The document discusses the Auckland Volcanic Field (AVF), focusing on its geological significance and volcanic risk, specifically through the study of olivine populations found in volcanic rock samples. Ongoing research under the DEVORA project aims to characterize these olivines and their inclusions to provide insights for hazard planning and eruption forecasting. Findings may enhance understanding of magma pathways and the structure of the crust beneath Auckland.

1. • Auckland is New Zealand’s most populated and economically-important city, however it is built
on the potentially active monogenetic, basaltic Auckland Volcanic Field (Fig. 1, above).
• Triggers, subsurface pathways, timing, magma ascent rates and consequently, warning times
and potential future eruption sites in the field are currently not well constrained.
• As part of the seven-year, multi-disciplinary DEtermining VOlcanic Risk in Auckland
(DEVORA) project, geochemical analyses of samples from the majority of the volcanic centers have
been completed, including whole rock major and trace elements and isotopic analyses.
• Preliminary observations from recent mineral analyses from several centers in the field reveal
several distinct olivine populations.
This study aims to:
1) characterize the olivine populations in the AVF, and
2) determine if they are suitable for further mineralogical analyses
which may help estimate critical hazard planning factors.
1. The Auckland Volcanic Field
Image courtesy of Tracy
Howe
Figure 2. The Auckland Volcanic
Field (AVF) sits on the North
Island of New Zealand, ~200 km
from the subduction zone
volcanoes of the Taupo Volcanic
Zone.
Tectonic Setting
Figure 3. Detailed magnetic surveys
reveal that Auckland is built on a
complex melange of ultramafic and
serpentinitic terranes, thought to
contain shear zones that the AVF
magma may be exploiting and
mining (Eccles et al., 2005). These
shear zones comprise the “Junction
Magnetic Anomaly” that runs along
the spine of New Zealand and
reaches its widest point underneath
Auckland. A detailed study of the
origin of the two olivine populations
is necessary in the investigation of
the AVF plumbing system.
Loose olivine crystals
Olivine phenocrysts
Olivine aggregates in lava
Olivine‘Types’
Bed 1
Bed 2
Bed 3
Bed 4
X X X
X
X
2. Finding and Analyzing the Olivines
3. Results
Fig. 5. Cores of olivine crystals reveal two populations of olivines in the
dataset. Phenocryst cores range from Fo69 - 84
. Aggregates and loose
olivines, or‘xenocrysts’have similar compositions ranging from Fo88 - 92
.
All olivines with chromium spinel inclusions fall in the xenocryst
population. Data from this study, Sinton 1977, and Spargo 2007.
Fig. 6. Xenocryst and phenocryst cores with liquid compositions at Pupuke
Volcano, further delineating the two olivine populations. Gray lines are
upper and lower bounds of equilibrium (+/- 0.5).
Figure 7. Spinel compositions within the AVF. Populations of high and low chromian spinel
inclusions in olivine and enstatite xenocrysts are evident, as well as a separate population of
spinel phenocrysts.
Fig. 8. a) Comparison of spinels in AVF samples with spinels from various ultramafic lithologies in
the Dun Mt Ophiolite complex found in the“Junction Magnetic Anomaly”(Fig. 3).‘Harz. dunite’=
harzburgitic dunite. Harz. dunite and harzburgite data from Sinton (1977); dunite data analysed
from a UoA collection sample. High chromian spinel inclusions from xenocrystic olivines match
closely with harzburgitic dunite. TO ADD: SpinelsmatchedXXandXXtectonic
settings??(Kamenetskyetal.,2001).
Olivine Spinel
• Detailed sampling of a stratigraphic column at Pupuke
Volcano indicate no differences in xenocryst olivine chemistry
throughout the eruption sequence.
• In addition to olivine and chromian spinel, identified minerals
included diopside, cr diopside, clinopyroxene, magnetite,
iron-nickel, plagioclase, enstatite, and orthopyroxene.
Other Findings
Acknowledgments
• Xenocrystic olivines and their inclusions
have potential for ascent rate modelling
and warning time estimations expected
during future Auckland eruptions.
• These minerals may also relay
information about the crustal structure
and potential magma pathways
underneath Auckland.
• Further work may inform hazard and
emergency management planning.
We are grateful to Tracy Howe, Ritchie Sims, Andres Arcila-Rivera, and John
Wilmshurst for their advice, sample preparation, and analytical assistance.
DEVORA is
funded by:
• As there are definite
differences in the major elements
in the olivines and spinel
inclusions, LA-ICP-MS trace
element analyses are planned to
elucidate the provenance of the
olivines.
• Transects of olivines and
spinels to further explore
core-rim differences and identify
samples for diffusion analyses
(c.f. Morgan and Blake, 2006) to
estimate ascent rates.
• There is potential for more
studies of xenocrysts and
xenoliths in the AVF, such as
amphibolites and quartz, which
will allow us to further discern the
nature and structure of the crust
and upper mantle underneath
Future Work
V31A-2674
Elaine R Smid1*
, Lucy E McGee2
, Ian E Smith1
, & Jan M Lindsay1
1
School of Environment, University of Auckland, Auckland, New Zealand
2
Andean Geothermal Centre of Excellence, Departamento Geología , Universidad de Chile, Santiago, Chile
*e.smid@auckland.ac.nz
A Tale of Two Olivines: Magma Ascent in the Auckland Volcanic Field, New Zealand
4. Conclusions and Future Work
Figure 4. (a) Although five volcanoes are represented in the
dataset, the majority of samples analyzed were systematically
sampled from various layers in the b) excellent Smales Quarry
exposure (b) of the eruption sequence from Pupuke Volcano
(Fig. 3).
• Samples from five volcanoes (Pupuke, Mt
Wellington, Puketutu, Maungataketake, and
Motukorea; Fig. 3) were sourced from the Auckland
University rock collection and through field
sampling (Fig. 4). Olivine ‘types’ were then discerned
from physical observations (see sidebar).
• Samples were thin-sectioned, polished, and
carbon-coated for electron microprobe. Loose and
aggregate olivines were mounted in epoxy and
ground down, polished, and carbon-coated for
analysis on a Jeol JXA-840 electron microprobe
fitted with a PGT Prism 2000 EDS detector at
University of Auckland. An absorbed current of 1.5
nA at 15 kV was used. Calibration was achieved by
Astimex™ mineral standards. For all major oxides,
errors were <5% except at low abundances (i.e. <0.5
wt. %), where error increased to >25% (data courtesy
of Tracy Howe).
• Two volcanic bombs were prepared for
whole-rock major element analysis on a Siemens
SRS3000 sequential X-ray spectrometer at the
University of Auckland. Samples were crushed in a
tungsten carbide ring grinder to <200 µm mesh,
ignited at 930 °C, homogenised and fused in glass
beads prepared with Spectrachem 12:22 lithium
tetraborate to lithium metaborate XRF flux in a ratio
of 2 g of ignited sample to 6 g of flux.
Fig. 1
Fig. 2
228 um
Fig. 4b
Pupuke
Motukorea
Mt Wellington
Puketutu
Maungataketake
Waitemata Harbour
Manukau Harbour
Rangitoto
channel
N
~5km
200km
N
AVF
JMA
TVZ
AVF Volcano (this study)
AVF Volcano
JMA (see text)
Fig. 3
• Olivine xenocrysts in at least one volcanic bomb from Pupuke
Volcano show evidence of interaction with the melt, both in
rim-to-core chemistry and visibly within the backscatter image
(see Figs. 10a and b, below)
Pupuke
Volcano
Smales
Quarry
250 m
N
• Xenocrystic olivine and enstatite contained
inclusions of low and high chromian spinel. The
high chromian spinel, in turn, contained rare
glass, clinopyroxene, olivine, and orthopyroxene
inclusions (see Fig. 9, right). This shows the
complexity of the inclusions and suggests that
they have been subjected to another
environment (melting and growing) at some
previous time.
gl
olv
olv
olv
olv
cr sp
Fig. 9
46 um
Fo90
Fo81
Fig. 10a
1258 um
Fig. 10b
Range of Pupuke
phenocryst liquid
compositions
Range of Pupuke
xenocryst liquid
compositions
KD = 0.3 (Roeder and Emslie, 1970)
Range of Pupuke
whole rock liquid
compositions
HIGH
LOW
PHENOCRYSTS
References
Eccles J, Cassidy J, Locke CA, and Spörli KB (2005). Aeromagnetic imaging of the
Dun Mountain Ophiolite Belt in northern New Zealand: insight into the fine
structure of a major SW Pacific terrane suture. J Geo Soc 162(4): 723-735.
Kamenetsky VS, Crawford AJ, Meffre S (2001). Factors controlling chemistry of
magmatic spinel: an empirical study of associated olivine, Cr-spinel and melt
inclusions from primitive rocks. J Pet 42(4): 655-671.
Morgan DJ and Blake S (2006). Magmatic residence times of zoned phenocrysts:
introduction and application of the binary element diffusion modelling (BEDM)
technique. Contrib Mineral Petrol 151: 58–70.
Roeder PL and Emslie RF (1970). Olivine-Liquid Equilibrium. Contrib Mineral Petrol
29: 275-289.
Sinton JM (1977). Equilibration History of the Basal Alpine-Type Peridotite, Red
Mountain, New Zealand. J Pet 18(2): 216-244.
Spörli KB and Black PM (2013). Catalogue of Crustal Xenoliths from the St. Heliers
Volcanoes, Auckland Volcanic Field, New Zealand. Institute of Earth Science and
Engineering Report 1-2013.01.
Spargo SRW (2007). The Pupuke Volcanic Centre, polygenetic magmas in a
monogenetic field. University of Auckland MSc Thesis 07-219.
Fo91
Fo80
1258 um
0
5
10
15
20
25
30
35
40
45
0 15 30
Al2O3
FeO
AVF High Cr Spinels vs Dun Mt Ophiolite
High Cr Spinel
Harz. Dunite
Harzburgite
0
10
20
30
40
50
60
70
0 15 30
Cr2O3
FeO
AVF High Cr Spinels vs Dun Mt Ophiolite
High Cr Spinel
Harz. Dunite
Harzburgite
Fig. 4a
CRUST
~30km
MANTLE
Zone of partial
melting within
mantle
>~100km