Petrographic and Geochemical Analysis of a NewlyDiscovered Desert Road Vent in the TVZ
Abigail Hubera,b,1 ([email protected])
Ben Kennedya ([email protected])
Elisabeth Bertoletta ([email protected])
Darren Gravleya ([email protected])
a
University of Canterbury, Geological Sciences Department, 20 Kirkwood Ave, Upper Riccarton,
Christchurch 8041, New Zealand
b
University of Minnesota Morris, Geology Department, 600 E 4th St, Morris, MN, 56267, United States
_________________________________________________________________________
Abstract
A new vent was discovered on the eastern side of Ruapehu’s ring plain within the boundaries of the
Taupo Volcanic Zone. The eastern side of Ruapehu is less volcanically active than the western side
where small-scale vents and scoria cones are common distally from Ruapehu. The vent is in-situ
and is composed of two types of volcanic material: lithics and spatter. Attempts to better understand
the vent, its origins, and the potential to place it within the surrounding volcanic activity on the
eastern ring plain of Ruapehu were carried out through petrographic and geochemical analyses of
five samples from the vent site. The lithic magma was more evolved than the spatter, and likely
come from two separate magma sources. Within the parameters of geochemistry, the vent XRF data
plots within data from Ruapehu as well as data from satellite vents on the western margin showing
that further work is required to better understand where the vent originated from.2
Keywords: Taupo Volcanic Zone, Ruapehu, andesite, spatter, lithics, ring plain
1. Introduction
The Taupo Volcanic Zone (TVZ) is a volcanic arc complex located in the central part of the
North Island of New Zealand. Beneath the North Island, oblique subduction of the Pacific plate
underneath the Australian plate forms an intra-arc rift known as the Taupo-Hikurangi arc-trench
system (Spinks, Acocella, Cole, and Bassett, 2005; Cole 1990). The present day extension focuses
the modern TVZ on a NNE-SSW trend with active volcanoes from White Island to Mt. Ruapehu
(Spinks et al., 2005). The TVZ lacks coherent eastern and western margins, which are covered by
ignimbrite deposits from caldera eruptions and show subsurface normal faulting, causing
hypothetical boundaries to arise (Cole, 1990). Within the theoretical borders of the TVZ, rhyolitic
calderas comprise the central portion of the TVZ and andesitic volcanoes are situated on the
northern and southern extremes of the zone (Cole, Gamble, Burt, Carroll, and Shelley, 2001).
1
Corresponding author
Formatted to be submitted to the Journal of Volcanology and Geothermal Research
2
1
Mt. Ruapehu is an andesitic volcano located at the southern termination of the TVZ. It is one
the largest active volcanoes in New Zealand and activity began approximately 250 ka (Cronin,
Neall, and Palmer, 1995). Ruapehu is surrounded by smaller, basaltic to basaltic-andesitic sites
(scoria cones, satellite vents, and craters) to the western and southwestern sides of the volcano in
Figure 1 (Houghton and Hackett, 1984). The eastern margin of the TVZ shows less volcanic
activity than the western margin near to Ruapehu, with the exception of a newly-discovered vent
site on the south-eastern end of Ruapehu’s ring plain (Figure 2a). The vent is distinguished from
other volcanic material carried to the ring plain via fluvial activity by parts of the deposit that are
exposed as topographic highs within Ruapehu’s ring plain and the rest covered beneath the 1.8 ka
Taupo ignimbrite deposit. The vent is in-situ while the surrounding material, although originally
volcanic, was transferred by fluvial activity to the ring plain around Ruapehu.
The purpose of this study is to associate the new vent using geochemistry and petrology of
the closest magmatic system to it, namely Ruapehu. In addition, exploration into the structure of the
TVZ and why this vent has occurred on the eastern margin of the zone instead of the more common
western margin will be examined. Finally, comparisons will be drawn between the other smaller
volcanics (Ohakune, Pukeonake, and Hauhungatahi) surrounding Ruapehu to see how the new vent
compares to already classified small-scale vents.
2. Geologic Setting
Located at the southern end of the TVZ is the Tongariro Volcanic Centre; comprised of five
andesitic stratovolcanoes and outer vents (Cronin et al., 1995). Mt. Ruapehu is the southern end of
the Tongariro Volcanic Centre and is an active volcano with an estimated volume of 110 km3 and
surrounded by a ring plain with nearly the same volume (Waight, Price, Stewart, Smith, and
Gamble, 1999). The new vent is located on the eastern side of Ruapehu’s ring plain, facing NW
from State Highway 1 (Desert Road). The vent sits on a ridge and is partially exposed, covered by
the 1.8 ka Taupo ignimbrite which has been rilled in a radial erosion pattern from the exposed unit
(Figure 2a).
3. Methods
Five samples were collected from the vent location; two lithics, two spatter bombs, and a
connected spatter and lithic. Hand sample descriptions of all five samples were done prior to cutting
samples, than repeated after cutting was completed. Eight total thin sections were produced from
five samples to ensure all components were obtained including boundaries between the attached
spatter/lithic sample. All samples were subject to a Philips PW 2400 Sequential 53 Wavelength
Dispersive X-ray Fluorescence 63 Spectrometer for X-ray florescence (XRF) to analyze bulk
2
chemistry of major elements at the University of Canterbury. XRF results were compared with
similar data from the TVZ using IgPet and Microsoft Excel software comparing % SiO2 against %
Na2O+ K2O. Samples were then compared with similar data from Ruapehu using IgPet and
Microsoft Excel software. Sample preparation followed the process laid out by Norrish & Hutton
(1969). Three lithic thin sections were carbon coated for analysis of phenocryst and microlite
content and subject to backscatter electron images (BSE), energy dispersive X-ray spectrometer
analysis (EDS), and secondary electron images (SEI) taken with a JEOL JSM6100 scanning
electron microscope and analyzed using Oxford Aztex SDDdispersive X-ray analysis system at the
University of Canterbury. High resolution aerial photographs from Koordinates were imported into
ArcGIS for a basic map of the vent and surrounding area (Figure 2a and 2b).
4. Results
4.1 Hand Sample Analysis
Sample DVR001 is a crystal-rich porphyritic lithic with plagioclase and pyroxene phenocrysts (12mm) and a rusty red groundmass exhibiting slight vesicularity. The sample is subangular to
subrounded with randomly oriented crystals (Figure 3). Sample DVR002 is a porphyritic spatter
sample with few phenocrysts present and rusty red groundmass with small but frequent vesicles
present (less than of equal to 0.5mm) and no definite shape (Figure 4). Sample DVR003 is a grey
porphyritic lithic with plagioclase and pyroxene phenocrysts present. The groundmass is grey and
the sample is subangular to subrounded with randomly oriented phenocrysts. This sample is
attached to DVR03B and was separated during the cutting process to examine the border between
the two compositions. Sample DVR03B was the attached spatter to DVR003 and appeared similar
to DVR002, although it was lighter in color than the other spatter sample. No phenocrysts were
visible in the sample, but displayed the same vesicularity as the other spatter (Figure 5). Sample
DVR004 is a crystal-rich porphyritic lithic with plagioclase and pyroxene as phenocrysts. The
phenocrysts are randomly oriented throughout the sample and appear subhedral to euhedral. The
groundmass is grey and red (Figure 6).
4.2 Petrographic Analysis
Three lithics (DVR001a, DVR001b, DVR003a, DVR003b, DVR003c, and DVR004) and
two spatter samples (DVR002a, DVR002b, DVR003a, DVR003b, and DVR003c) were examined
in thin section to determine the relationship between the two types of volcanic material and how
they related to one another (Figures 3-6). The lithics are crystal-rich with phenocrysts dominated by
plagioclase and pyroxene (1-2 mm). Groundmass of the lithics contained microlites of plagioclase
and pyroxene as well. No olivine was present in any of the lithic samples. Glomerocrysts,
comprised of wholly plagioclase or wholly pyroxene, are common in all three samples, although
3
some clots were mixed. Sieve texture and zonation were common in the plagioclase phenocrysts,
indicating some disequilibrium crystallization occurred.
Spatter samples differed from lithic samples in several areas. Although porphyritic,
groundmass was the dominant phase for all spatter samples with less than 5% phenocrysts
appearing in any sample. Microlites of plagioclase, pyroxene, Fe-Ti oxides and olivine were also
present. Phenocrysts of olivine were the dominant large minerals (1-2 mm) in size although
plagioclase was found in several instances and matched olivine in size. Vesicularity was common in
spatter samples and the glass content of spatter was much higher than the lithics.
EDS images were taken of the three lithic samples (DVR001, DVR003, DVR004) and the
border between an attached lithic and spatter sample (DVR003) to determine the element
compositions of the individual minerals in the lithics (Figure 7-9). Plagioclase phenocrysts showed
notable spikes for Ca and Na content and pyroxene phenocrysts had high Fe and Mg content.
Groundmass was not dominated by any particular element. The spatter sample groundmass does not
show particular spikes in any one element.
4.3 Geochemical Analysis
All samples were subject to XRF analysis to determine bulk rock chemistry of the major
elements (Table 1) Samples were classified using IgPet software to plot on an Alkalies-Silica
identification graph (Figure 10). All three lithics plotted as andesites, and both spatter samples
plotted as basaltic-andesite. The lithics show a relatively linear trend with increasing %SiO2. The
spatter samples plot closely together and fall into the linear trendline of the lithics showing they are
from a less evolved magma than the lithics. Figure 11 shows the vent geochemical data overlaying
other bulk rock major element data from Ruapehu and surrounding satellite vent sources. The vent
data plots most consistently within the realm of Ruapehu’s data, but there is overlap between the
new vent and data from Ohakune, Pukeonake, and Hauhungatahi.
4.4 Physical Analysis
The vent is an in-situ deposit at a topographic high with an approximate dip to the NW
following the downward sloping topography. Intense welding is visible at the main vent deposit and
decreases in intensity as the deposit heads downslope (Figure 2b). The deposit is incomplete and the
authors are unsure of how eroded the vent is. The presence of rills in a radial pattern surrounding
the slope where the vent is located indicates erosion has been an active force. This also shows that
sections of the vent are still buried beneath the overlying material and erosion is beginning to reveal
the underlying topography of the ridge, which includes parts of the vent. Portions of the vent have
broken off and are no longer in-situ because they sit above the Taupo ignimbrite deposit and have
broken off from the vent.
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5. Discussion
From the vent location two main types of rocks were found: crystal-rich lithics and spatter
bombs. The lithics are porphyritic and vary in groundmass color from a dark red/grey to grey
between the three lithic samples. The matrix of all three lithics was comprised of euhedral to
subhedral plagioclase, pyroxene, and iron-titanium oxide microlites. Glass content and vesicles
made up <5% of the groundmass. Glomerocrysts are common throughout all samples and are
composed of three or more phenocrysts of plagioclase or pyroxene. This is indicative of a long,
slow cooling period allowing large crystal clots to form. Some of the clots are a combination of
both minerals but most appear to be segregated. The lithics are all andesitic (Figure 11), but do not
plot especially close to each other, instead, they form a rough linear trend. Slight differences in the
percentage of phenocrysts from the three lithic samples shows that the more crystal-rich the sample
is the lower it plots on Figure 11. This could be indicative of the beginning of crystal fractionation
within the lithics, with more evolved mineral content being found in the higher plotting samples that
contain fewer crystals because the composition of the magma is increasing in incompatible
elements that need lower temperatures to crystallize.
The spatter samples are porphyritic with >95% of the rock composed of groundmass.
Olivine phenocrysts (1-2 mm) are the most common, but smaller plagioclase and pyroxene (<0.5
mm) are also present. Vesicles and subhedral to anhedral microlites are the main components in the
groundmass. The content of the spatter is indicative of a less-evolved magma due to a lack of large
phenocrysts. The ones present are the first to crystallize out at high temperatures, but phenocryst
percentage of the spatter compared to the lithics suggests that the spatter magma was still hot and
remained active, leaving a short amount of time to nucleate crystals. Both spatter samples plot as
basaltic-andesite and are relatively close to each other. Additionally they correlate well with the
linear trendline of the lithics at a less evolved end (Figure 11).
The vent shows proximal, medial, and distal lithofacies by the nature of its welded material
(Figure 2b). In the proximal zone, the vent deposit has hard coherent lithics surrounded by
deformed spatter curving around lithics similar to those described by Houghton and Hackett (1984)
at the Ohakune Craters. The lithics are not angular as if they were torn off a magma chamber wall
during eruption, and the spatter appears to have conformed to the lithics in a ductile manner (Figure
2b). Lithics are abundant in the proximal zone, but become less common in the medial zone where
the spatter slowly changes to a tephra or bomb like material (Figure 2-6) with significantly less
welding and a less consolidated appearance which continues into the distal zone. Based on the
topographic high the vent sits on and the apparent strike and dip of the vent deposited trending NW,
a suggestion on the eruptive event(s) of the vent point to a pre-existing degassed magma body with
nearly crystalline mush that was intruded into by less-evolved magma. A lack of frequent
5
disequilibrium textures in thin section show the two magmas were likely not mixed and quickly
erupted, shortly after the less-evolved magma intruded. The most crystalline mush was ripped apart
and cooled quickly, forming the proximal vent site with lithic blocks and spatter deforming around
the lithics and welding to itself, imbedding the lithics in spatter. The lack of lithics in less-welded,
tephra material could be associated with the ending of the eruption and less strength and heat to
remove more material from the crystal-rich mush.
6. Conclusions
The newly-discovered vent on the eastern side of Ruapehu’s ring plain contains crystal-rich
lithics imbedded in welded spatter. Situated on a topographic high, the vent has a rilled erosional
pattern downslope revealing the underlying topography and demonstrating that the vent is an in-situ
volcanic eruption. Proximal, medial, and distal lithofacies can be determined from aerial
photographs of the vent site and are distinguishable by the quality of welding the vent deposits
show. The proximal zone is highly welded with spatter deforming ductilely around the lithics. The
medial zone has less intensely welded spatter material that contains some tephra and fewer lithics in
the deposits. The distal zone continues the decline of welding intensity and disintegrates into
material not in-situ. Based on the geochemical and petrographic differences found between the
samples the lithics are likely not parental to spatter from crystal fractionation because the spatter is
from a less-evolved magma rather than being the remaining melt from the mostly crystallized lithic
magma. Geochemistry of the vent samples indicates a correlation with Ruapehu is possible because
the major elements coincide within the realm of Ruapehu’s geochemistry. The vent data also plots
within geochemistry related to other distal vents associated with Ruapehu: Ohakune, Pukeonake,
and Hauhungatahi. Future work includes manipulating geochemistry to better plot data and
correlate with other small-scale vents in the Tongariro Volcanic Center, better constraints on the
age of the vent location, and a more detailed approach to the volcanic activity at TVZ eastern
margin.
Acknowledgements
This project was conducted through the support of Frontiers Abroad and the University of
Canterbury under the excellent guidance of Sam Hampton, Darren Gravley, Ben Kennedy, and Liz
Bertolett which was greatly appreciated. Rob Spiers and Stephen Brown’s contributions to this
project by processing many of the samples used, was crucial to its completion. EDS and BSE data
was collected with the assistance of Mike Flaws in the use of the electron scanning microscope.
Chris Conway assisted with his knowledge of Ruapehu and his collected data was a major
contribution to the scope of this project.
6
References
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plutonic fragments from ignimbrites of the Okataina caldera complex, Taupo Volcanic
Zone, New Zealand: remnants of a partially molten intrusion associated with preceding
eruptions: Journal of Volcanology and Geothermal Research, v. 84, p. 209-237.
Cameron, E., Price, R., Smith, I., McIntosh, W., and Gardner, M., 2010, The petrology,
geochronology, and geochemistry of Hauhungatahi volcano, S. W. Taupo Volcanic Zone:
Journal of Volcanology and Geothermal Research, v. 190, p. 179-191.
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of Volcanology and Geothermal Research, v. 3, p. 121-153
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Zealand: Bulletin of Volcanology, v. 52, p. 445-459.
Cole, J. W., Brown, S. J. A., Burt, R. M., Beresford, S. W., and Wilson, C. J. N., 1998, Lithic types
in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New
Zealand: Journal of Volcanology and Geothermal Research, v. 80, p. 217-237.
Cole, J. W., Gamble, J.A., Burt, R. M., Carroll, L. D., and Shelley, D., 2001, Mixing and mingling
in the evolution of andesite-dacite magmas; evidence from co-magmatic plutonic
enclaves,Taupo Volcanic Zone, New Zealand: Lithos, v. 59, p. 25-46.
Conway, C., 2016, The Magmatic and Glaciovlcanic Evolution of Ruapehu Volcano [Ph. D. thesis]:
Victoria, University of Wellington.
Cronin, S. J., Neall, V. E., and Palmer, A. S., 1996, Geological History of the North-Eastern Ring
Plain of Ruapehu Volcano, New Zealand: Quaternary International, v. 34-36, p. 21-28.
Gamble, J. A., Wood, C. P., Price, R. C., Smith, I. E. M., Stewart, R. B., and Waight, T., 1999, A
fifty year perspective of magmatic evolution on Ruapehu Volcano, New Zealand:
verification of open system behaviour in an arc volcano: Earth and Planetary Science
Letters, v. 170, p. 301-314.
Houghton, B. F., and Hackett, W. R., 1984, Strombolian and Phreatomagmatic Deposits of
Ohakune Craters, Ruapehu, New Zealand: A Complex Interaction Between External Water
and Rising Basaltic Magma: Journal of volcanology and Geothermal Research, v. 21, p.
207-231.
Kilgour, G., Blundy, J., Cashman K., and Mader, H, M., 2013, Small volume andesite magmas and
melt-mush interactions at Ruapehu, New Zealand: evidence from melt inclusions:
Contributions to Mineralogy and Petrology, DOI:10.1007/s00410-013-0880-7.
Norrish, K., and Hutton, J. T., 1969, An accurate X-ray spectrographic method for the analysis of a
wide range of geological samples: Geochimica et Cosmochimica Acta, v. 33, p. 431-453,
DOI: 10.1016/0016-7037(69)90126-4.
Price, R. C., Gamble, J. A., Smith, I. E. M, Stewart, R. B., Eggins, S., and Wright, I. C., 2005, An
integrated model for the temporal evolution of andesites and rhyolites and crustal
development in New Zealand’s North Island: Journal of Volcanology and Geothermal
Research, v. 140. p. 1-24.
Price, R. C., Gamble, J. A., Smith, I. E. M., Maas, R., Waight, T., Stewart, R. B., and Woodhead, J.,
2012, Anatomy of an Andesite Volcano: a Straitigraphic Study of Andesite Petrogenesis and
Crustal Evolution at Ruapehu Volcano, New Zealand: Journal of Petrology, v. 53, p. 21392189.
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and caldera development in the transtensional Taupo Volcanic Zone, New Zealand: Journal
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Waight, T. E, Price, R. C., Stewart, R. B., Smith, I. E. M., and Gamble J.,1999, Straitigraphy and
geochemistry of the Turoa area, with implications for andesite petrogenesis at Mt. Ruapehu,
Taupo Volcanic Zone, New Zealand: New Zealand Journal of Geology and Geophysics, v.
42, p. 513-532.
Figure 1. (1.5 column)
8
Figure 2a. (2 column)
9
Figure 2b. (2 column)
Figure 3. (1 column)
10
Figure 4. (1 column)
Figure 5. (1 column)
Figure 6. (1 column)
11
Figure 7a. (1 column)
Figure 7b. (1 column)
Figure 7c. (1 column)
Figure 7d. (1.5 column)
12
Figure 7e. (1.5 column)
Figure 7f. (1.5 column)
Figure 8a. (1 column)
13
Figure 8b. (1 column)
Figure 8c. (1 column)
Figure 8d. (1.5 column)
Figure 8e. (1.5 column)
14
Figure 8f. (1.5 column)
Figure 9a. (1 column)
Figure 9b. (1 column)
Figure 9c. (1 column)
15
Figure 9d. (1.5 column)
Figure 9e. (1.5 column)
Figure 9f. (1.5 column)
16
Type
Lithic
Spatter
Spatter
Lithic
Sample
SiO2
TiO2
Al2 O3
Fe2 O3T
DVR001
56.27
0.63
18.33
8.01
DVR002
53.47
0.69
16.38
8.71
6.50
DVR03B
52.88
0.72
17.01
9.03
DVR004
58.47
0.58
19.74
5.93
MnO
MgO
CaO
Na2 O
K 2O
P2 O5
0.13
4.42
7.18
3.34
0.87
0.08
0.14
7.51
7.93
2.97
0.77
0.10
0.12
3.47
6.18
3.66
1.17
0.09
0.15
7.52
7.40
2.77
0.79
0.07
0.10
2.56
6.80
3.86
1.09
0.11
LOI
Total
0.62
99.88
1.20
99.88
0.09
99.88
1.54
99.89
0.67
99.91
Lithic
DVR003
60.80
0.54
17.27
Table 1. (1.5 column)
Lithic
Spatter
Figure 10. (2 column)
17
Figure 11. (2 column)
Figure and Table Captions
Figure 1. North Island of New Zealand with a schematic close-up of the Tongariro Volcanic Center
and the vent location marked out in red.
Figure 2a. Aerial photograph of the vent in Ruapehu’s ring plain marking out proximal, medial and
distal areas within the topographic high. Gives locations of ground photographs around the vent and
direction the pictures were taken from.
Figure 2b. White outlines vague layering within the deposits,+6174 shows thicker layers with less
welding and more distinct spatter chunks with some unconsolidated material, +3924 shows the
proximal deposit with variable thickness in its layering and much more welded appearance with
indistinct spatter and a lighter color than the medial deposit. Lithics appear in both deposits. +3930
illustrates the topographic high the vent sits on and looks downslope in a NE direction. +6173 shows
an imbedded lithic with deformed spatter bending around the lithic.
Figure 3. DVR001 in hand sample.
Figure 4. DVR002 in hand sample.
Figure 5. DVR003 and DVR03B in hand sample.
Figure 6. DVR004 in hand sample.
Figure 7a. EDS image with spectrum locations.
Figure 7b. EDS color image of DVR001.
Figure 7c. BSE image of DVR001.
Figure 7d. Element spectrum representative of pyroxene composition in DVR001.
Figure 7e. Element spectrum representative of groundmass content in DVR001.
Figure7f. Element spectrum representative of plagioclase composition in DVR001.
Figure 8a. EDS image with spectrum locations.
18
Figure 8b. EDS color image of the border between DVR003and DVR03B.
Figure 8c. BSE image of the border between DVR003and DVR03B.
Figure 8d. Element spectrum representative of pyroxene composition in DVR003.
Figure 8e. Element spectrum representative of groundmass content in DVR003.
Figure 8f. Element spectrum representative of groundmass content in DVR03B.
Figure 9a. EDS image with spectrum locations.
Figure 9b. EDS color image of DVR004.
Figure 9c. BSE image of DVR004.
Figure 9d. Element spectrum representative of pyroxene composition in DVR004.
Figure 9e. Element spectrum representative of plagioclase composition in DVR004.
Figure 9f. Element spectrum representative of groundmass content in DVR004.
Table 1. XRF bulk rock geochemistry of major elements for all five samples.
Figure 10. Alkalies-Silica IgPet plot to determine compositions of samples.
Figure 11. Geochemistry comparisons of major elements from Burt et al., 1998; Cameron et al.,
2010; Cole, 1978; Cole et al., 1998; Conway, 2016; Gamble et al., 1999; Kilgour et al., 2013; Price
et al., 2005; Price et al., 2012; and Waight et al., 1999.
19