Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

Heavy mineral content of ODP Leg 107 holes

Analyses of rock clasts and of heavy minerals in upper Miocene coarse detrital units drilled along the East Sardinia passive-type continental margin (Sites 654, 653, 652, and 656) reveal that the stretched basement contains quite complex rock suites. Taking also into account previous sampling data, in moving from west to east across the margin, the nature of the basement changes drastically. To the west there are mostly Hercynian basement rocks with their cover, referable to the alpine foreland of the Corsica-Sardinia block. To the east, along the lower margin, where crustal thinning is quite severe, the basement contains rock suites referable to a pre-upper Tortonian orogenized zone with units constituting parts of the Alpine and Apenninic chains (presumably with thickened continental crust prior to stretching). Largest thinning and ocean forming occurred then, in a rather short time, mostly at the expense of unstable crust just thickened by orogenetic/tectogenetic processes.

(Table 1) Chemical composition of plagioclase and amphibole-pyroxene containing samples from pyroxene-amphibole-asbestos rock collected in the Hess Deep

Recent sediments with distinct signs of hydrothermal alteration sampled in the Hess Deep(Galapagos Ridge, East Pacific Rise) contained a piece of ash-gray rock, which differed from other rock fragments by degree of consolidation, conchoidal fracture, and had properties of asbestos. Our studies found that the sample represented mixture of asbestos-like pyroxene of diopside-hedenbergite composition, amphibole of tremolite composition and a new mineral, which basic structure consisted of bands of triple pyroxene chains with the radical [Si6O16]. The latter can be regarded as intermediate between amphiboles and layered silicates. Also in some parts of the sample presence of trioctahedral vermiculite-chlorite was indicated. Genesis of the studied asbestos rock is considered from the standpoint of high-temperature hydrothermal-metasomatic alteration of sediment by post-magmatic mineralized halide solutions.

Geochemistry and stable isotopes of peridotites of ODP Hole 103-637A

A ridge of strongly serpentinized, plagioclase-bearing peridotite crops out at the boundary between the Atlantic oceanic crust and the Galicia continental margin (western Spain). These peridotites, cored at Hole 637A (ODP Leg 103) have been mylonitized at high-temperature, low-pressure conditions and under large deviatoric stress during their uplift (Girardeau et al., 1988, doi:10.2973/odp.proc.sr.103.135.1988). After this main ductile deformation event, the peridotite underwent a polyphase metamorphic static episode in the presence of water, with the crystallization of Ti- and Cr-rich pargasites at high-temperature (800°-900°C) interaction with a metasomatic fluid or alkaline magma. Introduction of water produced destabilization of the pyroxenes and the subsequent development of hornblendes and tremolite at temperatures decreasing from 750° to 350°C. The main serpentinization of the peridotite occurred at a temperature below 300°C, and possibly around 50°C, as a consequence of the introduction of a large amount of seawater, which is suggested by stable isotope (d18O and SD) data. Finally, calcite derived from seawater precipitated in late-formed fractures or locally pervasively impregnated the peridotite at low temperature (~10°C).