C. Mevel

Metasomatic hydrous fluids in amphibole peridotites from Zabargad Island (Red Sea)

Combining textural, petrological, chemical and isotopic (Sr, H and O) data for amphiboles and whole rocks from the Zabargad peridotite diapir allows three different events to be distinguished. During each event, which can be related to a specific tectonic process of the rifting of the Red Sea, hydrous fluids produced amphiboles. The first and the second generations of amphiboles have characteristics consistent with the involvement of mantle-derived hydrous fluids. The first generation consists of scarce Ti-pargasites which crystallized from small amounts of fluid at temperatures of around 900–1000°C. Their growth was linked to magma percolation in the peridotites before their deformation during diapiric uplift. The second generation consists of Cr-pargasites which crystallized locally (and abundantly) during reaction between the peridotites and a sodium/potassium-bearing hydrous fluid at temperatures of around 700–800°C. These amphiboles grew synchronously with the diapiric uplift. The hydrous fluids probably originated in the sub-continental mantle and were released during the diapiric uplift of the peridotites. The third generation consists of amphiboles (pargasitic hornblende, hornblende sensu lato and tremolite) which are localized in shear zones and veins. They crystallized at temperatures estimated between 700°C and 450°C, again from a sodium/potassium-bearing hydrous fluid. However, this fluid is extraneous to the peridotites, as shown by the Sr, H and O isotope compositions which suggest seawater penetration either during or after the final emplacement of the peridotite diapir. Although the peridotite diapir was emplaced in granulitic gneisses of the pan-African deep continental crust, no evidence was found for a contribution of hydrous continental fluids in the production of the amphiboles present in the peridotite bodies of Zabargad Island.

Occurrence of pumpellyite in hydrothermally altered basalts from the Vema fracture zone (mid-Atlantic ridge)

AbstractMetabasalts with abundant pumpellyite have been dredged in the Vema fracture zone, Atlantic ocean, and contain prehnite+pumpellyite±epidote+chlorite+white mica. The prehnite — pumpellyite association in these rocks differs from the prehnite-epidote association for most of the prehnite — pumpellyite facies metabasalts from the ocean crust described previously. The occurrence of pumpellyite is discussed in terms of temperature conditions,

AN ECLOGITISED OCEANIC PALAEO-HYDROTHERMAL FIELD FROM THE ST. MARCEL VALLEY (ITALIAN WESTERN ALPS)

\mu _{H_2 O}

Spreading Geometry and Melt Supply at the Ultraslow-Spreading Southwest Indian Ridge

and oxygen fugacity and the pumpellyiterich metabasalts are believed to be recrystallized by hydrothermal circulation of seawater at about 250° C under a very low pressure (<1 kb).The bulk composition of the rocks demonstrates a strong chemical modification during hydrothermal metamorphism, similar to what is observed under greenschist facies conditions, except for potassium which can be uptaken from seawater by the rocks.

Hydrothermal deposits at the central spteading axis of the North Fiji Basin(SW Pacific)

New analyses of rare earth elements (REEs) and abundances of K, Rb, Sr, Ba, Th, U, Ta, and Hf in peridotites from Zabargad Island, Red Sea, indicate that light REE-enriched amphibole peridotites in the northern and central bodies have undergone a complex, multistage, metasomatic history during Red Sea rifting. The metasomatism is partly attributed to the passage of alkalic basalts through the mantle lithosphere, which possibly occurred during the early, continental-rift history of the Red Sea. The metasomatism also results from the interaction of the uppermost lithosphere with hydrous crustal fluids released from thinning continental crust. The last metasomatic event is associated with the development of an oceanic hydrothermal system in the Red Sea, after the continental breakup. The available data also suggest that the Zabargad peridotites represent pieces of pan-African lithospheric mantle (the northern and central bodies) pierced by asthenospheric material (the southern body).