Element analyses and isotope composition of basalts from the West Valley segment, Juan de Fuca Ridge, northeast Pacific

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm en -0.3, 87Sr/86Sr = 0.70235 - 0.70242, and 206Pb/204Pb = 18.22 - 18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm en -1.8, 87Sr/86Sr = 0.70245 - 0.70260, and 206Pb/204Pb = 18.73 - 19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with "plums" of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the "plums", initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the "plums" and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the "plums" could evolve to their more evolved isotopic composition after a period of 1.5-2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a lessdepleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as "plums" throughout the upper mantle, transported from depth by mantle plumes.

Neodymium isotope signature of Holocene corals

The neodymium isotopic composition of marine precipitates is increasingly recognized as a powerful tool for identifying changes in ocean circulation and mixing on million year to millennial timescales. Unlike nutrient proxies such as ?13C or Cd/Ca, Nd isotopes are not thought to be altered in any significant way by biological processes, and thus they can serve as a quasi-conservative water mass tracer. However, the application of Nd isotopes in understanding the role of thermohaline circulation in rapid climate change is currently hindered by the lack of direct constraints on the signature of the North Atlantic end-member through time. Here we present the first results of Nd isotopes measured in U-Th-dated deep-sea corals from the New England seamounts in the northwest Atlantic Ocean. Our data are consistent with the conclusion that the Nd isotopic composition of North Atlantic deep and intermediate water has remained nearly constant through the last glacial cycle. The results address long-standing concerns that there may have been significant changes in the Nd isotopic composition of the North Atlantic end-member during this interval and substantiate the applicability of this novel tracer on millennial timescales for paleoceanography research.

Geochemistry and ages at DSDP Hole 19-192A

Conventional K-Ar and 40Ar/39Ar age data on altered basalts from DSDP Hole 192A on Meiji Guyot, Emperor Seamount chain, indicate a minimum age for the volcano of 61.9 ± 5.0 m.y. The K-Ar data are consistent with the early Maestrichtian date of the overlying sediments, but do not provide either a positive or negative test of the hypothesis that Meiji is older than Emperor volcanoes to the south. The most prominent alteration affecting the age measurements is potassium metasomatism, particularly of feldspar phenocrysts. The K-Ar apparent ages of feldspar separates from the Meiji basalts show that more than half of the potassium metasomatism occurred within the last 25 m.y. or so, and that if the potassium replacement rate has been constant, then the alteration of the Meiji basalts did not begin for 10 to 20 m.y. after the volcano formed.

Composition of marine phosphorites and phosphate-bearing sediments

The book deals with behavior of phosphorus and its concentration in oceanic phosphorites. The major stages of marine geochemical cycle of phosphorus including its supply to sedimentary basins, precipitation from sea water, distribution and speciation in bottom sediments, diagenetic redistribution, and relation to other elements are under consideration. Formation of recent phosphorites as a culmination of phosphate accumulation in marine and oceanic sediments is examined. Distribution, structure, mineral and chemical compositions of major phosphorite deposits of various age on continental margins, as well as on submarine plateaus, uplifts and seamounts and some islands are described. A summary of trace element abundances in oceanic phosphorites is presented. Problems of phosphorite origin are discussed.

Annotated record of the detailed examination of Mn deposits from the Albatross 1884 stations

he exploration of the Gulf Stream region was continued in 1884, under nearly the same conditions as in 1883, by the steamer Albatross, Lieut. Z. L. Tanner, commander. During the four trips, between July 20 and Sept. 13, sixty nine dredgings (at stations 2170 to 2238) were made. The results were highly satisfactory, both in the way of physical observatidns and zoological discoveries. In some localities, in 1000 to 1600 fathoms, the bottom was found covered with 0or largely composed of hard, very irregular, flattened, crust-like concretions of clay and iron-oxide, with more or less manganese?oxide in the crevices and worm-burrows with which they are filled. Sometimes a barrel-full, or more, of such masses were brought up, varying in size from a few ounces up to 20 pounds or more in weight alld from one inch to six inches in thickness.

Rhodora

v.80 (1978)

Rhodora

v.81 (1979)