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Durham University

Department of Earth Sciences


Publication details for Professor Yaoling Niu

Bach, W., Alt, J.C., Niu, Yaoling, Humphris, S.E., Erzinger, J. & Dick, H.J.B. (2001). The chemical consequences of late-stage hydrothermal circulation in an uplifted block of lower ocean crust at the Southwest Indian Ridge: Results from ODP Hole 735B (Leg 176). Geochimica et Cosmochimica Acta 65(19): 3267-3287.

Author(s) from Durham


Abstract—Chemical exchange between oceanic lithosphere and seawater is important in setting the chemical
composition of the oceans. In the past, budgets for chemical flux in the flanks of mid-ocean ridges have only
considered exchange between basalt and seawater. Recent studies have shown that lower crustal and upper
mantle lithologies make up a significant fraction of sea floor produced at the global mid-ocean ridge system.
Moreover, the rugged topography of slow spread crust exposing lower crust and upper mantle facilitates
prolonged fluid circulation, whereas volcanic ridge flanks are more rapidly isolated from the ocean by a
sediment seal. Hence, elemental fluxes during lower crust–seawater reactions must be assessed to determine
their role in global geochemical budgets.
ODP Hole 735B penetrates more than 1500 m into lower ocean crust that was generated at the very slow
spreading Southwest Indian Ridge and later formed the 5-km-high Atlantis Bank on the inside corner high of
the Atlantis II Fracture Zone. The gabbroic rocks recovered from Hole 735B preserve a complex record of
plastic and brittle deformation and hydrothermal alteration. High-temperature alteration is rare below 600 m
below seafloor (mbsf), but the lowermost section of the hole (500–1500 mbsf) has been affected by a complex
and multistage low-temperature (,250°C) alteration history probably related to the tectonic uplift of the
basement. This low-T alteration is localized and typically confined to fractured regions where intense
alteration of the host rocks can be observed adjacent to veins/veinlets filled with smectite, smectite– chlorite
mixed layer minerals, or chlorite 6 calcite 6 zeolite 6 sulfide 6 Fe-oxyhydroxide.
We have determined the bulk chemistry and O and Sr isotope compositions of fresh/altered rock pairs to
estimate the chemical fluxes associated with low-temperature interaction between the uplifted and fractured
gabbroic crust and circulating seawater. The locally abundant low-temperature alteration in crust at Site 735
has significantly changed the overall chemical composition of the basement. The direction of these changes
is similar to that defined for volcanic ridge flanks, with low-temperature alteration of gabbroic crust acting as
a sink for the alkalis, H2O, C, U, P, 18O, and 87Sr. The magnitudes of element fluxes are similar to volcanic
ridge flanks for some components (C, P, Na) but are one or two orders of magnitude lower for others. The flux
calculations suggest that low-temperature fluid circulation in gabbro massifs can result in S uptake (3% of
riverine sulfate input) in contrast to the S losses deduced for volcanic ridge flanks.