<p>The January 2022 eruption of Hunga Volcano, Tonga is likely the most explosive mafic eruption yet documented. It exhibited dynamics of ash plume expansion and atmospheric pressure waves unlike anything seen before. This is remarkable considering that it erupted crystal-poor and microlite-poor andesitic magma (57-63 wt% silica glass). The climactic phase produced an eruptive column of at least 39 km in height, however, the ash volume appears anomalously small for the explosive magnitude. Ash from nine different sites across the Kingdom of Tonga were analyzed for textural and morphological properties and grain size distribution. The tephra comprises light pumice (16%), dark pumice (44%), glassy microlite-rich grains (25%), lithics (7%) and free-crystals (Pl, Cpx, Opx) (8%). Specific gravity of particles range from 0.4 to ~2.5. Secondary electron images show that pumices have a variable vesicularity, from dense glassy blocky particles; glassy particles with isolated vesicles and weakly deformed, thick vesicle walls; and a smaller percentage of microvesicular pumices, coated in finer particles. The general characteristics imply a rapid decompression, fragmentation and chilling. This implies some form of phreatomagmatism but with high-efficiency to generate such a large blast &#8211; e.g., via propagation of stress waves and thermal contraction rapidly increasing a magma surface area for interaction. The ash is fine-grained and poorly sorted overall. Less than 20 wt.% of ash particles are >1 mm at 80 km SE of the volcano on the main island of Tongatapu, while 70 km NE of the volcano (Nomuka Island) has finer ash, with only 2% of particles >1 mm. It appears that the dispersion axis for the event was directed toward the E or ESE, across the main population centre of Nuku&#8217;alofa on Tongatapu. Of the fine fraction 20 wt.% is < 30 micron, 8 wt.% <10 micron but unusually few particles of very fine range (<0.05 wt.% finer than 1 micron). Variations in the mode and sorting of ash fall at different locations and angles from the vent show that there was potentially complex dispersal of ash from different phases of the 11-hour long eruption, and or different plume heights and fragmentation processes involved. Plume observations suggest at least two different plume levels during main phases of the eruption and the fragmentation mechanisms likely varied from the blast-generating phase and the lesser-explosive phases leading up to and following this.</p>
<p>The 15 Jan 2022 eruption of Hunga-Tonga-Hunga-Ha&#8217;apai was the largest explosive volcanic event in the last 30 years. These islands represent the subaerially exposed summit of the Hunga Volcano, merged into a single land mass during the most recent eruption in 2014-2015. The 2022 eruption likely represents a 1-in-1000-year event for the Hunga Volcano, with the previous large-magnitude eruption occurring in ~1100 CE during a series of caldera-forming events. The 2022 erupted magma is plagioclase-, orthopyroxene- and clinopyroxene-bearing basaltic andesite to andesite dominated by blocky, poorly vesicular glassy ash with lesser amounts of vesicular pumiceous ash and fine lapilli. Melt Inclusions (MIs) hosted in plagioclase, clinopyroxene and orthopyroxene are abundant and glassy, some displaying shrinkage bubbles, with no evidence of secondary crystallization along the walls or within the MI glass. The groundmass glass and MI in the three main phenocryst phases were analysed for major, trace and volatile element concentrations to enable identification of magmatic sources and to better constrain processes happening at depth. Preliminary data indicate that plagioclase phenocrysts range from An<sub>93</sub> to An<sub>78</sub>, and MI range from 54.1 to 58.7 wt % SiO<sub>2</sub>, with MgO from 2.5 to 5.3 wt %. Clinopyroxene phenocrysts range from En<sub>42</sub> to En<sub>50</sub>, and MI range from 51.6 to 65.1 wt % SiO<sub>2</sub>, with MgO from 1.1 to 5.7 wt %. Orthopyroxene phenocrysts range from En<sub>68</sub> to En<sub>77</sub>, and MI range from 55.7 to 59.6 wt % SiO<sub>2</sub>, with MgO from 2.5 to 5.3 wt %. Clinopyroxene MI span the full range of SiO<sub>2</sub> compositions observed from the Hunga Volcano, from the host 2022 event (SiO<sub>2</sub>: ~57.5 wt %), the 1100 CE event (SiO<sub>2</sub>: ~60 wt %), the 2014-2015 event (SiO<sub>2</sub>: ~60.5 wt %), and the most evolved 2009 event (SiO<sub>2</sub>: ~63 wt %) and extend an additional ~4 wt % SiO<sub>2</sub> to more mafic compositions. Orthopyroxene MI most closely resemble the 1100 CE event and the average groundmass glass compositions of the 2022 event. Plagioclase MI overlap the least silicic compositions observed in the 2022 groundmass glass (58.6 wt% SiO<sub>2</sub>) and extend down to 54 wt % SiO<sub>2</sub>, overlapping the main field of clinopyroxene MI. Both plagioclase and clinopyroxene MI tend to show higher MgO as compared to the 2022 groundmass glass at the same SiO<sub>2</sub> concentration, whereas orthopyroxene shows lower MgO than the groundmass glass. SO<sub>3</sub> in MI ranges up to 1600 ppm, significantly higher than the 2022 groundmass glass which averages 200 ppm, with both plagioclase and clinopyroxene MI preserving the highest observed concentrations. In contrast, Cl concentrations in MI extend to 2000 ppm, with the highest values in orthopyroxene and clinopyroxene, and plagioclase MI are lower and generally overlie the main groundmass glass concentrations (~1300 ppm). F was below detection limits. We postulate that clinopyroxene crystals reflect a more primitive basaltic andesite magma, whereas orthopyroxene crystals were likely derived from the magmatic remnants of the 2009 and 2014/2015 events in the upper magma system, and plagioclase crystals were sourced from the full range of magma sources.</p>
Abstract The Jemez Mountains volcanic field (JMVF) is the site of the two voluminous, caldera-forming members of the Bandelier Tuff, erupted at 1·60 and 1·25 Ma, following a long and continuous pre-caldera volcanic history (∼10 Myr) in this region. Previous investigations utilizing whole-rock geochemistry identified complex magmatic processes in the two major pulses of pre-caldera magmatism including assimilation–fractional crystallization (AFC) and magma mixing. Here we extend the petrological investigation of the pre-caldera volcanic rocks into the micro-realm and use mineral chemistry and textural information to refine magma evolution models. The results show an increasing diversity of mineral populations as the volcanic field evolved. A range of plagioclase textures (e.g. sieved cores and rims) indicate disequilibrium conditions in almost all pre-caldera magmas ranging from andesite to rhyolite, reflecting plagioclase dissolution and regrowth. Coarsely sieved or dissolved plagioclase cores are explained by resorption via water-undersaturated decompression during upward migration from a deep melting, assimilation, storage and homogenization (MASH) zone. Plagioclase crystals with sieved rims are almost ubiquitous in dacite-dominated magmatism (La Grulla Plateau andesite and dacite erupted at ∼8–7 Ma, as well as Tschicoma Formation andesite, dacite and rhyolite at ∼5–2 Ma), reflecting heating induced by magma mixing. These plagioclase crystals often have An-poor cores that are chemically distinct from their An-rich rims. The existence of different plagioclase populations is consistent with two distinct amphibole groups that co-crystallized with plagioclase: a low-Al, low-temperature, high-fO2 group, and a high-Al, high-temperature, low-fO2 group. Calculation of melt Sr, Ba, La, and Ce concentrations from plagioclase core and rim compositions suggests that these chemical variations are largely produced by magma mixing. Multiple mafic endmembers were identified that may be connected by AFC processes in the MASH zone in the middle to lower crust. The silicic component in an early andesite-dominated magmatic system (Paliza Canyon andesite, dacite and rhyolite, 10–7 Ma) is represented by contemporaneous early rhyolite (Canovas Canyon Rhyolite). A silicic mush zone in the shallow crust is inferred as both the silicic endmember involved in the dacite-dominant magmatic systems and source of the late low-temperature rhyolite (Bearhead Rhyolite, 7–6 Ma). Recharging of the silicic mush by mafic melts can explain observed diversity in both mineral disequilibrium textures and compositions in the dacitic magmas. Overall, the pre-caldera JMVF magmatic system evolved towards cooler and more oxidized conditions with time, indicating gradual thermal maturation of local crust, building up to a transcrustal magmatic system, which culminated in ‘super-scale’ silicic volcanism. Such conditioning of crust with heat and mass by early magmatism might be common in other long-lived volcanic fields.
Characterization of spatiotemporal variation of the stable isotopes δ 18 O and δD in surface water is essential to trace the water cycle, indicate moisture sources, and reconstruct paleoaltimetry. In this study, river water, rainwater, and groundwater samples were collected in the Yarlung Tsangpo River (YTR) Basin before (BM) and after the monsoon precipitation (AM) to investigate the δ 18 O and δD spatiotemporal variation of natural water. Most of the river waters are distributed along GMWL and the line of d-excess = 10‰, indicating that they are mainly originated from precipitation. Temporally, the δ 18 O and δD of river water are higher in BM series (SWL: δD = 10.26δ 18 O+43.01, R 2 = 0.98) than AM series (SWL: δD = 9.10δ 18 O + 26.73, R 2 = 0.82). Spatially, the isotopic compositions of tributaries increase gradually from west to east (BM: δ 18 O = 0.65Lon (°)-73.89, R 2 = 0.79; AM: δ 18 O = 0.45Lon (°)-57.81, R 2 = 0.70) and from high altitude to low (BM: δ 18 O = −0.0025Alt(m)-73.89, R 2 = 0.66; AM: δ 18 O = −0.0018Alt(m)-10.57, R 2 = 0.58), which conforms to the “continent effect” and “altitude effect” of precipitation. In the lower reaches of the mainstream, rainwater is the main source, so the variations of δ 18 O and δD are normally elevated with the flow direction. Anomalously, in the middle reaches, the δ 18 O mainstream and δD mainstream values firstly increase and then decrease. From the Saga to Lhaze section, the higher positive values of δ 18 O mainstream are mainly caused by groundwater afflux, which has high δ 18 O and low d-excess values. The δ 18 O mainstream decrease from the Lhaze to Qushui section is attributed to the combined action of the import of depleted 18 O and D groundwater and tributaries. Therefore, because of the recharge of groundwater with markedly different δ 18 O and δD values, the mainstream no longer simply inherits the isotopic composition from precipitation. These results suggest that in the YTR Basin, if the δ 18 O value of surface water is used to trace moisture sources or reconstruct the paleoaltimetry, it is necessary to rule out the influence from groundwater.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)