John R. Delaney

Infrared spectroscopic measurements of CO2 and H2O in Juan de Fuca Ridge basaltic glasses

Dissolved H_2O and CO_2 contents in basaltic glasses from the Juan de Fuca Ridge and neighboring seamounts were determined by infrared spectroscopy. CO_2 contents range from about 45 to 360 ppm by weight, with carbonate ion complexes the only detectable form of dissolved carbon. Samples erupted at a given depth exhibit a large range in dissolved CO_2 contents that we interpret to be the result of variable amounts of degassing. The lowest CO_2 contents at each depth are in reasonable agreement with the experimentally determined CO_2 solubility curve for basalt at low pressures. All glasses with CO_2 values higher than the experimentally determined solubility at the eruption depth are oversaturated because of incomplete degassing. The highest CO_2 contents are spatially associated with the local topographic highs for each ridge segment. Lavas from relatively deep areas may have had greater opportunity to degas during ascent from a magma chamber or during lateral flow in dikes or seafloor lava flows. The highest observed CO_2 concentrations are from the axial seamount and lead to an estimate of a minimum depth to the magma chamber of 2.7 km beneath the ridge axis. H_2O contents vary from 0.07 to 0.48 wt.%, with hydroxyl groups the only detectable form of dissolved water. Water contents correlate positively with FeO^*/MgO and the highest water contents are found in the incompatible element-enriched Endeavour segment lavas. Variations in ratios of water to other incompatible elements suggest that water has a bulk partition coefficient similar to La during partial melting (D ∼ 0.01).

Incidence and Diversity of Microorganisms within the Walls of an Active Deep-Sea Sulfide Chimney

ABSTRACT A large, intact sulfide chimney, designated Finn, was recovered from the Mothra Vent Field on the Juan de Fuca Ridge in 1998. Finn was venting 302°C fluids on the seafloor and contained complex mineralogical zones surrounding a large open central conduit. Examination of microorganisms within these zones, followed by community analysis with oligonucleotide probes, showed that there were variations in the abundance and diversity of eubacteria and archaea from the exterior to the interior of the chimney. The microbial abundance based upon epifluorescence microscopy and quantitative fatty acid analyses varied from >108 cells/g of sulfide 2 to 10 cm within the chimney wall to <105 cells/g in interior zones. Direct microscopic observation indicated that microorganisms were attached to mineral surfaces throughout the structure. Whole-cell hybridization results revealed that there was a transition from a mixed community of eubacteria and archaea near the cool exterior of the chimney to primarily archaea near the warm interior. Archaeal diversity was examined in three zones of Finn by cloning and sequencing of the 16S rRNA gene. The majority of sequences from the exterior of the chimney were related to marine group I of the Crenarchaeota and uncultured Euryarchaeota from benthic marine environments. In contrast, clone libraries from interior regions of the chimney contained sequences closely related to methanogens, Thermococcales, and Archaeoglobales, in addition to uncultured crenarchaeal phylotypes obtained from deep subsurface sites. These observations of microbial communities within an active hydrothermal chimney provide insight into the microbial ecology within such structures and may facilitate follow-up exploration into expanding the known upper temperature limits of life.

Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge

A high-precision, high-resolution geologic map explicitly documents relationships between tectonic features and large steep-sided, sulfide-sulfate-silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge-parallel normal faults and other fracture-fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well-weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (> 1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest-temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture-controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide-sulfate-silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep-walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development.

Petrologic consequences of rift propagation on oceanic spreading ridges

Abstract The production of anomalously differentiated lava compositions at several mid-ocean spreading centers can be attributed to magmatic processes associated with propagating rifts. The degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers. Low supply rates and moderate cooling rates allow advanced degrees of closed-system fractionation to occur. High supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma. Propagating rift tips are a special class of ridge-transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge. This balance is affected by the spreading rate, the propagation rate of the rift, the length of the bounding transform and proximity to hotspots. Maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance. Rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion. Geochemical anomalies may persist, though changing in degree and extent, for several million years on ridge segments that stop propagating. The concept of “magnetic telechemistry” is generally supported by our study, but in the vicinity of hotspots, magnetic anomaly amplitude may be controlled more by bathymetric and/or thickened magnetic layer effects than by geochemistry.

Mid-ocean ridge sulfide deposits: Evidence for heat extraction from magma chambers or cracking fronts?

Numerous seafloor observations show that the sizes of high-temperature hydrothermal sulfide edifices vary dramatically with spreading rate. On fast-spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m, while on slow-spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105–106 m3. We argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast-spreading ridges, where spreading occurs predominantly by magmatism, and slow-spreading ridges, where there is a significant component of tectonic extension. Along fast-spreading ridges, a steady-state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability. In such systems, episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis. Over a time scale of the order of a decade, mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions. On slow-spreading ridges, tectonic extension maintains the fluid pathways necessary to support vigorous convection. In the waning stages of magmatism, hydrothermal circulation driven by a downward-migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits. As the depth of circulation increases, overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively. Once hydrothermal fluids penetrate the Moho, serpentinization clogs the fluid pathways and high-temperature venting ceases until it is reactivated by a fresh magmatic intrusion.

Growth of large sulfide structures on the endeavour segment of the Juan de Fuca ridge

Abstract Mapping and sampling with DSRV “Alvin” has established that sulfide blocks 0.5 m across, dredged from the axial valley of the Endeavour Segment at 47°57′N, are samples of unusually large sulfide structures. The steep-sided structures, up to 30 m in length, 20 m in height, and 10–15 m across, are localized by venting along normal faults at the base of the western axial valley wall, and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest. High-temperature fluids (350 to more than 400°C) pass through the massive sulfide structures and enter seawater through small, concentric “nozzle-like” features projecting from the top or the sides of the larger vent structures. Diffuse, low-temperature flow is pervasive in the vicinity of the active sulfide structures, exiting from basalt and sulfide surfaces alike. Evidence of recent volcanic activity is sparse. The two largest samples taken with the dredge would not have been recoverable using the submersible. These samples represent massive, complex portions of the sulfide structures which were not closely associated with rapid high-temperature fluid flow at the time of sampling; they contain textural evidence of sealed hydrothermal fluid exit channels. Mineralogy is dominated by Fe sulfides nnd amorphous silica. Pyrite, marcasite, wurtzite, chalcopyrite, and iss are the most common sulfide phases. Pyrrhotite, galena, and sphalerite are present in trace amounts. Barite, amorphous silica, and chalcedony are the only non-sulfide phases; anhydrite is not observed in any of the dredge samples, although it is common in the chimney-like samples recovered by “Alvin”. Specific mineralogical-textural zones within the dredge samples are anaoogous to individual layers in East Pacific Rise at 21°N and southern Juan de Fuca Ridge samples, with two exceptions: a coarse-grained, highly porous Fe sulfide-rich interior containing sulfidized tubeworm casts, and a 2–5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica. The porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber. The amorphous silica deposited from a seawater/hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber; conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate. These silica-rich zones are the densest, most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features. Observations in these samples are consistent with two distinct phases of development. Phase 1 is analogous to chimney growth and construction at 21°N and ends when flow channels become sealed to rapid flow of through-going fluid. The flow is evidently redirected within the structure. Phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater/hydrothermal fluid mixtures. Evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures.

Two-phase separation and fracturing in mid-ocean ridge gabbros at temperatures greater than 700°C

Abstract Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events. The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1–2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C. Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000–1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapment depth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber. Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1–7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200–300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.

Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U– 230Th disequilibrium

A highly precise mass spectrometric method of analysis was used to determine238U—234U—230Th—232Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial230Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated usingδ234U and appear to be negligible; hence the230Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess230Th in off-axis samples.238U—230Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U—Th ages of 50 to 200 ka. The full spreading rate based on U—Th ages for Endeavour segment of JDF is 5.9 ± 1/2 cm/yr, with asymmetry in spreading between the Pacific (4.0 ± 0.6 cm/yr) and JDF (1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230Th/232Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial230Th/232Th ratios and230Th activity were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230Th/232Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large230Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between230Th excess and 230Th/232Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in230Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial230Th/232Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th—Sr isotope mantle array.

The nature and distribution of carbon in submarine basalts and peridotite nodules

Primary carbonaceous material has been identified in submarine basaltic glasses and mantle-derived peridotite nodules from alkali basalts using electron microprobe techniques. In the submarine rocks carbon occurs (1) in quench-produced microcracks in glasses and phenocrysts, (2) in vesicles, where it is preferentially concentrated on the sulfide spherules attached to vesicle walls, and (3) in microcracks and CO2-rich bubbles in inclusions of glass completely enclosed by phenocrysts. In peridotite nodules carbon exists in intergrain cracks, along grain boundaries, and on the walls of fluid inclusions disposed in two dimensional arrays. The carbonaceous material is believed to consist of a mixture of graphite, other forms of elemental carbon, and possibly small amounts of organic matter. It is suggested that carbon precipitates by disproportionation of CO according to the reaction 2 CO→C+CO2 and that this reaction is catalyzed by sulfide-oxide surfaces in vesicles. Once deposition has begun, the reaction continues on carbon surfaces as well. Based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them, it is proposed that carbon condensed from a magmatic vapor in which CO was a significant constituent. This implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the QFM buffer at equivalent temperatures. This is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials. Regardless of the mechanism of its formation, the presence of carbon in CO2-rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities. Condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks.

Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge

Massive-sulfide deposits rich in zinc and silver were recovered from the Juan de Fuca Ridge 500 km west of Oregon in September 1981. The samples recovered are composed largely of zinc sulfide, with lesser amounts of iron, lead, and copper sulfide. Most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1-km-wide valley along the ridge axis. The depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting. The axial valley floor outside the depression is underlain by fresh, glassy, ferrobasalt sheet and lobate flows. Two types of sulfide-mineral deposits were dredged from one of the hydrothermal vents: (1) angular slabs of dark-gray zinc sulfide; and (2) subrounded fragments of porous light-gray zinc sulfide. The samples contain fresh sphalerite, zoned wurtzite, pyrite, and minor amounts of marcasite, galena, and chalcopyrite-cubanite. The spreading rate of the Juan de Fuca Ridge and the composition of the sulfide samples are generally similar to the East Pacific Rise lat 21° N site; however, the texture and geologic setting of the sulfide deposits are significantly different.