Résumé
The peridotite massifs from the NorthWestern Pyrenean belt are fragments of a short-lived mid-Cretaceous hyper-extended rift inverted by the Pyrenean orogeny. We studied the petrology, mineralogy and geochemistry of 32 hydrothermally altered peridotites from the Montaut, Turon and Urdach massifs. This study aims to bring new constraints on the nature of serpentinization processes from domains having experienced various degrees of crustal hyper-extension and mantle exhumation in a context analogous to non-volcanic ocean-continent transitions. The three massifs studied are mainly composed of spinel-lherzolites and sample the same sub-continental mantle lithosphere as Eastern Pyrenean refertilized-peridotite massifs. They display contrasted degrees of hydrothermal alteration and serpentinization during cooling (e.g., serpentine contents ranging from 1 to 100 wt%) as well as variable reaction paths. The Montaut massif, located below an extremely thinned Paleozoic upper crust, records cooling from high temperature hydrothermalism with incipient serpentinization affecting mostly pyroxenes (serpentine, tremolite, talc, clinochlore assemblage; ~350-450 • C) to the formation of lizarditechrysotile and magnetite after olivine (~200 • C-350 • C). The Turon peridotites, which remained below the detached Mesozoic pre-rift cover after complete removal of the crust, are little serpentinized and have no magnetite, suggesting serpentinization during the latest stages of cooling (< 200 • C). The Urdach massif comprises magnetite-bearing ophicalcites as well as magnetite-poor serpentinites formed concurrently when mantle was exhumed to the sub-sea!oor at the margin toe (down to ~200 • C). Each massif shows selective enrichments in !uid mobile elements, in particular, Cs, Sb and Li, indicative of hydrothermal interactions with !uids deriving from neighboring sediments and continental crust, likely mixed with seawater-derived !uids for the Urdach peridotites. Our results show that the formation of magnetite in the studied samples is not tied solely to temperature, and point to a role of !uid composition in the development of the different serpentinization pathways observed from one massif to the other, and within a same massif in the case of Urdach. The development of complex sequences of temperature-, and composition-controlled reaction paths distinguish serpentinization in the Western Pyrenees peridotites from that commonly observed in the oceanic subsea!oor. It results in variable redistribution of iron between magnetite and serpentine minerals, with the predominance of Fe-rich lizardite in magnetite-poor serpentinized peridotites. It likely controls local redox conditions and H 2 production; suggesting that the impacts of serpentinization on the production of dihydrogen will vary from passive margins to oceanic sub-sea!oor.