Arsenic is a naturally occurring trace element that causes many health effects when present in drinking water. Elevated arsenic concentrations in water are often attributed to nearby felsic volcanic sequences; however, the specific rock units to which the groundwater anomalies can be accredited are rarely identified. The groundwater from wells around the city of Chihuahua, Mexico, contains high arsenic content. Arsenic in groundwater increases toward the base rock containing Tertiary volcanic rocks. Through detailed scanning electron microscope (SEM) and electron microprobe (EMP) work, arsenic minerals are identified in the cavities of the Tertiary volcanic tuff from the northeast part of the Tabalaopa Basin, city of Chihuahua. Arsenic minerals, the As–Sr–Al phase (a possible arsenogoyazite–arsenoflorencite group mineral) crystallized in the vesicles of the tuff and the As–Y bearing phase included in biotite, prevail in the studied Tertiary volcanic outcrops. Based on the current study, the arsenic anomaly in the Tabalaopa–Aldama aquifer corresponds to these arsenic phases in the Tertiary volcanic rocks.

Convergent margin igneous activity is generally limited to 100–200 km from the trench except where spreading ridges are subducted or in association with Subduction‐Transform Edge Propagators (STEP faults). The southernmost Mariana forearc, facing the Challenger Deep, subducts Mesozoic seafloor and is not in a STEP fault setting but includes at least one site where tholeiitic basalts recently erupted close to the trench, the SE Mariana Forearc Rift (SEMFR). We present evidence of young basaltic volcanism from ca. 100 km west of SEMFR. Shinkai 6500 diving during YK13‐08 (Dive 1363) recovered volcaniclastics from 5.5 to 6 km deep in the inner wall of the Mariana Trench, 50 km NE of the Challenger Deep. Glassy fragments are tholeiitic basalts similar to MORB except for much higher contents of magmatic water (approx. 2% H₂O vs. <0.2% H₂O in MORB) and enrichments in trace elements Rb ‐ Cs ‐ Ba , K , Pb , and Sr . Dive 1363 glasses are similar to basalts from SEMFR erupted near the trench and to Mariana Trough backarc basin basalts. Basalt fragments and palagonitized matrix dominate the studied samples, but small xenocrysts and xenoliths derived from mantle peridotite and Neogene volcanics are also present, probably torn from the vent walls. Dive 1363 hyaloclastites erupted at 3–6 km water depth accompanied by vigorous degassing of volatiles, most likely CO₂ . These results provide further evidence that the forearc adjacent to the Challenger Deep has been invaded by asthenospheric mantle and derivative hydrous melts. Extension, hydration, and melt invasion combine to further weaken Challenger Deep forearc lithosphere. Combined effects of: (i) absence of strong, cold lithosphere of the overriding plate; (ii) rapid rollback of a narrow, short subducted slab; and (iii) weak coupling between the subducting Pacific plate and the overriding Mariana plate may be responsible for the great depth of the Challenger Deep.

Carbon flux metasomatism in the subduction environment is an important process, but it remains poorly understood. The paucity of exposed lower crust and upper mantle rocks in continental arcs renders xenoliths a major target for studying the slab-derived carbon cycle. This study of the carbonate phases in volcanic rocks from three drill cores in Ulleung Island, South Korea, sheds light on the interaction of carbon flux in the upper mantle and lower crust in a back-arc setting. The volcanic rocks from Ulleung Island range in composition from trachybasalt to trachyte and contain abundant euhedral pseudomorphic carbonate grains, ulvöspinel-hosted and biotite-hosted carbonate-silicate melt inclusions, and irregular carbonate globules. Integrated petrographic and geochemical studies of a variety of phenocrysts, carbonate phases, and carbonate-silicate inclusions in biotite and ulvöspinel indicate that recharging of carbon flux affected magma evolution. Carbon and oxygen isotopes of the pseudomorphic carbonate grains overlap with mantle values, indicating a carbonatite-like origin of the carbonate phases. The (MgO, FeO, CaO)-rich silicates in ulvöspinel-hosted silicate inclusions and pseudomorphic carbonate grains likely represent a primary melt, which formed from the partial melting of carbonated eclogite of the subducted slab within the mantle wedge beneath Ulleung Island. A petrogenetic model is proposed to illustrate that the crystal mush in the magma chamber was intruded by carbonate-rich liquids and caused alteration of cumulate crystals to generate the euhedral pseudomorphic carbonate grains. The extrusive magma captured those pseudomorphic grains and erupted to form the trachybasalt-trachyte units. The observed carbonate phases and their geochemical characteristics indicate that carbon flux metasomatism played a fundamental role in this back-arc magmatism.

The youngest eruptions in the Jemez Volcanic field occurred in the southwestern moat of the Valles Caldera, producing two series: a younger El Cajete series (ECS) with lower SiO2 (72-76 wt%), consisting of three members; in ascending order: the El Cajete Pumice Fall (ECP), Battleship Rock Tuff (BAT), and Banco Bonito Lava Flow (BB); and an older South Mountain Rhyolite with higher SiO2 (76-79 wt%). Petrological, mineralogical, and geochemical data show that SMR and ECS belong to two different magmatic systems. ECS came from a hotter, less silicic, deeper magma chamber. Mineral equilibria calculations indicate that the ECS formed over a pressure interval of ~2.52 -3.68 kbar and a temperature range of ~834 to 868oC; SMR yielded a lower, narrower pressure range of about 1.24 kbar and a lower equilibration temperature of ~728 - 736oC. The dynamic difference of the eruptions caused the ECS to have a reverse compositional zonation. With increasing silica in ECP--BAT-BB, whole-rock trace elements show the following changes: Rb increases, Nb slightly increases, whereas commonly incompatible elements, such as Sr, Ba, and Zr, decrease. A moderately-deep subjacent magma chamber developed under the caldera and the magma built the compositional gradients during a low energy episode in the chamber. Magma recharge induced the eruption of ECS. Chemical variations in the different members of ECS correlated with the changing eruption energy. The strongest early eruption tapped into the deeper part of magma chamber and caused mixing of different chemical zones in the magma. Later eruptions were less energetic and only released the more felsic melt from the upper part of the magma chamber.