Basalt marine

Figure 1.61. Paleobathymetry of the NE Japan Are at 17-14 Ma (circled numbers) (Yamaji, 1990). The arc subsided rapidly from 16 to 15 Ma. The entire inner arc region became at middle bathyal depths until 15 Ma, except that Yuri hill remained at shallow marine depths. The hill was submerged until 14 Ma (Matoba, 1981). Stippled area, Aosawa basalt (Tsuchiya, 1988). Hatched area, Yuri hill. Note that submarine volcanoes are neglected in this figure because volcaniclastic rocks usually contain few fossils.

Chlorine isotope compositions vary by up to 15%o (Chapter 7 Stewart and Spivack 2004). These large variations in Cl isotope compositions are found in marine environments, including mid-ocean ridge basalts, seafloor and hydrothermal alteration products, and sedimentary pore... 

The first substantive report of Li isotopes in any Earth materials (Chan and Edmond 1988) largely presaged what was to come in Li isotope research in the oceans. The interpretations therein, based on a handful of data from seawater, fresh and altered basalt, hydrothermal fluids and lake waters, laid out the foundation to what has come since, in terms of natural and laboratory-based studies of the marine geochemistry of Li isotopes. 

Figure 14. Plot of Li isotopic composition vs. concentration of thermal waters from the continents and the oceans (see text for references). Differences between the isotopic range of marine versus non-marine fluids emphasizes the variability in 5 Li of continental rocks compared to oceanic basalt. The dilution of the continental fluids goes along with their lower temperatures vent fluids are the only truly geothermal samples here, with temperatures in excess of 300°C.

For a simple model of Ca cycling, where the Ca sources for the ocean are weathering of continental rocks, pore fluids in the marine environment, and ocean floor basalt (Gieskes and Lawrence 1981 Berner etal. 1983 Elderheld etal. 1999), and the primary sink is the biological fixation of Ca into sediments, the rate of change of 5 Ca (= 5sw ) of the oceans is given by ... 

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

T1 isotope ratios might be also used as a tracer in mantle geochemistry (Nielsen et al. 2006 2007). Since most geochemical reservoirs except Fe-Mn marine sediments and low temperature seawater altered basalts are more or less invariant in T1 isotope composition, admixing af small amounts of either of these two components into the mantle should induce small T1 isotope fractionations in mantle derived rocks. And indeed, evidence for the presence of Fe-Mn sediments in the mantle underneath Hawaii was presented by Nielsen et al. (2006). 

Poreda R, Schilling JG, Craig H (1986) Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland. Earth Planet Sci Lett 78 1-17 Poulson RL, Siebert C, McManus J, Berelson WM (2006) Authigenic molybdenum isotope signatures in marine sediments. Geology 34 617-620... 

Rouxel O, Galy A, Elderfield H (2006) Germanium isotope variations in igneous rocks and marine sediments. Geochim Cosmochim Acta 70 3387-3400 Rouxel O, Ono S, Alt J, Rumble D, Ludden J (2008) Sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at ODP Site 801. Earth Planet Sd Lett 268 110-123 Rozanski K, Sonntag C (1982) Vertical distribution of deuterium in atmospheric water vapour. Tellus 34 135-141... 

Fig. 16.4. Kolbeinsey Ridge. Network of hyphae of an unknown marine fungus in a pillow basalt pore from the Holocene oceanic crust of the Kolbeinsey Ridge (north of Iceland) from a water depth of nearly 1500 m. The old hyphae are partly covered with iron and manganese oxides. Fungi are very common in pillow basalts and play a central role during rock alteration and the formation of certain microbialites.

Fig. 16.5. Kolbeinsey Ridge. Terminal ends of an another type of unknown marine fungus from a pillow basalt of the Kolbeinsey Ridge. The thickened ends of the hyphae are probably conidia.

The Cl/Br ratio of seawater is 290, but that of evaporites is considerably higher (>3,000), such that the exospheric ratio is —400 50. The ratio in MORE and other basalts is the same (Jambon et al, 1995). Iodine in the Earth is concentrated in the organic matter of marine sediments this reservoir contains 1.2 X 10 kg I (O Neill and Palme, 1998), corresponding to 6 ppb if this iodine comes from 50% of the mantle. MORBs have —8 ppb I (Deruelle et al, 1992) implying —1 ppb in the depleted (degassed) mantle, for a PM abundance of 7 ppb. 

Figure 10 The Sr/ Sr ratio of the soil-exchangeable pool and foliage for a chronosequence of soils developed on Hawaiian basaltic lava flows. Also plotted is the result of a calculation of the percent weathering contribution needed to explain the Sr/ Sr ratio— assuming that all Sr is derived from basaltic weathering and marine atmospheric deposition. Sr isotopes clearly demonstrate the transition of the forest ecosystems from weathering dominated Sr to atmospherically dominated Sr as soils mature and easily weathered Sr is removed from soils (source Kennedy et al., 1998).

Lithium in seawater, with an average 5 Li value of —32%o, is therefore isotopically heavy. Marine sediments, with 5 Li values of +l%o to —15%o, are much lighter. Other reported ranges in 5 Li values for geological materials include those of mid-ocean-ridge basalts, —8%o to —21%o hemi-pelagic clays, —9%o to —15%o and continental crustal rocks, —8%o to —21%o (Huh et al., 1998). All lithium isotopes are referenced to the NBSL-SVEC Li2C03 standard (Clark and Fritz, 1997). 

---