Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Zoned Crystal Records of Transcrustal Magma Transport, Storage and Differentiation: Insights from the Shatsky Rise Oceanic Plateau

Magmatic processes occurring in the deepest parts of sub-volcanic plumbing systems remain poorly constrained. However, crystal mush fragments incorporated into ascending magmas can provide valuable insights into the processes and conditions of transcrustal magma transport, storage and differentiation. Here we use lava samples drilled from Tamu Massif, Shatsky Rise, to understand the magmatic processes taking place in a region of thickened oceanic crust. We observe correlations between crystal textures and compositional zones in plagioclase that reveal relationships between mechanisms of magmatic differentiation and the crustal depths at which they occurred. When combined with geothermobarometric models, our observations indicate that deep crustal crystal storage took place in high-crystallinity mushes at two discrete levels (∼17 and ∼27 km depth). Diffusive constraints from crystal zoning lengthscales indicate that the lifetime of crystals within the mushes exceeded several thousand years. Magmatic recharge was frequent and produced various dissolution textures in plagioclase. In contrast, shallow crystal storage (∼2·4 km depth) took place in a liquid-dominated domain where crystal residence times were much shorter. Crystal zoning patterns indicate that magmas transporting crystals from the deepest environment to the surface sometimes accumulated additional crystals from mid-crustal storage regions and sometimes did not, highlighting the complexity of magma assembly processes. Temperature contrasts in the lower crust at Shatsky Rise are probably low, owing to extensive magma input and a paucity of hydrothermal cooling at depth. Crystal growth morphologies are consequently relatively simple. Crystallization in thick and thermally mature crusts may therefore lead to less complexity in crystal textures than crystallization in thinner crusts where temperature contrasts are higher. Our observations indicate that combining thermobarometry with studies of crystal textures and crystal compositions is a powerful approach for improving our understanding of magmatic differentiation and magma ascent paths.

QEMSCAN as a Method of Semi-Automated Crystal Size Distribution Analysis: Insights from Apollo 15 Mare Basalts

Crystal size distribution analysis is a non-destructive, quantitative method providing insights into the crystallization histories of magmas. Traditional crystal size distribution data collection requires the manual tracing of crystal boundaries within a sample from a digital image. Although this manual method requires minimal equipment to perform, the process is often time-intensive. In this study we investigate the feasibility of using the Quantitative Evaluation of Minerals by SCANing electron microscopy (QEMSCAN) software for semi-automated crystal size distribution analysis. Four Apollo 15 mare basalt thin sections were analysed using both manual and QEMSCAN crystal size distribution data collection methods. In most cases we observe an offset between the crystal size distribution plots produced by QEMSCAN methods compared with the manual data, leading to differences in calculated crystal residence times and nucleation densities. The source of the discrepancy is two-fold: (1) the touching particles processor in the QEMSCAN software is prone to segmenting overlapping elongate crystals into multiple smaller crystals; (2) this segmentation of elongate crystals causes estimates of true 3D crystal habit to vary between QEMSCAN and manual data. The reliability of the QESMCAN data appears to be a function of the crystal texture and average crystal shape, both of which influence the performance of the touching particles processor. Despite these limitations, QEMSCAN is able to produce broadly similar overall trends in crystal size distribution plots to the manual approach, in a considerably shorter time frame. If an accurate crystal size distribution is required to calculate crystal residence time or nucleation density, we recommend that QEMSCAN should only be used after careful consideration of the suitability of the sample texture and average crystal shape.

An improved approach to age-modeling in deep time: Implications for the Santa Cruz Formation, Argentina

Accurate age-depth models for proxy records are crucial for inferring changes to the environment through space and time, yet traditional methods of constructing these models assume unrealistically small age uncertainties and do not account for many geologic complexities. Here we modify an existing Bayesian age-depth model to foster its application for deep time U-Pb and 40Ar/39Ar geochronology. More flexible input likelihood functions and use of an adaptive proposal algorithm in the Markov Chain Monte Carlo engine better account for the age variability often observed in magmatic crystal populations, whose dispersion can reflect inheritance, crystal residence times and daughter isotope loss. We illustrate this approach by calculating an age-depth model with a contiguous and realistic uncertainty envelope for the Miocene Santa Cruz Formation (early Miocene; Burdigalian), Argentina. The model is calibrated using new, high-precision isotope dilution U-Pb zircon ages for stratigraphically located interbedded tuffs, whose weighted mean ages range from ca. 16.78 ± 0.03 Ma to 17.62 ± 0.03 Ma. We document how the Bayesian age-depth model objectively reallocates probability across the posterior ages of dated horizons, and thus produces better estimates of relative ages among strata and variations in sedimentation rate. We also present a simple method to propagate age-depth model uncertainties onto stratigraphic proxy data using a Monte Carlo technique. This approach allows us to estimate robust uncertainties on isotope composition through time, important for comparisons of terrestrial systems to other proxy records.