The magmatic system beneath the Tristan da Cunha Island: Insights from thermobarometry, melting models and geophysics

doi:10.1016/j.tecto.2016.08.010

Tectonophysics

Volume 716, 2 October 2017, Pages 64-76

https://doi.org/10.1016/j.tecto.2016.08.010 Get rights and content

Highlights

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Thermobarometry results from melt inclusions in olivine and bulk rock compositions are presented.
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geophysical evidence for Moho depth
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REE inverse modelling imply 5% melt fraction at 60–100 km depth.
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mantle potential temperature of about 1360 °C for the Tristan hotspot

Abstract

This study provides new insights on the conditions of melt generation and of magma transport and storage beneath Tristan da Cunha Island in the South Atlantic. Situated at the seaward end of the Walvis Ridge-guyot hotspot track, this island is related to the evolving magmatic system of the Tristan plume. Much is known about the geochemical and isotopic composition of the alkaline lavas on Tristan, but the pressure–temperature conditions of the hotspot magmas are under-explored. This contribution reports new data from a suite of 10 samples collected during a geologic–geophysical expedition in 2012. The focus of this study is on the least-evolved, phenocryst-rich basanite lavas but we also included a sample of trachyandesite lava erupted in 1961. Mineral-melt equilibrium thermobarometry uses the composition of olivine, clinopyroxene and plagioclase phenocrysts. In addition to bulk-rock data we also analysed olivine-hosted melt inclusions for the P–T calculations. The results for olivine-melt and clinopyroxene-melt calculations suggest crystallization conditions of around 1200–1250 °C and 0.8–1.3 GPa for the least-evolved magmas (ankaramitic basanites). Combined with seismological evidence for a Moho depth of about 19 km, these results imply magma storage and partial crystallization of Tristan magmas in the uppermost mantle and at Moho level. The trachyandesite yielded values of about 1000 °C and 0.2–0.3 GPa (6 to 10 km depth), indicating further crystallization within the crust.

Constraints on the depth and degree of melting at the source of Tristan basanites were derived from REE inverse modelling using our new trace element data. The model predicts 5% melt generation from a melting column with its base at 80–100 km and a top at 60 km, which is consistent with the lithospheric thickness resulting from cooling models and seismological observations. The thermobarometry and melting models combined suggest a mantle potential temperature of about 1360 °C for the Tristan hotspot.

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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Hot primary melts and mantle source for the Paraná-Etendeka flood basalt province: New constraints from Al-in-olivine thermometry
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Citation 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.
Continental flood basalts (CFB) are amongst the most voluminous volcanic eruptions in Earth's history. They are rapidly emplaced, and in rare cases the thick lava piles are associated with primitive magmas that have high MgO contents. The compositions of these primitive melts are consistent with a deep-sourced, high-temperature mantle plume origin. Whilst the association of CFBs with impacting mantle plumes is widely accepted, the magnitude of the thermal anomaly is not yet resolved. The development of Al-in-olivine thermometry, however, allows the crystallisation temperature of (near-)liquidus olivine to be determined without knowing the composition of the co-existing melt. This provides both a robust minimum estimate of mantle temperature and a value from which potential temperature (T_P) can be back-calculated. This technique has previously confirmed that crystallisation temperatures in CFB settings can be a few hundred degrees greater than those estimated for MORB, and the results hint at a diversity in crystallisation temperatures between different CFB settings.
In this study, we re-assess the T_P of the mantle source of the Paraná-Etendeka CFB province by applying the Al-in-olivine thermometer to olivine–spinel pairs from picrites and ferropicrites. We show that the mean crystallisation temperature of olivine with Fo_>90 in the picrites is 1458 °C, with a maximum temperature of 1511 °C. Using the mean value, we calculate a preferred T_P of 1623 °C, for an assumed lithospheric thickness of 50 km and magma emplacement pressure of 0.5 GPa. This represents a thermal anomaly of around +300 °C relative to ambient mantle, and confirms that the mantle source of the Paraná-Etendeka CFB is the second hottest known from Phanerozoic Large Igneous Provinces, after the Caribbean Large Igneous Province. The ferropicrites record a cooler mean olivine crystallisation temperature of 1296 °C. Given that these low-volume melts derive from deeper and earlier melting of mantle pyroxenite, their temperature is not directly comparable to that of the picrites but they appear to require a somewhat cooler mantle source – perhaps found at the front or edges of a rising plume head.
New age and geochemical data from the Walvis Ridge: The temporal and spatial diversity of South Atlantic intraplate volcanism and its possible origin
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%; 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).
Long-lived spatial geochemical zonation of the Tristan-Gough and Discovery hotspot tracks and temporal variations from EMI-type basement to HIMU-type late-stage volcanism at the Walvis Ridge and Shona hotspot tracks point to a complex evolution and multiple source areas for the South Atlantic hotspots. Here we report ⁴⁰Ar/³⁹Ar age and geochemical (major and trace element, Sr-Nd-Pb-Hf isotope) data for samples from 16 new sites on the Walvis Ridge. This aseismic ridge is the oldest submarine expression of the Tristan-Gough mantle plume and represents the initial reference locality of the EMI end member in the South Atlantic Ocean. The EMI-type lavas display an excellent age progressive trend of ∼31 mm/a along the entire Tristan-Gough hotspot track, indicating constant plate motion over a relatively stationary melt anomaly over the last ∼115 Ma. The Gough-type EMI composition of the Tristan-Gough hotspot track is the dominant composition of the >70 Ma part of the Walvis Ridge, the Etendeka and Parana flood basalts, and along the Gough sub-track, extending from DSDP Site 525A on the SW Walvis Ridge to Gough Island, whereas Tristan-type EMI dominates on the Tristan sub-track, extending from DSDP Sites 527 and 528 to Tristan da Cunha Island. Gough-type EMI is also the dominant composition of the northern Discovery and Shona hotspot tracks, suggesting that these hotspots tap a common source reservoir. The continuous EMI-type supply over ≥132 Ma, coupled with high ³He/⁴He (>10 R_A), points to a deep-seated reservoir for this mantle material. The Tristan and Southern Discovery EMI-type flavors can be reproduced by mixing of the Gough-type component with (1) FOZO/PREMA to produce Tristan-type lavas, and (2) marine sediments or upper continental crust to generate the Southern Discovery-type composition. South Atlantic hotspots with EMI-type compositions overlie the margin (1% ∂V_s velocity contour) of the African Large Low Shear Velocity Province (LLSVP), which may promote the emergence of geochemical “zonation”. The St. Helena HIMU-type volcanism, however, is located above internal portions of the LLSVP, possibly reflecting a layered LLSVP.
Insights into the petrogenesis of low- and high-Ti basalts: Stratigraphy and geochemistry of four lava sequences from the central Paraná basin
2018, Journal of Volcanology and Geothermal Research
Citation Excerpt :
In Pitanga samples, Sr shows a negative anomaly with respect to Nd, whereas a Sr spike is visible in most Urubici samples. Notably, samples from Tristan da Cunha (Weaver et al., 1987; Weit et al., 2016), the hypothesized present-day hot spot of the Paraná plume, are markedly different in incompatible trace elements with respect to the here studied HTi samples. Tristan rocks mainly show a positive Nb-Ta anomaly, higher Large Ionic Litophile Elements (LILE) values (including K) and a higher La/Yb ratio.
Lava flow sequences were sampled in the central part of the Paraná basin aiming to verify the time-related evolution of the Paraná basaltic magmatism. It is shown that low- and high-Ti basalts were erupted synchronously. In particular, Esmeralda and Pitanga flows are interlayered, with the former prevailing in the upper part of the sequence. Evidence for synchronously active magma plumbing systems is also supported by mineralogical data, showing signs of mixing between the two groups.
Geochemical data, including Sr-Nd-Pb isotopic compositions are furthermore used to define the mantle source of various low- (Esmeralda and Gramado) and high-Ti (Pitanga and Urubici) magma types. Involvement of a carbonatitic component is proposed for the genesis of the basalts (particularly for the Urubici ones) as suggested by trace element enrichments unrelated to significant isotopic variations. This carbonatitic signature of the mantle source may be conveyed by CO₂-rich metasomatic fluids or melts percolating upwards within the sub-continental lithospheric mantle (SCLM) leading to rapid and selective enrichment of incompatible trace elements. Metasomatism was probably localized at the outskirts of the basin, were Urubici tholeiites and contemporaneous carbonatites were erupted. Geochemical data also suggest the occurrence of significant amounts of crustal contamination in the LTi magmas (mainly in the Gramado and in the late Esmeralda lavas) while crustal assimilation seems negligible in the HTi samples. Globally, a very complex picture arises for the genesis of the Paraná tholeiites, with near-synchronous and geographically coincident flows undergoing significantly different extents of interaction with the crust and tapping different mantle sources.
Thickness of the oceanic crust, the lithosphere, and the mantle transition zone in the vicinity of the Tristan da Cunha hot spot estimated from ocean-bottom and ocean-island seismometer receiver functions
2017, Tectonophysics
Citation 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.
The most prominent hotspot in the South Atlantic is Tristan da Cunha, which is widely considered to be underlain by a mantle plume. But the existence, location and size of this mantle plume have not been established due to the lack of regional geophysical observations. A passive seismic experiment using ocean bottom seismometers aims to investigate the lithosphere and upper mantle structure beneath the hotspot. Using the Ps receiver function method we calculate a thickness of 5 to 8 km for the oceanic crust at 17 ocean-bottom stations deployed around the islands. Within the errors of the method the thickness of the oceanic crust is very close to the global mean. The Tristan hotspot seems to have contributed little additional magmatic material or heat to the melting zone at the mid-oceanic ridge, which could be detected as thickened oceanic crust. Magmatic activity on the archipelago and surrounding seamounts seems to have only affected the crustal thickness locally. Furthermore, we imaged the mantle transition zone discontinuities by analysing receiver functions at the permanent seismological station TRIS and surrounding OBS stations. Our observations provide evidence for a thickened (cold) mantle transition zone west and northwest of the islands, which excludes the presence of a deep-reaching mantle plume. We have some indications of a thinned, hot mantle transition zone south of Tristan da Cunha inferred from sparse and noisy observations, which might indicate the location of a Tristan mantle plume at mid-mantle depths. Sp receiver functions image the base of lithosphere at about 60 to 75 km beneath the islands, which argues for a compositionally controlled seismological lithosphere-asthenosphere boundary beneath the study area.
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2017, Earth and Planetary Science Letters
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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).
According to classical plume theory, the Tristan da Cunha hotspot, located ∼400 km east off the Mid-Atlantic Ridge, is thought to have played a major role in the rifting while creating an aseismic Walvis Ridge during and after the breakup of the South Atlantic margins. Volcanic activity on the Tristan da Cunha Island shows that the hotspot might still be there influencing the upper mantle and crustal structure. In this study we present ambient noise data from 24 broadband OBS around Tristan da Cunha and a seismic station on Nightingale Island, which provide first constraints on the crustal and uppermost mantle structure around the island. By combining ambient noise techniques, dispersion curve analysis of Rayleigh waves, 2D tomographic inversion of travel times and 3D depth inversion of dispersion data we derived a 3D $V_{S}$ velocity model around the archipelago of Tristan da Cunha. The model shows an isolated, vertically sharp bounded thickened and modified crust beneath the islands surrounded by thin oceanic crust (<5 km). The velocity anomaly shows a typical volcanic structure with a shallow high velocity body and a low velocity root reaching into the upper mantle, which we interpret as shallow solidified magmatic material and volcanic feeding system/magmatic underplating, respectively. The observed simple and localized volcanic structure, embedded in a rather homogeneous crust and upper mantle indicates only minor and very localized magmatic overprinting of the existing lithosphere by the Tristan da Cunha hotspot. The uppermost mantle S wave velocity beneath nearby seamounts and to the SW of the islands is also slow and could indicate a thermal influence from a deeper source, whereas the Tristan da Cunha Fracture Zone shows no signs of modification.
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The magmatic system beneath the Tristan da Cunha Island: Insights from thermobarometry, melting models and geophysics

Highlights

Abstract

Introduction

Section snippets

Geologic background and previous work

Samples and methods

Petrography and mineral chemistry

Comparison with geophysical results from the Tristan group islands

Conclusions

Acknowledgements

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

J. Afr. Earth Sci.

Lithos

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Phys. Earth Planet. Inter.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

J. S. Am. Earth Sci.

The volcanological report of the Royal Society expedition to Tristan da Cunha, 1962

Philos. Trans. A

The magma plumbing system for the 1971 Teneguía eruption on La Palma, Canary Islands

Contrib. Mineral. Petrol.

Olivine-melt and orthopyroxene-melt equilibria

Contrib. Mineral. Petrol.

Re-equilibration of melt inclusions trapped by magnesian olivine phenocrysts from subduction-related magmas: petrological implications

Contrib. Mineral. Petrol.

Reference materials for geochemical studies: new analytical data by ICP-MS and critical discussion of reference values

Geostand. Newslett.

Etendeka volcanism of the Goboboseb Mountains and Messum Igneous Complex, Namibia. Part I: geochemical evidence of early cretaceous tristan plume melts and the role of crustal contamination in the Parana–Etendeka CFB

J. Petrol.

Petrology and geochemistry of early cretaceous bimodal continental flood volcanism of the NW Etendeka, Namibia. Part 1: introduction, mafic lavas and re-evaluation of mantle source components

J. Petrol.

Thickness of the oceanic crust and the mantle transition zone in the vicinity of the Tristan da Cunha hot spot estimated from ocean-bottom and ocean-island seismometer receiver functions

Tectophysics

Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid equilibria in magmatic systems at elevated temperatures and pressures

Contrib. Mineral. Petrol.

Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough island lavas: variation with fractional crystallization and evidence for assimilation

Contrib. Mineral. Petrol.

Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation

Geochem. Geophys. Geosyst.