ALBERT

Description: Forschungsschiff METEOR M196: Piräus (Griechenland) – Limassol (Zypern)

Description: Forschungsschiff METEOR M196: Piräus (Griechenland) – Limassol (Zypern)

Description: Forschungsschiff METEOR M196: Piräus (Griechenland) – Limassol (Zypern)

Description: The Tristan da Cunha (TDC) is a volcanic island located above a prominent hotspot in the Atlantic Ocean. Many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot. However, the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area. Marine magnetotelluric and seismological observations were conducted in 2012–2013 to examine the upper mantle structure adjacent to TDC. The electrical conductivity structure of the upper mantle beneath the area was investigated in this study. Three-dimensional inversion analysis depicted a high conductive layer at ~ 120 km depth but no distinct plume-like vertical structure. The conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the TDC islands are located compared to younger segment south of the TDC Fracture Zone, while the bathymetry is rather deeper than prediction for the northern segment. The apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and/or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area. Other observations suggest that 1) the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge, 2) the potential temperature of the segments north of the TDC Fracture Zone is lower than that of the southern segment at least during the past ~ 30 Myr.

Description: The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge – Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed. We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100km down to a depth of 250km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250km the low velocity structure ramifies into narrow veins, each with a radius of ∼50km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

Description: 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 effected 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.