Matching Items (10)

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The Mineralogy and Chemical Evolution of the Earth’s Deep Mantle

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The mineralogy of the deep mantle is one of the key factors for the chemical evolution of the Earth. The constituent minerals of the mantle rock control the physical properties

The mineralogy of the deep mantle is one of the key factors for the chemical evolution of the Earth. The constituent minerals of the mantle rock control the physical properties of the mantle, which have significant impacts on the large-scale processes occurring in the Earth's interior. In my PhD research, I adopted experimental approaches to investigate the mineralogy and the physical properties of the Earth's materials in the deep mantle by using the diamond anvil cells (DACs) combined with in-situ X-ray diffraction techniques.

First, I found that Ca-bearing bridgmanite can be stable in the deeper part of the Earth's lower mantle where temperature is sufficiently high. The dissolution of calcium into bridgmanite can change the physical properties of the mantle, such as compressibility and viscosity. This study suggests a new mineralogical model for the lower mantle, which is composed of the two layers depending on whether calcium dissolves in bridgmanite or forms CaSiO3 perovskite as a separate phase.

Second, I investigated the mineralogy and density of the subducting materials in the Archean at the P-T conditions near 670 km-depth. The experiments suggest that the major phases of Archean volcanic crust is majoritic garnet and ringwoodite in the P-T conditions of the deep transition zone, which become bridgmanite with increasing pressure. The density model showed that Archean volcanic crust would have been denser than the surrounding mantle, promoting sinking into the lower mantle regardless of the style of the transportation in the Archean.

Lastly, I further investigated the mineralogies and densities of the ancient volcanic crusts for the Archean and Proterozoic at the P-T conditions of the lower mantle. The experiments suggest that the mineralogy of the ancient volcanic crusts is composed mostly of bridgmanite, which is systemically denser than the surrounding lower mantle. This implies that the ancient volcanic crusts would have accumulated at the base of the mantle because of their large density and thickness. Therefore, the distinctive chemistry of the ancient volcanic crusts from the surrounding mantle would have given a rise to the chemical heterogeneities in the region for billions of years.

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  • 2020

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Modelling Geochemical and Geobiological Consequences of Low-Temperature Continental Serpentinization

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The hydrous alteration of ultramafic rocks, known as serpentinization, produces some of the most reduced (H2 >1 mmolal) and alkaline (pH >11) fluids on Earth. Serpentinization can proceed even at

The hydrous alteration of ultramafic rocks, known as serpentinization, produces some of the most reduced (H2 >1 mmolal) and alkaline (pH >11) fluids on Earth. Serpentinization can proceed even at the low-temperature conditions (<50°C) characteristic of most of Earth’s continental aquifers, raising questions on the limits of life deep in the subsurface and the magnitude in the flux of reduced volatiles to the surface. In this work, I explored the compositions and consequences of fluids and volatiles found in three low-temperature serpentinizing environments: (1) active hyperalkaline springs in ophiolites, (2) modern shallow and deep peridotite aquifers, and (3) komatiitic aquifers during the Archean.

Around 140 fluids were sampled from the Oman ophiolite and analyzed for their compositions. Fluid compositions can be accounted for by thermodynamic simulations of reactions accompanying incipient to advanced stages of serpentinization, as well as by simulations of mass transport processes such as fluid mixing and mineral leaching. Thermodynamic calculations were also used to predict compositions of end-member fluids representative of the shallow and deep peridotite aquifers that were ultimately used to quantify energy available to various subsurface chemolithotrophs. Calculations showed that sufficient energy and power supply can be available to support deep-seated methanogens. An additional and a more diverse energy supply can be available when surfacing deep-seated fluids mix with shallow groundwater in discharge zones of the subsurface fluid pathway. Finally, the consequence of the evolving continental composition during the Archean for the global supply of H2 generated through komatiite serpentinization was quantified. Results show that the flux of serpentinization-generated H2 could have been a significant sink for O2 during most of the Archean. This O2 sink diminished greatly towards the end of the Archean as komatiites became less common and helped set the stage for the Great Oxidation Event. Overall, this study provides a framework for exploring the origins of fluid and volatile compositions, including their redox state, that can result from various low-temperature serpentinizing environments in the present and past Earth and in other rocky bodies in the solar system.

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  • 2020

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Solar wind sodium and potassium abundance analysis in Genesis diamond-on-silicon and silicon bulk solar wind collectors, and how hydration affects the microtexture of olivine phase transformation at 18 GPa

Description

The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on

The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on high-pressure olivine phase transformations.

NASA’s Genesis mission collected SW samples for terrestrial analysis to create a baseline of solar chemical abundances based on direct measurement of solar material. Traditionally, solar abundances are estimated using spectroscopic or meteoritic data. This study measured bulk SW Na and K in two different Genesis SW collector materials (diamond-like carbon (DlC) and silicon) for comparison with these other solar references. Novel techniques were developed for Genesis DlC analysis. Solar wind Na fluence measurements derived from backside depth profiling are generally lower in DlC than Si, despite the use of internal standards. Nevertheless, relative to Mg, the average SW Na and K abundances measured in Genesis wafers are in agreement with solar photospheric and CI chondrite abundances, and with other SW elements with low first ionization potential (within error). The average Genesis SW Na and K fluences are 1.01e11 (+9e09, -2e10) atoms/cm2 and 5.1e09 (+8e08, -8e08) atoms/cm2, respectively. The errors reflect average systematic errors. Results have implications for (1) SW formation models, (2) cosmochemistry based on solar material rather than photospheric measurements or meteorites, and (3) the accurate measurement of solar wind ion abundances in Genesis collectors, particularly DlC and Si.

Deep focus earthquakes have been attributed to rapid transformation of metastable olivine within the mantle transition zone (MTZ). However, the presence of H2O acts to overcome metastability, promoting phase transformation in olivine, so olivine must be relatively anhydrous (<75 ppmw) to remain metastable to depth. A microtextural analysis of olivine phase transformation products was conducted to test the feasibility for subducting olivine to persist metastably to the MTZ. Transformation (as intracrystalline or rim nucleation) shifts from ringwoodite to ringwoodite-wadsleyite nucleation with decreasing H2O content within olivine grains. To provide accurate predictions for olivine metastability at depth, olivine transformation models must reflect how changing H2O distributions lead to complex changes in strain and reaction rates within different parts of a transforming olivine grain.

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  • 2015

Using micro-scale observations to understand large-scale geophysical phenomena: examples from seismology and mineral physics

Description

Earthquake faulting and the dynamics of subducting lithosphere are among the frontiers of geophysics. Exploring the nature, cause, and implications of geophysical phenomena requires multidisciplinary investigations focused at a range

Earthquake faulting and the dynamics of subducting lithosphere are among the frontiers of geophysics. Exploring the nature, cause, and implications of geophysical phenomena requires multidisciplinary investigations focused at a range of spatial scales. Within this dissertation, I present studies of micro-scale processes using observational seismology and experimental mineral physics to provide important constraints on models for a range of large-scale geophysical phenomena within the crust and mantle.

The Great Basin (GB) in the western U.S. is part of the diffuse North American-Pacific plate boundary. The interior of the GB occasionally produces large earthquakes, yet the current distribution of regional seismic networks poorly samples it. The EarthScope USArray Transportable Array provides unprecedented station density and data quality for the central GB. I use this dataset to develop an earthquake catalog for the region that is complete to M 1.5. The catalog contains small-magnitude seismicity throughout the interior of the GB. The spatial distribution of earthquakes is consistent with recent regional geodetic studies, confirming that the interior of the GB is actively deforming everywhere and all the time. Additionally, improved event detection thresholds reveal that swarms of temporally-clustered repeating earthquakes occur throughout the GB. The swarms are not associated with active volcanism or other swarm triggering mechanisms, and therefore, may represent a common fault behavior.

Enstatite (Mg,Fe)SiO3 is the second most abundant mineral within subducting lithosphere. Previous studies suggest that metastable enstatite within subducting slabs may persist to the base of the mantle transition zone (MTZ) before transforming to high-pressure polymorphs. The metastable persistence of enstatite has been proposed as a potential cause for both deep-focus earthquakes and the stagnation of slabs at the base of the MTZ. I show that natural Al- and Fe-bearing enstatite reacts more readily than previous studies and by multiple transformation mechanisms at conditions as low as 1200°C and 18 GPa. Metastable enstatite is thus unlikely to survive to the base of the MTZ. Additionally, coherent growth of akimotoite and other high-pressure phases along polysynthetic twin boundaries provides a mechanism for the inheritance of crystallographic preferred orientation from previously deformed enstatite-bearing rocks within subducting slabs.

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  • 2015

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Mineralogy of the Silicon-Rich Mantle: Implications for Mars and Exoplanets

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With the InSight mission deploying a seismometer , Martian bulk chemical compositional models are more important than ever. Three largely consistent models for the Martian mantle have been suggested over

With the InSight mission deploying a seismometer , Martian bulk chemical compositional models are more important than ever. Three largely consistent models for the Martian mantle have been suggested over the past two decades. Of these three, two are fairly similar and one is dramatically different. Of these three, the EH70 (Sanloup et al., 1999) models have the systematically lower divalent cation to silicon ratios as compared to the other model, the DW85 (Dreibus and Wanke, 1985) model. However, impact of such a low (Mg+Fe+Ca)/Si ratio on mineralogy has not been experimentally investigated. Measurements have been made of the mineralogy of the EH70 bulk mantle composition (Sanloup et al., 1999)) through in-situ laser-heated diamond anvil cell (LHDAC) and large volume press (LVP). Majorite-garnet (Mj) dominated mineralogy has been observed up to 25 GPa. Bridgmanite (Bm) begins to appear from 25.2 GPa and continues in a mixed phase with Mj up to 27 GPa at which point only Bm and calcium perovskite (CaPv) remain. Akimotoite (Ak) is stable up to 1873 K, higher by ≈300 K compared to numerical calculations (Connolly, 2009). This may result in an Ak layer in the Martian mantle, something missing in Earth’s mantle. The overall ratio of pyroxene to olivine polymorphs by volume is high, approaching pure pyroxene. This agrees with numerical calculations. Additionally, ferropericlase (Fp) is stable at lower temperatures, suggesting a higher dependence on temperature for its stability, something that is different from Perple_X calculations which show a strong dependence on pressure. Furthermore, Mj, which make up a majority of the volume of EH70 mantles, was measured to increase in Fe content as pressure increases. The more oxidizing conditions coupled with the silicon-rich composition resulted in three times higher Fe3+ content in Mj as opposed to a pyrolite model. This increased Fe3+ meant our Mj composition approached that of skiagite (Ski,Fe2+ 3 Fe3+ 2 Si3O12) and this caused Mj to have a very low compressibility of only 152.8 GPa, lower than any other Mj compositions in literature. This result suggests that a mantle with EH70 bulk composition would have lower than predicted seismic wave velocities , lower than Perple_X predicts. The Al content of Mj was also found to suppress the first derivative of compressibility to 4.45, lower than that of Ski100 at 6.7. Such differences compared with pyrolitic composition are important to estimate the velocity profiles and to model the dynamics of the Martian mantle. This dataset of mineralogy and composition can also model terrestrial exoplanetary mantles. Current measurements of stellar abundances show a wide range of compositions, and especially compositions with (Mg+Fe+Ca)/Si ratios approaching 1 (Brewer and Fischer, 2016). This experimental study of EH70 composition can fill-in this gap.

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  • 2019

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Shock metamorphism in ordinary chondrites: constraining pressure and temperature history

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Shock metamorphism in meteorites constrains the impact histories of asteroids and planets. Shock-induced high-pressure (HP) minerals can provide more precise estimates of shock conditions than shock-induced deformation effects. In this

Shock metamorphism in meteorites constrains the impact histories of asteroids and planets. Shock-induced high-pressure (HP) minerals can provide more precise estimates of shock conditions than shock-induced deformation effects. In this research, I use shock features, particularly HP minerals, in ordinary-chondrite samples to constrain not only shock pressures but also the pressure-temperature-time (P-T-t) paths they experienced.

Highly shocked L5/6 chondrites Acfer 040, Mbale, NWA 091 and Chico and LL6 chondrite NWA 757 were used to investigate a variety of shock pressures and post-shock annealing histories. NWA 757 is the only highly shocked LL chondrite that includes abundant HP minerals. The assemblage of ringwoodite and majoritic garnet indicates an equilibration shock pressure of ~20 GPa, similar to many strongly shocked L chondrites. Acfer 040 is one of the only two chondrite samples with bridgmanite (silicate perovskite), suggesting equilibration pressure >25 GPa. The bridgmanite, which is unstable at low-pressure, was mostly vitrified during post-shock cooling. Mbale demonstrates an example of elevated post-shock temperature resulting in back-transformation of ringwoodite to olivine. In contrast, majoritic garnet in Mbale survives as unambiguous evidence of strong shock. In these two samples, HP minerals are exclusively associated with shock melt, indicating that elevated shock temperatures are required for rapid mineral transformations during the transient shock pulse. However, elevated post-shock temperatures can destroy HP minerals: in temperature sequence from bridgmanite to ringwoodite then garnet. NWA 091 and Chico are impact melt breccias with pervasive melting, blackening of silicates, recrystallization of host rock but no HP minerals. These features indicate near whole-rock-melting conditions. However, the elevated post-shock temperatures of these samples has annealed out HP signatures. The observed shock features result from a complex P-T-t path and may not directly reflect the peak shock pressure. Although HP minerals provide robust evidence of high pressure, their occurrence also requires high shock temperatures and rapid cooling during the shock pulse. The most highly shocked samples lack HP signatures but have abundant high-temperature features formed after pressure release.

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  • 2016

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Amorphous weathering products: evidence for basalt-water interactions and the relevance to paleo-environments on Mars

Description

Amorphous phases are detected over large regions of the Martian surface from orbit and in more localized deposits by rovers on the surface. Amorphous silicates can be primary or secondary

Amorphous phases are detected over large regions of the Martian surface from orbit and in more localized deposits by rovers on the surface. Amorphous silicates can be primary or secondary in origin, both having formed through very different processes, so the unambiguous identification of these phases is important for understanding the geologic history of Mars. Secondary amorphous silicates are poorly understood and underrepresented in spectral libraries because they lack the long-range structural order that makes their crystalline counterparts identifiable in most analytical techniques. Fortunately, even amorphous materials have some degree of short-range order so that distinctions can be made with careful characterization.

Two sets of laboratory experiments were used to produce and characterize amorphous weathering products under probable conditions for the Martian surface, and one global spectral analysis using thermal-infrared (TIR) data from the Thermal Emission Spectrometer (TES) instrument was used to constrain variations in amorphous silicates across the Martian surface. The first set of experiments altered crystalline and glassy basalt samples in an open system under strong (pH 1) and moderate (pH 3) acidic conditions. The second set of experiments simulated a current-day Martian weathering scenario involving transient liquid water where basalt glass weathering solutions, formed in circumneutral (pH ~5.5 and 7) conditions, were rapidly evaporated, precipitating amorphous silicates. The samples were characterized using visible and near-infrared (VNIR) spectroscopy, TIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

All experiments formed amorphous silicate phases that are new to spectral libraries. Moderately acidic alteration experiments produced no visible or spectral evidence of alteration products, whereas exposure of basalt glass to strongly acidic fluids produced silica-rich alteration layers that are spectrally consistent with VNIR and TIR spectra from the circum-polar region of Mars, indicating this region has undergone acidic weathering. Circum-netural pH basalt weathering solution precipitates are consistent with amorphous materials measured by rovers in soil and rock surface samples in Gale and Gusev Craters, suggesting transient water interactions over the last 3 billion years. Global spectral analyses determine that alteration conditions have varied across the Martian surface, and that alteration has been long lasting.

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  • 2016

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Unearthing the antibacterial activity of a natural clay deposit

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The discovery and development of novel antibacterial agents is essential to address the rising health concern over antibiotic resistant bacteria. This research investigated the antibacterial activity of a natural clay

The discovery and development of novel antibacterial agents is essential to address the rising health concern over antibiotic resistant bacteria. This research investigated the antibacterial activity of a natural clay deposit near Crater Lake, Oregon, that is effective at killing antibiotic resistant human pathogens. The primary rock types in the deposit are andesitic pyroclastic materials, which have been hydrothermally altered into argillic clay zones. High-sulfidation (acidic) alteration produced clay zones with elevated pyrite (18%), illite-smectite (I-S) (70% illite), elemental sulfur, kaolinite and carbonates. Low-sulfidation alteration at neutral pH generated clay zones with lower pyrite concentrations pyrite (4-6%), the mixed-layered I-S clay rectorite (R1, I-S) and quartz.

Antibacterial susceptibility testing reveals that hydrated clays containing pyrite and I-S are effective at killing (100%) of the model pathogens tested (E. coli and S. epidermidis) when pH (< 4.2) and Eh (> 450 mV) promote pyrite oxidation and mineral dissolution, releasing > 1 mM concentrations of Fe2+, Fe3+ and Al3+. However, certain oxidized clay zones containing no pyrite still inhibited bacterial growth. These clays buffered solutions to low pH (< 4.7) and oxidizing Eh (> 400 mV) conditions, releasing lower amounts (< 1 mM) of Fe and Al. The presence of carbonate in the clays eliminated antibacterial activity due to increases in pH, which lower pyrite oxidation and mineral dissolution rates.

The antibacterial mechanism of these natural clays was explored using metal toxicity and genetic assays, along with advanced bioimaging techniques. Antibacterial clays provide a continuous reservoir of Fe2+, Fe3+ and Al3+ that synergistically attack pathogens while generating hydrogen peroxide (H2O¬2). Results show that dissolved Fe2+ and Al3+ are adsorbed to bacterial envelopes, causing protein misfolding and oxidation in the outer membrane. Only Fe2+ is taken up by the cells, generating oxidative stress that damages DNA and proteins. Excess Fe2+ oxidizes inside the cell and precipitates Fe3+-oxides, marking the sites of hydroxyl radical (•OH) generation. Recognition of this novel geochemical antibacterial process should inform designs of new mineral based antibacterial agents and could provide a new economic industry for such clays.

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  • 2015

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Use of X-ray diffraction to identify and quantify soil swelling potential

Description

Expansive soils impose challenges on the design, maintenance and long-term stability of many engineered infrastructure. These soils are composed of different clay minerals that are susceptible to changes in moisture

Expansive soils impose challenges on the design, maintenance and long-term stability of many engineered infrastructure. These soils are composed of different clay minerals that are susceptible to changes in moisture content. Expansive clay soils wreak havoc due to their volume change property and, in many cases, exhibit extreme swelling and shrinking potentials. Understanding what type of minerals and clays react in the presence of water would allow for a more robust design and a better way to mitigate undesirable soil volume change. The relatively quick and widely used method of X-ray Diffraction (XRD) allows identifying the type of minerals present in the soil. As part of this study, three different clays from Colorado, San Antonio Texas, and Anthem Arizona were examined using XRD techniques. Oedometer-type testing was simultaneously preformed in the laboratory to benchmark the behavior of these soils. This analysis allowed performing comparative studies to determining if the XRD technique and interpretation methods currently available could serve as quantitative tools for estimating swell potential through mineral identification. The soils were analyzed using two different software protocols after being subjected to different treatment techniques. Important observations include the formation of Ettringite and Thaumasite, the effect of mixed-layer clays in the interpretation of the data, and the soils being subject to Gypsification. The swelling data obtained from the oedometer-type laboratory testing was compared with predictive swelling functions available from literature. A correlation analysis was attempted in order to find what index properties and mineralogy parameters were most significant to the swelling behavior of the soils. The analysis demonstrated that Gypsification is as important to the swelling potential of the soil as the presence of expansive clays; and it should be considered in the design and construction of structures in expansive soils. Also, the formation of Ettringite and Thaumasite observed during the treatment process validates the evidence of Delayed Ettringite Formation (DEF) reported in the literature. When comparing the measured results with a proposed method from the University of Texas at Arlington (UTA), it was found that the results were somewhat indicative of swell potential but did not explain all causes for expansivity. Finally, it was found that single index properties are not sufficient to estimate the free swell or the swell pressure of expansive soils. In order to have a significant correlation, two or more index properties should be combined when estimating the swell potential. When properties related to the soil mineralogy were correlated with swell potential parameters, the amount of Gypsum present in the soil seems to be as significant to the swell behavior of the soil as the amount of Smectite found.

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  • 2014

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Fluorine partitioning between nominally anhydrous minerals (olivine, clinopyroxene, and plagioclase) and silicate melt using secondary ion mass spectrometry and newly synthesized basaltic fluorine microanalytical glass standards

Description

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the melt during magma differentiation and rendering F less subject to disturbance during degassing upon magma ascent. Hence, the association between fluorine in basalts and fluorine in the mantle source region is more robust than for other volatile species. The ionic radius of F- is similar to that of OH- and O2-, and F may substitute for hydroxyl and oxygen in silicate minerals and melt. Fluorine is also incorporated at trace levels within nominally anhydrous minerals (NAMs) such as olivine, clinopyroxene, and plagioclase. Investigating the geochemical behavior of F in NAMs provides a means to estimate the pre-eruptive F contents of degassed magmas and to better understand the degassing behavior of H. The partition coefficients of F were determined for clinopyroxene, olivine, plagioclase, and hornblende within melts of olivine-minette, augite-minette, basaltic andesite, and latite compositions. The samples analyzed were run products from previously-published phase-equilibria experiments. Fluorine was measured by secondary ion mass spectrometry (SIMS) using an 16O- primary beam and detection of negative secondary ions (19F-, 18O-, 28Si-). SIMS ion intensities are converted to concentrations by analyzing matrix-matched microanalytical reference materials and constructing calibration curves. For robust F calibration standards, five basaltic glasses (termed Fba glasses) were synthesized in-house using a natural tholeiite mixed with variable amounts of CaF2. The Fba glasses were characterized for F content and homogeneity, using both SIMS and electron-probe microanalysis (EPMA), and used as F standards. The partition coefficients for clinopyroxene (0.04-028) and olivine (0.01-0.16) varied with melt composition such that DF (olivine-minette) < DF (augite-minette) < DF (basaltic andesite) < DF (latite). Crystal chemical controls were found to influence the incorporation of F into clinopyroxene, but none were found that affected olivine. Fluorine partitioning was compared with that of OH within clinopyroxenes, and the alumina content of clinopyroxene was shown to be a strong influence on the incorporation of both anions. Fluorine substitution into both olivine and clinopyroxene was found to be strongly controlled by melt viscosity and degree of melt polymerization.

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Date Created
  • 2012