Pumice clasts were sampled from the deposits of the main Plinian phases of the eruptive units that represent contrasting lithofacies associations.
Each selected fallout deposit was sampled at regular intervals (separated in the vertical axis by 5 clasts), with a minimum total of 30 juvenile fragments between 8 and 32 mm per interval, completing 60 to 100 juvenile clasts per unit. In this study, the pumice textures are qualitatively described as finely vesicular if >90 % of vesicles are <2 mm and coarsely vesicular if larger. Vesicularity is described according to Houghton and Wilson (1989). These samples were further classified according to their macrotextures (Table 2.1), cleaned in distilled water, dried, and were kept in a desiccator for skeletal and bulk density determination. Ten samples of each eruptive unit were crushed with an agate mortar and only in the case of very dense clasts were milled with a tungsten carbide ring grinder. After classification of macrotextures within each fallout, a representative clast of each type within each unit was sectioned and polished for inspection under optical microscope (Appendix D), backscattered secondary electron (BSE) imaging, glass analysis with electron microprobe (Appendix F), and porosity measurements (Appendix G).
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Table 2.1 Macrotextural classification of pumice rocks collected from Mt. Ruapehu, Late Pleistocene, Plinian
eruptions.
Bulk Density:the selected pumice clasts are too small and irregular in shape to be cored to produce regular cylinders (e.g., Klug and Cashman 1994); therefore, Archimedes experiments were carried out following Houghton and Wilson (1989), in order to calculate the bulk density. All samples were previously dried at 50°C for 24 hrs, and their weight was then recorded as SD. For a first set of samples, each clast was impregnated in melted, ultra-refined paraffin wax of known density (0.9 g/cm3). The best results of rapid coating and minimum penetration of the wax into the pumice pores were obtained when the wax was not too hot and fluid, usually around 45°C-50°C; at that temperature, the wax rapidly solidifies when the clast is removed and it is viscous enough to avoid extensive penetration into the pumice pores. A thin thread was attached to the clast during this process to hang the coated pumice for further drying and weighing in a suspension balance.
A copper reference mass was used to ballast the coated pumice in distilled water (Density
δ=1.000 g/cm3), and the volume was estimated using the Archimedes Principle. Initially, the ballast was suspended in air on a precision balance and the resulting weight was recorded as BD. Then, it was submerged in a beaker containing distilled water and the
resulting weight was recorded as BW. The forces acting on the object are in equilibrium and
the buoyancy B1 is:
B1= BD - BW [13]
B1 is equivalent to the volume of the displaced fluid (i.e. Distilled water) and also equivalent to the volume of the ballast, recorded as Vb. Each coated sample was then
Type General characteristics Subtype
1
Highly vesicular, vitrophyric clasts with subspherical vesicles showing smooth outlines and thin walls
1a Foamy. highly vesicular clasts dominated by subspherical
vesicles <2 mm in diameter
1b Expanded. Subspherical to irregular large vesicles
2
Fluidal, highly to moderately
vesicular, crystal-bearing clasts with strongly orientated, elongated (i.e.
ellipsoidal) vesicles, commonly
aligned with the longest axes of phenocrysts
2a Finely vesicular to microvesicular (<2 mm in diameter). Vesicles
show irregular outlines and variable wall-thickness 2b
Coarsely vesicular (>2 mm in diameter). Vesicles show smooth outlines and thin walls
3 Fibrous, porphyritic, microlite-rich,
often colour-banded
3a Microvesicular, occasionally colour-banded
3b Coarsely vesicular (>2 mm in diameter), often colour-banded
4 Dense-microvesicular, microlite-rich clasts with highly distorted, often aligned vesicles only visible in the
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suspended in a precision balance and the resulting weight in air was recorded as CD. The
weight of the wax was obtained by:
WxD = CD - SD [14] Knowing the density of the wax, we calculated the respective volume, which was recorded as Vx.
Subsequently, the ballast was attached to the coated sample and the weight in air of the whole system was recorded as CSBD. This assemblage was further introduced in the
distilled water and the weight recorded as CSBW. The resulting buoyancy B2 is:
B2= CSBD - CSBW [15] B2 is equivalent to the volume of the coated sample with the ballast, recorded as Vcsb. The volume of the sample Vs is then obtained by:
Vs = Vcsb-Vb-Vx [16] This is the bulk volume of the sample and is used to calculate the bulk density, as:
δbulk = SD/Vs [17] During the last part of this project, a new Micrometrics GeoPyc 1360 envelope density analyzer was acquired by Massey University, which applies the Archimedes experiment within a quasi-fluid displacement medium composed of microspheres having a high degree of flowability (DryFlo-Micromeritics). Envelope volumes were assumed to be equivalent to bulk volumes and were further used to calculate the corresponding bulk density of each clast.
Skeletal and solid Density: these parameters were obtained on previously dried pumice
samples with a Quantachrome helium ultrapycnometer, using pure nitrogen as the flowing gas. Five to six runs per sample were carried out to minimize the standard deviation. The pressure of the equipment was set at 19-19.5 psi, with a gas flow rate fast enough to achieve that pressure between 30 and 60 s, but slow enough to establish an increasing rate of 0.1 psi per grade, once the desired pressure was achieved. Each analysis was carried out
31
using the automatic equilibration time, and either the vacuum flow system (1 minute of vacuum) or the pulse mode for the gas inflow (4 pulses per run), showing no significant differences in the results. For powder samples only the pulse mode was performed.
In a porous material, such as a pumice clast, the He-pycnometer measures the volume accessed by the flowing gas. It only accesses the connected pores of the sample and the resulting density is known as the skeletal density δskel. In order to consider the isolated
pores, it was necessary to crush some of the samples and run the analysis of the powder to obtain the solid density δsol, being always δsol > δskel. It is noteworthy that best results were
obtained when at least three-quarters of the sample container was filled with the powder.
δsol corresponds to the dense rock equivalent (DRE) used in the calculations of total
porosity performed by Houghton and Wilson (1989).
Standard deviations per analysis vary between 0.0004 and 0.2 %. However, errors between 1 % and 5 % were estimated most commonly. Rarely some irregularly-shaped and weathered samples gave variability up to 16 % from repeated measurements of the same sample, by changing its orientation. This issue is probably due to the heterogeneity in shape and texture of the samples, as well as the presence of a clay coat around the clasts (resulting from weathering), which can lead to variations in the accessible gas volumes with the orientation of the sample in the holder. This problem can be minimized when coring samples into regular cylinders. Further variations were also evidenced when slightly changing the flow rate into the He-pycnometer and room temperature, but in general, the experiments were considered as reproducible. The level of confidence in the measurements was tested by calibrating the pycnometer with steel, non-porous spheres of known volume and by frequently repeating the measurement of an andesitic pumice sample of Taranaki volcano, previously analyzed with a similar machine at the University of Oregon by Platz
(2007). Foliated or strongly anisotropic samples were positioned in at least three mutually
perpendicular orientations in the pycnometer.
Porosity: with all the different density data, the following calculations were carried out to
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by using the bulk volume, the volume accessible by the flowing gas (Hevol), and the solid density, following Klug and Cashman (1996), Klug et al. (2002), and Wright et al. (2007):
ϕୡ୭୬୬=(୳୪୩ ୴୭୪୳୫ୣିୌୣ౬ౢ)
୳୪୩ ୴୭୪୳୫ୣ [18]
ϕୠ୳୪୩= ቆ1 − ቀ୳୪୩ ୴୭୪୳୫ୣ
ୗ୭୪୧ୢ ୢୣ୬ୱ୧୲୷ቁቇ ∗ 100 [19]
ϕ୧ୱ୭=ϕୠ୳୪୩−ϕୡ୭୬୬ [20]
In the absence of a gas permeameter, the permeability of each sample could only be qualitatively considered, based on descriptions of bubble coalescence, shear, development of tubular textures, and presence of microfractures.
Pumice clasts showing inner vesicularity and/or crystallinity gradients were avoided in order to exclude post-fragmentation effects (e.g., Mangan and Cashman 1996). Thomas and
Sparks (1992) demonstrated that clasts larger than 1.6 cm in diameter do not cool
significantly while transported in the eruptive column, and for clasts close to 6.4 cm, it takes 10 to 15 s in the fallout to cross the glass transition. Similarly, Klug and Cashman
(1994) estimated that these lapilli sizes would have 50 s available for post-fragmentation
expansion and bubble coalescence.
2.2.4 Pyroclast 2D microtextures and ash morphology: optical and