Emerging studies are geared toward exploring new methods of nuclear rocket propulsion to provide more efficient space transit beyond Earth's orbit. One method is to employ a Fission Fragment Rocket Engine utilizing fissionable layers embedded in a low-density aerogel. A quantitative understanding of particle attenuation is essential for developing a functional prototype that permits fission fragments to escape the layers and contribute to specific impulse rather than being attenuated and generating waste heat. In this investigation, the MCNP code was used to theoretically analyze the attenuation of alpha particles from 241Am sources within aerogel materials. Simulations were conducted on aerogels with various densities and compositions. These simulations aimed to predict the expected intensity of alpha particles reaching a detector. CR-39 was employed as a Plastic Nuclear Track Detector to assess particle attenuation by the aerogels. The experimental and simulation results show that the threshold areal density of atoms was found to be high 1020 atoms/cm2 for the three materials studied in this project.

Determining accurate and precise ages for Quaternary volcanic centers is essential for reconstructing volcanic field histories, understanding magmatic processes, and assessing potential hazards or risk. Harrat Khaybar, western Saudi Arabia, is one of the youngest and potentially most active volcanic fields on the Arabian plate, has been active since ca. 1.7 Ma, and is characterized by a spectrum of rock compositions ranging from predominantly alkalic basalt to trachyte and comendite. Previous work in Harrat Khaybar utilizing 40Ar/39Ar incremental heating geochronology to constrain morphological preservation and superpositional relationships bracketed the volcanic activity into broad age groups in intervals of similar to 150 k.y., and the youngest and most compositionally evolved volcanoes, including Jabal Abyad, Jabal Bayda, and Jabal Qidr, were assigned to the age groups between ca. 300 ka and present. Herein, we establish a detailed chronology of prehistoric silicic and historical basaltic eruptions at central Harrat Khaybar using four independent eruption age determination techniques: zircon double-dating (ZDD), which combines 238U-230Th disequilibrium or U-Pb with (U-Th)/He dating; zircon U-Pb dating; cosmogenic 3He dating; and cosmogenic 36Cl geochronology. These were employed to accurately date six volcanic centers, including the comenditic Jabal Abyad, Jabal Bayda, Jabal Ibayl, and Jabal Alhayyirah, the trachytic Jabal Aluthmor, and the basaltic Jabal Qidr. Additionally, our previously published 40Ar/39Ar ages have been recalculated using isochron intercept (nonatmospheric) 40Ar/36Ar for the trapped Ar component. Our new results reveal that zircon rims from Jabal Abyad and Jabal Bayda define isochron 238U-230Th crystallization ages of 125 +/- 4 ka and 144 +/- 6 ka, concordant with ZDD eruption ages of 132 +/- 4 ka and 149 +/- 5 ka, respectively. Zircon U-Pb crystallization and (U-Th)/He eruption ages from Jabal Alhayyirah are concordant at 471 +/- 14 ka and 458 +/- 18 ka, respectively. Finally, zircons from the nearby Jabal Ibayl yield a U-Pb weighted mean crystallization age of 566 +/- 16 ka concordant with the corresponding (U-Th)/He eruption age of 554 +/- 12 ka, both of which are notably older than the previously proposed eruption ages of 300-150 ka. Recalculations of published 40Ar/39Ar ages for the youngest volcanoes at central Harrat Khaybar are now in excellent agreement with new geochronological data. Our new age data reveal several new insights into the development of Harrat Khaybar. It is now clear that the comenditic eruptions do not belong to the same eruptive phase and indicate an extended history during which comendites have episodically punctuated the basaltic volcanism since at least 600 ka. The data indicate that Jabal Ibayl and Jabal Alhayyirah represent separate older volcanic events, whereas the younger Jabal Abyad and Jabal Bayda volcanoes appear to be coeval, and their spatial proximity implies that they share a magmatic lineage and maybe a common plumbing system. Zircon age spectra of the comendites reveal obvious xenocrysts and antecrysts indicating assimilation of basement and plutonic progenitors, but otherwise they define broad unimodal populations of crystallization ages that overlap within error with the respective eruption ages. We interpret this to indicate that zircon crystallization continued up to the time of eruption. The cosmogenic ages of Jabal Aluthmor and Jabal Qidr reveal that these centers erupted as recently as ca. 2000 and 760 years ago, respectively. Broadly coeval young silicic and basaltic eruptions at northern Harrat Rahat, the harrat immediately south of Harrat Khaybar, may imply a shared geodynamic forcing between the two adjacent volcanic fields.

Heard Island, an active sub-Antarctic intraplate volcanic island on the Kerguelen Plateau, is mostly covered by glaciers. The amphitheatre shaped summit of the active volcanic centre, Big Ben (2813 m), has been interpreted to be the product of a significant volcanic landslide. Here we present the first offshore geomorphological and geological evidence supporting a volcanic landslide on Big Ben, including: (1) the seafloor to the southwest of Heard resembling a landslide deposit, covering at least 467 km2, (2) the spatial correlation between the onshore landslide scar and the offshore deposit and (3) the consistency in lithologies and compositions of rocks sampled from the deposit with the onshore in situ lithologies. 40Ar/39Ar geochronology constrains the maximum age of the volcanic landslide to 18.0 ± 1.4 ka, post the Last Glacial Maximum. Finally, we assess the risk of volcanic landslide at Heard Island in the future.

AbstractIt has long been recognised that spreading ridges are kept in place by competing subduction forces that drive plate motions. Asymmetric strain rates pull spreading ridges in the direction of the strongest slab pull force, which partially explains why spreading ridges can migrate vast distances. However, the interaction between mantle plumes and spreading ridges plays a relatively unknown role on the evolution of plate boundaries. Using a numerical model of mantle convection, we show that plumes with high buoyancy flux (>3000 kg/s) can capture spreading ridges within a 1000 km radius and anchor them in place. Exceptionally high buoyancy fluxes may fragment the overriding plate into smaller plates to accommodate more efficient plate motion. If the plume buoyancy flux wanes below 1000 kg/s the ridge may be de-anchored, leading to rapid ridge migration rates when combined with asymmetric plate boundary forces. Our results show that plume-ridge de-anchoring may have contributed to the rapid migration of the SE Indian Ridge from 43 million years ago (Ma) due to waning buoyancy flux from the Kerguelen plume, supported by magma flux estimates and radiogenic isotope geochemistry of eruption products. The plume-ridge de-anchoring mechanism we have identified has global implications for the evolution of plate boundaries near mantle plumes.

Palladium nanoparticles with two different ligands were used as electrocatalysts at low temperature in a hermetic, one-pot electrochemical cell with either a LiOH or LiOD electrolyte. During initial, low-pressure experiments involving palladium (Pd) nanoparticle electrocatalysts with Epigallocatechin Gallate (EGCG) ligands an abrupt, significant pressure drop occurred in the cell headspace air from 1.59 to 0.055 bar. This astounding, repeatable, and unexplained result initiated further experimentation. Synthetic methods for Pd nanoparticle electrocatalysts with EGCG ligands and with polyvinylpyrrolidone (PVP) ligands are described. These Pd nanoparticles are capable of simultaneously generating ammonium, nitrite, and nitrate ions. Ion chromatography was used for quantification of nitrogen-containing anions and cations in the alkaline electrolyte. The Pd nanoparticles with PVP ligands and a 5 bar 3:1 H2/N2 gas mixture resulted in a nitrate yield rate of 1.16 µg h−1 mg−1 with current applied for 132 hr. The Pd nanoparticles with PVP ligands also achieved the highest ammonium and nitrite ion yield rates at 10 bar under similar conditions while the Pd-EGCG nanoparticles achieved lower yield rates Faradaic efficiencies (FE) values. This study demonstrates that direct generation of nitrite and nitrate salts avoids the need to further process ammonia into ammonium nitrate.

The 180,000 km 2 of Arabian lava fields (“harrats” in Arabic) form one of the largest distributed basaltic provinces in the world. The most recent eruption in 1256 AD, on the outskirts of Medina, as well as shallow dike emplacement in 2009, ~ 200 km northeast of the city, suggest future volcanic threat to this area. Harrat Khaybar (~ 1.7 Ma to present) is one of the largest and most compositionally diverse Arabian lava fields; it is located ~ 137 km northeast of Medina and covers ~ 14,000 km 2 . Here, we present a new eruption event record and the first estimation of future potential locations and timing of volcanism in Harrat Khaybar. Volcanic vents and eruptive fissures were mapped using remote sensing and field studies, and categorized into a geospatial database, complemented by 16 new 40 Ar/ 39 Ar ages. Our analysis reveals that Harrat Khaybar developed over five eruptive phases, where vent locations over time focus towards the central axis forming a broad N-S trend, with a central group concentrated along an axis of the regional Makkah-Madinah-Nafud (MMN) line and wider spatial dispersion between vents outwards from there. For the whole field, we estimate a long-term average recurrence rate of ~ 2.3 eruptions per 10 kyr assuming a Poisson distribution for inter-event times, which indicates that Harrat Khaybar would belong to a global group of highly active distributed volcanic fields. Our analysis also reveals that the field likely had a “flare-up” period between 450 and 300 ka where the vast majority of eruptions occurred, with ~ 18 eruptions per 10 kyr. After this intense period, eruption rates fell to < 2 eruptions per 10 kyr. Based on our findings, we estimate cumulative probabilities of 1.09 and 16.3% as lower and upper bounds of at least one eruption occurring over the next 100 years somewhere in Harrat Khaybar, with the highest probabilities within the central axis region, in particular around Jabal Qidr, Bayda and Abyad.

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

We describe the design, operation, and performance of a new instrumental configuration capable of quantitative determinations with sub-picomole accuracy of dilute concentrations of low mass species, such as He-4, He-3, Ne-20, and Ar-40, in a balance of stable hydrogen (H-2, DH, and D-2) gas. This inexpensive system may realize important applications in fields ranging from climate studies to hydrogen fusion energy research, thereby providing an expanded availability of this diagnostic within emerging energy systems research and development. These spectra, calibration curves, and determinations were obtained by using a novel method for the purification and subsequent removal of the hydrogen matrix gas, and an extensively modified commercial Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer with an electron impact (EI) ionizer. These high-resolution FT-ICR mass spectrometers have routinely achieved a resolution, R = m/Delta m better than 10,000 Da at mass-3, with a mass resolution that scales as 1/ m. These devices have easily resolved D-2 from He-4, and DH from He-3. The performance of this upgraded instrument has demonstrated the ability to detect impurities from tiny air leaks, such as Ar-40 and Ne-20, in the presence of the hydrogen matrix gas. While no concentration measurements of radioactive species have been attempted to date with this system, it is expected to easily resolve DT from D2H (a 0.0059 Da mass difference) and HT from all other mass-4 species. (C) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The Snake River Plain of central Idaho represents the world's best example of a mantle hotspot track impinging upon continental crust and provides a record of bimodal volcanism extending over 12 Ma to the present. Project Hotspot recovered almost 2 km of continuous drill core from the Kimama borehole, located in central Idaho on the axial volcanic zone. The Kimama drill core represents the most complete record of mafic volcanism along the Yellowstone-Snake River Plain hotspot track. A total of 432 basalt flow units, representing 183 basalt flows, 78 basalt flow groups, and 34 super groups, along with 42 sediment interbeds are recognized using volcanic facies observations, stratigraphic relationships, borehole geophysical logs, and paleosecular variation in magnetostratigraphy. Rhyolite and other non-basaltic volcanic materials were not encountered in the drill core. Ages for six basalt lava flows were determined by Ar-40/Ar-3(9) using incremental heating experiments. Paleomagnetic inclination was measured on over 1200 samples collected at roughly 2-m-depth intervals, yielding mean values of paleosecular variation between +/- 50 degrees to +/- 70 degrees in Kimama flow groups, close to the expected 61 degrees axial dipole average for the Kimama borehole location. Twenty-three magnetic reversals were identified and correlated to dated geomagnetic chrons and subchrons and compared with the Ar-40/Ar-3(9) radiometric ages. A linear fit to Ar-40/Ar-3(9) dates, geomagnetic chron and subchron boundaries, and volcanogenic zircon U-Pb ages defines a mean accumulation rate of similar to 320 m/m.y. and extrapolates to a bottom hole age of 6.3 Ma. Average thicknesses of lithologic units increase from 2.7 m (sediment), 4 m (flow units), 10 m (flows), 23 m (flow groups), to 53 m (super groups). On average, one lava flow inundated the Kimama borehole location every 33 k.y. Intercalated sediments, ranging from 0.06 to 24.5 m thick, make up roughly 6% of the drill core and indicate lulls in local volcanic activity that may have lasted up to 77 k.y. Neutron and gamma-ray logs supplement observations from the drill cores: neutron logs document individual flow units through the contrast between massive flow interiors and more porous flow surfaces, and gamma-ray logs document the depth and thickness of sedimentary interbeds and high-K-Fe basalts. The 5.8 m.y. duration of basaltic volcanism in the Kimama drill core implies a steady rate of volcanism, indicating a relatively stable rate of mantle upflow along the lithosphere-mantle boundary in the wake of Yellowstone-Snake River Plain plume volcanism.

In this study, three field programs have been conducted to determine the distribution and structure of volcanic landforms (lava flows, cinder cones, eruptive centers), and collect samples for analytical studies (age determination, mineral and whole composition, isotope geochemistry). The first expedition was to Harrat Lunayyir, where recent seismic activity has indicated the possibility for developing new volcanic systems. The second expedition targeted Harrat Hutaymah, as a remarkable area for providing an extraordinary range of mantle and crustal xenoliths, which are rock fragments carried up from great depths (40–70 km) by magmas that fed lava flows and cinder cones.

The late Neogene Deschutes Formation of central Oregon preserves a remarkable volcanic and sedimentary record of the initial stages of High Cascades activity following an eastward shift in the locus of volcanism at ~7.5Ma. Numerous ignimbrite and tephra-fall units are contained within the formation, and since equivalent deposits are relatively rare for the Quaternary Cascades, the eruptions of the earliest High Cascade volcanoes were likely more explosive than those of the Quaternary arc. In this study, the timing and frequency of eruptions which produced 14 laterally extensive marker ignimbrites within the Deschutes Formation are established using 40Ar/39Ar geochronology. Plagioclase 40Ar/39Ar ages for the lowermost (6.25±0.07Ma) and uppermost (5.45±0.04Ma) marker ignimbrites indicate that all major explosive eruptions within the Deschutes Formation occurred within a period of 800±54k.y. (95% confidence interval). Minimum estimates for the volumes of the 14 ignimbrites, using an ArcGIS-based method, range from 1.0 to 9.4km3 and have a total volume of 62.5km3. Taken over the 50km of arc length, the explosive volcanic production rate of the central Oregon High Cascades during Deschutes Formation time was a minimum of 1.8km3/m.y./km of arc length. By including estimates of the volumes of tephra-fall components, as well as ignimbrites that may have traveled west, we estimate a total volume range, for these 14 eruptions alone, of 188 to 363km3 (~121 to 227km3 DRE), a rate of 4.7–9.1km3/m.y./km arc length. This explosive volcanic production rate is much higher than the average Quaternary eruption rates, of all compositions, estimated for the entire Cascade arc (1.5–2.5), Alaska Peninsula segment of the Aleutian arc (0.6–1.0), and the Andean southern volcanic zone (1.1–2.0). We suggest that this atypical explosive pulse may result from the onset of regional extension and migration of the magmatic arc, which had the combined effect of increasing magmatic flux and temporarily enhancing melting of more fusible crust.

New mapping, geochronology and petrological investigations reveal that the two volcanoes of the island of Moloka‘i, Hawai‘i exhibit striking contrasts in composition and volcanic production in the waning stages of their activity. Postshield lava extrusion on West Moloka‘i produced less than 1km3 of alkalic basalt and hawaiite, most likely in less than 100k.y, beginning ~1.8Ma. West Moloka‘i is unique among Hawaiian volcanoes that reached the postshield stage of evolution in lacking a protracted period of transitional volcanism in the late shield stage. Petrological modeling indicates that volatile-poor (<0.5 wt % H2O) West Moloka‘i postshield magmas evolved in reservoirs in the upper mantle, 20-26km beneath the summit. In contrast, following a protracted transitional period, East Moloka‘i extruded postshield magmas ranging from hawaiite to benmoreite that evolved in magma reservoirs lying within the crust, 11- 17km beneath the summit. Parental magmas to the East Moloka‘i postshield were hydrous and enriched in phosphorous and Sr (2.5 wt % P2O5, 2500 ppm Sr at 6 wt % MgO, 45 wt % SiO2), a magma type known previously only from Kohala volcano. Although the overall volume of East Moloka‘i volcano is less than that of West Moloka‘i, its proportion of postshield extrusives is greater by a factor of 8, and its postshield volumetric production rate (~155km3/m.y.) is greater than that at West Moloka‘i by a factor of 3. The lower volume and deeper magmatic evolution of the West Moloka‘i postshield compared to that of East Moloka‘i is consistent with a thermally controlled relationship between magma supply, as inferred from volcanic production rate, and magma reservoir depth.