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1 Nature and Origin of the Mozambique Ridge, SW Indian Ocean 1 2 3 2 4 5 3 G. Jacques1*,2, F. Hauff2, K. Hoernle2,3, R. Werner2, G. Uenzelmann-Neben4, D. Garbe- 6 3 4 7 4 Schönberg , M. Fischer 8 9 5 10 6 1 11 Bundesanstalt für Geowissenschaften und Rohstoffe, 30165 Hannover, Germany 12 7 2GEOMAR Helmholz Centre for Ocean Research Kiel, 24148 Kiel, Germany 13 14 8 3Kiel University, Institute of Geosciences. 24118 Kiel, Germany 15 4 16 9 Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, 27568 17 18 10 Bremerhaven, Germany 19 20 11 21 22 12 *Corresponding author: 23 24 25 13 Dr. Guillaume JACQUES 26 27 28 14 Bundesanstalt für Geowissenschaften und Rohstoffe 29 30 15 Stilleweg 2 31 32 33 16 30655 Hannover, Germany 34 35 36 17 [email protected] 37 38 39 18 40 41 19 42 43 44 20 45 46 47 21 48 49 22 50 51 52 23 53 54 55 24 56 57 58 25 59 60 26 61 62 63 64 65 27 ABSTRACT 1 2 3 28 4 5 29 The Mozambique Ridge (MOZR) is one of several bathymetric highs formed in the South 6 7 30 African gateway shortly after the breakup of the supercontinent Gondwana. Two major 8 9 31 models have been proposed for its formation - volcanic plateau and continental raft. In order 10 11 32 to gain new insights into the genesis of the Mozambique Ridge, R/V SONNE cruise SO232 12 13 33 carried out bathymetric mapping, seismic reflection studies and comprehensive rock sampling 14 34 of the igneous plateau basement. In this study, geochemical data are presented for 51 dredged 15 16 35 samples, confirming the volcanic origin of at least the upper (exposed) part of the plateau. The 17 18 36 samples have DUPAL-like geochemical compositions with high initial 87Sr/86Sr (0.7024- 19 143 144 176 177 20 37 0.7050), low initial Nd/ Nd (0.5123-0.5128) and low initial Hf/ Hf (0.2827-0.2831), 21 207 204 208 204 206 204 22 38 and elevated initial Pb/ Pb and Pb/ Pb at a given Pb/ Pb (Δ7/4= 2-16; Δ8/4= 13- 23 39 24 167). The geochemistry, however, is not consistent with exclusive derivation from an Indian 25 40 MORB-type mantle source and requires a large contribution from at least two components. 26 27 41 Ratios of fluid-immobile incompatible elements suggest the addition of an OIB-type mantle to 28 29 42 the ambient upper mantle. The MOZR shares similar isotopic compositions similar to 30 31 43 mixtures of sub-continental lithospheric mantle end members but also to long-lived, mantle- 32 33 44 plume-related volcanic structures such as the Walvis Ridge, Discovery Seamounts and Shona 34 45 35 hotspot track in the South Atlantic Ocean, which have been proposed to ascend from the 36 46 African Large Low Shear Velocity Province (LLSVP), a possible source for DUPAL-type 37 38 47 mantle located at the core-mantle boundary. Interestingly, the MOZR also overlaps 39 40 48 compositionally with the nearby Karoo-Vestfjella Continental Flood Basalt province after 41 42 49 filtering for the effect of interaction with the continental lithosphere. This geochemical 43 44 50 similarity suggests that both volcanic provinces may be derived from a common deep source. 45 51 46 Since a continuous hotspot track connecting the Karoo with the MOZR has not been found, 47 52 there is some question about derivation of both provinces from the same plume. In 48 49 53 conclusion, two possible models arise: (1) formation by a second mantle upwelling (blob or 50 51 54 mantle plume), possibly reflecting a pulsating plume, or (2) melting of subcontinental 52 53 55 lithospheric material transferred by channelized flow to the mid-ocean ridge shortly after 54 55 56 continental break-up. Based on geological, geophysical and geochemical observations from 56 57 this study and recent published literature, the mantle-plume model is favored. 57 58 59 58 60 61 62 63 64 65 59 Key words: Mozambique Ridge – Large Igneous Province (LIP) – Flood Basalts – 1 2 60 Continental breakup – Mantle plume – Submarine volcanism – Radiogenic Isotopes 3 4 61 5 6 7 62 1. INTRODUCTION 8 9 10 63 11 12 64 13 Continental breakup leads to the formation of new ocean basins floored by new ocean 14 65 crust. Breakup forms two types of rifted margins - magma-poor and volcanic-rifted margins 15 16 66 (Franke, 2013 and references therein). Magma-poor margins (e.g. the South China Sea) are 17 18 67 characterized by low magmatic activity and extensional features such as detachment faults 19 20 68 and rotated crustal blocks over wide domains (i.e., >1000 km in the South China Sea). The 21 22 69 crust breaks up before the lithospheric mantle extends, which contrasts with the formation of 23 70 24 volcanic rifted margins. In the second scenario, initial lithospheric extension preceding crustal 25 71 breakup leads to extensive magmatism over a short period of time. These large outpourings of 26 27 72 magma are generally referred to as Large Igneous Provinces (LIPs) (e.g., Coffin and Eldholm, 28 29 73 1994, Courtillot et al., 1999, Dalziel et al., 2000). As the continental fragments drift apart as a 30 31 74 result of new seafloor spreading, the continental crust along the margins cools and subsides, 32 33 75 such that the sub-aerially erupted flood basalts on the new continental margins dip seawards, 34 76 and are known as seaward-dipping reflectors. The super continent Gondwana started to 35 36 77 disperse in the Middle Jurassic when Africa, South America, Antarctica, India and Australia 37 38 78 rifted apart at different stages. The presence of the 183 Ma-old Karoo (Jourdan et al., 2005) 39 40 79 and the 132 Ma-old Paraná-Etendeka (Renne et al., 1996) Continental Flood Basalt (CFB) 41 42 80 provinces at the edge of the newly formed continents suggests that magmatic events may have 43 44 81 contributed to continental breakup. 45 46 82 The opening of the Southern Ocean results from the fragmentation of Eastern 47 48 83 Gondwana (i.e. Africa, Antarctica and Australia; 184-171 Ma, Nguyen et al., 2016). The first 49 50 84 oceanic crust between Africa and Antarctica formed at ca. 155 Ma (Jokat et al., 2003). 51 52 85 Numerous structures such as the Mozambique Ridge (MOZR), Agulhas Plateau (AGP), Maud 53 54 86 Rise, and Madagascar Rise (Figure 1a) were then emplaced between Southern Africa and 55 87 Antarctica on the newly formed oceanic crust, but little is known about possible links to the 56 57 88 Gondwana breakup. The continental margin of the Dronning Maud Land (Antarctica) and its 58 59 89 African counterpart have been classified as volcanic-rifted (Jokat et al., 2004, Eagles and 60 61 90 König, 2008, Mueller and Jokat, 2017). The nature and origin of these bathymetric highs are 62 63 64 65 91 enigmatic due to the lack of detailed marine-based investigations, which has led to opposing 1 2 92 models (volcanic plateau versus thinned continental crust) for their formation (e.g., Tucholke 3 93 4 et al., 1981, Ben Avraham et al., 1995; König and Jokat, 2010, Gohl et al., 2011). In order to 5 94 better understand the nature, origin and spatial and temporal evolution of the MOZR and its 6 7 95 relationship to Gondwana breakup, the Research Vessel SONNE (expedition SO232 in April- 8 9 96 May 2014) carried out a comprehensive bathymetric and seismic reflection survey on the 10 11 97 MOZR accompanied by a detailed sampling of the plateau basement, and preliminary 12 13 98 sampling of the NW part of the AGP. In this paper we present a comprehensive new 14 99 15 geochemical data set for samples from the MOZR and AGP, including major and trace 16 100 element and Sr-Nd-Hf-Pb (double spike) isotope data. 17 18 19 101 20 21 22 102 2. GEOLOGICAL SETTING 23 24 103 25 26 27 104 The MOZR is an elongated plateau striking roughly parallel to the SE coastline of 28 29 105 South Africa between 25° and 35°S (Figure 1). It rises ca. 4000 m above the abyssal plain 30 31 106 located at ca. 5000 m below sea level. It is composed of three sub-plateaus, divided by E-W 32 33 107 and NW-SE trending valleys: 1) southwestern, 2) central and 3) northern plateaus (Figure 1b). 34 108 A fourth sub-plateau, in the southeast, is less distinct. 35 36 37 109 The following section reviews the different models that have been proposed since the 38 39 110 early studies in the 1970s on the nature of the MOZR. Laughton et al. (1970) first proposed an 40 41 111 origin of the MOZR through thinning of continental crust based largely on its bathymetric 42 43 112 connection to the African shelf. This hypothesis was later supported by plate tectonic 44 113 reconstructions (Tucholke et al., 1981). Several dredges recovered continental rocks from 45 46 114 different parts of the MOZR that further supported this hypothesis (Mougenot et al., 1991, 47 48 115 Ben Avraham et al., 1995, Figure 1b). The authors described the samples (metapelites, gneiss, 49 50 116 metagabbros and anorthosites) as similar to Archean rocks occurring on the African craton. 51 52 117 There is some questions as to whether some or all of these samples may have been glacial 53 54 118 dropstones from Antarctica. Fresh volcanic glass was also recovered, which was believed to 55 119 reflect volcanism related to neo-tectonic activity (Ben Avraham et al., 1995). No ages, 56 57 120 however, were determined on any of these samples. The southern MOZR, AGP and 58 59 121 Madagascar Ridge were also considered to be thinned continental fragments that were 60 61 122 stranded after breakup (Ben Avraham et al. 1995). 62 63 64 65 123 On the other hand, Green (1972) suggested that the MOZR is an extinct north-south 1 2 124 spreading center responsible for the separation of Madagascar from Africa. In this context, 3 125 4 others found that the MOZR is in isostatic equilibrium with the neighboring oceanic crust, 5 126 despite its deep Moho (> 22 km) and thus proposed an oceanic origin (e.g., Hales and Nation, 6 7 127 1973, Chetty and Green, 1977, Maia et al., 1990). Other authors proposed a microplate origin 8 9 128 for the MOZR (e.g., Lawver et al., 1999, Marks and Stock, 2001, Marks and Tikku, 2001). 10 11 129 Strong evidence for a volcanic nature of at least a part of the northern MOZR came from 12 13 130 DSDP Leg 25 (Simpson, 1974; Figure 1b), which recovered Cretaceous tholeiitic basalts at 14 131 15 site 249 (Erlank and Reid, 1974, Thompson et al., 1982). 16 17 132 Geophysical data support a volcanic nature for the MOZR. Magnetic anomaly data 18 19 133 indicate a magmatic origin for the plateau (König and Jokat, 2010), in agreement with similar 20 21 134 studies of the AGP and Maud Rise showing that they are all composed of thickened ocean 22 23 135 crust (>15 km, which is more than twice the thickness of normal ocean crust), consistent with 24 136 25 a LIP origin (e.g., Gohl and Uenzelmann-Neben, 2001, Parsiglia et al., 2008, Gohl et al., 26 137 2011). Fischer et al. (2017) favored the LIP hypothesis for the MOZR based on seismic 27 28 138 reflection data from the SO232 cruise and estimated volume (3.1 x 106 km3), surface (0.15 x 29 6 2 30 139 10 km ) and duration of emplacement (10 Ma) compared to other oceanic LIPs. The MOZR 31 32 140 presumably formed 135-125 Ma ago, based on high resolution magnetic anomaly data and 33 34 141 tectonic reconstruction (König and Jokat, 2010, Fisher et al., 2017). The MOZR formed 35 142 during multiple events. It initially began to form in the north at 135 Ma and continued to grow 36 37 143 towards the southwest, where the central area formed mainly at ca. 131 Ma with volcanism 38 39 144 forming the southwestern sub-plateau lasting until ca. 126 Ma. The less prominent SE plateau 40 41 145 may have been emplaced at ca. 125 Ma. One prominent structure is a roughly N-S trending 42 43 146 scarp along the eastern margin, which is possibly an elongation of the Andrew Bain Fracture 44 45 147 Zone (König and Jokat, 2010). According to Mougenot et al. (1991) and König and Jokat 46 148 (2010), there is a crustal block located east of the northern plateau that is separated from the 47 48 149 main MOZR and may represent a continental fragment, which moved into the oceanic realm 49 50 150 during the initial stages of rifting. 51 52 53 151 54 55 152 3. SAMPLING AND ANALYTICAL METHODS 56 57 58 153 3.1 Petrography of the samples 59 60 61 154 62 63 64 65 155 Fifty-one successful dredges were carried out during SO232 covering the entire 1 2 156 MOZR except the northernmost part (Figure 1b). Massive lavas, mostly sheet and pillow 3 157 4 lavas, were recovered in 35 dredges and volcaniclastic rocks from a further 16 dredges. The 5 158 samples are variably altered with the groundmass ranging from brownish to grayish in color. 6 7 159 Mn crusts, present on most samples, and alteration halos were cut off with a rock saw onboard 8 9 160 to prepare the least altered inner core of individual rock samples for geochemical analyses. 10 11 161 Most samples are aphyric but a few contain relatively fresh millimeter-sized plagioclase 12 13 162 phenocrysts. The presence of vesicles is variable in the samples and they are generally filled 14 163 15 with secondary minerals. Samples with filled vesicles were avoided when preparing samples 16 164 for geochemistry. Thin sections of fresh cut blocks show that the matrix is generally 17 18 165 composed of plagioclase needles and pyroxene crystals. Some reddish and opaque minerals, 19 20 166 and some iddingsitized olivine are observed within the matrix or as phenocrysts. A full 21 22 167 detailed sample description and precise sample locations on the plateau basement can be 23 24 168 found in the SO232 cruise report (Uenzelmann-Neben, 2014; 25 169 http://hdl.handle.net/10013/epic.43724). 26 27 28 170 Dredge station DR65 exclusively recovered continental materials, in an area that has 29 30 171 been previously described as a continental splinter (Mougenot et al., 1991, König and Jokat, 31 32 172 2010, see also Fig. 5.44 in Uenzelmann-Neben, 2014). DR65-1 is a gneiss, whereas DR65-2 33 34 173 (one meter-sized block) is a Quartz-rich rock, possibly a schist. Both samples have thin Mn- 35 174 crusts, sharp angular edges and a few freshly broken surfaces (see Supplementary File A1 and 36 37 175 Uenzelmann-Neben, 2014). These observations suggest an in-situ origin. Only DR65-1 has 38 39 176 been geochemically characterized. 40 41 42 177 43 44 178 45 3.2 Analytical methods 46 47 179 48 49 50 180 In this study 55 samples were analyzed for major elements, trace elements, and Sr-Nd- 51 52 181 Pb double-spike (DS) isotopes. A representative subset (32 samples) was selected for Hf 53 54 182 isotopes. The full analytical methods including sample preparation and analytical procedures 55 183 can be found in the Supplementary File A2. All information and geochemical data can be 56 57 184 found in the Supplementary Table B1. 58 59 60 185 61 62 63 64 65 186 4. RESULTS 1 2 3 187 4.1 Effects of alteration 4 5 188 6 7 8 189 Because the samples are of submarine origin and have been exposed to seawater for up 9 10 190 to ~130 Ma, it is crucial to evaluate the degree of alteration and possible effects of alteration 11 12 191 on whole rock chemistry. As stated earlier the thin sections reveal that some of the samples 13 192 are heavily altered, as highlighted by reddish matrix and altered pyroxenes. 14 15 16 193 An important criteria for evaluating the freshness of whole rock chemistry is the loss 17 18 194 on ignition (LOI), which ranges from 0.5 to 13 wt. %, but 32 out of 55 samples have less than 19 20 195 5 wt. % LOI (Supplementary Table B1). A rough positive correlation between LOI and P2O5 21 22 196 is observed (P2O5 = 0.06 to 15.6 wt. %; plot not shown). Three samples possess LOI > 7.9 and 23 197 24 high P2O5 contents along with a fourth sample that displays an unusually high CaO contents 25 198 (~24 wt. %). All four samples have anomalously low SiO2 contents (< 41 wt. %). Therefore 26 27 199 they are excluded from further consideration in this manuscript. A multi-element diagram also 28 29 200 provides useful information about the effect of alteration on trace elements (Supplementary 30 31 201 Figure A3). The smooth multi-element pattern of glass sample DR14-1G can be considered a 32 33 202 reference sample to identify element depletions or enrichments caused by alteration of the 34 203 whole rock samples. Almost all the whole rock samples display pronounced peaks (Cs, Rb, U 35 36 204 and K) for the fluid-mobile trace elements that are commonly taken up by seafloor volcanic 37 38 205 rocks during alteration. The peaks in La and troughs in Ce in some samples are also likely to 39 40 206 reflect mobilization of these elements. To avoid these alteration effects, the interpretation of 41 42 207 this study focus on fluid-immobile incompatible elements and isotope ratios in order to derive 43 44 208 information about magmatic processes. 45 46 209 47 48 49 210 4.2 Major and trace elements 50 51 52 211 53 54 212 Due to the overall state of alteration of many samples, application of the Total Alkali 55 56 213 Silica (TAS) diagram is not suitable. Instead, the Zr/Ti vs. Nb/Y discrimination diagram from 57 58 214 Pearce (1996) is used to classify the volcanic rocks (Figure 2). Most of the samples are 59 60 215 basalts, but a few plot within the alkali basalt field. All the alkali basalts are from small cones 61 62 63 64 65 216 and/or structures at the summit of the southwestern and central MOZR plateaus likely to 1 2 217 represent late-stage volcanism (Figure 1b; Uenzelmann-Neben, 2014). The most evolved 3 218 4 sample (DR34-1) with SiO2 = 70 wt. % plots in the trachyte field. The gneiss sample DR65-1 5 219 has a SiO2 content of 69 wt. %. 6 7 8 220 All samples form positive correlations on plots of fluid-immobile incompatible 9 10 221 element ratios. On the Nb/Yb vs. TiO2/Yb diagram (Pearce, 2008, Figure 3a), the basalts 11 12 222 (henceforth called tholeiites) and the alkali basalts form distinct correlations. Most of the 13 223 14 tholeiites plot within the deep melting (OIB) field, suggestive of melting in presence of 15 224 residual garnet in the source and extend along the plume-ridge interaction trend of Pearce 16 17 225 (2008). On a Nb/Yb vs. Th/Yb diagram (Pearce, 2008, Figure 3b), the MOZR and AGP 18 19 226 basaltic samples largely overlap with Enriched Mid-Ocean Ridge Basalt (E-MORB) end 20 21 227 members with extensions to Normal (N) MORB and Ocean Island Basalt (OIB) end member, 22 23 228 whereas the alkali basalt samples lie near OIB. A single sample (DR21-1) has the lowest 24 229 25 fluid-immobile incompatible element ratios, below N-MORB composition. On the primitive- 26 230 mantle-normalized, immobile incompatible element diagram (Hofmann, 1988), all the 27 28 231 samples are enriched in incompatible elements (Figure 4). The degree of enrichment of light 29 30 232 rare earth elements (LREEs) for the tholeiites lies between E-MORB and OIB reference 31 32 233 patterns. From Tb to Yb, the patterns have negative slopes indicating the presence of residual 33 34 234 garnet in the source. Sample DR-21-1 has a pattern that runs sub-parallel to the N-MORB 35 235 reference with a diagnostic depletion in LREE of (La/Sm) = 0.88 and a flat HREE pattern 36 N 37 236 (Figure 4a). Ce depletions in some samples indicate post-magmatic mobilization of Ce under 38 39 237 oxidizing conditions. Samples collected from a large volcano with a flat top (henceforth 40 41 238 called “Tabletop Mountain”) display positive La anomalies reflecting both La uptake and Ce 42 43 239 removal. The alkali basalts generally have steeper negative REE slopes, i.e., higher ratios of 44 45 240 more to less incompatible elements compared to the tholeiites (Figure 4b) with (La/Sm)N > 2 46 241 and (La/Yb) > 6. Trachyte sample DR34-1 shows trace element enrichment similar to the 47 N 48 242 alkali basalt samples. DR65-1 has very depleted HREE with the highest (Sm/Yb)N ratio of 49 50 243 4.56 (Figure 4b). 51 52 53 244 54 55 245 56 4.3 Radiogenic isotopes 57 58 246 59 60 61 62 63 64 65 247 In this article, we present and compare radiogenic isotope data of magmatic rocks with 1 2 248 different ages, sources and histories. Therefore it is crucial to project all data to a common age 3 249 4 as for example in Homrighausen et al. (2019). The MOZR has been estimated to be 135-125 5 250 Ma-old (Fisher et al., 2017). A common formation age of 130 Ma is assumed for all samples 6 7 251 for the projection of initial isotope ratios. Initial isotope ratios presented in the Supplementary 8 9 252 Table B1 and plotted in the diagrams are calculated using the measured Parent/Daughter 10 11 253 (P/D) element ratios obtained from unleached powders (i.e. the inductively coupled plasma 12 13 254 mass spectrometry (ICPMS) data). This approach assumes that the isotope systems remained 14 255 15 closed over 130 Ma and that any postmagmatic alteration occurred shortly after eruption. In 16 256 order to project literature data of young and variably aged terranes to a common 130 Ma 17 18 257 formation age, the following measures were undertaken. For present-day Indian and Atlantic 19 20 258 MORB, together with SW Indian OIB, source P/D ratios of depleted mantle (DMM; 21 22 259 Workman and Hart, 2005) and OIB (Stracke et al., 2003) were applied to simulate a 130 Ma 23 24 260 formation age. Older terranes followed a two-step procedure that first calculates the initial 25 261 composition at the time of formation and then runs a source correction to 130 Ma using 26 27 262 source P/D’s from the above mentioned references and those of Willbold and Stracke (2006) 28 29 263 for Enriched Mantle 1 (EM-1) (Supplementary Table B7). 30 31 32 264 Low-temperature and hydrothermal alteration can affect the Rb-Sr, Th-Pb and U-Pb 33 34 265 isotope systems in submarine lavas, whereas the Sm-Nd and Lu-Hf systems are much less 35 266 affected due to their relatively fluid-immobile element behavior. Two major processes 36 37 267 contribute to elevated 87Sr/86Sr values in old, submarine altered rocks: 1) exchange/addition of 38 39 268 seawater Sr, and 2) Rb addition as evident from the pronounced Rb peaks in a multi-element 40 41 269 diagram (Supplementary Figure A3). Seawater contains ~7 ppm Sr and has strongly elevated 42 87 86 43 270 Sr/ Sr ratios in comparison to the sub-oceanic mantle (0.709 vs. 0.702-0.706). Mild 44 45 271 leaching of rock chips in 2N HCl at 70°C removes possible contamination from sample 46 272 handling and dissolves any carbonates present, but the treatment is unlikely to significantly 47 48 273 alter the overall P/D’s ratios. Hot leaching of sample powders in 6N HCL at 150°C will more 49 50 274 efficiently dissolve secondary mineral phases. The strong leaching of the powders in hot 6N 51 87 86 52 275 HCl yielded predominantly lower Sr/ Sr ratios compared to those from the mildly leached 53 54 276 rock chips (Supplementary Table B1). Two samples (DR87-1 and DR59-1), however, yielded 55 277 56 higher values. In order to evaluate effectively the leaching effects, we exclude the three highly 57 278 altered samples (DR06-1, DR59-1 and DR59-3) and the abnormal 87Sr/86Sr of DR87-1 58 _powder 59 279 from further consideration. The correlation coefficient (r²) for the measured 87Sr/86Sr vs. 60 61 280 measured 143Nd/144Nd or measured 176Hf/177Hf ratios obtained from the mildly leached rock 62 63 64 65 281 chips (r²= 0.22 and 0.16, respectively) and the strongly leached powders (r²= 0.30 and 0.28, 1 2 282 respectively) are not indicative of any correlation between Sr, Nd and Hf isotope ratios 3 283 4 (Supplementary Figures A4a-b). For Sr vs. Nd isotopes, r² improves to 0.38 (chips) and 0.80 5 284 (powders), when excluding trachyte DR34-1 (not shown). Similarly, r² increases to 0.35 6 7 285 (chips) and 0.65 (powders) for Sr vs. Hf isotopes. Trachyte sample DR34-1, expectedly, has 8 9 286 high Rb/Sr (=1.06), leading to high post-magmatic radiogenic ingrowth of 87Sr and 10 87 86 11 287 consequently high measured Sr/ Sr. Therefore it is crucial to also consider initial isotope 12 87 86 13 288 ratios. Correlations of Sr/ Sri of mildly leached chips and strongly leached powders against 14 289 143 144 176 177 15 Nd/ Ndi and Hf/ Hfi (Supplementary Figures A4c-d) are significantly better for the 16 290 mildly leached chips (r2= 0.74 and 0.71, respectively) than for the strongly leached powders 17 18 291 (r2= 0.43 and 0.51 respectively). The effect of age correction is best seen in sample DR34-1, 19 87 86 20 292 since its Sr/ Sr (from chips or powder) effectively decreases (Supplementary Table B1). 21 2 22 293 The lower r values for the strongly leached powders mainly stems from over-correction of 23 87 86 2 24 294 six samples to very unradiogenic Sr/ Sri ≤0.702. Even when filtered, r does not improve to 25 295 better than 0.61 and 0.60 respectively. 87Sr over-correction most likely reflects using the 26 27 296 Rb/Sr ratios from the unleached powders in conjunction with the possibility of significant 28 29 297 addition of Rb over the entire life-span of the rocks, and/or partial removal of ingrown 87Sr 30 31 298 during acid-leaching as secondary Rb is likely to be bound in secondary minerals. In 32 87 86 33 299 conclusion, although the net acid-leaching effects on the measured Sr/ Sr ratios are 34 35 300 significant for the strongly leached powders (Supplementary Table B1) implying efficient 36 301 removal of contaminating seawater-Sr, the calculation of the 87Sr/86Sr ratio using the Rb/Sr of 37 i 38 302 the unleached powders leads to over-correction in a few samples due to the inability to trace 39 40 303 the combined effects of later Rb addition and the loss of ingrown 87Sr into the acid leachate. 41 87 86 42 304 Since the Sr/ Sri values of mildly leached chips for all samples correlates better with the 43 87 86 44 305 initial Nd and Hf ratios compared to the Sr/ Sri from the strongly leached powders, the 45 306 87 86 46 Sr/ Sri values from the mildly leached chips represent the best possible approximation to 47 307 the initial magmatic values. Overall it is clear that the 87Sr/86Sr values in the bulk rock 48 i 49 308 analyses have large uncertainties and that the two approaches presented here define the lower 50 87 86 87 86 51 309 and upper limits of Sr/ Sri. The reasonably good correlations of Sr/ Sri using both 52 53 310 leaching approaches with the fluid-immobile isotope systems of Sm-Nd and Lu-Hf indicate 54 87 86 55 311 that primary petrological information can still be derived from Sr/ Sr in aged, seawater 56 312 57 altered igneous rocks. 58 59 313 Pb and Th are relatively immobile during low-temperature seafloor alteration 60 61 314 processes so that the 232Th-208Pb decay system can only be disturbed if Pb is mobilized early 62 63 64 65 315 in the history of a rock due to hydrothermal alteration. Since no obvious signs of 1 2 316 hydrothermal alteration are detected in the samples, the most significant process affecting Pb 3 317 4 isotope ratios is U uptake during low temperature alteration / weathering. This is observed as 5 318 significant positive U-spikes for most samples on a multi-element diagram (Supplementary 6 7 319 Figure A3) and is further underpinned by low and highly variable Nb/U ratios (= 30 ±15) for 8 9 320 the sample dataset (Supplementary Table B1) when compared to the canonical values for 10 11 321 oceanic basalts (Nb/U= 47 ±10, Hofmann et al., 1986) or fresh glass from tholeiite DR14-1G 12 13 322 (Nb/U= 46). Since the glass Nd/Pb (17) is very similar to the average whole rock sample 14 323 15 Nd/Pb ratio (13 ±6), we do not believe Pb has been significantly mobilized. In conclusion, U 16 324 uptake significantly influenced the post-magmatic evolution of Pb in the sample dataset. 17 18 325 Since most present-day U is 238U (238U/235U= 138.88), U uptake will primarily result in the 19 206 20 326 ingrowth of Pb throughout the history of these Cretaceous rocks. The reasonably good 21 206 204 207 204 208 204 22 327 correlation of Pb/ Pbi vs. Pb/ Pbi (r²= 0.68) and Pb/ Pb (r²= 0.74) (Figure 5), as 23 207 204 208 204 24 328 well as Pb/ Pbi vs. Pb/ Pbi r²= 0.79; plot not shown) support the assumption of closed 25 329 system behavior for U-Pb and Th-Pb in the majority of samples and indicate that most of the 26 27 330 alteration of these systems occurred early in the history of the samples. 28 29 30 331 The MOZR, Tabletop Mountain and AGP samples form a reasonable good negative 31 87 86 143 144 32 332 correlation (r²= 0.74) on a Sr/ Sri vs. Nd/ Ndi isotope diagram (Figure 6a) and a 33 143 144 176 177 34 333 similarly good positive correlation (r²= 0.75) on the Nd/ Ndi vs. Hf/ Hfi isotope 35 334 diagram (Figure 6b). The Tabletop Mountain samples have the most enriched isotope ratios 36 37 335 (radiogenic Sr and unradiogenic Nd-Hf), together with one tholeiite (DR44-2) and one alkali 38 39 336 basalt (DR72-1). On the other hand, the DR21-1 and AGP samples plot at the depleted end of 40 41 337 the array and two other alkali samples also have depleted isotope ratios. When considering the 42 207 204 208 204 43 338 initial Pb isotope data, the samples display elevated Pb/ Pbi and Pb/ Pbi ratios at a 44 206 204 45 339 given Pb/ Pbi ratio with respect to the Northern Hemisphere Reference Line (NHRL; i.e. 46 340 high Δ7/4 and Δ8/4 ratios, Hart, 1984). The AGP samples have the most radiogenic Pb 47 48 341 isotope ratios and sample DR21-1 has very low Δ7/4 and Δ8/4 ratios (Figure 5). The gneiss 49 87 86 143 144 50 342 sample (DR65-1) has radiogenic Sr/ Sri of 0.7097, unradiogenic Nd/ Ndi of 0.5115, 51 206 204 207 204 208 204 52 343 and very unradiogenic Pb ( Pb/ Pbi= 16.04, Pb/ Pbi= 15.29 and Pb/ Pbi= 36.69; 53 54 344 Figure 7). 55 56 345 57 58 59 346 5. DISCUSSION 60 61 62 63 64 65 347 5.1 Nature of the MOZR: Evidence for excess volcanism 1 2 3 348 4 5 349 The exact nature of the MOZR is controversial, and contradicting models (i.e., 6 7 350 volcanic versus continental) have been proposed. DSDP Leg 25 (Site 249) on the MOZR 8 9 351 northern plateau (Figure 1b) was the first sampling attempt of the plateau basement and 10 11 352 Cretaceous tholeiitic rocks were recovered (Thomson et al., 1982). In contrast, several cruises 12 13 353 in the late 1980s dredged continental crustal rocks (Figure 1b). Mougenot et al. (1991) 14 354 dredged gneiss, metagabbros and anorthosites samples, with similar features to the nearby 15 16 355 African cratons. Ben Avraham et al. (1995) described small rock fragments of garnet-bearing 17 18 356 metapelites and gneiss along with fresh volcanic glass from a single location at the SW 19 20 357 margin of the plateau. In the absence of radiometric ages, the glasses were interpreted to be 21 22 358 very young (not more than tens of thousands years old). However, none of these studies could 23 359 24 convincingly exclude a dropstone origin for the continental rocks recovered via dredging at 25 360 four locations. 26 27 28 361 The new study at hand presents the first comprehensive basement sampling of the 29 30 362 MOZR, which primarily recovered volcanic and volcaniclastic rocks (Uenzelmann-Neben, 31 32 363 2014). The gneiss sample (DR65-1), which has lower continental crust type isotopic 33 34 364 composition, was recovered close to the northern plateau in an elongated block-like structure 35 365 earlier identified as a continental block by Mougenot et al. (1991) and König and Jokat 36 37 366 (2010). This structure is clearly separated from the MOZR plateau, as evident from 38 39 367 bathymetric mapping (Fig. 5.53 in the cruise report; Uenzelmann-Neben, 2014). The 40 41 368 exclusive association with other continental like rocks in dredge 65 (seven samples - DR65-2 42 43 369 to -8 - in Uenzelmann-Neben, 2014) suggests that DR65 sampled the talus at the base of a 44 370 45 slope. Except for sample DR44-5, a rounded plutonic pebble interpreted as being an ice-rafted 46 371 dropstone, surprisingly no other continental rocks were recovered during SO232. Another 47 48 372 interesting sample is DR18-1 from the southern margin of the MOZR, which shows a 49 50 373 microcrystalline structure in thin section indicative of a plutonic or subvolcanic origin. Its 51 52 374 mantle-like geochemistry suggests that this dolerite formed in an oceanic setting. Numerous 53 54 375 morphologic structures, such as small cones, scattered on the plateau, probably reflect late- 55 376 56 stage volcanism, as they appear to intrude the sediment cover (Uenzelmann-Neben, 2014, 57 377 Fischer et al., 2017). 58 59 60 61 62 63 64 65 378 In summary, although a few presumably in-situ continental rocks were found at the 1 2 379 base of a morphologically separated ridge close to the northern plateau margin, extensive 3 380 4 sampling of the entire exposed (upper) plateau basement and bathymetric data obtained 5 381 during cruise SO232 provide strong evidence for a primarily volcanic origin of the MOZR. 6 7 382 Moreover, no serpentinites or related rocks were recovered precluding the model of Zhou and 8 9 383 Dick (2013) suggested for the nearby Marion Rise that such bathymetric highs could be 10 11 384 exhumed altered mantle rocks. 12 13 385 14 15 16 386 5.2 Mantle source components at the Mozambique Ridge 17 18 19 387 20 21 22 388 At the southernmost end of the MOZR, an unusual depleted rock sample (DR21-1) 23 389 24 was recovered from a seamount (Figure 1b). This sample is characterized by the lowest fluid- 25 390 immobile incompatible element ratios (Figures 2-4) and depletion in MORB-like LREEs. 26 27 391 Similarly, its isotopically depleted Sr-Nd-Hf ratios (Figure 6) and unradiogenic Pb along with 28 29 392 the lowest ∆7/4 and Δ8/4 (Figure 5) indicate derivation from a depleted source. Considering 30 31 393 that it was sampled at the lower edge of the SW plateau, it is possible that this sample is 32 33 394 derived from uplifted ocean crust and thus provides us with an idea of the upper mantle 34 395 composition close in time to the formation of the SW plateau but it does not necessarily 35 36 396 represent a component involved in the formation of the MOZR. In contrast, the Tabletop 37 38 397 Mountain samples have the most enriched Sr, Nd and Hf isotopic compositions, and the most 39 40 398 elevated Δ7/4 (Figures 5, 6). Finally, the AGP samples plot at the depleted end of the Sr-Nd- 41 42 399 Hf array (Figure 6) but have the most radiogenic Pb isotope ratios (Figure 5). 43 44 400 45 46 47 401 5.2.1 Comparison of the MOZR with (local) Indian Ocean sources (MORBs and OIBs) 48 49 50 402 51 52 53 403 The MOZR samples might be expected to have similar geochemical compositions 54 404 with surrounding Indian intraplate (OIB) volcanoes (e.g. Bouvet, Crozet, Marion, and Prince 55 56 405 Edward islands). Data from these Indian intra-plate volcanoes do not overlap with the MOZR 57 58 406 data on Sr-Nd (Figure 6a), Nd-Hf (Figure 6b) and uranogenic Pb isotope diagrams (Figure 59 60 407 5a). The MOZR samples overlap with the SW Indian MORB field on Sr-Nd-Hf and on the 61 62 63 64 65 408 thorogenic diagrams, but have distinctively higher Δ7/4 compared to most SW Indian 1 2 409 MORBs (Figure 5a). We also note that they have higher Δ8/4 at the unradiogenic end of the 3 410 4 thorogenic Pb isotope diagram than local MORB (Figure 5b). These observations are 5 411 surprising, especially when comparing nearby OIBs with the SW Indian MORBs, which 6 7 412 overlap in all isotope systems. Therefore the source of the 130 Ma MOZR volcanism was 8 9 413 different to what is now the source of SW Indian OIB volcanism. 10 11 12 414 13 14 415 5.2.2 Comparison of the MOZR with South Atlantic-type sources (MORBs and OIBs) 15 16 17 416 18 19 20 417 Elevated Δ7/4 values are a common feature of oceanic volcanic rocks in the southern 21 22 418 hemisphere compared to those of the northern hemisphere (e.g., Dupré and Allègre, 1983). 23 419 24 South Atlantic MORB has higher Δ7/4 but similar Δ8/4 compared to SW Indian MORB 25 420 (Figure 5). However, the higher Δ7/4 occur primarily at the unradiogenic 206Pb/204Pb end of 26 27 421 the MORB arrays, whereas the MOZR samples have elevated Δ7/4 for the entire 206Pb/204Pb 28 29 422 range. Therefore the more radiogenic Pb component is likely to come from a distinct source 30 31 423 (e.g., the subcontinental lithospheric mantle (SCLM) or a deep source) and thus excludes 32 33 424 mixing of the two MORB domains to form the MOZR data array. 34 35 425 Ocean island volcanoes are thought to be products of mantle plumes (e.g. Richards et 36 37 426 al., 1989). In classical models, the plumes rise from either the upper/lower mantle or 38 39 427 mantle/core boundary (e.g. Courtillot et al., 2003). In the South Atlantic, the Tristan-Gough 40 41 428 hotspot track, including the Walvis Ridge, Rio Grande Rise and the Paraná-Etendeka flood 42 43 429 basalts, are a prominent example of plume volcanism being associated with continental break 44 430 up (e.g. Hoernle et al., 2015, Stroncik et al., 2017, Homrighausen et al., 2019). Interestingly, 45 46 431 the MOZR samples have similar Sr-Nd-Pb-Hf isotopic compositions to most of the Walvis 47 48 432 Ridge (Figures 5, 6) and the Gough sub-track of the Guyot Province (Richardson et al., 1982, 49 50 433 Gibson et al., 2005, Salters and Sachi-Kocher, 2010, Rohde et al., 2013a, Hoernle et al., 2015, 51 52 434 Homrighausen et al., 2019). South of the Walvis Ridge, the Discovery Seamount chain 53 54 435 (Schwindrofska et al., 2016) and Shona hotspot (Hoernle et al., 2016) have isotopic 55 436 compositions that plot largely within the Gough component of the Tristan-Gough hotspot 56 57 437 track (Figures 5, 6; Homrighausen et al., 2019). Although there is considerable overlap, 58 59 438 overall the MOZR samples tend towards lower Δ8/4 than the South Atlantic array (Figure 5b), 60 61 439 suggestive of a different geochemical “flavor” with lower long-term Th/Pb ratios (i.e. 62 63 64 65 440 resulting in lower 208Pb/204Pb ratios) but similar U/Pb ratios (i.e. resulting in similar 1 207 204 2 441 Pb/ Pb ratios) and similar Sr-Nd-Hf isotope ratios. 3 4 442 The MOZR is located in the center of the so-called DUPAL anomaly (Dupré and 5 6 443 Allègre, 1983, Hart, 1984) and the MOZR and AGP samples have typical DUPAL-like 7 8 444 isotope signatures. The DUPAL anomaly is a geochemical domain located in the southern 9 87 86 10 445 hemisphere, characterized by enriched isotopic compositions (radiogenic Sr/ Sr and 11 12 446 elevated Δ7/4 and/or Δ8/4). The origin of the DUPAL anomaly is still controversial, but many 13 447 14 studies favor a recycled continental lithospheric component (lower crust and/or SCLM) to 15 448 explain the enrichment. Two opposing recycling models exist to explain the formation of the 16 17 449 DUPAL anomaly. The first envisions shallow recycling of the lower continental lithosphere 18 19 450 through the asthenospheric mantle resulting from delamination of the SCLM during the 20 21 451 breakup of Gondwana (Geldmacher et al., 2008, Hoernle et al 2011). The second model 22 23 452 argues for a deep origin of the enriched recycled lower continental lithospheric component 24 453 25 that roots DUPAL in the African low shear velocity province (LLSVP; Castillo, 1988, Class 26 454 and Le Roex, 2011, Rohde et al., 2013a, Hoernle et al., 2015, White, 2015). The models are 27 28 455 not mutually exclusive and both shallow and deep recycling of SCLM ± lower crust are 29 30 456 likely. 31 32 33 457 Over the last decade, new models have argued that the LLSVPs are the sources of 34 458 35 mantle plumes forming CFBs (e.g., Paraná-Etendeka, Karoo; Figure 1a). Upwelling of hot 36 459 material is postulated to have occurred at the margins of these long-lived thermo-chemical 37 38 460 anomalies at the base of the lower mantle (Torsvik et al., 2006, 2008, Burke et al., 2008, 39 40 461 Steinberger and Torsvik, 2012, French and Romanowicz, 2015). Plumes sampling the 41 42 462 boundary of the African LLSVP may be responsible for the long-lived geochemical zonation 43 44 463 of the 70 Ma Tristan-Gough and 40 Ma Discovery hotspots (Rohde et al., 2013a, Hoernle et 45 464 46 al., 2015, Schwindrofska et al., 2016). The Gough-type EM-1 component, believed to be 47 465 derived from the African LLSVP, is common to the Tristan-Gough, Discovery and Shona 48 49 466 hotspot tracks (Homrighausen et al., 2019). Two other EM-1-type components (Tristan and 50 51 467 Southern Discovery) may also be derived from the African LLSVP, suggesting that it is 52 53 468 heterogeneous (Schwindrofska et al., 2016). The overlap of the MOZR on most isotope 54 55 469 diagrams with the South Atlantic hotspot fields suggests that the MOZR source material could 56 470 also originate from the African LLSVP. 57 58 59 471 HIMU-type lavas have also been documented at the Walvis Ridge (Homrighausen et 60 61 472 al., 2018), which are 20-40 Ma younger than the Walvis basement. Therefore, the authors 62 63 64 65 473 suggested that the mantle plume source of Walvis volcanism first tapped the EM-1 component 1 2 474 in the LLSVP, then late-stage volcanism tapped a different reservoir, possibly HIMU. The 3 475 87 86 4 AGP samples have low Sr/ Sri ratios of ca. 0.7028 and have more radiogenic Pb than the 5 206 204 476 rest of the MOZR ( Pb/ Pbi > 18.72; Figures 5-6). Based on geophysical data, the AGP is 6 7 477 believed to be ~90 Ma-old, thus is ~30-40 Ma younger than the MOZR (Gohl et al., 2011). 8 9 478 This observation may point toward similar mantle plume dynamics beneath the MOZR, where 10 11 479 an EM-1 mantle component first dominates the geochemical signature, which then becomes 12 13 480 more influenced by a HIMU-like mantle component. Another possibility for the higher 14 481 206 204 15 Pb/ Pb component would be FOZO, an ubiquitous mantle component with isotopic 16 482 compositions intermediate between the mantle end members (DM, EM1, EM2 and HIMU) 17 18 483 that differs from HIMU by slightly less radiogenic Pb isotope signatures (Stracke et al., 2005). 19 20 484 However clear mixing trends to either HIMU or FOZO are not observed in both thorogenic 21 22 485 and uranogenic Pb isotope diagrams. 23 24 486 25 26 27 487 5.2.3 Comparison of the MOZR with subcontinental lithospheric mantle-type component 28 29 30 488 31 32 33 489 The enriched components, responsible for the MOZR geochemical variations, could 34 490 also be present in the upper mantle. In other places such as the Christmas Island Seamount 35 36 491 Province in the Eastern Indian Ocean, where there is no evidence for the presence of a long- 37 38 492 lived mantle plume, it has been shown that shallow recycling of subcontinental lithospheric 39 40 493 mantle (SCLM) can occur via off axis volcanism near mid-ocean ridges connected with 41 42 494 continental rifting (Hoernle et al., 2011). Notably, the Christmas Island Seamount Province 43 44 495 was formed in close connection with continental break-up and the movement of SCLM 45 496 material ± lower curst. The delaminated lithospheric material then welled up beneath the 46 47 497 newly formed spreading center and melted by decompression to form the Christmas Island 48 49 498 seamounts and former ocean island volcanoes (now guyot seamounts). Enriched lavas have 50 51 499 also been found on portions of the SW Indian Ridge (39-41°E, Mahoney et al., 1992) and the 52 53 500 SE Indian Ridge (123-130°E, Hanan et al., 2004). Geochemically, SCLM is difficult to trace 54 501 55 as it represents a reservoir beneath thick continental crust that is rarely tapped by magmatism. 56 502 Lamproites and/or kimberlites are infrequent volcanic rocks that are only found on continents 57 58 503 and are believed to derive from the SCLM (Fraser et al., 1985). They generally have very 59 60 504 enriched Sr-Nd isotopic compositions but possess variable Pb isotope ratios, ranging from 61 62 63 64 65 505 highly radiogenic to highly unradiogenic 206Pb/204Pb ratios but generally elevated 207Pb/204Pb 1 2 506 ratios (with the exception of e.g., the Smoky Butte lamproites, Fraser et al., 1985) and 3 507 208 204 206 204 4 elevated Pb/ Pb ratios compared to N-MORB at a given Pb/ Pb ratio. Lamproites and 5 508 kimberlites from Gondwana, i.e., Antarctica (Gaussberg; Murphy et al., 2002), South Africa 6 7 509 (Finsch Mine; Fraser et al., 1985, Nowell et al., 2004) and Australia (Eastern and Western 8 9 510 domain; Fraser et al., 1985, Nelson et al., 1986), all have higher Sr and lower Nd isotope 10 11 511 ratios than the MOZR (Figure 7a) and lie on the extension of the Sr-Nd MOZR array. The 12 13 512 Tabletop Mountain samples of the MOZR have the most enriched Sr-Nd-Hf isotopic 14 513 15 compositions and the highest ∆7/4, which could be explained by higher amounts of SCLM in 16 514 their magma source (Figure 7). On a Sr vs. Nd isotope diagram, any of these SCLM 17 18 515 compositions would be suitable as an enriched end member for all these previously mentioned 19 20 516 regions, except perhaps for the Eastern Australian lamproites. The Gaussberg samples lie 21 22 517 almost on an extension of the MOZR array except on the uranogenic Pb isotope diagram 23 24 518 (Figure 7). On both Pb-Pb isotope diagrams (Figures 7b-c), the Finsch Mine samples largely 25 519 overlap the low 206Pb/204Pb end of the MOZR array. The Western Australia and Gaussberg 26 i 27 520 samples have elevated ∆7/4 and ∆8/4 but none of the MOZR samples form a clear trend 28 29 521 toward one of these compositions (Figures 7b-c). Overall, Smoky Butte or Leucite Hills 30 31 522 (Mirnejad and Bell, 2006) have the closest EM-1 Pb isotope signature to serve as component 32 33 523 for the unradiogenic end of the MOZR array. In conclusion none of the SCLM components 34 35 524 alone can serve as mixing end member for the MOZR but a mixture of e.g. Leucite Hills ± 36 525 Gaussberg lamproites could potentially serve as the end member with the appropriate 37 38 526 unradiogenic Pb isotope composition. 39 40 41 527 42 43 44 528 Summarizing the last two sections, we note that the isotope and fluid-immobile 45 529 46 incompatible-element ratios (Figures 3-7) could be explained by mixing of three components: 47 48 530 (1) A component depleted in trace elements and similar to N-MORB (=D-MOZ), 49 50 531 with low 87Sr/86Sr, Δ7/4 and Δ8/4, and high 143Nd/144Nd and 176Hf/177Hf ratios, is 51 52 532 most likely similar to the composition of the ambient depleted upper mantle 53 54 533 (possibly SW Indian MORB). This component is best represented by the seamount 55 534 56 sample DR21-1. 57 535 (2) A component with low 87Sr/86Sr but radiogenic Pb isotopic ratios (=F-MOZ), 58 59 536 broadly similar to a FOZO- or HIMU-like component (Hart et al., 1992, Stracke et 60 61 537 al., 2005). This component is best represented by the AGP samples. 62 63 64 65 538 (3) A component with elevated 87Sr/86Sr and Δ7/4, low 143Nd/144Nd and 176Hf/177Hf 1 206 204 2 539 and Pb/ Pb ratios (=EM-MOZ), similar to the EM-1 composition. This 3 540 4 component is most clearly observed in the MOZR Tabletop Mountain samples. 5 541 Such source material could reflect either mixtures of different SCLM end member 6 7 542 compositions transferred into the upper oceanic mantle by delamination during 8 9 543 continental breakup (Hoernle et al., 2011) or EM/DUPAL deep mantle sources, as 10 11 544 observed for South Atlantic hotspot volcanics (Rohde et al., 2013a, Hoernle et al., 12 13 545 2015, 2016, Schwindrofska et al., 2016, Homrighausen et al., 2019). 14 15 546 16 17 18 547 5.3 Geodynamic implications 19 20 21 548 22 23 549 24 LIPs are often associated with continental breakup (e.g., the Greenland-North Atlantic 25 550 Magmatic Province in the North Atlantic, and the Paraná and Etendeka regions on both sides 26 27 551 of the South Atlantic Ocean; Coffin and Eldholm, 1994, Courtillot et al., 1999). Jokat et al. 28 29 552 (2004) and Eagles and König (2008) classified the Dronning Maud Land margin and its 30 31 553 African counterpart as volcanically rifted margins. Initial opening of the southern Ocean 32 33 554 occurred 184-171 Ma ago (Nguyen et al., 2016), and some authors have related the breaking 34 555 up of Eastern Gondwana to the 183 Ma Karoo CFB event (Courtillot et al., 1999, Dalziel et 35 36 556 al., 2000). Moreover, Mueller and Jokat (2017) estimated that the volcanic activity related to 37 38 557 the Gondwana breakup along the central coast of Mozambique could have lasted until 157 Ma 39 40 558 but then ceased due to southward motion of the magmatism. 41 42 43 559 In light of the new geochemical data from the MOZR, we will now address the 44 560 question of how the 140-120 Ma-old MOZR fits into the geodynamic evolution of the 45 46 561 Southern Ocean and the breakup of Gondwana. In the following sections, we will first 47 48 562 investigate potential links between the Karoo CFB and the MOZR before discussing the 49 50 563 preferred model for the formation of the volcanic basement of the MOZR. 51 52 53 564 54 55 565 5.3.1 Karoo CFB geological setting and geochemistry 56 57 58 566 59 60 61 62 63 64 65 567 The Karoo CFB province, which extends from South Africa to Botswana and 1 2 568 Mozambique, was formed by extensive volcanism between 183-178 Ma-ago (e.g., Jourdan et 3 569 4 al., 2005). The Karoo volcanism has typical intraplate geochemical characteristics with a 5 570 strong crustal and/or subcontinental lithospheric mantle component in the erupted tholeiitic 6 7 571 rocks (e.g., Hawkesworth et al., 1984, Sweeney et al., 1994, Jourdan et al., 2007). Some 8 9 572 authors suggest that the Karoo source contained small amounts of sediment and/or a weak 10 11 573 subduction component (e.g., Jourdan et al., 2007, Neumann et al., 2011, Heinonen et al., 12 13 574 2014). Karoo-related outcrops are also found in the western Dronning Maud Land of 14 575 15 Antarctica (Vestfjella) and the samples are mostly high MgO picrites, picrobasalts and 16 576 basalts. They are believed to be free of crustal interaction and therefore may provide insights 17 18 577 into the sub Gondwanan mantle reservoir from which they were extracted (e.g. Heinonen and 19 20 578 Luttinen, 2008, Heinonen and Kurz, 2015, Riley et al., 2005, Heinonen et al., 2010, 2013, 21 22 579 2014, 2015,). Two main source components are observed: a depleted component similar to the 23 24 580 SW Indian MORB, and a relatively enriched component that may be associated with a 25 581 pyroxenitic source, which is based on high Ni contents in olivine, high whole-rock Zn/Fe 26 27 582 ratios and low whole-rock CaO contents (Heinonen et al., 2013). On the other hand, Coltice et 28 29 583 al. (2009) proposed, and demonstrated through modeling, that long-lived mantle insulation 30 31 584 beneath super continents such as Gondwana leads to extreme internal heating below the 32 33 585 continental lithosphere and can trigger melting without the presence of a plume. Their 34 35 586 numerical model was successfully applied to the Karoo CFB. Nonetheless some authors favor 36 587 the plume model hypothesis based on geophysical data, which shows a plume currently 37 38 588 beneath southern Africa extending to the African LLSVP (Torsvik et al., 2006, 2008). 39 40 589 However, Heinonen and Kurz (2015) analyzed the most magnesian-rich lavas from the Karoo 41 3 4 42 590 and showed that the He/ He isotopic ratios (R/Ra= 7.03 ±0.23) are similar and 43 44 591 undistinguishable from the SW Indian MORBs (R/Ra= 6.4-7.6). They concluded that, 45 592 46 although a mantle plume cannot be totally excluded, a mantle insulation model seems more 47 593 appropriate. However, the lack of an elevated 3He/4He (i.e., >upper mantle values) does not 48 49 594 rule out the presence of a plume. As shown by Stroncik et al. (2017) for the Etendeka flood 50 51 595 basalts, only very rare picritic samples preserve elevated (>10) 3He/4He ratios and most of the 52 3 4 53 596 mafic rocks from Etendeka also have MORB-like He/ He ratios. 54 55 597 56 57 58 598 5.3.2 Relationship of the MOZR with the Karoo CFB 59 60 61 599 62 63 64 65 600 The MOZR data are now compared with published literature data from the Karoo- 1 2 601 Vestfjella CFB (Figure 7). The African Karoo data overlap with the MOZR data but extends 3 602 4 towards much more enriched compositions, e.g. higher Sr, and lower Nd and Pb isotope 5 603 ratios, which are typical for rocks interacting with the continental lithosphere (crust and/or 6 7 604 SCLM). The MOZR data plot at the depleted end of Sr-Nd isotopic array, overlapping the 8 9 605 Vestfjella data which are believed to represent the pristine mantle reservoir with little to no 10 11 606 continental lithospheric influence. On the Pb isotope diagrams, both Karoo and Vestfjella data 12 206 207 204 208 204 13 607 deviate towards lower Pb/204Pbi, Pb/ Pbi, and Pb/ Pbi ratios. The MOZR data 14 608 15 could represent the sublithospheric source mantle with no or little continental influence, 16 609 similar to what has been proposed for the extreme variations seen in the Paraná-Etendeka 17 18 610 CFB, i.e., interaction between the Tristan-Gough plume and the African lithosphere. For 19 20 611 Etendeka (Africa) and Paraná (South America), it has been shown that Sr isotope ratios 21 22 612 increase and Nd isotope ratios decrease with increasing degree of lava differentiation, 23 24 613 consistent with crustal assimilation (Hoernle et al., 2015 and references therein). Pb isotope 25 614 ratios also show evidence of continental lithospheric assimilation in more evolved samples. 26 27 615 The most mafic Etendeka and Parana samples overlap completely with the submarine lavas 28 29 616 from the Tristan-Gough-Walvis hotspot track and samples with high 3He/4He ratios (Stroncik 30 31 617 et al., 2017) have compositions similar to the submarine samples from the hotspot track. 32 33 618 Therefore there is strong evidence that the Paraná/Etendeka CFBs were formed by a mantle 34 35 619 plume from the lower mantle which interacted with the Gondwana continental lithosphere. 36 620 Considering the similarities in the composition and compositional range of the 37 38 621 Etendeka/Parana and the Karoo/Vestjfella CFBs, it is likely that the Karoo/Vestjfella CFBs 39 40 622 formed by a similar process. Plate tectonic reconstructions, however, do not place the 41 42 623 Karoo/Vestjfella CFBs near the MOZR or on an extension of it, but rather in the vicinity of 43 44 624 the Shona hotspot track (Figure 1a; see Torsvik et al., 2008 for further explanations), which 45 625 46 has a similar composition to the Walvis Ridge and thus could serve as a depleted (plume) end 47 626 member for the Karoo/Vestjfella CFBs, with the two enriched components reflecting 48 49 627 interaction with the upper and lower Gondwana continental crust and/or lithospheric mantle. 50 51 52 628 53 54 55 629 5.3.3 Nature and origin of the MOZR 56 57 630 58 59 60 61 62 63 64 65 631 The new geochemical data from the igneous portions of the MOZR and comparison 1 2 632 with literature data suggests mixing of at least two components: an enriched continental type 3 633 4 material similar to EM-1 (EM-MOZ) and a component with high Pb isotope ratios similar to 5 634 FOZO/HIMU (F-MOZ). Although the MOZR array forms a two-component mixing on the 6 7 635 Sr-Nd-Hf isotope diagrams (Figure 6), a third component (D-MOZ), reflecting the ambient 8 9 636 SW Indian upper mantle, is likely to be present on the Pb isotope diagrams (Figure 5). It 10 206 204 11 637 would explain the spread of the data in Δ7/4 and Δ8/4 at a given Pb/ Pb, which deviates 12 13 638 from the upper mantle toward more enriched compositions. This spread could be explained if 14 639 15 the MOZR was formed through interaction of a spreading ridge with plume or SCLM 16 640 material. We can model the MOZR array by mixing the F-MOZ with EM-MOZ components 17 18 641 (Figure 8; Supplementary Table B8). In addition, the D-MOZ could be mixed with any 19 20 642 proportion of the F-MOZ-EM-MOZ mixing. Two types of models are envisaged to achieve 21 22 643 mixing and melting the EM-MOZ component with the F-MOZ component: A) Shallow 23 24 644 (within the upper mantle), or B) deep (within the lower mantle, possibly LLSVP) through 25 645 recycling of the enriched component(s). In A), the EM-MOZ component could be similar to 26 27 646 different lamproitic compositions, e.g. Leucite Hills-type or Gaussberg-type. On the Sr vs Nd 28 29 647 (Figure 8a) and thorogenic Pb isotope diagram (Figure 8c), less than 10% of a Gaussberg-type 30 31 648 composition would explain the MOZR array. However, a Leucite Hills-type end member is 32 33 649 needed to explain the uranogenic diagram (Figure 8b). Therefore, none of these lamproitic 34 35 650 compositions work on all the diagrams. Of course, the SCLM beneath the MOZR at the time 36 651 of formation could be a mixture of these SCLM compositions. In B), the EM-MOZ 37 38 652 component could be similar to the EM-1 component sampled by South Atlantic hotspots, 39 40 653 tapping different reservoirs of the LLSVP (Homrighausen et al., 2019). When choosing a 41 42 654 composition close to the low-µ Gough described in Homrighausen et al. (2019), mixing lines 43 44 655 nicely overlap the MOZR data on all isotope diagrams (Figures 8d, e, f). We note, however, 45 656 46 that the isotopic composition of these end member could be much more different, as the 47 657 LLSVP is likely to be heterogeneous (e.g. Balmer et al., 2016). Therefore, the amount of 48 49 658 enriched material added is likely to significantly change. Finally, although we favor a FOZO- 50 51 659 like end member, an HIMU-like end member appears to be a possible mixing candidate, since 52 53 660 the Walvis-HIMU data lie on an extension of the MOZR array. In contrast to the Walvis 54 55 661 Ridge, however, no true HIMU signature has been sampled in the MOZR-AGP system. 56 57 662 58 59 60 61 62 63 64 65 663 The shallow recycling model involves delamination of the subcontinental lithospheric 1 2 664 mantle (SCLM) ± lower crust during continental breakup in the Cretaceous, followed by 3 665 4 successive upwelling and melting of the continental material beneath or near the mid-ocean 5 666 ridge. The MOZR could have formed at, or near, the SW Indian mid-ocean ridge, since it is 6 7 667 located on the Andrew Bain Fracture Zone (König and Jokat, 2010). This model is roughly 8 9 668 similar to that proposed for the origin of the Christmas Island Seamount Province (Hoernle et 10 11 669 al., 2011), except that a larger volume of excessive volcanism was produced over a smaller 12 13 670 region at the MOZR compared to the Christmas Island Seamount Province. The MOZR could 14 671 15 have formed at, or near, the SW Indian mid-ocean ridge, since it is located on the Andrew 16 672 Bain Fracture Zone (König and Jokat, 2010). Still this hypothesis does not account for the fact 17 18 673 that excessive volcanism occurred at the MOZR over a relatively short period of 10 Ma in a 19 6 20 674 fairly restricted area (450 x 450 km, i.e., ca. 0.15 x 10 km²; Fisher et al., 2017), in contrast to 21 22 675 the Christmas Island Seamount Province which formed over a much longer period (136-47 23 6 24 676 Ma) and covered a fivefold larger area (1,800 x 600 km, i.e. ca. 1 x 10 km²; Hoernle et al., 25 677 2011) at apparently much lower volumes. In conclusion, formation of the MOZR through 26 27 678 shallow recycling of SCLM would require high volume channelized flow of such material 28 29 679 into the ocean realm over a limited period unlike low volume detachment of SCLM over 30 31 680 longer periods as proposed for the Christmas Island Seamount province (Hoernle et al., 2011). 32 33 34 681 35 36 682 The second model calls upon the recycling of enriched continental material through 37 38 683 the lower mantle via subduction, possible storage in the deeper mantle (e.g., LLSVP), and its 39 40 684 return to the surface via a mantle plume fed from the LLSVP. Geophysical studies suggest 41 42 685 that other structures in the South African gateway (AGP, Maud Rise, and Georgia Rise in the 43 44 686 South Atlantic) could belong to a single magmatic unit, referred to the African Super LIP 45 687 46 (e.g., Gohl and Uenzelmann-Neben, 2001, König and Jokat, 2010, Parsiglia et al., 2008, Gohl 47 688 et al., 2011). The MOZR displays LIP characteristics such as large volume (3.1 x 106 km³) 48 49 689 and aerial extent (0.15 x 106 km²) comparable to other oceanic plateaus (Fischer et al., 2017). 50 51 690 According to Bryan and Ernst (2008), a LIP is characterized by short pulses (~1-5 Ma) with a 52 53 691 maximal lifespan of ~50 Ma. Fisher et al. (2017) demonstrated that the timing of 54 55 692 emplacement was short (10 Ma) with several magmatic pulses forming the sub-plateaus. 56 693 Moreover, most of the MOZR samples have elevated TiO /Yb ratios suggestive for deep 57 2 58 694 melting (Figure 3a, Pearce, 2008). The MOZR samples that trend toward the shallow melting 59 60 695 field may indicate plume-ridge interaction (Pearce, 2008) a likely scenario considering 61 62 63 64 65 696 formation of the MOZR shortly after continental breakup. The mantle plume hypothesis can 1 2 697 also better explain the positive bathymetric anomaly and excess volume of magmatism 3 698 4 produced to form the MOZR, and the similarity between the radiogenic isotopic compositions 5 699 (EM-1 type) with nearby South Atlantic volcanic structures and southern African CFBs. The 6 7 700 good to excellent age progressions of the Tristan-Gough, Discovery and Shona hotspot tracks 8 3 4 9 701 in the South Atlantic and evidence for high He/ He (>10 RA) in the Etendeka flood basalts 10 11 702 (Stroncik et al., 2017) and in lavas from the SMAR adjacent to the Discovery and Shona 12 13 703 hotspot tracks provide strong support that these volcanic tracks were formed by mantle 14 704 15 plumes (O’Connor et al., 2012; Rohde et al., 2013b; O’Connor and Jokat, 2015; 16 705 Schwindrofska et al., 2016). Plate reconstructions for the formation of the MOZR suggests a 17 18 706 southerly directed younging of the direction of plate motion, which is also consistent with 19 20 707 formation from a mantle plume (Fischer et al., 2017). Therefore the similarity in EM-1 type 21 22 708 composition of the MOZR to the Tristan-Gough, Discovery and Shona hotspot tracks, and to 23 24 709 the more depleted end of the Etendeka/Paraná and Karoo/Vestfella CFBs, is consistent with 25 710 derivation of all these volcanic tracks and provinces from mantle plumes that originated from 26 27 711 the margins of a chemically heterogeneous EM-1 type African LLSVP reservoir (Figure 1a). 28 29 30 712 31 32 33 713 6. CONCLUSIONS 34 35 714 36 37 38 715 This study presents the first comprehensive geochemical data set (major and trace 39 40 716 elements, and Sr-Nd-Pb-Hf isotopes) from an extensive sampling campaign (SO232) of the 41 42 717 Mozambique Ridge. We conclude the following: 43 44 718 (1) The evidence for a volcanic nature for the MOZR is clearly supported by material 45 46 719 recovered through dredging during the SO232 cruise. With the exception of exclusively 47 48 720 continental material from a suspect continental block immediately north of the MOZR, 49 50 721 volcanic rocks were recovered across the entire MOZR. The recovered samples, combined 51 6 6 52 722 with estimated volumes (3.1 x 10 km³) and aerial extent (0.15 x 10 km²) (Fisher et al., 53 54 723 2017), are consistent with the MOZR being a LIP. 55 724 (2) The geochemical data are consistent with the MOZR lavas being derived through 56 57 725 mixing of a FOZO-like and enriched (delaminated continental lithospheric ± lower 58 59 726 continental crustal) sources. The MOZR has an EM-1-type geochemical flavor similar to the 60 61 727 Tristan-Gough-Walvis, Discovery and Shona hotspot tracks that are interpreted to have been 62 63 64 65 728 formed by mantle plumes beneath the South Atlantic (Rohde et al., 2013a, Hoernle et al., 1 2 729 2015, 2016, Schwindrofska et al., 2016). The MOZR, however, has lower ∆8/4Pb than the 3 730 4 aforementioned locations, indicating lower time-integrated source Th/Pb ratio. 5 731 (3) The MOZR data plot at the depleted end of the Etendeka/Paraná and Karoo/Vestjfella 6 7 732 CFB arrays. 8 9 733 (4) Combined with recent studies (Fisher et al., 2017), the new data suggests two possible 10 11 734 models: Formation of the MOZR from (1) channelized transfer of subcontinental lithospheric 12 13 735 material to the mid-ocean ridge during continental break-up, in contrast to dispersed, low 14 736 15 volume outflow of such material as proposed for the Christmas Island Seamount Province 16 737 (Hoernle et al. 2011), or 2) a deep-seated mantle plume, derived from the African LLSVP, 17 18 738 which has also been proposed for the South Atlantic EM-1-type hotspots and the CFBs. The 19 20 739 overall LIP characteristics (aerial extent, volume, and timing), the excessive crustal thickness, 21 22 740 the (although vague) spatial age progression during plateau formation, and the geochemical 23 24 741 similarities with South Atlantic hotspots are indicative for a mantle plume to have caused 25 742 formation of the MOZR. 26 27 28 743 29 30 31 744 ACKNOWLEDGMENTS 32 33 34 745 35 36 746 We thank Captain Detlef Korte and his officers and crew of RV SONNE for their professional 37 38 747 and enthusiastic service during the SO232 cruise. We also thank S. Hauff and K. Junge from 39 40 748 GEOMAR and U. Westernströer from Kiel University for their support with the isotope and 41 42 749 trace element analytical processes, respectively. 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(2016) Origin of enriched components in the South Atlantic: 33 979 34 Evidence from 40 Ma geochemical zonation of the Discovery Seamounts, Earth 35 980 Planetary Sci. Lett. 441, 167-177, doi: 10.1016/j.epsl.2016.02.041 36 37 38 981 Simpson E.S.W. (1974) Initial reports of the Deep Sea Drilling Project, vol. 25, 259-346, U.S. 39 40 982 Governement Printing Office, Washington D.C., USA 41 42 43 983 Steinberger B. and Torsvik T.H. (2012) A geodynamic model of plumes from the margins of 44 984 Large Low Shear Velocity Provinces, Geochem. Geophys. Geosystems 13, doi: 45 46 985 10.1029/2011GC003808 47 48 49 986 Stracke A., Bizimis M. and Salters V.J.M. (2003) Recycling oceanic crust: Quantitative 50 51 987 constraints, Geochem. Geophys. Geosystems 4, doi: 10.1029/2001GC000223 52 53 988 54 Stracke A., Hofmann A.W. and Hart S.R. (2005) FOZO, HIMU, and the rest of the mantle 55 989 zoo, Geochem. Geophys. Geosystems 6, doi:10.1029/2004GC000824 56 57 58 990 Stroncik N.A., Trumbull R.B., Krienitz M.-S., Niedermann S., Romer R.L., Harris C. and Day 59 60 991 J. (2017) Helium isotope evidence for a deep-seated mantle plume involved in South 61 62 992 Atlantic breakup, Geology 45, 827-830, doi: 10.1130/G39151.1 63 64 65 993 Sweeney R.J., Duncan A.R., and Erlank A.J. (1994) Geochemistry and petrogenesis of central 1 2 994 Lebombo basalts of the Karoo igneous province, J. Petrol. 35, 95-125, doi: 3 995 4 10.1093/petrology/35.1.95 5 6 996 Thompson G., Bryan W.B., Frey F.A., Dickey J.S. and Davies H. (1982) Petrology, 7 8 997 geochemistry and original tectonic setting of basalts from the Mozambique Basin and 9 10 998 Ridge (DSDP sites 248, 249 and 250) and from the Southwest Indian Ridge (DSDP 11 12 999 site 251), Mar. Geology 48, 175-195, doi: 10.1016/0025-3227(82)90096-2 13 14 1000 Torsvik T.H., Smethurst M.A., Burke K. and Steinberger B. (2006) Large igneous provinces 15 16 1001 generated from the margins of the large low-velocity provinces in the deep mantle, 17 18 1002 Geophys. J. Int. 167, 1447-1460, doi: 10.1111/j.1365-246X.2006.03158.x 19 20 21 1003 Torsvik T.H., Steinberger B., Cocks L.R.M. and Burke K. (2008) Longitude: Linking Earth’s 22 23 1004 ancient surface to its deep interior, Earth Planetary Sci. Lett. 276, 273-282, doi: 24 1005 10.1016/j.epsl.2008.09.026 25 26 27 1006 Tucholke B.E., Houtz R.E. and Barrett D.M. (1981) Continental crust beneath the Agulhas 28 29 1007 Plateau, Southwest Indian Ocean, J. Geophys. Res. 86, 3791-3806, doi: 30 31 1008 10.1029/JB086iB05p03791 32 33 1009 34 Uenzelmann-Neben G. (2014) The expedition of the research vessel „Sonne“ to the 35 1010 Mozambique Ridhe in 2014 (SO232), Berichte zur Polar-und Meeresforschung = 36 37 1011 Reports on Polar and marine research, Bremerhaven, Alfred-Wegener-Institute for 38 39 1012 Polar and Marine Research 677, 206p. 40 41 42 1013 Willbold M. and Stracke A. (2006) Trace element composition of mantle end-members: 43 1014 44 Implications for recycling of oceanic and upper and lower continental crust, Geochem. 45 1015 Geophys. Geosyst. 7, doi: 10.1029/2005GC001005 46 47 48 1016 White W.M. (2015) Isotopes, DUPAL, LLSVPs, and Anekantavada. Chem. Geol. 419, 10-29, 49 50 1017 doi: 10.1016/j.chemgeo.2015.09.026 51 52 53 1018 Workman R. K. and Hart S. R. (2005) Major and trace element composition of the depleted 54 1019 MORB mantle (DMM), Earth Planet. Sci. Lett. 231, 42–53, doi: 55 56 1020 10.1016/j.epsl.2004.12.005 57 58 59 1021 Zhou H. and Dick H.J.B (2013) Thin crust as evidence for depleted mantle supporting the 60 61 1022 Marion Rise, Nature 494, doi: 10.1038/nature11842 62 63 64 65 1023 1 2 1024 FIGURE CAPTIONS 3 4 5 1025 Note: Color should not be used in the print version 6 7 8 1026 Figure 1 9 10 11 1027 (a) Regional map showing the South African gateway (modified from Torsvik et al., 2008). 12 1028 13 White labels show some of the prominent bathymetry anomalies mentioned in the paper. The 14 1029 black box shows the location of the Mozambique Ridge (MOZR). The orange circle shows 15 16 1030 the approximate location of dredges performed on the Agulhas Plateau (AGP). The white 17 18 1031 dotted line shows the approximate location of the Andrew Bain Fracture zone (see König and 19 20 1032 Jokat, 2010 for further details). The dark grey lines show the 1% slow contour of the African 21 22 1033 LLSVP. The two green circles show the reconstructed LIP eruption sites for Karoo (KR) and 23 1034 Paraná-Etendeka (P-E) discussed in the paper. (b) Bathymetric map showing the SO232 24 25 1035 dredge locations (blue circles = tholeiites; red circles = alkali basalts; brown squares = 26 27 1036 Tabletop Mountain). The purple stars and yellow triangle show the dredge locations of 28 29 1037 recovered continental blocks (Mougenot et al., 1991, Ben Avraham et al., 1995). The red 30 31 1038 square with a cross in it is the continental-like sample recovered from an elongated block 32 33 1039 distinct from the main plateau. The blue square shows the location of DSDP Leg 25 Site 249 34 1040 (Simpson, 1974). 35 36 37 1041 38 39 40 1042 Figure 2 41 42 1043 43 Nb/Y versus Zr/Ti classification diagram (after Pearce, 1996), showing the geochemical range 44 1044 of the recovered samples during cruise SO232. The samples are overall very mafic, ranging 45 46 1045 from basalt to alkali basalt. One sample (DR34-1; blue diamond with a bar in it) is more 47 48 1046 evolved with SiO2 = 70 wt. % and plots within the trachyte field. The blue star denotes DR21- 49 50 1047 1, the most depleted sample analyzed. 51 52 1048 53 54 55 1049 Figure 3 56 57 58 1050 (a) Nb/Yb versus TiO2/Yb diagram after Pearce (2008). The MOZR tholeiites reflect a 59 60 1051 mixture of shallow and deep melting material. Moreover, they form a separate trend from the 61 62 1052 alkali basalts. The arrow represent the plume-ridge interaction as shown in Pearce (2008). 63 64 65 1053 Note that trachyte DR34-1 plots at the lower limit TiO2/Yb due to fractionation of Ti bearing 1 2 1054 mineral phases. (b) Nb/Yb versus Th/Yb diagram after Pearce (2008). All samples fall on the 3 1055 4 mantle array, except the Tabletop Mountain samples and one alkali basalt sample, which have 5 1056 slightly elevated Th/Yb ratios. The tholeiites show E-MORB compositions, but the alkali 6 7 1057 basalts tend to OIB type compositions. DR21-1 has an N-MORB type composition. The solid 8 9 1058 squares represent the three end members of Pearce (2008). The red square with a cross is the 10 11 1059 gneiss sample (DR65-1) with continental affinities. 12 13 1060 14 15 16 1061 Figure 4 17 18 19 1062 Primitive-mantle normalized immobile incompatible element diagram (Hofmann, 1988) 20 21 1063 showing (a) the tholeiites from the MOZR plateau and the Tabletop Mountain, and (b) the 22 23 1064 alkali basalts, AGP plateau and continental sample DR65-1. The mafic samples have 24 1065 compositional characteristics between those of OIB and E-MORB. The alkali basalts have 25 26 1066 steeper slopes than the basalts. 27 28 29 1067 30 31 32 1068 Figure 5 33 34 1069 206Pb/204Pb versus (a) 207Pb/204Pb and (b) 208Pb/204Pb . The Pb isotope ratios 35 i_130Ma i_130Ma i_130Ma 36 1070 are typical for DUPAL-like volcanism with high Δ7/4 and Δ8/4. Almost all MOZR samples 37 38 1071 have higher Δ7/4 than the nearby Indian Ocean OIBs or SW Indian MORB field but overlap 39 40 1072 the field for the Walvis Ridge and other S Atlantic hotspots. The samples have similar Δ8/4 to 41 42 1073 the SW Indian OIBs and MORBs but extend to lower Δ8/4 than the Walvis Ridge and 43 1074 44 Discovery Seamounts but largely overlap the field for the Shona hotspot track. The Walvis 45 1075 Ridge data are from Rohde et al. (2013a), Hoernle et al. (2015), and Homrighausen et al. 46 47 1076 (2018, 2019). The Discovery Seamounts data are from Schwindrofska et al. (2016). Shona 48 49 1077 data are from Hoernle et al. (2016). Data sources for MORBs and OIBs are from PetDB 50 51 1078 (http://www.earthchem.org/petdb/) and Georoc (http://georoc.mpch-mainz.gwdg.de/georoc/) 52 53 1079 (Complete references in Supplementary File C). SW Indian MORB (39-41°E) values are from 54 1080 55 Mahoney et al. (1992). NHRL = Northern Hemisphere Reference Line from Hart (1984). The 56 1081 dashed field and the arrow represent the location of 130 Ma-corrected FOZO/HIMU end 57 58 1082 members of Stracke et al. (2005). All Literature data are also projected to 130 Ma. 59 60 61 1083 62 63 64 65 1084 Figure 6 1 2 87 86 143 144 3 1085 (a) Sr/ Sri_130Ma versus initial Nd/ Ndi_130Ma of the MOZR samples form a negative 4 1086 correlation (r²= 0.74), consistent with two-component mixing between a depleted and an 5 6 1087 enriched source. The Tabletop Mountain samples, as well as one basalt and one alkali basalt, 7 8 1088 have the most enriched isotopic compositions, whereas basalt sample DR21-2 has the most 9 10 1089 depleted composition. The compositional range of the MOZR is similar to the SW Indian and 11 143 144 12 1090 S Atlantic MORBs, nearby OIBs and the Walvis Ridge. (b) Nd/ Ndi_130Ma versus 13 1091 176 177 14 Hf/ Hfi_130Ma form a positive correlation (r²= 0.64), overlapping the SW Indian and S 15 1092 Atlantic MORB fields. Data sources are the same as Figure 5. All Literature data are projected 16 17 1093 to 130 Ma. 18 19 20 1094 21 22 23 1095 Figure 7 24 25 87 86 143 144 206 204 207 204 1096 (a) Sr/ Sri_130Ma versus Nd/ Ndi_130Ma, (b) Pb/ Pbi_130Ma versus Pb/ Pbi_130Ma and 26 206 204 208 204 27 1097 (c) Pb/ Pbi_130Ma versus Pb/ Pbi_130Ma. In all three diagrams, the Karoo samples extend 28 29 1098 to much more extreme isotopic compositions (e.g. radiogenic Sr and unradiogenic Pb), which 30 31 1099 is typical for magma interaction with continental lithosphere. The mantle source responsible 32 33 1100 for the Karoo volcanism could be the same for the MOZR and AGP (excluding continental 34 1101 crust interaction). Alternatively, SCLM similar in composition to Finsch Mine, South Africa 35 36 1102 and Smoky Butte lamproites/kimberlites could represent possible enriched end members. Data 37 38 1103 source for Karoo-Vestfjella are from Hawkesworth et al. (1984), Sweeney et al. (1994), 39 40 1104 Jourdan et al. (2007), Neumann et al. (2011), Heinonen and Luttinen (2008), Heinonen et al. 41 42 1105 (2010, 2013, 2014). Data from lamproites/kimberlites are from Fraser et al. (1985), Nelson et 43 44 1106 al. (1986), Murphy et al. (2002), and Nowell et al. (2004). All Literature data are projected to 45 1107 130 Ma. 46 47 48 1108 49 50 51 1109 Figure 8 52 53 1110 87 86 143 144 206 204 54 (a; d) Sr/ Sri_130Ma versus Nd/ Ndi_130Ma, (b; e) Pb/ Pbi_130Ma versus 55 1111 207Pb/204Pb , and (c; f) 206Pb/204Pb versus 208Pb/204Pb . The diagrams show 56 i_130Ma i_130Ma i_130Ma 57 1112 that the MOZR array can be explained by a two-component mixing between two end 58 59 1113 members: F-MOZ (orange stars) and variable EM-MOZ (geochemical composition in 60 61 1114 Supplementary Table B8). Model A: mixing lines between F-MOZ and E-MOZ represented 62 63 64 65 1115 by two types of lamproitic compositions, (1) Gaussberg-type (light grey lines and stars) or (2) 1 2 1116 leucite Hills-type (dark grey lines and stars). None of them fits as unique end member in multi 3 1117 4 Pb-Pb isotope space nor does a mixture of them. Model B: mixing lines between F-MOZ 5 1118 (orange stars) and a Walvis-type composition (black lines and stars) representing the E-MOZ 6 7 1119 end member. The SW Indian MORB (D-MOZ, dashed field) may play an additional role, 8 9 1120 especially in Pb isotopes (grey arrows) to cause deviation from the mixing lines. Each 10 11 1121 increment are 10%. 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figure 1 Figure 1

31˚E 32˚E 33˚E 34˚E 35˚E 36˚E 37˚E 38˚E a) 30˚S b) SO-232 DSDP 25 Mozambique Ridge northern plateau Bathymetry database: DR 65 DR 63 S Atlantic Africa ‘The GEBCO_08 Grid, v. 20091120, http://www.gebco.net 31˚S DR 62 Ridge Lines intervall: 200 m Tholeiites Alkali basalts DR 70 Tabletop Mountain Walvis Ridge MOZR Madagascar DR 72 Rise DSDP Leg 25 site 249 (Simpson, 1974) 32˚S Continental blocks from: This study DR 44 P-E DR 85-87 DR 60 Mougenot et al. (1991) DR 42-43 AGP Ben Avraham et al. (1995) DR 56 Discovery Smts DR 41 Marion & DR 59 DR 52 DR 39 33˚S Prince Edward DR 38 KR DR 51 Islands DR 53 central plateau DR 36-37

Andrew Bain FZ Shona Ridge SW DR 34 Indian DR 31 Bouvet Island DR 06 DR 30 34˚S Ridge DR 29 Triple Junction DR 08 DR 11 southwestern plateau DR 21 DR 19 DR 18 35˚S Astrid Ridge DR 14 Maud Rise DR 15

0 -1000 -2000 -3000 -4000 -5000 -6000 -7000m Antarctica 36˚S Figure 2 Figure 2

1 Agulhas Plateau Alkali- MOZR: rhyolite Basalts Alkali basalts Trachyte Phonolite Tabletop Mountain Rhyolite DR34-1 Dacite Tephri- 0.1 phonolite Trachy- andesite

Andesite

Basaltic andesite Zr/Ti Foidite 0.01

DR21-1 Alkali- Basalt basalt

0.001 0.01 0.1 1 10 Nb/Y Figure 3 0.1 10 Th/Yb TiO2/Yb 10 1 1 0.1 Figure 3 (a) (b)

MORB-OIB array deep melting (OIB) DR21-1 DR21-1 1 N-MORB Nb/Yb Tholeiites E-MORB DR65-1 DR65-1 10 OIB basalts Alkali shallow melting DR34-1 DR34-1 (MORB) Tabletop Mountain basalts Alkali Basalts MOZR: Agulhas Plateau 100 Figure 4 Figure 4

1000

100 OIB Tholeiites Tabletop Mountain

10 E-MORB N-MORB DR21-1 Sample / Primitive Mantle Sample / Primitive 1 1000

Alkali Basalts 100 DR34-1

10

AGP plateau DR65-1 Sample / Primitive Mantle Sample / Primitive 1 Th Nb Ta La Ce Pr Nd Sm Zr Hf Eu Gd Tb Dy Y Ho Er Tm Yb Lu Figure 5 Figure 5

15.80 (a) 15.75 HIMU 15.70

130 Ma 15.65 15.60  Pb 15.55 FOZO

SW Indian 15.50 DR34-1  Pb/ MORB (39-41°E) 15.45 EM-1 15.40 NHRL 15.35 (b)

39.0 HIMU

38.5 FOZO

Agulhas Plateau MOZR:

130 Ma 38.0 Basalts Alkali basalts Tabletop Mountain

 Pb 37.5 EM-1 Walvis Ridge Walvis Ridge HIMU DR21-1 Discovery smts Shona SW Indian OIBs  Pb/ 37.0 SW Indian MORB S Atlantic MORB 36.5 NHRL N Atlantic MORB 16.5 17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 Pb/ Pb 130 Ma Figure 6

Hf/ Hf 130 Ma Nd/ Nd 130 Ma Figure 6 0.2823 0.2825 0.2827 0.2829 0.2831 0.2833 0.5131 0.5129 0.5123 0.5119 0.5127 0.5121 0.5125 0.5119 0.702 (a) DR21-1 (b) EM-1 DR34-1 0.703 .110.5123 0.5121 r² =0.74 0.704 Nd/ Nd r² =0.75 Sr/Sr FOZO/HIMU 0.5125 0.705 130Ma 130Ma FOZO/HIMU 0.5127 0.706 0.5129 Tabletop Mountain basalts Alkali Basalts MOZR: .0 0.708 0.707 Agulhas Plateau SW Indian OIBs SW Indian Shona smtsDiscovery S Atlantic MORB MORB SW Indian Walvis HIMU Ridge Walvis Ridge EM-1 0.5131 Figure 7 Nd/ Nd 130 Ma Figure 7 0.5118 0.5126 0.5114 0.5130 0.5122 0.702 (a) FOZO/HIMU 0.706 Sr/Sr Smoky Butte Smoky Crust Lower

130 Ma 0.710 DR65-1 Crust Upper 0.714 Lamproites/Kimberlites: Paraná-Etendeka CFB Vestella CFB CFB Karoo Tabletop Mountain Moz. Ridge Agulhas Plateau Leucite Hills Seamount ProvinceSeamount Island Christmas Smoky Butte Smoky Finsch Mine Gaussberg Western Australia Eastern Australia

Pb/Pb 130 Ma Pb/Pb 130 Ma 36.5 37.0 37.5 38.0 38.5 39.0 15.6 15.5 15.2 15.3 15.4 15.8 15.7 16.0 DR65-1 DR65-1 Smoky Butte Smoky Smoky Butte Smoky 16.5 701. 801. 19.0 18.5 18.0 17.5 17.0 Pb/ Pb

130 Ma

HIMU FOZO FOZO HIMU 19.5 Figure 8 Figure 8 Figure 8 Nd/ Nd 130 Ma Pb/Pb 130 Ma Pb/Pb 130 Ma 0.5114 0.5118 0.5122 0.5126 0.5130 37.0 37.5 38.0 38.5 39.0 36.5 15.4 15.5 15.6 15.7 .0 .0 .0 .0 0.710 0.708 0.706 0.704 0.702 651. 751. 8519.0 18.5 18.0 17.5 17.0 16.5 (a) (c) (b) EM-MOZ D-MOZ EM-MOZ F-MOZ Sr/Sr Pb/ Model A Model 130Ma Pb EM-MOZ

130 Ma D-MOZ D-MOZ Walvis HIMU Ridge Tabletop Mountain Moz. Ridge Agulhas Plateau Leucite Hills Gaussberg Walvis Ridge F-MOZ F-MOZ

Nd/ Nd 130 Ma Pb/Pb 130 Ma Pb/Pb 130 Ma 0.5121 0.5123 0.5125 0.5127 0.5129 37.5 38.0 38.5 39.0 15.4 15.5 15.6 15.7 15.8 .0 .0 .0 .0 0.706 0.705 0.704 0.703 0.702 651. 851. 20.5 19.5 18.5 17.5 16.5 (d) D-MOZ (f) (e) EM-MOZ EM-MOZ F-MOZ Sr/Sr Pb/ Model B Model D-MOZ Pb 130Ma D-MOZ

130 Ma F-MOZ F-MOZ EM-MOZ