Alkali contents, Sr isotope and U-Th-Pb data for ocean crust vein minerals from DSDP Holes 27-261, 61-462A, and 72-516F

Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (Site 261 - 121.4+/-1.6 m.y.; Site 462A - 105.1+/-2.8 m.y.; Site 516F - 69.9+/-2.4 m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10-15 m.y.), or were reactivated by off-ridge igneous activity. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20-200.

Isotopic ratios and geochemical composition of ODP Hole 176-735B minerals

The mineralogy and stable (O and C) and Sr isotopic compositions of low-temperature alteration phases were determined in Hole 735B gabbroic rocks in order to understand the processes of low-temperature alteration in this uplifted block of lower oceanic crust. Phyllosilicates include smectite (saponite, Mg montmorillonite, and nontronite), chlorite/smectite, chlorite, talc, and serpentine. Other phases include prehnite, albite, K-feldspar, analcite, natrolite, thompsonite, pyrite, and titanite. The low-grade mineral assemblages mainly represent zeolite facies and lower-temperature "seafloor weathering" processes. Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress. Alteration temperatures were probably somewhat greater below 1300 meters below seafloor. Mineralogy and isotopic data indicate that conditions were mostly reducing and that seawater solutions were rock dominated. Carbonates formed late from cold and generally oxidizing seawater solution, however, as seawater penetrated downward as the result of fracturing and faulting in the uppermost portion of the uplifted crustal block.

Composition of rocks and minerals from the Bolshoy Lyakhovsky Island (New Siberian Islands)

In the southeast of the Bolshoi Lyakhovsky Island there are outcrops of tectonic outliers composed of low-K medium-Ti tholeiitic basic rocks represented by low altered pillow basalts, as well as by their metamorphosed analogs: amphibolites and blueschists. The rocks are depleted in light rare-earth elements and were melted out of a depleted mantle source enriched in Th, Nb, and Zr also contributed to the rock formation. The magma sources were not affected by subduction-related fluids or melts. The rocks were part of the Jurassic South Anyui ocean basin crust. The blueschists are the crust of the same basin submerged beneath the more southern Anyui-Svyatoi Nos arc to depth of 30-40 km. Pressure and temperature of metamorphism suggest a setting of "warm" subduction. Mineral assemblages of the blueschists record time of a collision of the Anyui-Svyatoi Nos island arc and the New Siberian continental block expressed as a counter-clockwise PT trend. The pressure jump during the collision corresponds to heaping of tectonic covers above the zone of convergence 12 km in total thickness. Ocean rocks were thrust upon the margin of the New Siberian continental block in late Late Jurassic - early Early Cretaceous and mark the NW continuation of the South Anyui suture, one of the main tectonic sutures of the Northeastern Asia.

Chemical composition of siderites and clay minerals from DSDP Holes 70-506G and 70-507B

As soon as they are emplaced on the sea floor, oceanic basalts go through a low-temperature alteration process which produces black halos concentrical with exposed surfaces and cracks, whereas the grey internal parts of the basaltic pieces apparently remain unaltered. This paper reports for the first time the occurrence of authigenic siderite and ankerite in oceanic basalts and more particularly in the grey internal parts of the latter. Small (8-50 µm) crystals of zoned siderite and ankerite have been observed in ten vesicles of two samples recovered from DSDP Holes 506G and 507B drilled south of the Galápagos Spreading Center (GSC). These Fe-carbonates show a large range of chemical composition (FeCO3 = 47-88%; CaCO3 = 5-40%; MgCO3 = 1-20%; MnCO3 = 0-11%). Most of them are Ca-richer than siderite reported in the literature. The chemical composition of the carbonate clearly reflects the fluctuation of the fluid chemical composition during crystallization. Mn and at least part of the Fe are thought to be hydrothermal in origin, whereas Mg and probably Ca were provided by seawater. It is proposed that siderite and ankerite formed at relatively low temperature (<85°C) and is metastable. The alteration of the GSC basalts seems to have proceeded in two stages: during the first, reducing stage, pyrite precipitated from hydrothermal fluids. A little further in the rock, siderite precipitated from the fluid which had already been modified by the formation of pyrite, and thus in a microenvironment where particular conditions prevailed (high P_CO2, increasing p_S**2- or increasing pH or increasing or decreasing pe). During the second, oxidizing, stage of alteration, a seawater-dominated fluid allowed crystallization of mixtures of Fe-rich smectites and micas, and Fe-hydroxides forming the black halos in the external portion of the basalt pieces and locally oxidizing pyrite and siderite in their innermost part. It is shown in this paper that, even at its earliest stage, and at low temperature, alteration of the upper oceanic crust (lavas) involves fluids enriched in Fe and Mn, interpreted to be of hydrothermal origin.