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.

(Table 1) Activity of alkali phosphatase in suspended matter at 20°C and environmental temperatures, and recycling time of organic phosphorus in the Barents and Norwegian seas

Alkali phosphatase activity and hydrochemical structure of waters in the Barents and Norwegian seas were investigated. In a sea with the seasonal bioproduction cycle alkali phosphatase activity is also seasonal, rising with trophic level of waters. At the end of hydrological and biological winter activity is practically zero. Alkali phosphatase activity is especially important in summer, when plankton has consumed winter supply of phosphate in the euphotic layer and nutrient limitation of primary production begins. In summer production and destruction cycle, apparent time for recycling of phosphorus by phosphatase in suspended matter in the euphotic layer of the Barents Sea and Norwegian Sea averages from 7 to 30 hours.

The alkali element and boron geochemistry of ODP Site 169-1038 in the Escanaba Trough, East Pacific

A suite of conjugate pore fluid and sediment samples were collected during Leg 169 of the ODP from within the clastic sedimentary sequences which host massive sulphides at Central Hill, Escanaba Trough (ODP Site 1038). We report the alkali element and boron, and Li and B isotope data for these samples. Relative to a reference site (Site 1037) located outside the zone of high heat flow, pore fluids from Site 1038 show a wide variation in Cl (300-800 mM), and have far higher concentrations of Li (up to 6.2 mM), B (up to 9.7 mM), Cs (up to 5.0 mM), and Rb (up to 97 mM). We show that the pore fluids are derived from hydrothermal circulation that has extended into the basement oceanic crust, with input of the alkali elements and B as the rising hydrothermal fluids interact geochemically with the overlying clastic sediments. There is, however, no marked depletion of these elements in the conjugate sediments, suggesting that there has been advective transport of fluids away from the primary hydrothermal reaction site. This is supported by modelling of the Li and B isotope systematics of the pore fluids, which shows that they record extensive formation of secondary minerals during cooling of the fluids from ~350 to ~20ºC. Precipitation of metal-rich sulphides would have occurred prior to the formation of these minerals, thus, the pore fluid Li and B isotope data can place important constraints on the locus of sulphide deposition beneath the seafloor at Escanaba.

Composition of alkali picrites from the southwestern Greenland continental margin

The Labrador Sea is a basin with oceanic crust in its deep part. Bottom morphology of the Labrador Sea is rather complicated. Data of seismic profiling in this region indicate presence of numerous submarine mountains and hills, which are dominated by volcanic rocks. Some chemical and mineral characteristics of the rocks, in particular, high concentrations of alkalis and phosphorus, and presence of high-titanium augite, ilmenite, and devitrified glass enriched in K and Na, allow us to attribute them to K-Na subalkaline picrites typical for ocean islands, seamounts, and oceanic plateaus. Rocks of the K-Na subalkaline series usually form submarine basements and subaerial volcanoes of ocean islands, seamounts, and oceanic plateaus. Thus, the suggestion on formation of the highs on the continental crust is not confirmed by petrographic data, which require a refinement of the tectonic model of the northern part of the Labrador Sea.