Where do Hot Spots Come From?

Claude Jean Allègre Laboratoire Géochimie et Cosmochimie, Université Paris 7 et IPGP, 4,

Place Jussieu, 75252 Paris Cedex 05, France


The idea that oceanic volcanics have two geodynamically different origins is known since Wilson and Morgan, the unresolved question is today: where do they come from?

Presently a general consensus concerning the origin of MORB is reached: they come from a shallow depleted and outgassed mantle. The idea that OIB originate from a hot spot source is also generally accepted but the location of such a hot spot source is debated.

Three different models are discussed:

a) Hot spots come from the core-mantle boundary. This hypothesis was suggested by Hofmann and White.

b) Hot spots come from the 670 km discontinuity. This model was proposed by Allègre and Turcotte and in a more restricted form by McKenzie and O'Nions.

c) Hot spots come from both locations as proposed by Courtillot. Big hot spots originate in the lower mantle, smaller hot spots come from the upper mantle. Recent studies of CFB have linked these basalts to some kind of hot spot activity, i.e. the Iceland or the Reunion hot spot.

We will discuss the question of the origin of hot spots and evaluate different pieces of information:

a) isotopic ratios of lithophile elements specially Pb

b) 187Os/186Os isotopic compositions

c) rare gases (including He, Ne, Ar, Xe)

d) ISOTER (Ta/Th-, Nb/U, Sm/Hf, Ce/u, Ba/Rb ratios)

e) geophysical observations

Rather than to answer this problem definitively, we will try to propose some tests that allow to distinguish between the different working hypothesis.

We will discuss the FOZO model from S. Hart and the model on ridges proposed by G. Schilling in more detail as well as the fundamental question of timing or interaction between Hot Spot and upper mantle materials. In this approach, we will try to distinguish between solved and open questions.