reproduce the trace element signature, but not the observed Sr-Nd-Pb isotopic compositions of Rodrigues. Instead, small proportions (<5%) of an EM1-like component provide a preferred endmember. The composition and origin of this component, which is exclusively reflected within Rodrigues lavas, may constitute a new geochemical feature of the Mascarene Islands and is consistent with geodynamical predictions that small-scale enriched mantle domains may be widespread in the source regions of hotspot volcanoes.

Classification of lava lakes based on their heat and SO2 emission: Implications for their formation and feeding processes

Lava lakes are a fascinating but somewhat rare form of volcanic activity. Lava lakes are large free surfaces of hot lava that discharge continuously, and almost quietly, heat and volcanic gases into the atmosphere. They are thought to be fed by convection processes that bring hot gas-rich magma to the surface and back downward, after its cooling and outgassing. A lava lake represents a latent threat for the populations living nearby, as it can drain suddenly through fissures and generate dangerously fast lava flows. We present time series of Volcanic Radiative Power (VRP) and SO2 flux measured from satellites (MODIS and OMI, respectively) from several lava lakes on Earth (Erta Ale, Nyiragongo, Kilauea, Nyamuragira, Ambrym, and Villarrica). Based on long-term trends plotted in a simple VRP versus SO2 flux diagram, we propose a new classification of lava lakes in three categories: small lakes, large foam-dominated lakes, and large melt-dominated lakes. Small lakes show a long-term correlation between VRP and SO2 emissions, while large lakes seem to show an anticorrelation between VRP and SO2 emissions. This at-first-glance surprising anticorrelation probably results from the limited heat transport capacity of the gas-rich foam that initially feeds the convection of these lakes. We also show that the formation of three large lava lakes in the last 2 decades at the rift and hotspot volcanoes followed a similar trend of transitioning, in a few months, from foam-dominated to melt-dominated. We deduce that lava lake formation at these volcanoes follows a common sequence of processes that includes the formation of a large shallow magma reservoir and its outgassing through a newly formed pit crater.

Temporal variations in plume flux: characterizing pulsations from tilted plume conduits in a rheologically complex mantle

SUMMARY Along age-progressive hotspot volcano chains, the emplacement rate of igneous material varies through time. Time-series analysis of changing emplacement rates at a range of hotspots finds that these rates vary regularly at periods of a few to several tens of millions of years, indicative of changing melt production within underlying mantle plumes. Many hotspots exhibit at least one period between ∼2 and 10 Myr, consistent with several proposed mechanisms for changing near-surface plume flux, and thus melting rate, such as small-scale convection, solitary waves and instability formation in tilted plume conduits. Here, we focus on quantifying instability growth within plumes tilted by overlying plate motion. Previous studies using fluids with constant or temperature-dependent viscosity suggest that such instabilities should not form under mantle conditions. To test this assertion, we use a modified version of the finite element code ASPECT to simulate 400 Myr of evolution of a whole-depth mantle plume rising through the transition zone and spreading beneath a moving plate. In a 2-D spherical shell geometry, ASPECT solves the conservation equations for a compressible mantle with a thermodynamically consistent treatment of phase changes in the mantle transition zone and subject to either a temperature- and depth-dependent linear rheology or a temperature-, depth- and strain-rate dependent non-linear rheology. Additionally, we examine plume evolution in a mantle subject to a range of Clapeyron slopes for the 410 km (1–4 MPa K–1) phase transitions. Results suggest that plume conduits tilted by &amp;gt;67° become unstable and develop instabilities that lead to initial pulses in the transition zone followed by repeated plume pulsing in the uppermost mantle. In these cases, pulse size and frequency depend strongly on the viscosity ratio between the plume and ambient upper mantle. Based upon our results and comparison with other studies, we find that the range of statistically significant periods of plume pulsing in our models (∼2–7 Myr), the predicted increase in melt flux due to each pulse (3.8–26 × 10−5 km3 km−1 yr−1), and the time estimated for a plume to tilt beyond 67° in the upper mantle (10–50 Myr) are consistent with observations at numerous hotspot tracks across the globe. We suggest that pulsing due to destabilization of tilted plume conduits may be one of several mechanisms responsible for modulating the melting rate of mantle plumes as they spread beneath the moving lithosphere.

Volcanic Outgassing of Volatile Trace Metals

Volcanoes play a key role in the cycling of volatile metals (e.g., chalcophile elements such as Tl, Pb, and Cu and metalloids such as As, Te, and Se) on our planet. Volatile metals and metalloids are outgassed by active volcanoes, forming particulate volcanic plumes that deliver them in reactive form to the environment, where they may be nutrients (e.g., Cu and Zn) or pollutants (e.g., Hg, As, Pb). Volcanic outgassing rates of these elements compare to those associated with building ore deposits in the crust and to anthropogenic emission rates. There are distinct compositional differences between volcanic plumes in different tectonic settings, related to the enrichment of arc magmas in metals transported in slab fluids, metal speciation, and partitioning between silicate melt, vapor, and magmatic sulfide. Volcanic gases have compositions similar to those of quartz-hosted fluid inclusions found in mineralized granites, albeit with a lower density and salinity. Volatile volcanic metals are transported as soluble aerosols in volcanic plumes and may persist for hundreds of kilometers in the troposphere. Volcanic metal chloride aerosols in tropospheric volcanic plumes at high latitudes are recorded in ice cores. ▪ Volcanoes emit significant fluxes of volatile trace metals such as Cu, Tl, and Pb, as gases and particulates, to the surface environment. ▪ There is a distinct metal compositional fingerprint in volcanic and hydrothermal plumes at subduction and hotspot volcanoes and mid-ocean ridges, controlled by magma and fluid chemistry. ▪ Volcanic gases are the less saline equivalent of the fluids forming economic porphyry deposits of chalcophile metals (e.g., Cu) in the crust. ▪ The metals in tropospheric volcanic plumes may be rained out near the vent, but in dry environments they may persist for thousands of kilometers and be deposited in ice cores.

Stability of a Mixed‐Valence Hydrous Iron‐Rich Oxide: Implications for Water Storage and Dynamics in the Deep Lower Mantle

AbstractIncorporation of water into mantle compositions can have significant effects on the phase relations in the systems. In this study, we synthesized an iron‐rich hexagonal hydrous phase (referred to as “HH1‐phase”) under the high pressure‐temperature (P‐T) conditions of the deep lower mantle and determined the crystal structure of the HH1‐phase at 79 GPa using the multigrain crystallography method. The chemical formula obtained was Fe12.76O18Hx (x ∼ 4.5) in the Fe‐O‐H system. To demonstrate the role of HH1‐phase for water storage in multicomponent systems relevant to mantle compositions, we investigated the stability of HH1‐phase in both MgO‐rich pyrolitic and SiO2‐rich basaltic compositions. Our results indicate that the HH1‐phase serves as major water storage in a pyrolitic composition, whereas the Al‐rich CaCl2‐type δ‐phase and SiO2 phase are major water storage phases in a SiO2‐rich basaltic composition. Incorporation of considerable amounts of SiO2, MgO, and Al2O3 into the HH1‐phase expands its stability field from 98 GPa in the Fe‐Al‐O‐H system to at least 108 GPa (corresponding to ∼2,400 km depth) in the Mg‐Si‐Al‐Fe‐O‐H system. Plumes of hot upwelling rock rooted at the base of the lower mantle have been proposed as a possible origin of hotspot volcanoes. The hydrous Fe‐rich HH1‐phase, if included into the material of upwelling plumes, will decompose on its rising to the upper part of the lower mantle and release water. Our results should provide constraints on water storage in the deep lower mantle and have implications for deep mantle dynamics.

Plumbing System Architecture of Late-Stage Hotspot Volcanoes in Eastern Australia

Abstract Eastern Australia encompasses the longest track (~2000 km) of age-progressive continental volcanoes on Earth. These so-called ‘central volcanoes’ are shield volcanoes considered as surficial expressions of Cenozoic mantle plume activity under the northward moving Australian continent. Here, we investigate three central volcanoes located in the southern, younger part of the volcanic track (Ebor, Nandewar, and Canobolas) with the aim of unraveling the plumbing system architecture during waning hotspot activity. We explore the duration of volcanic activity and compare long-term evolution of magmatic processes via 40Ar/39Ar geochronology, mineral and groundmass chemistry, mineral-melt thermobarometry, and Rhyolite-MELTS thermodynamic simulations. 40Ar/39Ar geochronology on groundmass and mineral separates indicates that Ebor is the oldest of the three volcanoes, with duration of at least ~1 Ma (20.4 ± 0.09 to 19.4 ± 0.07 Ma). Nandewar also lasted ~1 Ma (19.4 ± 0.03 to 18.5 ± 0.03 Ma). The Canobolas volcanic complex was younger and shorter lived at ~0.5 Ma (12.0 ± 0.02 to 11.55 ± 0.05 Ma). Interestingly, all three volcanoes share a repetitive tempo of ~0.1 Ma between eruptions. The volcanoes produced porphyritic to aphyric lavas with basalt to trachyte compositions. The phenocryst assemblage includes plagioclase and K-feldspar, pink and green clinopyroxene, rare olivine, and titanomagnetite. Textural and compositional zoning of phenocrysts reveals successive events of mafic replenishment and magma transport prior to eruption. Dissolution textures in plagioclase, coupled with increasing An and FeOt and decreasing Ba and Ce from crystal cores to mantles, indicate recharge with mafic, oxidised melt. Increasing Mg# and Cr from clinopyroxene cores to rims also supports primitive magma replenishment. Mineral–melt thermobarometry and Rhyolite–MELTS simulations indicate a main level of magma storage in the three volcanoes in the middle crust (18–25-km depth; ~1100°C), repeatedly replenished by undegassed, primitive melts. Green clinopyroxene cores crystallised in isolated pockets where magmas underwent extensive fractionation at depths of 15 to 30 km and ~800°C. The shallow level plumbing system was volumetrically minor and dominated by crystallisation of low-An plagioclase with large melt inclusions, possibly crystallised from degassed, reduced and evolved magma, as suggested by plagioclase hygrometry and fO2 modelling. Our combined geochronological and geochemical approach reveals that the three spatially separated but genetically linked volcanoes had comparable, complex plumbing system architectures. Fractionation and repeated magma rejuvenation were critical processes throughout the lifespans of volcanism, and eruptive tempos were controlled by recurrent mafic influx. The maficity of lavas and their crystal cargo correlate with the volume fraction of phenocrysts, suggesting mafic recharge was a key driver of mush remobilisation and eruption. Other volcanoes active during the late stages of plume activity in eastern Australia share similar textural and geochemical features, suggesting that waning hotspot activity may result in increased complexity in magma transport and storage.

Geodetic Monitoring at Axial Seamount Since Its 2015 Eruption Reveals a Waning Magma Supply and Tightly Linked Rates of Deformation and Seismicity

AbstractAxial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ∼1,500 m below sea level, superimposed on the Juan de Fuca spreading ridge, giving it a robust and continuous magma supply. Axial erupted in 1998, 2011, and 2015, and is monitored by a cabled network of instruments including bottom pressure recorders and seismometers. Since its last eruption, Axial has re‐inflated to 85%–90% of its pre‐eruption level. During that time, we have identified eight discrete, short‐term deflation events of 1–4 cm over 1–3 weeks that occurred quasi‐periodically, about every 4–6 months between August 2016 and May 2019. During each short‐term deflation event, the rate of earthquakes dropped abruptly to low levels, and then did not return to higher levels until reinflation had resumed and returned near its previous high. The long‐term geodetic monitoring record suggests that the rate of magma supply has varied by an order of magnitude over decadal time scales. There was a surge in magma supply between 2011 and 2015, causing those two eruptions to be closely spaced in time and the supply rate has been waning since then. This waning supply has implications for eruption forecasting and the next eruption at Axial still appears to be 4–9 years away. We also show that the number of earthquakes per unit of uplift has increased exponentially with total uplift since the 2015 eruption, a pattern consistent with a mechanical model of cumulative rock damage leading to bulk failure during magma accumulation between eruptions.

Identifying an Eruption “Tipping Point” in Hot Spot Volcanoes

By analyzing lava samples, researchers show how chemical processes that occur during magma’s ascent to the surface may contribute to a volcano’s eruptible state.

ВУЛКАНЫ ГОРЯЧИХ ТОЧЕК (НОВАЯ ВЫСТАВКА В МУЗЕЕ ЗЕМЛЕВЕДЕНИЯ МГУ)

The new exhibition is located in Hall 5 «Geotectonics» in the Earth Science Museum at Moscow State University and devoted to an urgent problem of volcanic manifestations in areas of hot spots. Among the large number of active volcanoes on our planet, a significant place is occupied by volcanoes of hot spots and mantle plumes, which are characterized by the eruption of a huge amount of pyroclastic material (Hawaii, Yellowstone) and often form large igneous provinces (Eastern Siberia, Kerguelen, Deccan, Iceland). The exhibition presents geological samples from the funds and collections of the museum staff. All samples are characteristic of hot spot volcanoes and mantle plumes from different regions of the world.

Mantle plumes and their role in Earth processes

The existence of mantle plumes was first proposed in the 1970s to explain intra-plate, hotspot volcanism, yet owing to difficulties in resolving mantle upwellings with geophysical images and discrepancies in interpretations of geochemical and geochronological data, the origin, dynamics and composition of plumes and their links to plate tectonics are still contested. In this Review, we discuss progress in seismic imaging, mantle flow modelling, plate tectonic reconstructions and geochemical analyses that have led to a more detailed understanding of mantle plumes. Observations suggest plumes could be both thermal and chemical in nature, can attain complex and broad shapes, and that more than 18 plumes might be rooted in regions of the lowermost mantle. The case for a deep mantle origin is strengthened by the geochemistry of hotspot volcanoes that provide evidence for entrainment of deeply recycled subducted components, primordial mantle domains and, potentially, materials from Earth’s core. Deep mantle plumes often appear deflected by large-scale mantle flow, resulting in hotspot motions required to resolve past tectonic plate motions. Future research requires improvements in resolution of seismic tomography to better visualize deep mantle plume structures at smaller than 100-km scales. Concerted multi-proxy geochemical and dating efforts are also needed to better resolve spatiotemporal and chemical evolutions of long-lived mantle plumes.

Petrography, mineralogy, and geochemistry of the Hemrin Basalt, Northern Iraq: Implications for petrogenesis and geotectonics

The studied Hemrin Basalt (HB) was previously mis-identified and called burnt hills. The HB is studied petrographically and geochemically as constraints for its provenance and geotectonics. The immobile trace element plot of Zr/TiO2–Nb/Y as well as mineralogy, textures and hand specimen appearance indicate that the studied rocks are basalts. This basalt occurs as surficial thin layer capping few hills along a NW/SE oriented direction within the Injana Formation in the Low-Folded Tectonic Zone, which is not related and far from the ophiolites of northeastern Iraq. It consists dominantly of diopside, labradorite and volcanic glass, and many accessory and secondary vesicle-filling minerals (amygdules). The basalt is very fine-grained with a massive and vesicular/amygdaloidal structures, and sub-ophitic as well as very rare and unusual textures, called firework and feathery in this study which are very similar to those of lunar basalts. The large ion lithophile elements (LILE) are enriched 4.8 times relative to the high field strength elements (HFSE), indicated from the MORB-normalized diagram for major and trace elements. The average content of rare earth elements (REE) in HB is 140 ppm with 10.6 times enrichment of LREE relative to HREE which is reflected in their chondrite-normalized diagram having a negative slope of 7.6 and a small Eu-anomaly of 0.7. The (La/Yb)n–La plot showed that the source magma was fractionally crystallized; whereas, the immobile trace element ratios and elemental values such as Nb/U, Ce/Pb, Nb/La, LREE/HREE, LILE/HFSE, Zr, Hf, Nb, and Ta indicated that source magma for HB was contaminated while rising through the Arabian continental crust. Many major, immobile trace and REE element discriminating diagrams showed that the studied rocks are high-K calc-alkaline basalts formed as a continental volcanic arc under orogenic convergent tectonic conditions because of the continent-continent collision between the Arabian and Turkish/Iranian plates. The surficial HB and the surrounding calc-alkaline tuffs are probably related and formed by the same volcanic eruption during Quaternary period, <0.5 Ma ago. The source of HB is most probably a hot spot volcano erupted in the area close to the studied hills, ejecting large amounts of volcanic ash which deposited the tuff beds and the associated bentonites in the surroundings, and culminated by a basaltic flow which formed the HB.

Geomorphometric descriptions of archipelagic aprons off the southern flanks of French Frigate Shoals and Necker Island edifices, Northwest Hawaiian Ridge

AbstractThis study describes the geomorphometries of archipelagic aprons on the southern flanks of the French Frigate Shoals and Necker Island edifices on the central Northwest Hawaiian Ridge that are hotspot volcanoes that have been dormant for 10–11 m.y. The archipelagic aprons are related to erosional headwall scarps and gullies on landslide surfaces but also include downslope gravitational features that include slides, debris avalanches, bedform fields, and outrunners. Some outrunners are located 85 km out onto the deep seafloor in water depths of 4900 m. The bedforms are interpreted to be the result of slow downslope sediment creep rather than products of turbidity currents. The archipelagic aprons appear to differ in origin from those off the Hawaiian Islands. The landslides off the Hawaiian Islands occurred because of oversteepening and loading during the constructive phase of the islands whereas the landslides off the French Frigate Shoals and Necker Island edifices may have resulted from vertical tectonics due to the uplift and relaxation of a peripheral bulge or isolated earthquakes long after the edifices passed beyond the hotspot. The lack of pelagic drape in water depths above the 4600 m depth of the local carbonate compensation depth suggests that the archipelagic apron off the French Frigate Shoals edifice is much younger, perhaps Quaternary in age, than that off the Necker Island edifice, which has a 50 m pelagic drape. The pelagic drape off the Necker Island edifice suggests that the landslides may be as old as 9 Ma. The lack of pelagic drape off the French Frigate Shoals edifice suggests that the most recent landslides are more recent, perhaps even Quaternary in age. The presence of a chute-like feature on the mid-flank of the French Frigate Shoals edifice appears to be the result of rejuvenated volcanism that occurred long after the initial volcanism ceased to build the edifice.

Extreme isotopic heterogeneity in Samoan clinopyroxenes constrains sediment recycling

Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high 87Sr/86Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high 87Sr/86Sr lava from Savai’i Island, Samoa for detailed analyses of 87Sr/86Sr and 143Nd/144Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of 87Sr/86Sr—including the highest observed in an oceanic hotspot lava—encompassing ~30% of the oceanic mantle’s total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene 87Sr/86Sr variability resulting from magma mixing between a high silica, high 87Sr/86Sr (up to 0.7316) magma, and a low silica, low 87Sr/86Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth’s mantle.

The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

AbstractThe locations of ultralow‐velocity zones (ULVZs) at the core‐mantle boundary (CMB) have been linked to a variety of features including hot spot volcanoes and large low‐velocity province (LLVP) boundaries, yet only a small portion of the CMB region has been probed for ULVZ existence. Here we present a new map of lower mantle heterogeneity locations using a global collection of highly anomalous SPdKS recordings based on a dataset of more than 58,000 radial component seismograms, which sample 56.9% of the CMB by surface area. The inference of heterogeneity location using the SPdKS seismic phase is challenging due to source‐versus receiver‐side ambiguity. Due to this ambiguity, we conducted an inversion using the principle of parsimony. The inversion is conducted using a genetic algorithm which is repeated several thousand times in order to construct heterogeneity probability maps. This analysis reveals that at probabilities 0.5, 0.25, and 0.125 up to 1.3%, 8.2%, or 19.7% of the CMB may contain ULVZ‐like heterogeneities. These heterogeneities exist in all lower mantle settings, including both high‐ and low‐velocity regions. Additionally, we present evidence that the Samoan ULVZ may be twice as large as previously estimated, and also present evidence for the existence of additional mega‐sized ULVZs, such as a newly discovered ULVZ located to the east of the Philippines. We provide new evidence for the ULVZ east of the Philippines through an analysis of ScP records.

Southward Drift of Eastern Australian Hotspots in the Paleomagnetic Reference Frame Is Consistent With Global True Polar Wander Estimates

Eastern Australian hotspots produce the longest continental tracks on Earth (>2000 km). These hotspots are generally assumed to be stationary with respect to one another, an observation reinforced by our analysis. If any motion between them occurred, it is within our 95% uncertainties of 370km of accumulated motion between hotspots. In contrast, motion between eastern Australian hotspots and the spin axis is indicated by changing paleolatitudes of hotspots through time. Reconstructing Australian hotspot paleolatitudes makes use of the global reference frame established by the geocentric axial dipole hypothesis. The classic paleomagnetic approach to establishing paleolatitude typically uses studies of discrete formations of known age, but the nearly continuous record of volcanism over ~34 Ma permit using paleomagnetic data from multiple, different studies that overlap in magnetization age in conjunction. This improves the robustness and precision of our paleolatitude estimates for Eastern Australian hotspot volcanoes. A northward shift of hotspot paleolatitudes between 15-24 Ma of ~300-1010 km (95% confidence limits) is observed for volcanoes along the Comboyne, Canobolas and Tasmantid tracks, and for interpolated positions along the Cosgrove and Lord Howe tracks but is significantly resolved only for volcanoes along Comboyne and Canobolas tracks, as well as for two ages from the Stradbroke and Britannia seamounts on the Tasmantid track. Before 24 Ma most data plot to the North of a fixed present day hotspot location in the paleomagnetic reference but 95% confidence limits overlap with it. Notable exceptions are an age from Hillsborough Volcano (Cosgrove Hotspot), and an age from Horse Head (Lord Howe Hotspot) that lie a small distance to the North 22-743km, and 167-988km North respectively. Importantly, paleolatitude changes observed along eastern Australian tracks are all explainable by the net rotation of the solid Earth, or true polar wander, with hotspots embedded in the global moving hotspot reference frame.

Paroxysmal eruptions tracked by variations of helium isotopes: inferences from Piton de la Fournaise (La R\xe9union island)

Helium (He) with its isotopes (3He, 4He) is a key tracer enabling the Earth’s mantle and dynamics to be characterized. Enrichment in primordial helium (3He) has been detected in volcanic gases of numerous magmatic systems in different geodynamic settings. Despite past use to monitor volcano-tectonic unrest, temporal 3He/4He variability in volcanic emissions is still poorly constrained. Here, we investigate noble gas chemistry of Piton de la Fournaise hotspot volcano, where temporal fluctuations of 3He/4He in response to the eruptive activity have never been studied. We compare the 3He/4He signature of volcanic gases and fluid inclusions and we highlight analogous evolution of the 3He/4He signature in both during the last decades of eruptive activity (1990–2017), even during the same eruption. We show that the maximum enrichment in 3He is found in magmatic fluids that fed the most voluminous eruptions which culminated in caldera collapse events. We argue that this enrichment in 3He mostly reflects a greater contribution of magmatic fluids from a primitive component of the mantle plume. These results emphasize that He isotopes may provide warnings of increases in deep magmatic contributions that potentially herald paroxysmal eruptions, as documented here at Piton de la Fournaise (2007) and also at Kilauea (2018).