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1. Mush, Melts and Metasediments: a History of Rhyolites from the Okataina Volcanic Centre, New Zealand, as Captured in Plagioclase

   https://doi.org/10.1093/petrology/egab038  

   Sas, May; Shane, Phil; Kuritani, Takeshi; Zellmer, Georg F; Kent, Adam J; Nakagawa, Mitsuhiro (August 2021, Journal of Petrology)

   null (Ed.)

   Abstract The Okataina Volcanic Centre (OVC), located in the Taupo Volcanic Zone, New Zealand, is a dominantly rhyolitic magmatic system in an arc setting, where eruptions are thought to be driven by mafic recharge. Here, Sr–Pb isotopes, and compositional and textural variations in plagioclase phenocrysts from 10 rhyolitic deposits (two caldera, one immediately post-caldera, four intra-caldera, and three extra-caldera) are used to investigate the OVC magmatic system and identify the sources and assimilants within this diverse mush zone. Plagioclase interiors exhibit normal and reverse zoning, and are commonly in disequilibrium with their accompanying glass, melt inclusions, and whole-rock compositions. This indicates that the crystals nucleated in melts that differed from their carrier magma. In contrast, the outermost rims of crystals exhibit normal zoning that is compositionally consistent with growth in cooling and fractionating melts just prior to eruption. At the intra-crystal scale, the total suite of 87Sr/86Sr ratios are highly variable (0·7042–0·7065 ± 0·0004 average 2SE); however, the majority (95 %) of the crystals are internally homogeneous within error. At whole-crystal scale (where better precision is obtained), 87Sr/86Sr ratios are much more homogeneous (0·70512–0·70543 ± 0·00001 average 2SE) and overlap with their host whole-rock Sr isotopic ratios. Whole-crystal Pb isotopic ratios also largely overlap with whole-rock Pb ratios. The plagioclase and whole-rock isotopic compositions indicate significant crustal assimilation (≥20 %) of Torlesse-like metasediments (local basement rock) by a depleted mid-ocean ridge mantle magma source, and Pb isotopes require variable fluid-dominant subduction flux. The new data support previous petrogenetic models for OVC magmas that require crystal growth in compositionally and thermally distinct magmas within a complex of disconnected melt-and-mush reservoirs. These reservoirs were rejuvenated by underplating basaltic magmas that serve as an eruption trigger. However, the outermost rims of the plagioclase imply that interaction between silicic melts and eruption-triggering mafic influx is largely limited to heat and volatile transfer, and results in rapid mobilization and syn-eruption mixing of rhyolitic melts. Finally, relatively uniform isotopic compositions of plagioclase indicate balanced contributions from the crust and mantle over the lifespan of the OVC magmatic system. 

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2. Origin and Evolution of Miocene Basanite to Comendite Lavas at the Mount Overlord Volcanic Field, Northern Victoria Land, Antarctica

   https://doi.org/10.1093/petrology/egaf014  

   Kim, Jihyuk; Kyle, Philip R; Lee, Mi Jung; Lee, Jong Ik; Heizler, Matthew; Im, Sunghwan; Gamble, John A; Noll, Mark R (March 2025, Journal of Petrology)

   Troll, Valentin (Ed.)

   Continental alkaline magmatism produces a wide variety of igneous rock types because of varying degrees of partial melting of heterogenous mantle sources, fractional crystallization, and magma contamination during transit through the continental crust. The Mount Overlord Volcanic Field (MOVF) is a continental alkaline volcanic province in northern Victoria Land, Antarctica. Mount Overlord and the associated vents that make up the volcanic field are some of the least-explored volcanic rocks in the western Ross Sea. The MOVF sits within the Transantarctic Mountains, which form the rift shoulder of the extensive West Antarctic Rift System. The compositions of volcanic rocks in the MOVF range widely from basanite to evolved trachyte and comendite with a suite of intermediate rock types. Here we present 40Ar/39Ar ages, petrography, and whole-rock and mineral geochemistry to establish the temporal and magmatic evolution of the magmatic system. Volcanic activity occurred from 21.2 to 6.9 Ma, making it one of the longest records of volcanism in the western Ross Sea area. Mount Rittmann, an active volcano that is part of the MOVF, is not discussed here but extends the timing of volcanism of the MOVF into the Holocene. At Mount Overlord and surrounding areas, there were eruptions of lava flows, domes, and pyroclastic rocks. Localized deposits of hyaloclastites formed by magma-ice interactions provide an insight into former ice levels. Geochemically the MOVF shows a single magma differentiation trend except for Navigator Nunatak lavas which have a potassic affinity rarely seen in northern Victoria Land. Partial melting of an amphibole-bearing mantle lithology at or near the base of the continental lithospheric mantle (CLM) was the main source of the parental basaltic magmas. Polybaric crystal fractionation of the primary basaltic magmas mainly occurred at lower crustal depths and involved fractionation of clinopyroxene, olivine, kaersutite, feldspars, biotite, Fe–Ti oxides, apatite, and sodalite. Crustal assimilation of c. 10% granite harbor igneous complex granitoids was important in the evolution of intermediate composition magmas. Trachyte, phonolite, and comendite magmas stagnated and evolved at shallow crustal depths (c. <8 km). Over 95% crystal fractionation was required to generate the comendites. Extraction of the comendite melt from a felsic crystal mush was an important process. The potassic Navigator Nunatak magma required partial melting of phlogopite-bearing metasomatized CLM. The metasomes had ‘HIMU-like’ or FOZO isotopic compositions that ultimately originated from recycling of materials in the mantle. The MOVF displays a stronger affinity toward FOZO than other northern Victoria Land basaltic rocks. This suggests that the interaction between parental melt and juvenile CLM was limited, which is similar to volcanic rocks from the oceanic Adare Basin seamounts. Our result emphasizes the critical importance of a thick CLM for the genesis of diverse alkaline magma compositions in a continental rift system. 

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3. Mobilizing mush: assembly and eruption of modern monotonous basaltic andesite magma at Momotombo Volcano, Nicaragua

   https://doi.org/10.1007/s00445-025-01815-2  

   Tramontano, Samantha; Zayac, John; Espinoza, Eveling; Aldebot, Silvio; Ming, Cissy; Feineman, Maureen; La_Femina, Peter; Saballos, Armando; Monteleone, Brian; Longpré, Marc-Antoine (May 2025, Bulletin of Volcanology)

   Abstract Documenting erupted products and estimating pre-eruptive magmatic conditions is critical in assessing future volcanic hazards, since the physical and chemical conditions of magma assembly influence eruption style. Here, we present a petrologic examination of eruptive products at Momotombo Volcano, Nicaragua, with a focus on the 2015–2016 eruption, to identify the dominant magmatic processes that modulate modern eruptive history. Basaltic andesite lavas and tephras from the 2015–2016, 1905 and earlier eruptive episodes largely show monotonous major and trace element bulk rock compositions. Erupted products contain abundant (34–48 vol.%) and texturally diverse crystals of plagioclase and pyroxenes, commonly forming glomerocrysts, suggesting that the incorporation of a crystal mush dominates the bulk rock composition. In contrast, the matrix glasses of tephra samples and melt inclusions from the opening phase of the 2015–2016 eruption cover a wider compositional range, from basaltic andesite to dacite. We apply plagioclase–melt K₂O partitioning and thermodynamic models to demonstrate that diverse melt compositions (basalt to dacite) are required to produce the observed range in plagioclase compositions (An47 to An95). Together, we interpret (1) relatively evolved tephra glass compositions, (2) thin, reversely zoned rims of pyroxene crystals, and (3) low volatile saturation melt inclusion pressures to record an influx of new intermediate melt to the shallow crust (0.5–2 km) within days of the 2015–2016 eruption, with precursory seismic activity enabling magma movement and extending timescales of unrest back to 2013. Our results provide context for the locally high geothermal potential and silicic magma generation and assembly underneath Momotombo. 

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4. Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon

   https://doi.org/10.1093/petrology/egad058  

   Pitcher, Bradley W; Grunder, Anita L; Kent, Adam J (August 2023, Journal of Petrology)

   Abstract Ignimbrite flare-ups are rare periods of intense silicic volcanism during which the pyroclastic volume and eruptive frequency is more than an order of magnitude higher than background activity. Investigating the compositional differences between flare-up and steady-state magmas provides critical constraints on the petrogenetic causes for the event and can offer unique opportunities to investigate the role of large-scale tectonic or geodynamic processes in arc magmatism. In this study, we focus on the bimodal Deschutes Formation ignimbrite flare-up of Central Oregon, which erupted unusually high volumes of pyroclastic material 6.25–5.45 Ma from a new axis of volcanism in the Cascades arc. This episode is marked by increased eruption rates and eruption of more silicic compositions relative to the Quaternary Cascade arc, which rarely erupts rhyolites. Ignimbrites are crystal-poor (<10%) dacite to rhyolites (mostly 65–77 wt.% SiO2) with anhydrous mineral assemblages and higher FeO/MgO, Y, Eu/Eu*, MREE and Zr/Sr, indicating drier magmatic evolution compared to the Quaternary arc, and are more similar to those from the rear-arc High Lava Plains (HLP) province that lies to the east. Magnetite-ilmenite oxybarometry indicates that Deschutes Formation felsic magmas tend to be hotter and more reduced (NNO-1 to NNO) than the Quaternary arc (NNO to NNO + 1.5). Rhyolite-MELTS geobarometry suggests complex storage of diverse Deschutes Formation magmas within the shallow crust (50–250 MPa), and the common co-eruption of multiple plagioclase populations, pumice compositions, and compositionally banded pumice suggest variable degrees of mixing and mingling of distinct magmas. Deschutes magmas also have low δ18Oplagioclase values that indicate partial melting and assimilation of hydrothermally altered shallow crust. Trace element systematics and rhyolite-MELTS modeling suggests that felsic pumice cannot be produced by simple fractionation of co-erupted mafic pumice or basaltic lavas, and requires a crustal melting origin, and trace elements and Pb isotopes suggest that young mafic crust may have been the primary protolith. We suggest that partial melting produced low-Si rhyolite melt (~72 wt.%) that acted as both a parent for the most evolved rhyolites, and as a mixing endmember to create the dacite to rhyodacite magmas with heterogenous plagioclase populations. Unlike the predominantly calc-alkaline basalts erupted in the Quaternary Cascade arc, Deschutes Formation primary basalts are mostly low-K tholeiites, indicative of decompression melting. These are similar to the compositions erupted during a contemporaneous pulse of low-K tholeiite volcanism across the whole HLP that reached into the Cascades rear-arc. We suggest that intra-arc extension focused decompression melts from the back-arc into the arc and that tensional stresses allowed this high flux of hot-dry-reduced basalt throughout the crustal column, causing partial melting of mafic protoliths and the production of hot-dry-reduced rhyolite melts. Depletion of incompatible elements in successive rhyolites implies progressive depletion in fertility of the protolith. Extension also allowed for the establishment of a robust hydrothermal system, and assimilation of hydrothermally-altered rocks by magmas residing in a shallow, complex storage network lead to low δ18O melts. Our findings suggest the integral role that extensional tectonics played in producing an unusual ignimbrite flare-up of hot-dry-reduced rhyolite magmas that are atypical of the Cascades arc and may be an important contributor to flare-ups at arcs worldwide. 

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5. Evolution of Mafic Tungnárhraun Lavas: Transcrustal Magma Storage and Ascent Beneath the Bárðarbunga Volcanic System

   https://doi.org/10.3390/min15070687  

   Furman, Tanya; Kincaid, Denali; Brady, Collin Oborn (July 2025, Minerals)

   Costa, Simone; Caracciolo, Alberto (Ed.)

   The Tungnárhraun basalts in southern Iceland record a transcrustal magma system formed during Holocene deglaciation. These large-volume (>1 km3) Early through Mid-Holocene lavas contain ubiquitous plagioclase feldspar macrocrysts that are too primitive to have grown from the host lavas. Thermobarometry based on plagioclase melt and clinopyroxene melt equilibrium reveals a transcrustal structure with at least three distinct storage regions. A lower-crustal mush zone at ~14–30 km is fed by primitive, low 87Sr/86Sr magmas with diverse Ti/K and Al/Ti signatures. Plagioclase feldspar growth is controlled by an experimentally determined pseudoazeotrope where crystals develop inversely correlated An and Mg contents. The rapid ascent of magmas to mid-crustal levels (~8–9 km) allows the feldspar system to revert to conventional thermodynamic phase constraints. Continued plagioclase growth releases heat, causing olivine and pyroxene to be resorbed and giving the magmas their characteristic high CaO/Al2O3 values (~0.8–1.0) and Sc contents (~52 ppm in matrix material). Mid-Holocene MgO-rich lavas with abundant plagioclase feldspar macrocrysts erupted directly from this depth, but both older and younger magmas ascended to a shallow-crustal storage chamber (~5 km) where they crystallized olivine, clinopyroxene, and plagioclase feldspar and evolved to lower MgO contents. The Sr isotope differences between the plagioclase macrocrysts and their carrier melts suggest that the fractionation involves the minor assimilation of country rock. This model does not require the physical disruption of an established and long-lived gabbroic cumulate mush. The transcrustal structures documented here existed in south Iceland at least throughout the Holocene and likely influenced much of Icelandic magmatism. 

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