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Hydrothermal alteration of basalts beneath the Bent Hill Massive Sulphide Deposit, Middle Valley, Juan de Fuca Ridge
No full textHydrothermal alteration of basalts beneath the Bent Hill massive sulfide deposit, Middle Valley, Juan de Fuca Ridge
No full textHydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801
No full textODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100 [deg]C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni.Secondary carbonate veins have 87Sr/86Sr=0.706337-0.707046, and a negative correlation with [delta]18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable [delta]13C values (-11.2[permil] to 2.9[permil]) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the [delta]13C of seawater over time.Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals
Thermal History of ODP Hole 1256D Lower Sheeted Dikes: Petrology, Chemistry and Geothermometry of the Granoblastic Dikes
No full textLithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP Sites 504B and 896A
No full textOcean Drilling Program (ODP) Hole 504B near the Costa Rica Rift is the deepest hole drilled in the ocean crust, penetrating a volcanic section, a transition zone and a sheeted dike complex. The distribution of Li and its isotopes through this 1.8-km section of oceanic crust reflects the varying conditions of seawater alteration with depth. The upper volcanic rocks, altered at low temperatures, are enriched in Li (5.6–27.3 ppm) and have heavier isotopic compositions (7Li=6.6–20.8‰) relative to fresh mid-ocean ridge basalt (MORB) due to uptake of seawater Li into alteration clays. The Li content and isotopic compositions of the deeper volcanic rocks are similar to MORB, reflecting restricted seawater circulation in this section. The transition zone is a region of mixing of seawater with upwelling hydrothermal fluids and sulfide mineralization. Li enrichment in this zone is accompanied by relatively light isotopic compositions (?0.8–2.1‰) which signify influence of basalt-derived Li during mineralization and alteration. Li decreases with depth to 0.6 ppm in the sheeted dike complex as a result of increasing hydrothermal extraction in the high-temperature reaction zone. Rocks in the dike complex have variable isotopic values that range from ?1.7 to 7.9‰, depending on the extent of hydrothermal recrystallization and off-axis low-temperature alteration. Hydrothermally altered rocks are isotopically light because 6Li is preferentially retained in greenschist and amphibolite facies minerals. The 7Li values of the highly altered rocks of the dike complex are complementary to those of high-temperature mid-ocean ridge vent fluids and compatible to equilibrium control by the alteration mineral assemblage. The inventory of Li in basement rocks permits a reevaluation of the role of oceanic crust in the budget of Li in the ocean. On balance, the upper 1.8 km of oceanic crusts remains a sink for oceanic Li. The observations at 504B and an estimated flux from the underlying 0.5 km of gabbro suggest that the global hydrothermal flux is at most 8×109 mol/yr, compatible with geophysical thermal models. This work defines the distribution of Li and its isotopes in the upper ocean crust and provides a basis to interpret the contribution of subducted lithosphere to arc magmas and cycling of crustal material in the deep mantle
Isotopic studies of gypsum in the Macquarie Island Ophiolite: tracing hydrothermal reactions (abstract of poster presented at AGU Fall Meeting, San Francisco, CA, 10-14 Dec 2001)
No full textRecharge flux to ocean-ridge black smoker systems: a geochemical estimate from ODP Hole 504b
No full textWe use the Sr-isotopic composition of rock and anhydrite, as a monitor of fluid composition, in DSDP/ODP Hole 504B to calculate the recharge flux to the axial high-temperature hydrothermal circulation. The fluid and rock Sr-isotope profiles are well fit by a tracer transport mass-balance model that approximates fluid-solid exchange by linear kinetics. The calculated time-integrated flux of 1.7+/-0.2 x 106 kg m-2 is significantly less than the 5 x 106 kg m-2 calculated from thermal models that assume all magma is intruded into a high-level magma chamber at the base of the sheeted dykes. Our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in Hole 504B and the intrusion of magma as lenses distributed through the lower oceanic crust. It leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balanc
Thallium isotope constraints on the water fluxes of ridge flank hydrothermal systems (abstract of paper presented at: 16th Annual V.M. Goldschmidt Conference 2006, Melbourne, Australia, 27 Aug-1 Sept 2006)
No full textHydrothermal activity in the oceans is not restricted to the high-temperature (T) vents that are found at mid-ocean ridge axes. Rather, ridge flanks have a total hydrothermal power output which exceeds the axial output by at least a factor of 3. As the temperatures are comparatively low at ridge flanks, the respective water flux must be several orders of magnitude larger than on-axis, to remove the inferred power output. Even small changes in the composition of the circulating fluids can therefore produce very significant chemical fluxes. Our understanding of the importance of these chemical fluxes with respect to global geochemical budgets is limited, however, because the partitioning of heat between warmer, more reactive fluids with T 40 °C and cooler fluids, which have T 20 °C, is only poorly constrained.In this study, we have used Tl isotope and concentration data for altered ocean crust and hydrothermal fluids to constrain the water fluxes and average fluid exit temperatures of ridge flank hydrothermal systems. The calculations are based on the observation that the upper ocean crust, which has been altered at low temperature, has fractionated Tl isotope compositions and elevated Tl concentrations. The observed systematics can be exploited to calculate the flux of low-T hydrothermal fluids (FLT) by mass balance:FLT×[Tl]sw×fupt=Fuoc×?[Tl]uoc Here [Tl]sw is the Tl concentration of seawater, fupt is the fraction of Tl removed from seawater by alteration, Fuoc is the mass flux of upper ocean crust affected by low-T hydrothermal alteration, and ?[Tl]uoc is the average change of Tl concentration observed for low-T altered ocean crust.The calculations indicate that the hydrothermal water flux of ridge flanks is (0.2–5.4) × 1017 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1–3.6° relative to ambient seawater. Such low temperatures should severely restrict the effect of ridge flank hydrothermal systems on the marine budgets of 87Sr/86Sr and Mg. This conclusion is in accord with the results of previous rock alteration studies, which concluded that the fluxes of low-T hydrothermal systems are insufficient to balance the oceanic budgets of these elements. <br/
Hydrothermal alteration of the lower oceanic crust: Sr isotopic constraints from the CCSP CY-4 drill hole, Troodos Ophiolite, Cyprus (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
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