Evolution Of Oceanic Crust Alteration From Deep Ocean Drilling (Invited)
Abstract
Understanding of submarine hydrothermal systems comes from fluid sampling and heat flow measurements, but drilling into basement is required to understand the subsurface distribution and evolution of alteration processes. ODP/IODP Hole 1256D penetrates through lavas, sheeted dikes, and into uppermost gabbros in crust formed at a superfast spreading rate at the EPR. The volcanic section is characterized by low temperature (<150C) alteration, with a step in thermal gradient up to hydrothermal conditions (250-400C) coinciding with the transition from lavas to dikes. Generally upwelling hydrothermal fluids in the sheeted dikes resulted in a mixing zone and sulfide mineralization at this horizon. Similar processes are observed in ODP Hole 504B, the only other basement hole to penetrate this lithologic transition. A significant feature of altered oceanic basement only revealed by drilling is the presence of anhydrite in these two basement holes. Isotopic analyses of anhydrite reveal chemical and biological processes that occur during seawater recharge into submarine hydrothermal systems. Intrusion of gabbros into sheeted dikes at the base of Hole 1256D resulted in a step in the thermal gradient up to contact metamorphism at temperatures of ~950C and upward migration of a conductive boundary layer above the axial melt lens. The low-temperature evolution of upper crustal lavas and its geochemical effects are revealed by drilling in different settings. Flow of cold seawater is partitioned into different zones, depending on crustal architecture that is a function of spreading rate. Crust formed at fast spreading rates (e.g., Holes 801C, 1256D) is less oxidized than at intermediate spreading rate (504B, 896A, 1149D), and oxidation is partitioned into discrete intervals in the former rather than decreasing with depth as in the latter. Late carbonates are a significant sink for CO2, and CO2 contents increase with age, suggesting greater uptake during higher levels of atmospheric CO2 in the Cretaceous (Teagle et al., 2010).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.U42A..07A
- Keywords:
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- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1034 GEOCHEMISTRY / Hydrothermal systems;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 3017 MARINE GEOLOGY AND GEOPHYSICS / Hydrothermal systems