Geochemistry and isotopic ratios of samples obtained during R/V Akademik Nikolaj Strakhov cruise ANS24

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V "Akademik Nikolay Strakhov" in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.

Inorganic geochemistry, epoch and water content in sediments at DSDP Site 91-596

Originally, we had planned to piston core at Site 595 in order to meet the sedimentologic and biostratigraphic objectives outlined in the introductory chapter. However, consultation with our colleagues, Thomas Jordan and John Orcutt on board Melville, indicated that coring near the ocean bottom seismometer (OBS) array around Hole 595B could alter the programmed signal to noise ratio above which teleseisms trigger recording in the OBSs. They requested that we core no closer than about 8 km from three OBSs nearest Hole 595B, and selected a target for us about that distance to the west. Since a new beacon was required at this distance, a new site number, 596, was designated. Briefly, we planned to obtain oriented hydraulic piston cores to the top of the cherts, then core through the cherts using the extended core barrel (XCB) to basement. With improved recovery, we hoped to reach the sediment/basalt contact, and thus obtain a reliable biostratigraphic determination of the basement age. We planned to obtain at least one core in basement, perhaps more, with time permitting. We planned no geophysical program for the hole.

Geochemistry of Ninetyeast Ridge basaltic glasses

The Ninetyeast Ridge (NER), a north-south striking, 5,000 km long, 77 to 43 Ma chain of basaltic submarine volcanoes in the eastern Indian Ocean formed as a hotspot track created by rapid northward migration of the Indian Plate over the Kerguelen hotspot. Based on the major and trace element contents of unaltered basaltic glasses from six locations along the NER, we show that the NER was constructed by basaltic magma derived from at least three geochemically distinct mantle sources: (1) a source enriched in highly incompatible elements relative to primitive mantle like the source of the 29-24 Ma flood basalts in the Kerguelen Archipelago; (2) an incompatible element-depleted source similar to the source of Mid-Ocean Ridge Basalt (MORB) erupted along the currently active Southeast Indian Ridge (SEIR); and (3) an incompatible element-depleted source that is compositionally and mineralogically distinct from the source of SEIR MORB. Specifically, this depleted mantle source was garnet-bearing and had higher Y/Dy and Nb/Zr, but lower Zr/Sm, than the SEIR MORB source. We infer that this third source formed as a garnet-bearing residue created during a previous melting event, perhaps an initial partial melting of the mantle hotspot. Subsequently, this residue partially melted over a large pressure range, from slightly over 3 GPa to less than 1 GPa, and to a high extent (~ 30%) thereby creating relatively high SiO2 and FeO contents in some NER basalts relative to SEIR MORB.

Petrology and geochemistry of basalts at DSDP Hole 86-581

Approximately 5 m of aphyric to sparsely phyric basalt was recovered from Hole 581, the only hole on Leg 86 where basement was cored. The occurrence of samples with altered glassy rinds indicates that at least three cooling units (pillows or thin flows) were sampled. The samples were moderately to intensely altered; groundmass crystals are generally fresh, but all glass is altered. Alteration is greatest in vesicular samples, but most of the samples have fractures filled with iron oxyhydroxide, clay, and/or calcite. All 13 samples analyzed are moderately fractionated aluminous N-type mid-ocean ridge basalts. The samples can be divided into two groups based on TiO2 and FeO contents. The least-evolved group may be derived from a more primitive mid-ocean ridge basalt by the crystallization of 18% plagioclase, 24% clinopyroxene, and 3% olivine. The more evolved group may be derived from the first group by the fractionation of 18% plagioclase, 11% clinopyroxene, and 3% olivine. However, higher Ce/Yb ratios in the more evolved group cannot be produced by fractionation and thus we must invoke a more complex process such as dynamic melting to relate the two groups to a common source.

(Appendix B) Carbonate fraction geochemistry at DSDP Hole 72-516F

This paper presents data on trace elements (Sr, Mg, Na, K, Mn, Fe, Ni, Cr) and isotopes (13C, 18O) on the carbonate fraction of bulk sediments from the Coniacian to Paleocene samples of Hole 516F. Relationships of trace elements to mineralogy and stratigraphic position are discussed at length, with special emphasis on 1) the differences between Hole 516F and other oceanic sites, and 2) the transitions observed at the Cretaceous/Tertiary boundary. Isotope data are compared to those obtained in other localities of the same age. The sections show the same major 13C variations at the Cretaceous/Tertiary boundary, indicating that this event is a planetary phenomenon.

(Table 1) Geochemistry of ODP Hole 130-807C carbonates across the K/T boundary

Paleontological, stable isotopic, trace elemental abundance, and magnetostratigraphic studies have been performed on limestones spanning the Cretaceous/Tertiary boundary transition at Ocean Drilling Program (ODP) Hole 807C. Paleontological evidence exists for considerable resedimentation, which we attribute to the fact that Hole 807C is located in a basement graben. Age estimates based on planktonic foraminiferal biostratigraphy, as well as magnetostratigraphy, indicate that sedimentation rates could have been on the order of 12-14 m/m.y. This is significantly higher than those documented in other important Deep Sea Drilling Project (DSDP) and ODP Cretaceous/Tertiary boundary sections using the same age control points (e.g., DSDP Hole 577 and ODP Hole 690B), although not as high as those documented from DSDP Hole 524. The expanded nature of this succession has resulted in the Cretaceous/Tertiary boundary d13C decrease occurring over approximately a 9-m interval. Ir analysis of these sediments do not show a single large anomaly, as has been found in other Cretaceous/Tertiary boundary sections, but trivial background levels instead. Ce data support the hypothesis that this section has been expanded by secondary sedimentological processes.