Enrico Bonatti

Postdepositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments

Abstract Deep sea hemipelagic sediments from the east Pacific show an oxidized upper zone of variable thickness and a reduced zone below. The geochemistry of a core from this region was studied in detail. The Eh of the upper zone is close to + 100 mV, while in the lower zone it reaches about −400 mV. Reducing conditions in the lower section of the core are related to the oxidation of organic matter in the sediment, as indicated by the gradual decrease with depth of the concentration of nitrogen in the upper part of the core. Various elements, such as Mn, Ni, Co, P and La, are enriched in the upper oxidized zone, while Cr, V, U and S are enriched in the lower reduced zone. Postdepositional mobility of the elements in question, mainly by diffusion in the interstitial solutions, can explain their distribution in the core. Redox reactions can account directly for the mobilization of Mn, Ni, Co, Cr, V and U, indirectly for that of P and La. Fe and Cu do not migrate significantly, since they are immobilized as sulfides in the reduced zone. Th appears not to be affected by diagenetical mobility. Some of the consequences of the Postdepositional mobility of elements in deep sea sediments are: 1. (1) The average elemental content of deep sea sediment, generally estimated from measurements of samples from the upper part of the sediment column, may be affected by errors; namely, an overestimation in the case of Mn, Ni, Co, P and La; an underestimation in the case of V, Cr and U. 2. (2) Postdepositional redistribution of U may introduce errors in age determinations of sediments by the Th 230 Th 232 and Th 230 Pa 231 methods. 3. (3) Some geographical variations in the composition of Fe-Mn nodules can be explained by postdepositional mobility of Mn, Ni and Co.

Dust in the Caribbean atmosphere traced to an African dust storm

Abstract Previous aerosol studies have shown that large quantities of dust are often present in the trade winds entering the Caribbean area; indirect evidence suggested that this material was derived from arid regions in Africa. We report here on a dust event which we observed at Barbados on June 12, 1967, and which we have traced to a dust storm covering a large area of West Africa on June 7, 1967; the storm is clearly visible in an ESSA 5 satellite photograph. The composition of the dust from this storm is similar to that of dusts collected during the summer and fall at Barbados except that the particle size distribution of the storm-derived dust is skewed sharply toward the larger particles: 25% of the sample mass consists of particles above 10 μm diameter and 4% above 20 μm diameter; we attribute the anomalous character of the size distribution to the unusually energetic nature of the dust storm. On the basis of similarities in composition and air parcel trajectories, we conclude that most of the airborne dust transported over the western Atlantic Ocean during the summer and early fall is derived from the same general area of the storm — Spanish Sahara, Mauritania, Mali and Senegal.

Peridotite-gabbro-basalt complex from the equatorial Mid-Atlantic Ridge

Rocks were dredged where the Mid-Atlantic Ridge is intersected by the Chain, Romanche, St Paul and Vema Fracture Zones, and from unfractured portions of the Ridge between 6 and 8° N. Peridotites are common at the fracture zones, but were found also in unfractured sections of the Ridge; harzburgites prevail, but lherzolites, dunites and plagioclase peridotites are also present. A variety of gabbros was recovered, generally above the peridotites, including norites, troctolites, quartz gabbros and theralites. The chemistry of these gabbros indicates a marked crystal-liquid fractionation, following both a ‘tholeiitic’ and an ‘alkali’ trend. The basalts show also both trends, but less markedly. Metamorphic rocks ranging from ‘greenschist’ to ‘amphibolite’ facies are found throughout the sections. Strontium isotopic data suggest that the peridotites (excluding St Peter-Paul rocks) are not related genetically to the associated gabbro-basalt, in a situation similar to that of alpine complexes on the continents. The peridotites are probably residual and were depleted of lithophile elements at some early stage of their history, before the opening of the Atlantic rift. It is postulated that in the upper mantle below the equatorial Atlantic a zone exists of residual, alpine-type peridotite, while the lower crust consists of a mixture of ultramafics and intrusive gabbros. The data indicate strong similarities between the Mid-Atlantic Ridge and alpine complexes from various parts of the world.

Deep-Sea Iron Deposit from the South Pacific

Along with specimens of manganese oxides and basalt, rocks containing more than 30 percent iron by weight and consisting mainly of poorly crystallized goethite have been dredged from the flanks of a seamount located on the East Pacific Rise. The Fe-Mn ratio varies widely among the various oxide rocks deposited at this locality and at another seamount in the same area. The deposit was probably formed by fractional precipitation of iron and manganese which had been introduced locally into the bottom water by hydrothermal solutions of volcanic origin, and by leaching from deep-sea basaltic lavas.

Ultramafic-carbonate breccias from the equatorial Mid Atlantic Ridge

Abstract Breccias consisting of fragments of serpentinized peridotite in a carbonate cement were dredged abundantly from the slopes of transverse ridges existing at the offsets of the Mid Atlantic Ridge in the Romanche and Vema fracture zones. The carbonate cement consists of microcrystalline, low-magnesium calcite and occasionally also of aragonite. The results of chemical and of 18 O/ 16 O, 87 Sr/ 86 Sr, and 234 U/ 238 U isotopic analyses of these carbonates suggest that in part they were deposited at high pH from interstitial sea water circulating in the spaces between the serpentinized peridotite fragments. Several possibilities are discussed as to the mechanisms which caused the ultramafic rocks to become brecciated. It is concluded that the breccias are probably tectonic in origin, and were formed in shear zones resulting from one of several possible types of differential motions between crustal blocks in the tectonically active offset zones. The breccias from the Mid Atlantic Ridge are similar to serpentinite-carbonate breccias associated with serpentinite bodies in the Apennine (Italy) ophiolite complex, which represents uplifted fragments of Mesozoic oceanic crust. The origin of these ophiolitic breccias may be similar to that of the Mid Atlantic Ridge breccias.

Excess argon in deep-sea rocks.

Excess radiogenic argon and helium were found in fresh, geologically young submarine basalts dredged from the crest of the East Pacific Rise and from several seamounts at varying distances from the crest. The presence of the excess gases is related to the glass content of the samples: the glassy outer rim retaining the greatest amount, the more crystalline interior of the flow showing less or no excess radiogenic gases. No definite criteria, other than glass content, could be established to judge the validity of measured K-Ar ages, although fission-track measurements on a limited number of samples provided reasonable upper limits.

Equatorial mid-atlantic ridge: petrologic and Sr isotopic evidence for an alpine-type rock assemblage

Abstract A preliminary petrological description is presented of rocks obtained where the Mid-Atlantic Ridge is intersected by the Chain, Romanche and St. Paul fracture zones and from unfractured sections of the Ridge between 6° and 8°N. Ultramafic rocks are predominant at the base of the sections; harzburgites prevail, but lherzolites, dunites and plagioclase-peridotites are also present. Gabbros, including norites, olivine gabbros and nepheline gabbros are found at intermediate levels, together with minor quantities of quartz-diorite. Dolerites and basalts, including some with alkali affinities, are recovered at the upper levels of the sections. Members of a metamorphic series ranging from greenschist to amphibolite facies are found throughout the sections. TheSr 87 /Sr 86 ratio of the basalts and gabbros ranges from 0.702 to 0.704, within the values previously obtained from oceanic basalts. The peridotites have values ranging from 0.706 and 0.723 and lowRb/Sr ratios. These values are similar to those found previously in alpine-type peridotites from various parts of the world. It is concluded that the peridotites from the Mid-Atlantic Ridge (excluding St. Peter-Paul rocks) are neither the parent material of modern oceanic basalts nor the residual product after basalt extraction. These peridotites are residual and were depleted of lithophile elements at some early stage of their history, before the opening of the Atlantic rift. The abundance of intrusive, alpine-type peridotites suggest that in the upper mantle below the Equatorial Atlantic a zone exists of residual, alpine-type peridotitic material, probably left over since the differentiation of a sialic crust. The data indicate strong similarities between the Mid-Atlantic Ridge and alpine-complexes from various parts of the world.

Nonspreading Crustal Blocks at the Mid-Atlantic Ridge

Transverse ridges consisting of protrusions into crustal fractures of ultramafic bodies derived from the upper mantle exist at the intersection of the Mid-Atlantic Ridge with equatorial fracture zones. Shallow-water limestones containing detrital grains of quartz, microcline, and orthoclase 1 millimeter in diameter were found on the summit of one such transverse ultramafic body at the Vema Fracture Zone; these findings are explained on the assumption that the limestones were deposited within a narrow, shallow proto-Atlantic and were left behind during the further opening of the Atlantic. Transverse ultramafic bodies from the offset zones of the Mid-Atlantic Ridge behave as nonspreading blocks plastered between spreading crustal plates.

Deep-Sea Authigenic Calcite and Dolomite

Crystals of calcite and, in one case, of dolomite up to 90 �m in size are dispersed in pelagic red clay at several sites on the floor of the South Pacific Ocean. They were analyzed by microscopic, x-ray diffraction, electron x-ray microprobe, and oxygen isotopic techniques. These carbonates are authigenic and were probably precipitated from hydrothermal solutions connected with deep-sea volcanic activity.

Uranium in submarine metalliferous deposits

Abstract Hydrothermal submarine metalliferous deposits, common in areas of the ocean floor with high heat flow, contain generally about 10 ppm U as an order of magnitude. The U 234 U 238 ratio is in the majority of cases close to that of seawater; only in a few cases is it anomalously high. Anomalous U 234 U 238 ratios are coupled with low U concentrations. These data are explained by a model where thermal water (essentially heated seawater) in its sub-bottom circulation often is unable to leach U from the basaltic oceanic crust; in fact, these thermal waters may in some cases lose U. When leaching of U from the basalt does take place, probably during shallow stages of the sub-bottom circulation, the resulting anomalous U 234 U 238 ratio can be preserved in the hydrothermal deposit only if mixing with ‘seawater’ U is prevented.