E. Julius Dasch

Origin of Metalliferous Sediments from the Pacific Ocean

Sediments from near the basement of a number of Deep Sea Drilling Project (DSDP) sites, from the Bauer Deep, and from the East Pacific Rise have unusually high transition metal-to-aluminum ratios. Similarities in the chemical, isotopic, and mineralogical compositions of these deposits point to a common origin. All the sediments studied have rare-earth-element (REE) patterns strongly resembling the pattern of sea water, implying either that the REEs were coprecipitated with ferromanganese hydroxyoxides (hydroxyoxides denote a mixture of unspecified hydrated oxides and hydroxides), or that they are incorporated in small concentrations of phosphatic fish debris found in all samples. Oxygen isotopic data indicate that the metalliferous sediments are in isotopic equilibrium with sea water and are composed of varying mixtures of two end-member phases with different oxygen isotopic compositions: an iron-manganese hydroxyoxide and an iron-rich montmorillonite. A low-temperature origin for the sediments is supported by mineralogical analyses by x-ray diffraction which show that goethite, iron-rich montmorillonite, and various manganese hydroxyoxides are the dominant phases present. Sr87/Sr86 ratios for the DSDP sediments are indistinguishable from the Sr87/Sr86 ratio in modern sea water. Since these sediments were formed 30 to 90 m.y. ago, when sea water had a lower Sr87/Sr86 value, the strontium in the poorly crystalline hydroxyoxides must be exchanging with interstitial water in open contact with sea water. In contrast, uranium isotopic data indicate that the metalliferous sediments have formed a closed system for this element. The sulfur isotopic compositions suggest that sea-water sulfur dominates these sediments with little or no contribution of magmatic or bacteriologically reduced sulfur. In contrast, ratios of lead isotopes in the metalliferous deposits resemble values for oceanic tholeiite basalt, but are quite different from ratios found in authigenic marine manganese nodules. Thus, lead in the metalliferous sediments appears to be of magmatic origin. The combined mineralogical, isotopic, and chemical data for these sediments suggest that they formed from hydrothermal solutions generated by the interaction of sea water with newly formed basalt crust at mid-ocean ridges. The crystallization of solid phases took place at low temperatures and was strongly influenced by sea water, which was the source for some of the elements found in the sediments.

Isotopic composition of strontium in Cretaceous-to-Recent, pelagic foraminifera

Abstract Variations in the strontium isotopic composition of Cretaceous-to-Recent pelagic foraminifera show that the87Sr/86Sr ratio for open ocean water has increased from about 0.7075 to about 0.7092 since Cretaceous time. These values agree, within analytical uncertainties, with those determined by Peterman et al. (1970) on epicontinental carbonate fossils.

Isotopic analysis of metalliferous sediment from the East Pacific Rise

Analysis of iron-rich sediments from the East Pacific Rise suggest that they originate as precipitates from hydrothermal solutions injected into seawater. Ultra-slow-scan X-ray diffraction and K-Ar data point to only trace amounts of biotic and detrital components. The sediment, which consists largely of discrete granules of siliceous and of ferruginous material, contains unusually high concentration of Pb, Zn, and Ni among other elements. An untreated sample and several water-rinsed, size-fractionated samples contain 669–880 ppm Sr with identical87Sr/86Sr ratios of 0.70905 ± 0.0001 - indistinguishable from Sr in modern seawater. After leaching a <20μ fraction of the sample with acetic acid, it retains 356 ppm Sr with87Sr/86Sr = 0.7088. Further leaching with HCl results in the removal of practically all of the ferruginous component; the degraded amorphous silica residue contains 34 ppm Sr with87Sr/86Sr = 0.7065. Volcanogenic Sr (87Sr/86Sr ∼ 0.7035) apparently contributes less than 3% of the total Sr. The sediment contains about 178 ppm Pb with isotopic ratios that are within the ranges of values for Pb from oceanic, tholeiitic basalts; the sediment Pb is markedly less radiogenic than Pb from marine manganese nodules, particularly with respect to207Pb/204Pb. Thus, the bulk of the Sr is derived from seawater, probably by absorption onto reactive Fe-Mn surfaces and incorporation in biogenic phosphate and carbonate, whereas most of the Pb ultimately is of magmatic origin.

Disequilibrium of strontium isotopes between mineral phases of parental rocks during magma genesis — a discussion

Abstract Consideration of available diffusion rate data at high temperatures and pressures casts serious doubt on disequilibrium melting as a mechanism by which variable 87Sr/86Sr ratios can be generated in basaltic melts whose parental rocks were at or below the low-velocity layer for more than a million years. Diffusion rates are orders of magnitude faster in these regions than in the upper lithosphere and lower crust. Ultramafic fragments with minerals in isotopic disequilibrium are pieces of the upper lithosphere, where elapsed time and comparatively low temperatures have resulted in the observed disequilibrium. Regional isotopic heterogeneity in the mantle is the most reasonable explanation for isotopic variability found in abyssal basalts.

Rb/Sr Study of the Stony Creek Granite, Southern Connecticut: A Case for Limited Remobilization

The Stony Creek granite is massive to migmatitic rock that crops out in the core of the Stony Creek dome, the most southerly exposed dome along the Branson Hill anti-clinorium. Rb/Sr data do not yield a precise isochron but indicate that the granite or its precursor is 616 ± 78 m.y. old, and perhaps originated during the Avalonian orogeny. The Stony Creek granite apparently was remobilized and emplaced at a higher structural level, possibly as a mantled gneiss dome, during one or more of the later Paleozoic orogenies (Taconian, Acadian, and perhaps Alleghanian) that formed the Bronson Hill anticlinorium and associated Oliverian domes. Local migration of Rb during Late Pennsylvanian to Permian (Alleghanian) metamorphism is sufficient to explain the major discordances in the data, but earlier redistribution of Rb and Sr cannot be disproven. We suggest that the remobilization of the Stony Creek granite was mainly by ductile flow of solid rock and did not involve extensive melting, mixing, or differentiation.

Metallogenesis in the southeastern Pacific: A progress report on the IDOE Nazca Plate project

Abstract Under the auspices of the International Decade of Ocean Exploration, scientists from Oregon State University and the Hawaii Institute of Geophysics, along with representatives from the several South American countries which border the Nazca Plate, are intensively studying the origin and economic importance of metalliferous sediments which form extensive deposits on and near the East Pacific Rise. The sediments, enriched in iron, manganese, copper, nickel, zinc, silver, molybdenum, and lead are mineralogically and chemically similar to deposits cored by the “Glomar Challenger”, implying a common origin. The metalliferous sediments are especially abundant in an extensive basin, the Bauer Deep, which occurs about 800 km east of the East Pacific Rise. Elemental and isotopic data, supported by geologic information provided by cores and by geophysical instrumentation, suggest that the enriched elements in the sediments originate by the interplay of volcanism and sea water, by precipitation from sea water alone, and perhaps by the influence of deep-ocean physical processes acting on detritus transported from the continents or from the basaltic East Pacific Rise. An additional objective of the project is to define the possible relationship between marine metalliferous sediments of the plate to ore deposits of the Andean Cordillera. As pointed out by several writers, much of the earths economic mineralization occurs along zones of convergence between subducted oceanic crust, including metalliferous sediments if present, and continental margins. Field and geochemical techniques are being applied to determine if sediments are in fact subducted beneath the Andean margin, and if they are remobilized into the continental ore deposits of the Andes. Isotopic tracers may prove most useful in testing this relation. Before the model can be tested rigorously, one must know not only the important chemical parameters of the ocean sediment that may be subducted, but also those of the pertinent rock and ore bodies of the South American Cordillera.

Metallogenesis in Southeast Pacific Ocean: Nazca Plate Project: ABSTRACT

The Nazca Plate Project is a detailed and systematic geological, geochemical, and geophysical investigation of a distinct and tectonically active lithospheric plate of the southeast Pacific Ocean basin. Major tectonic boundaries of the Nazca plate are defined by the East Pacific Rise on the west, along which new oceanic crust of basalt is being generated; the Peru-Chile Trench on the east, where older oceanic crust is being consumed by subduction beneath the overriding South American plate; and the Galapagos spreading center and the Chile Rise on the north and south, respectively. The motion of this plate relative to the Pacific plate is estimated to be eastward at more than 16 cm/yr, as documented by stratigraphic, structural, paleontologic, magnetic, and seismic evidenc . Normal sediments of the deep-sea floor cover basaltic basement rocks over much of the plate, but are locally dominated by a metal-rich component on and near the East Pacific Rise and within the Bauer Deep. These metalliferous sediments, which are composed of crystalline ferromanganese hydroxyoxides (goethite, psilomelane, etc) and Fe-montmorillonite, contain anomalously high concentrations of Fe, Mn, Cu, Ni, Zn, and other transition metals. Elemental abundances and ratios, extraordinary high rates of sedimentation, lead-isotope ratios, and statistical analysis of the data suggest that Fe, Mn, Cu, and possibly other metals were deposited by hydrothermal fluids emanating from sources of basaltic magmatism along the East Pacific Rise. However, isotopic abundances of O, Sr, S, and C and di tribution patterns of the rare-earth elements imply that deposition of the metalliferous sediments occurred at normally low temperatures of the ocean floor and that the nonmetallic components were derived from and/or equilibrated with seawater.