Résumé

Serpentine forms seamounts which rise as diapirs in the Mariana fore-arc of the western Pacific. These have been imaged by side-scan sonar and sampled by dredging, submersible dives and most recently by drilling in Leg 125 of the Ocean Drilling Program. The crust beneath the world's ocean basins is not dominated by serpentinite, so where do these serpentine seamounts come from? And where is this serpentine's water from? Water is incorporated into ocean crust in hydrous minerals like the serpentines and in sediments on the mid-ocean ridges, and then travels with the spreading sea floor to subduction zones, like the Mariana and Bonin Arcs. The water is released as sediment is squeezed and as hydrous minerals are heated in the down-going slab, and sometimes rises to serpentinize the ultra-mafic rocks of the overlying mantle wedge, so that serpentinite diapirs form which rise to the sea floor above. H.H. Hess once proposed that Layer 3 of the ocean crust was serpentinized mantle — serpentinized at mid-ocean ridges when water from inside the earth hydrated olivines and pyroxenes as the temperature cooled below 500°C near the sea floor. This hypothesis didn't stand up to subsequent tests, but Hess' proposal does seem to work in different circumstances in some island arc settings. And so, for example, we ask here if serpentinite-rich sediments are the products of ancient seamounts in Appalachian ophiolites? Serpentine doesn't merely affect island arcs, ocean floors and ophiolites — it is mined as asbestos. And so in the mines of the Eastern Townships of Quebec serpentine unwittingly set the scene for the meeting of Jean Marchand, Gérard Pelletier and Pierre Elliott Trudeau in the great asbestos strike of 1949 — "La Grève des Amiantes", an usher, a prelude a political revolution, Quebec's Quiet Revolution.