David E. Fisher

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.

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.

Distribution of mercury in East Pacific sediments

Abstract Fifty-seven sediments from the East Pacific have been analyzed for Hg. All high Hg-value are found on the crest of the East Pacific Rise. Either volcanic or absorption processes could explain this distribution pattern.

Volcanogenic uranium, vanadium and iron in Indian Ocean sediments

Abstract About one hundred U, V and Fe analyses of Indian Ocean sediments indicate that enrichments of U and V occur in hemipelagic sediments close to the continents, probably associated with biogen constituents. The highest concentration, however of U, V and Fe occur in active ridge sediments, suggesting that submarine volcanism is an important source of these elements.

URANIUM CONTENT AND RADIOGENIC AGES OF HYPERSTHENE, BRONZITE, AMPHOTERITE, AND CARBONACEOUS CHONDRITES.

Abstract U was measured by fission track analysis in 115 samples of hypersthene, bronzite, amphoterite and carbonaceous chondrites. On a weight basis the average values for the Cl carbonaceous and bronzite chondrites are similar to the “classic” value of 11 ppb, but the hypersthenes and amphoterites are ~50 per cent higher. Each class shows a well-determined peak in the U abundance distribution, allowing the calculation of radiogenic ages and comparison with other elements of interest.

Rare gas abundances in MORB

Abstract Data are presented for He, Ar and Xe in a number of MORB glasses and crystalline rocks. The rare gases in the latter group are characterized by atmospheric Ar and Xe, coupled with loss of trapped mantle He, Ar and Xe. The glass data also show clear signs of an atmospheric-like component for Ar and Xe. A reasonably constant world-wide average trapped mantle gas value of 40 Ar/ 36 Ar ~15000 ±3000 is suggested. Anomalies are seen in the 129 Xe/ 132 Xe ratio, but at least part of this might be due to mass fractionation rather than to 129 I decay. Correlating the data with those from oceanic island basalts in Ar-Xe space indicates that the latter samples are probably also greatly influenced by an atmospheric component rather than (or in addition to) trapping gases from an undepleted mantle source.

K-Ar geochronology of the Limestone Caribbees and Martinique, Lesser Antilles, West Indies

Abstract Twenty-seven K-Ar ages have been measured on igneous rocks from islands of the Limestone Caribbees (St. Martin, St. Bartholomew, Antigua, and Grande Terre, Guadeloupe) and Martinique in the Lesser Antilles arc. Earlier paleontological data indicated that rocks on several of these islands are Eocene in age and among the oldest known in the arc. The oldest igneous activity on the islands studied is 35–40 m.y. Bifurcation of the Lesser Antilles arc north of Guadeloupe took place between 7 and 20 m.y. ago. A distinct physical overlap of older and younger volcanic arcs exists on Martinique. Our results do not support previous suggestions that the pre-Tertiary basement of the Greater Antilles extends through the Limestone Caribbees to the anomalously old, Jurassic-Cretaceous, igneous rocks of La Desirade.

HELIUM, ARGON, AND XENON IN CRUSHED AND MELTED MORB

Helium, argon, and xenon were measured in vesicles of MORB from a variety of locations and in sequential crushing steps in two locations. Melting extractions, compared to these data, indicate that most of the trapped radiogenic gases are contained in the glass matrix and that vesicle/glass equilibrium has not been reached. 4He40Ar∗ ratios (where the asterisk indicates the radiogenic component) are clearly a function of eruption depth, a fact that I interpret in terms of the Klein/Langmuir model. Within individual samples, they correlate with 36Ar40Ar∗, indicating that some of the helium is included in the atmospheric-like contamination that is inherited in a diffusion process immediately prior to eruption. The previously documented 129Xe anomaly is not seen in these samples, suggesting that it may not be ubiquitous in MORB.

Oceanic basalts: Chemistry versus distance from oceanic ridges

Abstract The major elements chemistry, as expressed by the normative composition, and the degree of silica saturation of basalts erupted at various distances from the axis of the East Pacific Ridge do not differ systematically from the same parameters of basalts erupted at the axis of the Ridge. These results suggest that the conditions of magma generation are similar beneath Ridges and away from them.

Heavy rare gases in a Pacific seamount

Abstract Ar, Kr, and Xe were measured in a Pacific seamount and in some related samples. No isotopic anomalies were found, and the elemental ratios indicate sorption from a reservoir of atmospheric values.