D. SubmarineGeologyand Geophysics
There is abundant evidence that crustal contamination modifies the chemistry and isotopic characteristics of continental magmas. Calculations based on fluid mechanic considerations show, however, that assimilation near the roofs of large magma chambers is limited. A second major site for assimilation occurs in dikes with widths > 3 m, where flow becomes turbulent and the low-melting point components of the country rock are assimilated. Thermal erosion of dike walls may thus explain the higher SiO2, K20 and LREE contents and the 'more evolved isotopic characteristics' of continental tholeiites. Res. Lab. Earth and Planet. Sci., Univ. of Toronto, Mississauga, ON L5L 1C6, Canada. (hbf) 86:1595 Chert, C.-Y. and F.A. Frey, 1985. Trace element and isotopic geochemistry of lavas from Halcakala volcano, East MauL, Hawaii: implications for the origin of Hawaiian basalts. J. geophys. Res, 90(B 10):8743-8768. Geochemical analyses of core samples from numerous drill holes on the northeastern slope of the volcano reveal that although there is a wide range in the Sr and Nd isotopic ratios and the incompatible elements (P, K, Rb, Sr, Zr, Nb, Ba, REE, Hf, Ta and Th) of the tholeiites, transitional and alkalic basalts, both the isotope and incompatible element abundance ratios vary systematically with age, and an inverse correlation exists between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. A multi-component model involving mantle plumes from beneath the asthenosphere interacting with oceanic lithosphere best explains these findings. Dept. of Geol., Univ. of Illinois, Urbana, IL, USA.
(hbf) 86:1596 Condie, K.C., 1985. Secular variation in the ¢omposRion of basalts: an index to mantle evolution. J. Petrology, 26(3):545-563. At the same Mg-number, decreases in the average content of Ni, Co, and Cr in basalts between 2.5 and 2.0 b.y. can be accounted for by smaller degrees of melting in the mantle in response to falling geotherms. In terms of partially compatible and incompatible elements Archean basalts are similar to modern island-arc basalts. Basalts with large concentrations of these elements become important in the geologic record only after 1.0 b.y. ago. Varying degrees of melting, fractional crystallization, or crustal contamination are not capable of accounting for these changes. A greater availability of enriched mantle sources after the Archean may reflect recycling of ancient enriched mantle or continental
sediments into the mantle during catastrophic continental growth at 2.7 b.y. Dept. of Geosci., New Mexico Inst. of Mining and Tech., NM 87801, USA. 86:1597 Dmitriyev, L.V., G.C. Kharin, N.M. Sushchevskaya and S.A. Zapunnyy, 1985. South Atiaatie Imsalts (based on the 7th expedition of the R/V Professor Shtokman. Geochem. int. (a translation of Geokhimiya), 22(3):56-61. One of the principal objectives of the 1982 cruise was to obtain information on magmatism near the southern branch of the Mid-Atlantic Ridge between 20-30°S. It was shown that young, shallow (type 2) basalts formed in the mantle at the plagioclase facies level were restricted to a narrow belt associated with the axial zone. By analogy with features in the North Atlantic, the boundary between this basalt and those of the spinel depth facies is placed between the Walvis Ridge and Rio Grande Rise. Vernadskiy Inst. of Geochem. and Analyt. Chem., Acad. of Sci., Moscow, USSR. (hbf) 86:1598 Sheppard, S.M.F. and Chris Harris, 1985. Hydrogen and oxygen Isotope geochemistry of Aseemion Island lavas and granites: variation with crystal fractionatton and interaction with seawater. Contr. Miner. Petrology, 91(1):74-81. Oxygen isotope data on fresh samples of lavas and granites show that crystal fractionation has been the dominant differentiation process for the Ascension lava series, although 180 values >7% require additional processes as well. The H-isotope composition and low H20 and CI contents of the granites suggest major degassing of magma saturated with water interacting with H20-undersaturated comenditic melt. A seawater contamination model is proposed which requires the introduction of seawater into the residual magma before the system has had time to be sealed. The model can explain the combined presence of granite and pyroclastic volcanism. Centre de Rech. Petrogr. et Geochim, BP 20, 54501 Vandoeuvre-les-Nancy, France. (hbf) 86:1599 Tsuchide, Masakazu et al., 1985. Submarine volcanic activity at the Kaltoku Seamount in 1984. Rept hydrogr. Res., Tokyo, 20:47-82. (In Japanese, English abstract.) A submarine eruption occurred at 26°07.3"N, 141°06.1 "E, near a shoal of the Kaitoku Seamount called Higasi-Kaitoku-Ba; this is the first confirmation (since 1543) of submarine volcanic activity in this area. Progress of the eruption, observations