Jones
Meghan R.
Jones
Meghan R.
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ArticleThe mechanical response of a magma chamber with poroviscoelastic crystal mush(American Geophysical Union, 2021-01-28) Liao, Yang ; Soule, S. Adam ; Jones, Meghan R. ; Le Mével, HélèneImproved understanding of the impact of crystal mush rheology on the response of magma chambers to magmatic events is critical for better understanding crustal igneous systems with abundant crystals. In this study, we extend an earlier model by Liao et al. (2018); https://doi.org/10.1029/2018jb015985 which considers the mechanical response of a magma chamber with poroelastic crystal mush, by including poroviscoelastic rheology of crystal mush. We find that the coexistence of the two mechanisms of poroelastic diffusion and viscoelastic relaxation causes the magma chamber to react to a magma injection event with more complex time-dependent behaviors. Specifically, we find that the system’s short-term evolution is dominated by the poroelastic diffusion process, while its long-term evolution is dominated by the viscoelastic relaxation process. We identify two post-injection timescales that represent these two stages and examine their relation to the material properties of the system. We find that better constraints on the poroelastic diffusion time are more important for the potential interpretation of surface deformation using the model.
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ArticleSubmarine giant pumice: A window into the shallow conduit dynamics of a recent silicic eruption.(Springer, 2019-06-29) Mitchell, Samuel J. ; Houghton, Bruce ; Carey, Rebecca ; Manga, Michael ; Fauria, Kristen ; Jones, Meghan R. ; Soule, S. Adam ; Conway, Chris E. ; Wei, Zihan ; Giachetti, ThomasMeter-scale vesicular blocks, termed “giant pumice,” are characteristic primary products of many subaqueous silicic eruptions. The size of giant pumices allows us to describe meter-scale variations in textures and geochemistry with implications for shearing processes, ascent dynamics, and thermal histories within submarine conduits prior to eruption. The submarine eruption of Havre volcano, Kermadec Arc, in 2012, produced at least 0.1 km3 of rhyolitic giant pumice from a single 900-m-deep vent, with blocks up to 10 m in size transported to at least 6 km from source. We sampled and analyzed 29 giant pumices from the 2012 Havre eruption. Geochemical analyses of whole rock and matrix glass show no evidence for geochemical heterogeneities in parental magma; any textural variations can be attributed to crystallization of phenocrysts and microlites, and degassing. Extensive growth of microlites occurred near conduit walls where magma was then mingled with ascending microlite-poor, low viscosity rhyolite. Meter- to micron-scale textural analyses of giant pumices identify diversity throughout an individual block and between the exteriors of individual blocks. We identify evidence for post-disruption vesicle growth during pumice ascent in the water column above the submarine vent. A 2D cumulative strain model with a flared, shallow conduit may explain observed vesicularity contrasts (elongate tube vesicles vs spherical vesicles). Low vesicle number densities in these pumices from this high-intensity silicic eruption demonstrate the effect of hydrostatic pressure above a deep submarine vent in suppressing rapid late-stage bubble nucleation and inhibiting explosive fragmentation in the shallow conduit.
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ArticleIdentification of erosional terraces on seamounts : implications for interisland connectivity and subsidence in the Galápagos Archipelago(Frontiers Media, 2018-07-03) Schwartz, Darin M. ; Soule, Samuel A. ; Wanless, V. Dorsey ; Jones, Meghan R.Shallow seamounts at ocean island hotspots and in other settings may record emergence histories in the form of submarine erosional terraces. Exposure histories are valuable for constraining paleo-elevations and sea levels in the absence of more traditional markers, such as drowned coral reefs. However, similar features can also be produced through primary volcanic processes, which complicate the use of terraced seamounts as an indicator of paleo-shorelines. In the western Galápagos Archipelago, we utilize newly collected bathymetry along with seafloor observations from human-occupied submersibles to document the location and depth of erosional terraces on seamounts near the islands of Santiago, Santa Cruz, Floreana, Isabela, and Fernandina. We directly observed erosional features on 22 seamounts with terraces. We use these observations and bathymetric analysis to develop a framework to identify terrace-like morphologic features and classify them as either erosional or volcanic in origin. From this framework we identify 79 erosional terraces on 30 seamounts that are presently found at depths of 30 to 300 m. Although intermittent subaerial connectivity between the islands has been hypothesized, the depths of these erosional terraces in the Santiago region are the first direct evidence of paleo-connectivity in the modern archipelago. Collectively, the terraces have non-randomly distributed depths. We suggest that peaks in the distribution of terrace depths likely represent long durations of exposure (i.e., sea-level still or lowstands). By comparing these peaks to those of subsidence adjusted sea-level curves, we identify the average subsidence rate that best reproduces the observed terrace distributions. These rates are 0.2–0.4 m/ka for this portion of the central Galápagos, since the formation of the seamounts, consistent with previous independent estimates. Using these subsidence rates and evidence for erosional terraces at depths up to 300 m, we conclude that all islands in the central archipelago have been intermittently connected starting between 435 and 900 ka. Individual island pairs have likely been repeatedly subaerially connected for short intervals since that time.
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ArticleThe final stages of slip and volcanism on an oceanic detachment fault at 13°48′N, Mid‐Atlantic Ridge(John Wiley & Sons, 2018-09-14) Parnell-Turner, Ross ; Mittelstaedt, Eric ; Kurz, Mark D. ; Jones, Meghan R. ; Soule, Samuel A. ; Klein, Frieder ; Wanless, V. Dorsey ; Fornari, Daniel J.While processes associated with initiation and maintenance of oceanic detachment faults are becoming better constrained, much less is known about the tectonic and magmatic conditions that lead to fault abandonment. Here we present results from near‐bottom investigations using the submersible Alvin and autonomous underwater vehicle Sentry at a recently extinct detachment fault near 13°48′N, Mid‐Atlantic Ridge, that allow documentation of the final stages of fault activity and magmatism. Seafloor imagery, sampling, and near‐bottom magnetic data show that the detachment footwall is intersected by an ~850 m‐wide volcanic outcrop including pillow lavas. Saturation pressures in these vesicular basalts, based on dissolved H2O and CO2, are less than their collection pressures, which could be explained by eruption at a shallower level than their present depth. Sub‐bottom profiles reveal that sediment thickness, a loose proxy for seafloor age, is ~2 m greater on top of the volcanic terrain than on the footwall adjacent to the hanging‐wall cutoff. This difference could be explained by current‐driven erosion in the axial valley or by continued slip after volcanic emplacement, on either a newly formed or pre‐existing fault. Since current speeds near the footwall are unlikely to be sufficient to cause significant erosion, we favor the hypothesis that detachment slip continued after the episode of magmatism, consistent with growing evidence that oceanic detachments can continue to slip despite hosting magmatic intrusions.
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ArticleSubmarine deep‐water lava flows at the base of the western Galápagos Platform(John Wiley & Sons, 2018-10-25) Anderson, Molly ; Wanless, V. Dorsey ; Schwartz, Darin M. ; McCully, Emma ; Fornari, Daniel J. ; Jones, Meghan R. ; Soule, Samuel A.To investigate the initial phases of magmatism at the leading edge of the upwelling mantle plume, we mapped, photographed, and collected samples from two long, deep‐water lava flows located at the western base of the Galápagos Platform using the remotely operated vehicle Hercules. Lavas were recovered from four areas on the seafloor west of Fernandina volcano, including the western flow fronts of two deep‐water flows, heavily sedimented terrain between the two flows, and the eastern, shallower end of one flow. The sediment cover and morphologies are distinct between the western flow fronts and the eastern region based on seafloor imagery, suggesting that the long lava flows are not a single eruptive unit. Major and trace element concentrations reveal both tholeiitic and alkalic compositions and support the interpretation that multiple eruptive units comprise the deep‐water flows. Alkalic lavas have higher [La/Sm]N ratios (2.05–2.12) and total alkali contents (5.18–5.40) compared to tholeiitic lavas, which have [La/Sm]N ratios ranging from 1.64 to 1.68 and total alkali contents ranging from 3.07 to 4.08 wt%. Radiogenic isotope ratios are relatively homogeneous, suggesting a similar mantle source. We use petrologic models to assess three alternative mechanisms for the formation of the alkalic magmas: (1) high‐pressure crystallization of clinopyroxene, (2) mixing of high silica and mafic magmas, and (3) variable extents of melting of the same mantle source. Our modeling indicates that the alkalic samples form from lower extents of melting compared to the tholeiitic lavas and suggests that the deep‐water alkalic lavas are analogous to the initial, preshield building phase observed south of Hawaii and at the base of Loihi Seamount.
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ThesisGeophysical and geochemical constraints on submarine volcanic processes(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2019-09) Jones, Meghan R.Submarine volcanic systems form new oceanic crust, host unique chemosynthetic ecosystems, concentrate rare metals, and provide a conduit for chemical transfer from the Earth's interior to hydrosphere. Although our understanding of submarine volcanoes has been historically limited due to their relative inaccessibility, recent observations from active systems provide valuable opportunities to address key open questions in submarine volcanology. This thesis provides new insight into submarine volcanic processes using observations and samples from the 2011 Axial Seamount eruption, the 2012 Havre Volcano eruption, and the Mid-Atlantic Ridge near 14°N. In Chapter 2, I develop best practices for quantifying vesicle textures and reconstructing total CO2 concentrations in mid-ocean ridge basalts (MORB). Based on synthetic vesicle populations, 2D and 3D measurements, and Raman spectroscopy, I show that traditional methods overestimate MORB CO2 concentrations by as much as 50%, which has important implications for estimating ridge CO2 flux. In Chapter 3, I apply methods from Chapter 2, along with a bubble growth model, to samples from the 2011 Axial Seamount eruption in order to evaluate magma ascent and lava flow rates. I show that the variability in ascent rates during the 2011 eruption spans the range previously proposed over the global mid-ocean ridge system. I suggest that the variability in ascent rates relates to lateral dike propagation and evolving reservoir overpressures and that ascent rates influence flow morphology. In Chapter 4, I address the origin of highly vesicular MORB that pop upon recovery from the seafloor. I show that bubble accumulation produces the high volatile concentrations in these popping rocks and demonstrate that mantle carbon concentrations are lower and less heterogeneous than previously proposed. In Chapter 5, I evaluate models for the submarine dispersal of giant pumice clasts using observations from the 2012 Havre Volcano eruption. I show that the seafloor distribution of giant pumice is controlled by conductive cooling, the advective displacement of steam by water through highly permeable pathways, and clast breakup during transport and deposition. Together, these chapters provide critical constraints on the flux of volatiles at mid-ocean ridges and the processes governing the emplacement of volcanic products on the seafloor.