Rubidium and Strontium chemistry of oceanic crust

Times of vein mineral deposition in the ocean crust have been determined both by Rb-Sr isochron ages of vein smectites and by comparison of 87Sr/86Sr ratios of vein calcites with the known variations of seawater 87Sr/86Sr ratio with time. Results from drilling sites 105, 332B and 418A, Atlantic Ocean, which have basement formation ages of 155 m.y., 3.5 m.y., and 110 m.y., respectively, show that vein deposition is essenrially complete within 5-10 m.y. after formation of the basaltic crust. This provids direct evidence that hydrothermal circulation of sea-water through the oceanic crust is an important process for only 5-10 m.y. after crust formation.

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.

Chemical and isotopic compositions of volcanics from ODP Hole 210-1276

Site 1276, Leg 210 of the Ocean Drilling Program, was located on the Newfoundland margin in a seismically-defined ~128 Ma "transitional" crust just west of the presumed oceanic crust, and the M3 magnetic anomaly. The goal of drilling on this non-volcanic margin was to study the rifting, nature of basement, and post-rift sedimentation in the Newfoundland-Iberia rift. Drilling of this 1739 m hole was terminated 90-160 m above basement, in the lower of a doublet of alkaline diabase sills. We have carried out geochemical studies of the sill complex, in the hopes that they will provide proxy information regarding the nature of the underlying basement. Excellent 40Ar/39Ar plateau ages were obtained for the two sills: upper sill ~105.3 Ma; lower sill ~97.8 Ma. Thus the sills are substantially younger than the presumed age of the seafloor at site 1276 (~128 Ma), and were intruded beneath substantial sediment overburden (250 m for the upper, older sill, and 575 m for the lower younger sill). While some of the geochemistry of the sills has been compromised by alteration, the "immobile" trace elements show these sills to be hawaiites, differentiated from an enriched alkaline or basanitic parentage. Sr, Nd and Pb isotopes are suggestive of an enriched hotspot/plume mantle source, with a possible "added" component of continental material. These sills unequivocally were not derived from typical MORB (asthenospheric) upper mantle.

Post-glacial pollen record from Yorkshire

The stratigraphy and pollen analysis of the deposits show that this is a lake basin which during the Late-glacial period was partially filled by lake clays and muds. One of the main interests of the pollen diagrams lies in the division of zone i into three suh-zones showing a minor climatic oscillation which seems to be comparable with the Boiling oscillation of northern Europe. During Post-glacial time the greater part of the deposits has been muds but on one side a fen developed which in early zone VI was sufficiently dry to support birch and pine wood. Later in zone VI the water table must have risen slightly because the fen peats were gradually covered by a rather oxidized mud suggesting that the fen became replaced by a shallow swamp with a widely fluctuating water table. In the Atlantic period the basin was reflooded and the more central deposits were covered by a layer of mud. Later in the central region, swamp and eventually Sphagnum bog communities developed. The whole area is now covered by a sihy soil and forms a flat meadowland.