Articles https://doi.org/10.1038/s41561-018-0292-4

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

Sung-Hyun Park 1*, Charles H. Langmuir2, Kenneth W. W. Sims3, Janne Blichert-Toft4, Seung-Sep Kim5, Sean R. Scott3, Jian Lin6,7, Hakkyum Choi 1, Yun-Seok Yang1 and Peter J. Michael8

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 sepa- rate 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.

sotopic compositions of oceanic basalts constrain mantle com- Expeditions to terra incognita position, evolution and convective flow1,2. Basalts from Indian In 2011 and 2013, the Korean icebreaker RV Aaron conducted mid-ocean ridges, for example, have higher 208Pb/204Pb at a given three scientific expeditions to map and sample two segments in the I206 204 Pb/ Pb than those from the East Pacific Rise (EPR) and the middle of the AAR—KR1 and KR2 (Fig. 1). These cruises provided Pacific–Antarctic Ridge (PAR)3–5. Also, the Indian ridges are distin- the first samples and detailed mapping of the remote AAR, and guished by having higher 176Hf/177Hf for a given 143Nd/144Nd (refs. 5,6). filled an important gap in our knowledge of the crust and mantle These isotopic differences imply the existence of ocean-scale upper- in this region. mantle domains with limited interactions over long periods of time7–9, The KR1 and KR2 segments have intermediate spreading rates of and raise the question of the location of their boundaries. In the pres- ~70 mm yr–1. Segment KR1 is a 300 km long supersegment (Fig. 1a) ent case, the Australian–Antarctic Discordance (AAD) was proposed that exhibits large along-axis topographic differences from 1,800 to to be the boundary between the Indian and Pacific mantle domains 2,800 m. The axial depth and morphology of KR1 vary from west because it constitutes a remarkably deep portion of the global mid- to east, and range from an axial plateau in the westernmost region ocean ridge system that demarks abrupt changes in isotopic ratios to an axial rift, a narrow axial high, a flat axis and finally a well- from Indian to Pacific values. There remained, however, a broad developed rift valley in the easternmost region (Fig. 1b). Segment gap in sampling between the AAD and the Pacific ridges to the east, KR2, located northwest of KR1, has a western axial high and eastern which leaves the definitiveness of the boundary in question. In their rift valley. These segments exhibit shallower axial depths (1,800– study of the PAR, Vlastélic et al.4 demonstrated that the Pacific upper 2,800 m) than the mean axial depth (3,000 m) of ocean ridges of mantle is not homogeneous, but consists of two distinct subdomains intermediate spreading rates10. separated by the Pacific Superswell. However, this finding did not bring resolution to the unsettled demarcation of the striking isotopic A unique isotopic signature in the Southern Ocean contrast between the Indian and Pacific mantle domains. The isotopic compositions of Sr, Nd, Hf and Pb for the KR1 and KR2 The last gap in the mapping and sampling of seafloor spread- ridge segments are listed in Supplementary Data Table 1. 206Pb/204Pb ing centres worldwide was the Australian–Antarctic Ridge (AAR), values for KR1 vary between 18.7 and 19.1 from the west to the mid- a 2,000 km expanse in the most remote parts of the ocean ridge sys- dle of the segment and gradually decrease eastward to ~18.4 (Fig. 1c). tem. The collection and isotopic measurements of samples from the 206Pb/204Pb values for KR2 are ~18.4, except for the westernmost AAR provide answers to two key questions. Does the AAR, which sample which has a value of 19.2. Relative variations in the Hf isoto- lies to the east of the AAD, have a composition as predicted akin to pic compositions (expressed here as εHf =​ +​12.8 to +18.3)​ are almost 206 204 that of the Pacific? And is the AAD the major boundary between the a mirror image of the Pb/ Pb along-axis variations (Fig. 1d). εNd 2 Pacific and Indian suboceanic mantle domains? We show here that varies from +​7.4 to +​10.1 and is well correlated (R =​ 0.93) with εHf. "no" is the answer to both questions. Instead, our new data reveal εHf to the east of 159° 50′​ E is very high (up to +​18.3) compared to the the existence of a new mantle domain and constrain its origin. majority of Pacific-type depleted mantle (up to +​17)6,11. In contrast,

1Korea Polar Research Institute, Incheon, Republic of Korea. 2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA. 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. 7Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China. 8Department of Geosciences, University of Tulsa, Tulsa, OK, USA. *e-mail: [email protected]

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140° E 160° E 180° 120° E Tasmania AUS NZ 40° S AADZone A Zone C a SEIR AAR PAC 50° S KR2 KR1 59° 30 S ′ 61° 30′ S PAR 60° S Balleny Island Flat axis Rift valley ANT 70° S Ross Sea 62° 00′ S Mt Erebus Axial high Depth (m) 60° 00′ S 80° S

–2,000

–3,000 62° 30′ S

Axial plateau Axial rift

60° 30′ S 0 50 km 0 50 km 63° 00′ S

152° E 153° E 156° E 157° E 158° E 159° E 160° E 161° E b

–2 –2

Axial Depth (km) Axial high Axial rift Axial plateau Axial high Flat axis Rift valley rift

–3 –3 c

19 19 Pb 204

Pb/ All-MORB 206

18 18 d 18 18

16 16 Hf ε

14 14

12 12 152° E 153° E 156° E157° E 158° E 159° E 160° E 161° E

206 204 Fig. 1 | Bathymetry and along-axis variations in depth, Pb/ Pb and εHf 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 segments43. 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 50 Centre, which suggests the KR1 and KR2 segments are supplied by heterogeneous mantle as for ridges that interact with a plume . d, The εHf 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 εHf 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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a b 0.88

0.87 NHRL 39 NHRL 0.86 SPRL

0.85 Pb Pb 0.84 204 206 Pb/ Pb/ 0.83 208 38 207

KR1 and KR2 0.82 Indian ridges AAD 0.81 Zone A PAR EPR 0.80 Easter segment 37 0.79 17.6 18.0 18.4 18.8 19.2 19.6 2.00 2.02 2.04 2.06 2.08 2.10 2.12 206Pb/204Pb 208Pb/206Pb c d 24 22

22 20 Indian Pacific 20 18 18

16 Hf Hf ε

16 ε

14 14

12 12

10 Mantle array 10

8 567891011120.7022 0.7024 0.7026 0.7028 0.7030 0.7032

87 86 εNd Sr/ Sr

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 53 5 defined as εHf =​ 1.59εNd +​ 1.28 (ref. ). The dividing line for the Indian and Pacific mantle domains is from Kempton et al. . The KR1 and KR2 data mostly fall within the Indian mantle domain. In addition, the KR1 and KR2 samples have a higher εHf than the mantle array for a given εNd, 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 87 86 εHf, the PAR is the lowest in Sr/ Sr, the EPR the second lowest, the KR1 and KR2 segments the third lowest and the Indian MORB the highest. At a given 87 86 εHf, Zone A is slightly higher in Sr/ Sr than the PAR, yet lower than the KR1 and KR2 segments. Zone A is closer to the KR1 and KR2 segments in Pb 87 86 isotope composition and εHf and εNd, but different for εHf​ and Sr/ Sr, 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 Pacific and Indian mantle compositions. At a given 206Pb/204Pb, the 208 204 low values of εNd and εHf. Note that KR1 encompasses essentially the KR1 and KR2 samples have higher Pb/ Pb than Pacific samples, 208 204 full range of εHf values for Pacific mid-ocean ridge basalts (MORB) and lower Pb/ Pb than Indian samples (Fig. 2a). An effective 208 204 in just one segment. KR2 has εHf values from +15.6​ to +​ 16.7 and discriminant is Δ​8/4, which is the magnitude of the Pb/ Pb εNd values from +​9.0 to +​10.1 except for the westernmost sample, divergence from a regression line based on Northern Hemisphere 12 for which εHf is +​13.6 and εNd is +​7.7. The combined isotopic ratios samples (Fig. 2a). The KR1 and KR2 segments have Δ​8/4 of +​10 to of Sr, Nd, Hf and Pb permit a robust comparison of the KR1 and +​18, higher than both the PAR (−​30 to +10)​ and EPR (−​10 to +​10), KR2 segments with the Indian and Pacific ridges (Figs. 1 and 2), but lower than the Indian ridges (greater than +​22 and up to +​80) and show that the former are distinct from both the neighbouring (Fig. 3a). At a given 208Pb/206Pb, all the KR1 and KR2 samples have

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a 160 (Fig. 4), which suggests that the mixing zone does not extend to SWIR CIR SEIR AAD AAR PAR EPR 120 Zone A these regions. Zone A to the east of the AAD was previously considered to be 80 the geographical beginning of the Pacific mantle domain5–7, but this view is now no longer supported by the currently available isotope

∆ 8/4 40 data. Zone A samples are distinct from the Indian ridge samples 0 to the west (Figs. 2–4) and Pacific samples to the east, being inter- mediate in composition to the new AAR data reported here and −40 data from the EPR and PAR. The margin between Zone A and the b 80 KR2 segment establishes the western boundary of the mixing zone (thick black dashed lines in Fig. 5a). To determine more precisely 0 the extent and characteristics of the mixing zone will require future −80 systematic sampling between the east of the KR1 segment and the PAR, and between Zone A and the west of the KR2 segment (data ∆ 7/6 −160 IR gaps in Fig. 5a). AAD Zone A All the samples between Zone A and the west end of the PAR fall −240 AAR PAR below the SPRL with negative Δ7/6​ values. This region (inside the EPR −320 pink dashed lines in Fig. 5a) is entirely distinct from both the Indian and Pacific mantle domains as apparent from the discrete clusters 0° 60° E 120° E 180° 120° W in ​7/6– ​8/4 space as a function of longitudinal position (Figs. 3 Longitude Δ Δ and 5b). The large expanse in both space and time is consistent with an extensive, distinct mantle domain, for which we propose Fig. 3 | Δ8/4 and Δ7/6 variations. a,b, Variations in the Δ​8/4 the name 'Zealandia–Antarctic Swell'. The presence of this new (a) and Δ​7/6 (b) values for samples collected along mid-ocean mantle domain indicates that the AAD is not, as previously thought, ridges and plotted as a function of longitude. Δ​7/6 is defined as 207 206 207 206 4 the boundary between the Indian and Pacific suboceanic mantles. [( Pb/ Pb)Sample – ( Pb/ Pb)SPRL] ×​ 10 . CIR, Central Indian Ridge; SEIR, Southeast Indian Ridge; SWIR, Southwest Indian Ridge; IR, Indian ridges. 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 lower 207Pb/206Pb values than those of Pacific samples (Fig. 2b). and other volcanic rocks from neighbouring regions (Mt Erebus, Another powerful discriminant is Δ​7/6, which is the deviation of a Ross Island, WARS and so on), we propose that the Zealandia– given sample from a southern Pacific reference line (SPRL) defined Antarctic domain was initiated by the superplume associated with here as the best fit to the 207Pb/206Pb–208Pb/206Pb variations of the the break-up of Gondwana32,33. Subsequent active mantle upwell- PAR based on the least squares method (Fig. 2b). The Δ7/6​ of PAR ing in the region includes both rifting, as represented by the AAR, samples is between −​10 and +10,​ whereas KR1 and KR2 segments and asthenospheric channelling from present-day plumes, such as show negative values between −​70 and −​120 (Figs. 2b and 3b). The Balleny and Ross Islands14,34,35. KR1 and KR2 segments also plot on the Indian side of the 'Indian/ The plume model is supported by the isotopic compositions of 5 Pacific' dividing line in εNd–εHf space (Fig. 2c) , whereas at a given the Hikurangi Seamounts. The Hikurangi data lie along the mixing value of εHf, the KR1 and KR2 samples have Sr isotope composi- trajectories that connect the AAR and surrounding regions (Fig. 4). tions intermediate to those of the Pacific and Indian ridges (Fig. 2d). For example, the εNd–εHf diagram (Fig. 4c) shows that the isotopic These differences demonstrate that the KR1 and KR2 segments have variations of Hikurangi and WARS samples are better explained a unique isotopic signature that is distinct from both Pacific and by mixing with the depleted KR1 and KR2 component than with Indian ridge samples. the PAR source. Deviation of the Hikurangi Seamounts and some Comparison with other data in the region suggests a wide- Zealandia samples from the mixing relationship in Nd–Pb iso- spread distribution of this distinctive composition (Figs. 4 and 5a). tope space may be due to contamination by subcontinental litho- Volcanic samples from Ross Island13,14, the West Antarctic Rift spheric mantle (Fig. 4d)32, which is often observed in volcanic rocks System (WARS)15–18, Balleny and Scott Islands19, Macquarie Island20 erupted during initial stages of rifting. The Hikurangi Seamounts and Zealandia21–24 exhibit coherent relationships with the KR1 and were formed by the superplume that resulted in the break-up of KR2 segments (Fig. 4). The data are consistent with mixing an New Zealand and Antarctica at ~90 Ma32,33. Based on plate recon- enriched component with high Pb isotope ratios akin to high-mu struction models36, the Hikurangi Seamounts erupted at locations (HIMU), found almost ubiquitously in the region’s Cenozoic alka- similar to those of the KR1 and KR2 segments at ~90 Ma, which line volcanic rocks, with a depleted mantle component distinct from implies that this particular mantle domain may be associated with both the Pacific and Indian suboceanic mantle domains. Although the onset of the New Zealand and Antarctica break-up caused by most of the regional Cenozoic alkaline volcanism was not derived the superplume32,33. from an ocean ridge, it is still isotopically coherent with the AAR Alternatives to the plume model have been put forward by other samples. For example, the active Mt Erebus and neighbouring vol- studies that suggest the enriched component may be derived from canoes on Ross Island, which are part of the WARS13,14 and hence of the overriding continental lithosphere contaminated by the sub- the same isotopic grouping, display the same mixing relationship duction of the Pacific and Phoenix plates18,37. This metasomatically between an old enriched HIMU-like component sourced in the enriched lithospheric mantle model18,37, however, cannot explain the deep mantle and the depleted upper mantle, which thereby defines isotopic coherence among the KR1 and KR2 segments, Ross Island a mixing zone (Fig. 5a). and the Hikurangi Seamounts (Fig. 4c). As the Hikurangi Seamounts The geographical limits of this mixing zone can be delin- erupted on the oceanic lithosphere subducting along the former eated based on the isotopic compositions of lavas from Marie Gondwana margin32,38, the compositional influence of the subcon- Byrd Land25,26, East Australia27–31 and Zone A. The Marie Byrd tinental material on the Hikurangi volcanism would be minimal. Land and East Australia samples show slightly different trends Moreover, it is physically implausible that the lithospheric material from those of the KR1 and KR2 segments in multi-isotope space could influence midocean ridge volcanism 800 km away from both

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a b 41 0.90

HIMU 0.88 SPRL NHRL Approximate location of mixing line through AAR and Hikurangi 40 0.86 EM2 Approximate location of mixing line EM1 0.84 through AAR and Hikurangi Lithospheric Pb Lithospheric Pb interaction 204 39 interaction 206 EM2 Pb/ Pb/ 0.82 208 207

0.80 EM1 38 IR HIMU PAR 0.78 EPR AAD 0.76 37 17 18 19 20 21 22 1.92 1.96 2.00 2.04 2.08 2.12 2.16 206Pb/204Pb 208Pb/206Pb

c d 12 18 2% 16

14 2% 8 12 10% 10% HIMU 10 Hf Nd ε ε 20% 20% 8

6 4 40% 40% IR Lithospheric interaction 4 PAR 60% 60% 80% EPR 2 100% 80% AAD EM1 100% 0 0 98765432 10 11 17 18 19 20 21

εNd 206Pb/204Pb

KR1 and KR2 Balleny Zone A Zealandis−WARS−McQuarie Hikurangi Marie Byrd Land Ross Island (including Mt Erebus) Marie Byrd Seamounts East Australia

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​ (238U/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.

the Zealandia and Antarctica continents. In addition, the influence Zealandia–Antarctic domain corresponds to a widespread and deep of any pre-existing lithospheric component would be eliminated by province of low seismic velocities beneath the AAR (Fig. 6). The thermal erosion during rifting39 or deep mantle upwelling14. upper mantle (<250​ km deep) below this region has low S-wave Further support for a deep source of the Zealandia–Antarctic velocities40–42, which implies that it may be hotter than the neigh- Swell comes from seismic tomography. The isotopically defined bouring Pacific and Indian mantle domains. The shallow axial

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180° a 52.4 b 30° S S Pacific Hikurangi Plateau 50 150° E 4–0.9 Hikurangi Seamounts

Zealandia 98.7 150° W 35 3.2 89.2–66.9 92.3–90.6 Indian East Australia 38.1–36.4 63 85–82, 41–35, 6–3 55–44 8.7 Chatham Island −50 50–45 28 21.8-12.956.5-5.2 14.5–2.6 54–30 65 12–6

39.5–33.6 ∆ 7/6 26–14 20.7 55–36 Mixing zone 34.3–11.10.8 −150 Antipodes Island Hollister Ridge 30° S 'region of influence' 0.3–0.17 3.7–3.3 16.7–15.2 3.4 Zealandia 46–25 Auckland Island Campbell Island −250 –Antarctic ~16.8, 8.5–6.5 Vacquier FZ 8/4 Tasmania ∆ Udintesv FZ Macquarie Island 11.5–9.7 −80 −40 0 40 80 Macquarie George V FZ Balleny FZ Erebus FZ Tasman FZ T.J. PAR Menard FZ Tharp FZ

Heezen FZ 20 Scott Island AAR 20–10 Zone A Balleny ls. Marie Byrd Seamounts

120° E AAD 15–5 8.1–3.1 65–56 Tranant. Mountains 90° W Northern Ross Island Marie Victoria ~0 (Mt Erebus) Chile Ridge Byrd De Gerlache Land WARS Land Seamounts Zone C SEIR 9.3–1.8 2–0.23 22 Peter Island

KR1 and KR2 Zealandia–WARS–Macquarie Balleny–Scott Ross Island (and Mt Erebus) Depth (m) Hikurangi Seamounts East Australia Marie Byrd Land Marie Byrd Seamounts 0

IR AAD Zone A PAR PAR (∆7/6 < –25) EPR −8,000 −6,000 −4,000 −2,000

2,000

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 event24. The relatively young ages of the Cenozoic volcanism suggest geochemical signatures that correspond to their mantle source24. Furthermore, as the Antarctic Plate 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 region14,36. 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 model36, 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)32,38. 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 margin25,32,33,38. 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 Δ​8/4–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 upwellings would rise and mix with the shallower asthenospheric hotter mantle beneath this region43. Similarly, the mantle below the mantle, with some contamination by subcontinental lithospheric 44 14 WARS has isolated low P-wave and S-wave velocities (VS) down mantle where it is present. to a depth of ~1,200 km, apparently related to current Balleny/Ross Island volcanism. Mixing between plume components and depleted Domain boundaries and their significance asthenosphere in the middle of the newly identified mantle domain The new data demonstrate that the eastern boundary of the AAD is (the mixing zone discussed above) may well be driven by the not the boundary between the Indian and Pacific mantle domains, Balleny/Ross Island plume activity. as believed for the past three decades5–9. However, it remains the This model is broadly consistent with the concept of astheno- eastern boundary of the Indian Ocean mantle domain. It may spheric mixing induced by mantle upwelling34,35. Based on calcula- be that the AAD today reflects an earlier boundary between the tions of absolute plate motion and relative mantle plume strength, Pacific and Indian mantle regions, perhaps associated with sub- the plume-fed asthenosphere models34,35 predict a 'region of influ- duction along the Gondwana margin. The break-up of Gondwana ence' for the Balleny/Ross Island plume that covers at least the and the initiation of the Zealandia–Antarctic Swell created western extent of the newly proposed mixing zone. Deep mantle a new domain that separated the Pacific from the Indian mantle.

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30° S 30° S 30° S 150° E 150° E 150° E 30° S 30° S 30° S +3%

150° W 150° W 150° W S

PAR PAR PAR dln V Zone A Zone A Zone A AAR AAR AAR Scott AAD AAD Balleny AAD –3% SEIR SEIR Ross SEIR

30° S 30° S 150° E 150° E 30° S 150° E 30° S 30° S 30° S +2%

150° W 150° W 150 °W S PAR PAR PAR dln V Zone A Zone A Zone A AAR AAR AAR AAD AAD AAD –2% SEIR SEIR SEIR

55 40 Fig. 6 | Maps of shear-wave velocity variations dln VS in the Southern Ocean. Tomography models of SEMUCB-WM1 (left) , S40RTS (centre) and SP12RTS (right)42 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 VS anomalies.

It mixed both east and west to create a mixed Pacific/Zealandia– deep-seated, ancient origin for the enriched end members, with a Antarctic margin on the west adjacent to the AAD. In this context, flow and mixing shallow enough that the spatial extent of upwell- the compositions sampled in Zone A are of particular interest. ing is constrained by tectonic features near the surface. Further They form an intermediate field in radiogenic isotope space tests of this overall framework can be undertaken by further between the Pacific and Zealandia–Antarctic domains, which sampling along the AAR and Southeast Indian Ridge to deter- raises the possibility that the upwelling of Zealandia–Antarctic mine the exact nature and locations of the Zealandia–Antarctic compositions might have mixed with the pre-existing Pacific domain boundaries. mantle to create the distinctive composition observed in Zone A today. A full understanding of the distinct composition of Online content Zone A, however, requires additional sampling between Zone A Any methods, additional references, Nature Research reporting and the KR2 segment. summaries, source data, statements of data availability and asso- Although the current limited sampling along the west and east ciated accession codes are available at https://doi.org/10.1038/ of the AAR does not allow a precise definition of the boundaries s41561-018-0292-4. of the new mantle domain, it appears to be bounded by ancient, large-offset transform faults (Fig. 5). This feature is common to Received: 16 April 2018; Accepted: 17 December 2018; many mantle domains associated with hot spots. The North and Published online: 28 January 2019 South Atlantic basins, in particular, are separated by large equato- References rial transforms. For example, the Hayes Transform is the southern 1. Hofmann, A. W. in Treatise on Geochemistry 2nd edn, Vol. 3 (ed. Carlson, R. W.) boundary of the influence of the Azores hot spot, and the Iceland 67–101 (Elsevier, Amsterdam, 2014). hot spot is bounded by the large-offset Charlie–Gibbs Fracture 2. White, W. M. Probing the Earth’s deep interior through geochemistry. Zone to the south45,46 and the Tjörnes Fracture Zone to the north47. Geochem. Perspect. 4, 95–246 (2015). Similarly, the influence of the Galapagos plume has boundar- 3. Hamelin, B. D. & Allègre, C. J. Large scale regional units in the depleted 48,49 upper mantle revealed by an isotope study of the South-West Indian Ridge. ies associated with ridge offsets . Hence, there appears to be Nature 315, 196–199 (1985). a clear association between major tectonic boundaries, bathymetry 4. Vlastélic, I. et al. Large-scale chemical and thermal division of the Pacifc on the Earth’s surface and the underlying upper mantle domains mantle. Nature 399, 345–350 (1999). as reflected in the isotopic compositions of erupted basalts. 5. Kempton, P. D. et al. Sr–Nd–Pb–Hf isotope results from ODP Leg 187: This general phenomenon is further support for a deep origin of evidence for mantle dynamics of the Australian–Antarctic Discordance and origin of the Indian MORB source. Geochem. Geophys. Geosyst. 3, the Zealandia–Antarctic domain. 1074 (2002). Collectively, these observations suggest a common origin for 6. Hanan, B. B., Blichert-Tof, J., Pyle, D. G. & Christie, D. M. Contrasting many of the world’s mantle domains and their respective boundaries. origins of the upper mantle revealed by hafnium and lead isotopes from the Deep mantle upwellings feed and mix with upper mantle. These Southeast Indian Ridge. Nature 432, 91–94 (2004). upwellings have distinctive histories and inherited isotopic 7. Klein, E. M., Langmuir, C. H., Zindler, A., Staudigel, H. & Hamelin, B. Isotopic evidence of a mantle convection boundary at the Australian– compositions that are both old (Archaean and Proterozoic) and Antarctic Discordance. Nature 133, 623–629 (1988). derived from the deep mantle where mixing is inefficient and 8. Pyle, D. G., Christie, D. M., Mahoney, J. J. & Duncan, R. A. Geochemistry large-scale heterogeneities therefore can be created and preserved and geochronology of ancient southeast Indian and southwest Pacifc seafoor. for long periods of time. The upwellings mix with the astheno- J. Geophys. Res. 100, 22261–22282 (1995). sphere at a level shallow enough that the flow and mixing end up 9. Christie, D. M., West, B. P., Pyle, D. G. & Hanan, B. B. Chaotic topography, bounded by shallow features in the upper mantle that are associ- mantle fow and mantle migration in the Australian-Antarctic discordance. Nature 394, 637–644 (1998). ated with major tectonic boundaries, such as large-offset trans- 10. Gale, A., Dalton, C. A., Langmuir, C. H., Su, Y. & Schilling, J. G. Te mean forms. This overall process then creates mantle domains bounded composition of ocean ridge basalts. Geochem. Geophys. Geosyst. 14, by large tectonic features near the surface. The implications are a 489–518 (2013).

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Nature 525, 95–99 (2015). mantle interaction. Chem. Geol. 136, 33–54 (1997). 30. Zhang, M., O’Reilley, S. Y. & Chen, D. Location of Pacifc and Indian Acknowledgements mid-ocean ridge-type mantle in two time slices: evidence from Pb, Sr, and This study was supported by KOPRI grant nos PP13040 and PE18050 to S.-H.P. Support Nd isotopes for Cenozoic Australian basalts. Geology 27, 39–42 (1999). at Harvard, Wyoming, Woods Hole and Tulsa was provided by the National Science 31. McBride, J. S., Lambert, D. D., Nicholls, I. A. & Price, R. C. Osmium isotopic Foundation (OCE1259916). J.B.-T. was supported by the French Agence Nationale evidence for crust–mantle interaction in the genesis of continental intraplate de la Recherche through grant no. ANR-10-BLAN-0603 (M&Ms—Mantle Melting— basalts from the newer volcanics province, southeastern Australia. J. Petrol. 42, Measurements, Models, Mechanisms). S.-S.K. was supported by Basic Science Research 1197–1218 (2001). Program through the National Research Foundation of Korea (NRF) funded by the 32. Hoernle, K. et al. Age and geochemistry of volcanic rocks from the Ministry of Education (NRF-2017R1D1A1A02018632). J.L. was also supported by the Hikurangi and Manihiki oceanic plateaus. Geochim. Cosmochim. Acta 74, CAS through grant no.Y4SL021001 and NSFC through grant no. 91628301. We thank the 7196–7219 (2010). captain and crew of the icebreaker RV Aaron for support under difficult sea conditions. 33. Storey, B. C. et al. Mantle plumes and Antarctica–New Zealand rifing: We appreciate the constructive reviews by J. Morgan and P. Kempton. evidence from Mid-Cretaceous mafc dykes. J. Geol. Soc. Lond. 156, 659–671 (1999). 34. Yamamoto, M., Phipps Morgan, J. & Morgan, W. J. in Plates, Plumes, and Author contributions Planetary Processes (eds Foulger, G. R. & Jurdy, D. M.) 165–188 (Geol. Soc. S-H.P. led the KOPRIDGE project, which included three cruises, interpreted the data Am. Spec. Paper 430, Geological Society of America, 2007). and wrote the first draft of the manuscript. C.H.L. contributed to the initial stage of

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cruise planning, geochemical interpretations and manuscript preparation and editing, Competing interests and participated in the 2011 cruise. K.W.W.S. oversaw the isotopic analyses by S.R.S. The authors declare no competing interests. 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. Additional information S.-S.K. contributed to the cruise design, performed geophysical data analyses and Supplementary information is available for this paper at https://doi.org/10.1038/ interpretations, and contributed to manuscript writing and editing. S.R.S. performed s41561-018-0292-4. the Sr, Nd, Hf and Pb isotopic analyses and contributed to the table preparation, Reprints and permissions information is available at www.nature.com/reprints. geochemical interpretations and manuscript editing. J.L. contributed to the cruise design, Correspondence and requests for materials should be addressed to S.-H.P. 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 Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in produced the relevant figures and maps. P.J.M. was involved in the cruise planning, published maps and institutional affiliations. geochemical interpretation and manuscript editing. All authors discussed the results © The Author(s), under exclusive licence to Springer Nature and commented on the manuscript. Limited 2019

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Methods 208Pb/204Pb =​ 38.2165 ±​ 0.0013, 207Pb/206Pb =​ 0.836006 ±​ 0.000010 and 208 206 Samples were recovered by wax coring. Most samples are very fresh slightly phyric Pb/ Pb =​ 2.054926 ±​ 0.000032 (n =​ 1). The long-term reproducibility beginning 206 204 basaltic glasses. According to unpublished chemical data, all the samples are in June 2014 until the present for BIR-1a is Pb/ Pb =​ 18.8580 ±​ 0.0042, 207 204 208 204 tholeiietic MORB. Pb/ Pb =​ 15.6654 ±​ 0.0030, Pb/ Pb =​ 38.5111 ±​ 0.0106, 207Pb/206Pb =​ 0.830705 ±​ 0.000014 and 208Pb/206Pb =​ 2.042162 ±​ 0.000052 Sr, Nd and Pb isotopes. Sr, Nd and Pb isotopic compositions were measured at the (2 s.d., n =​ 3). Values for BIR-1a analysed during the AAR analytical sessions 206 204 University of Wyoming High-Precision Isotope Laboratory using a ThermoFisher from February to December 2014 were Pb/ Pb =​ 18.8556 ±​ 0.0005, 207 204 208 204 Scientific NEPTUNE Plus multicollector inductively coupled plasma mass Pb/ Pb =​ 15.6637 ±​ 0.0005, Pb/ Pb =​ 38.5051 ±​ 0.0013, 207 206 208 206 spectrometer. Analyses were done on 1–2 mm basalt glass chips leached in 6 M Pb/ Pb =​ 0.830721 ±​ 0.000008 and Pb/ Pb =​ 2.042105 ±​ 0.000026 (n =​ 1). HCl at 75 °C for 45 min to remove potential seawater precipitates or artefacts, then The long-term reproducibility beginning in February 2014 until the present 206 204 207 204 repeatedly rinsed in pure water. The efficacy of this leaching is discussed explicitly for W-2a is Pb/ Pb =​ 18.7490 ±​ 0.0035, Pb/ Pb =​ 15.6646 ±​ 0.0029, 208 204 207 206 in the supplemental information of Sims et al.56. Samples were dissolved using a Pb/ Pb =​ 38.6271 ±​ 0.0100, Pb/ Pb =​ 0.83549 ±​ 0.00023 and 208 206 mixture of concentrated HF/HNO3 followed by 6 M HCl. Pb was purified using Pb/ Pb =​ 2.06022 ±​ 0.00067 (2 s.d., n =​ 4). Values for W-2a analysed standard anion exchange chromatography in HBr following Strelow and Toerien57. during the AAR analytical sessions from February to December 2014 206 204 207 204 The bulk sample matrix was collected from the Pb purification procedure for were Pb/ Pb =​ 18.7481 ±​ 0.0013, Pb/ Pb =​ 15.6643 ±​ 0.0013, 208 204 207 206 analysis of Sr and Nd. Sr and rare earth element fractions were separated using Pb/ Pb =​ 38.6280 ±​ 0.0034, Pb/ Pb =​ 0.83551 ±​ 0.0001 and 208 206 a cation exchange column in HCl. The Sr and rare earth element fractions that Pb/ Pb =​ 2.06037 ±​ 0.00071 (n =​ 2). We estimate the long-term reproducibility 206 204 207 204 208 204 contained Nd were subsequently purified using a SrSpec resin column and an of Pb/ Pb, Pb/ Pb and Pb/ Pb at ~200–300 ppm based on these and other 208 206 207 206 LnSpec resin column, respectively. standard measurements. Reproducibilities for the Pb/ Pb and Pb/ Pb

Samples were dissolved in 5% HNO3 solutions for all isotopic analyses. Samples are correspondingly similar. All the USGS reference materials data described were introduced into the plasma using either an Apex or Aridus II desolvating above and measured during the AAR analytical sessions, GEOREM values and nebulizer. Sr isotopes were measured in the static mode using four Faraday additional literature values for the same USGS reference materials are given in collectors, with additional collectors used to monitor Rb and Kr interferences. Supplementary Table 2. 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 Hf isotopes. Hf isotopic compositions were measured at the Ecole Normale were measured in the static mode using seven Faraday collectors, with additional Supérieure de Lyon using a Nu Plasma high-resolution multicollector inductively collectors used to monitor the Ce and Sm interferences. Ratios were normalized coupled plasma mass spectrometer and following the protocol of Blichert-Toft et al.59. to 146Nd/144Nd =​ 0.7219 to correct for instrumental mass bias, and final data are Analyses were done on 1–2 mm basalt glass chips leached in hot 6 M HCl to reported relative to La Jolla 143Nd/144Nd =​ 0.51185. The total procedural blanks for remove potential seawater precipitates or artefacts, and then repeatedly rinsed in Sr and Nd were <​100 pg. pure water. Samples were dissolved in a 3:1 mixture of concentrated HF:HNO3, The United States Geological Survey (USGS) reference materials analysed and then Hf was leached from the residue using concentrated HF. The samples as unknowns were used for quality assurance and to establish realistic estimates were centrifuged and the HF solution, which contained Hf, decanted. The Hf of the University of Wyoming’s High Precision Laboratory’s long-term fraction was purified using an anion-exchange column in HCl/HF and Hf collected reproducibility. The long-term reproducibility beginning in June 2013 until with 6 M HCl followed by a cation-exchange column in HCl:H2O2 and Hf collected the present for BCR-2 is 87Sr/86Sr =​ 0.705002 ±​ 0.000020 (2 s.d., n =​ 53) and with HCl:HF. 143 144 Nd/ Nd =​ 0.512634 ±​ 0.000012 (2 s.d., n =​ 14). Values for BCR-2 analysed Samples were introduced into the plasma in 0.05 M HNO3 for analysis of during the AAR analytical sessions from February to December 2014 were the Hf isotopic compositions. Hf isotopes were measured in the static mode 87Sr/86Sr =​ 0.705006 ±​ 0.000013 (2 s.d., n =​ 6) and 143Nd/144Nd =​ 0.512629 ±​ 0.000005 using five Faraday collectors, with four additional collectors used to monitor (n =​ 2). The long-term reproducibility beginning in October 2013 until the the Lu, Yb, W and Ta isobaric interferences. Ratios were normalized to 179 177 present for BHVO-2 is 87Sr/86Sr =​ 0.703466 ±​ 0.000027 (2 s.d., n =​ 15) and Hf/ Hf =​ 0.7325, and repeat analysis of the JMC-475 Hf isotope standard gave 176 177 143Nd/144Nd =​ 0.512981 ±​ 0.000010 (2 s.d., n =​ 30). Values for BHVO-2 analysed Hf/ Hf =​ 0.282167 ±​ 0.000016 (2 s.d., n =​ 51). The long-term reproducibility during the AAR analytical sessions from February to December 2014 were is ~20–30 ppm. The in-run errors were half that for all samples. Hafnium isotope 87Sr/86Sr =​ 0.703482 ±​ 0.000006 (n =​ 1) and 143Nd/144Nd =​ 0.512985 ±​ 0.000006 data for USGS reference materials measured at the same time as the AAR samples (n =​ 3). The long-term reproducibility beginning in July 2014 until the are listed in Supplementary Table 2. present for BIR-1a is 87Sr/86Sr =​ 0.703092 ±​ 0.000034 (2 s.d., n =​ 9) and 143Nd/144Nd =​ 0.513089 ±​ 0.000017 (2 s.d., n =​ 4). Values for BIR-1a analysed Note on normalization. Supplementary Table 3 provides standard analytical during the AAR analytical sessions from February to December 2014 were information for the data sets used here. These data show that it is not 87Sr/86Sr =​ 0.703102 ±​ 0.000053 (2 s.d., n =​ 3) and 143Nd/144Nd =​ 0.513100 ±​ 0.000019 necessarily possible to renormalize all data to a single value due to differences in (n =​ 1). The long-term reproducibility beginning in March 2014 until the ‘standards’ used for analysis and in some cases a lack of reported isotopic the present for W-2a is 87Sr/86Sr =​ 0.706983 ±​ 0.000024 (2 s.d., n =​ 6) and values obtained for such standards. We chose not to renormalize literature 143Nd/144Nd =​ 0.512516 ±​ 0.000011 (2 s.d., n =​ 15). Values for W-2a analysed data for this reason. However, many of these data sets are highly important during the AAR analytical sessions from February to December 2014 were for our study and the comparison of isotopic compositions across ocean basins. 87Sr/86Sr =​ 0.706989 ±​ 0.000016 (n =​ 2) and 143Nd/144Nd =​ 0.512513 ±​ 0.000009 The isotopic variation observed across these ocean basins is much greater (2 s.d., n =​ 4). These and additional values measured during the AAR analytical (order of magnitude or greater) than the variation associated with different sessions and GEOREM values for the same USGS reference materials are given in standard normalizations, and our interpretations based on the reported data Supplementary Table 2. are robust. 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 Data availability The authors declare that the data supporting the findings of this study are available with ratios normalized to 203Tl/205Tl =​ 0.418922 to account for instrumental mass in Supplementary Tables 1 and 2. bias. A seventh Faraday collector was used to monitor Hg. Pb isotope ratios are reported relative to the NBS981 values of Thirlwall58. The total procedural blanks for Pb were <​40 pg. The internal precisions for 206Pb/204Pb, 207Pb/204Pb References and 208Pb/204Pb were generally <​60 ppm. Long-term reproducibility beginning 56. Sims, K. W. W. et al. Short length scale mantle heterogeneity beneath Iceland in June 2013 until the present for BCR-2 is 206Pb/204Pb =​ 18.7565 ±​ 0.0076, probed by glacial modulation of melting. Earth Planet. Sci. Lett. 379, 207Pb/204Pb =​ 15.6288 ±​ 0.0047, 208Pb/204Pb =​ 38.7333 ±​ 0.0201, 146–157 (2013). 207Pb/206Pb =​ 0.83325 ±​ 0.00015 and 208Pb/206Pb =​ 2.06505 ±​ 0.00085 (2 s.d., 57. Strelow, F. W. E. & Toerien, F. von S. Separation of lead(ii) from bismuth n =​ 21). Values for BCR-2 analysed during the AAR analytical sessions (iii), thallium (iii), cadmium(ii), mercury(ii), gold(iii), platinum(iv), from February to December 2014 were 206Pb/204Pb =​ 18.7660 ±​ 0.0411, palladium(ii), and other elements by anion exchange chromatography. 207Pb/204Pb =​ 15.6286 ±​ 0.0068, 208Pb/204Pb =​ 38.7570 ±​ 0.0822, Anal. Chem. 38, 545–548 (1966). 207Pb/206Pb =​ 0.83282 ±​ 0.00099 and 208Pb/206Pb =​ 2.06528 ±​ 0.00057 (2 s.d., 58. Tirlwall, M. Multicollector ICP-MS analysis of Pb isotopes using n =​ 5). The long-term reproducibility beginning in June 2014 until the present a 207Pb–204Pb double spike demonstrates up to 400 ppm/amu systematic for BHVO-2 is 206Pb/204Pb =​ 18.6284 ±​ 0.0398, 207Pb/204Pb =​ 15.5448 ±​ 0.0145, errors in Tl-normalization. Chem. Geol. 184, 255–279 (2002). 208Pb/204Pb =​ 38.2400 ±​ 0.0232, 207Pb/206Pb =​ 0.83447 ±​ 0.00123 and 59. Blichert-Tof, J., Chauvel, C. & Albarede, F. Separation of Hf and 208Pb/206Pb =​ 2.05278 ±​ 0.00182 (2 s.d., n =​ 7). Values for BHVO-2 analysed Lu for high-precision isotope analysis of rock samples by magnetic during the AAR analytical sessions from February to December 2014 sector-multiple collector ICP-MS. Contrib. Mineral. Petr. 127, were 206Pb/204Pb =​ 18.5975 ±​ 0.0005, 207Pb/204Pb =​ 15.5476 ±​ 0.0005, 248–260 (1997).

Nature Geoscience | www.nature.com/naturegeoscience n e w s & vi e w s

Strike-slip earthquakes are not co m monly i n standard seis mic hazard assess me nt. Published online: 4 February 2019 associated with tsu na mi ge neration because Regions such as the Mar mara Sea with https://doi.org/10.1038/s41561-019-0308-8 their grou nd displace me nts are pri marily its sub mari ne expression of t he Nort h h oriz o nt al. T he de a dl y ts u n a mi of t he A n at oli a n Fa ult, t he G ulf of A q a b a wit h t he References mag nitude 7.5 Palu eart hquake t herefore s o ut her n e xte nsi o n of t he L e v a nte Fa ult, or 1. S oc q u et, A., H olli n gs w ort h, J., Pat hier, E. & B o uc h o n, M. c a me as a s ur pris e t o s cie ntists. L a n dsli des the San Andreas Fault near Bodega N at. Ge osci. https://doi.org/10.1038/s41561-018-0296-0 (2019). and sub marine sedi ment slu mps may Bay a nd i n t he To males Bay, may be subject 2. B a o, H. et al. N at. Ge osci. https://doi.org/10.1038/s41561-018- 0297-z (2019). h a ve c o ntri b ute d t o l o c alize d ts u n a mi to si mil ar s ce n ari os as t he m a g nit u de 3. A rc h ulet a, R. J. Ge o p hys. Res. 8 9 , 4559–4585 (1984). w a ve ge ner ati o n, li ke wis e t he h oriz o nt al 7.5 Palu eart hquake. 4. B o uc h o n, M. & Vallée, M. Scie nce 3 0 1 , 824–826 (2003). displace me nt of r ugged seafloor topography. T h e t wo st u dies by S oc q uet et al. 1 a n d 5. D u n h a m, E. M. & Arc h ulet a, R. J. B ull. Seis m ol. S oc. A m. 9 4 , S256–S268 (2004). 2 H o we ver, t he s m all c o m p o ne nt of vertic al B a o et al. de monstrate t hat t he 2018 Palu 6. R o bi ns o n, D. P., D as, S. & S e arle, M. P. Tecto nophysics 4 9 3 , dis pl ace me nt of a n ot her wis e d o mi n a ntl y earthquake ruptured at supershear speeds. 236–243 (2010). strike-slip earthquake, co mbined with wave- The devasti ng local tsu na mi ge nerated i n 7. V allée, M. & D u n h a m, E. M. Ge o p hys. Res. Lett. 3 9 , L05311 (2012). 8. X ia, K., R osa kis, A. J. & Ka na m ori, H. Scie nce 3 0 3 , 1859–1861 (2004). i nterfere nce effects i n t he narro w Palu Bay, Palu must lea d us to reco nsi der t he hazard 9. B r u h at, L., Fa n g, Z. & D u n h a m, E. M. J. Ge o p hys. Res. S oli d E art h increased tsuna mi wave heights, and hence posed by eart hquakes occurri ng i n si milar 1 2 1 , 210–224 (2016). tsuna mi inundation and run-up. tect o nic s etti n gs. ❐ 1 0. K a m mer, D. S., S vetliz k y, I., C o he n, G. & Fi ne b er g, J. Sci. A d v. 4 , Whet her or not t he supershear r upture eaat5622 (2018). 1 1. D u n h a m, E. M. J. Ge o p hys. Res. 1 1 2 , B07302 (2007). aggravated the tsu na mi excitation re mai ns P. M arti n M ai 1 2. G a briel, A. A., A m p uer o, J.- P., D al g uer, L. A. & Mai, P. M. t o b e st u die d, b ut it cert ai nl y affecte d t he F ac ult y of E art h Scie nces, Divisi o n of P hysic al Scie nces J. Ge o p hys. Res. 1 1 7 , B09311 (2012). near-fault shaking levels. Such co mpounded a n d E ngi neeri ng, Ki ng A b d ull a h U niversit y of Scie nce 1 3. B o uc h o n, M. et al. Tecto nophysics 4 9 3 , 244–253 (2010). 1 4. M a d de n, E. H. et al. P h ysics- b as e d c o u ple d m o dels of t he 2 0 1 8 tsuna mi and shaking hazard in marine a n d Tec h n ol og y ( K A U S T), T u w al, S a u di Ar a bi a. Sula wesi earthquake and tsu na mi. In A n n u al F all Meeti ng of t he stri ke-sli p e n vir o n me nts is l ar gel y ne glecte d e- m ail: martin. mai @kaust.edu.sa A merican Geophysical Union Abstract N H23F-3546 ( A G U, 2018).

GEODYNA MICS Map of the under world

A che mically distinct region separates the Indian and Pacific mantle do mains as revealed by isotope analyses on rare sa mples fro m the Australian– Antarctic Ridge. Pa mela D. Ke mpton

art h s cie ntists 6 0 ye ars a g o c o ul d n ot J ul es Ver ne’s s ci e nc e fi cti o n n o vel Jo ur ney G e os cie ntists m a p t his m a ntle a ntici p ate t he p ar a di g m s hift t h at t he to the Ce nter of the Earth , ge os ci e ntists under world 3 a n d rel ate its fe at ures t o E a d ve nt of pl ate tect o nic t he or y w o ul d c a n’t si m pl y tr a vel t here; i nste a d, we rel y E art h’s l o n g hist or y. Ma ntle d o m ai ns bri n g 1 . Plate tecto nic t heory tra nsfor me d o n i n dire ct met h o ds, s u c h as m a ntl e reflect processes dee p wit hi n t he Eart h o ur vie w of b ot h t he E art h’s m a ntle a n d to mography or geoche mical analysis such as: superconti ne nt for mation, break-up geodyna mics. We no w kno w that the of m a ntl e- d eri ve d r o c ks. F or e x a m pl e, a n d dis p ers al; t he f ate of ass o ci ate d i nterior of our pla net is heteroge neous a nd the mineralogy and co mposition of the subducted slabs; and the re-e mergence c o m p os e d of c he mic all y disti nct re gi o ns ma ntle ca n be i nferre d fro m ocea nic of s o me rec ycle d m ateri als vi a m a ntle 4 or d o m ai ns. D ue t o differe nces i n de nsit y basalts, partic ularly t hose basalts t hat were p lu mes and hotspots . F or e x a m ple, bet wee n t hese do mai ns, t he mantle is pr o d u c e d b y p arti al melti n g of t he m a ntl e Indian mid-ocean ridge basalts ( M O R Bs) hi g hl y d y n a mic, m o vi n g t he ri gi d cr ust a n d and erupted along mid-ocean ridges. h ave lo ng bee n recog nize d to have a sh a pi n g t he s urf ace of t he pl a net. Writi n g Is otope ratios are es pecially helpf ul because differe nt c he mic al fi n ger pri nt c o m p are d i n Nat ure Geoscie nce , Par k et al. 2 re p ort t hey do not change duri ng t he process of wit h Pacific a n d Atl a ntic o nes 5 ,6 . A n t h at t he y h a ve i de ntifie d a m a ntle d o m ai n mag ma ge neration, and radioge nic isotopes i mporta nt first-order co nclusio n is t hat be neath a re mote region of the Souther n ca n be use d to place age co nstrai nts o n sec ular evolutio n has pro duced disti nct Ocean that adds to our u nderstandi ng m a ntl e e v ol uti o n. As a res ult of t hes e c o m p ositi o n al d o m ai ns wit hi n t he E art h of m a ntle c o m ple xit y a n d s u g gests a met h o ds, we n o w k n o w t h at E art h’s m a ntl e t hat ca n be observe d o n t he scale of ocea n c o m m o n ori gi n f or m a n y of t he w orl d’s is i ncre di bl y c o m pl e x a n d m a d e u p of b asi ns 7 ,8 . These co mpositional differe nces m a ntle d o m ai ns. c he mi c all y disti nct re gi o ns or m a ntl e must have existe d over lo ng perio ds of Wit h a t hi c k ness of 2, 9 0 0 k m, t he do mai ns t hat result fro m processes such ge ol o gic ti me, s o me ne arl y as l o n g as t he m a ntl e is t he l ar gest p art of o ur pl a net. as s u b d u cti o n a n d m a ntl e pl u mes. Ma ntl e age of t he Eart h 9 . Of c o urs e, disti nct m a ntle Nevert heless, studyi ng its co mpositio n a nd do mai ns are t herefore a record of our do mains must be separated by detectable str u ct ure is h ar d d u e t o its i n a c c essi bilit y pl a net’s d y n a mi c e v ol uti o n o ver billi o ns boundaries. One such boundary — the ( Fi g. 1 ). U nli ke Ott o Li d e n br o c k i n of ye ars ( Fi g. 1 ). Australian– Antarctic Discordance — has

N at ure Ge oscie N c e | V O L 1 2 | M A R C H 2 01 9 | 1 5 0 –1 5 3 | w w w .nature.co m/naturegeoscience 1 51 n e w s & vi e w s

re gi o n is s o re m ote, rel ati vel y fe w s a m ples t he precis e b o u n d aries of t his d o m ai n have bee n collecte d. h a ve yet t o b e l o c ate d, it a p p e ars t h at Par k et al. 2 t a ke a ste p t o w ar ds filli n g tr a nsf or m f a ults m a y b e a reflecti o n t his g a p wit h s a m ples c ollecte d o n t hree of its li mits at t he s urf ace, w hic h scie ntific cr uises aboard t he Korea n s u g gests a li n k b et wee n si g nific a nt icebreaker I B R V Aaro n . T he y s h o w t h at tectonic bou ndaries and u nderlyi ng sa mples fro m the Australian– Antarctic u p p er m a ntle d o m ai ns. Ri d ge, a 2, 0 0 0 k m e x p a ns e t o t he e ast of P ar k et al. 2 h a ve i de ntifie d a m a ntle the Australian– Antarctic Discordance are d o m ai n disti nct fr o m b ot h Pacific a n d disti nct fr o m b ot h t he Pacific a n d I n di a n Indian M O RBs beneath the Southern Ocean sig natures. I nstea d, t he sa mples appear to t h at li kel y ori gi n ate d fr o m t he bre a k- u p of res e m ble ol der C e n oz oic al k ali ne v olc a nic G o n d w a n a. A d diti o n al s a m pli n g fr o m ot her rocks fro m neighbouri ng regions to the A ustr ali a n – A nt arctic Ri d ge s e g me nts will north and south such as Mount Erebus, Ross likely be needed to resolve the geody na mic Isla nd a nd Zeala ndia. These alkali ne rocks s ce n ari o t h at g a ve ris e t o t he ne w m a ntle are wi del y distri b ute d i n t he are a a n d s o me d o m ai n, b ut t he w or k of Par k et al. re affir ms are milli o ns of ye ars ol der, w hic h s u g gests t he i mportance of conti nui ng to explore t he Fi g. 1 | c o m pl e x m a ntl e. E art h’s m a ntl e i s a t h at t his is ot o pic si g n at ure h as p ersiste d vast ex pa nse of Eart h. ❐ v a stl y c o m pl e x r e gi o n of t h e pl a n et t h at i s i n t he m a ntle f or a l o n g p eri o d of ti me. It virt u all y i m p o s si bl e t o s a m pl e dir e ctl y. I n st e a d is o n t his b asis t h at Par k et al. pr o p os e t he Pa mela D. Ke mpton we must rely on indirect sa mples such as e xiste nce of t he ‘ Z e al a n di a – A nt arctic S well’ 2 De p art me nt of Ge ol og y, K a ns as St ate U ni versit y, erupted M O RBs. Park et al. collected sa mples mantle do main: a heterogeneous do main M a n h att a n, K S, U S A. fro m the Australian– Antarctic Ridge in the t h at c o nsists of t w o c o m p o ne nts. T he first e- m ail: pke mpton @ksu.edu Southern Ocean and found that sa mples fro m is i nter me di ate i n c o m p ositi o n b et wee n t he the east of a proposed co mpositional boundary I n di a n a n d Pacific m a ntle a n d t he s ec o n d bet ween the Indian and Pacific mantle do mains is si mil ar t o t h at gi vi n g ris e t o t he ol der Published online: 27 February 2019 — the Australian– Antarctic Discordance — are Ce nozoic alkali ne volca nic rocks t hat occ ur https://doi.org/10.1038/s41561-019-0321-y co mpositionally distinct fro m both. They suggest throughout the region. that this, the Zealandia- Antarctic S well do main, It is n ot yet cle ar w h at t he i de ntific ati o n References for med during the break-up of Gond wana. of t he Z e al a n di a – A nt arctic S well d o m ai n 1. S c h ei de g ger, A. E. Principles of Geody na mics (Spri nger- Verlag, 4 I mage reproduced fro m ref. under a Creative mea ns for t he geo dy na mic history of t his B erli n, 1 9 5 8). Co m mons licence C C B Y 4. 0 . 2. P ar k, S.- H. et al. N at. Ge osci. https://doi.org/10.1038/s41561-018- re gi o n. B ut, w het her it f or me d i n res p o ns e 0292-4 (2019). t o dee p m a ntle u p welli n g a n d t he v olc a nis m 3. v a n der Meer, D. G., v a n Hi ns b er ge n, D. J. J. & S p a k m a n, W. Tecto nophysics 7 2 3 , 309–448 (2018). 2 that led to the break-up of Gond wana or 4. B arr y, T. L. et al. Sci. Re p. 7 , 1870 (2017). been docu mented along the mid-ocean as a res ult of c at astr o p hic sl a b det ac h me nt 5. M e yze n, C. M. et al. Geoche m. Geophys. Geosyst . 6 , Q11 K11 (2005). ri d ge s yste m s o ut h of A ustr ali a. To t he along the Gond wana margin and subsequent 6. K e m pt o n, P. D. et al. Geoche m. Geophys. Geosyst. 3 , 1074 (2002). 7. H art, S. R. N at ure 3 0 9 , 753–757 (1984). west of t his bou ndary, mid-ocea n ridge p arti al melti n g a n d mi xi n g of m ulti ple 8. Fi n n, C. A., M üller, R. D. & Pa nter, K. S. Geoche m. Geophys. b as alts h a ve is ot o pic si g n at ures t y pic al of mantle co mponents 1 1 , t he d o m ai n is li kel y a Ge osyst . 6 , Q02005 (2005). Indian M O R B, whereas to the east they have conseque nce of the separation of Zealandia 9. Jac ks o n, M. G. et al. N at ure 4 6 6 , 853–856 (2010). 1 0. K lei n, E. M. et al. N at ure 3 3 3 , 623–629 (1988). is ot o pic si g n at ures si mil ar t o b as alts fr o m fro m A ntarctica duri ng t he fi nal phases 1 1. T i m m, C. et al. E art h Sci. Re v. 9 8 , 38–64 (2010). Pacific M O R B 6 ,1 0 . Ho wever, because t his of Gond wana break-up 1 2 . A n d, alt h o u g h 1 2. P a nter, K. S. et al. J. Petr ol. 4 7 , 1673–1704 (2006).

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