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Geologists passionate about the "Dead Poets" of Amsterdam and St. Paul islands

The plateau of Amsterdam and St. Paul islands, and particularly its "Chain of the Dead Poets" are very interesting for the Earth sciences. Geochemistry unveils the origin of the lavas formed within this original tectonic context and brings new insight on wide-scale processes.


Deep magma is brought to the Earth surface along the 65,000-km long network of oceanic ridges and punctually as isolated plumes called "hotspots". As lithospheric plates move, these two magma sources can get closer to each other (quite a rare situation), drift apart (case of the Azores or Reunion island) or be active at a same location on the Earth surface (as in Iceland).

Oceanic ridges (white dotted line) and hotspots (triangles); in several cases hotspots come on top of ridges.

Studying Amsterdam-St Paul plateau (ASP) is particularly interesting because it results from the activity of a hotspot which successively had the three kinds of interaction with the ridge lying in the south-east of the Indian Ocean. The traces of these geometries are still visible because the relative displacement of the plate and the hotspot caused the formation of the two islands Amsterdam and St Paul, and of a set of seamounts called by names of French poets and writers and forming a chain therefore known as "Chain of the Dead Poets".

This work is the first attempt to describe and model in space and time the complete evolution of a ridge/hotspot interaction. To understand and characterize the sources of magmatism and their interaction under the ASP plateau, a cruise took place in 2006 on the R/V Marion-Dufresne. Following a detailed mapping of the area with sidescan sonar, forty dredging stations were done between depths of 700 and 3000 m. The best preserved rock samples, and volcanic glass when possible, were submitted to geochemical analysis of the main constituents (silica, iron, aluminium, etc.), on the trace elements present in very small amounts (25 elements including cesium, rubidium, baryum, thorium, etc.) and on the isotopes of a few elements (naturally decreasing radioactivity): hafnium (Hf), lead (Pb), strontium (Sr), neodymium (Nd)

Example of the analysis of the 25 trace elements: the shape of the profiles is an indicator of the possible origin of the sampled lavas.

All these results show that the lavas of the ASP plateau are geochemically complex. The analyses aim to discriminate the different constituents of this blend and their origin.

The two isotopic rations of lead (208Pb/204Pb et 206Pb/204Pb) allow a straightforward classification of the samples. All those from ASP plateau lie within a triangle or on its sides, which means that these lavas all result from the mixing, in different proportions, of three components whose isotopic compositions are indicated by the summits of the triangle.

Distribution of samples (color symbols) according to the two isotopic rations of lead. The triangle which contains them all is delimited by the red and blue dotted lines; the "ASP end-member") lies at their intersection.

Even though the lavas of the youngest seamount (Apollinaire) are probably close to it, the magmatic source feeding the hotspot ("ASP end-member") could not be sampled. Its composition is deduced from the most radiogenic summit of the triangle (the one with the highest isotopic ratios). Although less variable, the isotopic ratios of the other elements give information on the origin of this deep magmatic source. The analysis of the overall composition of the samples shows that the ASP end-member contains a significant part of pelagic sediments which covered the bottom of the oceans more than 1.5 billion years ago. These sediments were reincorporated into the Earth's mantle via a process called subduction, and then were recycled to participate into the formation of ASP hotspot.

Within the triangle, the two sides of the "ASP end-member" summit show the mixing of this source with two other ones. The scattering of CDP samples is mainly explained by the blending of the "ASP end-member" with a second source derived from the Indian Ocean upper mantle. However the composition of the samples coming from the whole plateau also shows the influence of a third source. This influence is extremely strong in the samples from the dredging station 24, where isotopic rations have extreme values for all the elements:
neodymium (143Nd/144Nd), hafnium (176Hf/177Hf), strontium (87Sr/86Sr) and lead (208Pb/204Pb, 206Pb/204Pb). This kind of signature is typical of the DUPAL anomaly, a chemical composition anomaly well known in Indian Ocean lavas and whose origin has been debated within the scientific community for almost thirty years.

Discussing the various possible origins of this anomaly and comparing these hypotheses with the observations lead to the conclusion that it is the incorporation of material of continental origin which is the most able to explain the composition of these samples. Continental rocks could have been stripped off and incorporated into the Indian upper mantle during the breakup of Gondwana supercontinent around 120 Myr ago.

The lavas of Amsterdam-St Paul plateau therefore derive from a blend of three components; this blend could be modeled and quantified, with 45 to 75 % of plume material ("ASP end-member") and 25 to 55 % of Indian Ocean upper mantle material, the latter itself containing 0 to 6 % of continental crust remains. This study thus showed and discussed global geodynamic processes whose traces are visible at a smaller scale.


The paper

Janin M., Hémond C., Maia M., Nonnotte P., Ponzevera E., Johnson K. T. M., 2012. The Amsterdam–St. Paul Plateau: A complex hot spot/DUPAL-flavored MORB interaction. Geochemistry Geophysics Geosystems 13(9).
See the first page


The authors

This works is the result of a collaboration between scientists from the laboratory Domaines océaniques of IUEM, the Department of Marine Geosciences of Ifremer and the University of Hawaii (USA).


The journal

Geochemistry, Geophysics, Geosystems (G3) is one of the scientific journals of the American Geophysical Union; this entirely electronic journal has no printed version. It publishes papers on the chemistry, physics and biology of Earth and planetary processes, particularly those pertaining to understanding the Earth as a system. Papers present observational, experimental, and theoretical investigations of the solid Earth, hydrosphere, atmosphere, and biosphere at all spatial and temporal scales.



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