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1. Constraints of boron and oxygen stable isotopes on dehydration fluids, sediment-derived melts, and crustal assimilation of the Toba volcanic system (Indonesia)

   https://doi.org/10.1130/G51690.1  

   Liu, Ping-Ping; Wang, Dian-Bing; Zhou, Mei-Fu; Li, Xian-Hua; Li, Qiu-Li; Gaetani, Glenn A; Monteleone, Brian; Kamenetsky, Vadim (December 2023, Geology)

   Abstract Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8–7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ~0.02‰. Approximately 6–9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide. 

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2. The Systematics of Olivine CaO + Cr-Spinel in High-Mg# Arc Volcanic Rocks: Evidence for in-Situ Mantle Wedge Depletion at the Arc Volcanic Front

   https://doi.org/10.1093/petrology/egad085  

   Straub, Susanne M.; Batanova, Valentina; Sobolev, Alexander; Gómez-Tuena, Arturo; Espinasa-Perena, Ramon; Fleming, W. Lindsey; Bindeman, Ilya N.; Stuart, Finlay M.; Widom, Elisabeth; Iizuka, Yoshiyuki (December 2023, Journal of Petrology)

   Abstract We investigated the state of the arc background mantle (i.e. mantle wedge without slab component) by means of olivine CaO and its Cr-spinel inclusions in a series of high-Mg# volcanic rocks from the Quaternary Trans-Mexican Volcanic Belt. Olivine CaO was paired with the Cr# [molar Cr/(Cr + Al) *100] of Cr-spinel inclusions, and 337 olivine+Cr-spinel pairs were obtained from 33 calc-alkaline, high-K and OIB-type arc front volcanic rocks, and three monogenetic rear-arc basalts that lack subduction signatures. Olivine+Cr-spinels display coherent elemental and He–O isotopic systematics that contrast with the compositional diversity of the bulk rocks. All arc front olivines have low CaO (0.135 ± 0.029 wt %) relative to rear-arc olivines which have the higher CaO (0.248 ± 0.028 wt %) of olivines from mid-ocean ridge basalts. Olivine 3He/4He–δ18O isotope systematics confirm that the olivine+Cr-spinels are not, or negligibly, affected by crustal basement contamination, and thus preserve compositional characteristics of primary arc magmas. Variations in melt H2O contents in the arc front series and the decoupling of olivine CaO and Ni are inconsistent with controls on the olivine CaO by melt water and/or secondary mantle pyroxenites. Instead, we propose that low olivine CaO reflects the typical low melt CaO of high-Mg# arc magmas erupting through thick crust. We interpret the inverse correlation of olivine CaO and Cr-spinel Cr# over a broad range of Cr# (~10–70) as co-variations of CaO, Al and Cr of their (near) primary host melts, which derived from a mantle that has been variably depleted by slab-flux driven serial melt extraction. Our results obviate the need for advecting depleted residual mantle from rear- and back-arc region, but do not upset the larger underlying global variations of melt CaO high-Mg# arc magmas worldwide, despite leading to considerable regional variations of melt CaO at the arc front of the Trans-Mexican Volcanic Belt. 

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3. The deep magmatic cumulate roots of the Acadian orogen, eastern North America

   https://doi.org/10.1130/G47887.1  

   Tassara, S.; Ague, J.J.; Valencia, V. (September 2020, Geology)

   Abstract The dearth of cumulate magmatic roots in accretionary orogens is a cornerstone of models that postulate redistribution of mass and energy within the crust for the genesis of intermediate to silicic magmatism. Likewise, the origin of the evolved Acadian (Devonian) plutonism in the New England Appalachians (northeastern USA) has long been explained by closed-system crustal melting due to the absence of associated coeval deep mafic counterparts. Here, we report the discovery of Acadian hydrous ultramafic cumulate rocks that formed by deep-seated (∼1.1 GPa) fractional crystallization processes from a mantle-derived parental melt (Connecticut, southern New England, USA). These rocks are the first of their kind identified in the Appalachian orogen, and one of only a handful of preserved deep subarc hydrous cumulates worldwide. We propose a genetic link between the studied rocks and the evolved coeval plutonism in central-southern New England, where the former represent the missing deep cumulate roots of the same magmatic arc. Our findings support the hypothesis that differentiation of mantle-derived hydrous magmas by fractional crystallization and assimilation processes in the deep crust is a fundamental process in the production of intermediate to silicic magmatism and the geochemical evolution of the continental crust. 

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4. Post-melting oxidation of highly primitive basalts from the southern Andes

   https://doi.org/10.1016/j.gca.2020.01.042  

   Tassara, S. (January 2020, Geochimica et cosmochimica acta)

   null (Ed.)

   The oxygen fugacity (fO2) of the Earth’s upper mantle and its melting products is an important parameter in the geochemical evolution of arc magmas and their connection with the continental crustal construction and growth. Several works have focused on the fO2 of peridotite xenoliths, primitive melts in relatively young arc settings, and mid-ocean ridge basalts (MORB) but few studies have attempted to examine the early redox history of primitive magmas in mature arcs. Hence, our understanding of the nature and evolution of fO2 during the subduction cycle remains limited. Here, we investigate the basaltic tephra from the Los Hornitos monogenetic cones in central-southern Chile, which are among the most primitive materials reported in the Southern Andes (olivine Mg#
   92.5, and Ni
   5000 mgg1). These features offer a unique opportunity to explore the fO2 conditions below the Andean arc by studying olivine phenocrysts and their contained crystal and melt inclusions. We integrated EPMA, LA-ICP-MS, and m-XANES analyses to constrain the redox conditions recorded in the basaltic tephra by three different and self-reliant methods. First, we determined the fO2 based on the olivine-spinel equilibrium, yielding average values DFMQ + 1.3 ± 0.4 (1r). Second, we constrained the fO2 conditions of melt inclusions using Fe m-XANES data and the redox dependent olivine-melt vanadium partitioning. After correcting for post-entrapment crystallization and diffusive iron loss, the Fe m-XANES data indicate that the melt inclusions were trapped in average at DFMQ +2.5 ± 0.5 (1r). Results using the olivine-melt vanadium partitioning oxybarometer in melt inclusions are in agreement with Fe m-XANES data, yielding average DFMQ values of +2.6 ± 0.3 (1r). In order to test the potential effects of other postentrapment modifications of the melt inclusions that could have affected the fO2 prior to eruption, we assessed the residence time of these magmas using Mg-Fe interdiffusion modelling in olivine. The short residence times (<200 days) compared to vanadium re-equilibration models strongly suggest that the melt inclusions preserve the prevailing fO2 conditions during their entrapment. Correlations between melt inclusions major element composition and their fO2 determined by Fe m-XANES, as well as V/Sc modelling reveal a case of post-melting oxidation of the LHC magmas. We argue that primitive arc magmas behave as an open system with respect to fO2 during their early geochemical evolution. Our data indicate a complex fO2 early history of primitive melts in the southern Andes and provide a cautionary note on the direct extrapolation of primitive melts fO2 values to that of their mantle source. 

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5. A preliminary assessment of olivine phenocrysts from the monogenetic basalt of the McCartys Flow, Zuni-Bandera Volcanic Field, New Mexico

   Gary S. Michelfelder, Lawrence K. (October 2021, New Mexico Geological Society 72nd Annual Fall Field Conference Guidebook)

   Frey, Bonnie A.; Kelley, Shari A.; Zeigler, Kate E.; McLemore, Virginia T.; Goff, Fraser; Ulmer-Scholle, Dana S. (Ed.)

   Monogenetic small-volume basaltic volcanoes are the most abundant subaerial volcanic landforms on Earth but are some of the most poorly understood systems. Their short durations, small volumes, and lack of recurrence make monitoring and hazard assessment difficult. The Zuni-Bandera volcanic field in western New Mexico contains small-volume basaltic centers erupting tholeiitic to alkalic basalts. Evidence shows no correlation of magma composition with eruption age, location, or volumetric output, prompting questions about the influence of magma ascent rates, magma storage conditions, and mantle source characteristics on lava compositions. Here, we present olivine major and minor element mineral chemistry from the 3200-year-old McCartys Flow, the youngest tholeiite basalt in the volcanic field. Olivine displays four phenocryst types with unique textures and major and minor element compositions. Multiple olivine types co-exist at the thin section scale. Major and minor element diffusion at frozen melt–phenocryst interfaces was modeled, revealing magma residence times ranging from 3–9 months. Type 3 olivine phenocrysts require step function initial conditions and record diffusion re-equilibration followed by magma mixing. These profiles indicate the magma resided in the reservoir for 10–15 years and accumulated from multiple batches of mixed magmas less than 10 days before the eruption. Our results show that primitive magmas in small-volume monogenetic volcanoes have complex lithospheric magmatic histories and stored in magma bodies influenced by an open system to develop different local chemical environments. 

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