Southern Juan De Fuca Ridge Research Articles

Calcium Isotopic Compositions of Normal Mid‐Ocean Ridge Basalts From the Southern Juan de Fuca Ridge

AbstractMantle peridotites show that Ca is isotopically heterogeneous in Earth's mantle, but the mechanism for such heterogeneity remains obscure. To investigate the effect of partial melting on Ca isotopic fractionation and the mechanism for Ca isotopic heterogeneity in the mantle, we report high‐precision Ca isotopic compositions of the normal Mid‐Ocean Ridge Basalts (N‐MORB) from the southern Juan de Fuca Ridge. δ44/40Ca of these N‐MORB samples display a small variation ranging from 0.75 ± 0.05 to 0.86 ± 0.03‰ (relative to NIST SRM 915a, a standard reference material produced by the National Institute of Standards and Technology), which are slightly lower than the estimated Upper Mantle value of 1.05 ± 0.04‰ and the Bulk Silicate Earth (BSE) value of 0.94 ± 0.05‰. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on δ44/40Ca of N‐MORB due to their low CaO contents, while plagioclase fractional crystallization cannot lead to light Ca isotopic compositions of the residue magma. Instead, the lower δ44/40Ca of N‐MORB samples compared to their mantle source is most likely caused by partial melting. The offset in δ44/40Ca between N‐MORB and BSE indicates that at least 0.1–0.2‰ fractionation would occur during partial melting and light Ca isotopes are preferred to be enriched in magma melt, which is in accordance with the fact that δ44/40Ca of melt‐depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease δ44/40Ca in basalts and induce Ca isotopic heterogeneity in Earth's mantle.

Seismic reflection images of a near-axis melt sill within the lower crust at the Juan de Fuca ridge

The oceanic crust extends over two-thirds of the Earth's solid surface, and is generated along mid-ocean ridges from melts derived from the upwelling mantle. The upper and middle crust are constructed by dyking and sea-floor eruptions originating from magma accumulated in mid-crustal lenses at the spreading axis, but the style of accretion of the lower oceanic crust is actively debated. Models based on geological and petrological data from ophiolites propose that the lower oceanic crust is accreted from melt sills intruded at multiple levels between the Moho transition zone (MTZ) and the mid-crustal lens, consistent with geophysical studies that suggest the presence of melt within the lower crust. However, seismic images of molten sills within the lower crust have been elusive. Until now, only seismic reflections from mid-crustal melt lenses and sills within the MTZ have been described, suggesting that melt is efficiently transported through the lower crust. Here we report deep crustal seismic reflections off the southern Juan de Fuca ridge that we interpret as originating from a molten sill at present accreting the lower oceanic crust. The sill sits 5-6 km beneath the sea floor and 850-900 m above the MTZ, and is located 1.4-3.2 km off the spreading axis. Our results provide evidence for the existence of low-permeability barriers to melt migration within the lower section of modern oceanic crust forming at intermediate-to-fast spreading rates, as inferred from ophiolite studies.

Geochemistry of basalt from the North Gorda segment of the Gorda Ridge: Evolution toward ultraslow spreading ridge lavas due to decreasing magma supply

High‐density, precisely located, dive and rock‐corer basalt samples from the 65‐km‐long North Gorda ridge segment reveal compositional diversity as great as for the entire Gorda Ridge. Lava compositions along the ridge axis show considerable major and minor element diversity (MgO 9.2–4.4%, K2O 0.04–0.36%) for lavas erupted in close proximity. Although they form a near‐continuum in the higher MgO range, the samples can be separated into two groups; one is typical N‐type mid‐ocean ridge basalt (MORB) (K2O/TiO2 < 0.09), and the other is a more enriched T‐MORB (K2O/TiO2 > 0.09). Incompatible elements also reflect this grouping with (Ce/Yb)N < 1 and Zr/Nb > 20 for N‐MORB and (Ce/Yb)N > 1 and Zr/Nb < 20 for T‐MORB. Samples collected from off‐axis, over a distance of 4 km up the eastern rift valley wall, are all light rare earth element (LREE)‐depleted N‐MORB with a narrower compositional range (MgO of glasses 7.7 ± 0.3%, Zr/Nb = 38–50; (Ce/Yb)N < 1), although isotopic ratios are comparable to those on‐axis. Lavas erupted in the past, before the present‐day deep axial valley formed on this part of the ridge, were more uniform N‐MORB, generated by larger degrees of melting when magma supply was greater. Basalts from the adjoining southern Juan de Fuca Ridge segment, with comparable spreading rate but distinctly different ridge morphology, are also all LREE‐depleted N‐MORB, but the narrow range of evolved compositions of the sheet flows covering the broad, U‐shaped valley suggests shallower, more steady state magma reservoirs underlying this ridge segment. Basalts from Escanaba, the slowest spreading segment of Gorda Ridge, include N‐, T‐, and E‐MORB that were erupted from isolated volcanic centers. The pattern of incompatible element enrichment, especially in LREE, K, Ba, and 87Sr/86Sr, with decreasing spreading rate and magma supply is even more pronounced at the ultraslow spreading Arctic ridges where most lavas are E‐MORB (Zr/Nb < 10, (Ce/Yb)N >1.0–3.0). Arctic E‐MORB compositions lie along a common mixing trend with those from North Gorda. As the magma budget and/or partial melting decreases, a similar enriched component, especially in K, Ba, and LREE, widely present in the oceanic mantle is apparently incorporated to a greater degree. At North Gorda, morphology and chemical characteristics appear to evolve with time toward that of ultraslow spreading ridges.

Seismic evidence for variations in axial magma chamber properties along the southern Juan de Fuca Ridge

Multichannel seismic data collected along the Cleft segment on the southern Juan de Fuca Ridge show that this intermediate-spreading center is underlain by a mid-crustal reflector interpreted as the top of an axial magma chamber (AMC). The AMC reflection is present along most of the segment, and deepens gently from 2.0 km near the southern end of the segment beneath the RIDGE Cleft Observatory Site, to 2.3 km at the northern end beneath the site of the mid-1980s submarine eruption. We analyzed the one-dimensional seismic structure of the AMC at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields. At the northern site, waveform modeling in the time intercept–slowness ( τ– p) domain indicates that the AMC is ∼ 100 m thick and it is characterized by a decrease in P-wave velocity from 6 km/s to 3.7 km/s. In contrast, the P-wave velocity within the shallower, ∼ 100-m-thick AMC at the southern site is higher (5.0 km/s). The decrease in seismic velocity within the AMC indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate-spreading segments. The data show a coherent seismic phase interpreted as the P- to S-wave conversion at the AMC ( P AMC S). Stacking of this event shows that the P AMC S is only present along the northern part of the segment. Our results thus suggest along-axis variations in the crystallinity of the AMC. The AMC along Cleft varies from a high crystal content (< 30% melt) sill at the southern end of Cleft, to a largely melt (60–75%) sill at the source of the 1980s eruption at the northern end. The variations in magma chamber properties inferred from our seismic data correlate with changes in lava chemistry and with the location of hydrothermal plumes, and they all suggest that focused, high-temperature hydrothermal venting along intermediate-spreading ridges is closely linked to the physical state of the underlying magma chamber.

The Cleft revealed: Geologic, magnetic, and morphologic evidence for construction of upper oceanic crust along the southern Juan de Fuca Ridge

The geology and structure of the Cleft Segment of the Southern Juan de Fuca Ridge (JdFR) have been examined using high‐resolution mapping systems, observations by remotely operated vehicle (ROV), ROV‐mounted magnetometer, and the geochemical analysis of recovered lavas. Bathymetric mapping using multibeam (EM300) coupled with in situ observations that focused on near‐axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the JdFR. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen ROV dives. Our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and/or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley. The geochemically evolved and heterogeneous nature of these near‐axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone. In contrast, the most recent axial eruptions are more primitive (higher MgO), chemically homogeneous lobate, sheet, and massive flows that generate a distinct magnetic high over the axial valley. We suggest that the syntectonic capping volcanics observed off‐axis were erupted from near‐axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data. This model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the Cleft Segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle.

Upper crustal structure and axial topography at intermediate spreading ridges: Seismic constraints from the southern Juan de Fuca Ridge

We use multichannel seismic reflection data to image the upper crustal structure of 0–620 ka crust along the southern Juan de Fuca Ridge. The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow (Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment to 2.3 km at the northern end near the site of the 1980s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250–300 m thick at the ridge axis along the Cleft segment and 300–350 m thick along the axis of the Vance segment. However, off‐axis layer 2A is similar in both segments (500–600 m), indicating ∼90% and ∼60% off‐axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3–4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.

Petrology, morphology, and geochemistry of the southern Juan de Fuca Ridge: Evidence for off-axis volcanism

In the 1960s, the discovery of a continuous Mid-Ocean Ridge (MOR) volcanic system rocked the geologic world. The MOR snakes through the seafloor as its constant, active eruptions give birth to seafloor spreading centers along the central axis of tectonic plates. For many years, the axis of the MOR has been subject to intense study because of its known active status.

Hydrothermal temperature changes at the southern Juan de Fuca Ridge associated with MW 6.2 Blanco Transform earthquake

The regional impact of transform seismicity on ridge-crest hydrothermal venting and ridge-transform dynamics is investigated using a June 1‐7, 2000, Blanco Transform earthquake sequence. The mainshock (Mw 6.2) and 170 foreshocks and aftershocks were located using T waves recorded on U.S. Navy hydrophones, and indicate that the active transform fault may be farther south than in previous tectonic models. During the earthquake sequence, two temperature probes were deployed in black smoker chimneys at the Vent1 and Plume hydrothermal fields along the southern Juan de Fuca Ridge. These two hydrothermal systems are ;39 km northwest of the mainshock’s acoustic location. Both probes show significant (.5 8C) temperature declines following the mainshock, the Vent1 temperature changes occurring over days to weeks while the Plume changes were coseismic. The Vent1 and Plume fluid temperature decreases are consistent with earthquakeinduced changes to permeability in the upper ocean crust. The evidence suggests that deep-ocean hydrothermal systems can be altered by large earthquakes even at a distance and across tectonic provinces, and ridge-crest seismic and magmatic activity are not the only causes of change to hydrothermal systems.

A deep-sea observatory experiment using acoustic extensometers: precise horizontal distance measurements across a mid-ocean ridge

In July 2000, an array of instruments called acoustic extensometers was deployed at the Cleft segment of the southern Juan de Fuca Ridge, a seafloor observatory site selected by the National Science Foundation RIDGE Program. These instruments are designed to precisely measure horizontal deformation across the axis of a mid-ocean ridge in order to detect and quantify seafloor spreading events. The instruments were deployed in semipermanent seafloor benchmarks in a linear array that is 1.2-km long and spans the floor of the axial valley. The instruments make daily measurements of distance to their neighbors in the array by recording the round trip travel time of 100-kHz acoustic pulses, and simultaneous temperature measurements are used to correct the ranges for sound speed variations. The instruments are expected to have lifetimes of at least five years. In addition, precise pressure measurements have been made at each benchmark with a remotely operated vehicle in order to monitor for vertical deformation across the array. Preliminary results show that the resolution of the acoustic measurements is /spl plusmn/1-2 cm and that no abrupt deformation events occurred during the first year.

Chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids

Concentrations of trace metals in hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) and the TAG Hydrothermal Field on the Mid-Atlantic Ridge are influenced by the combined effects of source rock composition, brine–seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys. Copper, Mo, and Co show predicted, sharp decreases in concentration for vent fluids as temperatures decrease below 350°C; however, concentrations of Cu in 363°C fluids at the TAG site are well above values determined from seawater/basalt interactions due to subseafloor-refining processes and remineralization of chalcopyrite. Concentrations of Zn, Cd, Pb, As, and Ga do not show any temperature dependence between 214 to 363°C. However, Zn values do correlate well with Cl on an area-by-area basis due to subseafloor mixing of brine with altered seawater. Zinc levels also correlate well with concentrations of Cd, Pb, and As for all vent fluid samples, as well as with basalt and Zn-rich sulfides, implying some continuity in behavior from source rock to vent fluid to sulfide minerals. Gallium values correlate with Zn and Cl levels for the SJFR, but are high relative to Zn at the TAG site due to subseafloor refining processes. Concentrations of Tl follow Cl, K, and Rb, consistent with the behavior predicted for Tl +. Overall, the Cl- and Zn-rich fluids from the SJFR are characteristic of a brine-dominated system formed during recent volcanism. In contrast, high-temperature fluids from the TAG area are representative of a system that has evolved over the past 100,000 yr with high concentrations of Cu, Co, and Ga that result from higher temperatures and complex refining processes within a large vent mound.

No spreading across the Southern Juan de Fuca Ridge axial cleft during 1994–1996

Direct‐path acoustic measurements between seafloor transponders observed no significant extension (−10±14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44°40′ N and 130°20′ W). The acoustic path for the measurement is a 691‐m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full‐spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (∼ 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft‐parallel 714‐m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5±7 mm/yr) between the 1994 and 1996 surveys.

A submersible study in the western Blanco fracture Zone, N.E. Pacific: Structure and evolution during the last 1.6 Ma

During July and August 1991, the French-American Blanconaute dive program used the French submersibleNautile to investigate the West Blanco Depression (WBD), a deep, elongate trough located at the intersection of the Blanco Transform Fault Zone with the southern Juan de Fuca Ridge (JdFR). Twenty dives were carried out along the north wall of the WBD, which exposes the upper oceanic crust over a 65 km distance, from the JdFR axis (to the west) to the oblique trace of an ancient propagator (to the east, crustal age around 2 Ma). Thirteen of these dives were precisely located within a 13 × 7 km zone of the north wall, covered by a high-resolution sonar mapping operation during the Blancotrough cruise in 1987. This series of geological traverses, plus 4 dives across the south wall of the WBD (one dive) and the adjacent Parks Plateau (3 dives), collected 242 rock samples. We report here the main results of the dive program and preliminary laboratory studies: 1. Transform-related tectonic activity has recently abandoned the southern margin of Parks Plateau, and is presently located inside the WBD area, mainly along its northern wall. The tectonic features observed are compatible with a right-lateral strike-slip system, with a NE-SW extensional component. 2. Three main lithological units are exposed along the north wall of the WBD. From top to bottom, they are: (1) a Volcanic Unit, forming a steep upper cliff, made of massive and pillow flows and basaltic dikes, with an estimated average thickness of 800 m; (2) a less steep Transition Zone, about 150 to 400 m thick, largely masked by rubble but exposing both diabase outcrops and pillow flows; and (3) a massive Diabase Unit, exposed over 700–800 m, with a dike complex structure visible from place to place, and cut by a net of hydrothermal veins. Deep crustal rocks such as gabbros were not observed. 3. Spectacular mass-wasting features are visible all along the north wall of the WBD. About 60% of the face of the wall is masked by talus cones, rubble, rock avalanche deposits and slide blocks. Three main landslides, of approximately one km3 in volume each, were tentatively identified. One of them was mapped in detail and consists of an approximately 300 m thick (0.85 km3), coherent slide block detached from a zone where intense hydrothermal alteration and faulting have obviously weakened the bedrock, that is in places entirely altered to blue clays. 4. The basaltic lavas of the WBD north wall show a remarkable evolution with time, from east to west. Around the tip of the ancient propagator, they are restricted to primitive, olivine-rich picritic basalts. Proceeding westward, they exhibit a wide range of differentiation, including highly fractionated, FeTi-rich ferrobasalts at about 35–45 km from the JdFR axis. When approaching the JdFR axis, the FeTi enrichment decreases gradually, and the ferrobasalts evolve towards slightly differentiated MORB-type basalts, typical of the southern JdFR. This magmatic evolution marks the transition from the end of a propagating rift regime to a steady-state accretion regime. 5. The WBD north wall also permits the study of weathering and hydrothermal alteration processes and their evolution in space and time. Vertically, the alteration products evolve from oceanic weathering and zeolite facies (Volcanic Unit) to the greenschist facies (Transition Zone and Diabase Unit). Horizontally, the evolution with time is mainly a general hydration of the crust that is, however, very irregularly distributed. 6. Several vertical magnetic traverses along the north wall of the WBD, using a bottom magnetometer attached to the basket of the submersible, have shown a sharp 5000 to 7000 nT positive anomaly at about 3500 m depth. This anomaly corresponds exactly to the first appearance of extrusive pillow-lava outcrops, and confirms the dramatic decrease in magnetic anomaly amplitude below that depth, detected during the Blancotrough cruise in 1987. The vertical magnetic profiles thus appear to have imaged the base of the magnetic source layer.

A three-component mixing model for ridge-crest hydrothermal fluids

The hydrothermal fluid chemistry data set for sediment starved ridge axes is used to estimate endmember compositions and to constrain a three-component mixing model based on that of Von Damm, using numerical methods to estimate the endmember compositions. The data set includes Fe, Cl, H 2S, Na, K, Ca, and Sr, at 11–13°N and 21°N East Pacific Rise, North and South Cleft Segment (Southern Juan de Fuca Ridge), TAG, and MARK. Q-mode factor analysis shows that the data can be explained (99.9%) by 3 factors. A constrained optimization of a weighted least-squares algorithm, assuming conservative mixing and with the constraints that all endmember concentrations and contributions are non-negative and that fractional contributions sum to one, is used to partition the data set into endmembers. Mixing of the calculated endmembers-consistently found to be phase-separated brine, vapor, and hydrothermally altered seawater -reproduces the composition of the vent fluids. The model is consistent with the presence of a convecting brine layer in the primary reaction zone at depth, as suggested by Bischoff and Rosenbauer, that transfers properties to the overlying circulating seawater by release of a “vapor” and by diffusion. We suggest that future experimental work examine solubility controls in conjunction with and subsequent to phase separation, and work with a wider range of fluid salinities.

Hydrothermal microseismicity at the megaplume site on the southern Juan de Fuca Ridge

Abstract Several thousand microearthquakes were recorded by ocean-bottom seismometers during 11 days of deployment on the Cleft segment of the southern Juan de Fuca Ridge. These microearthquakes primarily occurred in swarms lasting up to 1 hr, and are characterized by complex time series with typical single-event durations of just over 1 sec. The events were too small to be detected by more than one instrument at minimum instrument spacings of about 4 km. Clear P - and S -wave arrivals were not observed for most events, and the majority of the spectra are peaked at discrete frequencies that are not simply related (i.e., not integer multiples). These peaked spectra are quite similar to those observed from geysers and long-period events in volcanoes. Calibration tests performed during the deployment indicate that the event spectral peaks are not explained by instrument resonance or seafloor coupling. Based on these observations, the vigorous hydrothermal activity in the area, and thermodynamic considerations, we postulate that the majority of the microseismicity observed is generated within the seafloor hydrothermal system by pressure transients and hydraulic fracturing.

Evolution of a trench‐slope basin within the Cascadia subduction margin: the Neogene Humboldt Basin, California

ABSTRACTThe Neogene Humboldt (Eel River) Basin is located along the north‐eastern margin of the Pacific Ocean within the Cascadia subduction zone. This sedimentary basin originated near the base of the accretionary prism in post‐Eocene time. Subduction processes since that time have elevated strata in the south‐eastern portion of the basin above sea level. High‐resolution chronostratigraphic data from the onshore portion of the Humboldt Basin enable correlation of time‐equivalent lithofacies across the palaeomargin, reconstruction of slope‐basin evolution, and preliminary delineation of climatic and tectonic influence on lithological variation. Emergent basin fill is divided into five lithofacies which clearly document shoaling of the inner trench slope from deep‐water environments in early Miocene time to paralic environments in Pleistocene time. The oldest strata consist of hemipelagic mudstones and minor debris‐flow breccias deposited in a deep‐water setting during elevated sea level. These strata are overlain by glauconite‐rich, fine‐grained turbidites which heralded an increasing influx of terrigenous detritus. Water depths shoaled earlier in the eastern basin area as the palaeoshoreline prograded seaward. Turbidite deposition ceased in the eastern basin area at about 2‐2 Ma, whereas 22 km to the west, turbidite deposition continued until about 1‐8 Ma. Lithofacies at the western study site change abruptly across a middle Pleistocene unconformity from outer shelf to paralic deposits. In the east, a more complete Pleistocene section records transition from outer to inner shelf, beach and fluvial environments.The Humboldt Basin lithofacies sequence is overprinted by eustatic control of sediment source. Comparison of sediment character with palaeoceanographic conditions indicates dominance of hemipelagic facies during periods of elevated sea level in the middle Miocene and early Pliocene when depocentres were isolated from terrigenous sediment. Glauconite‐rich facies were mobilized from an upper slope setting following these periods of elevated sea level and redeposited in a deep‐marine environment. Pleistocene shoreline lithofacies display glacio‐esutatic control of depositional environment by recording several cycles of nearshore to fluvial progressions.General models of accretionary prism behaviour and trench‐slope basin evolution are compatible with the overall coarsening‐upward lithofacies sequence filling the Humboldt Basin. Early structural barriers precluded deposition of terrigenous material except from locally derived debris flows; subsequent shoaling and burial of deactivated thrust‐folds enabled turbidity flows to reach the basin floor.However, late‐stage tectonism apparently controlled the onset of coarse‐grained deposition in this sequence. Significant sand‐rich turbidite deposition began in the middle Pliocene, synchronous with tectonic uplift of the southern basin margin. Conversely, cessation of turbidite deposition in the eastern basin area in latest Pliocene time was synchronous with growth of anticlinal structures which again blocked widespread dispersal of turbidity flows. This middle Pliocene to Holocene period of crustal shortening is synchronous with continued reduction in spreading rate along the southern Juan de Fuca ridge, and probably reflects partial coupling between the subducting lithosphere and the overlying accretionary prism.

Effluent from diffuse hydrothermal venting: 2. Measurement of plumes from diffuse hydrothermal vents at the southern Juan de Fuca Ridge

An array of temperature and velocity sensors was deployed for two weeks in the bottom boundary layer at the Southern Juan de Fuca Ridge during the National Oceanic and Atmospheric Administration VENTS'90 field experiment. The instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal sources located 1–2 km distant. Over the duration of the field deployment, we detected effluent from two vent sources. The plume from the larger of the two sources was sensed at all of the instrument locations, permitting estimates of dilution based on the field measurements. Our data also permitted estimation of energy flux from diffuse vents (approximately 125±75 MW from a source to the north of the instrument array, and 12±8 MW from a small source to the east). The largest source of error in the data was estimation of the plume width. We have described a sensor array that would reduce this error in future work. Such an array would be suitable for a long‐term vent activity monitor.

Petrology and geochemistry of basalts from the southern Juan de Fuca Ridge: Controls on the spatial and temporal evolution of mid‐ocean ridge basalt

Three morphologically distinct regions within the neovolcanic zone of the Cleft segment of the southern Juan de Fuca Ridge were investigated and sampled in detail using the DSRV Alvin. Additional along‐strike and off‐axis samples were recovered by dredge. The southernmost region, the Southern Cleft site, is characterized by a 3‐km‐wide axial valley floored by ponded sheet flows and bisected by a 30‐ to 50‐m‐wide cleft. Farther north at the “Young Sheet Flow” site, the ridge axis is characterized by a distinct 500‐m‐wide inner graben that is largely covered by distinctly younger looking sheet flows. The northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the Cleft segment well into the zone of overlap with the neighboring Vance segment. The pillowed lavas at the “Young Pillow Mound” site represent the most recent episode of volcanism along the Cleft segment. Strong correlations exist between degrees of fractionation, relative ages of lavas, and latitude; lavas are progressively younger looking and more mafic to the north. The compositional range of mid‐ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35–40% low‐pressure fractional crystallization of relatively primitive, but not primary, depleted (N‐type) melts. Scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions, generated from small‐scale mantle heterogeneities. Furthermore, the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern Juan de Fuca Ridge. The more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along‐axis flow in a continuous axial magma chamber or conduit. Rather, the data suggest lavas were derived from discrete magma chambers or lenses, each in a different stage of evolution. The youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds. The oldest stage (∼40% additional crystallization of the most mafic composition) is associated with focused hydrothermal activity and tectonic extension, whereas the youngest events are characterized by ridge inflation, diking, and dispersed hydrothermal activity. Geochemical and tectonomagmatic features observed along the Cleft segment are similar to those recently documented along the East Pacific Rise from 9°30′N–10°N suggesting the scales, processes, and stages of magmagenesis are similar along medium to fast spreading ridge segments.

Trace metals in hydrothermal solutions from Cleft segment on the southern Juan de Fuca Ridge

Concentrations of trace metals in Fe‐ and Cl‐rich hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) have been determined and corrected for residual precipitates formed in the sampler. Precipitate corrections for Ag, Cd, Cu, Mo, Pb, Sb, and Zn were variable, contributing an average of 20% to total solution concentrations whereas corrections for Co averaged less than 5% and essentially no corrections were required for Fe and Mn. Values for Cu, Co, and Mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320°C. In addition, and unlike most other metals studied, all vent fluids from the SJFR were almost completely depleted in Mo relative to seawater values of about 110 nmol kg−1. In contrast to the Cu‐Co‐Mo group, concentrations of Ag, Cd, Sb, and Pb correlated well with those for Zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332°C range encountered in this study and more by the combined chemical processes that control Zn levels along the transport pathway from the deep reaction zone to the vent orifice.

Fine‐scale magnetic anomaly field over the southern Juan de Fuca Ridge: Axial magnetization low and implications for crustal structure

Sea surface, deep tow, and submersible magnetic surveys show that the spreading axis of the Cleft segment of the southern Juan de Fuca Ridge (SJDF) is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent Vance segment and most mid‐ocean ridge spreading centers. Forward iterative thickness modeling, based on the near‐bottom magnetic data, shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the Cleft axial valley. Thick crust is required to produce the magnetic anomaly high over the Vance axial valley. This magnetic modeling demonstrates that variations in the base of the magnetic source layer, which is assumed to be equivalent to the base of the extrusive lavas, can produce a significant proportion of the observed short‐wavelength magnetic anomaly signal. These magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2A thins beneath the axial valley of the Cleft and is thicker beneath the Vance axial valley. Recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading, which could produce significant magnetic contrasts. The results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration, first‐order change in upper crustal thickness is one of the most important variables to consider in the production of fine‐scale magnetic anomalies in young ocean crust.

Introduction to Special Section on Volcanic and Hydrothermal Processes on the Southern Juan de Fuca Ridge

Introduction to Special Section on Volcanic and Hydrothermal Processes on the Southern Juan de Fuca Ridge