Oceanic peridotites

Abstract

In the 1800s Charles Darwin recognized the existence of serpentine on the island of St. Paul’s rocks in the Atlantic Ocean and correctly concluded that the island was not of volcanic origin. Around the turn of the century peridotites broadly similar to those on St. Paul’s rocks were found as basalt-and kimberlite-borne xenoliths in ocean basins and on continental crust. Indeed it was the correlation between the seismic properties of such ultramafic xenoliths and earthquake seismic data that led to the suggestion by Bowen (1928) that much of the upper mantle was similar in composition to peridotite. Bowen (1928) synthesized much of the available geological and geophysical data and concluded that the sub-crustal regions were peridotitic and that partial melting of garnet peridotite would lead to the production of a basaltic magma and a ‘barren’ or residual peridotite. This idea of a partial melting continuum [i.e. garnet peridotite (source)-dunite (residue)-basalt (partial melt)] gained wide acceptance and was fundamental in any consideration of upper mantle evolution.

‘…… (the) mineralogical constitution (of St. Paul’s rocks) is not simple (being of) a felspathic nature,including thin veins of serpentine…’ (Darwin, 1845)