Toward an understanding of disequilibrium dihedral angles in mafic rocks

[1] The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109° ± 2°). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. First, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78°, rather than 60° which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intraplutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centers of oikocrysts, associated with rounding of chadacrysts

Textural equilibrium melt geometries around tetrakaidecahedral grains.

In textural equilibrium, partially molten materials minimize the total surface energy bound up in grain boundaries and grain-melt interfaces. Here, numerical calculations of such textural equilibrium geometries are presented for a space-filling tessellation of grains with a tetrakaidecahedral (truncated octahedral) unit cell. Two parameters determine the nature of the geometries: the porosity and the dihedral angle. A variety of distinct melt topologies occur for different combinations of these two parameters, and the boundaries between different topologies have been determined. For small dihedral angles, wetting of grain boundaries occurs once the porosity has exceeded 11%. An exhaustive account is given of the main properties of the geometries: their energy, pressure, mean curvature, contiguity and areas on cross sections and faces. Their effective permeabilities have been calculated, and demonstrate a transition between a quadratic variation with porosity at low porosities to a cubic variation at high porosities

Conduit and Eruption Dynamics of the 1912 Vulcanian Explosions at Novarupta, Alaska.

Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017

Cumulus processes and melt-migration in layered intrusions and the use of image analysis to quantify microscopic textures in cumulates.

This thesis presents three studies carried out to investigate the relationship between small scale structures and large scale processes in layered intrusions. The first is a field-based study from the Eastern Layered Series of the Rhum intrusion. Lateral variations were found to be significant in the allivalite of unit 12 on Hallival, and the cumulate stratigraphy was shown to thin by approximately 50% over less than 800 metres down-dip. This thinning was associated with the development of layeringparallel zones of intense deformation and the entire cumulate package is interpreted to have evolved as a sequence of alternating troctolitic and peridotitic mass flows initiated by the intrusion of basaltic melts into marginal cumulates. In addition, a series of gabbroic cumulates were interpreted as preserving melt-migration pathways within still-porous cumulates. Cumulus textures were found to control porosity-permeability relationships in the crystal pile. The effect of anisotropy on the movement of melts was studied. Core material from the Muskox layered intrusion was analysed to constrain compaction and showed it to be a widespread process which reduced melt contents to -10% over cumulate depths of 1000 metres, in less than 600 years. Progressive contamination of Muskox primary melts resulted in a change from olivine-rich to pyroxene-rich cumulates and this lea to an increase in matrix viscosity from <5x10 15 Pa s to -4x1016. The final part of the thesis concentrates on the quantification of interfacial curvature, a parameter which dominates the evolution of textures by surface energy minimization. Software is presented to allow the determination of variations in two dimensional grain boundary curvatures by curve-fitting method using a cubic spline. The software is tested on experimental olivinebasalt aggregates

Constraints on melt migration in the Earth's upper mantle

Melting and melt segregation are key processes in the geochemical evolution of the Earth. However, mechanism and time scale of melt transport from the source to the surface are still not well understood and are dependent on the grain-scale distribution of melt. A related question is the retention of melt in partially molten regions of the Earths upper mantle. Seismic observations from mid-ocean ridges (MOR) and subduction zones are interpreted to show in-situ melt contents up to 3%, while geochemical observations from MOR basalts are inferred to indicate very efficient extraction of melt (porosities of order 0.1%). Earlier theoretical models of the melt distribution were based on the balance of surface tension between melt and uniform crystalline grains, predicting a simple net- work of melt along three-grain edges. Analyses of experimentally produced samples of olivine and basaltic melt show that the melt geometry is much more complex, and includes wetted two-grain boundaries. I reconstructed the melt geometry of two experimentally produced samples by serial sectioning and 3-D rendering of the pore geometry which demonstrates for the first time that melt exists in thin layers on two-grain boundaries. This confirms the inferences from previous 2-D observations and has significant implications for physical properties of partially molten regions, for example seismic velocities and attenuation. The wetted two-grain boundaries are inferred to be a consequence of continuous grain growth. Due to the complexity of the 3-D melt geometry the perme- ability of partially molten rocks can not be predicted from simple models. I therefore investigated the permeability as a function of porosity for both synthetic and ex- perimentally determined pore geometries using a lattice-Boltzmann method. The calculated permeability is not a simple function of porosity, but increases rapidly at a critical fraction of wetted two-grain boundaries. In order to extrapolate the experimentally based findings to grain sizes expected in natural rocks I examined the geometry of secondary phases inferred to represent relict melt in mantle peridotites from the Krivaja massif in Bosnia. These findings corroborate the experimental observations of wetted two-grain boundaries

The geochemical behavior of metalloids and their effect on the highly siderophile elements during the crystallization of a magmatic sulfide liquid in relation to the formation of Ni-Cu-PGE magmatic sulfide deposits

The highly siderophile elements (HSE), which include Re, Au and the platinum-group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) are of scientific and economic interest because they are tracers of geological processes (e.g., planetary differentiation) or have industrial or investment value. In most ore deposits, HSE are usually mined as by-products of Ni-Cu mineralization, such as those of the Sudbury district (Canada). Although at high temperature HSE are expected to be in lattice of sulfides they are usually found as discrete phases or platinum-group minerals (PGM). Many of PGM have metalloids (As, Se, Sb, Te and Bi) as essential constituents but, despite this relationship, the role of metalloids on HSE mineralization is still unclear. This dissertation documents the results of three experimental studies undertaken to investigate the effect of metalloids on the geochemistry of HSE. The first study examined whether the metalloids can mobilize HSE in an anhydrous environment. The results show that metalloids do not transport the HSE but also that Au, Pd and Pt can be efficiently fractionated from the other HSE and transported away from a sulfide assemblage through a relatively low-porosity, low-permeability medium in absence of hydrothermal fluids. The second study investigated the solubility of metalloids in monosulfide solid solution (mss) and intermediate solid solution (iss), to asses the conditions required for the formation of immiscible metalloid liquids and their effect on HSE partitioning. The results show that the metalloids form two separate phases, one that is As-rich and strongly sequestered Pt from the sulfides and another that is Bi-Te rich and affected mostly Pd and Au. The results also show that the other HSE (Ru, Rh, Re, Os, Ir) are largely unaffected by the presence of metalloids. In addition the results show that mss can dissolve significant amounts of metalloids (hundreds of ppm) before metalloid V saturation can occur. The third study constrained the geochemical behavior of metalloids and HSE in a pyrite-bearing sulfide system. The results show that Co-Ni-rich and HSE bearing pyrite can form during cooling of a sulfide melt and also that the HSE partition roughly equally between mss and pyrite, whereas Pd that partitions preferentially into mss and Au partitions preferentially into iss. Moreover, pyrite showed compositional zonation of many elements similar to that documented in natural pyrite samples that are also Co-rich and HSE-bearing. Thus, identification of such pyrite in natural assemblages is consistent with a magmatic origin.Doctor of Philosophy (Ph.D.) in Mineral Deposits and Precambrian Geolog