Geochemistry of basalts from the Caribbean Large Igneous Province

Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92-74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr-Nd-Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ~3*10**6 km**2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96+/-0.16, n=64 out of 79 samples, 2sigma) and initial Nd-Pb isotopic compositions (e.g. 143Nd/144Ndin=0.51291+/-3, epsilon-Nd i=7.3+/-0.6, 206Pb/204Pbin=18.86+/-0.12, n=54 out of 66, 2sigma). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65-60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr-Nd-Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/4He in olivines of enriched picrites at Quepos are ~12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm-Nd and U-Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been <=500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

Petrology and geochemistry at DSDP Site 89-462

The 16 samples of Deep Sea Drilling Project (DSDP) Leg 89 basalts that we analyzed for whole rock major and trace elements and for mineralogic compositions are identical to some of the basalts recovered during Leg 61. Leg 89 samples are mostly olivine-plagioclase-clinopyroxene sparsely phyric basalts and exhibit a wide variety of textures. These basalts have lower TiO2 at a given Mg/(Mg+Fe2+)*100 than MORB (midocean ridge basalt). We recognize three major chemical types of basalts in the Nauru Basin. We believe that different degrees of partial melting, modified by fractional crystallization and possibly by magma mixing at shallow depths, can explain the chemical differences among the three groups. This petrogenetic model is consistent with the observed downhole chemical-chronostratigraphic relations of the samples. New 87Sr/86Sr and U3Nd/144Nd analyses of basalt samples from DSDP Site 462 indicate that the Nauru Basin igneous complex is within the Sr-Nd isotopic range of ocean island basalt. Thus the Nauru Basin igneous complex resembles MORB in many aspects of its chemistry, morphology, and secondary alteration patterns (Larson, Schlanger, et al., 1981), but not in its isotopic characteristics. If it were not for the unambiguous evidence that the Nauru Basin complex was erupted off-ridge, the complex could easily be interpreted as normal oceanic layer 2. For this reason, we speculate that the Nauru Basin igneous complex was produced in an oceanic riftlike environment when multiple, fast-propagating rifts were formed during the fast seafloor spreading episode in the Cretaceous.

(Table 1) Activity of strontium-90 in waters of the Northeast Atlantic at the end of 1965 and the beginning of 1966

In 1965-1966 R/V Mikhail Lomonosov conducted studies on concentrations of artificial radioactive products in the Northeast Atlantic. Concentration of strontium-90 at the end of 1965 and the beginning of 1966 was higher than the average level for the ocean and reached about 53 dpm/100 l in the surface layer. The most intense transport of artificial radioactive products out of the Irish Sea was detected in the northern and northeastern directions along the Hebrides and the Orkneys. In addition to radioactive fission products from nuclear weapons tests, radioactive wastes of atomic industrial facilities discharged into the ocean are an important source of radioactive contamination of some regions of the world ocean.