AN ABSTRACT OF THE THESIS OF
Nicole D. Rocco for the degree of Master of Science in Geology presented on June 15, 2020.
Title: Isotopic and Geochemical Signatures of Mantle and Crustal Contributions in Rhyolites from Okataina and Taupo Volcanoes, New Zealand
Abstract approved: ______Adam J.R. Kent
Silicic caldera-forming eruptions are some of the largest and most destructive volcanic eruptions known, and present significant local and global hazards. The underlying processes within crustal magma plumbing systems that lead to the accumulation and eruption of large volumes of evolved magma remain enigmatic, yet there is broad consensus that interaction between mantle-derived magmas and surrounding crust is crucial to the generation of many silicic magmas. Constraining these processes are key to understanding the evolution of caldera-forming systems.
Radiogenic isotopes are well-suited for deciphering mantle versus crustal contributions given they are not affected by fractionation and most sources have unique isotopic signatures. Here we present a suite of high precision whole pumice
Pb and Sr isotope measurements from two caldera-producing volcanic centers,
Okataina (OVC) and Taupo (TVC), in the world’s most active and voluminous rhyolitic volcanic system, Taupo Volcanic Zone (TVZ), New Zealand. Samples were collected from the most recent caldera-forming eruptions in the two volcanic centers along
with smaller yet significant eruptions over the last c. 50 ka, with the aim of investigating spatial and temporal changes throughout a caldera cycle – the caldera-
forming event and the smaller yet significant eruptions that occur between them.
Glass major and trace elements complement our isotopic data, and help to elucidate
contributions from mantle-derived and crustal sources and bring light to temporal changes surrounding a caldera cycle.
Strontium and lead isotope compositions from both the OVC and TVC show significant offset from local basalts toward local upper crustal terranes (Waipapa and Torlesse metasediments), representative lower crust, and an enriched mantle component. Potential mechanisms for this include melting of the lower crust by intruding basalts and assimilation of the upper crust by more evolved magmas during storage and transport. Mixing trends show these possibilities along with the complexities presented by variable compositions of source endmembers. Two hypotheses are proposed for explaining the observed OVC and TVC Sr and Pb isotopic compositions:
1. Local TVZ basaltic magmas are interacting with upper crustal rocks –
specifically, the Torlesse metasedimentary composite superterrane – that
comprise the eastern North Island, integrating radiogenic Pb and Sr from the
upper crust.
2. Local TVZ basalts are influenced by an enriched mantle component sourced
in the deep mantle and are melting the lower crust (represented by granulitic
xenoliths found in TVZ intermediate lavas) on their ascent, thus bringing
more radiogenic Pb from the source as well as Pb and Sr from the lower
crust.
The data also reveal distinct signatures for eruptions sourced from each volcanic
center. TVC eruptions are more radiogenic in both Sr and Pb isotope compositions,
suggesting either greater extent of crustal input or distinctly different crustal Pb and
Sr sources. Glass compositions support these notions, but on a more temporal scale.
For each volcanic center, there is a marked change in major and trace element
compositions after a large caldera-forming eruption, indicating a deepening of the magmatic system to less evolved compositions, higher pressures, and potentially more ability to melt and assimilate the crust. This is also reflected in Sr isotopic compositions; higher pressures correlate with more radiogenic Sr ratios, suggesting that deeper magmas may either have greater interaction with the crust or are interacting with more radiogenic crustal components.
©Copyright by Nicole D. Rocco June 15, 2020 All Rights Reserved
Isotopic and Geochemical Signatures of Mantle and Crustal Contributions in Rhyolites from Okataina and Taupo volcanoes, New Zealand
by Nicole D. Rocco
A THESIS
submitted to
Oregon State University
in partial fulfillment of the requirements for the degree of
Master of Science
Presented June 15, 2020 Commencement June 2021
Master of Science thesis of Nicole D. Rocco presented on June 15, 2020
APPROVED:
______Major Professor, representing Geology
______Dean of the College of Earth, Ocean, and Atmospheric Sciences
______Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request.
______Nicole D. Rocco, Author
ACKNOWLEDGEMENTS
I would like to express my deepest gratitude, first and foremost, to my advisor,
Adam Kent. Through the many challenges that graduate school presents, you have
continually been a support and staunch advocate. I have never once worried about
the many path changes that this degree has presented, as I have always known you
have had my back. You have provided insight when needed and allowed me to grow
as a critical thinker and scientist, pushed me when it was required, and reassured me in nebulous times. You have been the perfect advisor for me.
I am honored to have Dave Graham and Kari Cooper as my committee members.
Dave, your inbox and office door was continually open to all my isotope-related questions, and you have instilled in me deep wonder and love of isotope geochemistry. I am in constant amazement of your incredible depth of knowledge and am honored to have learned from you, in and away from the classroom. I am grateful to have Kari on my team as well. Spending two weeks in the field with you in
New Zealand was an opportunity of a lifetime – you are inspirational as a woman and a scientist.
The entire GeoPRISMS Tephra Team (Tyler Schlieder, Elizabeth Grant, Olivia Barbee, and Lydia Harmon) have been a continual support and resource. Our group chats always left me feeling stoked and inspired and I cannot wait to see the compilation of work our group produces. I was fortunate enough to meet Sarah Smithies three- quarters of the way through this project – thank you so very much for all our chats, sharing of data, and brainstorming. In addition to my GeoPRISMS collaborators, the
entire VIPER (and honorary VIPERS – Danielle Woodring and Kali Melby) group has
been a constant source of learning and community. I could not be prouder to call
myself a VIPER.
I could not have completed this project without the guidance of Frank Ramos. I
cannot express enough my deep appreciation for you opening your lab and home to
me to complete my data collection after the Burt 2 fire. I have learned more from
you about isotopic analysis than I ever imagined I would during this project and am
even more inspired to spend my career in a lab because of you. Thank you for
always answering my questions, no matter how small, and for pouring a mean Long
Island after a long day in the lab.
I would also like to acknowledge the GAIA team and Kelly Rose from the Department
of Energy National Energy Technology Laboratory in Albany, OR. My fellowship in
the GAIA lab has bolstered my growth as a scientist and as a collaborator. I am
grateful to have been a part of this team throughout my graduate work, and am
honored for the opportunities afforded to me at NETL.
I would have never made it through these last three years if not for my partner JJ.
Your unwavering support has held me afloat in even the most challenging times. You
have never questioned my drive to return to school later in life and have been my
biggest cheerleader every step of the way. Nikki Moore and Kit Kleinschmidt fill the
other slots on the cheerleading squad. Nikki – you are my biggest academic mentor
(and colleague). You have been available and with me at every turn, from calming me down over a beer or guiding me through the world of geochemistry, volcanoes,
and writing. Kit – you are always a pragmatic voice, a committed believer in facts and science, and the sister I never had. Our adventures in the wilderness have brought sanity, serenity, and purpose into my life. Thank you for both for reminding me to quiet the imposter voice in my head and to balance my work life with the other things that I love.
The biggest inspiration in my life is my mom. You have encouraged me to follow my dreams throughout my entire time on this planet, by your words and by your actions. Watching you return to school while I was in high school showed me that anything was possible if I was willing to work hard. I have witnessed you reinvent yourself again over the last two decades, all the while urging me to continue to do what I love, no matter what that entailed. You are my rock in every sense of the word and I would absolutely not have made it here without you.
Lastly, my research was made possible by funding from the National Science
Foundation (GeoPRISMS) and various grants and scholarships from Oregon State
University’s College of Earth, Ocean, and Atmospheric Sciences.
TABLE OF CONTENTS Page
1 Introduction ...... 1
2 Geologic Background ...... 7
2.1 Large scale structure and tectonics, North Island, New Zealand ...... 7
2.2 Taupo Volcanic Zone ...... 7
3 Analytical Methods ...... 15
3.1 Sampling ...... 15
3.2 Pumice glass major and trace elements ...... 15
3.3 Whole pumice Pb and Sr isotopes ...... 17
3.4 Rhyolite-MELTS geobarometry ...... 18
4 Results ...... 21
4.1 Glass major and trace elements ...... 21
4.2 Whole pumice Pb and Sr isotopes ...... 22
5 Discussion ...... 32
5.1 Comparison to previously published and other current data ...... 32
5.2 Mantle-derived and crustal contributions ...... 34 5.2.1 Mantle-derived parental melts ...... 37 5.2.2 Crustal contaminants ...... 38 5.2.3 Isotopic modeling outcomes ...... 40
5.3 Provinciality of caldera system isotopic signatures ...... 45
5.4 Changes through a caldera cycle ...... 49
6 Conclusions ...... 68
7 References Cited ...... 71
LIST OF FIGURES
Figure Page
2.1 Maps of New Zealand large scale tectonics ...... 12
2.2 Map of the North Island of New Zealand ...... 13
2.3 Map of the Taupo Volcanic Zone and sample locations ...... 14
4.1 Total alkali silica diagram ...... 26
4.2 SiO2, Ba, and Ti concentrations through time ...... 27
4.3 Extended trace element diagrams ...... 28
4.4 Rare earth element diagrams ...... 29
4.5 Three-isotope Pb diagrams ...... 30
4.6 Sr-Pb isotope diagram ...... 31
5.1 Pb and Sr isotope diagrams with published data ...... 54
5.2 Comparative Sr isotope diagram with published data ...... 55
5.3 Binary Pb-Pb mixing diagrams ...... 56
5.4 Binary Sr-Pb mixing diagram ...... 57
5.5 Comparative extended trace element diagram ...... 59
5.6 Ternary Pb-Pb mixing diagram ...... 60
5.7 Ternary Sr-Pb mixing diagram ...... 61
5.8 Schematic diagram of binary mixing ...... 62
5.9 Schematic diagram of ternary mixing ...... 63
5.10 Compiled map of New Zealand basement rocks ...... 64
LIST OF FIGURES (CONTINUED) Figure Page
5.11 Silica versus Ti plot ...... 65
5.12 Time and SiO2 versus pressure ...... 66
5.13 Strontium isotopic ratios versus pressure ...... 67
LIST OF TABLES
Table Page
3.1 Samples collected and analyses performed ...... 20
4.1 Representative glass major, trace, and isotopic compositions ...... 24
5.1 Isotopic mixing model parameters ...... 53
LIST OF SUPPLEMENTAL FILES
These files are attached separately in the Oregon State University Scholars Archive.
Supplement
1 Pumice glass major element compositions
2 Pumice glass trace element compositions
3 Whole pumice Pb isotopic compositions for samples and standards
4 Whole pumice Sr isotopic compositions for samples and standards
5 Major element oxide standards and uncertainties
6 Trace element standards and uncertainties
7 Published isotopic data 1
1 Introduction
Large, explosive caldera-forming eruptions (CFEs) are among the most catastrophic events that occur on the planet (Branney and Acocella, 2015a). They present complicated and widespread hazards, including population displacement, land destruction, and global climate perturbation (Branney and Acocella, 2015b). Conversely, the magmatic systems that produce such eruptions also produce important resources such as porphyry ore deposits and geothermal energy (Lipman and Sawyer, 1985;
Kennedy et al., 2018). In addition, crustal magmatism also plays an important role in the formation and evolution of continental crust (Taylor and McLennan, 1995; Plank and
Langmuir, 1998). Since a large CFE has not been directly observed in the modern era, a detailed investigation of past eruptions is critical to building a better foundation for how the magmatic systems that produce them evolve spatially and temporally. The Taupo
Volcanic Zone (TVZ), located on the North Island of New Zealand, is an ideal case study for the hazards and resources surrounding large caldera-forming systems: in addition to a small yet significant geothermal industry, three-quarters of the country’s 4.9 million residents (as of 2018 census) reside on the North Island, and at least 34 CFEs have occurred in the last c. 1.6 Ma (Giggenbach, 1995; Wilson et al., 1995). The TVZ is a region of active extension and subduction-related, caldera-forming volcanism (Cole and
Spinks, 2009a). The central TVZ is also characterized by voluminous bimodal volcanism consisting of basalt and rhyolite, with the latter representing >90% of the Quaternary eruptive products. The TVZ has the highest known silicic magma production rates on 2
Earth and has produced c. 15-20,000 km3 of magma in the last 2 Myr (Houghton et al.,
1995; Wilson et al., 1995).
While many aspects of this extremely active volcanic zone have been heavily studied, the origins of these prolific magmas are still the subject of much debate. An important question in this debate is what contributions of mantle-derived magmas, subducted
components, and overlying crust play a dominant part in the production of rhyolite and
other evolved magmas in the region, and how they vary over time and space. Over the
decades, numerous hypotheses regarding this problem have come forth (prominent
examples listed below), utilizing a broad range of data sets including major and trace
element geochemistry of major phases, glasses, and melt inclusions, whole rock and
mineral stable and radiogenic isotopes, U-Th and U-Pb dating of zircons, and numerical
modeling. Models for formation of silicic magmas in the TVZ include:
1. Direct melting of the local basement. It has been suggested that there is
wholesale melting of Torlesse and/or Waipapa metagreywacke composite
terranes based on trace element geochemistry (Reid, 1983) and U-Pb age spectra
from TVZ and xenocrystic zircons (Charlier et al., 2010). Reid (1983) explored the
notion, through non-modal melting models, that up to 35% melting of a granulite
assemblage with a bulk composition similar to the western (Waipapa) local
basement could produce the same REE patterns and similar trace element
concentrations as TVZ rhyolites. Charlier et al. (2010) asserted that a
metasedimentary xenolith in a 28 ka rhyolite dome has the same age as the
nearby exposed Early Cretaceous Pahau Terrane (a unit of the Torlesse). They 3
also noted that zircon ages from the Omega dacite (18.8 ka, sample 6 in this
study) align with the Jurassic Kaweka Terrane (also Torlesse). The authors
attributed the survival of euhedral xenocrystic zircons in Omega dacite magmas
as wholesale melting of crustal rocks and open-system processes.
2. Closed-system fractional crystallization of a contaminated parental magma. It
has also been suggested that TVZ rhyolite genesis takes place by closed system
fractionation of a contaminated primitive basaltic parent. McCulloch et al. (1994)
employed radiogenic (Sr-Nd-Pb) and stable (δ18O) isotopes in order to elucidate
fractionation and assimilation contributions in the TVZ. The authors used
isotopic compositions of plagioclase from the Maroa volcanic center, local basalt,
and basement terranes to conclude that TVZ rhyolites were produced by
magmatic differentiation of crustal (in this case, Torlesse) contaminated basalt.
Relatively low δ18O ‘mantle-like’ values, along with Sr-Nd isotopic compositions
that fall between primitive basalts and Torlesse, can be accounted for by 15-25%
contamination of the local parental basalt. Lead isotopic compositions of Maroa
plagioclase are controlled in this case by a small (c. 10%) contribution of
Torlesse, which has an order of magnitude higher Pb concentration than the
local basalt (2 ppm and 20 ppm, respectively). In short, they argued that crustal
contamination of parental basalt was then followed by c. 70% crystal
fractionation with a small amount of further crustal melting and assimilation.
3. Recycling of mature crust and andesitic restite. Graham et al. (1995) proposed
that melting of pre-existing volcanic material or granulitic lower crust could play 4
a role in TVZ rhyolite genesis, but noted that at the time, such sources were yet
to be defined. Price et al. (2005) utilized a large suite of previously published
data (field observations, whole-rock, groundmass, mineral, and glass
geochemistry of andesites and crustal xenoliths) to conclude that crustal
recycling is the dominant and common process producing intermediate and
rhyolitic rocks in the TVZ, and that TVZ rhyolites are the result of melting of
mature crust and andesitic restite due to increased heat flow from extension and
crustal thinning. The authors asserted that andesitic magma was dominant prior
to the onset of rhyolite eruptions in the central TVZ, and that as extension,
crustal thinning, and heat flow propagate southward, areas such as the
Tongariro Volcanic Complex, which is predominantly andesite, will evolve
towards rhyolitic composition. They posit that the central TVZ has been
overprinted with rhyolite and as heat flow and extension increased, melting of
mature, andesitic crust also increased. Through geochemistry of andesites,
rhyolites, and crustal xenoliths, the authors reveal that the andesites and
rhyolites of the TVZ are directly related and represent a continuum of
petrological processes.
4. Complex combination of the aforementioned processes. Other models suggest
various combinations of the above processes. For example, lead isotope and
trace element geochemistry have also been utilized to propose that these
voluminous and highly evolved magmas are assembled from local
metagraywacke-contaminated basalts and subsequent melting of early-formed, 5
crustal-contaminated andesites (Graham et al., 1992a). Deering et al (2010) used
polytopic vector analysis (PVA) to provide a basis for TVZ rhyolite
characterization and petrogenesis. They identified two endmember magma
types and conclude there is a continuum of compositions between the two. Their
data constrain the partial melting of amphibolite, as well as 50-60% equilibrium
crystallization of crustal contaminated, hornblende-bearing andesite in the
generation of TVZ rhyolites.
While multiple methods provide insight to the above hypotheses, radiogenic isotope geochemistry (specifically, Sr-Nd-Pb) is a common thread. The technique is ubiquitous for studying igneous evolution and has been widely used to investigate contributions from mantle and crustal sources in arc magmas (Davidson et al., 1987; Ben Othman et al., 1989; Hawkesworth et al., 1993). Since this study looks at contributions from both mantle-derived magmas and the overlying crust, Sr and Pb are well-suited systems.
Although some radiogenic isotope data (Sr, Nd, Hf, and Pb) are available for the TVZ, there is a dearth of modern high precision Pb isotope data. Much of the available data are older and were measured by thermal ionization mass spectrometry (TIMS; Graham et al., 1992; McCulloch et al., 1994; Schmitz and Smith, 2004) and thus less precise and plausibly, less accurate (White et al., 2000; Weis et al., 2005; Yuan et al., 2016). Even so, previous work shows that the different sources mentioned above should show unique
Pb isotopic signatures and the presence of three radiogenic isotopes (as compared to Sr) also provide further insight into possible sources – particularly 207Pb/204Pb, which also adds the potential for source age constraints. 6
Here we present a suite of high-precision Pb and Sr isotope compositions, as well as
complementary glass trace element chemistry, spanning the last c. 50 kyr from the two
most recently active volcanic centers in the TVZ, Taupo and Okataina. We have chosen
eruptions that represent the most recent caldera cycle – the CFE and eruptions that
surround the CFE – in each volcanic center in order to investigate how these magmatic systems change over time and space. Our data provide a deeper look into the derivation of TVZ rhyolites, from evolution of parental melts to the provenance of the overlying crust. Lead and strontium isotope measurements reveal distinct signatures in Okataina and Taupo, suggesting differing degrees of crustal melting and assimilation (or type of assimilant), and variability over relatively short distances, confirming that complex processes produce these highly evolved magmas.
7
2 Geologic Background
2.1 Large scale structure and tectonics, North Island, New Zealand
New Zealand is situated in the southwest Pacific and is dissected by the convergence of
the Pacific and Australian plates. The North Island is influenced by the subduction of the
Pacific plate beneath the Australian plate (c. 42 mm/yr – Fig 2.1a; DeMets, et al., 1990) and its subsequent arc and back-arc volcanism. Due to the clockwise rotation of the SE half of the North Island relative to the Australian plate (0.5 – 3.8 deg/Myr – Fig. 2.1b;
Wallace et al., 2004), subduction along the eastern margin of the North Island is slightly oblique and becomes progressively more so southward, contributing to interconnected arc-related calc-alkaline volcanism and intra-arc rifting (Graham et al., 1995a; Cole and
Spinks, 2009b; Rowland et al., 2010). It has been suggested that the rotation is caused by the progressive southward thickening of the Hikurangi Plateau, a Cretaceous-aged oceanic plateau that overlies the Pacific plate and is subducting beneath the North
Island (Wallace et al., 2004). The oblique subduction eventually transforms into a dextral strike-slip orientation in the South Island as the Alpine Fault (Sutherland et al., 2007).
Volcanism is confined to the North Island and mirrors the NE – SW striking subduction
zone, while rifting (c. 8 mm/yr; Darby et al., 2000) is approximately perpendicular to the
volcanic arc.
2.2 Taupo Volcanic Zone
The Taupo Volcanic Zone (TVZ) represents the southward expression of the Tonga-
Kermadec volcanic arc, extending into the continental crust of the North Island of New 8
Zealand (Graham et al., 1992a) and stretching approximately 300 km NE – SW through
the center of the North Island, from White Island in the Bay of Plenty to the Tongariro
Volcanic Complex (Fig. 2.2; Cole, 1979). It is a zone of dynamic lithospheric rifting in a
NW – SE direction (perpendicular to subduction) and is the most prolific rhyolite- producing system on Earth (Cole and Spinks, 2009b). The TVZ can be divided into three sections by types of volcanism: the northern (Whakatane Graben) and southern
(Tongariro Volcanic Complex) regions are predominantly andesitic (Cole and Spinks,
2009b), while the 125 km-long central region is characterized by the eruption of exceptionally large volumes of rhyolite, lesser dacite, and rare high-alumina basalt
(Rowland et al., 2010). The central volcanic region possesses anomalously high heat flow
(c. 800 mW/m3) and is underlain by c. 15 km of the Waipapa and Torlesse composite
metagraywacke basement terranes and a mafic, refractory lower crust extending to 30
km (Bibby et al., 1995; Brown et al., 1998a; Darby et al., 2000; Stratford and Stern, 2006;
Harrison and White, 2006). The Waipapa and Torlesse, potential crustal contributors to
TVZ rhyolites, are two components of the Eastern Province – a tectono-stratigraphic belt
of Carboniferous to Cretaceous forearc, backarc, and accretionary complex terranes that
underlie the TVZ (Adams et al., 2007; Mortimer et al., 2012).
Volcanism began in the TVZ c. 2 Ma and was andesitic, transitioning to predominantly
rhyolite in the central TVZ c. 1.6 Ma and was followed by active rifting c. 1.1 Ma (Wilson
et al., 1995). The exact timing of initiation remains unclear, as the early history of the
area is largely concealed by the 0.34 Ma, >1500 km3 Whakamaru eruption (Houghton et
al., 1995; Wilson et al., 2009a; Saunders et al., 2010). There is also an unclear 9
relationship, both temporally and spatially, between the TVZ and the NNW-striking
Coromandel Volcanic Zone, where andesitic volcanism commenced c. 18 Ma followed by
rhyolitic volcanism c. 10 Ma – younging southward. While there is no age overlap with
the Coromandel volcanism, the earliest TVZ eruptions (Mangakino Volcanic Center –
active c. 2 Ma-0.34 Ma; Wilson et al., 1995) could represent a transition between the
two volcanic zones (Cole and Spinks, 2009a).
Central Taupo Volcanic Zone
The 125 x 60 km central TVZ is dominated by eight rhyolitic volcanic centers (Fig. 2.3)
and has experienced at least 34 caldera-forming eruptions since c. 1.6 Ma (Houghton et al., 1995; Wilson et al., 1995). It is a broad structural depression surrounded by
Mesozoic and Cenozoic marine sedimentary rocks and underlain by Mesozoic composite
‘superterranes’ – Waipapa to the west and Torlesse to the east (Graham et al., 1992;
McCulloch et al., 1994; Houghton et al., 1995; Price et al., 2005, 2015; and many others).
Andesite was the dominant early manifestation of volcanism in the central TVZ from 2
Ma – 1.6 Ma, after which the transition to rhyolite took place (Wilson et al., 2009a).
Caldera forming eruptions have approximately 50 kyr – 100 kyr occurrence intervals with smaller, yet significant intra-caldera events between each CFE and can overlap in space and time (Sutton et al., 1995; Wilson et al., 2009a). The eruption record, however, is potentially incomplete due to dispersion to the east (higher erosion rates due to uplift east of the volcanic centers, along with offshore accumulation of air fall deposits) and 10
small- to medium-sized eruptions buried by larger eruptions or poorly exposed due to
climate/vegetation changes in the Pleistocene and Holocene (Wilson et al., 2009a).
The Taupo Volcanic Center
The Taupo volcanic center (TVC) represents the southwestern-most caldera complex in
the central TVZ (Fig. 2.3). The TVC, along with Maroa volcano to its north, overlies a
caldera complex formed c. 350-320 ka by several voluminous ignimbrites and fall
deposits of the Whakamaru group (Wilson et al., 1984; Houghton et al., 1995). The early
history of Taupo and Maroa (closely connected since c. 300 ka) is not well known, due to
a scarcity of age data and burial by younger deposits (Wilson et al., 2009a). Beginning c.
65 ka, explosive activity shifted to vents beneath Lake Taupo, culminating in Oruanui
(25.4 ka; Vandergoes et al., 2013), the most recent supereruption on Earth (>530 km3
DRE; Wilson, 2001). Two magma types have been described for Taupo volcano since c.
65 ka that are distinct in major elements and isotopic composition: Oruanui (these include at least three eruptions between c. 65 ka and 25.4 ka) and post-Oruanui (Sutton et al., 1995; Liu et al., 2006; Wilson and Charlier, 2009). It is worth noting there were
synchronous dome eruptions NE of Taupo volcano within the Whakamaru caldera
complex with yet another magma type, but those eruptions are beyond the scope of this
study.
The Okataina Volcanic Center
The Okataina Volcanic Center (OVC) contains the northeastern-most caldera complex in
the TVZ and is the most recently active of the eight designated volcanic centers (Fig. 2.3; 11
Cole et al., 2010, 2014). While the entirety of the TVZ is actively rifting, the rifting is asymmetric, with the fastest onshore rates at Okataina (12 mm/yr; Wallace et al., 2004).
Faulting is a major structural feature of the OVC and interplays directly with its
hyperactive volcanism via crustal thinning, high heat flow, and heavily intruded upper
crust (Cole and Spinks, 2009b; Cole et al., 2010). Okataina volcano is expressed by
clusters of rhyolitic cones and domes, younging inwards, bounded by large ignimbrite
sheets. OVC has a c. 625 kyr volcanic history with two definite periods of caldera
collapse – Matahina at c. 325 ka and Rotoiti at 45 ka (Manning, 1995; Cole et al., 2010;
Deering et al., 2011; Flude and Storey, 2016) – and a possible CFE c. 550 ka (Quartz-
biotite Ignimbrite; Cole et al., 2010; Charlier and Wilson, 2010). 12
(a) (b)
Figure 2.1. (a) Institute of Geological and Nuclear Sciences (GNS Science) map showing large-scale tectonic processes affecting New Zealand, including the convergence of the Pacific and Australian plates, which results in the subduction-related volcanism of the Taupo Volcanic Zone. (b) Velocity vectors demonstrate the clockwise rotation of the SE half of the North Island from Wallace et al. (2004). This rotation contributes to intra-arc rifting, thinning crust, and high heat flow of the TVZ.
13
Figure 2.2. Map of the North Island with the TVZ boundaries shown as red dashed lines and the northeastern and southwestern extent marked by White Island and the Tongariro Volcanic Complex, respectively.
14
Figure 2.3. The eight designated volcanic centers in the TVZ since c. 350 ka. Okataina and Taupo are colored blue and yellow, respectively. with yellow and blue numbered stars denoting sample locations (see Methods).
15
3 Analytical Methods
3.1 Sampling
Samples of flow and fall deposits were collected from 18 eruptions from Okataina and
Taupo in and around the Bay of Plenty, Rotorua, and Taupo regions (see Table 3.1 for ages, volumes, spatial data, and analyses performed; see Fig. 2.3 for sample locations).
In the TVC, we sampled from each of the subgroups (SG) within Taupo volcano: pre-
Oruanui (Okaia), Oruanui, post-Oruanui dacite group (Omega Dacite), SG1, SG2, and SG3
(Unit B, Waimahia, and Taupo Plinian, respectively). The sampling goal was to represent a full caldera cycle – pre-CFE, CFE, and post-CFE eruptions – in order to observe and analyze spatial and temporal evolution in a volcanic center. In Okataina, as in the TVC, our sampling was focused around the most recent CFE, Rotoiti, and the eruptions surrounding it. There are no known eruptions immediately prior to Rotoiti, so we chose to analyze Kakapiko, a pre-Rotoiti dome deposit (c. 127 ka; personal communication from Graham Leonard to K. Cooper and D. Gravley), as a representative pre-CFE eruption. Along with these eruptions, we sampled and analyzed five eruptions from the c. 40 ka – 28 ka Mangaone SG (Unit A, Unit B/C, Maketu, Hauparu, and Mangaone;
Jurado-Chichay and Walker, 2000; Smith et al., 2005), and three from the c. 25 ka to 0.7 ka Rotorua SG (Te Rere, Rotorua Upper and Lower, and Kaharoa; Smith et al., 2005).
3.2 Pumice glass major and trace elements
In-situ major and trace elements were measured in pumice glass fragments from the same suite of samples chosen for isotope analysis. Pumice clasts were crushed in a small 16
tungsten carbide jaw crusher, washed in ultrapure water in an ultrasonic three times,
dried for 3 days at 60° C and sieved. Ten to twelve 1 – 2 mm pieces of glass from each
eruption were hand-picked, mounted in epoxy, and polished to expose glass at the
surface. Glass pieces were chosen from the same subsamples analyzed for Pb and Sr
isotopic compositions (see below). Major elements were measured at Oregon State
University using a CAMECA SX-100 electron microprobe equipped with five wavelength-
dispersive spectrometers and high-intensity dispersive crystals for high-sensitivity
analysis. Analyses were performed with a 15 keV accelerating voltage, 10 nA sample
current, and 10 um beam size. Counting times ranged from 20-30 s depending on the
element and desired detection limit, and zero-time intercept functions were applied to
mitigate the effects of alkali migration. Analyses utilized a rhyolite glass calibration
standard (RHYO) that contained an average of 77.09 wt.% SiO2 over all runs. From repeat analyses of standard glass, the average relative percent uncertainties (1σ) for representative oxides were approximately 1% for SiO2 and Al2O3, 3% for K2O and Na2O,
and 5% for CaO. A complete table of all measured standard concentrations and uncertainties, along with accepted values, can be found in Supplement 3. Backscatter
images of each glass fragment were also taken.
A suite of 31 trace elements were measured for the same glass fragments by laser
ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Oregon State
University in the Keck Collaboratory for Plasma Mass Spectrometry. Analyses were
completed with a Photon Machines Analyte G2 193 ArF Excimer Laser connected to a
Thermo Scientific iCAP-RQ ICP-MS, using a 50 um spot size and 7 Hz pulse rate for all 17 glass analyses. Ablation occurred in a He atmosphere and ablated particulate was transferred to the ICP-MS using a flow rate of 0.6 L/min. Data were processed with the in-house LaserTram Visual Basic software (see Loewen and Kent, 2012, for details) using
29Si as the internal standard isotope and GSE-1G as the calibration standard. ATHO-G,
BCR-2G, BHVO-2G, GSE-1G, T1-G standards were also used as secondary standards for all analyses, see Supplement 4 for all standard concentrations and uncertainties.
3.3 Whole pumice Pb and Sr isotopes
Sub-samples, consisting of small pumice clasts or fragments of larger pumice clasts, for all analyses were chosen randomly to reduce bias and prepared in a uniform manner to ensure consistency. Sub-samples were washed in ultrapure (18 MΩ) water three times and dried at 60° C. Clean clasts and fragments were crushed in a clean agate mortar and pestle into a homogenized powder. The mortar and pestle were first primed with clean quartz grains and rinsed with ultrapure water and clean isopropanol between each sample to prevent contamination. Weighing, digestion, and purification were carried out in a Class 1000 metal-free clean room with ultrapure water and clean acids at Oregon
State University (OSU) and New Mexico State University (NMSU). Seven of 18 samples were weighed, digested, and purified at OSU (see Supplements 3 and 4 for distinctions), while the remaining samples were processed at NMSU. Approximately 100 mg of each powdered sample were completely dissolved in clean Teflon beakers in 3 mL of a 2:1 mixture (5 mL of a 2:3 mixture at OSU) of double-distilled hydrofluoric and nitric acid on a hot plate for 24 hours. All samples were visually inspected for total dissolution.
Strontium and Pb were then purified using 200-400 mesh AG 50W-X8 cation exchange 18
resin and 200-400 mesh AG1-X8 anion exchange resin, respectively. Ion exchange chromatography was performed at NMSU and OSU in accordance with the methods detailed in Ramos (1992) and Weis et al. (2006), respectively.
Strontium isotopes were measured on a VG Sector 54 thermal ionization mass spectrometer (TIMS) at NMSU using five Faraday collectors in multidynamic mode with
88Sr = 3.0 V and were corrected for mass fractionation using 86Sr/88Sr = 0.1194. NBS 987
standards were analyzed every ten samples (average = 0.710273; 1σ = 0.000017). The
accepted value for NBS 987 is 0.710250 ± 12 from Weis et al. (2005). This value is
preferable over the NIST-certified value of 0.71034 ± 26 due to improved analytical
techniques and precision. No correction was necessary, as measured NBS 987 values
were within uncertainty. Procedural blanks were c. 100 pg.
Lead isotopes were measured on a Thermo Scientific NeptunePlus multicollector
inductively coupled plasma mass spectrometer (MC-ICP-MS) at NMSU using seven
Faraday collectors in static mode while monitoring 202Hg, 203Tl, and 205Tl. NBS 981
standards were measured every three samples, normalized to the averaged measured
value for the first run, and corrected to 206Pb/204Pb = 16.9405 ± 0.0015, 207Pb/204Pb =
15.4963 ± 0016, and 208Pb/204Pb = 36.7219 ± 0.0044 (Galer and Abouchami, 1998).
Samples from this study were corrected with sample-standard bracketing using the normalized and corrected average standard value for each run. Procedural blanks were
<50 pg. See Supplement 7 for all Sr and Pb standard isotopic ratios and uncertainties.
3.4 Rhyolite-MELTS geobarometry 19
Pressures were calculated by Sarah Smithies at University of Canterbury using Rhyolite-
MELTS and following the methodology detailed in Gualda and Ghiorso (2014). Storage
depths were calculated from average glass compositions for each eruption and reflect
the last equilibration pressure between the observed glass compositions and quartz and
one feldspar (quartz – 1 feldspar geobarometer). Simulations were performed over a pressure-temperature grid with 1° C and 25 MPa intervals between 1100 – 700° C and
400 – 25 MPa. Pressures for successful simulations are listed in Table 4.1. 20
Table 3.1. Samples collected for analyses from 2017 TVZ field season.
quartz-hosted whole glass trace whole glass major melt inclusion pumice Pb elements pumice Sr sample ID mapped ID** eruption volcanic center age (ka) volume (km3) latitude longitude elements major isotopes (LA-ICP- isotopes (EMP) elements (MC-ICP- MS) (TIMS) (EMP) MS) GP-131217-22 1 Kaharoa* OVC 0.7 5 -38.2713 176.5152 x x x x x GP-111217-14 9 Rotorua (U) OVC 15.7 1 -38.1698 176.3247 x x x x GP-111217-13 10 Rotorua (L) OVC 15.7 -- -38.1698 176.3247 x x x x GP-111217-12 11 Te Rere^ OVC 25 13 -38.0441 176.4867 x x x x GP-081217-07 12 Mangaone OVC 33 12 -37.9659 177.0115 x x x x GP-081217-05 13 Hauparu (crystal poor) OVC 36.1 3.9 -37.8684 176.5236 x x x x GP-081217-04 14 Maketu OVC 36.8 2.8 -37.8684 176.5236 x x x x GP-091217-09 15 Unit B/C OVC 39 1.2 -38.1308 176.8054 x x x x GP-091217-08 16 Unit A OVC 40 0.1 -38.1308 176.8054 x x x x GP-121217-24 2 Earthquake Flat*^ OVC 45 10 -38.3093 176.3793 x x x x x GP-071217-03 3 Rotoiti*^ OVC 45 120 -37.8408 176.6285 x x x x x GP-102 18 Kakapiko^^ OVC 127 ? -38.2062 176.2820 x x x GP-151217-25 4 Taupo Plinian TVC 1.8 13.4 -38.7457 176.1998 x x x x GP-151217-26 9 Waimahia TVC 3.55 5.1 -38.9273 176.4862 x x x x GP-151217-27 5 Unit B TVC 11.8 0.4 -38.7039 176.1026 x x x x GP-171217-28 6 Omega Dacite TVC 18.8 0.03 -38.6962 175.9988 x x x x GP-121217-15 7 Oruanui*^ TVC 25.4 530 -38.5957 175.9789 x x x x GP-121217-18 8 Okaia TVC 28.5 3 -38.6198 175.8935 x x x x note: All units are air fall unless otherwise noted: *lava dome block-and-ash flow; ^ignimbrite; *^pyroclastic flow; ^^lava dome deposit. Volumes are reported as dense rock equivalent. **mapped ID in reference to Figure 2.3 Eruption ages and volumes compiled from Froggett and Lowe (1990), Wilson (1993), Jurado-Chichay and Walker (2000), Nairn (2002), Kilgour (2002), Wilson and Charlier (2009), Danisik et al. (2012), Vandergoes et al. (2013), and Flude and Storey (2016). Kakapiko age was obtained by Kari Cooper in peronal communication with Graham Leonard while in the field in 2019.
21
4 Results
4.1 Glass major and trace elements
Major element concentrations for OVC and TVC pumice glass fragments are listed in
Supplement 1 (see Table 4.1 for summary of anhydrous eruption average
concentrations). All samples analyzed are rhyolitic (>69 wt.% SiO2; Fig 4.1), and each
eruption exhibits homogeneity in major and minor elements for all glass fragments
analyzed (1σ < 1 wt.% for SiO2 and Al2O3, < 0.1 wt. % for K2O, CaO, Na2O, and FeO, and <
0.05 wt.% for TiO2, MnO, and MgO). There is, however, a clear temporal evolution through each center’s caldera cycle, with a significant drop in SiO2 following each
caldera forming event (Fig. 4.2a).
Average glass trace element concentrations for OVC and TVC eruptions are shown in
Table 4.1 (see Supplement 2 for all analyses). Most eruptions in both volcanic centers have relatively homogenous concentrations (within c. 10%), but there are some noteworthy deviations, and some elements exhibit distinct variations associated with the caldera cycle (see Fig. 4.2). For example, in the OVC, Ti concentrations range from c.
600-800 µg/g for pre-caldera, syn-caldera, and CFE (Kakapiko, Earthquake Flat, and
Rotoiti), increases to c. 2000-2800 µg/g for the Old Mangaone SG (Unit A, Unit B/C,
Maketu, Hauparu), decreases to c. 1100-1300 µg/g for Mangaone (Young Mangaone SG) and early Rotorua SG (Te Rere and Rotorua), and is the lowest (c. 500 µg/g) in Kaharoa dome block-and-ash flow samples (Fig. 4.2b). Titanium in the TVZ does not exhibit the same large variations as the OVC (Fig. 4.2b), but also shows a temporal trend with lower 22
concentrations in the pre-caldera eruption and CFE (700-800 µg/g) and in this case,
steadily increasing through time to the youngest eruption analyzed (Taupo Plinian –
1400-1500 µg/g). Zirconium (variable) and Sr (depleted) show similar trends in both
volcanic centers. In the OVC, Ba concentrations are the highest (up to 1800 µg/g) in
Kakapiko (pre-caldera dome) and Rotoiti (CFE; c. 1000 µg/g) and then are consistently c.
700-900 µg/g in all the post-CFE samples (Fig. 4.2c). In the TVC, Ba concentrations are
consistent throughout all samples analyzed (c. 550-650 µg/g; Fig. 4.2c). Okataina’s pre-
caldera dome, Kakapiko, displays the most heterogeneous compositions for Ti, Ba, and
Zr, with some glass fragments as much as 2x more or less concentrated than others.
Barium, in particular, spans the entire range of all the other eruptions.
Extended trace element and rare earth element (REE) diagrams (Figs. 4.3 and 4.4) show
overall broad similarities and typical arc patterns such as slight light rare earth
enrichment, flat heavy rare earths, enrichment in large ion lithophile elements (namely,
Rb, Ba, and Pb), and Ti, Nb, Sr, and Eu depletions.
4.2 Whole pumice Pb and Sr isotopes
Whole pumice strontium and lead isotopic ratios for OVC and TVC are listed in Table 4.1
(and Supplement 3 and 4). Average isotopic compositions for each eruption from both volcanic centers are shown in Figures 4.5 (Pb only), and 4.6 (Pb and Sr). Each volcanic center is distinct from the other in 87Sr/86Sr (OVC = 0.705172 – 0.705484 ± 0.000011 average 2σ; TVC = 0.705580 – 0.706698 ± 0.000011 average 2σ) and 206Pb/204Pb (OVC =
18.845 – 18.872 ± 0.001 average 2σ; TVC = 18.870 – 18.879 ± 0.001 average 2σ). There is 23
significant overlap for the two volcanic centers in 207Pb/204Pb (OVC = 15.634 – 15.672 ±
0.001 average 2σ; TVC = 15.649 – 15.670 ± 0.002 average 2σ) and 208Pb/204Pb (OVC =
38.743 – 38.890 ± 0.003 average 2σ; TVC = 38.811 – 38.876 ± 0.004 average 2σ). In
general, the TVC is more radiogenic than the OVC with the exception of Rotorua (L),
which is more radiogenic in 207Pb/204Pb and 208Pb/204Pb.
24
Table 4.1. TVC (top) and OVC (bottom) glass major, trace element, and isotopic compositions
sample ID GP-151217-25 GP-151217-26 GP-151217-27 GP-171217-28 GP-121217-15 GP-121217-18 eruption Taupo Plinian Waimahia Unit B Omega Dacite Oruanui Okaia volcanic center TVC TVC TVC TVC TVC TVC age (ka) 1.8 3.55 11.8 18.8 25.4 28.5 volume (km3) 13.4 5.1 0.4 0.03 530 3 latitude -38.7457 -38.9273 -38.7039 -38.6962 -38.5957 -38.6198 longitude 176.1998 176.4862 176.1026 175.9988 175.9789 175.8935 n = 14 n = 17 n = 11 n = 11 n = 15 n = 17
SiO2 75.21 75.76 75.99 75.40 77.68 77.81
Al2O3 13.46 12.98 13.34 13.25 12.53 12.40
TiO2 0.24 0.20 0.18 0.14 0.12 0.13
K2O 2.81 2.88 2.98 3.36 3.08 3.23 Cl 0.17 0.16 0.15 0.16 0.18 0.17 CaO 1.50 1.29 1.47 1.53 1.17 1.14 MnO 0.09 0.07 0.08 0.05 0.06 0.05 FeO 1.96 1.90 1.73 2.18 1.25 1.24
Na2O 4.31 4.58 3.90 3.79 3.79 3.69 MgO 0.23 0.18 0.17 0.12 0.13 0.13 S 0.006 0.004 0.002 0.002 0.004 0.004 n = 27 n = 17 n = 15 n = 17 n = 13 n = 18 Rb 107.60 112.36 105.22 128.81 112.38 119.07 Ba 615.66 599.23 574.13 634.43 604.94 650.70 Th 11.81 11.97 10.09 12.38 10.86 11.57 U 2.46 2.49 2.32 2.53 2.43 2.76 Nb 8.16 8.32 6.47 6.54 6.03 5.87 La 28.27 28.71 23.23 25.53 21.84 21.88 Ce 55.88 54.41 46.13 54.73 41.94 42.72 Pb 19.76 19.96 18.26 21.58 15.04 14.75 Pr 6.69 6.51 5.47 6.33 4.73 4.61 Sr 150.20 124.99 103.96 94.90 87.55 86.74 Nd 26.13 26.16 21.26 25.64 17.80 16.52 Zr 251.30 261.80 202.00 218.44 127.49 119.39 Hf 6.24 6.73 5.28 6.04 3.68 3.51 Sm 5.74 5.75 4.60 5.65 3.60 3.20 Eu 1.17 1.16 0.80 0.73 0.63 0.54 Ti 1471.04 1196.22 979.08 890.86 764.84 804.60 Gd 5.80 6.07 4.79 5.59 3.60 3.05 Tb 0.93 0.98 0.75 0.91 0.55 0.49 Dy 5.88 6.08 4.85 5.89 3.66 3.21 Y 36.79 39.15 29.74 35.69 23.18 20.25 Ho 1.30 1.34 1.03 1.25 0.77 0.69 Er 4.36 4.03 3.07 3.76 2.43 2.17 Yb 3.58 3.82 3.04 3.78 2.46 2.30 Tm 0.55 0.59 0.45 0.55 0.35 0.32 Lu 0.60 0.65 0.49 0.60 0.42 0.37 Cu 2.38 2.88 2.97 5.00 2.37 2.41 Pm 27.00 26.44 21.63 25.11 18.00 16.89 Sc 21.50 19.52 22.18 23.19 18.59 17.61 V 0.81 0.84 2.44 10.58 1.80 1.98 87Sr/86Sr 0.706050 0.706183 0.706070 0.706698 0.705580 0.705637 206Pb/204Pb 18.879 18.874 18.873 18.877 18.870 18.872 207Pb/204Pb 15.667 15.660 15.649 15.670 15.657 15.669 208Pb/204Pb 38.861 38.882 38.811 38.876 38.855 38.866 25
Table 4.1 (continued)
sample ID GP-131217-22 GP-111217-14 GP-111217-13 GP-111217-12 GP-081217-07 GP-081217-05 GP-081217-04 GP-091217-09 GP-091217-08 GP-121217-24 GP-071217-03 GP-102 eruption Kaharoa Rotorua (U) Rotorua (L) Te Rere Mangaone Hauparu Maketu Unit B/C Unit A Earthquake Flat Rotoiti Kakapiko volcanic center OVC OVC OVC OVC OVC OVC OVC OVC OVC OVC OVC OVC age (ka) 0.7 15.7 15.7 25 33 36.1 36.8 39 40 45 45 127 volume (km3) 5 1 -- 13 12 3.9 2.8 1.2 0.1 10 120 ? latitude -38.2713 -38.1698 -38.1698 -38.0441 -37.9659 -37.8684 -37.8684 -38.1308 -38.1308 -38.3093 -37.8408 -38.2062 longitude 176.5152 176.3247 176.3247 176.4867 177.0115 176.5236 176.5236 176.8054 176.8054 176.3793 176.6285 176.2820 n = 20 n = 15 n = 16 n = 15 n = 13 n = 8 n = 11 n = 7 n = 12 n = 19 n = 12 n = 18
SiO2 77.78 76.28 76.64 76.74 76.12 73.79 72.76 73.47 73.75 77.74 78.11 77.76
Al2O3 12.42 13.16 12.82 12.90 13.21 14.01 14.11 14.20 14.08 12.41 12.35 12.26
TiO2 0.08 0.23 0.22 0.17 0.20 0.37 0.44 0.36 0.36 0.11 0.12 0.09
K2O 4.11 2.98 2.97 2.96 2.75 2.66 2.33 2.39 2.40 4.36 3.43 4.13 Cl 0.17 0.15 0.15 0.19 0.17 0.15 0.18 0.17 0.16 0.21 0.22 0.18 CaO 0.59 1.32 1.30 1.05 1.18 1.76 2.09 2.11 2.08 0.79 0.79 0.69 MnO 0.07 0.06 0.05 0.08 0.09 0.10 0.11 0.09 0.12 0.04 0.06 0.05 FeO 0.82 1.36 1.35 1.22 1.31 2.04 2.40 1.97 1.99 0.87 0.90 0.95
Na2O 3.89 4.22 4.26 4.50 4.75 4.62 5.07 4.74 4.58 3.38 3.86 3.80 MgO 0.07 0.24 0.23 0.18 0.22 0.41 0.52 0.49 0.49 0.09 0.14 0.08 S 0.004 0.006 0.005 0.006 0.009 0.005 0.004 0.010 0.002 0.005 0.01 0.006 n = 29 n = 19 n = 20 n = 16 n = 20 n = 21 n = 15 n = 16 n = 16 n = 13 n = 22 n = 17 Rb 136.46 96.24 98.85 94.59 87.03 91.45 81.16 77.13 79.34 145.44 104.38 144.66 Ba 832.39 831.74 756.79 744.98 878.73 734.06 702.47 726.95 769.13 783.26 1016.62 1119.60 Th 15.01 9.42 9.06 9.09 9.51 9.52 7.67 8.04 8.24 14.52 10.86 18.18 U 3.35 2.28 2.03 2.08 2.25 1.97 1.85 1.67 1.85 3.40 2.35 4.18 Nb 7.96 7.12 6.62 8.18 9.13 8.54 8.41 7.33 7.38 6.88 7.37 10.10 La 29.16 22.38 21.16 25.72 30.80 26.97 31.96 23.66 24.42 25.06 26.90 32.26 Ce 58.45 42.44 38.19 51.57 50.35 47.24 48.12 40.87 43.95 47.37 50.26 58.42 Pb 17.74 16.08 15.29 16.32 15.90 13.49 12.51 12.19 13.17 17.75 12.17 18.47 Pr 6.28 4.90 4.37 6.14 7.27 6.15 7.70 5.49 5.63 4.72 5.44 6.52 Sr 36.30 116.21 107.63 103.75 112.38 155.81 178.75 175.42 192.88 47.49 65.68 47.97 Nd 22.40 18.46 16.89 24.64 29.49 24.18 31.82 22.45 22.52 16.00 19.30 22.01 Zr 77.72 204.64 191.05 169.36 201.31 304.90 324.01 250.76 241.32 83.78 99.96 112.29 Hf 2.96 5.04 4.74 4.69 5.59 7.04 7.58 6.20 6.12 2.76 3.00 3.86 Sm 4.44 3.77 3.43 5.32 6.30 5.39 6.95 4.77 4.72 2.81 3.69 4.29 Eu 0.49 0.85 0.72 1.07 1.34 1.16 1.44 1.15 1.16 0.34 0.53 0.48 Ti 490.15 1331.43 1237.03 1126.88 1286.27 2288.52 2610.09 2275.29 2234.73 603.61 796.22 761.06 Gd 4.35 3.85 3.47 5.49 6.83 5.63 7.55 5.06 4.92 2.71 3.54 4.08 Tb 0.71 0.59 0.56 0.86 1.07 0.90 1.24 0.81 0.76 0.43 0.57 0.68 Dy 4.58 3.90 3.67 5.66 7.09 5.81 8.08 5.28 4.93 2.76 3.63 4.30 Y 29.47 24.23 23.89 35.20 48.31 38.65 50.80 34.24 31.60 19.23 25.05 29.24 Ho 1.02 0.84 0.80 1.24 1.57 1.29 1.71 1.18 1.09 0.60 0.82 0.94 Er 3.56 2.59 2.54 3.84 4.95 3.91 5.27 3.73 3.48 2.02 2.83 3.00 Yb 3.27 2.66 2.55 3.71 4.73 3.68 4.88 3.56 3.43 2.19 2.66 3.16 Tm 0.48 0.38 0.36 0.54 0.69 0.57 0.73 0.51 0.50 0.31 0.37 0.47 Lu 0.55 0.44 0.41 0.59 0.75 0.63 0.80 0.59 0.56 0.39 0.45 0.54 Cu 3.79 1.74 2.20 1.58 2.21 2.15 2.45 2.65 2.27 1.78 1.31 4.22 Pm 22.70 18.86 17.36 24.81 29.89 24.38 31.75 22.80 22.46 16.07 19.33 22.32 Sc 15.12 18.93 11.52 12.55 11.91 19.70 18.98 16.10 12.94 16.68 14.45 10.96 V 0.48 4.92 4.44 1.34 1.70 10.40 9.88 9.68 10.79 1.25 2.03 1.25 87Sr/86Sr 0.705393 0.705415 0.705427 0.705459 0.705409 0.705484 0.705282 0.705215 0.705172 0.705342 0.705330 0.705314 206Pb/204Pb 18.864 18.852 18.872 18.859 18.849 18.853 18.849 18.848 18.852 18.855 18.845 207Pb/204Pb 15.664 15.634 15.672 15.660 15.636 15.634 15.635 15.636 15.657 15.650 15.655 208Pb/204Pb 38.858 38.770 38.890 38.871 38.750 38.745 38.743 38.748 38.841 38.812 38.853 note: Major elements are reported in weight percent (wt.%) and trace elements are reported in micrograms per gram (µg/g). All values are averaged, see Supplements 1 – 4 for a complete tabulation of all analyses. 26
Figure 4.1. Total alkali silica (TAS) diagram for OVC (dark blue circles) and TVC (gold circles) glasses analyzed (anhydrous eruption averages). Caldera-forming eruptions Oruanui and Rotoiti are denoted by gold and dark blue stars, respectively. One TVC and two OVC data points with the highest SiO2 wt.% aside from each respective CFE are pre- or syn-caldera eruptions. See discussion for details.
27
(a)
(b)
(c)
Figure 4.2. SiO2 (a), Ti (b), and Ba (c) concentrations through time for the most recent CFE – denoted by dark blue (OVC) and gold (TVC) stars – and subsequent eruptions for OVC and TVC. Symbols for OVC and TVC subgroups defined in (a). 28
(a)
(b)
Figure 4.3. Primitive mantle normalized extended trace element diagrams for OVC (a) and TVC (b) glasses. Averages for each eruption are shown. Normalization values from Sun and McDonough (1989). 29
(a)
(b)
Figure 4.4. Primitive mantle normalized rare earth element diagrams for OVC (a) and TVC (b) glasses. Averages for each eruption are shown. Normalization values from Sun and McDonough (1989). 30
Figure 4.5. Three-isotope Pb diagrams showing both volcanic centers. Internal uncertainties for all isotope ratios are smaller than sample symbol (± 0.001 – 0.004).
31
Figure 4.6. 87Sr/86Sr and 206Pb/204Pb diagram showing the two volcanic centers analyzed in this study. Internal uncertainties for both isotope systems are smaller than sample symbol (Sr: ± 0.000008-0.000015; Pb: ± 0.001). 32
5. Discussion
5.1. Comparison to previously published and other current data
There has been much debate regarding the origin of TVZ rhyolites (and arc rhyolites in
general) and much work has been done with the aim of settling the debate (Reid, 1983;
Graham et al., 1992a; McCulloch et al., 1994; Graham et al., 1995b; Sutton et al., 1995;
Smith et al., 2005; Price et al., 2005; Liu et al., 2006; Wilson et al., 2009b; Charlier et al.,
2010; Deering et al., 2010; Smith et al., 2010; Rooney and Deering, 2014). While this
debate has involved many modes of investigation, radiogenic isotopes have been
employed for many hypotheses regarding TVZ rhyolite genesis (Graham et al., 1992a;
McCulloch et al., 1994; Schmitz and Smith, 2004). Figure 5.1 compares previously
published whole rock and mineral Pb and Sr isotopic compositions with this study, as
well as newly collected and unpublished whole rock and mineral data. The most striking
observation in these figures is the clear offset between our new data and older data sets
– specifically in terms of Pb isotopes. While the complex details behind these
discrepancies are beyond the scope of this study, it is worth noting that variability in the
older data is most likely due to methodological and analytical advances in isotope
measurement techniques over the last few decades. Older (pre-1990s) Pb isotope
measurements by thermal ionization mass spectrometry (TIMS) were notoriously
difficult for controlling mass bias due to the lack of a stable Pb isotope ratio that can be
used for internal normalization. The advent of the double spiked mass bias correction method (developed in the 1960s but not commonly used until the late 1990s; Taylor et al., 2015 and references therein) to account for continuous mass fractionation for use 33 with Pb isotope analysis via TIMS greatly improved precision (Woodhead et al., 1995;
Galer and Abouchami, 1998; White et al., 2000; Taylor et al., 2015; Yuan et al., 2016). In addition, the development of techniques for measuring Pb isotopic compositions via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) has also greatly improved data quality. As a result, measurement precision has improved by at least an order of magnitude due to better control over mass bias correction and more streamlined analytical methods. These technical improvements are welcome, but make it challenging to directly compare older Pb isotope data with more modern measurements. For example, Yuan et al. (2016) presented a collection of published NIST
NBS 981 standard data, along with their own, showing that unspiked TIMS ratios were less radiogenic than spiked TIMS and MC-ICPMS data. Figure 1 in Yuan et al. (2016) shows a similar pattern as Figure 5.1a here – an offset to the lower left of older TIMS generated Pb isotope ratios. Although published methods are unclear for some of the literature sources for published data from the TVZ and related units, the majority are non-double spiked TIMS measurements; except for McCulloch et al. (1994) who list double spike calibration in their methods, Graham et al. (1992) note a spike for some of their samples (but do not mention a double spike, so this was likely a spike for concentration measurements only), Brown et al. (1998) and Schmitz and Smith (2004) do not mention it at all. This speaks to a historical variance in methodology, at least with regard to the Pb isotope data available prior to what is presented in this study (and other very recent analyses). In addition to precision, inter-lab variability has improved markedly over the last few decades as well (Ramos, F.C., personal communication). Due 34
to these reasons, our interpretations are largely limited to the data from this study, together with newer data from the literature and unpublished sources. The older data is shown for reference only.
In contrast to Pb, Sr isotope measurements have typically been more accurate and precise throughout the decades, as mass bias is controlled by internal normalization using the stable 86Sr/88Sr ratio (0.1194), which can be applied using TIMS and MC-ICP-
MS alike. In this study, our pumice Sr isotopic compositions (Figs. 5.1b – showing
published Sr isotopic data that has complementary Pb isotopic data – and 5.2) are in
very good agreement with existing data. Small differences between our measurements
and previous data probably relate to heterogeneities in erupted material (e.g., the range
of 87Sr/86Sr reported for the Oruanui eruption from Wilson et al., 2006). As a result, we compare our data with all available (and relevant) Sr isotope data for the TVZ. We do
note that our measurement for one sample from the Taupo Volcanic Center (Omega
Dacite) is significantly more radiogenic (0.7067) compared to the reported composition
(0.7060) in Sutton et al. (1995) for another sample from the same unit. Replicate
analysis (also 0.7067) confirmed the higher 87Sr/86Sr ratio for our sample. Though this
could also reflect heterogeneity within erupted material, the difference is larger than
what is observed in other much larger eruptions, such as Oruanui (Wilson et al., 2006).
5.2 Mantle-derived and crustal contributions
Radiogenic isotopic ratios in evolved magmas are important tools for teasing out the complex processes that occur in continental arc magma systems. Due to long term 35
differences in parent-daughter ratios, mantle and crustal sources typically have quite
diverse radiogenic isotopic compositions, and different mantle and crustal domains can
also have discrete signatures (e.g. (Zindler and Hart; White and Patchett, 1984; Plank
and Langmuir, 1998). Parental magmas and crustal assimilants can thus imprint their unique isotopic signatures on the final magmatic composition, even where magmas have undergone significant differentiation. There are additional complexities – for
example, Davidson et al. (2005) pressed the point that these processes can be muted or
difficult to resolve with whole-rock geochemistry alone, due to effects of ‘element
weighted averages of heterogeneous crystal cargo’. However, this stance is typically
applied to crystal-rich intermediate arc rocks. We argue that for the crystal-poor (<15%,
as defined by Bachmann, 2004; Hildreth, 2004; Bachmann and Bergantz, 2008) rhyolites of the TVZ, the whole-rock approach is not only sufficient, but provides valuable insight to the last melt compositions prior to eruption, as the samples are predominantly made up of pumice glass (rather than a crystalline groundmass).
We present a suite of whole-rock isotopic data (Sr and Pb) for the two most recently active volcanic centers, Taupo and Okataina, in the TVZ. Our goal is to investigate contributions from and variation (if any) of mantle-derived magma and the surrounding
crust in the generation of rhyolites in the TVZ. Strontium and Pb isotopic ratios from this
study (Figs. 5.3 and 5.4) suggest notable crustal contributions but the source and amount of crustal input into these magmas remains less clear. In effort to quantify these variables, our OVC and TVC isotopic compositions are evaluated in terms of mixing 36
between potential mantle-derived and crustal endmembers (discussed in more detail in
sections 5.2.1 and 5.2.2; see table 5.1 for parameters):
• TVZ basalts and representative lithospheric mantle. These two possible mantle-
derived endmembers have similar isotopic signatures (Figs. 5.3 and 5.4), but are
considered as different potential contributing sources (Graham et al., 1992a;
Gamble et al., 1993; Storey et al., 1999).
• An enriched mantle component thought to be supplying radiogenic Pb to
Zealandia intraplate volcanism (Timm et al., 2010; Hoernle et al., 2010).
• Torlesse and Waipapa metasedimentary composite superterranes. The majority
of the North Island upper crust is composed of these two metagreywackes
(Mortimer, 2004; Price et al., 2015).
• A lower crustal component. This possible endmember is represented by
metaigneous xenoliths found in intermediate lavas in the SW TVZ (Graham et al.,
1990; Price et al., 2005, 2012)
• Sediments from the Tonga-Kermadec and Hikurangi trenches. Trench sediments
are thought to be a component of arc magmas world-wide (Plank and Langmuir,
1993, 1998; Elliott et al., 1997) and these represent the sediments local to the
TVZ.
We employ a simple two-component mixing model (DePaolo, 1981; modified by
Graham et al., 1992b) to investigate the possible roles of these Pb and Sr sources. We acknowledge that concurrent assimilation-fractional crystallization (AFC) may produce 37
somewhat different results than binary mixing alone, however mixing of two
components still provides a reasonable way to explore the range of possible isotope
compositions provided by these sources. In addition, Pb-Pb isotopic mixing lines are
straight and thus AFC and two component mixing trends will only differ by the
proportions of each endmember required to produce a given composition. We also note
that the modeling approach we have taken also assumes restricted endmember
compositions – but existing data and our modeling suggest that heterogeneity exists in
both mantle source and crustal contributions between each volcanic center, and
possibly within eruptive cycles in both the TVC and OVC (Figs. 5.3 – 5.6). It should also
be noted that unpublished data (S. Smithies, 2020) from the TVZ ignimbrite flare-up (c.
350 – 280 ka) is shown for comparative purposes in multiple figures, but is not tabulated or discussed here in detail as it is not part of this study.
5.2.1 Mantle-derived parental melts
The bulk of erupted material in the central TVZ is rhyolitic, and it is widely accepted that silicic volcanism is driven by underplating and intrusion of mantle-derived magmas
within the lower crust (Hildreth, 1981; Bachmann and Bergantz, 2008a). There are also a
number of small basaltic eruptions known in the TVZ, and they have been noted to have
a subduction-related component (Gamble et al., 1993, 1996; Waight et al., 2017).
Therefore, TVZ basalts are a logical source for mafic parental material contributing to
TVZ rhyolites. In addition to local basalts, we also explore the possible contributions of
an inferred lithospheric mantle component (Storey et al., 1999) as another parental
endmember. These compositions are those of Marie Byrd Land dykes (West Antarctica) 38 and have been hypothesized to be representative of the Gondwanan-aged lithospheric mantle beneath New Zealand (Hoernle et al., 2006). There is significant overlap of the local basalt and lithospheric mantle isotopic compositions in both Sr-Pb and Pb-Pb space, but the lithospheric mantle may contribute a slightly more radiogenic component in 207Pb/204Pb (Fig. 5.3).
Lastly, Zealandia intraplate volcanism – specifically seamounts atop the Hikurangi
Plateau – may provide an additional source for mantle-derived material. It has been suggested that a large scale low seismic velocity zone (at >600 km depth) that spans an area beneath West Antarctica and the Zealandia microcontinent may be indicative of a
Cretaceous-aged reservoir that has been furnishing the upper mantle beneath Zealandia with HIMU-type material throughout the Cenozoic (Timm et al., 2010; Hoernle et al.,
2010). This potential component offers a more radiogenic parental component than local basalts, and while TVZ rhyolites may not descend directly from it, this enriched mantle source could be supplying material to TVZ basalts.
5.2.2. Crustal contaminants
Two commonly called upon crustal assimilants for TVZ rhyolites are the Waipapa and
Torlesse composite superterranes (Graham et al., 1992a; McCulloch et al., 1994; Price et al., 2005). Available data suggests the TVZ is both flanked and underlain by these metasedimentary terranes; they constitute the upper crust in which the magmatic systems of the TVZ are thought to occur (Price et al., 2015). While they are both
Mesozoic-aged composite metasedimentary terranes, existing data show that the 39
Waipapa and Torlesse have variable isotopic compositions – both from each other and regionally within each terrane (e.g. Fig. 5.3 and 5.4). Isotopic data from McCulloch et al.
(1994) and Price et al. (2015) were chosen for our mixing models, as these studies include both Sr and Pb isotope compositions.
Another potential crustal source is represented by metaigneous xenoliths found in
Tongariro Volcanic Complex andesites, thought to reflect a granulitic lower crust in the
SW boundary of the TVZ (Graham et al., 1990, 1995b; Price et al., 2005, 2012). This source is less well-known as it does not outcrop at the surface. Isotopically, this material represents a more radiogenic component in 207Pb/204Pb and 208Pb/204Pb (Fig. 5.3), which
is lacking in other potential crustal sources. These xenoliths have not been seen in
central TVZ rhyolites, but should not be discounted, as the composition of the middle
and lower crust beneath the TVZ is largely unknown. It is worth noting that Price et al.’s
(2012) study included Pb and Sr isotopic compositions for one metasedimentary
xenolith, considered to be a high-grade version of the Torlesse terrane. This sample has distinct isotopic ratios from both the existing Torlesse and granulitic xenoliths, and could
be yet another lower crustal component – albeit characterized by only a single sample.
However, this sample attests to the variability of potential sources for crustal material.
Lastly, subducting sediments along the Tonga-Kermadec and Hikurangi margins could
also contribute Pb and Sr to TVZ rhyolites. It has long been suggested that sediments
play a role in subduction zone magmatism (Armstrong, 1968; Plank and Langmuir, 1993,
1998). While it is possible sediments from the greater Pacific basin could be influential,
we chose sediments from along the greater Tonga-Kermadec arc (including the 40
Hikurangi trench) as representative compositions due to their proximity to the TVZ.
Although two of the three data points taken from Plank (2014) have much less radiogenic 206Pb/204Pb ratios, the Hikurangi trench sample falls within the same range as
the Torlesse, making it a viable contributing factor, albeit difficult to resolve.
5.2.3. Isotopic modeling outcomes
Two-component mixing lines for Pb-Pb and Sr-Pb are shown in figures 5.3 and 5.4. The
compositions of endmembers used for these calculations are shown in Table 5.1. These
models show that our TVC and OVC data lie broadly between TVZ basalt and Torlesse
compositions, but are also displaced to higher 207Pb/204Pb and 208Pb/204Pb. Thus, subduction-modified (Gamble et al., 1993; Waight et al., 2017) TVZ basalts represent one reasonable mantle-derived source of the Pb and Sr in rhyolites and the Torlesse metasedimentary terrane serves as a sensible crustal source. However, we also note that representative lithospheric mantle is also possible parental melt, specifically due to slightly more radiogenic 207Pb/204Pb ratios. In reality, there is significant overlap of the
two in 206Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr, so even if that is the case, it is difficult to
distinguish between these – and it is also likely TVZ basalts are probably derived in part
from local upper mantle, so these two endmembers could be considered as broadly the
same source.
Our data also show that Pb and Sr in OVC and TVC samples also require significant contributions from one or more crustal sources. However, the data appear to rule out contributions from the Waipapa terrane, as samples from both volcanic centers have 41
distinctly higher 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb (Fig. 5.3) than the range of
Waipapa compositions (see below). Subduction zone trench sediments represent yet another potential source as they have 206Pb/204Pb and 87Sr/86Sr ratios that overlap some
of our data in both volcanic centers, but fall short in 207Pb/204Pb and 208Pb/204Pb (Figs.
5.3 and 5.4). The Torlesse serves as a more reasonable crustal source (Figs. 5.3 and 5.4).
Although this terrane is variable in composition, it does extend to the higher 206Pb/204Pb
and 87Sr/86Sr as also seen in OVC and TVC samples. In detail, however, we also note that
available Torlesse data does not contain samples with high enough 207Pb/204Pb and
208Pb/204Pb to completely explain our data. This could be due to a relatively small
number of samples measured to represent a large and variable rock unit, but could also
mean additional crustal sources are required. In light of this, a viable additional crustal
source could be material from deeper in the crust, as represented by metaigneous xenoliths found in intermediate lavas from the Tongariro Volcanic Complex at the SW boundary of the TVZ. While they also have variable compositions, some samples from this suite have 207Pb/204Pb and 208Pb/204Pb that overlap with our data from both volcanic
centers (Fig. 5.3). In addition to isotopic data and mixing trends, trace element concentrations for both volcanic centers provide insight into potential crustal sources.
Figure 5.5 reveals that trace element data favor the Torlesse as a crustal input over the representative lower crust, as the Torlesse data is in very good agreement with our data for both volcanic centers.
Another way to produce more radiogenic 207Pb/204Pb and 208Pb/204Pb in OVC and TVC
samples is by contributions from an enriched mantle source. Mantle material supplied 42
to seamounts on the Hikurangi Plateau (along with other intraplate volcanism in the
region; Hoernle et al., 2006, 2010; Timm et al., 2010) have significantly more radiogenic
Pb than other suggested parental endmembers. As a result, in combination with the lower crustal xenolith source, it is possible to produce a three component mixing model that explains most of the Sr and Pb isotope compositions we see in the OVC and TVC magmas. Figures 5.6 and 5.7 show scenarios that invoke mixing between this enriched component and TVZ basalts followed by a lower crustal assimilant (here, represented by metaigneous xenoliths). This results in a set of more complex mixing trends for TVZ magmas but is a plausible model for generating the extent of Pb isotopic compositions we see in our TVC and OVC samples.
Despite limitations, there are some useful conclusions that can be drawn from the mixing models (see Figs. 5.8 and 5.9 for schematic representations of both the two- and three-component mixing scenarios):
• Our data further confirm that the Waipapa metasedimentary terrane does not
contribute substantially to TVZ rhyolites. Although the Waipapa shows significant
overlap with the Torlesse and our data in 87Sr/86Sr, its Pb isotopic composition is
not radiogenic enough to be considered as a crustal endmember for OVC and
TVC samples. This interpretation is consistent with the conclusions of earlier
workers (Graham et al., 1992a; McCulloch et al., 1994), and our data supports
this (Figs. 5.3 and 5.4).
• While the Torlesse is a plausible contributor to isotopic compositions of some
TVZ rhyolites, the large range of values shown by existing Torlesse samples make 43
it challenging to define a particular endmember composition, especially for Pb
isotopic ratios. In addition, our data also show that TVC and OVC rhyolites have
distinctly higher 207Pb/204Pb and 208Pb/204Pb values than the currently known
range of Torlesse compositions (Figs. 5.3 and 5.6). One possibility is that higher
207Pb/204Pb in our OVC and TVC samples reflect contributions from older
sediments in the Torlesse terrane that are not currently represented in the
known range of Pb compositions. There is some evidence that such older
sediments may exist. For example, detrital zircons from Torlesse metasediments
include populations with Paleozoic and Proterozoic ages – similar to those found
in Gondwanan-aged sediments from Eastern Australia (Ireland, 1992; Pickard et
al., 2000; Wandres et al., 2004; Adams et al., 2007). More detailed sampling of
the Torlesse would be required to identify if there are sedimentary components
that have more radiogenic 207Pb/204Pb. In addition, if such a component exists, it
would also need to be preferentially sampled by OVC and TVC magmas relative
to the remainder of the Torlesse composition to retain the restricted range of
compositions seen in the OVC and TVC samples.
• Another viable source of Pb and Sr within OVC and TVC rhyolites appears to be
three way mixing between lower crustal granulites, TVZ basalts/lithospheric
mantle, and enriched seamount compositions (Fig. 5.6 and 5.7). This model
would require that the crustally-derived Pb come from the most radiogenic
existing xenolith compositions, and would also suggest mixing between different 44
mantle sources contributing to TVZ eruptives. The latter has been proposed by
other workers (e.g. Waight et al., 2017; Barker et al., 2020).
Our data also allow us to investigate the relative amounts of crustal vs. mantle sources
in OVC and TVC magmas. It has been suggested by other workers that a notable (up to
35%) contribution from an upper crustal source (e.g. the Torlesse terrane) could
produce TVZ rhyolites (Reid, 1983; Charlier et al., 2010). Assuming that Torlesse
sediments do constitute the crustal Pb and Sr source in OVC and TVC rhyolites, our
binary mixing trend for 206Pb/204Pb vs. 87Sr/86Sr suggest that up to c. 30% crustal Pb and
Sr is required for the genesis of TVZ rhyolites. However, for 206Pb/204Pb vs. 207Pb/204Pb
and 208Pb/204Pb binary mixing with this amount of crust is more difficult to
accommodate with our relatively high 207Pb/204Pb and 208Pb/204Pb. In fact, to represent
this in our model, the most radiogenic ratios from TVZ basalt or lithospheric mantle
sources were chosen – anything less than that from the given data would suggest much greater crustal contributions (60-80%). Such contributions are energetically more difficult to accommodate using an AFC scenario – where the amount of assimilation is limited by heat released from cooling and crystallization of mafic magma – and would
likely require mixing between crustal melts produced by the heat from intrusion of
additional basalt (Annen et al., 2006). If we consider the lower crust as a potential
crustal source, then the ternary mixing plots (Figs. 5.6 and 5.7) suggest much greater
proportions of Sr and Pb derive from the crust. Although this proportion is greater, it is easier to incorporate material within the lower crust where hotter ambient 45
temperatures make assimilating or melting substantial amounts of crust more
energetically feasible (Annen et al., 2006).
5.3 Provinciality of caldera system isotopic signatures
It is generally accepted that TVZ rhyolites, as with most rhyolites in subduction zones and other settings, are generated by some combination of fractional crystallization from a parental basalt along with melting and assimilation of the surrounding crust (Hildreth et al., 1991; Riley et al., 2001; Hildreth, 2004; Price et al., 2005). Thus, as discussed in the previous section, the likely sources and controls on Sr and Pb isotopic compositions are the mantle-derived melt (in this case, local basalts), together with material from melting and assimilation of crust beneath the TVZ. The contribution of these different components to the production of rhyolite in the OVC and TVC are paramount to understanding how such giant volumes of silicic magma are produced. These roles are explored here on a regional scale using our Pb and Sr isotope data.
One of the more striking observations in our data set is the clear distinction in 87Sr/86Sr
and 206Pb/204Pb in the two volcanic centers (Figs. 4.5 and 4.6). Only one Okataina
eruption overlaps with the TVC (the upper unit of Rotorua in 206Pb/204Pb, not in Sr), otherwise the two centers are unique in these two ratios. Further, although we do not discuss it in detail, we also note that the data in our plots for older ‘flare-up’ magmas
(350-280 ka, from Whakamaru, Okataina, Kapenga, and Reporoa volcanic centers) share many of the Sr and Pb isotopic features of our TVC and OVC sample suites but in detail, also show distinct ranges of compositions. This difference is also seen in previously 46
published data from discrete studies of each volcanic center (Fig. 5.1 and 5.2), but our
more precise data (together with measurements of Pb and Sr isotopic compositions on
the same material) makes this difference clearer. We suggest a few plausible
explanations for the observed differences in 206Pb/204Pb and 87Sr/86Sr below.
Heterogeneous mantle-derived sources
It is likely that there are differing mantle sources or melting regimes beneath the two
volcanic centers. As discussed above (section 5.2.3), binary mixing models are inconsistent with a single mantle-derived source for TVZ rhyolites. This has also been noted in studies of more primitive material from the TVZ and is seen in the large variability in previously published Pb and Sr isotopic compositions (Figs. 5.3 and 5.4;
Graham et al., 1992a; Gamble et al., 1993; Hiess et al., 2007; Waight et al., 2017;
Zellmer et al., 2020; Barker et al., 2020). Barker et al. (2020) also observed major compositional differences in olivine-hosted melt inclusions from Taupo and Okataina
basalts compared to inter-caldera basalts. They suggest a more highly depleted source
via previous melt extraction, along with higher melt fractions, beneath the calderas as
opposed to inter-caldera eruptions. While this comparison is between inter- and intra-
caldera inputs, if there are such differences in such small spatial areas in the TVZ (< 10
km in some instances) it is entirely possible that the two volcanic centers also have
heterogeneous mantle-derived contributions. Zellmer et al. (2020) note differences
through time in basaltic compositions from the TVZ, and suggest a less depleted source
for TVZ basalts (and an enriched source for basaltic andesites) erupted in active volcanic
centers, while extinct systems have more depleted mantle-derived inputs. Primary 47
mantle heterogeneities beneath the TVZ are also supported through Hf-Nd isotope
compositions which suggest variability in primary magma composition as a result of
differing degrees of sediment input at the slab-wedge interface (Waight et al., 2017). Hf-
Nd isotopic systems are less effected by crustal contamination due to similar behavior during mantle melting and fractional crystallization (Waight et al., 2017). Although mantle inputs tend to be muted in such highly evolved magmas, some heterogeneity in source could contribute to the unique isotopic signatures in the OVC and TVC. In terms of the three endmember mixing scenario described above (and seen in Fig. 5.6 and 5.7), it is then possible that the mantle sources for OVC and TVC magmas contain consistently different ratios of Pb derived from TVZ basalts versus enriched intraplate sources.
Heterogeneous compositions of crustal terranes
An additional prospect is that each volcanic center is sampling a unique part of the upper or lower crustal package, thus incorporating different crustal isotopic signatures.
Upper crustal Torlesse metasediments have highly variable compositions and it is reasonable that different subunits and packages within the unit also have distinctive compositions. In addition, lower crustal rocks also appear to have variable compositions as seen by the available data (see Figs. 5.3 and 5.4), although these are far less well- characterized. Figure 5.10 (from Price et al., 2015), shows a compiled map and schematic cross-section highlighting the litho-stratigraphic terranes of the Eastern
Province, reveals what the subsurface may look like beneath the TVZ – and show a minimum of three different subsets of metasedimentary crust. These units are from both the Waipapa (Morrinsville) and Torlesse (Kaweka and Pahau). Since the TVZ is 48
covered by thick units of volcanic material, sampling of the basement rocks is reserved
for outside the TVZ boundaries and may not be representative of what overlies each
magmatic system. This interpretation of the local crust, coupled with the wide range of
isotopic compositions reported for both the Torlesse and xenoliths from the lower crust
(Figs. 5.3 and 5.4), suggests that each volcanic center could be sampling different components of a crustal package. The existing data for the Torlesse and the lower crust permits this hypothesis due to the large spread in the data. It is important to note, however, that our potential crustal sources must be much more limited in isotopic composition than the range of compositions shown by both the Torlesse terrane and the representative lower crust overall, otherwise the data from TVC and OVC eruptives
would be significantly more scattered.
Different thickness of the crustal package beneath Taupo Volcano
Extension in Okataina is occurring more rapidly than in Taupo (Wallace et al., 2004), so it
is plausible that the crustal package above the TVC magma system is thicker and this
could lead to differences in the amount of crustal melting and assimilation. If thicker
crust is indeed present beneath Taupo Volcano, the magmatic system could also be
subject to passage through the same metaigneous crust present beneath the Tongariro
Volcanic Complex. Strontium isotopic ratios in the lower crustal xenoliths are relatively
high and overlap with the higher values we observe in the TVC and thus could represent
material that is also supplying radiogenic Sr and Pb (specifically, 207Pb/204Pb and
208Pb/204Pb) to the TVC but not to the OVC. We also note that it has been suggested that
the TVC is a hotter system then Okataina, and tends towards hot-dry-reduced magmas 49
(as compared to the cold-wet-oxidized magmas of the OVC; (Charlier et al., 2008;
Deering et al., 2010), possibly contributing to more bulk assimilation and/or melting of the crust.
5.4. Changes through a caldera cycle
Major and trace elements were measured in the same pumice glass fragments as measured for Pb and Sr isotopic compositions in order to complement radiogenic isotope analyses. While isotopic compositions primarily reflect regional source variability, major and trace elements are also sensitive to a range of magmatic processes and thus can also record temporal trends related to magmatic evolution. Combining the two data sets produces some interesting observations, and these are discussed here as we consider the implications of the caldera cycle – including pre-, syn-, and post-caldera events, as well as the caldera-forming eruption itself.
One of the more conspicuous features in our major and trace element data is a clear temporal evolution through each center’s caldera cycle. This is shown as a significant drop in SiO2 post-CFE (Fig. 4.2a). Changes to lower SiO2 and the presence of more mafic and hotter mineral assemblages following major caldera-forming eruptions have been noted previously (e.g. Smith et al., 2005; Deering et al., 2010) and have been related to shifts to a less evolved magmatic system post-CFE. We also observe consistent trends in other elements (e.g. Ti, Ba; Fig. 4.2b and c, Fig. 5.11).
We can also relate these changes in composition to changes in pressure of last mineral- melt equilibration using the rhyolite-MELTS geobarometer (Gualda and Ghiorso, 2014), 50
which shows deeper equilibration pressures immediately following a CFE (Fig. 5.12a),
showing a deepening of the pre-eruptive magma body along with the shift to less
evolved compositions (Fig. 5.12b). Similar trends were also highlighted in Smith et al.
(2005) and Pamukçu et al. (2020). In the OVC, the pre-caldera, syn-caldera, and CFE all
exhibit the highest SiO2 concentrations (77.74 – 78.11 wt.%) and the shallowest depths
(c. 40 – 90 MPa). The TVC shows a slightly different evolution, with more variability post-CFE (Figs. 4.2a and 5.12). These trends support the idea of a magmatic cycle revolving around the caldera-forming events, where the accumulation of magma prior to CFEs result in greater extent of magmatic evolution and magma storage at shallower crustal levels, therefore producing more evolved magmas. After caldera-forming events,
the magmatic system is largely emptied and commences being ‘reset’ and rebuilt by
deep introduction of mafic magmas (e.g. Bachmann et al., 2012; Forni et al., 2018).
While similar to the OVC regarding a significant deepening of the magmatic system and
shift to less-evolved compositions, the TVC still appears to still be in a deeper stage of
evolution than the OVC – the most recent eruption, Taupo, shows depth and SiO2
concentrations similar to Omega Dacite, the eruption directly post-CFE. The caldera
cycle represented in our dataset is also longer in the OVC (c. 45 ka, not including
Kakapiko, the c. 127 ka dome eruption) than in the TVC (c. 27 ka). These differences
likely reflect one of two scenarios (or possibly both): (1) the two centers are in different
stages of a similar evolution (the cycle observed in our dataset is longer in the OVC than
in the TVC), or (2) the two centers are evolving on different timescales. 51
Although the temporal variation of SiO2 through caldera-forming events has been
described before (e.g. Smith et al., 2005), it still remains unclear as to whether these
shallow silicic compositions are solely due to greater extents of fractional crystallization
or whether accumulation of these large volumes of magma also requires greater crustal
contributions to the magmatic system. If the latter is the case, then the additional
observation that evolved magmas are equilibrated at relatively low pressures (50 – 100
MPa, or c. 2 – 3 km depth) has important consequences for the thermal state of the system – as more heat is required to melt or assimilate the crust at shallower depths.
Although it is difficult to determine the extent of crustal input by elemental compositions alone, our isotopic data provide a method for gauging this, as mantle and crust have different isotopic compositions (as discussed above and seen in figures 5.3 and 5.4; figures 5.6 and 5.7). As we have shown, this is more complex for Pb isotopes where there is considerable variation in both crustal and mantle sources. However, Sr isotopes are more straightforward and reveal interesting trends for TVC and OVC magmas. In figure 5.13, we compare measured OVC and TVC Sr isotopic compositions
with pressure. While SiO2 shows less evolved compositions with increasing pressure,
87Sr/86Sr ratios are distinctly more radiogenic with increasing pressure. This suggests that deeper magmas either have greater extents of interaction with the crust and/or interact with more radiogenic crustal components. This surprising observation suggests that large volumes of silicic magma that reside in the shallow crust and erupted during caldera-forming events may be more related to fractionation of magmas that contain overall larger proportions of mantle-derived Sr, whereas the hotter, deeper magmas 52 erupted after CFEs have greater crustal contributions, as they are smaller volume, but able to effectively assimilate crustal material. 53
Table 5.1. Compositions used for isotopic mixing models Parental Melts Crustal assimilants TVZ basaltsa Lithospheric Mantleb EMc Torlesse Metasedimentsd MIXe Sr (µg/g ) 313 746 859 295 378 Pb (µg/g ) 3.7 15 6 27 7 87Sr/86Sr 0.70429 0.70470 0.70472 0.70280 0.71182 0.70800 0.70737 206Pb/204Pb 18.792 18.84018.785 18.789 18.737 18.829 19.389 18.914 18.955 18.840 207Pb/204Pb 15.624 15.608 15.582 15.628 15.68 15.649 15.680 208Pb/204Pb 38.735 38.668 39.096 38.839 38.850 note : EM - enriched mantle component, MIX - metaigneous xenoliths. See Figure 5.1 caption for all other references. For models, some ratios were used in multiple models and are listed once here. aTrace element concentrations averaged from Graham et al. (1992a), Price et al. (1992), and Heiss et al. (2007) bTrace element concentrations averaged from Storey et al. (1999) cTrace element concentrations averaged from Hoernle et al. (2010) dTrace element concentrations averaged from McCulloch et al. (1994) and Price et al. (2015) eTrace element concentrations from Graham et al. (1990) 54
(a)
(b)
55
Figure 5.1 (previous page). Comparative plots for 206Pb204Pb vs. 207Pb/204Pb and 208Pb/204Pb (a), and 87Sr/86Sr (b) showing a compilation of previously published data for the TVZ and data from this study. All data is whole rock unless otherwise noted. Ignimbrite flare-up data is from S. Smithies from University of Canterbury and is unpublished. Published data are from the following sources: OVC (Rotoiti and Earthquake Flat) – Schmitz and Smith (2004); Flare-up - Brown et al. (1998); TVZ rhyolites - (Graham et al., 1992a); Maroa plagioclase - McCulloch et al. (1994). Internal precision for data from this study (dark blue and gold circles) and the Ignimbrite flare-up (grey triangles) is smaller than symbol size (see Results for values), and for previously published work is highly variable (2σ = 0.01 – 0.05). See text for discussion. 56
Figure 5.2. Comparative plot of published Sr isotope data not shown in Fig. 5.1 that highlights the unique isotopic signatures (represented by grey dashed line) for each volcanic center, as well as the excellent reproducibility of Sr isotopes from different studies over the last two and a half decades. Data compiled from Sutton et al. (1995), Wilson et al. (2006), and Wilson and Charlier (2010). Internal precision is smaller than symbol size for all data shown.
57
Figure 5.3. Lead isotopic compositions from Okataina (dark blue circles), Taupo (gold circles) and the ignimbrite flare-up (grey triangles – unpublished data from S. Smithies at University of Canterbury), along with two simple isotopic mixing trends (DePaolo, 1981; modified by Graham et al., 1992b). Regional TVZ basalts were used as parental melts (purple field; data from McCulloch et al., 1994; Gamble et al., 1996; Graham et al., 1992a), as well as representative lithospheric mantle (red field; data from Storey et al., 1999), and Torlesse metasediments as assimilants (teal field; data from McCulloch et al., 1994; Price et al., 2015). Other possible contaminants are also included and consist of Waipapa metasediments (orange field; data from McCulloch et al. 1992; Price et al., 2015), metaigneous xenoliths (MIX – brown field; data from Price et al., 2012), and sediments from the Tonga-Kermadec and Hikurangi trenches (green field; data from Plank 2014). Parental melt and assimilant compositions used for modeling are listed in Table 5.1 and all data used for contaminant and mixing fields are listed in Supplement 7. 58
Figure 5.4. Strontium and Pb isotopic compositions from this study and others (see Fig. 5.3 for all references), along with simple binary mixing lines. Parental and assimilant endmembers are TVZ basalts, representative lithospheric mantle, and Torlesse metasediments. See Table 5.1 for compositions used for mixing and Supplement 7 for all data for representative fields. 59
Figure 5.5. Extended trace element diagram for all OVC and TVC samples from this study (colored lines) with comparative lines for N-MORB (grey dashed line; Sun and McDonough, 1989), TVZ basalts (red dashed line; Gamble et al., 1990 and 1993), Torlesse metasedimentary terrane (black dashed line; Price et al., 2015), and metaigneous xenoliths (MIX; black dotted line; Price et al., 2012). All lines, including data from this study, are averaged concentrations from all available data. Data normalized to primitive mantle (Sun and McDonough, 1989). All trace element data for this study can be found in Supplement 2. 60
Figure 5.6. Three component Pb-Pb diagram showing mixing trends between an enriched mantle source (EM; averaged isotopic compositions of Hikurangi seamounts from Hoernle et al., 2010), TVZ basalts, and representative lower crust (MIX – metaigneous xenoliths – see Fig. 5.1). Parental endmembers are a 90:10 and 30:70 mixture of TVZ basalts and an EM source, respectively. Each of these mixtures is then considered with a lower crustal assimilant, as represented by metaigneous xenoliths from the southwestern-most boundary of the TVZ. Torlesse terrane shown for comparative purposes. All data and references for representative data are listed in Figure 5.1 and Supplement 7. 61
Figure 5.7. Three component Sr-Pb mixing trends between TVZ basalts, an enriched mantle (EM) component, and representative lower crust (MIX). Parental endmembers are a 90:10 and 30:70 mixture of TVZ basalts and an EM source, respectively. Each of these mixtures is then considered with a lower crustal assimilant, as represented by metaigneous xenoliths from the southwestern-most boundary of the TVZ. Torlesse terrane shown for comparative purposes. All data and references for representative data are listed in Figure 5.1 and Supplement 7.
62
Figure 5.8. Schematic representation of TVZ basalts ascending through the upper crust and melting and assimilating the Torlesse metasedimentary terrane to create TVZ rhyolites. The upper mantle consists of the representative lithospheric mantle beneath New Zealand and contributes to the production of TVZ basalts. This scenario requires c. 30% crustal assimilation to contribute Pb and Sr to TVZ rhyolites (see text for details).
63
Figure 5.9. Schematic showing a deep enriched mantle component contributing to TVZ basalts, along with the upper (lithospheric) mantle. The basalts stage in the lower crust, represented by granulitic xenoliths found in the SW TVZ, where they melt and assimilate the deeper crustal material. The magma then ascends through the upper crust before becoming an eruptible body. This scenario requires c. 70% crustal material to contribute Pb and Sr to TVZ rhyolites (see text for details).
64
Figure 5.10. Compiled map (A) showing distribution of the litho-stratigraphic terranes of the Eastern Province from Price et al. (2015) and references therein. (B) is a record of the sample locations from Price et al.’s study, and represents the dataset used for comparative purposes here. (C) is a schematic cross-section showing the relationship between the composite terranes of the central North Island and the TVZ. See Price et al. (2015) for a detailed description of their geochemical study.
65
Figure 5.11. Averaged Ti and SiO2 compositions for each OVC and TVC eruption, showing a decrease in Ti concentrations with increasing SiO2.
66
(a)
(b)
Figure 5.12. Calculated pressures vs. time (a) and average SiO2 concentrations (b) for TVC and OVC eruptions, showing a drop in SiO2 correlating with deepening of each magmatic system. CFEs represented by gold (TVC) and dark blue (OVC) stars. 67
(a)
(b)
Figure 5.13. Calculated pressures vs. Sr isotopic ratios for the OVC (a) and TVC (b). Each volcanic center shown separately to highlight trends. Error bars represent ± 25 MPa.
68
6 Conclusions
New high precision Pb and Sr isotope ratios and glass chemistry from Taupo and
Okataina volcanoes reveal distinctions both between volcanic centers and throughout caldera cycles in each center and have brought to light the following points:
1. Our data help elucidate potential mantle-derived and crustal source
contributions toward the production of TVZ rhyolites. We find that two general
scenarios may explain the observed Pb and Sr compositions in OVC and TVC
rhyolites. The first is that they contain a mixture of Pb and Sr derived from local
TVZ basalts (or lithospheric mantle) and from the Torlesse metasedimentary
terrane. This requires that some of the Torlesse terrane have higher 207Pb/204Pb
and 208Pb/204Pb compositions than are shown in the existing data. This more
radiogenic component may relate to an older detrital zircon population evident
is some Torlesse samples, but must also be preferentially sampled by OVC and
TVC magmas. This scenario requires 20 – 40% crustal contribution to erupted
magmas. Trace element data from both volcanic centers supports this scenario,
as Torlesse trace element concentrations are in very good agreement with our
data.
2. The second scenario is that Pb and Sr in OVC and TVC magmas derive from three-
component mixing between local basalts (or lithospheric mantle), an enriched
mantle source (represented by intraplate seamounts on the Hikurangi Plateau),
and the lower crust (represented by granulitic xenoliths from the SW TVZ). This
model can explain the range of Pb and Sr isotopic compositions observed in our 69
data, but require Pb and Sr to be derived discretely from the most radiogenic
end of the array of known lower crustal compositions. Our three component
mixing model necessitates significant contributions (up to 70%) from the crust,
although invoking the lower crust makes this more feasible energetically.
3. Relative to each other, the TVC and OVC have distinctive isotopic signatures in
87Sr/86Sr and 206Pb/204Pb. This provinciality could be due to distinct and
heterogeneous mantle-derived and/or crustal sources, or each volcanic center
may be sampling a unique part of the crust. Higher 87Sr/86Sr in the TVC also
points to the possibility that the magmatic system is assimilating more crust than
the OVC.
4. Temporal changes are observed in both volcanic centers, albeit on different
timescales or cycles. There is a shift to less evolved compositions and deeper
crustal storage after large caldera-forming eruptions in both the OVC and TVC,
but on different timescales – the TVC is in a deeper and less evolved stage of
evolution than the OVC. Strontium isotopic ratios also increase with higher
pressures and decreasing SiO2 – this supports the notion that the TVC magmatic
system has the ability to melt and assimilate more crust than the OVC.
Overall, the data presented here has shown that crustal interaction with mantle-derived melts is influential in the genesis of TVZ rhyolites. Although magmatic processes in evolved systems are inherently complex, our data assist in interpreting source contributions in TVZ rhyolites. Moving forward, deeper isotopic investigation into the upper and lower crustal rocks (e.g. Torlesse and lower crustal xenoliths), as well as TVZ 70 basalts, through more sampling and analysis, would add insight to mantle-derived and crustal contributions. Oxygen isotope measurements of OVC and TVC rhyolites would also aid in teasing out contributions from mantle-derived parental melts and crustal inputs, as they are an important indicator of the presence and amount of crustal contamination. Lastly, Nd isotope measurements for our suite of samples from each volcanic center would create a more comprehensive data set and allow for more comparative analysis between isotopic systems.
71
7 References Cited
Adams, C.J., Campbell, H.J., and Griffin, W.L., 2007, Provenance comparisons of Permian to Jurassic tectonostratigraphic terranes in New Zealand: perspectives from detrital zircon age patterns: Geological Magazine, v. 144, p. 701–729, doi:10.1017/S0016756807003469.
Annen, C., Blundy, J.D., and Sparks, R.S.J., 2006, The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones: Journal of Petrology, v. 47, p. 505– 539, doi:10.1093/petrology/egi084.
Armstrong, R.L., 1968, A model for the evolution of strontium and lead isotopes in a dynamic Earth: Reviews of Geophysics, v. 6, p. 175, doi:10.1029/RG006i002p00175.
Bachmann, O., 2004, On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes: Journal of Petrology, v. 45, p. 1565–1582, doi:10.1093/petrology/egh019.
Bachmann, O., and Bergantz, G.W., 2008a, Rhyolites and their Source Mushes across Tectonic Settings: Journal of Petrology, v. 49, p. 2277–2285, doi:10.1093/petrology/egn068.
Bachmann, O., and Bergantz, G., 2008b, The Magma Reservoirs That Feed Supereruptions: Elements, v. 4, p. 17–21, doi:10.2113/GSELEMENTS.4.1.17.
Bachmann, O., Deering, C.D., Ruprecht, J.S., Huber, C., Skopelitis, A., and Schnyder, C., 2012, Evolution of silicic magmas in the Kos-Nisyros volcanic center, Greece: a petrological cycle associated with caldera collapse: Contributions to Mineralogy and Petrology, v. 163, p. 151–166, doi:10.1007/s00410-011-0663-y.
Barker, S.J., Rowe, M.C., Wilson, C.J.N., Gamble, J.A., Rooyakkers, S.M., Wysoczanski, R.J., Illsley-Kemp, F., and Kenworthy, C.C., 2020, What lies beneath? Reconstructing the primitive magmas fueling voluminous silicic volcanism using olivine-hosted melt inclusions: Geology, v. 48, p. 504–508, doi:10.1130/G47422.1.
Ben Othman, D., White, W.M., and Patchett, J., 1989, The geochemistry of marine sediments, island arc magma genesis, and crust-mantle recycling: Earth and Planetary Science Letters, v. 94, p. 1–21, doi:10.1016/0012-821X(89)90079-4.
Bibby, H.M., Caldwell, T.G., Davey, F.J., and Webb, T.H., 1995, Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation: Journal of Volcanology and Geothermal Research, v. 68, p. 29–58, doi:10.1016/0377-0273(95)00007-H. 72
Branney, M., and Acocella, V., 2015a, Calderas, in The Encyclopedia of Volcanoes, Elsevier, p. 299–315, doi:10.1016/B978-0-12-385938-9.00016-X.
Branney, M., and Acocella, V., 2015b, Calderas: Elsevier Inc., 299–315 p., doi:10.1016/B978-0-12-385938-9.00016-X.
Brown, S.J.A., Burt, R.M., Cole, J.W., Krippner, S.J.P., Price, R.C., and Cartwright, I., 1998a, Plutonic lithics in ignimbrites of Taupo Volcanic Zone, New Zealand; sources and conditions of crystallisation: Chemical Geology, v. 148, p. 21–41, doi:10.1016/S0009-2541(98)00026-6.
Brown, S.J.A., Wilson, C.J.N., Cole, J.W., and Wooden, J., 1998b, The Whakamaru group ignimbrites, Taupo Volcanic Zone, New Zealand: evidence for reverse tapping of a zoned silicic magmatic system: Journal of Volcanology and Geothermal Research, v. 84, p. 1–37, doi:10.1016/S0377-0273(98)00020-1.
Charlier, B.L.A., and Wilson, C.J.N., 2010, Chronology and Evolution of Caldera- forming and Post-caldera Magma Systems at Okataina Volcano, New Zealand from Zircon U–Th Model-age Spectra: Journal of Petrology, v. 51, p. 1121–1141, doi:10.1093/petrology/egq015.
Charlier, B.L.A., Wilson, C.J.N., and Davidson, J.P., 2008, Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand: Contributions to Mineralogy and Petrology, v. 156, p. 799–813, doi:10.1007/s00410-008-0316-y.
Charlier, B.L.A., Wilson, C.J.N., and Mortimer, N., 2010, Evidence from zircon U-Pb age spectra for crustal structure and felsic magma genesis at Taupo volcano, New Zealand: Geology, v. 38, p. 915–918, doi:10.1130/G31123.1.
Cole, J.W., 1979, Structure, petrology, and genesis of Cenozoic volcanism, Taupo Volcanic Zone, New Zealand—a review: New Zealand Journal of Geology and Geophysics, v. 22, p. 631–657, doi:10.1080/00288306.1979.10424173.
Cole, J.W., Deering, C.D., Burt, R.M., Sewell, S., Shane, P.A.R., and Matthews, N.E., 2014, Okataina Volcanic Centre, Taupo Volcanic Zone, New Zealand: A review of volcanism and synchronous pluton development in an active, dominantly silicic caldera system: Earth-Science Reviews, v. 128, p. 1–17, doi:10.1016/j.earscirev.2013.10.008.
Cole, J.W., and Spinks, K.D., 2009a, Caldera volcanism and rift structure in the Taupo Volcanic Zone, New Zealand: Geological Society, London, Special Publications, v. 327, p. 9–29, doi:10.1144/SP327.2.
Cole, J.W., and Spinks, K.D., 2009b, Caldera volcanism and rift structure in the Taupo Volcanic Zone, New Zealand: Geological Society, London, Special Publications, v. 327, p. 9–29, doi:10.1144/SP327.2. 73
Cole, J.W., Spinks, K.D., Deering, C.D., Nairn, I.A., and Leonard, G.S., 2010, Volcanic and structural evolution of the Okataina Volcanic Centre; dominantly silicic volcanism associated with the Taupo Rift, New Zealand: Journal of Volcanology and Geothermal Research, v. 190, p. 123–135, doi:10.1016/j.jvolgeores.2009.08.011.
Darby, D.J., Hodgkinson, K.M., and Blick, G.H., 2000, Geodetic measurement of deformation in the Taupo Volcanic Zone, New Zealand: The north Taupo network revisited: New Zealand Journal of Geology and Geophysics, v. 43, p. 157–170, doi:10.1080/00288306.2000.9514878.
Davidson, J.P., Dungan, M.A., Ferguson, K.M., and Colucci, M.T., 1987, Crust-magma interactions and the evolution of arc magmas: The San Pedro-Pel I ado volcanic complex, southern Chilean Andes: , p. 4.
Deering, C.D., Cole, J.W., and Vogel, T.A., 2011, Extraction of crystal-poor rhyolite from a hornblende-bearing intermediate mush: a case study of the caldera-forming Matahina eruption, Okataina volcanic complex: Contributions to Mineralogy and Petrology, v. 161, p. 129–151, doi:10.1007/s00410-010-0524-0.
Deering, C.D., Gravley, D.M., Vogel, T.A., Cole, J.W., and Leonard, G.S., 2010, Origins of cold-wet-oxidizing to hot-dry-reducing rhyolite magma cycles and distribution in the Taupo Volcanic Zone, New Zealand: Contributions to Mineralogy and Petrology, v. 160, p. 609–629, doi:10.1007/s00410-010-0496-0.
DeMets, C., Gordon, R.G., Argus, D.F., and Stein, S., 1990, Current Plate Motions: Geophysical Journal International, v. 101, p. 425–478.
DePaolo, D.J., 1981, Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization: Earth and Planetary Science Letters, v. 53, p. 189–202, doi:10.1016/0012-821X(81)90153-9.
Elliott, T., Plank, T., Zindler, A., White, W., and Bourdon, B., 1997, Element transport from slab to volcanic front at the Mariana arc: Journal of Geophysical Research: Solid Earth, v. 102, p. 14991–15019, doi:10.1029/97JB00788.
Flude, S., and Storey, M., 2016, 40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals: Quaternary Geochronology, v. 33, p. 13–23, doi:10.1016/j.quageo.2016.01.002.
Forni, F., Degruyter, W., Bachmann, O., De Astis, G., and Mollo, S., 2018, Long-term magmatic evolution reveals the beginning of a new caldera cycle at Campi Flegrei: Science Advances, v. 4, p. eaat9401, doi:10.1126/sciadv.aat9401.
Galer, S.J.G., and Abouchami, 1998, Practical Application of Lead Triple Spiking for Correction of Instrumental Mass Discrimination: Mineralogical Magazine, v. 62A, p. 491–492, doi:10.1180/minmag.1998.62A.1.260. 74
Gamble, J.A., Smith, I.E.M., McCulloch, M.T., Graham, I.J., and Kokelaar, B.P., 1993, The geochemistry and petrogenesis of basalts from the Taupo Volcanic Zone and Kermadec Island Arc, S.W. Pacific: Journal of Volcanology and Geothermal Research, v. 54, p. 265–290, doi:10.1016/0377-0273(93)90067-2.
Gamble, J., Woodhead, J., Wright, I., and Smith, I., 1996, Basalt and Sediment Geochemistry and Magma Petrogenesis in a Transect from Oceanic Island Arc to Rifted Continental Margin Arc: the Kermadec—Hikurangi Margin, SW Pacific: Journal of Petrology, v. 37, p. 1523–1546, doi:10.1093/petrology/37.6.1523.
Giggenbach, W.F., 1995, Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand: Journal of Volcanology and Geothermal Research, v. 68, p. 89–116, doi:10.1016/0377-0273(95)00009-J.
Graham, I.J., Blattner, P., and McCulloch, M.T., 1990, Meta-igneous granulite xenoliths from Mount Ruapehu, New Zealand: Fragments of altered oceanic crust? Contributions to Mineralogy and Petrology, v. 105, p. 650–661, doi:10.1007/BF00306531.
Graham, I.J., Cole, J.W., Briggs, R.M., Gamble, J.A., and Smith, I.E.M., 1995a, Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review: Journal of Volcanology and Geothermal Research, v. 68, p. 59–87, doi:10.1016/0377-0273(95)00008-I.
Graham, I.J., Cole, J.W., Briggs, R.M., Gamble, J.A., and Smith, I.E.M., 1995b, Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review: Journal of Volcanology and Geothermal Research, v. 68, p. 59–87, doi:10.1016/0377-0273(95)00008-I.
Graham, I.J., Gulson, B.L., Hedenquist, J.W., and Mizon, K., 1992a, Petrogenesis of Late Cenozoic volcanic rocks from the Taupo Volcanic Zone, New Zealand, in the light of new lead isotope data: Geochimica et Cosmochimica Acta, v. 56, p. 2797–2819, doi:10.1016/0016-7037(92)90360-U.
Graham, D.W., Jenkins, W.J., Schilling, J.-G., Thompson, G., Kurz, M.D., and Humphris, S.E., 1992b, Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic: Earth and Planetary Science Letters, v. 110, p. 133–147, doi:10.1016/0012-821X(92)90044-V.
Gualda, G.A.R., and Ghiorso, M.S., 2014, Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 1: Principles, procedures, and evaluation of the method: Contributions to Mineralogy and Petrology, v. 168, p. 1033, doi:10.1007/s00410-014-1033-3.
Harrison, A., and White, R.S., 2006, Lithospheric structure of an active backarc basin: the Taupo Volcanic Zone, New Zealand: Geophysical Journal International, v. 167, p. 968–990, doi:10.1111/j.1365-246X.2006.03166.x. 75
Hawkesworth, C.J., Gallagher, K., Hergt, J.M., and McDermott, F., 1993, Mantle and Slab Contributions in ARC Magmas: Annual Review of Earth and Planetary Sciences, v. 21, p. 175–204, doi:10.1146/annurev.ea.21.050193.001135.
Hiess, J., Cole, J.W., and Spinks, K.D., 2007, Influence of the crust and crustal structure on the location and composition of high‐alumina basalts of the Taupo Volcanic Zone, New Zealand: New Zealand Journal of Geology and Geophysics, v. 50, p. 327–342, doi:10.1080/00288300709509840.
Hildreth, W., 1981, Gradients in Silicic Magma Chambers: Implications for Lithospheric Magmatism: Journal of Geophysical Research, v. 86, p. 10153–10192.
Hildreth, W., 2004, Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: several contiguous but discrete systems: Journal of Volcanology and Geothermal Research, v. 136, p. 169–198, doi:10.1016/j.jvolgeores.2004.05.019.
Hildreth, W., Halliday, A.N., and Christiansen, R.L., 1991, Isotopic and Chemical Evidence Concerning the Genesis and Contamination of Basaltic and Rhyolitic Magma Beneath the Yellowstone Plateau Volcanic Field: Journal of Petrology, v. 32, p. 63–138, doi:10.1093/petrology/32.1.63.
Hoernle, K., Hauff, F., van den Bogaard, P., Werner, R., Mortimer, N., Geldmacher, J., Garbe-Schönberg, D., and Davy, B., 2010, Age and geochemistry of volcanic rocks from the Hikurangi and Manihiki oceanic Plateaus: Geochimica et Cosmochimica Acta, v. 74, p. 7196–7219, doi:10.1016/j.gca.2010.09.030.
Hoernle, K., White, J.D.L., van den Bogaard, P., Hauff, F., Coombs, D.S., Werner, R., Timm, C., Garbe-Schönberg, D., Reay, A., and Cooper, A.F., 2006, Cenozoic intraplate volcanism on New Zealand: Upwelling induced by lithospheric removal: Earth and Planetary Science Letters, v. 248, p. 350–367, doi:10.1016/j.epsl.2006.06.001.
Houghton, B.F., Wilson, C.J.N., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M., and Pringle, M.S., 1995, Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand: , p. 4.
Ireland, T.R., 1992, Crustal evolution of New Zealand: Evidence from age distributions of detrital zircons in Western Province paragneisses and Torlesse greywacke: Geochimica et Cosmochimica Acta, v. 56, p. 911–920, doi:10.1016/0016- 7037(92)90036-I.
Jurado-Chichay, Z., and Walker, G.P.L., 2000, Stratigraphy and dispersal of the Mangaone Subgroup pyroclastic deposits, Okataina Volcanic Centre, New Zealand: Journal of Volcanology and Geothermal Research, v. 104, p. 319–380, doi:10.1016/S0377-0273(00)00210-9. 76
Kennedy, B.M., Holohan, E.P., Stix, J., Gravley, D.M., Davidson, J.R.J., and Cole, J.W., 2018, Magma plumbing beneath collapse caldera volcanic systems: Earth-Science Reviews, v. 177, p. 404–424, doi:10.1016/j.earscirev.2017.12.002.
Lipman, P.W., and Sawyer, D.A., 1985, Mesozoic ash-flow caldera fragments in southeastern Arizona and their relation to porphyry copper deposits: , p. 5.
Liu, Y., Anderson, A.T., Wilson, C.J.N., Davis, A.M., and Steele, I.M., 2006, Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand: evidence from volatiles and trace elements in melt inclusions: Contributions to Mineralogy and Petrology, v. 151, p. 71–87, doi:10.1007/s00410-005-0046-3.
Manning, D.A., 1995, Late Pleistocene tephrostratigraphy of the eastern Bay of Plenty region, New Zealand.: Victoria University.
McCulloch, M.T., Kyser, T.K., Woodhead, J.D., and Kinsley, L., 1994, Pb-Sr-Nd-O isotopic constraints on the origin of rhyolites from the Taupo Volcanic Zone of New Zealand: evidence for assimilation followed by fractionation from basalt: Contributions to Mineralogy and Petrology, v. 115, p. 303–312, doi:10.1007/BF00310769.
Mortimer, N., 2004, New Zealand’s Geological Foundations: Gondwana Research, v. 7, p. 261–272, doi:10.1016/S1342-937X(05)70324-5.
Mortimer, N., McLaren, S., and Dunlap, W.J., 2012, Ar-Ar dating of K-feldspar in low grade metamorphic rocks: Example of an exhumed Mesozoic accretionary wedge and forearc, South Island, New Zealand: K-FELDSPAR AR-AR IN LOW GRADE ROCKS: Tectonics, v. 31, p. n/a-n/a, doi:10.1029/2011TC003057.
Pamukçu, A.S., Wright, K.A., Gualda, G.A.R., and Gravley, D., 2020, Magma residence and eruption at the Taupo Volcanic Center (Taupo Volcanic Zone, New Zealand): insights from rhyolite-MELTS geobarometry, diffusion chronometry, and crystal textures: Contributions to Mineralogy and Petrology, v. 175, p. 48, doi:10.1007/s00410-020-01684-2.
Pickard, A.L., Adams, C.J., and Barley, M.E., 2000, Australian provenance for Upper Permian to Cretaceous rocks forming accretionary complexes on the New Zealand sector of the Gondwanaland margin: Australian Journal of Earth Sciences, v. 47, p. 987–1007, doi:10.1046/j.1440-0952.2000.00826.x.
Plank, T., 2014, The Chemical Composition of Subducting Sediments, in Treatise on Geochemistry, Elsevier, p. 607–629, doi:10.1016/B978-0-08-095975-7.00319-3.
Plank, T., and Langmuir, C.H., 1998, The chemical composition of subducting sediment and its consequences for the crust and mantle: Chemical Geology, v. 145, p. 325– 394, doi:10.1016/S0009-2541(97)00150-2. 77
Plank, T., and Langmuir, C.H., 1993, Tracing trace elements from sediment input to volcanic output at subduction zones: Nature, v. 362, p. 739–743, doi:10.1038/362739a0.
Price, R.C., Gamble, J.A., Smith, I.E.M., Maas, R., Waight, T., Stewart, R.B., and Woodhead, J., 2012, The Anatomy of an Andesite Volcano: a Time–Stratigraphic Study of Andesite Petrogenesis and Crustal Evolution at Ruapehu Volcano, New Zealand: Journal of Petrology, v. 53, p. 2139–2189, doi:10.1093/petrology/egs050.
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, doi:10.1016/j.jvolgeores.2004.07.013.
Price, R., Mortimer, N., Smith, I., and Maas, R., 2015, Whole-rock geochemical reference data for Torlesse and Waipapa terranes, North Island, New Zealand: New Zealand Journal of Geology and Geophysics, v. 58, p. 213–228, doi:10.1080/00288306.2015.1026832.
Ramos, F.C., 1992, Isotope geology of the Central Grouse Mountains, Box Elder County, Utah [Master’s Thesis]: University of California, Los Angeles.
Reid, F., 1983, Origin of the rhyolitic rocks of the taupo volcanic zone, New Zealand: Journal of Volcanology and Geothermal Research, v. 15, p. 315–338, doi:10.1016/0377-0273(83)90105-1.
Riley, T.R., Leat, P.T., Pankhurst, R.J., and Harris, C., 2001, Origins of Large Volume Rhyolitic Volcanism in the Antarctic Peninsula and Patagonia by Crustal Melting: v. 42, p. 23.
Rooney, T.O., and Deering, C.D., 2014, Conditions of melt generation beneath the Taupo Volcanic Zone: The influence of heterogeneous mantle inputs on large-volume silicic systems: Geology, v. 42, p. 3–6, doi:10.1130/G34868.1.
Rowland, J. V., Wilson, C.J.N., and Gravley, D.M., 2010, Spatial and temporal variations in magma-assisted rifting, Taupo Volcanic Zone, New Zealand: Journal of Volcanology and Geothermal Research, v. 190, p. 89–108, doi:10.1016/j.jvolgeores.2009.05.004.
Saunders, K.E., Morgan, D.J., Baker, J.A., and Wysoczanski, R.J., 2010, The Magmatic Evolution of the Whakamaru Supereruption, New Zealand, Constrained by a Microanalytical Study of Plagioclase and Quartz: Journal of Petrology, v. 51, p. 2465–2488, doi:10.1093/petrology/egq064.
Schmitz, M.D., and Smith, I.E.M., 2004, The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic 78
System: Journal of Petrology, v. 45, p. 2045–2066, doi:10.1093/petrology/egh047.
Smith, V., Shane, P., and Nairn, I., 2010, Insights into silicic melt generation using plagioclase, quartz and melt inclusions from the caldera-forming Rotoiti eruption, Taupo volcanic zone, New Zealand: Contributions to Mineralogy and Petrology, v. 160, p. 951–971, doi:10.1007/s00410-010-0516-0.
Smith, V.C., Shane, P., and Nairn, I.A., 2005, Trends in rhyolite geochemistry, mineralogy, and magma storage during the last 50 kyr at Okataina and Taupo volcanic centres, Taupo Volcanic Zone, New ZealandB: Journal of Volcanology and Geothermal Research, p. 35.
Storey, B.C., Leat, P.T., Weaver, S.D., Pankhurst, R.J., Bradshaw, J.D., and Kelley, S., 1999, Mantle plumes and Antarctica-New Zealand rifting: evidence from mid- Cretaceous mafic dykes: Journal of the Geological Society, v. 156, p. 659–671, doi:10.1144/gsjgs.156.4.0659.
Stratford, W.R., and Stern, T.A., 2006, Crust and upper mantle structure of a continental backarc: central North Island, New Zealand: Geophysical Journal International, v. 166, p. 469–484, doi:10.1111/j.1365-246X.2006.02967.x.
Sun, S. -s., and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes: Geological Society, London, Special Publications, v. 42, p. 313–345, doi:10.1144/GSL.SP.1989.042.01.19.
Sutherland, R. et al., 2007, Do great earthquakes occur on the Alpine Fault in central South Island, New Zealand?, in Okaya, D., Stern, T., and Davey, F. eds., Geophysical Monograph Series, Washington, D. C., American Geophysical Union, v. 175, p. 235–251, doi:10.1029/175GM12.
Sutton, A.N., Blake, S., and Wilson, C.J.N., 1995, An outline geochemistry of rhyolite eruptives from Taupo volcanic centre, New Zealand: Journal of Volcanology and Geothermal Research, v. 68, p. 153–175, doi:10.1016/0377-0273(95)00011-I.
Taylor, R.N., Ishizuka, O., Michalik, A., Milton, J.A., and Croudace, I.W., 2015, Evaluating the precision of Pb isotope measurement by mass spectrometry: Journal of Analytical Atomic Spectrometry, v. 30, p. 198–213, doi:10.1039/C4JA00279B.
Taylor, S.R., and McLennan, S.M., 1995, The geochemical evolution of the continental crust: Reviews of Geophysics, v. 33, p. 241, doi:10.1029/95RG00262.
Timm, C., Hoernle, K., Werner, R., Hauff, F., den Bogaard, P. van, White, J., Mortimer, N., and Garbe-Schönberg, D., 2010, Temporal and geochemical evolution of the Cenozoic intraplate volcanism of Zealandia: Earth-Science Reviews, v. 98, p. 38– 64, doi:10.1016/j.earscirev.2009.10.002. 79
Vandergoes, M.J. et al., 2013, A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand: Quaternary Science Reviews, v. 74, p. 195–201, doi:10.1016/j.quascirev.2012.11.006.
Waight, T.E., Troll, V.R., Gamble, J.A., Price, R.C., and Chadwick, J.P., 2017, Hf isotope evidence for variable slab input and crustal addition in basalts and andesites of the Taupo Volcanic Zone, New Zealand: Lithos, v. 284–285, p. 222– 236, doi:10.1016/j.lithos.2017.04.009.
Wallace, L.M., Beavan, J., McCaffrey, R., and Darby, D., 2004, Subduction zone coupling and tectonic block rotations in the North Island, New Zealand: Journal of Geophysical Research, v. 109, p. B12406, doi:10.1029/2004JB003241.
Wandres, A.M., Bradshaw, J.D., Weaver, S., Maas, R., Ireland, T., and Eby, N., 2004, Provenance of the sedimentary Rakaia sub-terrane, Torlesse Terrane, South Island, New Zealand: the use of igneous clast compositions to define the source: Sedimentary Geology, v. 168, p. 193–226, doi:10.1016/j.sedgeo.2004.03.003.
Weis, D. et al., 2006, High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS: ISOTOPIC STUDY OF USGS REFERENCE MATERIALS: Geochemistry, Geophysics, Geosystems, v. 7, p. n/a-n/a, doi:10.1029/2006GC001283.
Weis, D., Kieffer, B., Maerschalk, C., Pretorius, W., and Barling, J., 2005, High- precision Pb-Sr-Nd-Hf isotopic characterization of USGS BHVO-1 and BHVO-2 reference materials: Pb-Sr-Nd-Hf CHARACTERIZATION: Geochemistry, Geophysics, Geosystems, v. 6, doi:10.1029/2004GC000852.
White, W.M., Albarède, F., and Télouk, P., 2000, High-precision analysis of Pb isotope ratios by multi-collector ICP-MS: Chemical Geology, v. 167, p. 257–270, doi:10.1016/S0009-2541(99)00182-5.
White, W.M., and Patchett, J., 1984, HfNdSr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution: Earth and Planetary Science Letters, v. 67, p. 167–185, doi:10.1016/0012-821X(84)90112-2.
Wilson, C.J.N., 2001, The 26.5 ka Oruanui eruption, New Zealand: an introduction and overview: Journal of Volcanology and Geothermal Research, v. 112, p. 133–174, doi:10.1016/S0377-0273(01)00239-6.
Wilson, C.J.N., and Charlier, B.L.A., 2009, Rapid Rates of Magma Generation at Contemporaneous Magma Systems, Taupo Volcano, New Zealand: Insights from U–Th Model-age Spectra in Zircons: Journal of Petrology, v. 50, p. 875–907, doi:10.1093/petrology/egp023.
Wilson, C.J.N., Gravley, D.M., Leonard, G.S., and Rowland, J.V., 2009a, Volcanism in the central Taupo Volcanic Zone, New Zealand: tempo, styles and controls, in 80
Studies in volcanology: the legacy of George Walker., Special Publications of IAVCEI, v. 2, p. 225–247.
Wilson, C.J.N., Gravley, D.M., Leonard, G.S., and Rowland, J.V., 2009b, Volcanism in the central Taupo volcanic zone, New Zealand: tempo, styles and controls, in Thordarson, T., Self, S., Larsen, G., Rowland, S.K., and Hoskuldsson, A. eds., Studies in Volcanology: The Legacy of George Walker, The Geological Society of London on behalf of The International Association of Volcanology and Chemistry of the Earth’s Interior, p. 225–247, doi:10.1144/IAVCEl002.12.
Wilson, C.J.N., Houghton, B.F., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., and Briggs, R.M., 1995, Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review: Journal of Volcanology and Geothermal Research, v. 68, p. 1–28, doi:10.1016/0377-0273(95)00006-G.
Wilson, C.J.N., Rogan, A.M., Smith, I.E.M., Northey, D.J., Nairn, I.A., and Houghton, B.F., 1984, Caldera volcanoes of the Taupo Volcanic Zone, New Zealand: Journal of Geophysical Research, v. 89, p. 8463, doi:10.1029/JB089iB10p08463.
Woodhead, Jon.D., Volker, F., and McCulloch, M.T., 1995, Routine lead isotope determinations using a lead-207–lead-204 double spike: a long-term assessment of analytical precision and accuracy: The Analyst, v. 120, p. 35–39, doi:10.1039/AN9952000035.
Yuan, H., Yuan, W., Cheng, C., Liang, P., Liu, X., Dai, M., Bao, Z., Zong, C., Chen, K., and Lai, S., 2016, Evaluation of lead isotope compositions of NIST NBS 981 measured by thermal ionization mass spectrometer and multiple-collector inductively coupled plasma mass spectrometer: Solid Earth Sciences, v. 1, p. 74– 78, doi:10.1016/j.sesci.2016.04.001.
Zellmer, G.F., Kimura, J.-I., Stirling, C.H., Lube, G., Shane, P.A., and Iizuka, Y., 2020, Genesis of Recent Mafic Magmatism in the Taupo Volcanic Zone, New Zealand: Insights into the Birth and Death of Very Large Volume Rhyolitic Systems? Journal of Petrology, p. egaa027, doi:10.1093/petrology/egaa027.
Zindler, A., and Hart, S. Chemical Geodynamics: , p. 80.