Rifting of the oceanic Azores Plateau with episodic volcanic activity

Extension of the Azores Plateau along the Terceira Rift exposes a lava sequence on the steep northern flank of the Hirondelle Basin. Unlike typical tholeiitic basalts of oceanic plateaus, the 1.2 km vertical submarine stratigraphic profile reveals two successive compositionally distinct basanitic to alkali basaltic eruptive units. The lower unit is volumetrically more extensive with ~ 1060 m of the crustal profile forming between ~ 2.02 and ~ 1.66 Ma, followed by a second unit erupting the uppermost ~ 30 m of lavas in ~ 100 kyrs. The age of ~ 1.56 Ma of the youngest in-situ sample at the top of the profile implies that the 35 km-wide Hirondelle Basin opened after this time along normal faults. This rifting phase was followed by alkaline volcanism at D. João de Castro seamount in the basin center indicating episodic volcanic activity along the Terceira Rift. The mantle source compositions of the two lava units change towards less radiogenic Nd, Hf, and Pb isotope ratios. A change to less SiO2-undersaturated magmas may indicate increasing degrees of partial melting beneath D. João de Castro seamount, possibly caused by lithospheric thinning within the past 1.5 million years. Our results suggest that rifting of oceanic lithosphere alternates between magmatically and tectonically dominated phases.

Oceanic plateaus with a crustal thickness to 30 km cover large areas in the oceans and these bathymetric swells affect oceanic currents and marine life 1,2 . Most oceanic plateaus have complex magmatic histories with several volcanic phases erupting tholeiitic to alkaline basaltic lavas over time scales of tens of millions of years [3][4][5][6] . For example, drilling of Pacific oceanic plateaus revealed that the Ontong-Java Plateau apparently formed between 121 and 37 Ma by four volcanic episodes, whereas the Shatsky Plateau erupted continuously between 144 and 129 Ma 4 . The main magmatic episode forming oceanic plateaus is believed to reflect the initial arrival of a deep mantle plume head e.g. 5 , but the overall sequence that follow the mantle and volcanic processes in oceanic plateau remains poorly understood 4 . Stratigraphic sampling of continental flood basalt lava flows yields important insight into petrogenetic processes 7 , but similar studies at oceanic plateaus have been limited by the depths of drill cores that typically sampled the uppermost few hundred meters 4,6 . Oceanic plateaus frequently show evidence of rifting phases like, for example, the Manihiki and Kerguelen Plateaus 8,9 . The Azores Plateau formed 10 to 4 million years ago 10 and is rifted by the NW-SE striking ultraslow Terceira Rift [11][12][13] . Seismic work suggested an opening of the Terceira Rift ~ since 25-20 Ma ago 14 , whereas tectonic studies suggested rifting initiation 1to 2 Ma ago 15,16 . Deep submarine rift basins of the Terceira Rift are results of the extension and expose the earlier volcanic stages along the 1 to 2 km high escarpments of the rift flanks. Volcanic edifices with ages < 1.5 Ma formed within the Terceira Rift 17-20 causing a morphology that resembles the magmatic and amagmatic segments at ultraslow-spreading centers such as the Southwest Indian Ridge and the Gakkel Ridge in the Arctic Ocean 21,22 . Large volcanic structures imply short-lived melt focusing at the magmatic segments, whereas mantle peridotite occurs in deep sediment-covered amagmatic ridge segments 21,23 . Magmatic segments with average lengths of 25 to 60 km are also typical for the continental Main Ethiopian rift system with a significantly thicker lithosphere than slow-spreading mid-ocean ridges 24 . The magmatic intrusions reduce the strength of the lithosphere and thus play an important role in the rifting process 25 .
Here, we present geochronological and geochemical data on the upper 1.2 km of the Azores Plateau crust that give evidence for episodic volcanic activity at the Terceira Rift. The new data show that the Terceira Rift opened after 1.56 Ma with tectonic extension followed by volcanism in the rift basin. The basanitic to alkali basaltic magmas form by low degree (< 5%) partial melting beneath thick lithosphere and the increasing SiO 2 contents Geological setting. The Azores Plateau covers an area of ~ 4 × 10 5 km 2 26 with a minimum crustal thickness of ~ 16 km 27,28 , thus representing a slightly smaller oceanic plateau than Shatsky Rise in the NW Pacific with an area of 5.33 × 10 5 km 2 29 . Large portions of the Azores Plateau probably formed by enhanced melt production close to the Mid-Atlantic Ridge (MAR) between 10 and 4 Ma ago, possibly with the abundant eruption of tholeiitic basalts from large melt volumes in the head of a deep mantle plume 10,30 . In contrast, most of the Azores islands are younger than 1.5 million years and erupt alkaline lavas [17][18][19][20] . The abundant volcanism may be caused by a small thermal mantle anomaly 31,32 , or by decompression melting of a volatile-enriched mantle 33,34 . The anomalously thick oceanic crust of the eastern Azores Plateau is bounded by the roughly N-S striking MAR in the west (Fig. 1). Extension within the Azores Plateau occurs along several NW-SE and WNW-ESE striking fault zones with the Terceira Rift being the most pronounced 11,12 . Several authors suggested the formation of new oceanic lithosphere along the Terceira Rift but no systematic magnetic anomaly pattern parallel to the Terceira Rift is observed 11,13,35 . The extension may have occurred in two phases with the first by normal faulting of existing crust of the entire Azores plateau, and the second very recent phase with magmatic intrusions along the Terceira Rift 35 . Seismic studies reveal an extended crust with numerous normal faults and suggest a NE directed migration of the rifting in the SE part of the Terceira Rift 14 . The oblique ultraslow extension of the Terceira Rift opened the Hirondelle Basin with later formation of the volcanic islands of Terceira and São Miguel 15,36 , and the large D. João de Castro seamount that occurs in the northwestern portion of the basin (Fig. 1). D. João de Castro seamount is an active volcano with reported eruptive activity in 1720 and active shallow hydrothermal venting 37 . The Hirondelle Basin is less than 35 km wide and extends ~ 100 km from SE to NW and is bounded by rift flanks rising from ~ 2500 to 1300 m below sea level (mbsl, Fig. 1). The northern rift flank is steeper than the southern flank probably reflecting the existence of several faulted blocks in the south (Fig. 1). The Hirondelle Basin is seismically active implying ongoing tectonic extension in this area 38 .  Hafnium was separated using a modified version of published methods 43,44 . Titanium (using an oxidation mixture) and Zr were separated from the Hf fractions through further steps on Ln-Spec columns. The isotopes were measured with a Thermo Scientific Neptune Plus High Resolution MC-ICP-MS, at the GeoZentrum Nordbayern in Erlangen, Germany. We measured the AMES Grenoble standard yielding a 176 Hf/ 177 Hf 0.282171 ± 3 (n = 6) compared to a published value of 0.282169 ± 22 42 . All measured standard values and the Hirondelle Basin dataset are listed in the Supplementary Material III. 40 Ar/ 39 Ar ages. All 40 Ar/ 39 Ar age determinations (groundmass and plagioclase phenocrysts, see Table 1) for the Hirondelle Basin samples were carried out at the Oregon State University (OSU) Argon Geochronology Laboratory, USA (described in detail in Supplementary Material IV). The separated grain size fraction between 150 and 300 μm was washed (ultrapure water), dried at 55 °C and plagioclase phenocrysts were separated by hand-picking from groundmass material. The density fractions were acid-leached with 1 M HCl, then 6 M HCl, 1 M HNO 3 , 3 M HNO 3 and ultra-pure deionized water (all for about 60 min) in an ultrasonic bath heated to ~ 50 °C. The plagioclase phenocrysts were leached using 5% HF for 5-15 min. The leached samples were irradiated for 6 h in the TRIGA nuclear reactor at OSU, together with the FCT sanidine flux monitor 45 . The individual J-values for each sample were calculated by parabolic extrapolation of the measured flux gradient against irradiation height and typically give 0.1-0.2% uncertainties (1σ). The 40 Ar/ 39 Ar incremental heating age was determined with two multicollector ARGUS-VI mass spectrometers. After loading the irradiated samples into Cu-planchettes in an ultra-high vacuum sample chamber, they were incrementally heated by scanning a defocussed 25 W CO 2 laser beam in preset patterns across the sample, in order to release the Ar evenly. Each pass involved incremental heating of 15-20 mg of separated groundmass material or plagioclase phenocrysts. The sample material was 'pre-cleaned' for 60 s, while released gasses were pumped away directly at two low (0.5%, 1.8%) laser power settings to remove any loosely-held atmospheric Ar adsorbed onto grain surfaces. After heating, the reactive gases were cleaned out using a SAES Zr-al ST101 getter operated at 400 °C and two SAES Fe- Ar (k) ≥ 50%. (2) The isochron has a spreading factor > 5% (S-factor 51 ), MSWD < 1 + 2 (2/ƒ) 1/2 52 , where f = n-2 and n is number of steps in the isochron, and (3) the 40 Ar/ 36 Ar intercept is within error or greater than 295.5 ± 0.7 1σ. If experiments had no resolvable isochron but yielded highly radiogenic Ar, the initial trapped 40 Ar/ 36 Ar was assumed to equal 295.5 53 , and a plateau model age was calculated.

Results
Sampling and age determinations of lavas from crustal profile. The samples from the northern Hirondelle Basin wall (Fig. 1) were recovered by the Remotely Operated Vehicle (ROV) 'Quest 4000' (MARUM Bremen), during research cruise M128 in 2016 with the German RV Meteor. We stratigraphically sampled a ~ 1.2 km vertical profile of the northern flank of the Hirondelle Basin between 2510 and 1308 mbsl (Fig. 2). All samples were obtained from submarine pillow lava flows (Supplementary Material I) and thus represent eruptive units rather than intrusive rocks. Whereas the lower part of the profile consists only of lavas and dikes, volcaniclastic rocks and pelagic sediments become more abundant shallower than 1690 mbsl depth where they alternate with pillow lavas.
Four samples were selected for 40 Ar/ 39 Ar age dating (on groundmass and plagioclase phenocrysts) at Oregon State University, USA ( Table 1). The lowermost sample IEAZO0903 (2427 mbsl) has a groundmass plateau age of 2.020 ± 0.010 Ma (Fig. 2). Sample IEAZO1054 (1760 mbsl) from the central part of the profile reveals a groundmass plateau age of 1.958 ± 0.008 Ma. The uppermost samples IEAZO1064 (1367 mbsl) and IEAZO1065 (1338 mbsl) show groundmass plateau ages of 1.657 ± 0.004 Ma and 1.558 ± 0.005 Ma, respectively. The groundmass ages are interpreted as eruption ages, yet from the inverse isochron 40 Ar/ 36 Ar intercept calculations, the samples do show evidence for (minor amounts of) excess Ar, which has been corrected accordingly 54 (Fig. 2a). Lavas with low Nb/Zr also display low TiO 2 contents (< 4.2 wt% at > 4 wt% MgO) and relatively high 176 Hf/ 177 Hf isotope ratios (Fig. 2b). The lavas from the Hirondelle Basin wall are alkali basalts, basanites, tephrites, trachybasalts, and phonotephrites with 8.5 to 3.3 wt% MgO (Fig. 3a). Most of the lavas from the Hirondelle Basin wall have lower www.nature.com/scientificreports/ SiO 2 contents at a given MgO concentration compared to lavas from the young volcanoes along the Terceira Rift (Fig. 3b). All lavas are enriched in light relative to heavy Rare Earth Elements (REE) with chondrite-normalized Ce/Yb ratios between 7 and 11 which is similar to basalts from Terceira, whereas lavas from Sete Cidades on São Miguel and from D. João de Castro seamount are more enriched (Fig. 4a). The basalts from the Hirondelle Basin have relatively high (Dy/Yb) N that are comparable to Sete Cidades and Terceira lavas but the D. João de Castro alkali basalts have lower (Dy/Yb) N and higher SiO 2 than those from the Hirondelle Basin wall (Fig. 4b). The lavas of the upper unit between 1390 and 1274 mbsl with high Nb/Zr have low 143 Nd/ 144 Nd and 176 Hf/ 177 Hf ratios but high 87 Sr/ 86 Sr relative to lavas from the lower unit (Figs. 2 and 5). In terms of Nb/Zr and Nd isotope ratios the upper basalts resemble those from D. João de Castro and Sete Cidades whereas the lower lavas overlap with compositions of Terceira basalts (Fig. 5b). The 206 Pb/ 204 Pb ratios of the Hirondelle Basin lavas range from 19.46 to 19.77 where the lower unit generally has higher ratios than the upper unit (Fig. 6). The isotopic composition of the Hirondelle Basin flank lavas overlaps with those of rocks from Terceira but the low Nd and Hf isotope ratios of the upper unit basalts resemble Sete Cidades lavas. Samples from the young D. João de Castro seamount have even lower Nd, Hf, and Pb isotope ratios than the Hirondelle Basin flank basalts.

Discussion
Magmatic evolution of the Azores Plateau. Oceanic plateaus typically consist of tholeiitic lavas reflecting large degrees of melting in the shallow mantle 4,6 . The alkali basaltic to basanitic lavas forming the upper > 1 km of the crust at the Hirondelle Basin (Fig. 3a) are unlikely to represent the initial magmatic plateauforming stage and differ significantly from > 5 Ma old tholeiitic lavas found on the western Azores Plateau 30 . www.nature.com/scientificreports/ Experimental results indicate that alkali basaltic to basanitic melts form by low degrees of partial melting (< 5%) of carbonated garnet peridotite at high pressures > 3 GPa 63,64 . The light REE enrichment supports low degrees of melting and the relative depletion of heavy REE in the Hirondelle Basin lavas (Fig. 4a) suggests a deep melting regime of the magmas in garnet peridotite stability field 65,66 . Thus, the alkaline composition of the lavas from the Hirondelle Basin crustal profile reflects deep partial melting beneath thick lithosphere, unlike the tholeiitic mid-ocean ridge basalts at ultraslow-spreading axes 67 . We conclude that the lavas from the Hirondelle Basin flank represent an alkaline magmatic phase suggesting formation of deep magmas beneath the lithosphere between ~ 2.0 and 1.5 Ma ago, rather than extensive shallow melting producing tholeiitic melts. The primitive basalts of the young D. João de Castro seamount have higher SiO 2 contents and lower (Dy/Yb) N than lavas of the Hirondelle Basin flank (Fig. 4b) which implies larger degrees of melting at lower pressures. Consequently, deep melting apparently formed the magmas prior to ~ 1.5 Ma, followed by lithospheric thinning due to tectonic rifting of the Hirondelle Basin, and finally the generation and eruption of the D. João de Castro magmas.
Change of mantle sources with time. The Nb/Zr ratios are not affected by fractional crystallization processes because they remain constant over a large range of MgO contents (Fig. 5a). At similar MgO the upper unit lavas have higher Nb/Zr than the lower unit basalts (Fig. 5a). Additionally, Nd isotope ratios correlate with Nb/Zr implying that Nb/Zr variations reflect mantle source compositions (Fig. 5b). The lower Nb/Zr ratios of the Hirondelle Basin flank lavas compared to those of the islands indicate a more depleted source. Radiogenic isotope compositions suggest that volcanoes of the Azores are typically fed by distinct mantle sources 57,68 . The Hf and Nd isotope ratios are insensitive to alteration and thus imply different mantle sources between the two lava units of the Hirondelle Basin flank (Fig. 6). Most lavas from the lower unit have higher 143 Nd/ 144 Nd, 208 Pb/ 204 Pb, and 206 Pb/ 204 Pb compositions than those from the upper unit. The isotopes indicate a transition from a source  13,35 . However, seismic profiles across the southeastern Terceira Rift show faulted crust but no evidence for young magmatic spreading 14 which is in agreement with structural observations on São Miguel island 15 . A teleseismic receiver function study reveals that the lithosphere beneath Terceira and São Miguel islands has a thickness of ~ 80 km implying rifting did not cause significant thinning of the plate 27 . Additionally, the alkaline basaltic composition of the lavas erupting at the Hirondelle Basin in the past 2 million years implies melting beneath thick lithosphere, i.e. there is no geochemical evidence for lithospheric thinning with production of  69 . Ultraslow-spreading axes show alternating amagmatic extensional phases and magmatic phases with extension by dike intrusion 23 . Although we do not find evidence for the formation of new lithosphere by magmatic processes in the Hirondelle Basin, we agree with Sibrant et al. 15 that the extension of Terceira Rift follows patterns similar to other ultraslow mid-ocean ridges 70 . The crust exposed at the Hirondelle Basin may thus represent the early magmatic phase in the building of a volcanic ridge (Fig. 7). This volcanic ridge was split by tectonic rifting younger than 1.56 Ma that formed the Hirondelle Basin (Figs. 1 and 7 cross section: A-A') and at a time when the Terceira Rift in this region became volcanically inactive. More recently, the formation of volcanic edifices like D. João de Castro seamount along the Terceira Rift (Figs. 1 and 7 cross section: B-B') indicates that magmas are focusing beneath this portion of the rift leading to volcanism and lateral dike intrusion, potentially with some magmatic spreading in the shallow crust. Our new age of < 1.56 Ma for the opening of Hirondelle Basin is in agreement with previous estimates of the onset of Terceira Rift extension between 1.8 and 0.8 Ma further to the west 16 , and between 2.7 and 1.4 Ma further to the east 15 . The onset of volcanic activity in the Hirondelle Basin is unknown and we assume that D. João de Castro seamount formed within the past 500 kyrs similar to the youngest volcanoes on Terceira and São Miguel 17,19,71 . Rifting of volcanic structures followed by formation of young volcanic cones has also been observed at the eastern end of Terceira 36 and on several other islands with the Terceira Rift like on Graciosa 72 . Similar episodic magmatic phases along an ultraslow-spreading axis exist at the Southwest Indian Ridge 23 . We conclude that the lavas from the northern rift shoulder of the Hirondelle Basin neither represent formation of new ocean floor by magmatic spreading as previously suggested 13,35 , nor do the samples represent an initial phase of formation of the Azores Plateau by high degrees of melting in a mantle plume. Rather, the lavas of the uppermost crust exposed along the Hirondelle Basin represent a rifted volcanic structure that formed by episodic deep and low degrees of partial melting. The volcanic succession implies that much of the thickening (> 1 km) of the eastern Azores Plateau occurred by late addition of lavas. Dike intrusions into the crust and potential magmatic spreading are probably restricted to the volcanic centers of the Azores islands 16 and D. João de Castro www.nature.com/scientificreports/ seamount. We speculate that the wide zone of extension observed in the Azores Plateau 12,17 may become focused along the narrow Terceira Rift with four magmatic segments at western São Miguel, D. João de Castro seamount, Terceira, and Graciosa (Fig. 1). The magma intrusions weaken the oceanic lithosphere which in turn causes strain localization 24,25 . Thus, the general pattern of extension of the Azores Plateau resembles that of continental rifts where tectonic extension starts in a relatively wide area along boundary faults with later narrowing of the zone of deformation and active volcanism 73 .

Conclusions
The upper ~ 1.2 km of the Azores Plateau crust along the Hirondelle Basin formed within ~ 500 kyrs with the lower 1000 m-thick portion erupting within 350 kyrs. Thus, magmatic eruption volumes decreased significantly to the top while the magma source compositions changed. The Hirondelle Basin shows similar episodic volcanic phases to ultraslow-spreading axes although the lithosphere is much thicker and the alkaline basaltic magmas suggest deep melting at relatively low degrees. The formation of volcanoes with heights of > 1 km is followed by tectonic extension with normal faulting but there is no evidence for magmatic spreading with production of new basaltic crust. Slight changes in basalt composition from mainly basanites prior to 1.56 Ma to recent alkali basalts at the D. João de Castro seamount may indicate increasing degrees of melting due to thinning of the lithosphere associated with the formation of the Terceira Rift. The episodic volcanism along the Terceira Rift with breaks of perhaps 1 million years reflects variations of magma formation in the mantle possibly reflecting the ascent of fertile mantle into the melting zone. The tectonic and magmatic evolution of the Hirondelle Basin of the Terceira Rift thus resembles that known from narrow continental rift systems 24 .

Data availability
The dataset we used in the study can be found in Supplementary Information of the manuscript. www.nature.com/scientificreports/