Determination of crystallization temperatures in fractional crystallization series by nickel partitioning equations

Four high-quality seismic refraction profiles were recorded parallel to the structural grain in the Cuvier Basin and adjacent Wharton Basin to study the nature of the earth's crust in this area. The principal result of this experiment is that this area is generally floored with oceanic crust. No transitional velocity structure exists at the base of the continental slope. Departures from a standard oceanic crustal section are observed on an intermediate profile that are attributed to structural complications on the flank of an abandoned spreading ridge. Additional magnetic anomaly profiles in the eastern Cuvier Basin are used to correlate the lineations in that area with Early Cretaceous reversals M-5 to M-10. This correlation dates the onset of plate separation at 120–125 m.y., essentially contemporaneous with the opening of the Perth Basin to the south. However, it leaves a 220-km gap between M-4 and M-5 in the Cuvier Basin that suggests a ridge jump of that magnitude occurred nominally at 118 m.y. Early Cretaceous magnetic lineations northwest of the Exmouth Plateau suggest that spreading at the seaward edge of the Exmouth Plateau began 120 m.y. ago, while Late Jurassic marine sediments and fault structures landward of the Exmouth Plateau suggest rifting in that area at 155 m.y.     

Comportement géochimique des éléments métalliques en milieu réducteur et diagrammes (log S,pH)

Résumé L'existence de relations entre les teneurs en carbonates et en sulfures dans les eaux des bassins réducteurs, ainsi que l'irreversibilité de la réaction sulfate-sulfure, conduisent à proposer le remplacement des diagrammes Eh. pH, par les diagrammes log S, pH (S désignant la concentration totale de S (-II)). Nous donnons ici le principe de construction, des exemples de diagrammes et quelques applications.

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The existence of relations between the amounts of carbonates and the sulfides in the water of anoxic basins and the irreversibility of the reaction sulphate-sulfide lead us to propose the substitution of diagrams Eh, pH by diagrams log S, pH (S being the total concentration of S (-II)). The principle of how diagrams should be drawn is outlined and some examples and applications are given.

     

Statistical analysis of isotopic ratios in MORB: the mantle blob cluster model and the convective regime of the mantle

A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations.     

Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data,I. Theoretical methods

While the chemical structure of the earth's mantle is probably rather complex, multi-box models have been used as a first approximation to evaluate this structure. Most commonly, a three-box model is used, involving the continental crust, the upper mantle and the lower mantle. The depleted upper mantle and the continental crust are assumed to represe1nt complementary reservoirs, related by crust formation processes occurring during geologic history.Here we investigate the Rb/Sr and Sm/Nd isotopic systematics of several three-box models, using mass balance equations and the definition of the mean age of the reservoirs. The geochemical uniqueness of the models, chosen from a large family of possible models, is evaluated from elementary graph theory, and these models are then solved using a total inversion approach. This paper (Part I) describes the methodology of the procedure; the companion paper (Part II) discusses the application of this approach to multi-box mantle models.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

230Th-238U radioactive disequilibria in tholeiites from the FAMOUS zone (Mid-Atlantic Ridge, 36°50′N): Th and Sr isotopic geochemistry

We analyzed, U, Th and²³⁰Th/²³²Th activity ratios for a few tholeiites from the Mid-Atlantic Ridge FAMOUS zone at 36°50′N. The results show a fairly wide scatter for both Th/U and (²³⁰Th/²³²Th) ratios. Seawater contamination appears to be responsible for this scatter and, for the uranium, produces an increase in content yielding a (²³⁴U/²³⁸U) ratio greater than 1 and, for the Th, an increase of the (²³⁰Th/²³²Th) ratio which is a very sensitive indicator for contamination. Also, the latter often is selective: U, Th and Sr are not affected in the same manner.When discarding all data for contaminated samples, the FAMOUS zone appears to be very homogeneous with aTh/U ratio value of 3.05 and a (²³⁰Th/²³²Th) ratio value of 1.24. Comparison with other active volcanic areas reveals a negative correlation between (²³⁰Th/²³²Th) and⁸⁷Sr/⁸⁶Sr ratios for present lavas which is indicative of a consistency in Th-U and Rb-Sr fractionation in the source regions of these magmas. The Th isotopic geochemistry can thus provide useful information for the study of present volcanism, information as valuable as that from Sr, Pb or Nd isotopes.