Geochemical composition and atomic formulas of saponites, celadonites and clays from ODP Leg 168 sites

A drilling transect across the sedimented eastern flank of the Juan de Fuca Ridge, conducted during Leg 168 of the Ocean Drilling Program, resulted in the recovery of samples of volcanic basement rocks (pillow basalts, massive basalts, and volcanic glass breccias) that exhibit the effects of low-temperature hydrothermal alteration. Secondary clays are ubiquitous, with Mg-rich and Fe-rich saponite and celadonitic clays commonly accounting for several percent, and up to 10%-20% by volume. Present-day temperatures of the basement sites vary from 15° to 64°C, with the coolest site being about 0.8 Ma, and the warmest site being about 3.5 Ma. Whereas clays are abundant at sites that have been heated to present temperatures of 23°C and higher, the youngest site at 15°C has only a small trace of secondary clay alteration. Alteration increases as temperatures increase and as the volcanic basement ages. The chemical compositions of secondary clays were determined by electron microprobe, and additional trace element data were determined by both conventional nebulization inductively coupled plasma-mass spectroscopy (ICP-MS) and laser-ablation ICP-MS. Trioctahedral saponite and pyrite are characteristic of the interior of altered rock pieces, forming under conditions of low-oxygen fugacity. Dioctahedral celadonite-like clays along with iron oxyhydroxide and Mg-saponite are characteristic of oxidized haloes surrounding the nonoxidized rock interiors. Chemical compositions of the clays are very similar to those determined from other deep-sea basalts altered at low temperature. The variable Mg:Fe of saponite appears to be a systematic function both of the Mg:Fe of the host rock and the oxidation state during water-rock interaction.

Trace element contents in zircons from oceanic crust

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

Composition of sedimemts from the Transpacific profile

The book is devoted to comprehensive study of composition of sediments from the North Pacific Ocean. The sediments have been divided characterized by their lithologic and facial types, grain size composition and mineralogy. Influence of volcanism on formation of mineral and chemical composition of these sediments has been shown. Regularities of distribution of sediment accumulation rates and of a number of chemical elements on the Transpacific profile have been found. Determining role of mechanical fractionation in their localization has been shown.

Toba Tephra across India

A controversy currently exists regarding the number of Toba eruptive events represented in the tephra occurrences across peninsular India. Some claim the presence of a single bed, the 75,000-yr-old Toba tephra; others argue that dating and archaeological evidence suggest the presence of earlier Toba tephra. Resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples, allowing comparison of the Indian samples to those from the Toba caldera in northern Sumatra, Malaysia, and, importantly, the sedimentary core at ODP Site 758 in the Indian Ocean - a core that contains several of the earlier Toba tephra beds. In addition, two samples of Toba tephra from western India were dated by the fission-track method. The results unequivocally demonstrate that all the presently known Toba tephra occurrences in peninsular India belong to the 75,000 yr B.P. Toba eruption. Hence, this tephra bed can be used as an effective tool in the correlation and dating of late Quaternary sedimentary sequences across India and it can no longer be used in support of a middle Pleistocene age for associated Acheulian artifacts.

Geochemistry at DSDP Leg 92 Holes

The major-element and most of the trace-element data from the different laboratories that contributed to the study of samples recovered during Leg 82 are presented in the following tables. The different basalt groups, identified on the basis of their chemical properties (major and trace elements), were defined from the data available on board the Glomar Challenger as the cruise progressed (see site chapters, all sites, this volume). Most of the data obtained since the end of the cruise and presented in these tables confirm the classification that was proposed by the shipboard party (see site chapters, all sites, this volume). Nevertheless, special mention should be made about Site 564. The shipboard party proposed a single chemical group at this site but noticed significant variations down the hole, mainly in trace-element data. However, the range of variation was small compared to the precision of the measurements. These variations were confirmed by the onshore studies (see papers in Part IV of this volume, especially Brannon's paper, partly devoted to this topic).