Received: 15 October 2019 | Revised: 31 March 2020 | Accepted: 27 April 2020 DOI: 10.1111/ter.12468 RESEARCH ARTICLE Origin of isolated seamounts in the Canary Basin (East Atlantic): The role of plume material in the origin of seamounts not associated with hotspot tracks Xiaojun Long1 | Jörg Geldmacher1 | Kaj Hoernle1,2 | Folkmar Hauff1 | Jo-Anne Wartho1 | C.-Dieter Garbe-Schönberg2 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany Abstract 2Institute of Geosciences, Kiel University, In contrast to seamount chains, small solitary seamounts/seamount groups have Kiel, Germany rarely been sampled despite their large number and therefore their origins remain 40 39 Correspondence enigmatic. Here we present new Ar/ Ar, trace element and Nd-Hf-Pb isotope data Xiaojun Long, GEOMAR Helmholtz Centre from the solitary Demenitskoy Seamount, the isolated Tolkien seamount group and for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany. the Krylov Seamount and Ridge in the Canary Basin, Central Atlantic Ocean. Their Email: [email protected] chemical compositions range from intraplate ocean-island-basalt (Demenitskoy) to Funding information mid-ocean-ridge-basalt (Tolkien and Krylov) types. Lavas from all three seamount China Scholarship Council, Grant/Award groups, however, show geochemical evidence for involvement of enriched Canary/ Number: [2015]0672 Cape Verde plume material. Seismic tomography shows that large areas around these mantle plumes consist of dispersed low-velocity material, which could represent dif- fusely-upwelling plume mantle. Melts from such upwelling mantle could form isolated seamounts. Diffuse upwelling of plume material is likely to be extremely widespread but has been poorly studied to date. Significance Statement: A fundamental question concerns the origin of the hundreds of thousands of solitary seamounts and small isolated clusters of such seamounts on the seafloor of the world's ocean basins. Most of them do not fit into any cur- rently accepted models (e.g. they are not associated with a linear hotspot track or plate boundary processes). Their formation could therefore represent a new kind of intraplate volcanism that in fact could be extremely widespread but has been thus far largely neglected. In this manuscript, we report geochemical data from three isolated seamount sites in the Canary Basin and propose a provocative model for their forma- tion that can also be applied to isolated seamounts elsewhere. Our study is therefore also a plea for the long overdue systematic investigation of small seamount volcanism in the world's ocean basins. I hereby confirm that all the data and interpretations are new and have not been published elsewhere. All co-authors have been actively involved in this work, have approved the manuscript and agreed to this submission. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2020 The Authors. Terra Nova published by John Wiley & Sons Ltd 390 | wileyonlinelibrary.com/journal/ter Terra Nova. 2020;32:390–398. LONG ET AL. | 391 1 | INTRODUCTION material brought to the base of the lithosphere by a mantle plume (Doucelance, Escrig, Moreira, Gariépy, & Kurz, 2003), as does an Linear chains of intraplate volcanic islands and seamounts in the age progression for the Canary Islands and associated seamounts ocean basins are generally attributed either to mantle plumes (e.g. to the northeast (Geldmacher, Hoernle, Bogaard, Duggen, & Morgan, 1971, 1972; Wilson, 1963) or to shallow tectonically driven Werner, 2005). processes (e.g. Hieronymus & Bercovici, 2000; Koppers, Staudigel, The abyssal plain of the Canary Basin hosts several small and medi- Pringle, & Wijbrans, 2003; Natland & Winterer, 2005). The earth's um-sized isolated seamounts or small clusters of seamounts (Figure 1), ocean floor, however, is dotted with hundreds of thousands of sol- which do not directly relate to any known hotspot tracks. The solitary itary seamounts/seamount clusters with heights of >1 km (Wessel, Demenitskoy Seamount rises ~3,200 m from the sea floor and is lo- Sandwell, & Kim, 2010), which are not directly associated with cated ~800 km to the southwest of the Canary Islands on ~110 Ma old age-progressive hotspot tracks and thus their origins remain enig- oceanic crust (Müller et al., 2016). Bathymetric profiles indicate that matic. In this study, we present 40Ar/39Ar age, trace element and Demenitskoy Seamount is a guyot (see Figure S1) and thus appears Nd-Hf-Pb isotope data of dredged rock samples from isolated sea- to have once formed an ocean island that was eroded to wave base. mounts at three locations in the Canary Basin (Figure 1). Their origin The Tolkien Seamount cluster consists of six unnamed seamounts lo- does not fit any currently accepted models and could represent a cated ~670 km southwest of Demenitskoy Seamount and ~750 km new kind of intraplate volcanism that in fact could be very common northwest of the Cape Verde Islands (Figure 1) on ~95 Ma old crust in the world's ocean basins but has not been extensively researched (Müller et al., 2016). The largest seamount "Tolkien" (Duggen, 2009) thus far. in the cluster rises to a height of ~2,750 metres-below-sea-level (m.b.s.l.). Krylov system is located ~300 km west of the Cape Verde Islands (Figure 1). Multi-beam mapping revealed a roundish ~960 m 2 | GEOLOGICAL BACKGROUND AND high cone, abutting a 15–20 km long E-W oriented ridge (Figure S1), SAMPLING that are named Krylov Seamount and Krylov Ridge respectively (Duggen, 2009). Krylov Ridge is oriented perpendicular to the MAR The Canary Basin in the central East Atlantic Ocean is bordered axis, subparallel to the prevailing strike of the fracture zones in the by the Canary Islands to the northeast, the Azores Archipelago Canary Basin. The age of the oceanic crust beneath Krylov is ~95 Ma to the northwest and the Cape Verde Islands to the southeast (Müller et al., 2016). (Figure 1). Seismic tomography studies show that broad (up to Since the ages and geochemical compositions of these seamounts 800 km across) low-velocity anomalies beneath the Canary, Cape are unknown, it is unclear if these volcanic edifices originally formed Verde and Azores Islands can be traced into the deep mantle, near the MAR, along fracture zones, or in an intraplate setting. During where they coalesce (Montelli, Nolet, Dahlen, & Masters, 2006; cruise POS397/1, volcanic rocks were dredged from the solitary Saki, Thomas, Nippress, & Lessing, 2015). Primitive (high) 3He/4He Demenitskoy Seamount, the Tolkien Seamount cluster and the Krylov ratios detected in phenocrysts of lavas from the Cape Verde Seamount and Ridge (Duggen, 2009). Dredging information are sum- Islands provide further support for the presence of lower mantle marized in Table S1. (a) FIGURE 1 (a) Bathymetric map of the Canary Basin with the location of solitary seamounts and isolated small seamount clusters sampled for this study. *Working name Tolkien was given to a nameless seamount by the POS379/1 scientific party (Duggen, 2009). White seafloor basement isochrons (in Ma) are based on Müller et al. (2016). Map created using (b) GeoMapApp. The inset (b) shows the location of (a) within the central Atlantic Ocean. [Colour figure can be viewed at wileyonlinelibrary.com] 392 | LONG ET AL. 3 | RESULTS 69.55 ± 0.76 Ma (Figure 2) consistent with inverse isochron ages of 68.54 ± 0.69 and 62.4 ± 7.8 Ma. Combining the HT-WM ages from 40 39 3.1 | Ar- Ar age determinations these two samples yielded a weighted mean age of 69.02 ± 0.56 Ma (2σ uncertainties), which we interpret to be the best age estimate for Only a single sample each from Demenitskoy and the Krylov sea- this Krylov Seamount sample. mounts were considered fresh enough to attempt age determination, but unfortunately neither of the samples yielded statistically valid plateau ages (Figure 2). Replicate analyses of sample DR5-1 from 3.2 | Trace element and isotopic variations Demenitskoy Seamount yielded a complex age spectra and inverse isochrons that suggest 39Ar recoil, a problem commonly encoun- Thin section examination shows considerable submarine alteration. tered in Cretaceous altered seafloor basalts (e.g., Koppers, Staudigel, Therefore, we restrict this study to largely alteration-resistant immobile & Wijbrans, 2000). Total fusion ages from the two splits were trace elements (Table S3) and to the Nd-Hf-Pb isotope systems (Table 87.31 ± 0.35 and 91.07 ± 0.39 Ma (all ages are quoted with 2σ exter- S4). A description of sample petrography, standard analytical methods nal uncertainties, unless otherwise stated), which are similar but do and data quality information is given in the Supplemental Material. not overlap within errors. Applying the method of Fleck, Hagstrum, Immobile trace element data are shown in Figures 3 and 4. Lavas Calvert, Evarts, and Conrey (2014), 50% Ar recoil model ages were de- from Demenitskoy Seamount show patterns with an overall enrich- termined for these two replicated analyses, giving ages of 86.8 ± 2.8 ment of more over less incompatible elements and depletion in the and 89.6 ± 3.6 Ma (see Table S2 for more details). Combining these heavy rare earth elements (HREE), characteristic of alkali ocean-is- recoil model ages, we obtained an age of 87.8 ± 2.2 Ma, which is the land basalts (OIBs). The steep HREE patterns can be explained by best age estimate for this Demenitskoy Seamount sample. the presence of residual garnet in the source. In contrast, Tolkien Replicate analyses of basaltic groundmass sample DR9-3 from Seamount lavas display relatively flat patterns without signifi- Krylov Seamount yielded a similarly complex age spectra, with old cant HREE depletion, overlapping normal mid-ocean-ridge basalts low-temperature ages that were influenced by high levels of atmo- (N-MORBs).