# An isotopically distinct Zealandia–Antarctic mantle domain in the Southern Ocean

Sung-Hyun Park, Charles Langmuir, Kenneth Sims, Janne Blichert-Toft, Seung-Sep Kim, Sean Scott, Jian Lin, Hakkyum Choi, Yun-Seok Yang, Peter Michael

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## To cite this version:

Sung-Hyun Park, Charles Langmuir, Kenneth Sims, Janne Blichert-Toft, Seung-Sep Kim, et al.. An isotopically distinct Zealandia–Antarctic mantle domain in the Southern Ocean. Nature Geoscience, 2019, 12 (3), pp.206-214. 10.1038/s41561-018-0292-4. hal-02109913

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https://hal.science/hal-02109913v1

Submitted on 2 Oct 2025

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# An isotopically distinct Zealandia-Antarctic mantle domain in the Southern Ocean

Sung-Hyun Park ⌠1*, Charles H. Langmuir², Kenneth W. W. Sims<fnref n="3" />, Janne Blichert-Toft⁴, Seung-Sep Kim⁵, Sean R. Scott<fnref n="3" />, Jian Lin⁶,<fnref n="7" />, Hakkyum Choi ⌠1, Yun-Seok Yang<fnref n="1" /> and Peter J. Michael<fnref n="8" />

The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian-Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth's upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian-Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian-Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia-Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian-Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain.

Isotopic compositions of oceanic basalts constrain mantle composition, evolution and convective flow<fnref n="1" />,². Basalts from Indian mid-ocean ridges, for example, have higher ²⁰⁸Pb/²⁰⁴Pb at a given ²⁰⁶Pb/²⁰⁴Pb than those from the East Pacific Rise (EPR) and the Pacific-Antarctic Ridge (PAR)<fnref n="3" />⁻⁵. Also, the Indian ridges are distinguished by having higher ¹⁷⁶Hf/¹⁷⁷Hf for a given ¹⁴³Nd/¹⁴⁴Nd (refs. ⁵,⁶). These isotopic differences imply the existence of ocean-scale upper-mantle domains with limited interactions over long periods of time<fnref n="7" />⁻⁹, and raise the question of the location of their boundaries. In the present case, the Australian-Antarctic Discordance (AAD) was proposed to be the boundary between the Indian and Pacific mantle domains because it constitutes a remarkably deep portion of the global mid-ocean ridge system that demarks abrupt changes in isotopic ratios from Indian to Pacific values. There remained, however, a broad gap in sampling between the AAD and the Pacific ridges to the east, which leaves the definitiveness of the boundary in question. In their study of the PAR, Vlastélík et al.⁴ demonstrated that the Pacific upper mantle is not homogeneous, but consists of two distinct subdomains separated by the Pacific Superswell. However, this finding did not bring resolution to the unsettled demarcation of the striking isotopic contrast between the Indian and Pacific mantle domains.

The last gap in the mapping and sampling of seafloor spreading centres worldwide was the Australian-Antarctic Ridge (AAR), a 2,000 km expanse in the most remote parts of the ocean ridge system. The collection and isotopic measurements of samples from the AAR provide answers to two key questions. Does the AAR, which lies to the east of the AAD, have a composition as predicted akin to that of the Pacific? And is the AAD the major boundary between the Pacific and Indian suboceanic mantle domains? We show here that "no" is the answer to both questions. Instead, our new data reveal the existence of a new mantle domain and constrain its origin.

## Expeditions to terra incognita

In 2011 and 2013, the Korean icebreaker RV *Aaron* conducted three scientific expeditions to map and sample two segments in the middle of the AAR—KR1 and KR2 (Fig. 1). These cruises provided the first samples and detailed mapping of the remote AAR, and filled an important gap in our knowledge of the crust and mantle in this region.

The KR1 and KR2 segments have intermediate spreading rates of ~70 mm yr⁻<fnref n="1" />. Segment KR1 is a 300 km long supersegment (Fig. 1a) that exhibits large along-axis topographic differences from 1,800 to 2,800 m. The axial depth and morphology of KR1 vary from west to east, and range from an axial plateau in the westernmost region to an axial rift, a narrow axial high, a flat axis and finally a well-developed rift valley in the easternmost region (Fig. 1b). Segment KR2, located northwest of KR1, has a western axial high and eastern rift valley. These segments exhibit shallower axial depths (1,800–2,800 m) than the mean axial depth (3,000 m) of ocean ridges of intermediate spreading rates¹⁰.

## A unique isotopic signature in the Southern Ocean

The isotopic compositions of Sr, Nd, Hf and Pb for the KR1 and KR2 ridge segments are listed in Supplementary Data Table 1. ²⁰⁶Pb/²⁰⁴Pb values for KR1 vary between 18.7 and 19.1 from the west to the middle of the segment and gradually decrease eastward to ~18.4 (Fig. 1c). ²⁰⁶Pb/²⁰⁴Pb values for KR2 are ~18.4, except for the westernmost sample which has a value of 19.2. Relative variations in the Hf isotope compositions (expressed here as ε<sub>Hf</sub> = +12.8 to +18.3) are almost a mirror image of the ²⁰⁶Pb/²⁰⁴Pb along-axis variations (Fig. 1d). ε<sub>Nd</sub> varies from +7.4 to +10.1 and is well correlated (R² = 0.93) with ε<sub>Hf</sub>. ε<sub>Hf</sub> to the east of 159° 50' E is very high (up to +18.3) compared to the majority of Pacific-type depleted mantle (up to +17)⁶,¹¹. In contrast,

<footnote n="1">¹Korea Polar Research Institute, Incheon, Republic of Korea. ²Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.</footnote>

<footnote n="3">³Department of Geology and Geophysics, University of Wyoming, Laramie, WY, USA. ⁴Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, CNRS UMR 5276, Lyon, France. ⁵Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon, Republic of Korea. ⁶Departement of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.</footnote>

<footnote n="7">⁷Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China.</footnote>

<footnote n="8">⁸Department of Geosciences, University of Tulsa, Tulsa, OK, USA. *e-mail: shpark314@kopri.re.kr</footnote>
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**Fig. 1** | Bathymetry and along-axis variations in depth, ²⁰⁶Pb/²⁰⁴Pb and εₕH for the KR1 and KR2 segments of the AAR. **a**, Bathymetric maps for the KR1 (right) and KR2 (left) segments were produced using multibeam echo sounder data (Kongsberg EM122) acquired during three Korean Polar Ridge program (KOPRIDGE) cruises. Inset: important tectonic features in the vicinity of the AAR. Circles show the sampling locations of the wax corer. These maps indicate that the AAR mainly consists of a series of first-order segments bounded by parallel transform faults. ANT, Antarctic Plate; AUS, Australian Plate; PAC, Pacific Plate; NZ, New Zealand. **b**, The KR1 segment (right) is a 300-km-long supersegment that exhibits large topographic variation from west to east: an axial plateau in the west, followed by an axial rift, a narrow axial high, a flat axis and a well-developed rift valley. The KR2 segment (left) also shows topographic variation with a western axial high and an eastern rift valley. Both segments are shallower than the mean axial depth of global mid-ocean ridges, which implies a more buoyant mantle beneath the KR1 and KR2 segments¹³. **c**, All-MORB (mid-ocean ridge basalts) indicates the total composition of the crust excluding back-arc basins¹⁰. Most of the KR1 and KR2 samples have higher ²⁰⁶Pb/²⁰⁴Pb than All-MORB, which suggests they are enriched compared to the global average. The variation of ²⁰⁶Pb/²⁰⁴Pb (18.3–19.1) is comparable to that of the Galapagos Spreading Centre, which suggests the KR1 and KR2 segments are supplied by heterogeneous mantle as for ridges that interact with a plume⁵⁰. **d**, The εₕH variation of the KR1 (right) and KR2 (left) segments (about +12.8 to +18.3) is comparable to that of the Galapagos Spreading Centre (about +9 to +16)¹²,⁵⁰. However, the εₕH of the KR1 and KR2 extends to higher values than the Galapagos data, which suggests the mantle beneath the KR1 and KR2 segments is more depleted than the Galapagos mantle.
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**Fig. 2** | Comparisons of isotope ratios from the KR1 and KR2 segments and other mid-ocean ridge basalts. Available isotope data from the Indian ridges, AAD, Zone A, PAR, EPR and the Easter segment are compared with the KR1 and KR2 segments. The data are mostly from Gale et al.¹⁰ and references therein. Data from Hanan et al.⁵¹ are added for the Indian ridges. PAR data are from Vlastélic et al.⁴ and Hamelin et al.⁵². **a.** The KR1 and KR2 segment data plot above the Northern Hemisphere reference line (NHRL)¹². At a given ²⁰⁶Pb/²⁰⁴Pb, the PAR data mostly plot below the NHRL, on a trend with shallower slope. **b.** The SPRL is estimated as ²⁰⁷Pb/²⁰⁶Pb = 0.6253 × (²⁰⁸Pb/²⁰⁶Pb) – 0.4441 with R² = 0.92. The slope of SPRL is shallower than that of the NHRL (see the EPR samples), which supports the hypothesis that the northern and southern Pacific mantle domains are isotopically distinct. **c.** The mantle array is defined as ε<sub>Hf</sub> = 1.59ε<sub>Nd</sub> + 1.28 (ref. ⁵³). The dividing line for the Indian and Pacific mantle domains is from Kempton et al.⁵. The KR1 and KR2 data mostly fall within the Indian mantle domain. In addition, the KR1 and KR2 samples have a higher ε<sub>Hf</sub> than the mantle array for a given ε<sub>Nd</sub>, whereas the PAR and EPR plot below the mantle array. **d.** Clear separations between the EPR, PAR, KR1 and KR2 segments, Zone A and Indian MORB are evident. At a given ε<sub>Hf</sub>, the PAR is the lowest in ⁸⁷Sr/⁸⁶Sr, the EPR the second lowest, the KR1 and KR2 segments the third lowest and the Indian MORB the highest. At a given ε<sub>Hf</sub>, Zone A is slightly higher in ⁸⁷Sr/⁸⁶Sr than the PAR, yet lower than the KR1 and KR2 segments. Zone A is closer to the KR1 and KR2 segments in Pb isotope composition and ε<sub>Hf</sub> and ε<sub>Nd</sub>, but different for ε<sub>Hf</sub> and ⁸⁷Sr/⁸⁶Sr, which suggests that Zone A differs from both the KR1 and KR2 segments and PAR. Thus, Zone A is no longer considered as belonging to the Pacific mantle. However, it appears that Zone A and the KR1 and KR2 segments share a similar depleted component.

the middle of KR1 is relatively enriched, with a high ²⁰⁶Pb/²⁰⁴Pb and low values of ε<sub>Nd</sub> and ε<sub>Hf</sub>. Note that KR1 encompasses essentially the full range of ε<sub>Hf</sub> values for Pacific mid-ocean ridge basalts (MORB) in just one segment. KR2 has ε<sub>Hf</sub> values from +15.6 to +16.7 and ε<sub>Nd</sub> values from +9.0 to +10.1 except for the westernmost sample, for which ε<sub>Hf</sub> is +13.6 and ε<sub>Nd</sub> is +7.7. The combined isotopic ratios of Sr, Nd, Hf and Pb permit a robust comparison of the KR1 and KR2 segments with the Indian and Pacific ridges (Figs. 1 and 2), and show that the former are distinct from both the neighbouring

Pacific and Indian mantle compositions. At a given ²⁰⁶Pb/²⁰⁴Pb, the KR1 and KR2 samples have higher ²⁰⁸Pb/²⁰⁴Pb than Pacific samples, and lower ²⁰⁸Pb/²⁰⁴Pb than Indian samples (Fig. 2a). An effective discriminant is Δ8/4, which is the magnitude of the ²⁰⁸Pb/²⁰⁴Pb divergence from a regression line based on Northern Hemisphere samples¹² (Fig. 2a). The KR1 and KR2 segments have Δ8/4 of +10 to +18, higher than both the PAR (−30 to +10) and EPR (−10 to +10), but lower than the Indian ridges (greater than +22 and up to +80) (Fig. 3a). At a given ²⁰⁸Pb/²⁰⁶Pb, all the KR1 and KR2 samples have
<figure><img src="image_10.png" /><figcaption>**Fig. 3** | Δ8/4 and Δ7/6 variations. **a,b.** Variations in the Δ8/4 (a) and Δ7/6 (b) values for samples collected along mid-ocean ridges and plotted as a function of longitude. Δ7/6 is defined as [(207Pb/206Pb)sample - (207Pb/206Pb)SPRL] × 10⁴. CIR, Central Indian Ridge; SEIR, Southeast Indian Ridge; SWIR, Southwest Indian Ridge; IR, Indian ridges.</figcaption></figure>

lower 207Pb/206Pb values than those of Pacific samples (Fig. 2b). Another powerful discriminant is Δ7/6, which is the deviation of a given sample from a southern Pacific reference line (SPRL) defined here as the best fit to the 207Pb/206Pb-208Pb/206Pb variations of the PAR based on the least squares method (Fig. 2b). The Δ7/6 of PAR samples is between -10 and +10, whereas KR1 and KR2 segments show negative values between -70 and -120 (Figs. 2b and 3b). The KR1 and KR2 segments also plot on the Indian side of the 'Indian/ Pacific' dividing line in ε<sub>Na</sub>-ε<sub>Hf</sub> space (Fig. 2c)⁵, whereas at a given value of ε<sub>HP</sub> the KR1 and KR2 samples have Sr isotope compositions intermediate to those of the Pacific and Indian ridges (Fig. 2d). These differences demonstrate that the KR1 and KR2 segments have a unique isotopic signature that is distinct from both Pacific and Indian ridge samples.

Comparison with other data in the region suggests a wide-
spread distribution of this distinctive composition (Figs. 4 and 5a).
Volcanic samples from Ross Island<sup>13,14</sup>, the West Antarctic Rift
System (WARS)<sup>15-18</sup>, Balleny and Scott Islands<sup>19</sup>, Macquarie Island<sup>20</sup>
and Zealandia<sup>21-24</sup> exhibit coherent relationships with the KR1 and
KR2 segments (Fig. 4). The data are consistent with mixing an
enriched component with high Pb isotope ratios akin to high-mu
(HIMU), found almost ubiquitously in the region's Cenozoic alka-
line volcanic rocks, with a depleted mantle component distinct from
both the Pacific and Indian suboceanic mantle domains. Although
most of the regional Cenozoic alkaline volcanism was not derived
from an ocean ridge, it is still isotopically coherent with the AAR
samples. For example, the active Mt Erebus and neighbouring vol-
canoes on Ross Island, which are part of the WARS<sup>13,14</sup> and hence of
the same isotopic grouping, display the same mixing relationship
between an old enriched HIMU-like component sourced in the
deep mantle and the depleted upper mantle, which thereby defines
a mixing zone (Fig. 5a).

The geographical limits of this mixing zone can be delin-
eated based on the isotopic compositions of lavas from Marie
Byrd Land<sup>25,26</sup>, East Australia<sup>27-31</sup> and Zone A. The Marie Byrd
Land and East Australia samples show slightly different trends
from those of the KR1 and KR2 segments in multi-isotope space

(Fig. 4), which suggests that the mixing zone does not extend to
these regions.

Zone A to the east of the AAD was previously considered to be the geographical beginning of the Pacific mantle domain⁵⁻⁷, but this view is now no longer supported by the currently available isotope data. Zone A samples are distinct from the Indian ridge samples to the west (Figs. 2–4) and Pacific samples to the east, being intermediate in composition to the new AAR data reported here and data from the EPR and PAR. The margin between Zone A and the KR2 segment establishes the western boundary of the mixing zone (thick black dashed lines in Fig. 5a). To determine more precisely the extent and characteristics of the mixing zone will require future systematic sampling between the east of the KR1 segment and the PAR, and between Zone A and the west of the KR2 segment (data gaps in Fig. 5a).

All the samples between Zone A and the west end of the PAR fall below the SPRL with negative Δ7/6 values. This region (inside the pink dashed lines in Fig. 5a) is entirely distinct from both the Indian and Pacific mantle domains as apparent from the discrete clusters in Δ7/6-Δ8/4 space as a function of longitudinal position (Figs. 3 and 5b). The large expanse in both space and time is consistent with an extensive, distinct mantle domain, for which we propose the name 'Zealandia-Antarctic Swell'. The presence of this new mantle domain indicates that the AAD is not, as previously thought, the boundary between the Indian and Pacific suboceanic mantles. It remains, however, the boundary of the Indian Ocean domain.

Origin and dynamics of the Zealandia-Antarctic Swell

From the coherency in isotopic characteristics between the AAR and other volcanic rocks from neighbouring regions (Mt Erebus, Ross Island, WARS and so on), we propose that the Zealandia-Antarctic domain was initiated by the superplume associated with the break-up of Gondwana³²,³³. Subsequent active mantle upwell-ing in the region includes both rifting, as represented by the AAR, and asthenospheric channelling from present-day plumes, such as Balleny and Ross Islands¹⁴,³⁴,³⁵.

The plume model is supported by the isotopic compositions of the Hikurangi Seamounts. The Hikurangi data lie along the mixing trajectories that connect the AAR and surrounding regions (Fig. 4). For example, the ε<sub>Na</sub>-ε<sub>Hf</sub> diagram (Fig. 4c) shows that the isotopic variations of Hikurangi and WARS samples are better explained by mixing with the depleted KR1 and KR2 component than with the PAR source. Deviation of the Hikurangi Seamounts and some Zealandia samples from the mixing relationship in Nd-Pb isotope space may be due to contamination by subcontinental lithospheric mantle (Fig. 4d)³², which is often observed in volcanic rocks erupted during initial stages of rifting. The Hikurangi Seamounts were formed by the superplume that resulted in the break-up of New Zealand and Antarctica at ~90 Ma³²,³³. Based on plate reconstruction models³⁶, the Hikurangi Seamounts erupted at locations similar to those of the KR1 and KR2 segments at ~90 Ma, which implies that this particular mantle domain may be associated with the onset of the New Zealand and Antarctica break-up caused by the superplume³²,³³.

Alternatives to the plume model have been put forward by other
studies that suggest the enriched component may be derived from
the overriding continental lithosphere contaminated by the sub-
duction of the Pacific and Phoenix plates<sup>18,37</sup>. This metasomatically
enriched lithospheric mantle model<sup>18,37</sup>, however, cannot explain the
isotopic coherence among the KR1 and KR2 segments, Ross Island
and the Hikurangi Seamounts (Fig. 4c). As the Hikurangi Seamounts
erupted on the oceanic lithosphere subducting along the former
Gondwana margin<sup>32,38</sup>, the compositional influence of the subcon-
tinental material on the Hikurangi volcanism would be minimal.
Moreover, it is physically implausible that the lithospheric material
could influence midocean ridge volcanism 800 km away from both
<figure><img src="image_11.png" /><figcaption>**Fig. 4** | Comparison of isotope ratios from KR1 and KR2 with Gondwana margin locations (Zealandia, WARS, Balleny and Scott Islands, Marie Byrd Land and East Australia). a, The WARS, which includes Ross Island (and Mt Erebus), Balleny and Scott Islands, Macquarie Island and Zealandia, lies on the same mixing line as the KR1 and KR2 segments as do the Hikurangi Seamounts that erupted during the initial stages of rifting of New Zealand and Antarctica at ~90 Ma during the break-up of Gondwana. In contrast, Marie Byrd Land and Zone A define a different slope relative to the KR1 and KR2 segments and to East Australia. EM1, enriched mantle 1; EM2, enriched mantle 2; HIMU, high-μ (²²⁸U/²⁰⁴Pb). b, The data from the Hikurangi Seamounts, Zealandia, the WARS and Balleny and Scott Islands lie on the KR1 and KR2 segment mixing line, but the data from Zone A and the Marie Byrd Land do not. All the data, however, plot below the SPRL and differ from those of the Indian and Pacific basins. c, The two mixing curves were calculated between the most enriched Hikurangi and the most depleted AAR (black dashed line) and PAR (grey dashed line) data, respectively, using the general mixing equation of Langmuir et al.⁵⁴. We used unpublished Nd (7.19 ppm) and Hf (1.84 ppm) concentrations for AAR. For PAR, we inferred Nd (14.7 ppm) and Hf (3.83 ppm) concentrations from published data in PetDB. Hikurangi data are from Hoernle et al.³² (Nd = 75.3 ppm and Hf = 11.5 ppm). d, The WARS and most of Zealandia are part of the mixing relationship with the KR1 and KR2 segments, whereas Hikurangi Seamounts and some of Zealandia and East Australia deviate from it. This deviation can be accounted for by contamination by the subcontinental lithosphere, which is often observed in volcanism from the initial stages of continental rifting³².</figcaption></figure>

the Zealandia and Antarctica continents. In addition, the influence of any pre-existing lithospheric component would be eliminated by thermal erosion during rifting³⁹ or deep mantle upwelling¹⁴.

Further support for a deep source of the Zealandia-Antarctic Swell comes from seismic tomography. The isotopically defined

Zealandia-Antarctic domain corresponds to a widespread and deep province of low seismic velocities beneath the AAR (Fig. 6). The upper mantle (<250 km deep) below this region has low S-wave velocities⁴⁰⁻⁴², which implies that it may be hotter than the neighbouring Pacific and Indian mantle domains. The shallow axial
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**Fig. 5** | Location of the Zealandia-Antarctic mantle domain. a, Sample locations and domain boundaries. Circles show the sample locations of this study. Numbers around the circles, squares and triangles of the Cenozoic volcanoes erupted on the former Gondwana margin (pink, white and green) are the average ages (millions of years ago (Ma)) for each volcanic event. Note that the age distribution of volcanism is irregular, which implies that this volcanism cannot be explained by a single plume event<sup>24</sup>. The relatively young ages of the Cenozoic volcanism suggest geochemical signatures that correspond to their mantle source<sup>24</sup>. Furthermore, as the Antarctic Plateau did not move significantly during the Cenozoic, the volcanic rocks from the Marie Byrd Seamounts and Balleny and Scott Islands provide a temporal perspective on the variations of the mantle source beneath this region<sup>14,36</sup>. The pink dashed lines show the proposed mantle domain boundaries among the Pacific (blue dots), Indian (yellow and gold dots) and Zealandia-Antarctic (red, pink, green and white dots) mantles. The thick black dashed line indicates the mixing zone in the middle of the Zealandia-Antarctic domain. The samples within this zone are part of the multi-isotopic mixing relationship discussed in the text. According to a tectonic reconstruction model<sup>36</sup>, Zealandia, Australia and Antarctica merged as the Gondwana margin and the Phoenix and Pacific plates were subducting beneath it prior to the break-up of Gondwana at ~90 Ma (refs. 33,36,38). Around 90 Ma, the Hikurangi Plateau (area inside the grey dotted lines) on the Pacific Plate started to collide along the Pacific-Gondwana margin (the present Chatham Rise and Marie Byrd Land)<sup>32,38</sup>. Simultaneously, voluminous volcanic activity (Hikurangi Seamounts) occurred over the Hikurangi Plateau and Marie Byrd Land, which signalled the onset of a plume event with the arrival of a superplume head below the Pacific-Gondwana margin<sup>25,32,33,38</sup>. Thus, the Hikurangi Seamounts are considered to originate from the initial superplume event. The mixing relationship between Hikurangi and the KR1 and KR2 segments implies that the mantle domain may be traced back to the plume activity that caused the Gondwana break-up. b, In this discriminant diagram, the Zealandia-Antarctic, Pacific and Indian mantles are clearly clustered in Δ8/4-Δ7/6 space. The Zealandia-Antarctic mantle is divided into several domains, which include the mixing zone. The PAR data plotting in the Zealandia-Antarctic area (purple) are mostly from the region that extends from the Udintsev Fracture Zone (FZ) to the Heezen FZ, which was influenced by the Hollister Ridge.

depths of the KR1 and KR2 segments are also consistent with a hotter mantle beneath this region<sup>43</sup>. Similarly, the mantle below the WARS has isolated low P-wave<sup>44</sup> and S-wave<sup>14</sup> velocities ($V_s$) down to a depth of ~1,200 km, apparently related to current Balleny/Ross Island volcanism. Mixing between plume components and depleted asthenosphere in the middle of the newly identified mantle domain (the mixing zone discussed above) may well be driven by the Balleny/Ross Island plume activity.

This model is broadly consistent with the concept of asthenospheric mixing induced by mantle upwelling<sup>34,35</sup>. Based on calculations of absolute plate motion and relative mantle plume strength, the plume-fed asthenosphere models<sup>34,35</sup> predict a 'region of influence' for the Balleny/Ross Island plume that covers at least the western extent of the newly proposed mixing zone. Deep mantle

upwellings would rise and mix with the shallower asthenospheric mantle, with some contamination by subcontinental lithospheric mantle where it is present.

## Domain boundaries and their significance

The new data demonstrate that the eastern boundary of the AAD is not the boundary between the Indian and Pacific mantle domains, as believed for the past three decades<sup>5-9</sup>. However, it remains the eastern boundary of the Indian Ocean mantle domain. It may be that the AAD today reflects an earlier boundary between the Pacific and Indian mantle regions, perhaps associated with subduction along the Gondwana margin. The break-up of Gondwana and the initiation of the Zealandia-Antarctic Swell created a new domain that separated the Pacific from the Indian mantle.
<figure><img src="image_13.png" /><figcaption>**Fig. 6** | Maps of shear-wave velocity variations dln V<sub>s</sub> in the Southern Ocean. Tomography models of SEMUCB-WM1 (left)<sup>55</sup>, S40RTS (centre)<sup>40</sup> and SP12RTS (right)<sup>42</sup> at depths of 250 km (top) and 600 km (bottom) are compared. Three hotspots in the vicinity of the Southern Ocean mantle domain are denoted as circles: Balleny (white), Scott (yellow) and Ross (red) islands. The upper mantle anomalies in the Southern Ocean are consistently imaged by three tomography models and appear to be closely related with the Zealandia-Antarctic mantle domain. Moreover, the newly proposed mixing zone that may be driven by the Balleny/Ross Island plume activity coincides with the upper mantle V<sub>s</sub> anomalies.</figcaption></figure>

It mixed both east and west to create a mixed Pacific/Zealandia-
Antarctic margin on the west adjacent to the AAD. In this context,
the compositions sampled in Zone A are of particular interest.
They form an intermediate field in radiogenic isotope space
between the Pacific and Zealandia-Antarctic domains, which
raises the possibility that the upwelling of Zealandia-Antarctic
compositions might have mixed with the pre-existing Pacific
mantle to create the distinctive composition observed in Zone
A today. A full understanding of the distinct composition of
Zone A, however, requires additional sampling between Zone A
and the KR2 segment.

Although the current limited sampling along the west and east of the AAR does not allow a precise definition of the boundaries of the new mantle domain, it appears to be bounded by ancient, large-offset transform faults (Fig. 5). This feature is common to many mantle domains associated with hot spots. The North and South Atlantic basins, in particular, are separated by large equatorial transforms. For example, the Hayes Transform is the southern boundary of the influence of the Azores hot spot, and the Iceland hot spot is bounded by the large-offset Charlie-Gibbs Fracture Zone to the south<sup>45,46</sup> and the Tjörnes Fracture Zone to the north<sup>47</sup>. Similarly, the influence of the Galapagos plume has boundaries associated with ridge offsets<sup>48,49</sup>. Hence, there appears to be a clear association between major tectonic boundaries, bathymetry on the Earth's surface and the underlying upper mantle domains as reflected in the isotopic compositions of erupted basalts. This general phenomenon is further support for a deep origin of the Zealandia-Antarctic domain.

Collectively, these observations suggest a common origin for
many of the world's mantled domains and their respective boundaries.
Deep mantle upwellings feed and mix with upper mantle. These
upwellings have distinctive histories and inherited isotopic
compositions that are both old (Archaean and Proterozoic) and
derived from the deep mantle where mixing is inefficient and
large-scale heterogeneities therefore can be created and preserved
for long periods of time. The upwellings mix with the astheno-
sphere at a level shallow enough that the flow and mixing end up
bounded by shallow features in the upper mantle that are associ-
ated with major tectonic boundaries, such as large-offset trans-
forms. This overall process then creates mantle domains bounded
by large tectonic features near the surface. The implications are a

deep-seated, ancient origin for the enriched end members, with a
flow and mixing shallow enough that the spatial extent of upwell-
ing is constrained by tectonic features near the surface. Further
tests of this overall framework can be undertaken by further
sampling along the AAR and Southeast Indian Ridge to deter-
mine the exact nature and locations of the Zealandia-Antarctic
domain boundaries.

Online content

Any methods, additional references, Nature Research reporting summaries, source data, statements of data availability and associated accession codes are available at https://doi.org/10.1038/s41561-018-0292-4.

Received: 16 April 2018; Accepted: 17 December 2018;
Published online: 28 January 2019

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### Acknowledgements

This study was supported by KOPRI grant nos PP13040 and PE18050 to S.-H.P. Support at Harvard, Wyoming, Woods Hole and Tulsa was provided by the National Science Foundation (OCE1259916). J.B.-T. was supported by the French Agence Nationale de la Recherche through grant no. ANR-10-BLAN-0603 (M&Ms—Mantle Melting—Measurements, Models, Mechanisms). S.-S.K. was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A02018632). J.L. was also supported by the CAS through grant no.Y4SL021001 and NSFC through grant no. 91628301. We thank the captain and crew of the icebreaker RV *Aaron* for support under difficult sea conditions. We appreciate the constructive reviews by J. Morgan and P. Kempton.

### Author contributions

S-H.P. led the KOPRIDGE project, which included three cruises, interpreted the data and wrote the first draft of the manuscript. C.H.L. contributed to the initial stage of
cruise planning, geochemical interpretations and manuscript preparation and editing, and participated in the 2011 cruise. K.W.W.S. oversaw the isotopic analyses by S.R.S. and contributed to the geochemical interpretations and manuscript preparation and editing. J.B.-T. oversaw and participated in the Hf isotopic analyses by S.R.S. and contributed to the geochemical interpretations and manuscript preparation and editing. S.-S.K. contributed to the cruise design, performed geophysical data analyses and interpretations, and contributed to manuscript writing and editing. S.R.S. performed the Sr, Nd, Hf and Pb isotopic analyses and contributed to the table preparation, geochemical interpretations and manuscript editing. J.L. contributed to the cruise design, performed geophysical data analyses and interpretations, participated in the 2011 cruise and contributed to manuscript editing. H.C. and Y.-S.Y. contributed to the cruises and produced the relevant figures and maps. P.J.M. was involved in the cruise planning, geochemical interpretation and manuscript editing. All authors discussed the results and commented on the manuscript.

**Competing interests**

The authors declare no competing interests.

**Additional information**

**Supplementary information** is available for this paper at https://doi.org/10.1038/s41561-018-0292-4.

**Reprints and permissions information** is available at www.nature.com/reprints.

**Correspondence and requests for materials** should be addressed to S.-H.P.

**Publisher's note:** Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

© The Author(s), under exclusive licence to Springer Nature Limited 2019
## Methods

Samples were recovered by wax coring. Most samples are very fresh slightly phytic basaltic glasses. According to unpublished chemical data, all the samples are tholeiitic MORB.

**Sr, Nd and Pb isotopes.** Sr, Nd and Pb isotopic compositions were measured at the University of Wyoming High-Precision Isotope Laboratory using a ThermoFisher Scientific NEPTUNE Plus multicollector inductively coupled plasma mass spectrometer. Analyses were done on 1–2 mm basalt glass chips leached in 6 M HCl at 75 °C for 45 min to remove potential seawater precipitates or artefacts, then repeatedly rinsed in pure water. The efficacy of this leaching is discussed explicitly in the supplemental information of Sims et al.⁵⁶. Samples were dissolved using a mixture of concentrated HF/HNO₃ followed by 6 M HCl. Pb was purified using standard anion exchange chromatography in HBr following Strelow and Toerien⁵⁷. The bulk sample matrix was collected from the Pb purification procedure for analysis of Sr and Nd. Sr and rare earth element fractions were separated using a cation exchange column in HCl. The Sr and rare earth element fractions that contained Nd were subsequently purified using a SrSpec resin column and an LnSpec resin column, respectively.

Samples were dissolved in 5% HNO₃ solutions for all isotopic analyses. Samples were introduced into the plasma using either an Apex or Aridus II desolvating nebulizer. Sr isotopes were measured in the static mode using four Faraday collectors, with additional collectors used to monitor Rb and Kr interferences. Ratios were normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 to correct for instrumental mass bias, and final data are reported relative to NBS987 ⁸⁷Sr/⁸⁶Sr = 0.71024. Nd isotopes were measured in the static mode using seven Faraday collectors, with additional collectors used to monitor the Ce and Sm interferences. Ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 to correct for instrumental mass bias, and final data are reported relative to La Jolla ¹⁴³Nd/¹⁴⁴Nd = 0.51185. The total procedural blanks for Sr and Nd were <100 pg.

The United States Geological Survey (USGS) reference materials analysed as unknowns were used for quality assurance and to establish realistic estimates of the University of Wyoming's High Precision Laboratory's long-term reproducibility. The long-term reproducibility beginning in June 2013 until the present for BCR-2 is ⁸⁷Sr/⁸⁶Sr = 0.705002 ± 0.000020 (2 s.d., *n* = 53) and ¹⁴⁹Nd/¹⁴⁴Nd = 0.512634 ± 0.000012 (2 s.d., *n* = 14). Values for BCR-2 analysed during the AAR analytical sessions from February to December 2014 were ⁸⁷Sr/⁸⁶Sr = 0.705006 ± 0.000013 (2 s.d., *n* = 6) and ¹⁴⁳Nd/¹⁴⁴Nd = 0.512629 ± 0.000005 (*n* = 2). The long-term reproducibility beginning in October 2013 until the present for BHVO-2 is ⁸⁷Sr/⁸⁶Sr = 0.703466 ± 0.000027 (2 s.d., *n* = 15) and ¹⁴³Nd/¹⁴⁴Nd = 0.512981 ± 0.000010 (2 s.d., *n* = 30). Values for BHVO-2 analysed during the AAR analytical sessions from February to December 2014 were ⁸⁷Sr/⁸⁶Sr = 0.703482 ± 0.000006 (*n* = 1) and ¹⁴⁴Nd/¹⁴⁴Nd = 0.512985 ± 0.000006 (*n* = 3). The long-term reproducibility beginning in July 2014 until the present for BIR-1a is ⁸⁶Sr/⁸⁶Sr = 0.703092 ± 0.000034 (2 s.d., *n* = 9) and ¹⁴⁹Nd/¹⁴⁴Nd = 0.513089 ± 0.000017 (2 s.d., *n* = 4). Values for BIR-1a analysed during the AAR analytical sessions from February to December 2014 were ⁸⁷Sr/⁸⁶Sr = 0.703102 ± 0.000053 (2 s.d., *n* = 3) and ¹⁴³Nd/¹⁴⁴Nd = 0.513100 ± 0.000019 (*n* = 1). The long-term reproducibility beginning in March 2014 until the present for W-2a is ⁸⁷Sr/⁸⁶Sr = 0.706983 ± 0.000024 (2 s.d., *n* = 6) and ¹⁴⁳Nd/¹⁴⁴Nd = 0.512516 ± 0.000011 (2 s.d., *n* = 15). Values for W-2a analysed during the AAR analytical sessions from February to December 2014 were ⁸⁷Sr/⁸⁶Sr = 0.706989 ± 0.000016 (*n* = 2) and ¹⁴⁹Nd/¹⁴⁴Nd = 0.512513 ± 0.000009 (2 s.d., *n* = 4). These and additional values measured during the AAR analytical sessions and GEOREM values for the same USGS reference materials are given in Supplementary Table 2.

For the measurement of Pb isotopic compositions, Tl was added to each sample to correct for mass fractionation with a Pb/Tl ratio target of ~3/1. Pb and Tl isotopes were analysed in the static mode using six Faraday collectors with ratios normalized to ²⁰³Tl/²⁰⁵Tl = 0.418922 to account for instrumental mass bias. A seventh Faraday collector was used to monitor Hg. Pb isotope ratios are reported relative to the NBS981 values of Thirlwall³⁰. The total procedural blanks for Pb were <40 pg. The internal precisions for ²⁰⁰Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb were generally <60 ppm. Long-term reproducibility beginning in June 2013 until the present for BCR-2 is ²⁰⁰Pb/²⁰⁴Pb = 18.7565 ± 0.0076, ²⁰⁷Pb/²⁰⁴Pb = 15.6288 ± 0.0047, ²⁰⁸Pb/²⁰⁴Pb = 38.7333 ± 0.0201, ²⁰⁷Pb/²⁰⁶Pb = 0.83325 ± 0.00015 and ²⁰⁸Pb/²⁰⁶Pb = 2.06505 ± 0.00085 (2 s.d., *n* = 21). Values for BCR-2 analysed during the AAR analytical sessions from February to December 2014 were ²⁰⁶Pb/²⁰⁴Pb = 18.7660 ± 0.0411, ²⁰⁷Pb/²⁰⁴Pb = 15.6286 ± 0.0068, ²⁰⁸Pb/²⁰⁴Pb = 38.7570 ± 0.0822, ²⁰⁷Pb/²⁰⁶Pb = 0.83282 ± 0.00099 and ²⁰⁸Pb/²⁰⁶Pb = 2.06528 ± 0.00057 (2 s.d., *n* = 5). The long-term reproducibility beginning in June 2014 until the present for BHVO-2 is ²⁰⁶Pb/²⁰⁴Pb = 18.6284 ± 0.0398, ²⁰⁷Pb/²⁰⁴Pb = 15.5448 ± 0.0145, ²⁰⁸Pb/²⁰⁴Pb = 38.2400 ± 0.0232, ²⁰⁷Pb/²⁰⁶Pb = 0.83447 ± 0.00123 and ²⁰⁸Pb/²⁰⁶Pb = 2.05278 ± 0.00182 (2 s.d., *n* = 7). Values for BHVO-2 analysed during the AAR analytical sessions from February to December 2014 were ²⁰⁶Pb/²⁰⁴Pb = 18.5975 ± 0.0005, ²⁰⁷Pb/²⁰⁴Pb = 15.5476 ± 0.0005,

²⁰⁸Pb/²⁰⁴Pb = 38.2165 ± 0.0013, ²⁰⁷Pb/²⁰⁶Pb = 0.836006 ± 0.000010 and

²⁰⁸Pb/²⁰⁶Pb = 2.054926 ± 0.000032 (*n* = 1). The long-term reproducibility beginning in June 2014 until the present for BIR-1a is ²⁰⁶Pb/²⁰⁴Pb = 18.8580 ± 0.0042,

²⁰⁷Pb/²⁰⁴Pb = 15.6654 ± 0.0030, ²⁰⁸Pb/²⁰⁴Pb = 38.5111 ± 0.0106,

²⁰⁷Pb/²⁰⁶Pb = 0.830705 ± 0.000014 and ²⁰⁸Pb/²⁰⁶Pb = 2.042162 ± 0.000052 (2 s.d., *n* = 3). Values for BIR-1a analysed during the AAR analytical sessions from February to December 2014 were ²⁰⁸Pb/²⁰⁴Pb = 18.8556 ± 0.0005,

²⁰⁷Pb/²⁰⁴Pb = 15.6637 ± 0.0005, ²⁰⁸Pb/²⁰⁴Pb = 38.5051 ± 0.0013,

²⁰⁷Pb/²⁰⁶Pb = 0.830721 ± 0.000008 and ²⁰⁸Pb/²⁰⁶Pb = 2.042105 ± 0.000026 (*n* = 1).

The long-term reproducibility beginning in February 2014 until the present for W-2a is ²⁰⁶Pb/²⁰⁴Pb = 18.7490 ± 0.0035, ²⁰⁷Pb/²⁰⁴Pb = 15.6646 ± 0.0029,

²⁰⁷Pb/²⁰⁴Pb = 38.6271 ± 0.0100, ²⁰⁷Pb/²⁰⁶Pb = 0.83549 ± 0.00023 and

²⁰⁸Pb/²⁰⁶Pb = 2.06022 ± 0.00067 (2 s.d., *n* = 4). Values for W-2a analysed during the AAR analytical sessions from February to December 2014 were ²⁰⁸Pb/²⁰⁴Pb = 18.7481 ± 0.0013, ²⁰⁷Pb/²⁰⁴Pb = 15.6643 ± 0.0013,

²⁰⁸Pb/²⁰⁴Pb = 38.6280 ± 0.0034, ²⁰⁷Pb/²⁰⁶Pb = 0.83551 ± 0.0001 and

²⁰⁸Pb/²⁰⁶Pb = 2.06037 ± 0.00071 (*n* = 2). We estimate the long-term reproducibility of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at ~200–300 ppm based on these and other standard measurements. Reproducibilities for the ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb are correspondingly similar. All the USGS reference materials data described above and measured during the AAR analytical sessions, GEOREM values and additional literature values for the same USGS reference materials are given in Supplementary Table 2.

**Hf isotopes.** Hf isotopic compositions were measured at the Ecole Normale Supérieure de Lyon using a Nu Plasma high-resolution multicollector inductively coupled plasma mass spectrometer and following the protocol of Blichert-Toft et al.⁵⁹. Analyses were done on 1–2 mm basalt glass chips leached in hot 6 M HCl to remove potential seawater precipitates or artefacts, and then repeatedly rinsed in pure water. Samples were dissolved in a 3:1 mixture of concentrated HF:HNO₃, and then Hf was leached from the residue using concentrated HF. The samples were centrifuged and the HF solution, which contained Hf, decanted. The HF fraction was purified using an anion-exchange column in HCl/HF and Hf collected with 6 M HCl followed by a cation-exchange column in HCl:H₂O₂ and Hf collected with HCl:HF.

Samples were introduced into the plasma in 0.05 M HNO₃ for analysis of the Hf isotopic compositions. Hf isotopes were measured in the static mode using five Faraday collectors, with four additional collectors used to monitor the Lu, Yb, W and Ta isobaric interferences. Ratios were normalized to ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325, and repeat analysis of the JMC-475 Hf isotope standard gave

¹⁷⁹Hf/¹⁷⁷Hf = 0.282167 ± 0.000016 (2 s.d., *n* = 51).

The long-term reproducibility is ~20–30 ppm. The in-run errors were half that for all samples. Hafnium isotope data for USGS reference materials measured at the same time as the AAR samples are listed in Supplementary Table 2.

**Note on normalization.** Supplementary Table 3 provides standard analytical information for the data sets used here. These data show that it is not necessarily possible to renormalize all data to a single value due to differences in the 'standards' used for analysis and in some cases a lack of reported isotopic values obtained for such standards. We chose not to renormalize literature data for this reason. However, many of these data sets are highly important for our study and the comparison of isotopic compositions across ocean basins. The isotopic variation observed across these ocean basins is much greater (order of magnitude or greater) than the variation associated with different standard normalizations, and our interpretations based on the reported data are robust.

## Data availability

The authors declare that the data supporting the findings of this study are available in Supplementary Tables 1 and 2.

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