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1. Insights for crystal mush storage utilizing mafic enclaves from the 2011–12 Cordón Caulle eruption

   https://doi.org/10.1038/s41598-022-13305-y  

   Winslow, Heather ; Ruprecht, Philipp ; Gonnermann, Helge M. ; Phelps, Patrick R. ; Muñoz-Saez, Carolina ; Delgado, Francisco ; Pritchard, Matthew ; Amigo, Alvaro ( December 2022 , Scientific Reports)

   Abstract Two distinct types of rare crystal-rich mafic enclaves have been identified in the rhyolite lava flow from the 2011–12 Cordón Caulle eruption (Southern Andean Volcanic Zone, SVZ). The majority of mafic enclaves are coarsely crystalline with interlocking olivine-clinopyroxene-plagioclase textures and irregular shaped vesicles filling the crystal framework. These enclaves are interpreted as pieces of crystal-rich magma mush underlying a crystal-poor rhyolitic magma body that has fed recent silicic eruptions at Cordón Caulle. A second type of porphyritic enclaves, with restricted mineral chemistry and spherical vesicles, represents small-volume injections into the rhyolite magma. Both types of enclaves are basaltic end-members (up to 9.3 wt% MgO and 50–53 wt% SiO 2 ) in comparison to enclaves erupted globally. The Cordón Caulle enclaves also have one of the largest compositional gaps on record between the basaltic enclaves and the rhyolite host at 17 wt% SiO 2 . Interstitial melt in the coarsely-crystalline enclaves is compositionally identical to their rhyolitic host, suggesting that the crystal-poor rhyolite magma was derived directly from the underlying basaltic magma mush through efficient melt extraction. We suggest the 2011–12 rhyolitic eruption was generated from a primitive basaltic crystal-rich mush that short-circuited the typical full range of magmatic differentiation in a singlemore »step.« less
2. Crystal Mush Storage Constraints for the Puyehue-Cordon Caulle Volcanic Complex

   Heather Winslow, Philipp Ruprecht ( January 2021 , AGU Fall Meeting New Orleans, LA & Online Everywhere, 13-17 December 2021)

   Puyehue-Cordon Caulle (PCC) is an active volcanic complex located in the SVZ of the Andes that has had three major historic rhyodacitic eruptions with the most recent event in 2011-12. We provide petrologic and geochemical evidence that PCC is underlain by a crystal mush using recently identified basaltic mafic enclaves that highlights the involvement of distinct mafic magma components during the 2011-12 eruption. We suggest the mafic enclaves represent remnants of the crystal-rich mush that get entrained during eruption of the crystal-poor rhyodacite melt lens cap. This architecture requires the basaltic mush to produce rhyodacite through efficient fractionation. The dominant population of enclaves are equigranular, crystal-rich (45-55%), vesiculated (10-20%), and display interlocking grains between phases. Vesicles have complex shapes filling the irregular interlocking textures, while phenocrysts show stepwise normal zoning (uniform plagioclase cores, ~An90, overgrown with weakly zoned rims, ~An60). A second porphyritic population may represent mafic recharge into the system that bypasses the mush unperturbed. The porphyritic enclaves have spherical vesicles and tightly bound primitive mineral compositions (Fo80-86 vs Fo70-86 in the equigranular enclaves). Published geothermobarometry from the 2011-12 rhyodacite suggests shallow magma storage (5-7 km, 100-140 MPa, 895°C), which we compare against newly determined mineral-mineral trace-element partitioning basedmore »thermometry. Our thermometry indicates the equigranular enclaves were stored at ~900-1000°C at the time of eruption suggesting both a compositionally and thermally zoned magma system. We combine this temperature information with trace element data and mass balance calculations from various minerals phases and melt to substantiate our previous hypothesis that the basaltic enclaves can produce rhyodacite given their crystallinity. These estimates may support a spatially connected basaltic crystal-mush underlying a rhyodacite melt lens cap further proving highly efficient rhyolite formation at PCC. PCC’s enclaves present one of the largest compositional gaps on record globally. We compare them to other enclave-bearing systems and how PCC is an important end-member to understand enclaves as well as rhyolite formation.« less
3. The Atlantis Bank Gabbro Massif, Southwest Indian Ridge

   https://doi.org/org/10.1186/s40645-019-0307-9  

   Dick, Henry J. ; Robinson, Paul T. ; MacLeod, C.J. MacLeod ; Kinoshita, Hajimu ( November 2019 , Progress in earth and planetary science)

   This paper presents the first detailed geologic map of in situ lower ocean crust; the product of six surveys of Atlantis Bank on the SW Indian Ridge. This combined with major and trace element compositions of primary magmatic phases in 99 seafloor gabbros shows there are both significant vertical and ridge-parallel variations in crustal composition and thickness, but a continuity of the basic stratigraphy parallel to spreading. This stratigraphy is not that of magmatic sedimentation in a large crustal magma chamber. Instead, it is the product of dynamic accretion where the lower crust formed by episodic intrusion, large-scale upward migration of interstitial melt due to crystal mush compaction, and continuous tectonic extension accompanied by hyper- and sub-solidus, crystal-plastic deformation. Five crossings of the gabbro-peridotite contact along the transform wall show that massive mantle peridotite is intruded by cumulate residues of moderately to highly evolved magmas, few of which could be even close to equilibrium with a primary mantle magma. This contact then does not represent the crust-mantle boundary as envisaged in the ophiolite analog for ocean crust. The residues of the magmas parental to the shallow crust must also lie beneath the center of the complex. This, and the nearlymore »complete absence of dunites in peridotites from the transform wall, shows that melt transport through the shallow lithosphere was largely restricted to the central region of the paleo-ridge segment. There is almost no evidence for a melt lens or high-level storage of primitive melt in the upper 1500 m of Atlantis Bank. Thus, the composition of associated mid-ocean ridge basalt appears largely controlled by fractional crystallization of primitive cumulates at depth, near or at the base of the crust, modified somewhat by melt-rock reaction during transport through the overlying cumulate pile to the seafloor. Inliers of the dike-gabbro transition show that the uppermost gabbros crystallized at depth and were then emplaced upward, as they cooled, into the zone of diking. ODP and IODP drilling along the center of the gabbro massif also found few primitive gabbros that could have been in equilibrium with the original overlying lavas. Evidence of large-scale upward, permeable transport of interstitial melt through the gabbros is ubiquitous. Thus, post-cumulus processes, including extensive reaction, dissolution, and re-precipitation within the cumulate pile have obscured nearly all evidence of earlier primitive origins. We suggest that many of the gabbros may have started as primitive cumulates but were hybridized and transformed by later, migrating melts to evolved compositions, even as they ascended to higher levels, while new primitive cumulates were emplaced near the base of the crust. Mass balance for a likely parental melt intruded from the mantle to form the crust, however, requires that such primitive cumulates must exist at depth beneath Atlantis Bank at the center of the magmatic complex. The Atlantis Bank Gabbro Massif accreted by direct magma intrusion into the lower crust, followed by upward diapiric flow, first as a crystal mush, then by solid-state, crystal-plastic deformation, and finally by detachment faulting to the sea floor. The strongly asymmetric spreading to the south, parallel to the transform, was due to fault capture, with the bounding faults on the northern rift valley wall cut off by the detachment fault, which extended across the zone of intrusion causing rapid migration of the plate boundary to the north.« less
4. Stacked sills forming a deep melt-mush feeder conduit beneath Axial Seamount

   https://doi.org/10.1130/G47223.1  

   Carbotte, Suzanne M. ; Arnulf, Adrien ; Spiegelman, Marc ; Lee, Michelle ; Harding, Alistair ; Kent, Graham ; Canales, Juan Pablo ; Nedimović, Mladen ( April 2020 , Geology)

   Abstract Magmatic systems are composed of melt accumulations and crystal mush that evolve with melt transport, contributing to igneous processes, volcano dynamics, and eruption triggering. Geophysical studies of active volcanoes have revealed details of shallow-level melt reservoirs, but little is known about fine-scale melt distribution at deeper levels dominated by crystal mush. Here, we present new seismic reflection images from Axial Seamount, northeastern Pacific Ocean, revealing a 3–5-km-wide conduit of vertically stacked melt lenses, with near-regular spacing of 300–450 m extending into the inferred mush zone of the mid-to-lower crust. This column of lenses underlies the shallowest melt-rich portion of the upper-crustal magma reservoir, where three dike intrusion and eruption events initiated. The pipe-like zone is similar in geometry and depth extent to the volcano inflation source modeled from geodetic records, and we infer that melt ascent by porous flow focused within the melt lens conduit led to the inflation-triggered eruptions. The multiple near-horizontal lenses are interpreted as melt-rich layers formed via mush compaction, an interpretation supported by one-dimensional numerical models of porous flow in a viscoelastic matrix.
5. Stacked sills forming a deep melt-mush feeder conduit beneath Axial Seamount

   https://doi.org/https://doi.org/10.1130/G47223.1  

   S. Carbotte, A. F. ( April 2020 , Geology)

   Magmatic systems are composed of melt accumulations and crystal mush that evolve with melt transport, contributing to igneous processes, volcano dynamics, and eruption triggering. Geophysical studies of active volcanoes have revealed details of shallow-level melt reservoirs, but little is known about fine-scale melt distribution at deeper levels dominated by crystal mush. Here, we present new seismic reflection images from Axial Seamount, northeastern Pacific Ocean, revealing a 3–5-km-wide conduit of vertically stacked melt lenses, with near-regular spacing of 300–450 m extending into the inferred mush zone of the mid-to-lower crust. This column of lenses underlies the shallowest melt-rich portion of the upper-crustal magma reservoir, where three dike intrusion and eruption events initiated. The pipe-like zone is similar in geometry and depth extent to the volcano inflation source modeled from geodetic records, and we infer that melt ascent by porous flow focused within the melt lens conduit led to the inflation-triggered eruptions. The multiple near-horizontal lenses are interpreted as melt-rich layers formed via mush compaction, an interpretation supported by one-dimensional numerical models of porous flow in a viscoelastic matrix.