ALBERT

Description: 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 +/- calcite +/- zeolite +/- sulfide +/- 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.

Description: The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (~850°C) drilled in Hole 735B is estimated at 11.93F0.14 Ma, based on U-Pb analyses of zircon by ion probe. This crystallization age is older than the 'crustal age' from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949-11.935 Ma). 40Ar/39Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42 +/- 0.21 Ma), implying cooling rates of 〉800°C/m.y. over the first 500,00 years to temperatures below ~330-400°C. Fission-track ages on zircon (mean 9.35 +/- 1.2 Ma) and apatite reveal less rapid cooling to 〈110°C by ~7 Ma, some 4-5 m.y. off axis. Comprehensive thermochronometric data from the structurally intact block of gabbro between ~700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range ~ 900-330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T〈=280°C) are not consistent with these predictions and record anomalously high temperatures for crust 〉700 m below sea floor at 8-10 Ma (i.e. 2-4 m.y. off axis). We offer two hypotheses for this thermal anomaly: (i) Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from ~19.5 to 7.5 Ma. (ii) Late detachment faulting, which led to significant crustal denudation (2.5-3 km removed), further from the ridge axis than conventionally thought.

Description: The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#〉61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2〈7 wt.% and FeOtotal〈18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and U〈/=0.005 ppm in the primitive rocks and up to 2 ppm Pb and 0.2 ppm U in the evolved. The range of isotopic compositions of the unleached rocks is: 87Sr/86Sr=0.70280-0.70299, average 0.70287+/-0.00005 (1 S.D., N=30 samples) (except one felsic vein with 87Sr/86Sr=0.7045), 143Nd/144Nd=0.51304-0.51314, average 0.51310+/-0.00002 (1 S.D., N=28), 206Pb/204Pb=17.43-18.55, 207Pb/204Pb=15.40-15.61 and 208Pb/204Pb=37.19-38.28. The range of Sr and the almost constant Nd isotopic composition resemble that found in the upper 500 m of Hole 735B, while Pb ranges to more radiogenic compositions. In general, there is a decrease in isotopic variation of Sr and Pb as well as ? (238U/204Pb), U and Pb with depth, with a trend towards relatively unradiogenic compositions. This correlates with a decrease in alteration and frequency of evolved rock-types in the core. Leached samples generally have less radiogenic Pb with values trending towards 206Pb/204Pb=17.35, 207Pb/204Pb=15.35 and 208Pb/204Pb=37.0, while their 87Sr/86Sr ratios deviate less systematically from unleached rocks and reach both higher, 0.70307, and lower values, 0.70276. Separated clinopyroxene has elevated 87Sr/86Sr up to 0.7035, while plagioclase generally has close to whole rock Sr. Leaching reduced 87Sr/86Sr in clinopyroxene and in two (out of nine) cases leached separates and whole rock display isotopic equilibrium. Relatively minor hydrothermal seawater alteration is thought to have increased 87Sr/86Sr in the rocks, while a secondary high temperature percolation of a mantle-derived agent is thought to be the cause for the trend towards radiogenic Pb. This material had intermediate 87Sr/86Sr and may have originated from non-MORB off axis mantle. The main primary igneous isotopic variation of the gabbros is suggested to have been derived from the MORB-mantle and is defined mainly by leached samples from both ODP Leg 176 and Leg 118 and can be explained by two-component mixing of an end-member with composition like Central Indian Ridge basalts and an end-member with composition unlike any MORB. The latter is characterized by very unradiogenic Pb, in particular 207Pb/204Pb, and may have an origin with affinity to old depleted mantle (DM). The isotopic composition of the magmas parental to the FeTi-oxide rich rocks cannot be distinguished from the magmas parental to the primitive gabbros and an intimate relationship is indicated. The small-scale inhomogeneity indicated for the SWIR MORB-mantle at the Atlantis II Fracture Zone was probably inherited by the lower crustal rocks due to small-scale melting and monogenetic magma chambers at this slow spreading ridge.