Basement Fluids Research Articles

A helium, argon, and nitrogen record of the upper continental crust (KTB drill holes, Oberpfalz, Germany): implications for crustal degassing

An extensive set of bulk rock samples and mineral separates from the German Deep Drilling Project (KTB) has been analyzed for nitrogen, helium, and argon concentrations. The samples comprise metabasites, paragneisses, and hornblende–gneisses from a 7-km section in the crystalline basement metamorphosed during the Variscan orogeny. The data indicate a dependence of the N and Ar concentrations on the mica content of the rocks. The He–N 2–Ar composition of fluid inclusions in quartz from felsic veins suggests that the fluids are old basement brines with large proportions of radiogenic He and Ar. There is no systematic down-hole variation in N and Ar concentrations, but He concentrations appear to increase with depth. Calculated degassing rates suggest near complete degassing of He from the gneisses, but some He retention in the metabasites. Nonetheless, overall degassing fluxes of He (1.2–4.0×10 10 atoms m −2 s −1) are not in conflict with estimates of the He flux in continental crust based on the steady-state degassing model. In contrast, our estimates indicate that Ar fluxes are significantly lower than predicted by a steady-state degassing model which is thus probably inappropriate to describe Ar fluxes correctly. The down-hole trend of increasing He concentration is interpreted as a degassing profile. A model of simultaneous radiogenic ingrowth and diffusive loss of He has been applied to determine the bulk transport coefficient of He in crystalline basement, which was found to be between 10 −10 and 10 −7 m 2 s −1. This is much higher than reasonable diffusion rates, suggesting that episodes of enhanced degassing, probably related to hydrothermal or tectonic activities, played an important role in the degassing history of the crust. The relationship between the transport of heat and He in the crust in the KTB area is examined. Under the assumption that the ratio of heat flow and He flux from the mantle is the same as in ocean basins, the He isotopic composition of the basement fluids in the KTB boreholes is consistent with the inferred reduced heat flow at the KTB site. On a global scale, however, heat flow and flux of mantle He are probably not correlated.

Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge

Research Article| January 01, 1998 Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge M. J. Mottl; M. J. Mottl 1School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar G. Wheat; G. Wheat 2Institute of Ocean Sciences, University of Alaska, Fairbanks, Alaska 99775 Search for other works by this author on: GSW Google Scholar E. Baker; E. Baker 3Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way, Seattle, Washington 98115 Search for other works by this author on: GSW Google Scholar N. Becker; N. Becker 1School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar E. Davis; E. Davis 4Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, British Columbia V8L 4B2, Canada Search for other works by this author on: GSW Google Scholar R. Feely; R. Feely 3Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way, Seattle, Washington 98115 Search for other works by this author on: GSW Google Scholar A. Grehan; A. Grehan 5Marine Resources Institute, University College, Galway, Ireland Search for other works by this author on: GSW Google Scholar D. Kadko; D. Kadko 6Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar M. Lilley; M. Lilley 7School of Oceanography, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar G. Massoth; G. Massoth 3Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way, Seattle, Washington 98115 Search for other works by this author on: GSW Google Scholar C. Moyer; C. Moyer 8Department of Biology, Western Washington University, Bellingham, Washington 98225 Search for other works by this author on: GSW Google Scholar F. Sansone F. Sansone 1School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822 Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (1): 51–54. https://doi.org/10.1130/0091-7613(1998)026<0051:WSDOMO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. J. Mottl, G. Wheat, E. Baker, N. Becker, E. Davis, R. Feely, A. Grehan, D. Kadko, M. Lilley, G. Massoth, C. Moyer, F. Sansone; Warm springs discovered on 3.5 Ma oceanic crust, eastern flank of the Juan de Fuca Ridge. Geology 1998;; 26 (1): 51–54. doi: https://doi.org/10.1130/0091-7613(1998)026<0051:WSDOMO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We have located warm springs on an isolated basement outcrop on 3.5 Ma crust on the eastern flank of the Juan de Fuca Ridge in the northeast Pacific Ocean. These are the first ridge-flank hydrothermal springs discovered on crust older than 1 Ma. The springs are venting altered seawater at 25.0 °C along a fault near the summit of Baby Bare outcrop, a high point along a ridge-axis-parallel basement ridge that is otherwise buried by turbidite sediment. Baby Bare is a small volcano that probably erupted off-axis ca. 1.7 Ma; it is thermally extinct, but acts as a high-permeability conduit for venting of basement fluids. The springs have been sampled from the manned submersible Alvin. Compared with the ambient ocean bottom water, they are heavily depleted in Mg, alkalinity, CO2, sulfate, K, Li, U, O2, nitrate, and phosphate, and enriched in Ca, chlorinity, ammonia, Fe, Mn, H2S, H2, CH4, 222Rn, and 226Ra. The springs appear to support a community of thysirid clams. Although we saw no obvious bacterial mats, the surficial sediments contain the highest biomass concentrations ever measured in the deep sea, based on their phospholipid phosphate content. Areal integration of Alvin heat-flow and pore-water velocity data yields flux estimates of 4–13 L/s and 2–3 MW for the total (diffuse and focused) hydrothermal output from Baby Bare, comparable to that from a black smoker vent on the ridge axis. Warm springs such as those on Baby Bare may be important for global geochemical fluxes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major- and trace-element systematics

Quartz-rich synfolial veins and wallrocks from different areas of a Western Alps cross-section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains. Different portions of this cross-section underwent distinct P-T evolutions as reflected by the mineralogy of veins: (a) in the greenschist external Dauphinois domain, veins are characterized by quartz and calcite, together with an aluminosilicate (pyrophyllite), NaK-bearing phyllosilicates and a FeMg-bearing silicate (chlorite); (b) in narrow domains of the Briançonnais and Piémontais zones, with a non-eclogitic HPLT evolution followed by rapid cooling, veins are characterized by quartz, calcite, Ca- and FeMg-bearing silicates (lawsonite and FeMg-carpholite, respectively); and (c) in the major part of the Briançonnais and Piémontais zones, where low-grade blueschistfacies metamorphism is followed by a greenschist-facies evolution, veins are characterized by the replacement of earlier lawsonite and carpholite by Na- and K-bearing micas. In order to constrain the provenance of fluids involved in vein formation, we determined stable isotope (C, O, H), and major- and trace-element compositions in minerals and whole rocks for a large set of veins and wallrocks. These rocks are compared with some non-metamorphic shales from the northern Paris Basin, considered as protoliths of the Alpine schists. Stable isotope compositions of calcites are regionally distinct: δ 18O ≈ 28% (SMOW) in the northern Paris Basin, +26% in the Briançonnais zone, +22% in the Piémontais zone and + 18% in the Dauphinois zone. Within each studied region, calcite δ 18O-values are quite homogeneous regardless of rock type (vein or wallrock). This feature is explained as an isotopic “homogenization” with the most abundant lithology in each zone (“rock-buffered system”) — where the overall ratio of calcite (of marine origin) to detrital silicate minerals determines the final isotopic composition of the whole rock (w.r.) after equilibration during Alpine metamorphism. δ 18O/ δD-values of analysed hydrated minerals and whole rocks ( δ 18O ≈ + 15 to + 25% and δD ≈ −60% SMOW) lie within the field of Alpine cover rocks and of fluids in equilibrium with these rocks: interaction with an external 18O- or D-depleted fluid, such as hydrothermal waters, meteoric water or “basement fluid”, can be ruled out. Major- and trace-element systematics indicate an enrichment in Si, Ca, P, Sr, Y, Ta and locally Eu in the veins with respect to the wallrock, related to massive crystallization of quartz and carbonates, together with phosphate and oxide. Potential fluid sources, chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed. Fluid flow takes place which tectonic units, at both hectometric and decimetric scale. Fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented.

Hydrothermal flow through the Mariana Mounds: Dissolution of amorphous silica and degradation of organic matter on a mid-ocean ridge flank

We apply one-dimensional advection-diffusion-reaction equations to interpret amorphous silica, dissolved silica, organic carbon, and dissolved nitrate concentrations from the Mariana Mounds ridgeflank hydrothermal system as a means of elucidating the effects of convectively driven porewater flow on the dissolution of amorphous silica and degradation of organic carbon in sediments. We develop these mass transfer equations into models that predict an increase in the dissolution of amorphous silica with increasing porewater upwelling velocity, rate of reaction, and difference between the concentration of dissolved silica in basement from the steady-state dissolved silica concentration in the sediment column. One prediction of models for the degradation of organic matter is the depth of oxygen penetration, and thus the depth at which dissolved, reduced chemical species will oxidize and precipitate. The models can also be used to determine the porewater upwelling velocity. Our dissolution and degradation models are applied to two mid-ocean ridge-flank hydrothermal systems, the Mariana Mounds and the Galapagos Mounds, which have different basement fluid compositions. In the Mariana Mounds, seawater enters basement through faults and outcrops and the resultant basement fluid has a similar composition to that of bottom seawater. As this fluid upwells through the sediment, it enhances the rate of dissolution and degradation. In contrast, seawater enters the basement in the southern section of the Galapagos Mounds by downwelling through the sediment column, resulting in a basement fluid that is saturated with amorphous silica and reducing. As this fluid upwells through the sediment, it limits dissolution and degradation. The amounts of amorphous silica dissolution and organic matter degradation in ridge-flank hydrothermal systems are greater on an areal basis than those that would occur in nonhydrothermal settings. Fluid flow through the sediments of these hydrothermal systems may alter indicators of paleoproductivity and impede the formation of cherty deposits, thereby yielding a potential indicator of ancient ridge-flank hydrothermal systems. Ridge-flank hydrothermal systems also may introduce old dissolved organic carbon and large organic molecules to the deep ocean.

Hydrothermal circulation, Juan de Fuca Ridge eastern flank: Factors controlling basement water composition

Pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the Juan de Fuca Ridge as part of FlankFlux 90, a study of hydrothermal circulation through mid‐ocean ridge flanks. Seismic reflection and heat flow surveys (Davis et al., 1992a) indicate that the three areas differ in sediment thickness, basement topography, abundance of outcrops, basement temperature, and fraction of heat lost by advection versus conduction. Area 1 is on 0.6 Ma crust with nearly continuous basement outcrop, area 2 is on 1.3 Ma crust over the first buried ridge parallel to the present ridge axis, and area 3 is on 3.5–3.8 Ma crust over two axis‐parallel buried ridges that penetrate the sediment cover in three locations. Each area includes a hydrothermal system in which seawater flows into basement, reacts with crustal basalt, and then exits basement either through the sediment or directly into the overlying water column. As constrained by concentrations of sulfate and lithium in the pore waters, at least some seawater enters basement in all three areas without reacting fully with the overlying sediment, even where no outcrops are known nearby. Speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity, which behave conservatively in these areas, to numerical time‐dependent transport models. The estimated velocities range from &lt;0.1 to 7.4 cm/yr; faster flows probably occur but were not sampled. Upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness. The correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment. We have documented pore water upwelling through sediment up to 100 m thick. We estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick, corresponding to a heat flow of 0.44 W/m2 in area 2 and 0.3 W/m2 in area 3. Concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next. Both species remain at the bottom seawater concentration in area 1, where basement is cooled to &lt;10°C at the base of the sediments mainly by advection. The concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2.5 mmol/kg at about 90°C in area 3. The transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement, as the uppermost basement is heated from &lt;10° to 40–50°C. Chlorinity of the basement fluid increases above the present‐day bottom seawater concentration in areas 2 and 3 and in nearly all other mid‐ocean ridge flanks studied to date, as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period. While chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems, it reaches a maximum in area 2 at only 40°C, probably because alteration there occurs at a lower water/rock ratio than elsewhere. For all mid‐ocean ridge flanks studied to date, the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age.

FlankFlux: an experiment to study the nature of hydrothermal circulation in young oceanic crust

The sediment-buried eastern flank of the Juan de Fuca Ridge provides a unique environment for studying the thermal nature and geochemical consequences of hydrothermal circulation in young ocean crust. Just 18 km east of the spreading axis, where the sea-floor age is 0.62 Ma, sediments lap onto the ridge flank and create a sharp boundary between sediment-free and sediment-covered igneous crust. Farther east, beneath the nearly continuous turbidite sediment cover of Cascadia Basin, the buried basement topography is extremely smooth in some areas and rough in others. At a few isolated locations, small volcanic edifices penetrate the sediment surface. An initial cruise in 1978 and two subsequent cruises in 1988 and 1990 on this sedimented ridge flank have produced extensive single-channel seismic coverage, detailed heat flow surveys co-located with seismic lines, and pore-fluid geochemical profiles of piston and gravity cores taken over heat flow anomalies. Complementary multichannel seismic reflection data were collected across the ridge crest and eastern flank in 1985 and 1989. Preliminary results of these studies provide important new information about hydrothermal circulation in ridge flank environments. Near areas of extensive basement outcrop, ventilated hydrothermal circulation in the upper igneous crust maintains temperatures of less than 10–20 °C; geochemically, basement fluids are virtually identical to seawater. Turbidite sediment forms an effective hydrologic and geochemical seal that restricts greatly any local exchange of fluid between the igneous crust and the ocean. Once sediment thickness reaches a few tens of metres, local vertical fluid flux through the sea floor is limited to rates of less than a few millimetres per year. Fluids and heat are transported over great distances laterally in the igneous crust beneath sediment however. Heat flow, basement temperatures, and basement fluid compositions are unaffected by ventilated circulation only where continuous sediment cover extends more than 15–20 km away from areas of extensive outcrop. Where small basement edifices penetrate the sediment cover in areas that are otherwise fully sealed, fluids discharge at rates sufficient to cause large heat flow and pore-fluid geochemical anomalies in the immediate vicinity of the outcrops. After complete sediment burial, hydrothermal circulation continues in basement. Estimated basement temperatures and, to the limited degree observed, fluid compositions are uniform over large areas despite large local variations in sediment thickness. Because of the resulting strong relationship between heat flow and sediment thickness, it is not possible, in most areas, to detect any systematic pattern of heat flow that might be associated with cellular hydrothermal circulation in basement. However, an exception to this occurs at one location where the sediment thickness is sufficiently uniform to allow detection of a systematic variation in heat flow that can probably be ascribed to cellular circulation. At that location, temperatures at the sediment–basement interface vary smoothly between about 40 and 50 °C, with a half-wavelength of about 700 m. A permeable-layer thickness of similar dimension is inferred by assuming that circulation is cellular with an aspect ratio of roughly one. This thickness is commensurate with the subbasement depth to a strong seismic reflector observed commonly in the region. Seismic velocities in the igneous crustal layer above this reflector have been observed to be low near the ridge crest and to increase significantly where the transition from ventilated to sealed hydrothermal conditions occurs, although no associated reduction in permeability can be ascertained from the thermal data.

Isotope Geochemistry of Calcite and Clay Minerals in Volcanogenic Rocks, Great Valley Sequence, Northern California: Implications for Organic Diagenesis: ABSTRACT

Petrographic and isotopic data from an 8,500-m thick section of the Great Valley sequence indicate that widespread calcite cement in sandstones and mudstones was precipitated during at least two distinct stages that are linked to organic and clay-mineral diagenesis. The range of ^dgr13C values for calcite in mudstone is -11.6 to +1.0 ppt PDB, in sandstones is -9,7 to +4.5 ppt PDB, and in veins is -16.7 to -2.2 ppt PDB. The ^dgr13C values are heavier in progressively older and, presumably, more deeply buried strata. A strong shift to heavier 13C/12C ratios of calcite in mudstones and sandstones corresponds with an abrupt 20 ppt shift to more enriched ^dgrD values of OH-hydrogen in diagenetic smectite and a 10A clay-mineral from mudstones. Clay D/H ratios range between -69 to -49 ppt SMOW. The stratigraphic position of the shift corresponds to a modeled burial temperature estimate of about 80 to 100°C. This is interpreted to be the burial temperature for late-stage dehydration and conversion of smect te to a 10A clay-mineral phase. Theoretical considerations indicate that a shallow burial phase of predominantly pore-filling calcite formed in association with bacterial production of methane ( 80°C) that contained thermogenically produced low C2+ gases. Release of dry gas at increasingly elevated temperatures was characterized by continuous 13C-enrichment in CH4. Depleted calcite carbon in the most basal strata was derived from CH4-rich basement fluids. Estimation of ^dgrD values of formation waters indicates that CH4-H2O equilibrium was not attained. End_of_Article - Last_Page 999------------