Geochemistry of basalts of the Indian Ocean

Indian Ocean crust formed at Sites 765 and 766 is geochemically comparable to that presently forming in the Red Sea. In both cases, we interpret the crust as reflecting high degrees of mantle melting that are associated with an enhanced thermal gradient below recently rifted continental lithosphere. Asthenospheric melts formed in this environment are rich in CaO and FeO, poor in Na2O and Al2O3, and characterized by depleted rare earth element (REE) profiles ([La/Sm]n approximately 0.5-0.6). Both the Red Sea basalts and the basalts at Sites 765 and 766 are distinct from those erupted at the present Mid-Indian Ocean Ridge. The isotope characteristics of the Site 765 basalts define a geochemical signature similar to that of the present-day Mid-Indian Ocean Ridge basalts (MIORB). The Indian Ocean mantle domain is distinct from that of the Atlantic and Pacific oceans, and this distinction has persisted since Jurassic time, when the Site 765 oceanic crust was formed.

Geochemistry at DSDP Hole 84-567A

Dismembered ophiolitic rocks including abundant sheared, serpentinized peridotite (mostly harzburgite) and minor basalts, dolerites, gabbros, and altered metabasites (mainly altered amphibolite) were drilled at most of the sites on the upper to lower Middle America Trench landward slope off Guatemala during Leg 84 of the Deep Sea Drilling Project. These rocks show characteristic Cataclastic deformation with zeolite facies metamorphism and alteration after amphibolite and greenschist facies metamorphism. These features indicate that the rocks originated in mid-oceanic ridge, offridge, and possibly other areas including island arc areas and were metamorphosed under a high geothermal gradient at low pressure. They were then structurally deformed and mixed within a serpentinite melange. Such ophiolite melanges may have been emplaced onto the Trench landward slope area during the initiation of subduction of the Cocos Plate. The emplacement seems to be connected to that of the Nicoya Complex in Costa Rica. The slope cover from early Eocene to Recent shows no history of these metamorphic and deformational events, therefore the emplacement of the dismembered ophiolitic rocks occurred at least before the early Eocene. The dismembered ophiolite-based Trench landward slope off Guatemala is a newly documented style of subduction, which has also recently been found at the easternmost edge of the Philippine Sea Plate along the Izu-Mariana-Yap Trench landward slope.

(Table 1) Composition of magmatic rocks from the Iceland-Faeroe Rise

Devoted to composition of magmatic rocks from the Iceland-Faeroe Rise and their origin.

(Table 1) Chemical composition of Fe-rich layered silicates from the East Korean Rise, Japan Sea

Mineral and chemical compositions of highly ferruginous layered silicates (HLS) of glauconite sands occurred on the East Korean Rise outside volcanic structures and on an unnamed volcano and the Chentsov Volcano have been studied. The use of cluster and discriminant analyses has resulted to more objectively distinguished groups among HLS, and the use of factor analysis - to illustrate correlations between chemical elements in different groups. It has been found that green mineral assemblages of the East Korean Rise are heterogeneous in terms of morphology, composition and origin, and their formation is a complex multistage process including both neoformation and degradation.