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1. Connecting Magma Chamber Mush Dynamics to Post Eruptive Reinflation at Cordón Caulle

   Patrick Phelps, Helge Gonnermann ( January 2021 , AGU Fall Meeting, New Orleans, LA & Online Everywhere, 13-17 December, 2021)

   Volcano inflation, for durations of months to years immediately following an eruption, has been observed at a number of volcanoes, including the 2011/12 eruption of Cordón Caulle, Chile. Such reinflation is often explained by replenishment of the magma reservoir from a deeper source. Whether and why that is the case remains uncertain in most instances, but the implications for renewed eruptive potential may be profound. Here, we posit redistribution of melt within a zoned magma reservoir consisting of a crystal rich mush overlain by an eruptible layer of crystal poor rhyolite as an alternate mechanism for reinflation. Such a zoned magma body is consistent with conceptual models for how crystal poor rhyolites form and with the presence of mafic enclaves within the Cordón Caulle rhyolite. The enclaves can be interpreted as pieces of mush entrained into the overlying rhyolite during its withdrawal from the reservoir. We test the hypothesis that melt from the inter-crystalline pores of the mush can redistribute by porous flow into the overlying crystal poor rhyolite, causing inflation after an eruption. We simulate the flow of melt within the zoned reservoir during and after eruption with a numerical model. As crystal poor rhyolite is erupted, magma pressuremore »within the rhyolite layer above the mush decreases. Consequently, interstitial melt flows upward within the mush, toward the reduced pressure at the interface of mush and crystal poor rhyolite. The mush is treated as a poroelastic material, with interstitial melt flow governed by Darcy's law. Thus, the change in pressure caused by withdrawal from the overlying rhyolite diffuses downward into the mush as the interstitial melt flows upward. The change in pore pressure results in an elastic deformation of the mush matrix. Because pore pressure diffusivity is small, melt redistribution can persist for years after eruptive activity ends, leading to slow inflation compared to fast eruptive deflation. We predict a partial recovery of volume lost from the eruption. Reinflation occurs because the expansion of decompressing melt flowing from the mush into the crystal poor rhyolite exceeds compression of the poroelastic mush. For cases where the interstitial melt is moderately compressible due to exsolved volatiles, our model reproduces the deformation observed at Cordón Caulle.« less
2. 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
3. Constraints on deep, CO2-rich degassing at arc volcanoes from solubility experiments on hydrous basaltic andesite of Pavlof Volcano, Alaska Peninsula, at 300 to 1200 MPa

   https://doi.org/10.2138/am-2021-7531  

   Mangan, Margaret T. ; Sisson, Thomas W. ; Hankins, W. Ben ; Shimizu, Nobumichi ; Vennemann, Torsten ( May 2021 , American Mineralogist)

   Abstract The solubility of CO2 in hydrous basaltic andesite was examined in fO2-controlled experiments at a temperature of 1125 °C and pressures between 310–1200 MPa. Concentrations of dissolved H2O and CO2 in experimental glasses were determined by ion microprobe calibrated on a subset of run glasses analyzed by high-temperature vacuum manometry. Assuming that the solubility of H2O in mafic melt is relatively well known, estimates of XH2Ofluid and PH2Ofluid in the saturating fluid were modeled, and by difference, values for XCO2fluid and PCO2fluid were obtained (XCO2 ~0.5–0.9); fCO2 could be then calculated from the fluid composition, temperature, and pressure. Dissolved H2O over a range of 2.3–5.5 wt% had no unequivocal influence on the dissolution of CO2 at the pressures and fluid compositions examined. For these H2O concentrations, dissolved CO2 increases with fCO2 following an empirical power-law relation: dissolved CO2 (ppmw) = 14.9−3.5+4.5[fCO2 (MPa)]0.7±0.03. The highest-pressure results plot farthest from this equation but are within its 1 standard-error uncertainty envelope. We compare our experimental data with three recent CO2-H2O solubility models: Papale et al. (2006); Iacono-Marziano et al. (2012); and Ghiorso and Gualda (2015). The Papale et al. (2006) and Iacono-Marizano et al. (2012) models give similar results, both over-predicting themore »solubility of CO2 in a melt of the Pavlof basaltic andesite composition across the fCO2 range, whereas the Ghiorso and Gualda (2015) model under-predicts CO2 solubility. All three solubility models would indicate a strong enhancement of CO2 solubility with increasing dissolved H2O not apparent in our results. We also examine our results in the context of previous high-pressure CO2 solubility experiments on basaltic melts. Dissolved CO2 correlates positively with mole fraction (Na+K+Ca)/Al across a compositional spectrum of trachybasalt-alkali basalt-tholeiite-icelandite-basaltic andesite. Shortcomings of current solubility models for a widespread arc magma type indicate that our understanding of degassing in the deep crust and uppermost mantle remains semi-quantitative. Experimental studies systematically varying concentrations of melt components (Mg, Ca, Na, K, Al, Si) may be necessary to identify solubility reactions, quantify their equilibrium constants, and thereby build an accurate and generally applicable solubility model.« less
4. Trace and Rare Earth Element Compositions of Lawsonite as a Chemical Tracer of Metamorphic Processes in Subduction Zones

   https://doi.org/10.1093/petrology/egac065  

   Kang, Patricia ; Whitney, Donna L ; Martin, Laure A ; Fornash, Katherine F ( August 2022 , Journal of Petrology)

   Abstract Lawsonite is a major host mineral of trace elements (TEs; e.g. REE, Sr, Pb, U, Th) and H2O in various rock types (metabasite, metasediment, metasomatite) over a wide range of depths in subduction zones. Consequently, the composition of lawsonite is a useful archive to track chemical exchanges that occurred during subduction and/or exhumation, as recorded in high-pressure/low-temperature (HP/LT) terranes. This study provides an extensive dataset of major element and TE compositions of lawsonite in HP/LT rocks from two mélanges (Franciscan/USA; Rio San Juan/Dominican Republic), two structurally coherent terranes (Tavşanlı/Turkey; Alpine Corsica/France), and the eclogite blocks of the Pinchi Lake/Canada complex. Bulk major and TE compositions were also determined for lawsonite-bearing host rocks to understand petrogenesis and assess compositional evolution. Most analyzed mélange and coherent-terrane metabasalts have normal mid-ocean ridge/back-arc basin basalt signatures and they preserve compositional evidence supporting interactions with (meta)sediment ± metagabbro/serpentinite (e.g. LILE/LREE enrichments; Ni/Cr enrichments). Most lawsonite grains analyzed are compositionally zoned in transition-metal elements (Fe, Ti, Cr), other TEs (e.g. Sr, Pb), and/or REE, with some grains showing compositional variations that correlate with zoning patterns (e.g. Ti-sector zoning, core-to-rim zoning in Fe, Cr-oscillatory zoning). Our results suggest that compositional variations in lawsonite formed in responsemore »to crystallographic control (in Ti-sector zoning), fluid–host rock interactions, modal changes in minerals, and/or element fractionation with coexisting minerals that compete for TEs (e.g. epidote, titanite). The Cr/V and Sr/Pb ratios of lawsonite are useful to track the compositional influence of serpentinite/metagabbro (high Cr/V) and quartz-rich (meta)sediment (low Sr/Pb). Therefore, lawsonite trace and rare earth element compositions effectively record element redistribution driven by metamorphic reactions and fluid–rock interactions that occurred in subduction systems.« less
5. CO2 and H2O in Plagioclase-Hosted Melt Inclusions from Ocean-Ridge Lavas: An Indicator of Crystallization in the Lower Oceanic Crust

   • Kotash, A. ; • Tepley, F. J. ; • Nielsen, R. ; • Bodnar, R. J. ( December 2019 , American Geophysical Union, Fall Meeting 2019, abstract #V13C-0172)

   Interpretation of erupted products we observe on the seafloor requires that we understand the petrogenesis of melts in the oceanic crust and where crystallization initially takes place. Our work focuses on estimating depth of crystallization of the plagioclase megacrysts using CO2 and H2O concentrations from plagioclase ultraphyric basalts (PUBs). Samples were analyzed from the Lucky Strike segment on the Mid-Atlantic Ridge and from three locations on the Juan de Fuca Ridge (West Valley, Endeavor Segment, and Axial Segment). Melt inclusions were re-homogenized to remove the effects of post-entrapment crystallization. The CO2 in the vapor bubbles present in the melt inclusions were analyzed at Virginia Tech using Raman spectroscopy, and associated glassy melt inclusions were analyzed at WHOI using the ion microprobe for CO2 and H2O. Vapor-saturation pressures calculated from these volatiles stored in melt inclusions and vapor bubbles range from 359-3994 bars, corresponding to depths of 1.0-11.4 km below the sea floor. The proportion of CO2 partitioned in the bubbles range from 11-98%. In summary, about 14% of the melt inclusions from Lucky Strike record crystallization depths of 3-4 km, consistent with the depth of the seismically imaged melt lens, whereas ~55% of melt inclusions crystallized at depths >4 kmmore »with a maximum at 9.8 km. These data are similar to depths of formation determined through olivine-hosted melt inclusions from the same segment (Wanless et al., 2015), although a greater portion of plagioclase-hosted melt inclusions record crystallization below the melt lens. At the Juan de Fuca ridge, ~24% of the melt inclusions record crystallization depths of 2-3 km, consistent with a seismically imaged mid-crustal magma chamber at the Endeavor Segment, while an additional ~62% crystallize at depths >3 km with a maximum at 11.4 km. This suggests that while crystallization can be focused within the melt lenses and magma chambers at these ridge localities, a significant and greater proportion of the megacrysts were sampled from the lower crust or upper mantle.« less