September 2020 LIP of the Month
Temporal geochemical evolution of the Tristan-Gough hotspot track
S. Homrighausen1, H. Zhou1, K. Hoernle1,2
1 GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany
2 Institut für Geowissenschaften, Christian-Albrechts Universität zu Kiel, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany
Email: S. Homrighausen: shomrighausen@geomar.de
Introduction
This contribution is a short summary of selected publications which are based on expeditions with German research vessels into the South Atlantic during the last 10 years. For a more detailed description of the Tristan-Gough (Rohde et al., 2013a; Rohde et al., 2013b; Hoernle et al., 2015; Homrighausen et al., 2018a; Homrighausen et al., 2019; Homrighausen et al., 2020; Zhou et al., 2020), Discovery (Schwindrofska et al., 2016), and Shona (Hoernle et al., 2016) hotspot tracks, and a general overview of intraplate volcanism (Homrighausen et al., in press), the interested reader is directed to the respective publications. In the following, we will focus on the volcanic activity along the Tristan-Gough volcanic lineament, which could serve as type locality of the temporal and geochemical evolution of South Atlantic hotspot tracks.
In general, it is widely accepted that age-progressive volcanic chains represent the surface expression of deep-rooted mantle plumes. The mantle plume model explains the origin of this volcanism as the result of melting related to a relatively stationary focused (conduit-like) mantle upwelling that rises from a fixed source beneath the moving tectonic plates (Morgan, 1971; Wilson, 1973). Accordingly, “hotspots” are considered as the surface expression of mantle plumes that rise from a thermal boundary layer within the Earth. The majority of oceanic hotspots appear to be linked with two continent-sized seismic anomalies at the base of the lower mantle (e.g., Castillo, 1988; Thorne et al., 2004; Torsvik et al., 2006; Jackson et al., 2018a, b), known as the Pacific and African Large Low Shear Velocity Provinces (LLSVPs). Although the relation between mantle plumes and LLSVPs is generally accepted, the nature of their relationship is controversial, as well as the origin and composition of the LLSVP (primordial and/or continuous accumulation of recycled material; see Garnero et al. (2016) for discussion).
The Tristan-Gough seamount chain represents the most prominent bathymetric feature on the South Atlantic seafloor extending more than 3000 km from the Namibian coast to the active volcanic island groups of Tristan da Cunha and Gough (Fig. 1). The volcanic track comprises the voluminous Paraná-Etendeka flood basalts in Brazil and Namibia, the submarine Rio Grande Rise and Walvis Ridge oceanic plateaus in the South Atlantic and a chain of volcanic ridges and seamounts (“Guyot Province”), which further links the Walvis Ridge with the Gough and the Tristan da Cunha Island groups (Fig. 1; e.g. O'Connor and Duncan, 1990; Renne et al., 1996; Hoernle et al., 2015). The entire magmatism along the Tristan-Gough seamount chain was attributed to the Tristan mantle plume (e.g., Courtillot et al., 2003). The initial “plume head” stage is believed to have formed the Paraná-Etendeka flood basalts contemporaneous with continental break-up between Africa and South America. The Rio Grande Rise and Walvis Ridge were subsequently formed through plume-ridge interaction while the plume was located close to the young Mid-Atlantic Ridge. After several major ridge jumps, the Rio Grande Rise was separated from the Walvis Ridge, and continuing volcanism above the presumed plume tail formed the Guyot Province on the African Plate.
Fig. 1: Bathymetric map of the South Atlantic with sample locations along the Tristan-Gough, Discovery and Shona hotspot tracks. Note, sample sites with Tristan-type composition are indicated by blue dots and Southern Discovery by white dots, whereas the Gough-type composition is marked by red dots. Both the Tristan-Gough (since 70 Ma; Rohde et al., 2013a) and Discovery (since 40 Ma; Schwindrofska et al., 2016) show geographical geochemical zonation. The St. Helena HIMU Late-Stage volcanism is marked by yellow triangles. The dashed white line indicates the margin of the African LLSVP. Source of
bathymetric map: http://www.geomapapp.org.
Although this simplified model most easily explains the evolution of the Tristan-Gough chain since ~135 Ma (Renne et al., 1996; Renne, 2015), more recent studies have shown that this hotspot track is much more complex than previously thought (Salters and Sachi-Kocher, 2010; Rohde et al., 2013a; Hoernle et al., 2015; O'Connor and Jokat, 2015b, a; Gassmöller et al., 2016; Schlömer et al., 2017; Homrighausen et al., 2018a; Geissler et al., 2020; Homrighausen et al., 2020; Zhou et al., 2020). Detailed geochronological and geochemical studies, for example, revealed a complex geochemical evolution with spatially and temporally distinct geochemical fingerprints, possibly providing insights into the composition and evolution of the lower mantle/ African LLSVP (e.g., Hoernle et al., 2015; Homrighausen et al., 2020; Zhou et al., 2020).
The age-progressive Tristan-Gough volcanic lineament
The Tristan-Gough volcanic chain has been regarded as textbook example of a hotspot track, which connects flood basalt provinces at its older end with two active volcanic island groups at its younger end (e.g., Morgan, 1972; Richardson et al., 1982; Humphris and Thompson, 1983; Richardson et al., 1984; Thompson and Humphris, 1984; O'Connor and Duncan, 1990). Although the Tristan-Gough volcanic track is believed to represent a type locality for a classical mantle plume (e.g., Richards et al., 1989; Courtillot et al., 2003), the age-progression and thus hotspot origin has been questioned (e.g., Foulger, 2002; Fairhead and Wilson, 2005; Baksi, 2007; Foulger, 2018). Lavas from the Walvis Ridge, for example, have ages up to 40 Ma younger than expected from the linear age progression through the oldest submarine lavas of the Tristan-Gough hotspot track (Fig. 2; O'Connor and Duncan, 1990; Rohde et al., 2013b; Homrighausen et al., 2018a; Homrighausen et al., 2019).
Fig. 2: Distance from the active volcanic islands (Tristan da Cunha and Gough) versus ages for samples along the Tristan-Gough volcanic lineament, displaying two age-progressive trends with distinct (EMI and HIMU) compositions. Note that one sample with EMI composition falls on the HIMU trend and most likely represents resetting of the Ar/Ar age of this sample by the younger HIMU-type volcanism. For sources of data used to construct this figure, refer to Homrighausen et al. (2020).
The combination of morphological, geochemical and geochronological information indicates that the unusually young ages from the Walvis Ridge are derived from post-erosional seamounts (Homrighausen et al., 2018a), henceforth referred as St. Helena type HIMU (high time-integrated µ = 238U/204Pb ratio) Late-Stage volcanism. These alkalic lavas have a distinct St. Helena type HIMU incompatible-element and isotopic composition, compared to the preceding primarily tholeiitic EMI-type (enriched mantle one) composition of the flat-top (eroded) main volcanic edifices of the Walvis Ridge (Fig. 3; Homrighausen et al., 2018a; Homrighausen et al., 2018b; Homrighausen et al., 2019). The long volcanic hiatus (20-40 Ma) and different geochemical composition, which has not been recorded in the age-progressive EMI-type lavas along the volcanic chain (Hoernle et al., 2015; Homrighausen et al., 2019), demonstrate that the St. Helena HIMU-type Late-Stage volcanism is not directly related to the Tristan-Gough hotspot volcanism.
Fig. 3: Geochemical diversity of the Tristan-Gough volcanic lineament on Pb isotope diagrams projected to a common age of 60 Ma. For comparison the composition of St. Helena Island and the Mid-Atlantic Ridge basalts (MARB) are shown with compositions also projected back to 60 Ma. For sources of data used to make this figure, refer to Homrighausen et al. (2019), Homrighausen et al. (2020), and Zhou et al. (2020).
All available ages with EMI-type composition, however, show a spatially continuous age progression along the entire submarine Tristan-Gough volcanic chain (Fig. 2). The age-progressive array ends near a recently discovered conduit-like low shear-wave-velocity anomaly in the upper mantle located ~200 km to the west of Tristan da Cunha (Schlömer et al., 2017), consistent with it being the Tristan mantle plume. In general, the NE-SW-oriented age-progressive submarine Tristan-Gough, Discovery and Shona volcanic tracks are consistent with having formed by the NE movement of the African plate over relatively stationary (or synchronously moving) hotspots / mantle plumes (O'Connor et al., 2012).
A common lower mantle compositional type in South Atlantic hotspots
Apart from the St. Helena type HIMU Late-Stage volcanism, the South Atlantic intraplate lavas along the Tristan-Gough, Discovery and Shona hotspot tracks are characterized by an EMI-type composition (e.g., Class and le Roex, 2011; Hoernle et al., 2015; Hoernle et al., 2016; Schwindrofska et al., 2016; Homrighausen et al., 2019). Several distinct EMI-type flavors are recognized when considering multiple isotope systems (Fig. 3). The origin of these distinct EMI-like flavors is controversial, but generally are interpreted as distinct lower mantle signatures (Rohde et al., 2013a; Hoernle et al., 2015; Schwindrofska et al., 2016; Homrighausen et al., 2019; Zhou et al., 2020). The Gough-type composition is of special interest, since this EMI-type flavor is the long-lived compositional type along the Tristan-Gough track (~135 Ma) and represents the common component in all South Atlantic EMI-type hotspot tracks (Hoernle et al., 2015; Hoernle et al., 2016; Schwindrofska et al., 2016; Homrighausen et al., 2019).
The combination of: 1) a systematic age progression (pointing to a relatively stationary long-term melt anomaly; e.g., O'Connor and Jokat, 2015b; Homrighausen et al., 2019), 2) a constant Gough-type composition over ~135 Ma, which is distinct from the depleted upper mantle composition recorded by Mid-Atlantic Ridge Basalts (Fig. 3; Hoernle et al., 2015; Homrighausen et al., 2019; Zhou et al., 2020), 3) primitive mantle signatures (high helium isotopes ratios) in some plume-related Gough-type lavas (indicating that this material is derived from the lower mantle; Sarda et al., 2000), and 4) low-velocity anomalies beneath the South Atlantic EMI-type hotspots that extend to the base of the lower mantle, i.e. margin of the African LLSVP, provide strong evidence for the origin of the Gough-type plume material in the Tristan- Gough, Discovery and Shona hotspots from a common reservoir in the lower mantle, possibly the LLSVP.
Geochemical zonation along the Tristan-Gough hotspot track
Apart from the common Gough-type EMI composition, distinct EMI-type flavors, such as Tristan-, Southern Discovery- and Doros-types, are documented by multiple isotope ratios along the South Atlantic hotspot tracks (Fig. 1 and 3; Rohde et al., 2013a; Hoernle et al., 2015; Schwindrofska et al., 2016; Zhou et al., 2020). One fundamental observation is that the distinct EMI-type flavors do not appear randomly along the volcanic chains, but instead are clearly spatially separated from each other (Rohde et al., 2013a; Hoernle et al., 2015; Schwindrofska et al., 2016; Zhou et al., 2020). Along the submarine Tristan-Gough hotspot track, for example, the Tristan- and Gough-type composition (Fig. 3), define two distinct sub-tracks (Fig. 1; Rohde et al., 2013a; Hoernle et al., 2015). At the southwest end of the Walvis Ridge, the Tristan-Gough hotspot track becomes geochemically zoned and then bifurcates into two sub-tracks toward (1) Gough island with a Gough-type composition, which also dominates the older hotspot volcanism (e.g., Walvis Ridge), and (2) the Tristan da Cunha island group (Fig. 1 & 3; Rohde et al., 2013a; Hoernle et al., 2015). The seamounts on the Discovery Plateau can also be divided into northern (Gough type) and southern (Southern Discovery type) groups, which are consistent with the geochemical zonation along the adjacent mid-Atlantic Ridge.
In general, two models are considered to explain the spatial geochemical zonation along hotspot tracks: 1) laterally-zoned plume conduit that preserves heterogeneities sampled at the base of the plume through laminar flow (e.g., Farnetani and Hofmann, 2009; Weis et al., 2011; Hoernle et al., 2015) or a plume consisting of enriched fertile lithologies (e.g., pyroxenite/eclogite) within a relatively depleted peridotitic matrix with lateral variations in melting conditions (temperature/pressure), the so-called ‘‘plum pudding model” (Bianco et al., 2008, 2011; Jones et al., 2017). The plum pudding model, however, is inconsistent with the temporal and spatial geochemical evolution along the Tristan-Gough hotspot track and with the Pb isotope systematics (see Hoernle et al., 2015), and thus the laterally zoned plume model is preferred (Hoernle et al., 2015; Zhou et al., 2020).
Since several zoned hotspot tracks worldwide are associated with the margins of the LLSVPs, most recent studies assign the source of the more enriched material to the main body of the LLSVP, e.g. Loa- (at Hawaii), north Galapagos- or Gough-type, and the less-enriched component to ambient mantle entrained from outside of the LLSVP, e.g. Kea- (at Hawaii), south Galapagos-, or Tristan-type (e.g., Hoernle et al., 2000, Huang et al., 2011; Weis et al., 2011; Harpp et al., 2014; Hoernle et al., 2015; Harrison et al., 2017). The appearance of the Tristan-type composition at ~70 Ma could be triggered by continuous material loss from the margin of the LLSVP (Gough-type) until the LLSVP material has been exhausted and the plume stem taps the ambient mantle and/or the plume stem migrates towards the LLSVP boundary with decreasing age (see Hoernle et al. (2015) for detailed description of this model).
Interestingly, the Etendeka lavas show a similar geochemical zonation. The high-Ti (Khumib) basalts in northern Etendeka show identical trace element and isotopic composition as Gough-type lavas in the plume tail stage (Zhou et al., 2020), indicating that the high-Ti Gough-type composition was already present in the plume head stage. Although most of low-Ti basalts in southern Etendeka are strongly contaminated by assimilation of lithospheric components, Doros (Tafelkop) and Horingbaai tholeiitic formations are the least contaminated and reflect the mantle source composition. Opposite to the Tristan-type, the most uncontaminated Doros-type basalts (low Pb/Ce, Th/Nb, 87Sr/86Sr ratios, mantle-like δ18O and primitive 3He/4He ratios) show similar 207Pb/204Pb but lower 208Pb/204Pb than the Gough-type lava at a given 206Pb/204Pb ratio, indicating another distinct magma composition (Fig. 3), which is only documented in the plume head stage (Zhou et al., 2020). Interestingly, this composition has also been found on the Mozambique Ridge extending southwards from the southern tip of Africa (Jacques et al., 2019). It remains unclear why the Doros-type was only present in the initial plume-head stage but three options are possible (Zhou et al., 2020): 1) the Doros-type may have been completely exhausted in the plume source during the plume head stage; 2) the base of the plume stem in the lower mantle may have migrated and thus may no longer have tapped the low-Ti Doros component (Hoernle et al., 2015); or 3) only plume heads may be able to carry dense deep mantle material to the surface (Jones et al., 2019).
Late-stage volcanism and compositionally distinct overlying hotspot tracks
The unexpected St. Helena type HIMU Late-Stage volcanism on the Walvis Ridge is 30-40 Ma younger than the oldest reported EMI-type lavas at a given locality (Fig. 2). Recent geochemical and geochronological data from the Namibian coast demonstrates that the HIMU volcanism extended onto the continent, possibly along the entire Mocamedes Arch in central Angola, which lies on the northeast extension of the Walvis Ridge in the direction of plate motion (Fig. 1; Homrighausen et al., 2020). Combining all available geochronological data for samples with a “HIMU affinity”, the combined “Walvis-Mocamedes” HIMU lineament forms an age-progressive trend with a similar plate velocity as reported from the submarine EMI-type Tristan-Gough hotspot track but displaced to ~30 Ma younger ages at any given location along the volcanic track (Fig. 2; Homrighausen et al., 2020). Since the HIMU volcanism is long-lived, age-progressive, in the direction of and at a similar rate as plate motion and crosses the continent-ocean boundary, the HIMU volcanism is most likely also derived from a deep-seated HIMU mantle plume (Homrighausen et al., 2020). A second paired hotspot on the oldest portion of the Shona hotspot track, with a similar temporal offset between the EMI and HIMU volcanism, suggests that the pairing of Gough-type-EMI primary and St.-Helena-type-HIMU secondary hotspot tracks is not fortuitous and reflects similar spatial relationships in the plume sources (Homrighausen et al., 2020).
Since the South Atlantic EMI-type hotspots overlie the margin of the African LLSVP and the HIMU hotspots (including St. Helena) over a more central portion of the LLSVP, the spatial arrangement between the paired EMI-HIMU hotspots reflect large-scale and long-lasting geochemical zonation within the LLSVP or on the its surface subparallel to the outer margin of the African LLSVP. The location of the distinct reservoirs and mechanism of hotspot pairing is speculative (Homrighausen et al., 2018a; Homrighausen et al., 2020). The secondary HIMU-type volcanism on both lineaments, however, is younger than the primary EMI-type volcanism, which initiated with the Etendeka/Parana and Karoo flood basalt provinces at the Tristan-Gough and Shona track, respectively. The removal of large masses of EMI-type material from the LLSVP or its surface by the plume heads believed to be responsible for these two flood basalt events could initiate instabilities within or on the LLSVPs (Ballmer et al., 2016; Garnero et al., 2016), generating the two secondary HIMU-type mantle upwellings from an internal part of the LLSVP (see Homrighausen et al. (2020) for detailed description of this model).
Conclusion
During the last 10 years, the increasing geochemical and geochronological data sets have provided new perspectives on South Atlantic intraplate volcanism, which has changed the understanding of hotspot volcanism in this region, while constantly raising new questions. Previously, the Tristan da Cunha islands groups were believed to represent the recent hotspot location, but the long-lived Gough-type compositions indicates that the long-lived hotspot should be located beneath Gough island. The composition of the nearly unsampled Rio Grande Rise, which could extend the occurrence of the Tristan-type component in the plume tail stage, is crucial for determining the initiation of geochemical zonation. The origin of: 1) geochemical zonation in the flood basalts and hotspot tracks, 2) compositionally distinct paired hotspot tracks, and 3) distinct EMI-type flavors will continue to raise questions in the coming years. Also, the composition and age of the seamounts between the apparent hotspot tracks is almost unknown, which once again could significantly change our understanding of intraplate volcanism in this region.
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