Geochemical and Petrologic Data for Basalts from Sites 756, 757, and 758: Implications for the Origin and Evolution of the Ninetyeast Ridge

Abstract

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.

Data Report: Assessment of the Precision of Leg 135 Shipboard XRF Analyses and the Contamination Introduced by Crushing in Tungsten Carbide

“…In contrast, all other elements presented here appear to be free from a significant crushing blank, and the shipboard powders should be adequate for future studies involving these elements. We note here in agreement with Joron et al (1990) and in contrast with Frey et al (1991) that we found no evidence for the introduction of Zn caused by crushing in the tungsten mill.…”

“…We acknowledge that differences may be present between the absolute abundances of elements introduced into the basalts and our test silica as a result of the different abrasive properties of the two materials. Other studies have sought to eliminate this by comparing splits of the same samples prepared in agate and tungsten carbide (e.g., Frey et al, 1991). In a similar manner, we have compared the data from two samples powdered on the ship with additional samples taken from exactly adjacent pieces of the core.…”

Building of the Amsterdam-Saint Paul plateau: A 10 Myr history of a ridge-hot spot interaction and variations in the strength of the hot spot source

“…The compositional signature of the basalts of the area requires the existence of a heterogeneous ASP component that mixes with the upper mantle in variable proportions [ Doucet et al , 2004; Nicolaysen et al , 2007]. Evidence of the ASP component at the southern end of the Ninetyeast Ridge on 38 Myr old volcanism, [ Frey et al , 1991; Weis and Frey , 1991] suggests that this source is long‐lived. The ASP hot spot anomaly is however supposed to be small.…”

Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard Island, Indian Ocean

“…In the bottom panel C, the time axis is not linear. The Cretaceous lavas drilled from the Kerguelen Plateau and Ninetyeast Ridge are tholeiitic basalts with relatively high Zr/Nb, with the exception of ODP Site 748 where they are alkalic [ Storey et al , 1992; Frey et al , 1991]. The Oligocene flood basalts from the Kerguelen Archipelago have transitional major element compositions with intermediate Zr/Nb ratios [ Yang et al , 1998] whereas the alkalic Miocene flood basalts from the Kerguelen Archipelago have lower Zr/Nb [ Frey et al , 2000b].…”