# The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas

Erin Phillips, Kenneth W.W. Sims, Janne Blichert-Toft, Richard Aster, Glenn Gaetani, Philip Kyle, Paul Wallace, Daniel Rasmussen

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## To cite this version:

Erin Phillips, Kenneth W.W. Sims, Janne Blichert-Toft, Richard Aster, Glenn Gaetani, et al.. The nature and evolution of mantle upwelling at Ross Island, Antarctica, with implications for the source of HIMU lavas. Earth and Planetary Science Letters, 2018, 498, pp.38-53. 10.1016/j.epsl.2018.05.049. hal-02109978

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https://hal.science/hal-02109978v1

Submitted on 25 Jul 2025

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Highlights

* Isotope compositions of Ross Island volcanoes are variable mixtures of HIMU and DMM.

* Length scale of mantle isotopic heterogeneity comparable to inter-volcano distances.

* Ross Island volcanics are geochemically distinct from older CAMP volcanics.

* Seismic tomography shows low-velocity mantle extending to below the transition zone.

* Mantle upwelling eroded away metasomatic signature relict from Gondwana subduction.
1 The nature and evolution of mantle upwelling at Ross Island,
2 Antarctica, with implications for the source of HIMU lavas

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4 Erin H. Phillips¹#; Kenneth W.W. Sims¹*; Janne Blichert-Toft²; Richard C. Aster³; Glenn

5 A. Gaetani⁴; Philip R. Kyle⁵; Paul J. Wallace⁶; Daniel J. Rasmussen⁷

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7 ¹Department of Geology and Geophysics, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071 USA

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²Laboratoire de Géologie de Lyon, CNRS UMR 5276, Ecole Normale Supérieure de
Lyon and Université Claude Bernard Lyon 1, 46 Allée d'Italie, 69007 Lyon, France

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³Department of Geosciences, Warner College of Natural Resources, Colorado State
University, Fort Collins, CO 80523 USA

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⁴ Department of Geology and Geophysics, Woods Hole Oceanographic Institution,
Woods Hole, MA 02543 USA

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⁵ Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801 USA

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⁶Department of Earth Sciences, University of Oregon, Eugene, OR 97403 USA

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⁷Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 USA

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*Corresponding author: ksims7@uwyo.edu

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#Present address: Center for Economic Geology Research, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071 USA

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The multiple, proximal, young and/or active volcanic centers of Ross Island, Antarctica, provide a unique opportunity to investigate both deep and shallow processes of alkaline magma genesis and the length scales of mantle heterogeneity. Ross Island, Antarctica is an assembly of four silica-undersaturated alkaline volcanic centers, including the active phonolitic Erebus volcano (1.2 to 0 Ma). Mt. Terror, Mt. Bird, and Hut Point Peninsula surround Erebus on the periphery of Ross Island and are mostly older (~0.3 to 4 Ma) and mainly basanitic in composition. While the geochemical compositions and HIMU isotopic signature of Erebus lavas are well characterized, the geochemistry of the peripheral volcanic centers was, until this study, poorly known. To further investigate the nature of mantle sources and magmatic processes beneath Ross Island, we therefore measured major and trace element and Sr, Nd, Hf, and Pb isotope compositions on fifty-seven samples from the three volcanoes peripheral to Erebus volcano and compared the results to previous geochemical and isotopic studies of the latter. Mt. Terror, Mt. Bird, and Hut Point Peninsula have ⁸⁷Sr/⁸⁶Sr ranging from 0.702907 to 0.703147, εNd ranging from +4.3 to +6.3, εHf ranging from +5.6 to +8.6, and ²⁰⁶Pb/²⁰⁴Pb ranging from 19.282 to 20.241. The Sr, Nd, Hf, and Pb isotope compositions of all four volcanoes (Erebus, Terror, Bird, and Hut Point Peninsula) fall on a mixing line between the HIMU and DMM mantle end-members, with a minor contribution from an EM component. Small, but distinct differences in the isotopic compositions of the four volcanoes, most notably Mt. Bird, imply mantle source heterogeneity beneath Ross Island on a length scale comparable to or smaller than inter-volcano distances (i.e., less than 50 km). The major and trace element systematics and isotopic signatures of the Ross Island volcanic samples are distinct from those for samples from Zealandia and older Antarctic samples that typify magmas from the Cenozoic alkaline magmatic province. The chemical and isotopic compositions of the Ross Island lavas and tephras are best explained by small-degree melting of an ancient asthenospheric source containing residual garnet.

Additionally, recent global tomographic modeling indicates low shear wave velocity anomalies with depths down to ~1200 km beneath Ross Island. This suggests a deep-seated upwelling plume impinging on shallower metasomatized mantle which has been gradually eroded away and is no longer a potential source of Ross Island magmatism.
Rather, the source of Ross Island lavas is old (Archean to early Proterozoic) material upwelling from the deep mantle as opposed to melting of subcontinental lithospheric mantle metasomatized by Paleozoic subduction. While our findings have local significance in terms of understanding Ross Island volcanism and providing a new perspective on the evolution of volcanism in the surrounding region during the Phanerozoic, they also have important global implications for gauging the nature, age, and evolution of the HIMU mantle source.

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64 **Keywords:** Ross Island, Antarctica, alkaline magmatism, Sr-Nd-Hf-Pb isotopes, mantle upwelling, HIMU

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## 67 Highlights

68 • Isotope compositions of Ross Island volcanoes are variable mixtures of HIMU and DMM

70 • Length scale of mantle isotopic heterogeneity comparable to inter-volcano distances

72 • Ross Island volcanics are geochemically distinct from older volcanics in the region

74 • Seismic tomography shows low-velocity mantle extending to below the transition zone

76 • Mantle upwelling eroded away metasomatic signature relict from Gondwana subduction

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## 79 1. Introduction

80 Alkaline basalts represent deep, small-degree melts of the mantle. As such, they provide a unique window into some of the mantle's subordinate reservoirs. Here we focus on 81 alkaline HIMU lavas at Ross Island, Antarctica, which is part of a region known as the 82 Cenozoic alkaline magmatic province. It encompasses the easternmost part of the Indo- 83 Australian Plate, the micro-continent of Zealandia, the southwest portion of the Pacific 84
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Plate, and West Antarctica (Finn et al., 2005). Finn et al. (2005) actually used the acronym DAMP for “diffuse alkaline magmatic province” but given that “diffuse” is somewhat vague and this acronym infers a process, we hereinafter prefer to use the more neutral and descriptive “Cenozoic alkaline magmatic province,” which refers to a region rather than a process. The Cenozoic alkaline magmatic province is dominantly alkaline, has a widespread “HIMU”-like Pb isotopic signature (e.g., Sun and Hanson, 1975; Hart, 1988; Rocholl et al., 1995; Hart et al., 1995; 1997; Panter et al., 2000; 2006; Rocchi et al., 2002; Finn et al., 2005; Sims and Hart, 2006; Sims et al., 2008), and is generally underlain by low-velocity mantle that extends to depths of ~600–1450 km (Ritzwoller et al., 2001; Montelli et al., 2006; Nolet et al., 2006; Li et al., 2008; Hansen et al., 2014; French and Romanowicz, 2015; Bozdağ et al., 2016). The Cenozoic alkaline magmatic province also has a complex tectonic history, including subduction along the margin of Gondwana during the Paleozoic-Mesozoic and the Mesozoic break-up of Gondwana (Seton et al., 2012).

While the compositions of the Cenozoic alkaline magmatic province lavas generally are very similar, there are significant geochemical and isotopic distinctions between the region's different volcanic centers (e.g., Timm et al., 2010). These differences likely are a manifestation of the non-uniformity of Phanerozoic tectonic processes across the region. Hence, while it is crucial to recognize the all-encompassing characteristics of the Cenozoic alkaline magmatic province, the details of individual volcanic systems help discern more explicitly relative source contributions and the nature of the processes controlling lava compositions.
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An important and well-studied active volcanic system within the Cenozoic alkaline

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magmatic province is Ross Island in West Antarctica. Ross Island comprises four

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alkaline volcanic centers: the active, centrally located Erebus volcano and three older

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(~0.3-4 Ma) volcanic loci, Mt. Terror, Mt. Bird, and Hut Point Peninsula, which are

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situated on the periphery of the island (Fig. 1). Erebus, the world's southernmost active

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volcano, erupts phonolitic lavas from a persistently open magma-filled conduit system,

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and has been the object of sustained volcanological, geological, and geochemical interest

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(e.g., Sun and Hanson, 1975; Kyle et al., 1992; Kelly et al., 2008; Sims et al., 2008, 2013;

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Zandomeneghi et al., 2013; Knox et al., 2018). In contrast, the peripheral Mt. Terror, Mt.

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Bird, and Hut Point Peninsula volcanoes, which are also silica-undersaturated but

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primarily composed of primitive basanites, have attracted surprisingly little attention

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despite their close association with and proximity to Erebus volcano.

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A long-standing conceptualization for the magmatic evolution of Ross Island is that it is

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the expression of a mantle plume (Kyle et al., 1992). The large eruptive volume at Erebus

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volcano and the presence of kaersutite in the peripheral volcanoes led Kyle et al. (1992)

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to propose that Ross Island is a concentrically zoned thermal anomaly with the interior

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being hot asthenospheric upwelling, and the periphery being cooler and incorporating an

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increased amount of the surrounding asthenosphere and lithosphere. In this model, the

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peripheral region is represented by the volcanoes Mt. Terror, Mt. Bird, and Hut Point

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Peninsula, which radially surround Erebus volcano, located in the middle above the

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center of the thermal anomaly. While the chemical and isotopic compositions of Mt.
131 Erebus are well established (Sims et al., 2008), this is not the case for Mt. Terror, Mt.

132 Bird, and Hut Point Peninsula. Without chemical and isotopic constraints on these
133 volcanoes, the simple concentrically zoned plume hypothesis remains untested.

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135 In order to remedy this shortcoming, and to further advance our understanding of the
136 formation and evolution of Ross Island magmatism, we analyzed major and trace element
137 concentrations and Sr, Nd, Hf, and Pb isotope compositions for 57 volcanic rocks
138 collected from Mt. Terror, Mt. Bird, and Hut Point Peninsula, and compared them to
139 similarly analyzed samples from Erebus volcano (e.g., Sun and Hanson, 1975; Kyle et al.,
140 1992; Sims and Hart, 2006; Kelly et al., 2008; Sims et al., 2008). We further present new
141 geophysical transects extracted from recent deep mantle seismic tomography models
142 (Becker and Boschi, 2002; Simmons et al., 2010; Ritsema et al., 2011; Auer et al., 2014;
143 Chang et al., 2015; French and Romanowicz, 2015) to assess the evidence for a deep
144 plume structure beneath Ross Island and the volcanic provinces of West Antarctica. Our
145 geochemical and isotopic data for Mt. Terror, Mt. Bird, and Hut Point Peninsula, coupled
146 with the existing Erebus data and an improving tomographic resolution of mantle
147 structure in the region, provide for a better understanding of alkaline volcanism on the
148 local scale at Ross Island and also within regional and global contexts.

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150 **2. Background**

151 Ross Island is located in the Ross Sea, near the western edge of the Transantarctic

152 Mountains that separate the stable East Antarctic craton from tectonically active West

153 Antarctica (Fig. 1). Lithospheric thickness in East Antarctica is broadly estimated to be >
154 220 km, while in West Antarctica it is ~70-100 km (Heeszel et al., 2016). Ross Island lies
155 at the southern end of the Terror Rift, which is the locus of the most recent extension
156 within the West Antarctic Rift System (WARS) (e.g., Behrendt, 1999). Ross Island is part
157 of the Erebus volcanic province (Kyle, 1990a), which is a subdivision of the regional
158 McMurdo Volcanic Group (Kyle, 1990b). Crustal thickness beneath the Ross Island
159 region ranges from ~19 to 25 km (Bannister et al., 2003; Finotello et al., 2011; Chaput,
160 2014). Approximately 4520 km³ of volcanic rocks have been emplaced on Ross Island
161 since the oldest dated volcanism at ~4 Ma (Esser et al., 2004).

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163 A simple but complete differentiation sequence from basanite to anorthoclase phonolite is
164 displayed by the Erebus lineage; major crystallizing phases include olivine,
165 clinopyroxene, oxides, feldspar, and apatite (Kyle et al., 1992). Exposed volcanic rocks at
166 Erebus volcano are predominantly anorthoclase-phyric phonotephrite and phonolite
167 (Kyle, 1990a; Kyle et al., 1992; Esser et al., 2004; Kelly et al., 2008). The enriched iron
168 series is a minor lineage of iron-rich lavas on Erebus characterized by Kyle et al. (1992).
169 The oldest dated basanite lavas associated with Erebus are ~1.3 Ma; the more recent and
170 more evolved anorthoclase-phyric rocks are less than 0.25 Ma (Esser et al., 2004;
171 Parmelee et al., 2015). Lava compositions at Erebus shifted to a largely phonolitic
172 composition at approximately 36 ka (Harpel et al., 2004).

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174 In addition to the Erebus lineage, Ross Island hosts the lineage of the Dry Valley Drilling
175 Project (DVDP), which cored three holes on Hut Point Peninsula in 1973 (Kyle, 1981).
176 Kyle et al. (1992) modeled the evolution of the Erebus and DVDP lineages as originating
177 from a similar basanite parent and evolving along different trends. DVDP and Erebus
178 lineage mineralogies are comparable except that the DVDP lineage also includes
179 kaersutite. The presence of kaersutite reflects wetter conditions and slightly lower
180 temperatures which is a function of different degassing conditions in the peripheral
181 volcanoes as compared to Erebus (Iacovino et al., 2016; Rasmussen et al., 2017). The
182 three peripheral volcanic centers belong to the DVDP lineage; their eruptive products
183 lack modern high-precision age dates, but they are mostly older than Erebus eruptive
184 products and their lavas generally less evolved than at Erebus volcano. Mt. Terror is a
185 stratovolcano to the east of Erebus volcano with eruptive products between ~0.5 and 2
186 Ma in age (Armstrong, 1978). Mt. Bird is a shield volcano to the northwest of Erebus
187 volcano with eruptive products ranging in age from ~3 to 4 Ma (Armstrong, 1978). Hut
188 Point Peninsula, located to the south of Erebus volcano, is a 20 km long linear trend of
189 small volcanic centers that appeared between ~0.3 and 1.5 Ma (Kyle, 1981; Kyle, 1990a).

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## 3. Samples and analytical methods

192 Ross Island lava and tephra samples were collected in 2012 and 2014 (Supplement A).

193 For complete coverage, select archived samples from Cape Crozier (Cole et al., 1971)

194 and Hut Point Peninsula, including the DVDP core (Kyle, 1981), were included in the

195 sample suite to be analyzed for major and trace element abundances and long-lived

196 radiogenic Sr, Nd, Hf, and Pb isotope compositions. The data from this study are

197 compared to existing data of a large suite of Erebus samples (Sims and Hart, 2006; Sims

198 et al., 2008).

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200 Major and trace element concentrations were measured at Michigan State University by
201 X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS),
202 respectively (Supplements B and C). Chemical separation of Sr, Nd, Hf, and Pb was
203 carried out at the University of Wyoming (UW) High Precision Isotope Laboratory and
204 the Ecole Normale Supérieure (ENS) in Lyon, France, on the same dissolutions for each
205 sample (Supplement D). Strontium and Nd isotope compositions were measured at UW
206 and Hf and Pb isotope compositions at ENS Lyon (Table 1; Supplement A). Analyses of
207 mineral separates were done at the UW Materials Characterization Laboratory
208 (Supplement E). Tomographic Antarctic images referred to here utilized global velocity
209 models SEMUCB-WM1 (French and Romanowicz, 2015), SGLOBE-rani (Chang et al.,
210 2015), GyPSuM (Simmons et al., 2010), and SMEAN2 (Becker and Boschi, 2002;
211 Simmons et al., 2010; Ritsema et al., 2011; Auer et al., 2014) (Supplement F).
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213 **4. Results**

214 *4.1 Whole-rock major and trace element geochemistry*

215 The majority of Mt. Terror, Mt. Bird, and Hut Point Peninsula samples analyzed in this
216 study are basanites, with some ranging to phonolites on a total alkali versus silica
217 diagram (Fig. 2). Erebus lineage lavas range from basanite to phonolite (Kyle et al., 1992;
218 Sims et al., 2008) with most analyses being on recently erupted phonolite bombs (Kelly
219 et al., 2008; Sims et al., 2008, 2013). MgO concentrations in Mt. Terror, Mt. Bird, and
220 Hut Point Peninsula lavas and tephras range from 0.12 to 12.72 wt.%. Samples with less
221 than 1 wt.% MgO are phonolites or fall along the phonolite-tephriphonolite divide (Fig.
222 2). Primitive mantle-normalized trace element patterns for volcanic rocks from Ross
223 Island are shown in Figure 3. The highest Mg# samples all show prominent negative K and Pb anomalies and depletions in Cs and Rb relative to Ba and Th, whereas the lowest Mg# samples lack significant negative K and Pb anomalies and have negative Ti anomalies.

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228 4.2 *Long-lived radiogenic isotopes*

229 The Sr, Nd, Hf, and Pb isotope compositions measured on samples from Ross Island include the 57 samples from Mt. Terror (28), Mt. Bird (12), and Hut Point Peninsula (17) mentioned above, as well as one sample from the Erebus lineage at Cape Barne, and one enriched iron series sample from Fang Ridge (Table 1; Supplement A) that were analyzed here to complement existing Erebus data. These are compared with the Sr, Nd, Hf, and Pb isotope compositions of 42 samples from the Erebus lineage and four samples from the DVDP lineage (Sims and Hart, 2006; Sims et al., 2008).

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237 The overall ranges in Sr, Nd, Hf, and Pb isotope compositions for the three peripheral volcanic centers are greater than for the Erebus lineage (Figs. 4-6). On a global scale, all of the Ross Island rocks are isotopically similar to the HIMU mantle end-member in Sr-Nd-Hf-Pb isotope space and plot on a mixing line between the HIMU and depleted mid-ocean ridge basalt (MORB) mantle (DMM) end-members, possibly with a small contribution from an enriched mantle (EM) component (Figs. 4, 5, and 7).

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244 4.3 *Mineral separates*
To assess the possible presence of xenocrystic material, we analyzed the isotope compositions of four mineral separates from Mt. Terror. The Sr, Nd, Hf, and Pb isotope compositions of the four mineral separates (plagioclase, pyroxene, and two amphiboles), are within the range of whole-rock isotope compositions from Mt. Terror, except for one amphibole that has Pb isotope compositions slightly outside this range (Fig. 4).

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## 5. Discussion

### 5.1 The significance of isotopic variability at Ross Island

Significant isotopic variability is observed at Ross Island with a general clustering of isotopic compositions within each volcanic center and an overall higher degree of variability within the peripheral volcanic centers relative to the central Erebus volcano (Figs. 4-6). These characteristics point to small-scale mantle source heterogeneities beneath Ross Island on the scale of less than ~50 km (which is the approximate distance between each of the peripheral volcanic centers and Erebus volcano and therefore the smallest scale that can be observed).

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The extent to which the observed isotopic variability results from crustal contamination can be evaluated using the relative abundances of key trace elements. Hart et al. (1997) pointed out that negative K and Pb anomalies, along with depletions in Cs and Rb and relatively low ⁸⁷Sr/⁸⁶Sr, are evidence against significant crustal contamination. At Ross Island, samples with different Mg# are characterized by different trace element patterns (Fig. 3). The highest Mg# samples all show prominent negative K and Pb anomalies and depletions in Cs and Rb relative to Ba and Th. These systematics are consistent with the
268 HIMU mantle end-member, indicating a lack of crustal contamination (Fig. 3).

269 Conversely, the lowest Mg# samples lack significant negative K and Pb anomalies and
270 have negative Ti anomalies, suggesting that they have been affected by crustal
271 contamination. Such crustal contamination is particularly evident in the Erebus trachytes
272 of the enriched iron series, which show distinctive isotopic characteristics, namely high
273 <sup>87</sup>Sr/<sup>86</sup>Sr and low <sup>208</sup>Pb/<sup>204</sup>Pb (Sims et al., 2008).

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275 Given the observed isotopic variability at Ross Island, the evolved nature of some of the
276 samples (Fig. 2) further raises the question of whether shallow differentiation and
277 assimilation have had a systematic effect on the isotope composition of the lavas and
278 tephras. If variations in the isotope compositions at Ross Island were significantly
279 impacted by shallow differentiation (i.e., fractional crystallization) and assimilation of
280 felsic crust, one would expect a correlation between isotope compositions and both Mg#
281 (Supplement G) and trace element ratios such as Nb/La, which is not observed for any of
282 the peripheral volcanic centers. This, therefore, indicates that the isotope compositions
283 are not systematically controlled by assimilation or differentiation processes. Even if only
284 the most primitive samples (Mg# > 50) are considered, the general observation of overall
285 greater compositional variability of the peripheral volcanic centers compared to Erebus
286 volcano and the clustering of isotope compositions for each peripheral volcanic center,
287 indicate small, but analytically significant source variability at Ross Island. Additionally,
288 samples from the peripheral volcanic centers do not have the same isotopic characteristics
289 as the Erebus enriched iron series trachytes, which have been shown unambiguously to
290 have evolved along a different liquid line of descent than the majority of Erebus lineage
291 lavas and to have been affected by significant crustal contamination (Kyle et al., 1992; Sims et al., 2008). Therefore, we conclude that the statistically significant variations in isotope compositions observed at Ross Island are the result of partial melting of a heterogeneous mantle source with assimilation playing a subordinate role only for the more evolved rocks.

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297 These observations provide important insights into the meaning of the isotopic variability across Ross Island (Figs. 4-6). Mt. Terror lavas and tephras have the highest $\epsilon_{Nd}$ and $\epsilon_{Hf}$ and least radiogenic Pb isotope compositions and show a relatively wide range in Sr isotope compositions. Hut Point lavas and tephras are the most similar to Erebus volcano in terms of their Nd and Hf isotope compositions and have the most radiogenic Pb. Mt. Bird shows a range similar to Mt. Terror for its Sr isotope compositions, whereas divergent trends emerge in Nd-Hf-Pb isotope space (Figs. 4b, c). Two samples from the Mt. Bird sea cliffs are outliers when compared to other Mt. Bird samples with respect to their Hf isotope compositions ($\epsilon_{Hf}$ = +6.0 and +6.1), suggesting they tapped a slightly different mantle component than the other rocks from Mt. Bird. With the exception of these two cliff lavas, Mt. Bird lavas and tephras exhibit unusually radiogenic Hf isotope compositions for a given Nd isotope composition, resulting in a shallowing of the Nd-Hf trend for Mt. Bird. One potential explanation for this observation is the presence of deep-sea pelagic sediments in the source of Mt. Bird eruptive products, an occurrence first identified by Blichert-Toft et al. (1999) in Hawaiian lavas based on their shallow slope in Nd-Hf isotope space compared to the mean global OIB array. It should be noted, however, that Ross Island rocks display a relatively narrow range in Nd and Hf isotope
314 compositions compared to Hawaii, suggesting only minor influence of pelagic sediments
315 in the source of Mt. Bird eruptive products. We further note that other common indicators
316 of pelagic sediment, such as relatively low Ce/Pb (see e.g., Sims and DePaolo, 1997), are
317 not observed in Mt. Bird rocks. Thus, if pelagic sediments are indeed at the origin of the
318 isotopic differences at Mt. Bird compared to the rest of Ross Island, Hf isotopes are more
319 sensitive than trace element proxies in identifying them.

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321 Most Mt. Bird samples also have high $^{208}$Pb/$^{204}$Pb for a given $^{206}$Pb/$^{204}$Pb when compared
322 to the rest of the Ross Island suite (Fig. 4c), indicating that the Mt. Bird samples
323 incorporated source material with a different long-term U/Th ratio than the other Ross
324 Island rocks. This overall slight divergence of Mt. Bird lavas and tephras from the main
325 Ross Island trend suggests that the majority of Mt. Bird samples have incorporated a
326 source component, or components, distinct from the rest of Ross Island.

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328 Ross Island lavas and tephras primarily plot between the DMM and HIMU mantle end-
329 members (Figs. 4, 5, 7, 8), suggesting they derive from partial melting of mantle
330 peridotite containing these two components. A small but significant variability within the
331 Ross Island volcanics, which creates a slight deviation from a simple two-component
332 mixing relationship between DMM and HIMU, is consistent with the admixing of small
333 amounts of an enriched mantle (EM) type of component.

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335 Finally, we note that the isotopic compositions of four mineral separates demonstrate that
336 they are co-genetic with their host lavas and tephras, lending additional credence to the
337 notion that the isotope compositions of the whole-rock samples are good indicators of
338 source composition (Fig. 4).

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340 5.2 *Sources and processes responsible for Ross Island magmatism*

341 5.2.1 *Rifting as a cause of Ross Island volcanism*

342 Ross Island is located within a rift setting, and studies such as those of Rocchi et al.

343 (2002; 2003) contend that magmatic activity in the West Antarctic Rift is closely linked
344 in space and time to fault systems and is the result of decompression melting unrelated to
345 mantle plumes. However, Sleep (2006) argued against a correlation between magmatism
346 and rift tectonics in the West Antarctic Rift, noting that extension in Marie Byrd Land
347 and Victoria Land had largely ceased before intraplate magmatism began in these
348 regions. Present magmatic and recent eruptive activity (Lough et al., 2013) exists in
349 Marie Byrd Land despite very low levels of ongoing rift extension (Bevis et al., 2009).

350 Thus, we conclude that the large volume of magmatism expressed at Ross Island is not
351 due to rift-associated extensional melting.

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353 5.2.2 *Ross Island's HIMU isotopic signature: A deep mantle plume or metasomatically
354 enriched subcontinental lithosphere?*

355 Cenozoic, dominantly alkaline volcanism in West Antarctica is widespread and a HIMU-
356 like signature is ubiquitous in the Antarctic and southwest Pacific region, often referred
357 to as Zealandia (Figs. 5 and 7). Ross Island is part of this volcanic province and is
358 noteworthy because of its prolific and focused magmatism since ~4 Ma (~4500 km³;
359 Esser et al., 2004), pointing to a local thermal anomaly. The isotope compositions of
regional volcanic rocks such as the Ferrar Group (~177 Ma; Fleming et al., 1995) and the distant minettes from East Antarctica (~455 Ma; Hoch et al., 2001) are more variable and enriched than at Ross Island, whereas at Ross Island all four long-lived isotope systems (Sr, Nd, Hf, and Pb) closely resemble HIMU (Figs. 4, 5, 7, 8). Three main hypotheses have been proposed to explain the HIMU signature at Ross Island. The first is that Ross Island is the manifestation of a localized mantle thermal anomaly centered beneath Erebus volcano (Kyle et al., 1992; Storey et al., 1999). Sims et al. (2008) argued that Erebus lineage lavas (basanite through phonolite) and the restricted Sr, Nd, Hf, and Pb isotopic variations of Erebus volcano are best explained by melting of deep-seated mantle peridotite consisting of DMM and HIMU mantle components that mix to various extents (Figs. 4, 5, 7, 8). The other two hypotheses draw on the widespread nature of the HIMU-like signature in the Antarctic and southwest Pacific. Hart et al. (1997) studied HIMU-like lavas from Marie Byrd Land and proposed that a large deep-seated mantle plume with a HIMU signature impinged on the base of the Gondwana lithosphere prior to its breakup at ~85 Ma and noted the plausibility of its large plume head being associated with the Ferrar Large Igneous Province. In the model of Finn et al. (2005), the widespread regional HIMU-like signature is explained by melting of a metasomatized lithospheric source, with the presumed metasomatism occurring between 100 and 500 Ma in a sustained subduction regime. Slab detachment in the late Cretaceous caused a change in mantle flow followed by onset of magmatism in the region at ~55 Ma. The metasomatized lithospheric source was later sampled by small degrees of partial melting. Panter et al. (2006) discussed the origin of the HIMU-like signature found in basalts from the Antipodes, Campbell, and Chatham Islands and likewise advocated for a widespread
subduction-related metasomatized lithosphere as their primary source. We argue below that our new geochemical and isotopic data and geophysical modeling collectively point to a focused plume origin for Ross Island magmatism.

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### 5.2.2.1 Physiographic observations

Kyle et al. (1992) were the first to propose the presence of a mantle plume beneath Ross Island and noted that a large volume of mantle must have melted to produce the basanites that in turn differentiated into the large volume of phonolites erupted at Erebus volcano. Additionally, they pointed out that the geometry of volcanic upwelling at Ross Island is indicative of a cylindrical upwelling, in which the active Erebus volcano resides at the center with the peripheral volcanoes Mt. Terror, Mt. Bird, and Hut Point Peninsula radially surrounding Erebus at roughly ~120° angles, an arrangement predicted by the modeling of Burke and Dewey (1973). Finally, Kyle et al. (1992) pointed out that the physical and geochemical characteristics of the Mt. Discovery complex, its current location and age, and the motion of the Antarctic plate are consistent with Mt. Discovery and its peripheral volcanoes being a previous locus of magmatism that resulted from the Ross Island thermal anomaly.

400

Geochemical and fluid mechanical models (e.g., Watson and McKenzie, 1991; Ribe and Christensen, 1994; Sims et al., 1999) suggest that mantle plumes have a clearly defined upwelling structure, which includes higher levels of entrainment on the cooler edges of plumes than at the center (Hauri et al., 1994). The overall greater variability in Sr, Nd, Hf, and Pb isotope compositions at the peripheral volcanic centers compared to Erebus
406 volcano point to increased extents of mixing with surrounding heterogeneous lithosphere
407 and/or asthenosphere at the periphery of Ross Island, consistent with the plume models of
408 Hauri et al. (1994), which, as mentioned above, predict increased incorporation of
409 ambient asthenosphere on the fringes of plumes (Fig. 4-6).

410

411 **5.2.2.2 Geochemical observations**

412 A first-order observation is that Ross Island volcanics do not exhibit the same anomalous
413 major element chemistry attributed to melting of a metasomatically altered source that is
414 seen in unusual alkaline lavas such as lamprophyres and minettes (e.g., the minettes of
415 the Schirmacher Oasis in East Antarctica (Hoch et al., 2001)). Instead, the Ross Island
416 lavas and tephras are basanitic or differentiates of parental basanites (e.g., phonolites)
417 which have long been interpreted to represent small-degree partial melts of mantle
418 peridotite (Green et al., 1967; Green, 1973). Further, depletion in heavy REE compared
419 to middle REE in Ross Island lavas and tephras (mean DYN/YbN ≈ 1.4; Fig. 3) and the
420 observation that (230Th/238U) > 1 in Erebus lineage lavas require melting in the presence
421 of residual garnet (Sun and Hanson, 1975; Sims and Hart, 2006; Sims et al., 2013). These
422 observations imply that the Ross Island lavas and tephras were derived from a relatively
423 deep source, below the garnet-spinel transition zone (i.e., below ~80 km depth). The
424 seismologically estimated thickness of the lithosphere beneath West Antarctica is ~70-
425 100 km (Heeszel et al., 2016; Lloyd et al., 2015). As such, the simplest interpretation of
426 the Ross Island magmas is that they were derived by low-degree partial melting of garnet
427 peridotite in the mantle asthenosphere.

428
429 From an isotopic viewpoint, the long-lived radiogenic isotope compositions of the Ross Island lavas and tephras are similar to the St. Helena HIMU mantle source (interpreted to be a deep mantle plume located in the mid-Atlantic) and do not resemble what would be expected from a metasomatically enriched subcontinental lithospheric source (Figs. 4, 5, 7, 8). Lavas interpreted to be lithospheric melts usually have more enriched isotopic signatures (i.e., lower $\epsilon_{Nd}$ and $\epsilon_{Hf}$) than those found at Ross Island (e.g., Daley and DePaolo, 1992; Leat et al., 2005) because metasomatic enrichment will decrease Sm/Nd and Lu/Hf which as a consequence will grow in less radiogenic Nd and Hf over time leading to lower values of $\epsilon_{Nd}$ and $\epsilon_{Hf}$ (Reid et al., 2012). In this regard, we note that the older Antarctic samples, such as the minettes of the Schirmacher Oasis (~455 Ma; Hoch et al., 2001) and the Ferrar Group (~177 Ma; Fleming et al., 1995), show much more enriched and variable isotopic signatures (Fig. 7 and 8), consistent with melting of metasomatized lithosphere during Paleozoic subduction.

442

443 The quintessential aspect of the HIMU isotopic end-member is its radiogenic Pb isotope compositions, which reflect long-term enrichment of U and Th relative to Pb. While it is conceivable that a metasomatic enrichment of U, if strong enough, could lead relatively quickly (i.e., within < 500 m.y.) to HIMU $^{206}$Pb/$^{204}$Pb isotopic signatures because of the relatively short timescale of $^{238}$U decay ($t_{1/2}$ = 4.5x10$^9$ yr), $^{207}$Pb/$^{204}$Pb would not be significantly influenced in the last 500 m.y. because of Earth's current low abundance of $^{235}$U, which has mostly decayed away. Preferential enrichment of Th relative to Pb likewise could lead to HIMU $^{208}$Pb/$^{204}$Pb isotopic signatures, but on much longer timescales than the 500 m.y. in question here ($^{232}$Th $t_{1/2}$ = 1.4x10$^{10}$ yr). Similarly, it
would take far longer for a metasomatic event to also affect the Sm-Nd and Lu-Hf
isotope systems because of the significantly longer half-lives of ¹⁴⁷Sm (parent of ¹⁴³Nd;
t₁/₂ = 1.06x10¹¹ yr) and ¹⁷⁶Lu (parent of ¹⁷⁶Hf; t₁/₂ = 3.71x10¹⁰ yr) which are orders of
magnitude longer than the half-lives of ²³⁸U and ²³²Th. Given these long half-lives, the
relatively short time window since the alleged metasomatism (< 500 m.y.), and the fact
that all four long-lived isotope systems (Sr, Nd, Hf, and Pb) are similar to the St. Helena
HIMU end-member, it is improbable that the HIMU characteristics of Ross Island
represent melting of a relatively young (< 500 m.y.) metasomatically altered lithosphere.
Rather, the HIMU mantle end-member is consistent with an ancient (Archean to early
Proterozoic) recycled source component thought to have become isolated in the lower
mantle (Hanyu et al., 2011; Weiss et al., 2016), thereby allowing enough time for the
characteristic HIMU isotopic signature to grow in for all isotopic systems over billions of
years. We therefore argue that the Ross Island lavas contain a deep-seated, long-lived
component that was brought to the surface by deep mantle upwelling.

466

Furthermore, the U-Th systematics of Erebus volcano are not consistent with arc-related
metasomatic signatures. Arc-related metasomatism likely would enrich U over Th (e.g.,
Reubi et al., 2014, and references therein) resulting in high ²³⁸U/²³²Th and high
²³⁰Th/²³²Th due to the decay of ²³⁸U to ²³⁰Th (over ~500 k.y.). Erebus samples have
relatively low ²³⁰Th/²³²Th compared to global MORB and OIB compilations (Sims and
Hart, 2006; Sims et al., 2013) and show ²³⁸U/²³²Th similar to the HIMU end-member
(Sims and Hart, 2006), indicating that the parental basanites were not derived from a
source that had been enriched in U relative to Th as would be expected from arc-related metasomatism.

Moreover, the trace element characteristics of the Ross Island volcanics also are not consistent with their derivation from a metasomatized lithospheric source altered by subduction processes. For example, in subduction regimes, the subcontinental lithosphere is expected to be depleted in high field strength elements (HFSE; e.g., Nb) relative to other incompatible elements because HFSE are retained in downgoing slabs (Kelemen et al., 1993); at the same time, fluid mobile elements such as La will be enriched. As such, ratios like Nb/La are important discriminants. In arcs Nb/La is universally < 0.5 (Gill, 1981). The Ross Island lavas and tephras that we have analyzed (this study and Sims et al., 2008) all have Nb/La ratios in excess of 1.25, implying that little involvement of a subduction metasomatized lithospheric component has taken place (Figs. 3 and 8). In a study of Zealandia volcanism, Timm et al. (2010) used New Zealand's Mount Somers Volcanic Group as an end-member composition for Zealandia magmas generated from subduction-related metasomatically altered lithosphere. In stark contrast to the Ross Island samples, the Mount Somers Volcanic Group samples (Tappenden, 2003) and the Schirmacher Oasis minettes from East Antarctica (Hoch et al., 2001) all have Nb/La < 0.5, suggesting they are either sourced from or have assimilated significant amounts of metasomatized lithospheric mantle (Fig. 8). We thus interpret the Nb/La > 0.5 of the Ross Island volcanics as a contraindication of their being derived by melting or assimilation of a lithospheric source altered by subduction processes. Finally, we note that the Ross Island samples from Mt. Bird, Mt. Terror, and Hut Point Peninsula lie on a mixing curve
497 between DMM and HIMU (Fig. 8), consistent with trends also observed among Sr, Nd, Hf, and Pb isotopes. The high Nb/La trend seen in the Erebus lavas relative to lavas from its peripheral volcanoes reflects variable extents of fractionation of apatite, a phase which is present in Erebus lavas at ~0.5% modal abundance (Kyle, 1981; Kyle et al., 1992).

501
502 In further support of a deep HIMU plume hypothesis, we note that eleven olivines from
503 hyaloclastites from the DVDP core at Ross Island have ³He/⁴He (R/Rᵃ values with an
504 error-weighted mean of 6.91; Parmelee et al., 2015) similar to St. Helena (R/Rᵃ of 5.24 to
505 5.97; Hanyu et al., 2014). While high ³He/⁴He is thought to be indicative of undegassed
506 primitive mantle due to the primordial nature of ³He, lavas derived from the HIMU
507 mantle end-member at plumes such as St. Helena typically have low ³He/⁴He, which is
508 attributed to the presence of radiogenic ⁴He in the recycled materials that form the deep
509 HIMU mantle reservoir (Graham et al., 1992; Parai et al., 2009; Hanyu et al., 2014).

510
511 5.2.2.3 Geophysical observations

512 Regional seismic data collected across Antarctica reveal a prominent low-velocity zone
513 in the upper mantle beneath Ross Island in particular (Watson et al., 2006; Zhao, 2007;
514 Gupta et al., 2009; Heeszel et al., 2016) and the WARS in general (Ritzwoller et al.,
515 2001), implying elevated temperatures. The regional tomographic body wave inversion of
516 Hansen et al. (2014) highlights extensive low velocities within the upper ~200-300 km of
517 the mantle but has insufficient resolution to image low velocities at greater depths.

518 However, global tomographic modeling using finite-frequency kernels (Montelli et al.,
519 2006) and full-waveform inversion (Bozdağ et al., 2016; Fig. 9; see French and
Romanowicz, 2015), although at much broader resolution, indicate a sub-mantle transition zone low shear wave seismic velocity anomaly that extends to ~1200 km depth beneath Ross Island, with linked features that extend eastward beneath the WARS and below the Marie Byrd Land volcanic province. Low-velocity sub-transition zone structure is also corroborated in the independent models SGLOBE-rani (Chang et al., 2015) and GyPSuM (Simmons et al., 2010), and in the composite model SMEAN2 (Becker and Boschi, 2002; Simmons et al., 2010; Ritsema et al., 2011; Auer et al., 2014) (Supplement F). These global images, which benefit from vastly expanded recent seismographic data recorded in Antarctica (e.g., Anthony et al., 2015), are consistent with the hypothesis that a deep mantle thermal structure is driving the melting that ultimately produced the observed HIMU signature.

531

532 5.3 Evolution of the magma source of Antarctic volcanics

533 Noting the widespread regional HIMU-like signature of Ross Island and greater West Antarctica and the finding that plate reconstruction models indicate prolonged subduction in the Paleozoic-Mesozoic (~500–100 Ma) along the Pacific margin of Gondwana (Finn et al., 2005; Seton et al., 2012), Finn et al. (2005) suggested that “common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long-lived, low-volume, diffuse alkaline magmatic province (which the authors termed DAMP) encompassing the easternmost part of the Indo-Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate”. As such they suggested that the ubiquitous HIMU-like signature seen in
542 the lavas represents melting of a metasomatized lithospheric source, which is the product
543 of long-lived subduction along Gondwana's Pacific margin.

544

545 However, as discussed in detail above (section 5.2.2.2; Figs. 7 and 8), an important
546 distinction exists between the trace element and isotopic characteristics of the older
547 Zealandia and Antarctic volcanic samples and those of Ross Island lavas and tephras.

548 Given these marked geochemical differences and the recent geophysical observation that
549 an active sub-transition zone is upwelling below Ross Island and the WARS (Fig. 9), we
550 argue that, while ancient subduction processes occurring along the Pacific margin of
551 Gondwana undoubtedly influenced the lithosphere beneath the southwest Pacific and
552 Antarctica and may have produced radiogenic Pb isotope compositions akin to HIMU,
553 this metasomatized material is no longer the source of lavas being erupted at Ross Island.

554 Rather, the Ross Island volcanics are best explained as deriving from an asthenospheric
555 source that is a mixture of DMM and HIMU, as attested to by all four long-lived
556 radiogenic isotopic systems (Sr, Nd, Hf, and Pb), not just the Pb isotopes alone. We
557 hence conclude that Ross Island lavas and tephras represent melting of a deep-seated,
558 long-lived component, such as the HIMU mantle end-member thought to be ancient
559 (Archean to early Proterozoic) recycled oceanic crust isolated in the lower mantle for
560 billions of years (Hanyu et al., 2011; Weiss et al., 2016), thereby allowing the
561 characteristic HIMU isotopic signature to grow in over time in all four isotopic systems
562 (Sr, Nd, Hf, and Pb), even the longest-lived among them. We further contend that, by
563 now, large-scale mantle upwelling has eroded away the metasomatic signature relict from
564 the Paleozoic to Mesozoic subduction that took place along the margins of Gondwana. It
565 is not surprising that the relict metasomatic signature has been completely removed
566 beneath Ross Island given the large amount of magma that has been produced under this
567 island (4520 km³ of erupted material over the last four million years; Esser et al., 2004)
568 and the slow motion of the Antarctic lithosphere (14 mm/yr in a south-southeasterly
569 direction using the hotspot reference frame HS2-NUVEL 1A; Argus and Gordon, 1991)
570 causing the lithosphere to be left in place for long enough to become thoroughly eroded.

571

572 **6. Conclusions**

573 We provide an extensive geochemical and isotopic data set for Mt. Terror, Mt.
574 Bird, and Hut Point Peninsula to complement an equally extensive and similar existing
575 data set for Erebus (Kelly et al., 2008; Sims et al., 2008). Using geophysical,
576 geochemical, and isotopic constraints and examining all four volcanic centers at Ross
577 Island together, this integrated, multi-disciplinary study sheds new light on the driving
578 forces of alkaline volcanism beneath Ross Island and the nature and evolution of its
579 HIMU source. Three new holistic perspectives on this unique and important volcanic
580 complex have emerged from the present study.

581

582 First, although the Sr, Nd, Hf and Pb isotope compositions of all four volcanic centers,
583 (Erebus, Mt. Terror, Mt. Bird, and Hut Point Peninsula) can be broadly modeled as a
584 mixture of HIMU and DMM, there is significant variability between these centers and a
585 general clustering of isotope compositions among them. This inter-volcano isotopic
586 variability implies a length scale of mantle heterogeneity beneath Ross Island comparable
587 to or smaller than that of inter-volcano distances (i.e., < 50 km). Further, Mt. Bird lavas
588 and tephras have isotope compositions distinct from the other three volcanoes, pointing to
589 a separate source component, possibly deep-sea pelagic sediments, tapped only by Mt.
590 Bird.

591

592 Second, several lines of evidence suggest that Ross Island is the surface manifestation of
593 a deep upwelling mantle plume: (1) The three peripheral volcanic centers radially
594 surround the active Erebus volcano at 120° angles, consistent with the plume structure
595 modeling of Burke and Dewey (1973); (2) the greater overall Sr, Nd, Hf, and Pb isotopic
596 variability of Mt. Terror, Mt. Bird, and Hut Point Peninsula lavas and tephras compared
597 to Erebus volcano samples suggests increased asthenospheric and/or lithospheric mixing
598 at the periphery of Ross Island consistent with fluid dynamical models of plume
599 upwelling (Hauri et al., 1994); and (3) independent seismic tomography models resolve a
600 low shear wave velocity zone that extends to ~1200 km beneath Ross Island and into
601 greater West Antarctica, suggesting that the mantle melting responsible for the HIMU
602 signature has a deep-seated origin.

603

604 Third, an important distinction is observed between both the geochemical and isotopic
605 characteristics of the older Zealandia and Antarctic volcanic samples and the Ross Island
606 lavas and tephras. Namely: (1) All four isotope systems (Sr, Nd, Hf, and Pb), not just Pb
607 isotopes alone, show compositions similar to the HIMU mantle end-member (e.g., as
608 represented by St. Helena); and (2) trace element signatures of the Ross Island lavas such
609 as Nb/La are inconsistent with a metasomatized mantle influence, unlike the Zealandia
610 Mount Somers Volcanic Group and older Antarctic samples such as the Ferrar group and
611 the minettes from East Antarctica.

612

613 We therefore posit that the source of the Ross Island lavas is old (Archean to early
614 Proterozoic) material upwelling from the deep mantle rather than subcontinental
615 lithospheric mantle that was metasomatized by Paleozoic subduction. Thus, while the
616 process of Paleozoic subduction along Gondwana likely strongly affected the lithosphere
617 beneath the southwest Pacific and Antarctica, we argue that this metasomatized material
618 has been thermally eroded away beneath Ross Island and is not the source of lavas
619 currently being erupted there.

620

621

622

623 Acknowledgements

624 This work was funded by National Science Foundation awards OPP-ANT-1141167
625 (KWWS), NSF-OPP ANT-1644013 (KWWS), EAR-0960270 (KWWS), ANT-1141534
626 (PRK) and OPP-ANT-1419268 (RCA). JBT acknowledges financial support from the
627 French Agence Nationale de la Recherche through grant ANR-10-BLAN-0603 (M&Ms
628 — Mantle Melting — Measurements, Models, Mechanisms). Geospatial support for this
629 work was provided by the Polar Geospatial Center under NSF OPP awards 1043681 &
630 1559691. Additional funding was provided to EHP by the Dick and Lynne Cheney
631 Fellowship for Excellence in Study Abroad, the University of Wyoming Department of
632 Geology and Geophysics, the Roy J. Schlemon Center for Quaternary Studies, and a
633 Goldschmidt Student Travel Grant. Global model sections were facilitated by the IRIS
634 Earth Modeling Collaboration (Trabant et al., 2012). Code and data for the SEMUCB-
635 WM1 model was provided by Barbara Romanowicz and colleagues at the University of
636 California, Berkeley. NSF-directed Antarctic support personnel made fieldwork in
637 Antarctica smooth and efficient. EHP is grateful to ENS Lyon for its hospitality and to
638 Philippe Telouk and Francis Albarède for help and guidance. We further thank Susan
639 Swapp, Tyler Brown, Sean Scott, and Evan Soderberg for assistance with sample
640 preparation and analyses. We appreciate constructive discussions with Sean Scott and
641 Mark Reagan. Comments by two anonymous reviewers and editor Tamsin Mather
642 significantly improved the manuscript.

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960 **Figure Captions**

961 **Figure 1.** (a) Map of Antarctica. (b) Map of the West Antarctic Rift System, the
962 Transantarctic Mountains, and Ross Island. (c) Map of Ross Island. Locations of new
963 samples are shown with red circles and sample numbers are highlighted in yellow; “RI”
964 precedes sample numbers 1 through 117; “T” denotes tephra. Dry Valley Drilling Project
965 (DVDP) holes 2 and 3 were drilled in the same location; sample numbers for Hut Point
966 lavas from these drill holes are preceded by 2- or 3- based on the hole from which they
967 were collected. Locations of samples reported by Sims et al. (2008) are shown with blue
968 stars; 39 of these samples are from Erebus crater or summit area and four are from DVDP
969 hole 2; samples from the enriched iron series reported by Sims et al. (2008) are neither
shown nor used in comparison with data for Erebus because they show clear signs of crustal contamination. Mt. Erebus, Mt. Terror, Mt. Bird, and Terra Nova are marked by green triangles. Dark gray boxes enclose samples from each of the volcanic centers (Erebus volcano, Mt. Terror, Mt. Bird, and Hut Point Peninsula). See Supplement A for GPS coordinates of samples from this study.

**Figure 2.** Total alkalis vs. silica diagram for Ross Island lavas and tephras. Most samples from the peripheral volcanic centers examined in this study are basanites. The Mt. Terror, Mt. Bird, and Hut Point Peninsula samples examined in this study contain $SiO_2$ ranging from 40.86 to 56.75 wt.% and $Na_2O+K_2O$ ranging from 3.74 to 14.73 wt.%. They range from basanite to phonolite, but the majority of samples plot in the basanite field. MgO ranges from 0.12 to 12.72 wt.%, with an average of 6.43 wt.%. All samples with MgO less than 1 wt.% are phonolites or plot on or close to the phonolite-tephriphonolite divide. Erebus lineage rocks in this study are primarily from anorthoclase phonolite lavas and bombs. Erebus lineage samples plotted from Sims et al. (2008) are those for which there are isotope data, and exclude enriched iron series samples, some of which show clear signs of crustal contamination. The major and trace element data for these samples, including both whole-rock analyses and matrix glass analyses, are presented in Kelly et al. (2008) and Kyle et al. (1992).

**Figure 3.** Primitive mantle-normalized trace element diagram, showing that light REE are enriched over heavy REE in Mt. Terror, Mt. Bird, and Hut Point Peninsula samples, with primitive mantle-normalized La/Yb ranging from 14.5 to 26.2 (see Supplement C).
993 Trace element patterns for the highest Mg# samples from the three peripheral volcanic centers are very similar to each other as well as to the HIMU mantle end-member, with 994 negative K and Pb anomalies. The lowest Mg# samples from Mt. Terror, Mt. Bird, and 995 Hut Point Peninsula show greater enrichments in the most incompatible trace elements, 996 lack significant K and Pb anomalies, and have negative Ti anomalies; these samples more 997 closely resemble Erebus lineage lavas, especially a trachyandesite that likely was affected 998 by crustal contamination. HIMU is an average of 12 samples from St. Helena from 999 Willbold and Stracke (2006). Averages for Erebus volcano are for samples reported in 1000 Sims et al. (2008). Major and trace element concentrations for these Erebus samples are 1001 reported in Kelly et al. (2008) and Kyle et al. (1992). Historical Erebus lineage (EL) lavas 1002 from Kelly et al. (2008) are for matrix glass and older Erebus lineage lavas from Kelly et 1003 al. (2008) and Kyle et al. (1992) are for whole-rock samples. Primitive mantle values are 1004 from McDonough and Sun (1995). The highest and lowest Mg# samples from Mt. Terror, 1005 Mt. Bird, and Hut Point are indicated. 1006

1007

1008 **Figure 4.** Isotope plots showing the similarity of Ross Island lavas and tephras to OIB 1009 and the HIMU mantle end-member, the overall greater variability of the peripheral 1010 volcanic centers at Ross Island compared to Erebus volcano, and the general clustering of 1011 isotope compositions within each volcanic center. Plots on the left-hand side include 1012 MORB in green and OIB in gray. Plots on the right-hand side show Ross Island lavas and 1013 tephras in detail. The Sr, Nd, Hf, and Pb isotope data from this study for 28 samples from 1014 Mt. Terror, 12 samples from Mt. Bird, 17 samples from Hut Point Peninsula, 1 sample 1015 from the Erebus lineage, 1 sample from the Erebus enriched iron series, and 4 mineral
separates from the Terror area are presented in Supplement A. Erebus lineage samples
from Sims et al. (2008) exclude enriched iron series samples, which show clear signs of
crustal contamination. (a) $^{87}$Sr/$^{86}$Sr versus $\varepsilon_{Nd}$. Mantle end-members are shown with
yellow symbols; depleted MORB mantle (DMM) from Hart et al. (1992; circle), Salters
and Stracke (2004; square), and Workman and Hart (2005; triangle); HIMU and enriched
mantle 1 (EM1) from Hart et al. (1992); enriched mantle 2 (EM2) from Hart et al. (1992;
circle) and Workman et al. (2004; triangle). (b) $\varepsilon_{Nd}$ versus $\varepsilon_{Hf}$. Mantle end-members,
shown with yellow circles, are inferred for Hf isotope compositions from their Nd isotope
compositions in Hart et al. (1992). (c) $^{206}$Pb/$^{204}$Pb versus $^{208}$Pb/$^{204}$Pb. Mantle end-
members are shown with yellow symbols; depleted MORB mantle (DMM), HIMU, and
enriched mantle 1 (EM1) from Hart et al. (1992; circles); enriched mantle 2 (EM2) from
Hart et al. (1992; circle) and Workman et al. (2004; triangle). Error bars shown are $2\sigma$
external precision based on replicate measurements of standards (See Supplements A and
D). The isotope compositions of the Ross Island lavas and tephras plot on a mixing line
between the HIMU and DMM end-members.

1031

**Figure 5.** $^{206}$Pb/$^{204}$Pb versus $^{87}$Sr/$^{86}$Sr showing the similarity of Ross Island lavas and tephras to those from the West Antarctic Rift and the greater southwest Pacific. (a) Ross Island volcanics in a global context (data from Sun and Hanson, 1975; Hart (1988; Balleny); Hart et al., 1995, 1997; Rocholl et al., 1995; Hoernle et al., 2006, 2010 [Hikurangi]; Panter et al., 2000, 2006; Timm et al., 2010; see also Finn et al., 2005).

Mantle end-members are shown with yellow symbols; depleted MORB mantle (DMM) from Hart et al. (1992; circle) and Workman and Hart (2005; triangle); HIMU and
enriched mantle 1 (EM1) from Hart et al. (1992); enriched mantle 2 (EM2) from Hart et al. (1992; circle) and Workman et al. (2004; triangle). Ross Island lavas and tephras are on a mixing line between the DMM and HIMU mantle end-members with a small possible contribution from an EM component. (b) Close-up of (a) showing Ross Island samples in detail. Error bars shown are 2σ external precision based on replicate measurements of standards (See Supplements A and D).

**Figure 6.** Three-dimensional plots of εNd (a) and εHf (b) versus $^{206}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb showing the greater variability of the peripheral volcanic centers on Ross Island compared to Erebus volcano and the general clustering within each volcanic center. Mt. Terror lavas and tephras have the highest εNd and εHf and least radiogenic Pb isotope compositions, whereas Hut Point lavas and tephras have the most radiogenic Pb.

For Mt. Bird, divergent trends emerge in Nd-Hf-Pb isotope space (see also Fig. 4), indicating that Mt. Bird tapped a slightly different mantle component. See Figure 4 for error bars.

**Figure 7.** $^{87}$Sr/$^{86}$Sr versus $^{143}$Nd/$^{144}$Nd comparing Ross Island lavas and tephras to rocks from the greater Antarctic and southwest Pacific region, highlighting the highly enriched isotopic signature of the older Mount Somers Volcanic Group (data from Tappenden, 2003), the Ferrar Group (Fleming et al., 1995), and the Schirmacher Oasis minettes of East Antarctica (Hoch et al., 2001), which are likely influenced by subduction-related metasomatically altered lithosphere. West Antarctic and southwest Pacific/Zealandia data from Futa et al. (1983), Hart (1988; Balleny), Hart et al. (1995; 1997), Rocholl et al.
1062 (1995), Hoernle et al. (2006; 2010 [Hikurangi]), Panter et al., (2000; 2006), Rocchi et al.

1063 (2002), and Timm et al. (2010). Ross Island data include samples from this study and
1064 from Sims et al. (2008), excluding enriched iron series samples. The isotopic

1065 dissimilarity between the Ross Island volcanics and the older Zealandia and Antarctic
1066 volcanic samples indicates that the source of the Ross Island lavas is old (Archean to

1067 early Proterozoic) material upwelling from the deep mantle rather than subcontinental

1068 lithospheric mantle metasomatized by Paleozoic subduction. Also shown are the values
1069 for the mantle end-members (yellow symbols; see Figure 4 caption for references).

1070

1071 **Figure 8.** Nb/La versus (a) $^{206}$Pb/$^{204}$Pb and (b) $\varepsilon_{Hf}$, showing the difference between Ross

1072 Island volcanics and the Mount Somers Volcanic Group (MSVG; Tappenden (2003)) and
1073 the Schirmacher Oasis minettes (Hoch et al. (2001)), and the position of Ross Island lavas

1074 and tephras on a mixing line between the HIMU and DMM mantle end-members. Left-
1075 hand side panels show Ross Island lavas and tephras in a global context. Also shown is

1076 St. Helena (yellow HIMU circle; Willbold and Stracke (2006)), which represents the
1077 HIMU mantle end-member, and Depleted MORB Mantle (yellow DMM circle;

1078 Workman and Hart (2005)) and the mixing compositions, with marked increments,
1079 between these two sources. The blue shaded region (Nb/La < 0.5) represents Nb/La ratios

1080 that reflect arc-related processes (Gill, 1981). Right-hand side panels are close-ups
1081 showing the Ross Island samples in detail. Timm et al. (2010) use the MSVG as a

1082 possible composition for Zealandia lithosphere enriched by subduction, and therefore its
1083 dissimilarity to Ross Island volcanics underlines the lack of influence from

1084 metasomatized subcontinental lithospheric mantle at Ross Island.
1085

1086 **Figure 9.** Seismic tomography transects through Ross Island for three azimuths (upper

1087 left) displaying the isotropically averaged seismic shear wave velocity anomaly from the

1088 crust to the core-mantle boundary in global velocity model SEMUCB-WM1 (French and

1089 Romanowicz, 2015). This and other recent tomography models of the region indicate a

1090 low seismic shear wave velocity anomaly that extends below the mantle transition zone

1091 to near 1200 km depth beneath Ross Island, with linked features that extend eastward

1092 beneath the West Antarctic Rift System into the volcanic province of Marie Byrd Land.

1093 The color scale indicates the relative perturbation of shear wave velocity compared to the

1094 corresponding model 1-dimensional depth average. SEMUCB-WM1 was calculated

1095 using full-waveform inversion of a large data set of long period body (> 32 s) and surface

1096 (> 60 s) wave global seismograms. Comparable lower mantle features are also present in

1097 the full waveform inversion model GLAD-M15 of Bozdağ et al. (2016) and in other

1098 global models (see Supplement F).
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 9</figcaption></figure>
<table><caption>Table 1: Locations and Sr, Nd, Hf, and Pb isotope compositions of selected Ross Island samples. See Supplement A for complete table of results.</caption><thead><tr><th>Sample</th><th>Location</th><th><sup>87</sup>Sr/<sup>86</sup>Sr</th><th><sup>143</sup>Nd/<sup>144</sup>Nd</th><th>ε<sub>Nd</sub></th><th><sup>176</sup>Hf/<sup>177</sup>Hf</th><th>ε<sub>Hf</sub></th><th><sup>206</sup>Pb/<sup>204</sup>Pb</th><th><sup>207</sup>Pb/<sup>204</sup>Pb</th><th><sup>208</sup>Pb/<sup>204</sup>Pb</th></tr></thead><tbody><tr><td colspan="10"><em>Mt. Terror</em></td></tr><tr><td>RI 3</td><td>77°21′59″S<br>168°19′38″E</td><td>0.703133 ± 9</td><td>0.512897 ± 4</td><td>5.0</td><td>0.282991 ± 5</td><td>7.7</td><td>19.7659</td><td>15.6401</td><td>39.3475</td></tr><tr><td>RI 14T</td><td>77°29′56″S<br>169°08′34″E</td><td>0.703098 ± 8</td><td>0.512901 ± 5</td><td>5.1</td><td>0.282979 ± 4</td><td>7.3</td><td>19.7038</td><td>15.6395</td><td>39.2722</td></tr><tr><td>RI 25</td><td>77°30′07″S<br>169°03′15″E</td><td>0.703119 ± 8</td><td>0.512907 ± 4</td><td>5.5</td><td>0.282971 ± 5</td><td>7.0</td><td>19.7009</td><td>15.6430</td><td>39.2787</td></tr><tr><td>RI 45</td><td>77°29′53″S<br>168°42′33″E</td><td>0.703069 ± 7</td><td>0.512919 ± 5</td><td>5.5</td><td>0.282998 ± 4</td><td>8.0</td><td>19.6632</td><td>15.6298</td><td>39.2056</td></tr><tr><td>RI 59T</td><td>77°30′22″S<br>168°51′18″E</td><td>0.703041 ± 10</td><td>0.512926 ± 6</td><td>5.6</td><td>0.283008 ± 3</td><td>8.3</td><td>19.5820</td><td>15.6280</td><td>39.1661</td></tr><tr><td>RI 60</td><td>77°30′23″S<br>168°51′19″E</td><td>0.702969 ± 9</td><td>0.512942 ± 5</td><td>5.9</td><td>0.283003 ± 5</td><td>8.2</td><td>19.5679</td><td>15.6189</td><td>39.1097</td></tr><tr><td>RI 82</td><td>77°30′51″S<br>167°57′46″E</td><td>0.702907 ± 9</td><td>0.512959 ± 3</td><td>6.3</td><td>0.283013 ± 5</td><td>8.5</td><td>19.4578</td><td>15.6055</td><td>38.9735</td></tr><tr><td>CC 03</td><td>77°30′46″S<br>169°19′56″E</td><td>0.703048 ± 7</td><td>0.512920 ± 4</td><td>5.5</td><td>0.282980 ± 3</td><td>7.4</td><td>19.6812</td><td>15.6309</td><td>39.2126</td></tr><tr><td>22962<sup>a</sup></td><td>77°29′50″S<br>169°14′02″E</td><td>0.703088 ± 10</td><td>0.512911 ± 4</td><td>5.3</td><td>0.282975 ± 6</td><td>7.2</td><td>19.2820</td><td>15.6127</td><td>38.8590</td></tr><tr><td colspan="10"><em>Mt. Bird</em></td></tr><tr><td>RI 93</td><td>77°14′49″S<br>166°44′45″E</td><td>0.702932 ± 12</td><td>0.512896 ± 4</td><td>5.0</td><td>0.283002 ± 4</td><td>8.1</td><td>19.6796</td><td>15.6266</td><td>39.4553</td></tr><tr><td>RI 98T</td><td>77°17′28″S<br>166°52′03″E</td><td>0.703065 ± 9</td><td>0.512858 ± 4</td><td>4.3</td><td>0.282981 ± 5</td><td>7.4</td><td>19.8412</td><td>15.6363</td><td>39.7039</td></tr><tr><td>RI 117</td><td>~77°19′01″S<br>~166°32′23″E</td><td>0.703156 ± 11*</td><td>0.512898 ± 4</td><td>5.1</td><td>0.282943 ± 4</td><td>6.0</td><td>19.7376</td><td>15.6244</td><td>39.4391</td></tr><tr><td>LB 02</td><td>77°18′04″S<br>167°06′10″E</td><td>0.703126 ± 7</td><td>0.512872 ± 5</td><td>4.6</td><td>0.282983 ± 2</td><td>7.5</td><td>19.8001</td><td>15.6329</td><td>39.6306</td></tr><tr><td colspan="10"><em>Hut Point Peninsula<sup>b</sup></em></td></tr><tr><td>RI 34</td><td>~77°44′39″S<br>~166°46′01″E</td><td>0.703083 ± 11</td><td>0.512895 ± 4</td><td>5.1</td><td>0.282957 ± 5</td><td>6.5</td><td>19.8778</td><td>15.6472</td><td>39.4490</td></tr><tr><td>2-45.86<sup>a</sup></td><td>DVDP-2</td><td>0.703035 ± 7</td><td>0.512889 ± 6</td><td>4.9</td><td>0.282939 ± 3</td><td>5.9</td><td>19.8597</td><td>15.6498</td><td>39.4383</td></tr><tr><td>2-105.53<sup>a</sup></td><td>DVDP-2</td><td>0.702996 ± 10</td><td>0.512919 ± 6</td><td>5.5</td><td>0.282955 ± 4</td><td>6.5</td><td>19.7861</td><td>15.6393</td><td>39.2850</td></tr><tr><td>3-267.38</td><td>DVDP-3</td><td>0.702969 ± 7</td><td>0.512904 ± 4</td><td>4.9</td><td>0.282955 ± 5</td><td>6.5</td><td>20.0773</td><td>15.6488</td><td>39.6953</td></tr><tr><td>3-306.33</td><td>DVDP-3</td><td>0.702972 ± 8</td><td>0.512905 ± 3</td><td>5.2</td><td>0.282956 ± 4</td><td>6.5</td><td>20.1309</td><td>15.6648</td><td>39.7147</td></tr><tr><td>3-372.27</td><td>DVDP-3</td><td>0.702991 ± 10</td><td>0.512905 ± 3</td><td>5.1</td><td>0.282951 ± 6</td><td>6.3</td><td>20.1292</td><td>15.6613</td><td>39.7235</td></tr><tr><td>22888<sup>a</sup></td><td>77°50′48″S<br>166°41′59″E</td><td>0.703051 ± 7</td><td>0.512889 ± 5</td><td>4.9</td><td>0.282942 ± 5</td><td>6.0</td><td>20.2406</td><td>15.6679</td><td>39.7960</td></tr><tr><td colspan="10"><em>Standard<sup>c</sup></em></td></tr><tr><td>BCR-2 (unleached)</td><td></td><td>0.705002 ± 11</td><td>0.512632 ± 5</td><td>-0.1</td><td>0.282871 ± 4</td><td>3.5</td><td>18.7502</td><td>15.6229</td><td>38.7251</td></tr><tr><td></td><td></td><td>0.704999 ± 11</td><td>0.512637 ± 5</td><td>0.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td>0.705013 ± 9</td><td>0.512630 ± 5</td><td>-0.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td></td><td>0.705012 ± 8</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Footnotes (See Supplements A and D for complete footnotes and analytical methods):

<footnote><sup>a</sup>Archived powders ground in a tungsten-carbide (WC) shatterbox were used for isotopic analysis for these samples; no W or Ta interferences were observed during Hf isotope analysis. All other samples were ground in a ceramic shatterbox.</footnote>

<footnote><sup>b</sup>Hut Point sample names that are preceded by 2- or 3- are from the DVDP drill core. The DVDP-2 and DVDP-3 holes were located within 2 m of each other at 77°50′53″S, 166°40′53″E.</footnote>

<footnote><sup>c</sup>Analysis of BCR-2 rock standard that was purified and analyzed along with Ross Island samples. Multiple listings are analyses of a single purified solution. Additional rock standard analyses reported in Supplement A.</footnote>

<footnote>* Sr measured on two separate dissolutions and chemical purifications. Duplicate analyses agree within analytical error.</footnote>

<footnote><sup>#</sup>Duplicate Nd isotope measurement at ENS Lyon on same solution measured at UW; duplicate measurements agree within analytical error.</footnote>