Lucy Porritt | The University of British Columbia (original) (raw)

Papers by Lucy Porritt

Journal of Volcanology and Geothermal Research, 2008

The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pip... more The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pipes occupied by a wide variety of rock types including coherent, magmatic material and a range of fragmental, volcaniclastic deposits of both pyroclastic and resedimented origin. Geochemical analysis of a suite of samples from several of these bodies provides valuable insight into the nature of their components

Journal of Volcanology and Geothermal Research, 2009

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled th... more An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. >40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al2O3 and Na2O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~30% loss of fines whereas the column collapse deposit exhibits ~50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.

Bulletin of Volcanology, 2011

The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced ... more The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.

Journal of Volcanology and Geothermal Research, Jun 1, 2008

Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1)... more Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1) kimberlite geology is very model driven; (2) a highly genetic terminology drives deposit or facies interpretation; (3) the effects of alteration on preserved depositional textures have been grossly underestimated; (4) the level of understanding of the physical process significance of preserved textures is limited; and, (5) some inferred processes and deposits are not based on actual, modern volcanological processes. These issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits. The traditional, steep-sided southern African pipe model (Class I) consists of a steep tapering pipe with a deep root zone, a middle diatreme zone and an upper crater zone (if preserved). Each zone is thought to be dominated by distinctive facies, respectively: hypabyssal kimberlite (HK, descriptively called here massive coherent porphyritic kimberlite), tuffisitic kimberlite breccia (TKB, descriptively here called massive, poorly sorted lapilli tuff) and crater zone facies, which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite (RVK). Porphyritic coherent kimberlite may, however, also be emplaced at different levels in the pipe, as later stage intrusions, as well as dykes in the surrounding country rock. The relationship between HK and TKB is not always clear. Sub-terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis; modern in-vent volcanological processes should initially be considered to explain observed deposits. Crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes, indicating that the pipe was largely empty at the end of the eruption, and subsequently began to fill-in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents. Classes II and III Canadian kimberlite models have a more factual, descriptive basis, but are still inadequately documented given the recency of their discovery. The diversity amongst kimberlite bodies suggests that a three-model classification is an over-simplification. Every kimberlite is altered to varying degrees, which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in-vent context; few preserve original textures. The effects of syn- to post-emplacement alteration on original textures have not been adequately considered to date, and should be back-stripped to identify original textural elements and configurations. Applying sedimentological textural configurations as a guide to emplacement processes would be useful. The traditional terminology has many connotations about spatial position in pipe and of process. Perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies-mining terminological scheme because it is so entrenched. However, for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics, important variations in these, and the process origins. For example every deposit of TKB is different in componentry, texture, or depositional structure. However, because so many deposits in many different pipes are called TKB, there is an implication that they are all similar and that similar processes were involved, which is far from clear.

Journal of Volcanology and Geothermal Research, 2016

Pele's tears are a well known curiosity commonly associated with low viscosity basaltic explosive... more Pele's tears are a well known curiosity commonly associated with low viscosity basaltic explosive eruptions. However, detailed studies of these pyroclasts are rare and, thus, there is no full explanation for their formation. These intriguing pyroclasts have smooth glassy surfaces, vesiculated interiors ( $ 30%), and fluidal morphologies trending towards teardrops and then spheres as they decrease in size to o 2 mm. Detailed characterisation of Pele's tears from the 1959 fire-fountaining eruption of Kilauea Iki has led to a reassessment of the mechanisms of magma disruption and fragmentation, timescales of relaxation, and cooling rates that are responsible for their formation. We conclude that the particle size distributions and vesicularities of Pele's tears are representative of the magma properties at the moment of explosive disruption. However, the morphology of these unique pyroclasts results from reshaping through viscous relaxation, driven by surface tension forces, on a time scale fast enough to compete with cooling times.

The dominant type of pyroclasts found in kimberlite deposits are crystals of rounded to subrounde... more The dominant type of pyroclasts found in kimberlite deposits are crystals of rounded to subrounded olivine that have no apparent quenched melt or magma attached. We refer to these as free-olivine pyroclasts. Rounded to sub-rounded juvenile lapilli comprising olivine grains mantled by a thin rim of (micro) crystalline coherent kimberlite are also common but they are generally subordinate to free olivine grains. Within the juvenile lapilli vesicles are rare to absent, and the rims of quenched kimberlite can show alignment of microlites. The rounded nature of these juvenile pyroclasts is generally considered to be a manifestation of surface tension processes operating on a very low viscosity melt (i.e. kimberlite melt) although there are no direct measurements of physical properties for kimberlite melt. We suggest that the properties of kimberlite pyroclasts can be used to constrain the physical properties of kimberlite magmas and the styles of volcanic eruption. Specifically, we consider the implications that the morphology and internal structure of these pyroclasts has for the melt properties of kimberlite magmas. We also explore how pyroclast shape and size distributions can be used to constrain eruption dynamics and depositional processes. Lastly, we assess the significance of the highly abundant free olivine crystals in terms of the transport and eruption processes assumed to be responsible for liberating these crystals from their host melt.

Proceedings of 10th International Kimberlite Conference, 2013

American Mineralogist, 2015

We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjac... more We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjacketed cores of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland, under atmospheric pressure. The obsidian is bubble-and crystal-free with an H 2 O content of 0.11(4) wt%. The obsidian cores were heated above the glass transition temperature (T g ), held for 0.25-24 h, then quenched. During each experiment the volume of the samples increased as H 2 O vapor-filled bubbles nucleated and expanded. Uniquely, the bubbles did not nucleate on the surface of the core, nor escape, conserving mass during all experiments. Within each isothermal experimental suite, the cores increased in volume with time until they reached a maximum, after which continued heating caused no change in volume (measured by He-pycnometry). We interpret these T-t conditions as representing thermochemical equilibrium between the melt and exsolved vapor. These experiments are modeled to recover the 1-atm, temperature-dependent solubility of water in the rhyolite melt. Our results define the magnitude of retrograde solubility (-7.1 × 10 -3 wt% H 2 O per 100 °C) and provide estimates of the enthalpy and entropy of the H 2 O exsolution reaction [ΔH° = 17.8 kJ/mol, ΔS° = 107 J/(K·mol)]. We conclude by modeling the implications of retrograde solubility for the glass transition temperatures (T g ) of cooling volcanic systems at pressures relevant to volcanic conduits and the Earth's surface. All volcanic systems cool; the effects of retrograde solubility are to allow melts to rehydrate by H 2 O dissolution as they cool isobarically, thereby depressing T g and expanding the melt window. Ultimately, the melt is quenched at higher H 2 O contents and lower temperatures where the isobaric retrograde solubility curve "catches" the evolving T g .

Trials and Tribulations of Life on an Active Subduction Zone: Field Trips in and around Vancouver, Canada, 2014

Applied Earth Science, 2004

ABSTRACT

Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains ... more Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains that are derived mainly from disaggregation of peridotite. Here, we describe the shapes, sizes and surfaces of olivine grains recovered from kimberlite lavas erupted from the Quaternary Igwisi Hills volcano, Tanzania. The Igwisi Hills kimberlitic olivine grains are compared to phenocrystic olivine, liberated from picritic lavas, and mantle olivine, liberated from a fresh peridotite xenolith. Image analysis, scanning electron microscopy imagery and laser microscopy reveal significant differences in the morphologies and surface features of the three crystal populations. The kimberlitic olivine grains form smooth, rounded to ellipsoidal shapes and have rough flaky micro-surfaces that are populated by impact pits. Mantle olivine grains are characterised by flaked surfaces and indented shapes consistent with growth as a crystal aggregate. Phenocrystic olivine exhibit faceted, smooth-surfaced crystal faces. We suggest that the unique shape and surface properties of the Igwisi Hills kimberlitic olivine grains are products of the transport processes attending kimberlite ascent from mantle source to surface. We infer that the unique shapes and surfaces of kimberlitic olivine grains result from three distinct mechanical processes attending their rapid transport through the thick cratonic mantle lithosphere:

Proceedings of 10th International Kimberlite Conference, 2013

Quaternary Science Reviews, 2014

We present a descriptive genetic classification scheme and accompanying nomenclature for glaciovo... more We present a descriptive genetic classification scheme and accompanying nomenclature for glaciovolcanic edifices herein defined as tuyas: positive-relief volcanoes having a morphology resulting from ice confinement during eruption and comprising a set of lithofacies reflecting direct interaction between magma and ice/melt water. The combinations of lithofacies within tuyas record the interplay between volcanic eruption and the attending glaciohydraulic conditions. Although tuyas can range in composition from basaltic to rhyolitic, many of the characteristics diagnostic of glaciovolcanic environments are largely independent of lava composition (e.g., edifice morphology, columnar jointing patterns, glass distributions, pyroclast shapes). Our classification consolidates the diverse nomenclature resulting from early, isolated contributions of geoscientists working mainly in Iceland and Canada and the nomenclature that has developed subsequently over the past 30 years. Tuya subtypes are first recognized on the basis of variations in edifice-scale morphologies (e.g., flat-topped tuya) then, on the proportions of the essential lithofacies (e.g., lava-dominated flat-topped tuya), and lastly on magma composition (e.g., basaltic, lavadominated, flat-topped tuya). These descriptive modifiers potentially supply additional genetic information and we show how the combination of edifice morphologies and lithofacies can be directly linked to general glaciohydraulic conditions. We identify nine distinct glaciovolcanic model edifices that potentially result from the interplay between volcanism and glaciohydrology. Detailed studies of tuya types are critical for recovering paleo-environmental information through geological time, including: ice sheet locations, extents, thicknesses, and glaciohydraulics. Such paleo-environmental information represents a new, innovative, underutilized resource for constraining global paleoclimate models.

Physics and Chemistry of the Earth, Parts A/B/C, 2012

Kimberlite pipes represent the conduits and vents of eroded volcanoes and are commonly filled wit... more Kimberlite pipes represent the conduits and vents of eroded volcanoes and are commonly filled with both coherent and volcaniclastic kimberlite including pyroclastic deposits, resulting from explosive volcanic eruptions. The properties of pyroclastic deposits, including grain size and grain shape distributions of pyroclasts, provide insight into the intensity and style of explosive volcanic eruptions. Pyroclastic kimberlite deposits generally comprise abundant coarse

Journal of Volcanology and Geothermal Research, 2008

The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pip... more The Ekati property, Northwest Territories, Canada, hosts in excess of 150 volcanic kimberlite pipes occupied by a wide variety of rock types including coherent, magmatic material and a range of fragmental, volcaniclastic deposits of both pyroclastic and resedimented origin. Geochemical analysis of a suite of samples from several of these bodies provides valuable insight into the nature of their components

Journal of Volcanology and Geothermal Research, 2009

An integrated approach involving volcanology, geochemistry and numerical modelling has enabled th... more An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. >40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al2O3 and Na2O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~30% loss of fines whereas the column collapse deposit exhibits ~50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.

Bulletin of Volcanology, 2011

The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced ... more The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.

Journal of Volcanology and Geothermal Research, Jun 1, 2008

Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1)... more Five significant problems hinder advances in understanding of the volcanology of kimberlites: (1) kimberlite geology is very model driven; (2) a highly genetic terminology drives deposit or facies interpretation; (3) the effects of alteration on preserved depositional textures have been grossly underestimated; (4) the level of understanding of the physical process significance of preserved textures is limited; and, (5) some inferred processes and deposits are not based on actual, modern volcanological processes. These issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits. The traditional, steep-sided southern African pipe model (Class I) consists of a steep tapering pipe with a deep root zone, a middle diatreme zone and an upper crater zone (if preserved). Each zone is thought to be dominated by distinctive facies, respectively: hypabyssal kimberlite (HK, descriptively called here massive coherent porphyritic kimberlite), tuffisitic kimberlite breccia (TKB, descriptively here called massive, poorly sorted lapilli tuff) and crater zone facies, which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite (RVK). Porphyritic coherent kimberlite may, however, also be emplaced at different levels in the pipe, as later stage intrusions, as well as dykes in the surrounding country rock. The relationship between HK and TKB is not always clear. Sub-terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis; modern in-vent volcanological processes should initially be considered to explain observed deposits. Crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes, indicating that the pipe was largely empty at the end of the eruption, and subsequently began to fill-in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents. Classes II and III Canadian kimberlite models have a more factual, descriptive basis, but are still inadequately documented given the recency of their discovery. The diversity amongst kimberlite bodies suggests that a three-model classification is an over-simplification. Every kimberlite is altered to varying degrees, which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in-vent context; few preserve original textures. The effects of syn- to post-emplacement alteration on original textures have not been adequately considered to date, and should be back-stripped to identify original textural elements and configurations. Applying sedimentological textural configurations as a guide to emplacement processes would be useful. The traditional terminology has many connotations about spatial position in pipe and of process. Perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies-mining terminological scheme because it is so entrenched. However, for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics, important variations in these, and the process origins. For example every deposit of TKB is different in componentry, texture, or depositional structure. However, because so many deposits in many different pipes are called TKB, there is an implication that they are all similar and that similar processes were involved, which is far from clear.

Journal of Volcanology and Geothermal Research, 2016

Pele's tears are a well known curiosity commonly associated with low viscosity basaltic explosive... more Pele's tears are a well known curiosity commonly associated with low viscosity basaltic explosive eruptions. However, detailed studies of these pyroclasts are rare and, thus, there is no full explanation for their formation. These intriguing pyroclasts have smooth glassy surfaces, vesiculated interiors ( $ 30%), and fluidal morphologies trending towards teardrops and then spheres as they decrease in size to o 2 mm. Detailed characterisation of Pele's tears from the 1959 fire-fountaining eruption of Kilauea Iki has led to a reassessment of the mechanisms of magma disruption and fragmentation, timescales of relaxation, and cooling rates that are responsible for their formation. We conclude that the particle size distributions and vesicularities of Pele's tears are representative of the magma properties at the moment of explosive disruption. However, the morphology of these unique pyroclasts results from reshaping through viscous relaxation, driven by surface tension forces, on a time scale fast enough to compete with cooling times.

The dominant type of pyroclasts found in kimberlite deposits are crystals of rounded to subrounde... more The dominant type of pyroclasts found in kimberlite deposits are crystals of rounded to subrounded olivine that have no apparent quenched melt or magma attached. We refer to these as free-olivine pyroclasts. Rounded to sub-rounded juvenile lapilli comprising olivine grains mantled by a thin rim of (micro) crystalline coherent kimberlite are also common but they are generally subordinate to free olivine grains. Within the juvenile lapilli vesicles are rare to absent, and the rims of quenched kimberlite can show alignment of microlites. The rounded nature of these juvenile pyroclasts is generally considered to be a manifestation of surface tension processes operating on a very low viscosity melt (i.e. kimberlite melt) although there are no direct measurements of physical properties for kimberlite melt. We suggest that the properties of kimberlite pyroclasts can be used to constrain the physical properties of kimberlite magmas and the styles of volcanic eruption. Specifically, we consider the implications that the morphology and internal structure of these pyroclasts has for the melt properties of kimberlite magmas. We also explore how pyroclast shape and size distributions can be used to constrain eruption dynamics and depositional processes. Lastly, we assess the significance of the highly abundant free olivine crystals in terms of the transport and eruption processes assumed to be responsible for liberating these crystals from their host melt.

Proceedings of 10th International Kimberlite Conference, 2013

American Mineralogist, 2015

We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjac... more We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjacketed cores of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland, under atmospheric pressure. The obsidian is bubble-and crystal-free with an H 2 O content of 0.11(4) wt%. The obsidian cores were heated above the glass transition temperature (T g ), held for 0.25-24 h, then quenched. During each experiment the volume of the samples increased as H 2 O vapor-filled bubbles nucleated and expanded. Uniquely, the bubbles did not nucleate on the surface of the core, nor escape, conserving mass during all experiments. Within each isothermal experimental suite, the cores increased in volume with time until they reached a maximum, after which continued heating caused no change in volume (measured by He-pycnometry). We interpret these T-t conditions as representing thermochemical equilibrium between the melt and exsolved vapor. These experiments are modeled to recover the 1-atm, temperature-dependent solubility of water in the rhyolite melt. Our results define the magnitude of retrograde solubility (-7.1 × 10 -3 wt% H 2 O per 100 °C) and provide estimates of the enthalpy and entropy of the H 2 O exsolution reaction [ΔH° = 17.8 kJ/mol, ΔS° = 107 J/(K·mol)]. We conclude by modeling the implications of retrograde solubility for the glass transition temperatures (T g ) of cooling volcanic systems at pressures relevant to volcanic conduits and the Earth's surface. All volcanic systems cool; the effects of retrograde solubility are to allow melts to rehydrate by H 2 O dissolution as they cool isobarically, thereby depressing T g and expanding the melt window. Ultimately, the melt is quenched at higher H 2 O contents and lower temperatures where the isobaric retrograde solubility curve "catches" the evolving T g .

Trials and Tribulations of Life on an Active Subduction Zone: Field Trips in and around Vancouver, Canada, 2014

Applied Earth Science, 2004

ABSTRACT

Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains ... more Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains that are derived mainly from disaggregation of peridotite. Here, we describe the shapes, sizes and surfaces of olivine grains recovered from kimberlite lavas erupted from the Quaternary Igwisi Hills volcano, Tanzania. The Igwisi Hills kimberlitic olivine grains are compared to phenocrystic olivine, liberated from picritic lavas, and mantle olivine, liberated from a fresh peridotite xenolith. Image analysis, scanning electron microscopy imagery and laser microscopy reveal significant differences in the morphologies and surface features of the three crystal populations. The kimberlitic olivine grains form smooth, rounded to ellipsoidal shapes and have rough flaky micro-surfaces that are populated by impact pits. Mantle olivine grains are characterised by flaked surfaces and indented shapes consistent with growth as a crystal aggregate. Phenocrystic olivine exhibit faceted, smooth-surfaced crystal faces. We suggest that the unique shape and surface properties of the Igwisi Hills kimberlitic olivine grains are products of the transport processes attending kimberlite ascent from mantle source to surface. We infer that the unique shapes and surfaces of kimberlitic olivine grains result from three distinct mechanical processes attending their rapid transport through the thick cratonic mantle lithosphere:

Proceedings of 10th International Kimberlite Conference, 2013

Quaternary Science Reviews, 2014

We present a descriptive genetic classification scheme and accompanying nomenclature for glaciovo... more We present a descriptive genetic classification scheme and accompanying nomenclature for glaciovolcanic edifices herein defined as tuyas: positive-relief volcanoes having a morphology resulting from ice confinement during eruption and comprising a set of lithofacies reflecting direct interaction between magma and ice/melt water. The combinations of lithofacies within tuyas record the interplay between volcanic eruption and the attending glaciohydraulic conditions. Although tuyas can range in composition from basaltic to rhyolitic, many of the characteristics diagnostic of glaciovolcanic environments are largely independent of lava composition (e.g., edifice morphology, columnar jointing patterns, glass distributions, pyroclast shapes). Our classification consolidates the diverse nomenclature resulting from early, isolated contributions of geoscientists working mainly in Iceland and Canada and the nomenclature that has developed subsequently over the past 30 years. Tuya subtypes are first recognized on the basis of variations in edifice-scale morphologies (e.g., flat-topped tuya) then, on the proportions of the essential lithofacies (e.g., lava-dominated flat-topped tuya), and lastly on magma composition (e.g., basaltic, lavadominated, flat-topped tuya). These descriptive modifiers potentially supply additional genetic information and we show how the combination of edifice morphologies and lithofacies can be directly linked to general glaciohydraulic conditions. We identify nine distinct glaciovolcanic model edifices that potentially result from the interplay between volcanism and glaciohydrology. Detailed studies of tuya types are critical for recovering paleo-environmental information through geological time, including: ice sheet locations, extents, thicknesses, and glaciohydraulics. Such paleo-environmental information represents a new, innovative, underutilized resource for constraining global paleoclimate models.

Physics and Chemistry of the Earth, Parts A/B/C, 2012

Kimberlite pipes represent the conduits and vents of eroded volcanoes and are commonly filled wit... more Kimberlite pipes represent the conduits and vents of eroded volcanoes and are commonly filled with both coherent and volcaniclastic kimberlite including pyroclastic deposits, resulting from explosive volcanic eruptions. The properties of pyroclastic deposits, including grain size and grain shape distributions of pyroclasts, provide insight into the intensity and style of explosive volcanic eruptions. Pyroclastic kimberlite deposits generally comprise abundant coarse