Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256

作者：Gao Y^*; Vils F; Cooper K M; Banerjee N; Harris M; Hoefs J; Teagle D A H; Casey J F; Elliott T; Laverne C; Alt J C; Muehlenbachs K

来源：Geochemistry Geophysics Geosystems, 2012, 13: Q10001.

DOI：10.1029/2012GC004207

• hydrothermal systems
• lithium isotope
• ocean drilling
• stable isotopes

Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have delta O-18 values from +6.1% to +9.2%, reflecting alteration of oceanic crust by seawater at low temperatures (%26lt;200-250 degrees C). Bulk rocks from the sheeted dike complex and plutonic section have overall lower delta O-18 values(+3.0 parts per thousand+5.5 parts per thousand). In the sheeted dike complex bulk rock delta O-18 values gradually decrease with depth, and then increase toward the fresh MORB delta O-18 value after reaching a minimum of +3.0% at similar to 1350 m below seafloor (mbsf). The entire sampled crust is dominated by rocks with low lithium contents relative to fresh MORBs except for a few localized Li enrichment. The upper volcanic zone is characterized by a spread of delta Li-7 from low to high values relative to average unaltered MORB values (delta Li-7 = +3.4 +/- 1.4%). The presence of rocks with low delta Li-7 values in the upper crust most likely indicates zones of upwelling of relatively hot (similar to 200-250 degrees C) hydrothermal fluids. In the sheeted dike complex, bulk rock delta Li-7 values show wide range of variation, but exhibit a general trend from enriched to depleted values at similar to 1280 mbsf and then return to that for fresh MORB within the upper tens of meters of the plutonic section at the bottom of the after reaching a minimum at similar to 1350 mbsf (delta Li-7 = -1.6 parts per thousand). The downhole pattern of delta Li-7 principally reflects variations in water-rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.

• 出版日期2012-10-5