Phase-equilibria modeling of magma mixing: Petrogenetic case studies featuring the Magma Chamber Simulator
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Phase-equilibria modeling of magma mixing: Petrogenetic case studies featuring the Magma Chamber Simulator

  • Author(s): Scruggs, Melissa
  • Advisor(s): Spera, Frank J
  • et al.
Abstract

In the last 40 years or so, there has been a renaissance regarding the commonality and importance of magma bodies as open systems, where energy and mass are exchanged between a magma body and its surroundings over its lifetime. Paralleling this has been the advancement of computing technologies and significant developments in the fields of petrology and petrochemistry, resulting in a plethora of phase-equilibria modeling options available to the modern day igneous petrologist. This dissertation demonstrates how the additional constraints imposed by considering phase-equilibria when examining variations in major oxide, trace element, and isotope compositions can provide further insight into the different magmatic processes at play in an active volcanic system. This dissertation consists of four chapters: 1) software development and packaging of MCS-PhaseEQ, a phase-equilibria modeling program with application to the major element systematics of open magmatic systems; 2) software development and packaging of MCS-Traces, a trace element and isotope systematics calculator for open magmatic systems, to be used in conjunction with MCS-Traces; 3) A case study of Kilauea Volcano, where MCS-PhaseEQ is used to address questions of magma identity, magma volumes, and the petrogenetic processes required to produce the evolved lavas erupted during the January 1997 Episode 54 fissure eruption; and 4) A case study of Chaos Crags, where MCS-PhaseEQ and MCS-Traces are used to resolve seemingly conflicting geochemical signatures of Chaos Crags mafic enclaves.Chapters 1 and 2 present a user-friendly, refined software package for the Magma Chamber Simulator (MCS; see Bohrson et al. 2014)—a rigorous, energy- and mass-constrained thermodynamic model for evaluating the relative contributions of different magmatic processes in complex, open systems. MCS updates include a new, streamlined user interface, comprehensive data production and visualization, the ability to simulate up to 30 recharge events, a suite of online tutorials and new case studies, the addition of stoping as a method of assimilation, and the application of trace element and isotope calculations to MCS-PhaseEQ model outputs to track changes in geochemical compositions during mixing, assimilation, and crystallization. The case studies presented herein demonstrate the importance of examining variations in trace element and isotope compositions—in addition to major oxide and mineral compositions and relationships—as they record geochemical signatures that might be otherwise obfuscated if only the major oxide compositions of a magma are investigated. Chapter 3 presents an investigation of the petrogenetic origin of unusually evolved lavas erupted at a mantle-plume fed, intraplate ocean-island basalt volcano. Chapter 3 examines whether the geochemical and petrological signatures of low-MgO lavas erupted along the East Rift Zone of Kilauea Volcano on 30-31 January 1997 (Episode 54) can be explained by mixing between juvenile basaltic magmas and partially crystalline material from earlier eruptions, and also whether calculated mixing proportions are consistent with GPS-based geodetic inversions of ground deformation and intrusion growth. Open-system phase-equilibria thermodynamic models were used to constrain the composition, degree of differentiation, and thermodynamic state of the rift-stored, two Pyx-Pl saturated low-MgO magma body immediately preceding its mixing with high-MgO recharge and degassed drainback magmas, shortly before disruptive fissure activity within Napau Crater began. Mixing models constructed using the Magma Chamber Simulator reproduce the mineralogy and compositions of Episode 54 lavas within uncertainties, and suggest that the identity of the low-MgO magma body may be either variably differentiated remnants of un-erupted magmas intruded into Napau Crater in October 1968, or another spatially and compositionally similar magma body. Fissure A-E lavas can be replicated by a 43:57 mixture of mafic magmas with a low-MgO magma produced by ~23% fractionation of the 1968 intrusion. Fissure F lavas are reproduced by a 40:60 mixture of the same mafic recharge with a more evolved magma produced by ~35% fractionation of the 1968 intrusion. The resultant mineral assemblages and compositions suggest that this rift-stored magma body was ~40-50% crystalline at the time of mixing, and likely compositionally stratified. Phase-equilibria model results corroborate field and geochemical relationships indicating that sub-edificial intrusions at intraplate shield volcanoes crystallize and evolve, only to be remobilized later by mafic recharge magma—and also demonstrate that the pre-eruptive conditions of an intrusive body may be recovered by examining mineral compositions within mixed lavas. Discrepancies between the geodetically-consistent mixing proportions and those used in our mixing models highlight the uneven and complex nature of incomplete mixing on shorter, more localized scales reflected in erupted lavas, whereas geodetically-determined mixing proportions may reflect large-scale contributions to an entire, un-erupted intrusive body. Chapter 4 affirms the importance of including phase-equilibria constrained trace element and isotopic calculations when evaluating the petrogenesis of mafic enclaves. This chapter presents new trace element and 87Sr/86Sr and 143Nd/144Nd isotope data for mafic enclaves and host lavas of the ~1100 BP eruption of Chaos Crags (a series of six lava domes in the Lassen Volcanic Center, Northern California), which documents the contamination of mafic endmember magmas by anatectic melts derived by low degrees of partial melting of a Sierra Nevada granitoid (sensu lato) wallrock at mid-crustal depths, prior to their later mixing with a rhyodacitic mush that dominated the system at shallow depth. Enclaves whose isotopic signatures suggest higher extents of contamination are found in the latter half of the eruptive sequence (Domes D-F), consistent with a meltback of the conduit walls during the latter stages of magma ascent, after significant wallrock heating. The contamination event is followed by recharge of now-variably contaminated mafic magmas into the shallow-level rhyodacitic magma body. This two-stage petrogenetic sequence of Assimilation and Fractional Crystallization (AFC) followed by Recharge (R) reproduces the isotopic trends, and closely approximates the mineralogical, trace element and major oxide compositions of Chaos Crags mafic enclaves. The results of our study allow for a more nuanced and quantified temporal view of magmatic evolution for this system and serve as a reminder of the importance and spatially heterogeneous pattern of upper crustal assimilation processes in volcanic arc lavas when mafic magmas ascend through thick sections of older and colder ‘granitoid’ crust.

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