The magmatic system beneath the Tristan da Cunha Island: Insights from thermobarometry, melting models and geophysics
Introduction
The island of Tristan da Cunha in the South Atlantic belongs to a volcanic archipelago at the southwestward end of a seamount chain which, together with the Ridge, reflects a hotspot track that formed during the Cretaceous separation of South America from Africa (O'Connor and Duncan, 1990, O'Connor et al., 2012, Rohde et al., 2012). Many studies of magma genesis in the Paraná–Etendeka Large Igneous Province have taken Tristan da Cunha lavas to reflect the present-day composition of the Tristan mantle plume (e.g. Ewart et al., 1998, Ewart et al., 2004, Harris et al., 2000; Hawkesworth et al., 1999, Thompson et al., 2001, Trumbull et al., 2003, Gibson et al., 2005, Rocha-Junior et al., 2012, Rocha-Junior et al., 2013). However, recent work has shown that the mantle source(s) and melting regime along the hotspot track have evolved over time since the early Cretaceous Tristan plume (Le Roex et al., 1990, Harris et al., 2000, Gibson et al., 2005, Hicks et al., 2012, Rohde et al., 2013, Hoernle et al., 2015). The picture emerging is one of a diminishing intensity of the plume with time, along with a change in the proportions of mantle components that are melting within it. Rohde et al. (2013) and Hoernle et al. (2015) demonstrated geochemical differences between the Tristan and Gough islands and their respective seamount chains that suggest a compositionally-zoned plume has persisted since separation of the Walvis Ridge from the Atlantic spreading center at about 80 Ma. Gibson et al. (2005) argued from geochemical–petrologic models that the potential temperature in the mantle source had decreased from around 1500 °C in the Etendeka Province to 1450 °C before hotspot-ridge separation at 80 Ma to around 1350 °C for the Tristan archipelago today.
None of the previous studies addressed the question of magma temperatures and pressures recorded in the phenocryst assemblage of Tristan lavas and their implications for the geometry of the “plumbing system” under the hotspot island. That is the main goal of a new geochemical and petrologic study of Tristan da Cunha mafic lavas reported here, where we shed light on the conditions of magma crystallization based on analyses of olivine and clinopyroxene phenocrysts in Tristan da Cuhna lavas and, crucially, of the melt inclusions preserved in olivine. In their detailed geochemical study of Tristan lavas le Roex et al. (1990) established the main geochemical and isotopic features of Tristan magmas and proposed a conceptual model of magma genesis and evolution. Our new trace element analyses include the full REE spectrum and concentrations of other incompatible trace elements not previously determined, and we use those data for inversion modelling to better understand the melting regime in the mantle below the Tristan archipelago. Finally, we compare the petrological constraints on magma generation and storage depths with seismic S-wave velocity variations in the crust and uppermost mantle beneath the Tristan group islands based on new seismological studies (Geissler et al., 2017).
Section snippets
Geologic background and previous work
Tristan da Cunha is the largest of three neighboring islands comprising the Tristan da Cunha group, the others being Inaccessible and Nightingale (Fig. 1). With its highest point at 2060 m above sea level, the nearly conical volcano rises about 5.5 km above the seafloor. Tristan lavas are all younger than 0.5 Ma (Hicks et al., 2012) and the island is still volcanically active: the most recent eruption being in 1961, which caused the island to be evacuated and motivated the volcanologic expedition
Samples and methods
The samples used in this study were collected in 2012 during a stop on the island by the research vessel RV Maria S. Merian, which also conducted geophysical experiments and detailed bathymetry surveys in the region around the Tristan and Gough islands. The emphasis of sampling was on olivine-rich akaramitic basanites and four localities of these were chosen based on previous work: Sandy Point in the east; Big Point, Pigbite (Plantation) Gulch and Wash Gulch in the north and north-west (Fig. 1
Petrography and mineral chemistry
The samples collected for this study are comprised of basanite to trachyandesite, whose bulk compositions cover most of the range established by Le Roex et al. (1990). The ankaramite samples (e.g., crystal-rich basanite with dominant clinopyroxene) contain up to 50% by volume of phenocrysts (clinopyroxene and olivine, minor plagioclase and Ti-magnetite). By definition, clinopyroxene is more abundant than olivine, but in some samples the volume fraction of olivine phenocrysts reaches 20–25%. The
Comparison with geophysical results from the Tristan group islands
Geissler et al. (2017) describe the results of a teleseismic study based on seismometer stations on Tristan da Cunha (station TRIS) and Nightingale Island (NIG01) and an array of ocean-bottom seismometers deployed around the Tristan group islands. Of relevance to this paper are the estimations of the Moho depth and the lithospheric thickness in this region, which helps put the depths of magma generation, storage and crystallization into a geodynamic context. For this purpose, the S-wave
Conclusions
The results of petrologic and geochemical studies on a suite of crystal-rich ankaramite–basanite samples collected from Tristan da Cunha provide estimates of magmatic crystallization temperatures and pressures. We confirm the suggestion of Le Roex et al. (1990) that most Mg-rich ankaramite samples are affected by crystal accumulation and do not represent melt compositions. Olivine-hosted melt inclusions from these rocks provide better information on the melt composition. Thermobarometry
Acknowledgements
We are grateful to the people of Tristan and the island administration for permission to work on the island, for hospitality and help in sample collection. We thank Captain Ralf Schmidt, the crew of R/V Maria S. Merian and the Scientific Party of cruise MSM20/2 for their skilled and friendly work in support of this study. Oona Appelt (Potsdam) provided expert support for the microprobe analyses. Constructive reviews by Sverre Planke and Abigail Barker helped improve the paper. This work was
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2019, Chemical GeologyCitation Excerpt :Such high crystallisation temperatures and mantle potential temperature estimates, together with 3He/4He ratios up to 26 times RA (Stroncik et al., 2017) in other Paraná-Etendeka picrites are consistent with conceptual models that invoke a mantle plume starting head in their generation, as well as in the generation of other comparable CFB provinces. Measurements of a more recently-erupted sample from the Tristan mantle plume indicate a cooler TP of around 1360 °C (Weit et al., 2017) to 1440 °C (Herzberg and Asimow, 2008). These values are broadly consistent with estimates of the mantle around Tristan da Cuhna of TP = 1410–1430 °C from inversion of seismic velocities from an ocean-bottom seismometer array (Bonadio et al., 2018): our results therefore support a model of secular cooling of the Tristan mantle plume.
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2019, Geochimica et Cosmochimica ActaCitation Excerpt :%; Figs. 2 and 3). Overall, the differences between the tholeiitic and alkaline lavas reflect the change from plume-ridge interaction during the Walvis Ridge formation (e.g., Humphris and Thompson, 1983; O'Connor and Duncan, 1990), characterized by low-pressure and high-degrees (15–24%) of melting (Gibson et al., 2005; Salters and Sachi-Kocher, 2010), to intraplate volcanism during formation of the Guyot Province, characterized by an increase in lithospheric thickness resulting in higher pressures and lower degrees (5–8%) of melting (Gibson et al., 2005; Weit et al., 2017). The Gough-type samples form a positive array on the uranogenic Pb isotope diagram (Fig. 4; R2 = 0.90).
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2017, TectonophysicsCitation Excerpt :A minor contribution or later thickening by plume-related magmatism in the order of 0.5 to 1 km cannot be ruled out. The crustal thickness beneath and close to the islands indicate also only minor overprint by the hotspot, but there might be a more complicated Moho structure beneath Nightingale Island due to ongoing magmatic processes (see also Weit et al., in press). From the Ps receiver functions, we do not have clear observations of the base of the lithosphere.
Uppermost mantle and crustal structure at Tristan da Cunha derived from ambient seismic noise
2017, Earth and Planetary Science LettersCitation Excerpt :Weit et al. (2016) studied the magmatic system beneath Tristan da Cunha analyzing thermo-barometric data and found rock crystallization depths of 6–10 and 24–36 km, representing potential levels of magmatic intrusions into the crust and uppermost mantle. Furthermore, Weit et al. (2016) modeled the origin of the associated magmas in a depth range of ∼60 km to ∼80–100 km. Between February 2012 and January 2013 a network of 24 broadband ocean-bottom seismometers (OBS) was in operation around the volcanic archipelago of Tristan da Cunha, complemented by a temporary land station on Nightingale Island (Fig. 1).