The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches

<p>The Bishop Tuff is the product of one of the largest eruptions on Earth in the last 1 Myr. This thesis studies the Bishop Tuff in order to better understand the nature of the pre-eruptive magma body, with an emphasis on the processes that occurred within it and the timescales over which they operated. In situ geochemical analyses of crystals and glass from samples collected throughout the Bishop Tuff stratigraphic succession yields insights into the nature of zoning and mixing within this supervolcanic system. Timescales for zircon growth (inferred to represent longevity of the magma chamber) are investigated using U-Pb dating of zircons. Zircon textural and trace element data obtained by SIMS (SHRIMP-RG) are presented from 15 stratigraphically controlled Bishop Tuff samples and two older Glass Mountain (GM) lava samples. The resulting eruption age estimate derived from the weighted mean of 166 rim ages of 766.6±3.1 ka (95% confidence) is identical within uncertainty to published values from ID-TIMS and 40Ar/39Ar techniques. An eruption age is also derived for GM dome YA (the youngest GM dome) of 862±23 ka (95% confidence), significantly older than the widely used 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~80 kyr, and had effectively no temporal overlap with the GM system. Bishop zircon textures are divided into four suites whose proportions change systematically through the eruptive sequence. Trace element variations in Bishop zircons are influenced strongly by sector zoning for many elements, and thus restrict the value of trace element variations in discerning compositional stratification within the magma chamber. In later-erupted units, bright-rim overgrowths are common, and are inferred to have crystallized from the same „bright-rim‟ magma as generated the contrasting rims seen in CL or BSE imaging on quartz, feldspar and orthopyroxene. From zircon zonation patterns, this less-evolved, slightly hotter magma invaded deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption. In order to better quantify the timescales of interaction with the „bright-rim‟ magma, two-feldspar thermometry data are presented on multiple Bishop Tuff samples to constrain temperature variations within the pre-eruptive magma body and yield values for diffusion modelling. Two-feldspar thermometry agrees well with published Fe–Ti-oxide thermometry and reveals a ~80 °C uniform thermal gradient between the upper and lower regions of the magma chamber. Using this thermometry, diffusion of Ti in quartz, Ba in sanidine, Sr in sanidine and Fe-Mg interdiffusion in orthopyroxene are modelled to estimate timescales for the formation of overgrowth rims on crystals. Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims, although differing by about an order of magnitude in their timing. However, Ba and Sr diffusion modelling in sanidine yields disparate timescales 1-2 orders of magnitude longer than for Ti in quartz. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. the profile is influenced by growth zoning). Using the comparison with Sr, constraints are placed on the initial width of the core-rim interface and the initial conditions can be refined, bringing Ba and Sr diffusion timescales into mutual alignment and closer to the values from Ti in quartz. This modelling shows that piecemeal rejuvenation of lower Bishop Tuff magma chamber occurred over a period of ~500 years leading up to eruption. In situ major and trace element analyses of sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene, zircon and matrix glass from the Bishop Tuff and two GM lavas are presented to investigate the pre-eruptive stratification of the Bishop magma chamber and its chemical relationship to the GM system. Analyses of samples from the entire Bishop stratigraphy confirm that the magma chamber was thermally and compositionally zoned prior to growth of crystals and the intrusion of the „bright-rim‟ forming magma. Study of rare mixed swirly and dacitic pumice samples shows enrichments in Ba, Sr and Ti (the elements responsible for bright-rim overgrowths in phenocryst phases) and identifies these pumices as possible representatives of the „bright-rim‟ magma. This integrated study of phenocrysts and glass from the Bishop Tuff leads to development of a revised magma chamber model, in which there is a unitary chamber with a stepped or sloping roof. The chamber has an upper, volumetrically dominant (~2/3) part showing no evidence for convection and with unzoned crystals, and a lower part which had experienced mixing of crystals and interaction with the „bright-rim‟ magma. Intrusion of the „bright-rim‟ magma introduced orthopyroxene and dominantly bright zircon crystals, and caused overgrowth of bright rims enriched in Ti, Sr and Ba on sanidine and quartz phenocrysts. Chemical compositions of GM and Bishop Tuff materials show a shared consanguinity, implying common modes of magma generation, yet the generation of GM and Bishop eruptible magma bodies were physically and temporally separate events.</p>

The Bishop Tuff, California: New Insights into Magmatic Timescales and Processes from Micro-Analytical Approaches

<p>The Bishop Tuff is the product of one of the largest eruptions on Earth in the last 1 Myr. This thesis studies the Bishop Tuff in order to better understand the nature of the pre-eruptive magma body, with an emphasis on the processes that occurred within it and the timescales over which they operated. In situ geochemical analyses of crystals and glass from samples collected throughout the Bishop Tuff stratigraphic succession yields insights into the nature of zoning and mixing within this supervolcanic system. Timescales for zircon growth (inferred to represent longevity of the magma chamber) are investigated using U-Pb dating of zircons. Zircon textural and trace element data obtained by SIMS (SHRIMP-RG) are presented from 15 stratigraphically controlled Bishop Tuff samples and two older Glass Mountain (GM) lava samples. The resulting eruption age estimate derived from the weighted mean of 166 rim ages of 766.6±3.1 ka (95% confidence) is identical within uncertainty to published values from ID-TIMS and 40Ar/39Ar techniques. An eruption age is also derived for GM dome YA (the youngest GM dome) of 862±23 ka (95% confidence), significantly older than the widely used 790±20 ka K-Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean of 838.5±8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~80 kyr, and had effectively no temporal overlap with the GM system. Bishop zircon textures are divided into four suites whose proportions change systematically through the eruptive sequence. Trace element variations in Bishop zircons are influenced strongly by sector zoning for many elements, and thus restrict the value of trace element variations in discerning compositional stratification within the magma chamber. In later-erupted units, bright-rim overgrowths are common, and are inferred to have crystallized from the same „bright-rim‟ magma as generated the contrasting rims seen in CL or BSE imaging on quartz, feldspar and orthopyroxene. From zircon zonation patterns, this less-evolved, slightly hotter magma invaded deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption. In order to better quantify the timescales of interaction with the „bright-rim‟ magma, two-feldspar thermometry data are presented on multiple Bishop Tuff samples to constrain temperature variations within the pre-eruptive magma body and yield values for diffusion modelling. Two-feldspar thermometry agrees well with published Fe–Ti-oxide thermometry and reveals a ~80 °C uniform thermal gradient between the upper and lower regions of the magma chamber. Using this thermometry, diffusion of Ti in quartz, Ba in sanidine, Sr in sanidine and Fe-Mg interdiffusion in orthopyroxene are modelled to estimate timescales for the formation of overgrowth rims on crystals. Ti in quartz and Fe-Mg in orthopyroxene diffusion both yield timescales of <150 years for the formation of overgrowth rims, although differing by about an order of magnitude in their timing. However, Ba and Sr diffusion modelling in sanidine yields disparate timescales 1-2 orders of magnitude longer than for Ti in quartz. The main cause for this discrepancy is inferred to be an incorrect assumption for the initial profile shape for Ba and Sr diffusion modelling (i.e. the profile is influenced by growth zoning). Using the comparison with Sr, constraints are placed on the initial width of the core-rim interface and the initial conditions can be refined, bringing Ba and Sr diffusion timescales into mutual alignment and closer to the values from Ti in quartz. This modelling shows that piecemeal rejuvenation of lower Bishop Tuff magma chamber occurred over a period of ~500 years leading up to eruption. In situ major and trace element analyses of sanidine, plagioclase, biotite, orthopyroxene, clinopyroxene, zircon and matrix glass from the Bishop Tuff and two GM lavas are presented to investigate the pre-eruptive stratification of the Bishop magma chamber and its chemical relationship to the GM system. Analyses of samples from the entire Bishop stratigraphy confirm that the magma chamber was thermally and compositionally zoned prior to growth of crystals and the intrusion of the „bright-rim‟ forming magma. Study of rare mixed swirly and dacitic pumice samples shows enrichments in Ba, Sr and Ti (the elements responsible for bright-rim overgrowths in phenocryst phases) and identifies these pumices as possible representatives of the „bright-rim‟ magma. This integrated study of phenocrysts and glass from the Bishop Tuff leads to development of a revised magma chamber model, in which there is a unitary chamber with a stepped or sloping roof. The chamber has an upper, volumetrically dominant (~2/3) part showing no evidence for convection and with unzoned crystals, and a lower part which had experienced mixing of crystals and interaction with the „bright-rim‟ magma. Intrusion of the „bright-rim‟ magma introduced orthopyroxene and dominantly bright zircon crystals, and caused overgrowth of bright rims enriched in Ti, Sr and Ba on sanidine and quartz phenocrysts. Chemical compositions of GM and Bishop Tuff materials show a shared consanguinity, implying common modes of magma generation, yet the generation of GM and Bishop eruptible magma bodies were physically and temporally separate events.</p>

Behavioural and neural indices of perceptual decision-making in autistic children during visual motion tasks

Many studies report atypical responses to sensory information in autistic individuals, yet it is not clear which stages of processing are affected, with little consideration given to decision-making processes. We combined diffusion modelling with high-density EEG to identify which processing stages differ between 50 autistic and 50 typically developing children aged 6-14 years during two visual motion tasks. Our pre-registered hypotheses were that autistic children would show task-dependent differences in sensory evidence accumulation, alongside a more cautious decision-making style and longer non-decision time across tasks. We tested these hypotheses using hierarchical Bayesian diffusion models with a rigorous blind modelling approach, finding no conclusive evidence for our hypotheses. Using a data-driven method, we identified a response-locked centro-parietal component previously linked to the decision-making process. The build-up in this component did not consistently relate to evidence accumulation in autistic children. This suggests that the relationship between the EEG measure and diffusion-modelling is not straightforward in autistic children. Compared to a related study of children with dyslexia, motion processing differences appear less pronounced in autistic children. Our results also provide weak evidence that ADHD symptoms moderate perceptual decision-making in autistic children.

Speeding up Time: Hierarchical Bayesian Drift Diffusion Modelling Evidence for Accelerating Temporal Accumulation

Time perception is malleable ‒ it can be made to speed up and slow down by various experimental manipulations including the presentation of a sequence of auditory clicks and also angry facial expressions. Recent evidence supports the idea that auditory click trains increase accumulation of evidence across time. Here, we test this idea for both angry expressions and auditory clicks by modelling response times (and choice responses) using Bayesian Hierarchical Drift Diffusion Modelling. Two separate groups of participants (Experiment 1; n = 29; Experiment 2; n = 38) judged the duration of angry and neutral facial expressions preceded by either a 3-s sequence of auditory clicks or silence. In both experiments, standard psychophysical analyses showed that both clicks and angry expressions lengthened the perception of time. The original finding came from the analyses of the Drift Diffusion Modelling parameter that represents the speed of information accumulation ‒ the drift rate parameter. Drift rates grew in magnitude with the duration of the face and moreover this effect was larger when the faces were either preceded by clicks or appeared angry ‒ evidence for accelerating temporal accumulation. This novel insight would not have been possible from traditional psychophysical analyses and therefore, the results highlight the potential value of Bayesian Hierarchical Drift Diffusion Modelling as a tool for understanding how we perceive time.

Financial incentives facilitate the neural computation of prosocial decisions stronger in low empathic individuals

Financial incentives are commonly used to motivate behaviours. There is also evidence that incentives can decline the behaviour they are supposed to foster, for example, documented by a decrease in blood donations if a financial incentive is offered. Based on these findings, previous studies assumed that prosocial motivation is shaped by incentives. However, so far, there is no direct evidence showing an interaction between financial incentives and a specific prosocial motive. Combining drift-diffusion modelling and fMRI, we investigated the effect of financial incentives on empathy, i.e., one of the key motives driving prosocial decisions. In the empathy-alone condition, participants made prosocial decisions based on empathy, in the empathy-bonus condition, they were offered a financial bonus for prosocial decisions, in addition to empathy induction. On average, the bonus enhanced the information accumulation in empathy-based decision. On the neural level, this enhancement was related to the anterior insula, the same region that also correlated with empathy ratings. Moreover, the effect of the financial incentive on anterior insula activation was stronger the lower a person scored on empathy. These findings show that financial incentives enhance prosocial motivation in the absence of empathy but have little effect on high empathic individuals.