Magma chambers versus mush zones: constraining the architecture of sub-volcanic plumbing systems from microstructural analysis of crystalline enclaves

Holness, Marian B.; Stock, Michael J.; Geist, D.

doi:10.1098/rsta.2018.0006

Cited by 44 publications

(54 citation statements)

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“…2). The five olivine groups identified in the Floreana samples have distinct morphologies and zoning patterns (see Section 2 above), suggesting chemically heterogeneous magma storage (Holness et al, 2019). In particular, the most evolved crystals (Group 4; Fo~70-75) are in equilibrium with melts that are more evolved than the Floreana erupted basalts (likely basaltic andesites).…”

Section: Mush Disaggregation Prior To Eruptionmentioning

confidence: 98%

“…In contrast, plagioclase triple junctions in the gabbroic xenoliths have ~120 o dihedral angles (Fig. 3A), indicating a high degree of textural equilibration (Holness et al, 2019(Holness et al, , 2005. Plagioclase textural equilibrium, along with the two-pyroxene phase assemblage, suggests that the gabbroic xenoliths This manuscript has been re-submitted to Journal of Petrology following peer-review (submitted 24 th of August 2020).…”

Section: Mush Crystallisation and Textural Equilibrationmentioning

confidence: 99%

“…Such extensive fractional crystallisation would be expected to result in the saturation and crystallisation of plagioclase and other accessory phases (e.g. apatite, magnetite/ilmenite, quartz), which are observed in more evolved xenoliths from Rabida island in the central Galápagos (Holness et al, 2019). However, these phases are absent in the Floreana xenoliths, indicating that either…”

Section: Reactive Porous Flow Within a Cumulate Mushmentioning

confidence: 99%

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Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

Gleeson

Gibson

Stock

2020

Journal of Petrology

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The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterised by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits, indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e., mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveals that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveal that the Floreana mush zones are located in the upper mantle, at a depth of 23.7 ± 5.1 km. Our barometric results are compared to recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.

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Section: Mush Disaggregation Prior To Eruptionmentioning

confidence: 98%

Section: Mush Crystallisation and Textural Equilibrationmentioning

confidence: 99%

Section: Reactive Porous Flow Within a Cumulate Mushmentioning

confidence: 99%

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Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

Gleeson

Gibson

Stock

2020

Journal of Petrology

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“…While volcanism clearly attests to the transit of heat and mass through the earth's crust, the rate at which this process occurs and the details of the mechanism by which the most viscous and explosive magmas evolve remain contentious. Most agree that such magmas are dominantly derived from extensive, fractional-crystallization-driven, long-lived "mushy" reservoirs (Hildreth, 2004), as demonstrated by thermal models (Dufek and Bachmann, 2010;Gelman et al, 2013;Karakas et al, 2017), geochronology (Schmitt et al, 2010;Schoene et al, 2012;Wotzlaw et al, 2013;Barboni et al, 2015;Szymanowski et al, 2017) and petrology/geochemistry (Deering and Bachmann, 2010;Pamukcu et al, 2013;Ellis et al, 2014;Wolff et al, 2015;Holness et al, 2019). Such reservoirs, in addition to generating large-volume, crystal-rich ignimbrites, produce crystal-poor eruptible melt batches through slow extraction over 10's to 100's of kyr (Bachmann and Bergantz, 2004;Bachmann and Huber, 2018;Jackson et al, 2018), though many contend that such melt-dominant reservoirs are ephemeral, and the production of large volumes therefore necessitates rapid assembly from smaller melt lenses (Annen, 2009;Wilson and Charlier, 2009;Druitt et al, 2012;Allan et al, 2013;Caricchi et al, 2014;Wotzlaw et al, 2014;Cooper et al, 2017;Flaherty et al, 2018;Shamloo and Till, 2019).…”

Section: Introductionmentioning

confidence: 94%

Rapid Magma Generation or Shared Magmatic Reservoir? Petrology and Geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

et al. 2019

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Large-volume silicic volcanism poses global hazards in the form of proximal pyroclastic density currents, distal ash fall and short-term climate perturbations, which altogether warrant the study of how silicic magma bodies evolve and assemble. The southern rocky mountain volcanic field (SRMVF) is home to some of the largest super-eruptions in the geological record, and has been studied to help address the debate over how quickly eruptible magma batches can be assembled-whether in decades to centuries, or more slowly over 100's of kyr. The present study focuses on the San Luis caldera complex within the SRMVF, and discusses the paradigms of rapid magma generation vs. rapid magma assembly. The caldera complex consists of three overlapping calderas that overlie the sources of three large-volume mid-Cenozoic ignimbrites: first, the Rat Creek Tuff (RCT; zoned dacite-rhyolite, 150 km 3), followed by the Cebolla Creek Tuff (mafic dacite, 250 km 3) and finally, the Nelson Mountain Tuff (NMT; zoned daciterhyolite, 500 km 3), which are all indistinguishable in age by 40 Ar/ 39 Ar dating. We argue for a shared magmatic history for the three units on the basis of (1) similar mineral trace element compositions in the first and last eruptions (plagioclase, sanidine, biotite, pyroxene, amphibole, titanite, and zircon), (2) overlapping zircon U-Pb ages in all three units, and (3) similar thermal rejuvenation signatures visible in biotite (low-Mn, high-Ba) and zircon (low-Hf, low-U) geochemistry within the RCT and NMT. It is postulated that the NMT was sourced from a pre-existing magma reservoir to the northeast, which is corroborated by the formation of the nearby Cochetopa Caldera during the eruption of the NMT. The inferred lateral magma transport has two important implications: (1) it demonstrates long-distance transport of highly viscosity magmas at volumes (100's of km 3) not previously recorded, and (2) the sourcing of magma from a nearby pre-existing magma reservoir greatly reduces the rate of magma generation necessary to explain the close coincidence of three overlapping, large-volume magma systems. Additionally, the concept of magmatic "flux" (km 3 kyr −1) is discussed in this context, and it is argued that an area-normalized flux (km 3 kyr −1 km −2) provides a more useful number for measuring magma production rates: it is expected that magmatic volumes will scale with footprint of the thermal anomaly, and not taking this into account may lead to errant volumetric

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“…Thus, microstructures formed during in situ nucleation and growth with no subsequent mobilization of grains, and in the absence of strong compositional or temperature gradients, are characterized by the absence of an SPO. They are furthermore characterized by the prevalence of irregular grain boundaries that are not generally parallel to expected growth faces, by the presence of small mineral inclusions on the irregular grain boundaries [termed "impingement lenses" by Holness et al (2007a)], by a wide range of grain sizes indicative of an extended nucleation period and the absence of hydrodynamic sorting (Holness et al, 2019), by the absence of any correlation between the location of compositional zoning and crystallographic orientation, and by the presence of partially included grains of the same phase as the enclosing grain (Holness et al, 2019) (Figure 1a). Within a group of samples from the same body, the average apparent aspect ratio of plagioclase will have a negative correlation with the median value of the clinopyroxeneplagioclase-plagioclase dihedral angle, cpp (Holness, 2015).…”

Section: Fabrics In Igneous Rocksmentioning

confidence: 99%

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

et al. 2020

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The strength of foliations defined by shape preferred orientation of plagioclase in troctolitic cumulates from the Layered Series of the Skaergaard intrusion, and the Rum Eastern Layered Intrusion, increases as the grains become more tabular, due either to the greater propensity of highly non-equant grains to be rearranged by magmatic currents or tectonic disruption of poorly consolidated mush, or by the effects of a pre-existing shape preferred orientation on final grain shape in fully solidified rocks. The stratigraphic evolution of grain shape, microstructures and fabrics in the lowest 320 m of the Skaergaard Layered Series records the progressive inflation of the chamber to its final size. During the earliest stages of solidification, the extent of in situ nucleation and growth on the chamber floor decreased upward through the stratigraphy, due to the development of a thermally insulating blanket of mush on the floor. An upward increase in foliation strength as the chamber inflated to its final size was a result of the increasing strength of convection of the bulk magma and an increasing contribution to the floor mush of crystals derived from the walls of the enlarging magma chamber. Plagioclase in the troctolites in the open-system magma chamber of the Rum Eastern Layered Intrusion is generally more equant than that in the Skaergaard intrusion, perhaps related to the slower crystal growth on the margins of the continuously replenished Rum chamber. Significant sub-solidus modification of original igneous microstructures is observed in Rum troctolites from parts of the stratigraphy recording frequent replenishment events.

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Magma chambers versus mush zones: constraining the architecture of sub-volcanic plumbing systems from microstructural analysis of crystalline enclaves

Abstract: One contribution of 15 to a Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Cited by 44 publications

References 84 publications

Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

Upper Mantle Mush Zones beneath Low Melt Flux Ocean Island Volcanoes: Insights from Isla Floreana, Galápagos

Rapid Magma Generation or Shared Magmatic Reservoir? Petrology and Geochronology of the Rat Creek and Nelson Mountain Tuffs, CO, USA

Insights Into Magma Chamber Processes From the Relationship Between Fabric and Grain Shape in Troctolitic Cumulates

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