Theoretical petrology

The central issues in petrology have remained remarkably unchanged in the last 50 years. In igneous petrology, the focus is on understanding the nature and cause of diversity in igneous rocks: on identifying primary magma types and constraints on the compositional and mineralogical characteristics, the physical conditions, and the evolutions of their source regions and on establishing the processes by which derivative magmas evolve from primary magmas. In metamorphic petrology, the major concern is with understanding the conditions and processes experienced by a rock in reaching its present state. In both igneous and metamorphic petrology, the ultimate goal is the integration of petrological constraints with those from other branches of earth science into regional and global theories of earth history. What has changed over the years, however, is the framework within which these issues are addressed: the backdrop provided by plate tectonics and geophysical constraints, the growing sophistication of chemical and physical models of rock systems, the ever increasing inputs from trace element and isotopic geochemistry, the sophistication and complexity of experimental approaches to petrological problems, and the growing body of detailed petrological studies of specific rock suites and associations from all over the world. What I will attempt in this report is to pinpoint and briefly review those areas of growing interest and emphasis in American efforts in petrology during the 1975–1978 quadrennium and the ways in which they were shaped by this framework

An experimental study of the grain-scale processes of peridotite melting : implications for major and trace element distribution during equilibrium and disequilibrium melting

Author Posting. © Springer, 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 156 (2008): 87-102, doi:10.1007/s00410-007-0275-8.The grain-scale processes of peridotite melting were examined at 1340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P– T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxenemelt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt.This work was supported by NSF grants EAR-0208141 and EAR-0510606 to Yan Liang

Failure Simulations at Multiple Length Scales in High Temperature Structural Alloys

A number of computational methodologies have been developed to investigate the deformation and damage mechanism of various structural materials at different length scale and under extreme loading conditions, and also to provide insights in the development of high-performance materials. In microscopic material behavior and failure modes, polycrystalline metals of interest include heterogeneous deformation field due to crystalline anisotropy, inter/intra grain or phase and grain boundary interactions. Crystal plasticity model is utilized to simulate microstructure based polycrystalline materials, and micro-deformation information, such as lattice strain evolution, can be captured based on crystal plasticity finite element modeling (CPFEM) in ABAQUS. The comparison of advanced experimental measurement and numerical simulation facilitates the understanding of the deformation and stress partitioning mechanisms in dual phase steel (DP980) and multilayered steel. For corrosion or oxidation induced failure in high temperature alloys, a cohesive zone model (CZM) is introduced to describe the interfacial traction and separation behavior. By coupling diffusion process with CZM, impurity degradation effect at grain boundary can be studied to predict intergranular failure mechanism under corrosive environments. On the other hand, microscopic numerical methods are not efficient or applicable in the damage predictions for structural components. To this end, elastic perfect plastic (EPP) model has been proposed as an efficient tool to evaluate creep and fatigue damage for structural material (nickel based superalloy A617, SS316 etc.) at elevated temperatures. This methodology will be applied in numerous finite element simulations. By comparing with simplified method test data, the feasibility of EPP methodology at elevated temperatures can be verified

High pressure experiments on kinetic and rheological properties of primitive alkaline magmas: constraints on deep magmatic processes at the Campi Flegrei Volcanic District

Defining the timescales of magma storage and ascent beneath active volcanoes is a fundamental tool in volcanological investigation of the last decade to constrain pre-eruptive magmatic processes and magma chamber dynamics, since it is able to provide the basis for volcanic hazard assessment. This Ph.D. project focuses on the investigation of the deep portion of the Campi Flegrei Volcanic District plumbing system (crustal-mantle boundary; ~25 km of depth), in correspondence of which the presence of a possible crystallization zone has been hypothesized on the basis of melt inclusion studies, seismic data interpretations, gravimetric and petrological modelling and experimental data. The Campi Flegrei Volcanic District, which includes the Campi Flegrei and the islands of Ischia and Procida, represents one of the most active volcanic areas in the Mediterranean region and one of the most dangerous volcanic complexes on Earth owing to the intense urbanization of the area. Many petrological, geochemical and geophysical surveys were carried out in the Campania Active Volcanic Area that have helped to define the main architecture and the development of the sub-volcanic system. Nevertheless, the dynamic processes that operate during the earliest, deepest differentiation steps of primitive magmas that fed all Campi Flegrei eruptions are yet poorly constrained. The knowledge of the dynamics and residence and ascent timescales of magma at deep levels, indeed, may be the key to understand the triggering mechanisms of volcanic eruptions, and are essential for understanding the rates at which magmas are supplied to volcanic complexes. In this thesis, the investigation of the kinetic and rheological properties of a K-basaltic magma at Moho depth, together with the partitioning of trace elements between crystal and melts, has allowed to fill some gaps relative to the knowledge of the deep portion of the Campi Flegrei Volcanic District plumbing system, providing magma residence time and ascent timescales, and models for deep magmatic differentiation processes

Pre-eruptive storage constraints of an active crystal mush using mineral-scale techniques

Crystal mushes are common conceptual models used to interpret the structure of a magmatic reservoirs in many volcanic environments and are drivers of differentiation that can generate explosive silicic eruptions. They are composed of a crystal-rich (45-65 vol%) framework, that may represent a plexus of intrusions, and their high crystallinity produces differentiated interstitial melt that is extracted to form an eruptible crystal-poor silicic melt lens cap. Here, I present a comprehensive study to determine petrologic constraints of an active crystal mush system that is hosted within a continental arc. I focus on crystal-rich mafic enclaves that are interpreted to represent fragments of a crystal mush and were hosted in a crystal-poor rhyolite flow from the 2011-12 eruption of Cordón Caulle, Chile. Mafic enclaves are commonly associated with mafic injection origins, but this study utilizes mineral textures, whole-rock geochemistry, and mineral chemistry and chemical zonation to argue for a more nuanced investigation of highly crystalline enclaves that points toward crystal mush origins. The Cordón Caulle mafic enclaves are basaltic endmembers in comparison to enclaves globally and display interlocking grain textures with simple zonation patterns indicative of slow continual growth within a crystal mush. Melt chemistry from the interstitial glass of the mafic enclaves determined a genetic relationship between the enclaves and their host lavas. This further corroborated the mush conceptual model as opposed to mafic injection magmas that intrude chemically distinct and pre-existing reservoirs. I determine quantitative storage constraints of the crystal mush using a variety of thermobarometers (Mg in plagioclase thermometer, Al in olivine-spinel thermometer, clinopyroxene-liquid barometer) that utilize chemical zonation in individual crystals. These methods reveal the basaltic mush resides at shallow crustal levels and relatively cooler temperatures compared to typical basalts (~100-350 MPa, ~920-970°C). Diffusion chronometry, which takes advantage of the chemical gradients in crystals, is used to determine timescales of magma residence (~104 yr), cooling paths (~103 yr), and the final stages of differentiation prior to eruption (~months to years) at Cordón Caulle. This study reports first-ever parameters on the underlying crystal mush that generated explosive silicic eruptions but can also be used as a model to interpret crystal mush systems globally and better understand the dynamics of rhyolite formation in continental arcs

Kinetic processes of mantle minerals

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999This dissertation discusses the experimental results designed to constrain the processes of MORB generation. The main focus of this study is to investigate the location and the related processes of the transformation boundary from spinel to garnet peridotite facies at subsolidus conditions, because the presence of garnet in melting residues has significant influence to the conclusion drawn from geochemical/geophysical observations. Using an approach that monitors the rate of reaction progresses, the experimental results confirmed the presence of a region that garnet and spinel coexist in peridotite compositions. The trace element distribution among the product phases (opx and cpx) subsequent to the garnet breakdown reaction is in disequilibrium, due to the differences of diffusivity between major and trace elements. The presence of disequilibrium distribution in nature may be used to infer time scales of geodynamic processes. Diffusion coefficients of A1 in diopside are experimentally determined, and used for modeling the equilibration of major elements in pyroxene during MORB genesis. In summary, this dissertation contributes two major inferences: the location of the transformation boundaries of the gamet-spinel peridotite; the presence of disequilibrium trace elements distribution with equilibrium major elements distribution in mantle pyroxenes

Compositional Zoning in Kilauea Olivine: A Geochemical Tool for Investigating Magmatic Processes at Hawaiian Volcanoes.

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

DFENS: Diffusion Chronometry Using Finite Elements and Nested Sampling

Abstract: In order to reconcile petrological and geophysical observations of magmatic processes in the temporal domain, the uncertainties in diffusion timescales need to be rigorously assessed. Here, we present a new diffusion chronometry method: Diffusion chronometry using Finite Elements and Nested Sampling (DFENS). This method combines a finite element numerical model with a nested sampling Bayesian inversion, meaning that uncertainties in the parameters contributing to diffusion timescale estimates can be obtained and that observations from multiple elements can be used to better constrain individual timescales. Uncertainties associated with diffusion timescales can be reduced by accounting for covariance in the uncertainty structure of diffusion parameters rather than assuming that they are independent of each other. We applied the DFENS method to the products of the Skuggafjöll eruption from the Bárðarbunga volcanic system in Iceland, which contains zoned macrocrysts of olivine and plagioclase that record a shared magmatic history. Olivine and plagioclase provide consistent pre‐eruptive mixing and mush disaggregation timescales of less than 1 year. The DFENS method goes some way toward improving our ability to rigorously address the uncertainties of diffusion timescales, but efforts still need to be made to understand other systematic sources of uncertainty such as crystal morphology, appropriate choice of diffusion coefficients, initial conditions, crystal growth, and the petrological context of diffusion timescales