Compositions, sources and depositional environments of organic matter from the Middle Jurassic clays of Poland

The heat flow map derived from 550 measurements collected in a the southern portion of the sedimented rift in Middle Valley, northern Juan de Fuca Ridge, displays kilometer-sized quasi-circular regions of very high heat flow. Some of these domains, explored during Ocean Drilling Program (ODP) leg 139, are thought to be discharge zones of large-scale hydrothermal plumes. To understand this unique data set, we modeled the kilometer-scale hydrothermal circulation within both the sedimentary and the igneous crust, using a set of two- and three-dimensional models that use a numerical technique based on horizontal spectral decomposition of the flow equations. These models include variations in the viscosity and density of the hydrothermal fluids with temperature. We examine the variations in flow patterns due to different permeability-versus-depth distribution within sediment and pillow layers. Models with the same permeability in both layers do not match the seafloor heat flow field in Middle Valley. When the permeability decreases from the bottom to the top of the simulation domain by a factor greater than 20, convection assumes a plume pattern to produce surface heat flow comparable to that observed in Middle Valley. Within the models the ratio of the heat flux above the recharge and discharge domains is directly related to the vertical harmonic mean of the permeability field. A value of 7×10−16 m2 provides a good match to the heat flow observations. The Darcy velocities of the hydrothermal fluids in the discharge areas approach 16 cm/yr while in the recharge areas they are lower than 3 cm/yr. These rates and the temperature inside the plumes are sufficiently high to produce the observed massive sulfide deposits and mineral alterations in 1–2 × 105 years. The dynamic pressure produced by the hydrothermal flow matches the pressure measured in drill sites. This process may play a major role in compaction, fracturing, and uplift of the sediment cover. For example, the dynamic pressure in the ascending plume equals the lithostatic pressure at a depth of 50 m. Resulting hydrofracturing could explain the genesis of the vent fields associated with the hydrothermal discharge.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB01484

Two‐ and three‐dimensional modeling of hydrothermal convection in the sedimented Middle Valley segment, Juan de Fuca Ridge

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24 A stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14 A with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ18O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300° C±30° for chlorite-quartz at 32 m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley, Juan de Fuca)

Isotopic evidence of magmatism and seawater bicarbonate removal at the endeavour hydrothermal system

The temperature dependence of Sr coprecipitation with calcite was determined experimentally in solutions with Sr/Ca ratios and ionic strengths closely resembling marine pore fluids Aragoniteto-calcite and dolomite-to-calcite transformations were conducted over a temperature range from 40° to 200°C Temperature dependence of the distribution coefficient of strontium in calcite (DcSr) is significant DcSr in aragonite-to-calcite experiments varied from 0046 at 40°C to 0068 at 200°C DcSr in dolomite-to-calcite experiments varied from 0034 at 40°C to 0062 at 200°C Experimental values of DcSr are somewhat dependent on the precursor phase The experimental results are applied to natural diagenetic and hydrothermal calcites recovered from the sediment-covered Middle Valley part of the Juan de Fuca Ridge spreading center (Ocean Drilling Program Holes 857C and 858D) Low-Mg calcites from Hole 858D have Sr contents appropriate for their present-day in situ temperatures and pore-water Sr values In contrast, high-Mg calcites from Hole 858D that precipitated from supersaturated pore waters at inferred high growth rates have elevated Sr contents relative to calculated equilibrium values The Sr contents of calcites from Hole 857C are close to predicted equilibrium values in some instances but are substantially different in others Deviations from predicted equilibrium Sr contents, especially in nodules, are attributed to the formation of mixed generations of carbonate precipitating with increasing burial depths and temperatures through pore waters with variable Sr composition

Temperature dependence of the strontium distribution coefficient in calcite; an experimental study from 408 degrees to 2008 degrees C and application to natural diagenetic calcites

Stable isotope and geochemical record of convective hydrothermal circulation in the sedimentary sequence of Middle Valley, Juan de Fuca Ridge, Leg 139

Petrographic and mineralogic studies of the sedimentary cover drilled at Middle Valley on the Juan de Fuca Ridge allow characterization of the newly formed silicate and carbonate phases and a determination of their origin. At all drill sites, the sediments are composed of a mixture of detrital and authigenic minerals in various proportions. The unmodified sediments forming the surficial cover consist of detrital minerals, including quartz, feldspar, amphibole, mica, clays (smectite, chlorite), and biogenic remains. Quartz, feldspar, and mica are still present in hydrothermally modified sediments where they coexist with newly formed minerals, except in Hole 858B, which is located close to an active hydrothermal vent where pure hydrothermal layers consist solely of authigenic minerals. The main authigenic phases in the modified sediments are silicate minerals and calcite concretions with minor sulfides. The most common authigenic silicate in the deepest layers downhole is Mg-chlorite. At Site 858, a mineralogic sequence is clearly established from the seawater/sediment interface to the deeper parts of the hole. The shallowest hydrothermal unit from Hole 858B is composed of Mg-smectite and authigenic pyrite, which downhole sequentially overlies corrensite-rich layers which contain some talc and anhydrite. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. All of these silicates seem to be the product of a direct precipitation from hydrothermal fluids. This sequence reflects a thermal gradient related to the vertical influx of hydrothermal fluids. Secondary precipitation of carbonate concretions was generated from Ca released by biogenic phases and hydrothermal fluids. The concretions show several stages of crystallization and seem to occur preferentially in oxidizing areas and in the shallowest deposits. The occurrence of the concretions is favored by high porosity in the sediments. The presence of pure hydrothermal layers locally interbedded with unmodified sediments, and the variable mineralogy of the carbonate nodules suggest that convective circulation of fluids acts at various scales in this hydrothermal system and that the sedimentary cover favors lateral fluid circulation.

Mineralogic and petrographic records of sediment-fluid interaction in the sedimentary sequence at Middle Valley, Juan de Fuca Ridge, Leg 139

Carbonate nodules and associated sediments from Ocean Drilling Program Leg 139 sites of the Middle Valley, Juan de Fuca Ridge, were studied using a combination of inorganic and organic geochemical techniques. The mineralogy and stable isotopic compositions defined three types of nodules, reflecting the conditions of carbonate precipitation. Nodule ingrowth has entrapped the sedimentary organic matter, and concomitant displacement of the sedimentary mineral matrix and associated organic matter resulted in decreased bitumen extract yields from the nodules. Molecular organic geochemical analyses of the bitumen by gas chromatography-mass spectrometry revealed that polycyclic aromatic hydrocarbons are major constituents in the total extracts, reflecting the high-temperature origin of the entrapped hydrothermal organic products. Strong reducing conditions are evident during the hydrothermal alteration of biomarker precursors to products. The oxygen isotope data reflect the lower fluid temperatures during carbonate precipitation.

http://www-odp.tamu.edu/publications/139_SR/VOLUME/CHAPTERS/sr139_15.pdf

J. A. Mckenzie

Papers

Stable isotope and geochemical record of convective hydrothermal circulation in the sedimentary sequence of Middle Valley, Juan de Fuca Ridge, Leg 139

Mineralogic and petrographic records of sediment-fluid interaction in the sedimentary sequence at Middle Valley, Juan de Fuca Ridge, Leg 139

Organic matter and carbon isotope composition of carbonate nodules and associated sediments from middle valley, leg 139