Geochemistry and petrology of ODP Leg 121 basalts

Basement lavas from Sites 756, 757, and 758 on Ninetyeast Ridge are tholeiitic basalts. Lavas from Sites 756 and 757 appear to be subaerial eruptives, but the lowermost flows from Hole 758A are pillow lavas. In contrast to the compositional variation during the waning stages of Hawaiian volcanism, no alkalic lavas have been recovered from Ninetyeast Ridge and highly evolved lavas were recovered from only one of seven drill sites (DSDP Site 214). All lavas from Site 758 have relatively high MgO contents (8-10 wt%), and they are less evolved than lavas from Sites 756 and 757. Although abundances of alkali metals in these Ninetyeast Ridge basalts were significantly modified by postmagmatic alteration, abundances of other elements reflect magmatic processes. At Site 757 most of the lavas are Plagioclase cumulates, but lava compositions require two compositionally distinct, AhCb-rich parental magmas, perhaps segregated at relatively low mantle pressures. In addition, at both Sites 756 and 758 more than one compositionally distinct parental magma is required. The compositions of these Ninetyeast Ridge lavas, especially those from Site 758, require a source component with a depleted composition; specifically, the abundance ratios Th/Ta, Th/La, Ba/Nb, Ba/La, and La/Ce in these lavas are generally less than the ratios inferred for primitive mantle. Lavas from Ninetyeast Ridge and the Kerguelen Archipelago have very different chondrite-normalized REE patterns, with lower light REE/heavy REE (LREE/HREE) ratios in lavas from Ninetyeast Ridge. However, lavas from Sites 757 and 758 have Pb isotope ratios that overlap with the field defined by lavas from the Kerguelen Archipelago (Weis and Frey, this volume). Therefore, these Ninetyeast Ridge lavas contain more of a component that is relatively depleted in LREE and other highly incompatible elements, but have similar amounts of the component that controls radiogenic Pb isotopes. A model involving mixing between components related to a depleted source and an enriched plume source has been proposed for the oldest Kerguelen Archipelago basalts and Ninetyeast Ridge lavas. Although the incompatible element characteristics of the Ninetyeast Ridge lavas are intermediate between depleted MORB and Kerguelen Archipelago basalts, these data are not consistent with a simple two-component mixing process. A more complex model is required.

Major-oxides and trace-elements at DSDP Hole 78-543A

Four chemically distinct basalts were cored in 44 m of basement penetration at Deep Sea Drilling Project Site 543, in Upper Cretaceous crust just seaward of the deformation front of the Barbados Ridge and north of the Tiburon Rise. All four types are moderately fractionated abyssal tholeiites. The four types have different magnetic inclinations, all of reversed polarity, suggesting eruption at different times which recorded secular variation of the earth's magnetic field. Extensive replacement of Plagioclase by K-feldspar has occurred at the top of the basalts, giving analyses with K2O contents up to 5 %. The earliest stages of alteration were dominantly oxidative, resulting in fractures lined with celadonite and dioctahedral smectite, and pervasive replacement of olivine and most intersertal glass with iron hydroxides and green clay minerals. Latef, non-oxidative alteration resulted in formation of olive-green clays and pyrite veins in a portion of the rocks. Basalts affected by this alteration actually lost K2O (to abundances lower than in adjacent fresh basalt glasses), and gained MgO (to abundances higher than in the glasses). Finally, fractures and interpillow voids were lined with calcite, sealing in much fresh glass. Oxygen-isotope measurements on the calcite indicate that this occurred at 12 to 25C. Either altering fluids were warm or the basalts had become buried with a considerable thickness of sediments, such that temperatures increased until a conductive thermal gradient was established, when the veining occurred.