Southern Explorer Ridge Research Articles

Picrite “Intelligence” from the Middle-Late Triassic Stikine arc: Composition of mantle wedge asthenosphere

Primitive, near-primary arc magmas occur as a volumetrically minor ≤100 m thick unit in the Canadian Cordillera of northwestern British Columbia, Canada. These primitive magmas formed an olivine-phyric, picritic tuff near the base of the Middle-Late Triassic Stuhini Group of the Stikine Terrane (Stikinia). A new 40Ar/39Ar age on hornblende from a cross-cutting basaltic dyke constrains the tuff to be older than 221 ± 2 Ma. An 87Sr/86Sr isochron of texturally-unmodified tuff samples yields 212 ± 25 Ma age, which is interpreted to represent syn-depositional equilibration with sea-water. Parental trace element magma composition of the picritic tuff is strongly depleted in most incompatible trace elements relative to MORB and implies a highly depleted ambient arc mantle. High-precision trace element and Hf-Nd-Pb isotopic analyses indicate an origin by mixing of a melt of depleted ambient asthenosphere with ≤2% of subducted sediment melt. Metasomatic addition of non-conservative incompatible elements through melting of subducted Panthalassa Ocean floor sediments accounts for the arc signature of the Stuhini Group picritic tuff, enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and high field strength elements (HFSE), and anomalous enrichment in Pb.The inferred Panthalassan sediments are similar in composition to the Neogene-Quaternary sediments of the modern northern Cascadia Basin. The initial Hf isotopic composition of the picritic tuff closely approximates that of the ambient Middle-Late Triassic asthenosphere beneath Stikinia and is notably less radiogenic than the age-corrected Hf isotopic composition of the Depleted (MORB) Mantle reservoir (DM or DMM). This suggests that the ambient asthenospheric mantle end-member experienced melt depletion (F ≤ 0.05) a short time before picrite petrogenesis. The mantle end-member in the source of the Stuhini Group picritic tuff is isotopically similar to the mantle source of enriched mid-ocean ridge basalts (E-MORB) erupted today at the southern end of the Explorer Ridge in northeastern Pacific Ocean. The isotopic similarity between the Middle-Late Triassic ambient mantle under Stikinia, and mantle presently tapped at the southern Explorer Ridge suggests that enriched domains in the northeastern Pacific mantle are long-lived (≥222 million years).

Radiogenic isotopes in enriched mid-ocean ridge basalts from Explorer Ridge, northeast Pacific Ocean

Extreme gradients in topography related to variations in magma supply are observed on the Southern Explorer Ridge (SER), part of the northern Juan de Fuca ridge system. We report radiogenic isotope (Pb, Sr, Nd, Hf) and geochemical data for twenty-four basalt whole-rock and glass samples collected from the length of the SER and from Explorer Deep, a rift to the north of the SER. Lavas from the SER form a north-south geochemical gradient, dominated by E-MORB at the northern axial high, and range from T-MORB to N-MORB towards the southern deepest part of the ridge. Linear relationships between incompatible element ratios and isotopic ratios in MORB along the ridge are consistent with mixing of magmas beneath the ridge to generate the geographic gradient from E- to N-MORB. The E-MORB have high Sr and Pb, and low Nd and Hf isotopic ratios, typical of enriched mantle that includes a FOZO or HIMU isotopic component. The West Valley and Endeavour segments of the northern Juan de Fuca ridge also include this isotopic component, but the proportion of the FOZO or HIMU component is more extreme in the SER basalts. The FOZO or HIMU component may be garnet-bearing peridotite, or a garnet pyroxenite embedded in peridotite. Recycled garnet pyroxenite better explains the very shallow SER axial high, high Nb/La and La/Sm, and the “enriched” isotopic compositions.

Waning magmatic activity along the Southern Explorer Ridge revealed through fault restoration of rift topography

We combine high‐resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the remotely operated vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief‐forming processes over the last 11,000–14,000 years. The restored topography indicates that the axial morphology evolved from a smooth constructional dome >500 m in diameter, to a fault‐bounded graben, ~500 m wide and 30–70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited‐extent low‐effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley postdate the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low‐effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes.

Pb and other ore metals in modern seafloor tectonic environments: Evidence from melt inclusions

Many modern seafloor tectonic environments are host to hydrothermal systems and associated polymetallic sulfide deposits. Metal transport and precipitation are controlled by magmatic processes such as pre-eruptive degassing and the hydrothermal cycle. The original availability of Pb and other ore metals in a given setting is dependent on concentrations in the original magmatic source or additional enrichment processes. We have examined the Pb budget of melt inclusions from nine modern seafloor settings representing back-arcs, mid-ocean ridges and seamounts. Melt inclusions provide information on the characteristics of parental magmas, including insights into metal budgets. Trace element data in melt inclusions hosted in plagioclase, olivine and pyroxene were obtained by laser-ablation inductively-coupled mass-spectrometry. Results from back-arcs emphasize the impact of slab-subduction and dehydration processes on the chemical characteristics of generated magmas. Volatile- and fluid-mobile element-rich melt inclusions at Manus basin and Okinawa trough reflect a robust contribution of elements from the subducting slab as evidenced by relatively low Ce/Pb ratios. At Bransfield strait, on the other hand, melt inclusions are volatile poor, and fluid-mobile element ratios are similar to mid-ocean ridge values indicating little or no contribution from the slab. High Cu concentrations at Manus basin and Okinawa trough can be explained by fluxing of ferric iron from the subducting slab benefiting the production of sulfate over sulfide. Metal budgets for seamounts located on and nearby the axis of mid-ocean ridge segments appear to be independent of any input of mantle plume material. Results from the southern Explorer ridge (strong lower mantle influence, transitional- and enriched-MORBs), Pito and Axial seamounts (moderate lower mantle influence, transitional-MORBs) and a Foundation near-ridge seamount (little to no mantle influence, normal-MORB) show that, despite similar tectonic environments and varying contributions of mantle plume material, Cu, Zn and Pb values do not vary significantly between the enriched and non-enriched magma components of a given setting.

Quantitative study of the deformation at Southern Explorer Ridge using high-resolution bathymetric data

We present preliminary results of a morphological study of the summit of the Southern Explorer Ridge (SER). The SER is an inflated intermediate-rate spreading center located in the northeast Pacific off the West coast of Canada, that hosts a large hydrothermal vent complex known as “Magic Mountain”. A quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high-resolution, near-bottom, bathymetric data. These data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor. Structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures. The ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults, highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer. Populations of small faults exhibit an exponential size-frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain (3.7 to 18.4%) accommodated by the normal faults. We propose that the 500 to 600-m wide and ∼ 60-m deep asymmetric axial summit graben of the SER formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben. We find a well-defined geographic distribution in the types of lava flows, which indicates a general decrease of the eruption rate through time. We also find that the “Magic Mountain” hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile-transition depth. This fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature.

Formation of Fe-silicates and Fe-oxides on bacterial surfaces in samples collected near hydrothermal vents on the Southern Explorer Ridge in the Northeast Pacific Ocean

Formation of Fe-silicates and Fe-oxides on bacterial surfaces in samples collected near hydrothermal vents on the Southern Explorer Ridge in the Northeast Pacific Ocean

Formation of Fe-silicates and Fe-oxides on bacterial surfaces in samples collected near hydrothermal vents on the Southern Explorer Ridge in the Northeast Pacific Ocean

Samples collected in low-temperature (2-50 degrees C) waters near hydrothermal vents of the Southern Explorer Ridge, in the northeast Pacific Ocean, contained fine (<500 nm) Fe -and Mn-oxide and Fe-silicate particles coating bacterial surfaces. Partially to totally mineralized bacteria, along with bacterial exopolymers, were covered with a mixture of poorly ordered Si-rich Fe-oxides (possibly ferrihydrite), Mn-oxides, and Fe-silicates (possibly nontronite). Minerals occur as very fine (2-20 nm) granular material, fine (20-100 nm) needles and sheets, small (200-500 nm) nodules and filaments (i.e., mineralized exopolymers). Under saturation conditions, we infer that bacterial surfaces provided nucleation sites for poorly ordered oxides and silicates. The formation of Fe- and Mn-oxides was likely initiated by the direct binding of soluble Fe and Mn species to reactive sites (like carboxyl, phosphate, and hydroxyl groups) present within the bacterial cell wall and the exopolymers. Fe-silicate formation involved a more complex binding mechanism, whereas metal ions, such as Fe, possibly bridged reactive sites within the cell walls to silicate anions to initiate silicate nucleation.

Population genetics and biogeography of vestimentiferan tube worms

Molecular markers were used to assess the geographical ranges and estimate rates of gene flow in several species of vestimentiferan tube worm from the eastern Pacific Ocean. Small specimens (<5 cm) of Tevnia jerichonana and Oasisia alvinae are difficult to distinguish morphologically; however, species-specific allozymes allowed unequivocal identification of specimens from sites along the East Pacific Rise (EPR). Both T. jerichonana and O. alvinae were found at 9°, 11° and 13°N latitude; only O. alvinae occurred at 21°N; and neither occurred along the Galapagos Rift. Estimated rates of gene flow were sufficient (i.e. Nm>1.0) to counteract significant genetic drift in each species, and dispersal appeared to occur in a predominantly stepping-stone fashion. In addition, DNA sequence information from a 637 bp region of the mitochondrial COI gene was used to examine genetic diversity and geographical continuity in new Ridgeia piscesae specimens from the northeastern Pacific. The range of this species is now known to include sites ranging between 41°N on the Gorda Ridge and 49°N on the Southern Explorer Ridge. Sequence divergence throughout this range did not exceed 0.8%, which is consistent with intraspecific levels of sequence variation in other vent and seep species. Finally, the global distribution of vestimentiferan tube worms is reviewed.

Comparative mineralogy and geochemistry of gold-bearing sulfide deposits on the mid-ocean ridges

A comparative study of the mineralogy and geochemistry of sulfide deposits on mid-ocean ridges in the Northeast Pacific and the Mid-Atlantic reveals common characteristics associated with primary gold enrichment. Average gold contents of 0.8 to 5 ppm Au occur in sulfides from Southern Explorer Ridge and Axial Seamount (Northeast Pacific) and from the TAG hydrothermal field and Snakepit vent field (Mid-Atlantic Ridge). The enrichment of gold in these deposits is consistently related to a phase of late-stage, low-temperature (< 300°C) venting. Concentrations > 1 ppm Au occur exclusively in pyritic assemblages and commonly with abundant Fe-poor sphalerite and a suite of complex Pb—Sb—As sulfosalts. Amorphous silica and, locally, barite or carbonate are important constituents of the gold-rich precipitates but do not contain gold themselves. High-temperature (350°C) black smoker assemblages, consisting dominantly of pyrite, chalcopyrite, pyrrhotite, isocubanite and abundant anhydrite are uniformly gold-poor (≤0.2 ppm Au). To the extent that individual sulfides can be mechanically separated, chemical analyses by neutron activation indicate that gold is most abundant in sphalerite (up to 5.7 ppm Au) but also occurs in pyrite and marcasite. Samples of sphalerite with abundant inclusions of fine-grained sulfosalts locally contain up to 18 ppm Au, suggesting that sulfosalts may be repositories for gold. No free gold has been observed at 4000 × magnification of polished specimens, indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides. Samples from gold-rich deposits in the Northeast Pacific and Mid-Atlantic are compared with similar but relatively gold-poor sulfides from the Galapagos Rift and 13°N on the East Pacific Rise (EPR), and with barren sulfides from 11°N EPR, 21°N EPR, the Endeavour Ridge, and the Southern Juan de Fuca Ridge. Trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of Ag, As, Sb, Pb and Zn, and locally with high Cd, Hg, Tl, and Ga. In contrast, gold is typically depleted in samples with high Co, Se, and Mo. The close association of Au with Ag, As, Sb, and Pb may reflect the common behavior of these metals as aqueous sulfur complexes (e.g., [Au(HS) − 2]) at low temperatures. Similar mineralogical and geochemical associations are observed in sulfide deposits from modern back-arc settings and in the ancient geologic record.

Rare earth element geochemistry of massive sulfides-sulfates and gossans on the Southern Explorer Ridge

Massive sulfide-sulfate deposits on the Southern Explorer Ridge were analyzed for 14 rare earth elements (REE) by a modified inductively coupled plasma-mass spectrometric technique that included a correction for high Ba content. Bulk samples of finely intermixed sulfides, sulfate, and amorphous silica contain ΣREE concentrations of ≤6 ppm. REE patterns range from (1) strongly enriched in light REE with positive Eu anomalies, to (2) relatively flat with positive Eu anomalies and slightly negative Ce anomalies, to (3) slightly enriched in light REE with moderately negative Ce anomalies. Pattern 1 is similar to that of 300-350 °C solutions discharging at vents on the East Pacific Rise and the Mid-Atlantic Ridge, whereas pattern 3 resembles REE distributions in normal oceanic bottom waters. The sulfide-sulfate patterns are interpreted to result from variable mixtures of hydrothermal and normal seawater. Barite in gossans capping the mounds has an REE pattern almost identical to patterns of high-temperature vent solutions. Hydrothermal barite has lower REE contents and a different REE pattern relative to hydrogenous barite formed slowly on the sea floor.

Petrologic and geologic variations along the Southern Explorer Ridge, northeast Pacific Ocean

Mid‐ocean ridge basalts (MORB) from the well‐defined Southern Explorer Ridge segment (SER) in the northeast Pacific Ocean are moderately to strongly enriched in incompatible elements. Enriched MORB were erupted at the highest and widest part of the segment (culmination) where the magma supply may be the greatest and also at the northern and southern ends of the SER. Variations in basalts' incompatible element enrichment occur over short distances and suggest that the underlying mantle is heterogeneous on a small scale. The variations also preclude the existence of a long‐lived, well‐mixed magma chamber beneath the robust culmination. Instead, magma chambers are probably temporally and spatially isolated, as evidenced by the presence of highly differentiated lavas. Less enriched (transitional) MORB were erupted along the central part of the SER, 11–27 km south of the culmination, and were supplied to the ridge separately from a less enriched pan of the mantle. A more continuous magma chamber cannot be ruled out for this section on geochemical grounds. All of the MORB from the SER have undergone significant amounts of fractional crystallization. Unlike certain segments of the East Pacific Rise, the most evolved MORB were erupted near the culmination of the ridge segment. Aphyric lavas have been recovered only from the vicinity of the culmination, while phyric lavas have erupted along the entire length of the SER. There is no correlation between composition and total crystal abundances. Despite the requirement of multiple parental magmas, most of the lavas from the culmination and the distal ends of the ridge fall on a common liquid line of descent, indicating that they formed at similar depths and extents of partial melting. The transitional MORB from the central part of the ridge have probably formed by lesser extents of partial melting and separated from their mantle source at greater depths based on abundances of Na and moderately incompatible elements such as Sm and Zr. Enriched, high‐melt fraction lavas at the magma‐starved southern end of the SER closely resemble lavas from the culmination. Either the southernmost lavas have flowed laterally within the crust from near the culmination, or there is another melting anomaly that supplies the southern end of the SER. If the latter case is true, correspondence between ridge morphology and mantle thermal state is poor.

Relationships between geologic development of ridge crests and sulfide deposits in the Northeast Pacific Ocean

A tectonic model developed for the northeast Pacific spreading centers explains many of the diverse morphological and structural attributes of medium-spreading ridges and may provide a basis for explaining the distribution and composition of their associated base metal massive sulfide deposits. Stage 1 of the tectonic model is a period of excessive extrusive axial volcanism that builds the axial crestal ridge; an elongate summit depression (axial valley) is narrow or absent during this stage and inflated pillow lavas are dominant (e.g., southern Explorer Ridge). Sulfide deposition during stage 1 occurs immediately adjacent to the principal bounding faults of the elongate summit depression. The deposits are large in comparison with those of the East Pacific Rise and are composed of pyritic mounds containing copper and zinc. During stage 2, volcanism is rare to absent and the elongate summit depression develops and widens by collapse of the summit of the crestal ridge, as a result of tectonic stretching (e.g., southern Endeavour Ridge). Sulfide deposits forming during stage 2 are also the bounding faults and the deposits are similar in size and composition of those of stage 1. Renewed volcanism along axial fissures occurs during stage 3; sheet and lobate flow forms dominate within the elongate summit depression (e.g., southern Juan de Fuca Ridge). Sulfide deposition during stage 3 is in axial fissures in the floor of the summit depression. Deposits are very small relative to those in stages 1 and 2 and are very zinc rich, and copper and iron poor. Deposits in both stages 1 and 2 more closely resemble deposits preserved in ophiolitic assemblages than do those in stage 3.Axial Seamount, which straddles Juan de Fuca Ridge and is the western termination of the Cobb-Eickelberg seamount chain, has small zinc-rich deposits within its caldera. The caldera floor is composed of disrupted and fissured sheet lava flows and the deposits are in fissures and along fractures near the caldera wall. They are similar in composition and setting to those in stage 3 areas. Deposits in heavily sedimented failed rifts (e.g., Middle Valley of Endeavour Ridge) are the largest (tens of millions of metric tons) and are formed of pyrrhotitic, zinc-rich mounds. The heat source for the latter deposits is similar to that for their volcanic-hosted counterparts, but heat is more effectively conserved in the high-temperature reaction zone because the impermeable sediments inhibit cooling of the heat source by inflow of cold seawater.The largest volcanic-hosted deposits thus occur in those segments that formed closest in time to a period of major constructional volcanism. Upwelling hydrothermal fluid is focused along the principal marginal faults of the elongate summit depression and rises from the reservoir without significant cooling or mixing as a result of the relative impermeability of the cap rocks (i.e., inflated pillow lava flows, stages 1 and 2); the major volcanic edifices in the latter areas also provide some insulation from the overlying seawater, thus inhibiting cooling and promoting the growth of a relatively large high-temperature reservoir. In the stage 3 areas, and at Axial Seamount, the exceptionally high permeability in the sheet lava flows promotes the cooling and mixing of hydrothermal fluid immediately prior to venting. This, and the evident shallow position of the hydrothermal reaction zone (at the southern Juan de Fuca site), inhibits the formation of a large, stable reservoir. Also, some sulfide precipitation probably occurs immediately beneath the vent area, further contributing to the size of the deposits. Deposits are much larger in heavily sedimented rifts than in volcanic-associated areas of ridge crest that have relatively little sediment cover because of the broad insulating and protective nature of the sediments.

Tectonic evolution of the Explorer‐Northern Juan de Fuca Region from 8 Ma to the present

New magnetic anomaly data, in conjunction with Sea Beam and SeaMARC II data, has been used to schematically model the tectonic development of the Explorer‐Northern Juan de Fuca Region with a series of five ridge‐offset propagators during the last 8 m.y. The Sovanco Fracture Zone played a major role in the evolution of this area since its origin as a spreading center offset between the Explorer and Juan de Fuca Ridges approximately 7.4 Ma. Between this time and 2.5 Ma, the transform migrated southwards due to southward propagation of Explorer Ridge, and lengthened by the addition of three propagating ridge offsets as well as by asymmetric spreading to the east on Explorer Ridge. Since the independence of Explorer Plate at 4 Ma, there has been clockwise rotation of the Explorer Spreading Center relative to the Juan de Fuca Ridge. In response to forces acting at the convergent margin, the Sovanco Fracture Zone also underwent clockwise reorientation, and in the process developed into a broad zone of shear. The tectonic deformation associated with this rotation distorted the previously existing N‐S magnetic anomaly pattern. Explorer Seamount, a major geological feature in the region, is modeled as a presently extinct southern spreading segment of the Explorer Ridge system which was isolated by the northward migration of the west end of the Sovanco Fracture Zone. The recent (&lt;1 Ma) tectonic evolution of Explorer Ridge involved both northward and southward propagation of Southern Explorer Ridge, as well as an eastward ridge jump during the Brunhes. Ih the northern portion of the study area, the axis of spreading jumped 40 km to the NW around 0.3 Ma, from the eastern Explorer Deep to the western Explorer Rift.

Gold in sea-floor polymetallic sulfide deposits

Bulk chemical analyses of 48 samples of hydrothermal precipitates from seven polymstallic sulfide deposits at sea-floor spreading centers in the eastern Pacific reveal distinct patterns of gold enrichment. High gold contents are found in samples from two sulfide deposits: the Axial Seamount (45057 ' N, 130002 ' W), and the southern Explorer Ridge (49045.6 ' N, 130016.2 ' W). A large, 160-kg sample composed of silica, barite, and sphalerite from the top of the Axial Seamount deposit gives analyses up to 6,700 ppb Au, averaging 4,900 ppb Au. Similar material from the southern Explorer Ridge gives analyses up to 1,500 ppb Au, averaging 660 ppb Au. Detailed mineralogical studies together with bulk chemical analyses of a wide range of samples from these two sites reveal strong elemental associations and paragenetic controls on gold deposition. Gold at concentrations of about 200 ppb is associated with high Cu (>1 wt %) and Mo (up to 470 ppm); at concentrations > 800 ppb, with high Zn (>10 wt %), Ba (>3 wt %), and SiO (>20 wt %); and at concentrations > 1,200 ppb, with high Pb (>0.1 wt %), Ag (>100 ppm), As (>300 ppm), and Sb (50-100 ppm). Samples with the highest gold values from both locations contain late, sinterlike, low-temperature sulfosalts of Pb, As, Sb, Ag, and S in a matrix of amorphous silica. These sulfosalts are the probable repositories of the gold. Published analyses of Au-poor ( 300oC), Cu-Fe-rich sulfides and subsequently remobilized by late, sustained, low-temperature (<250oC) fluids and concentrated in SiO-Ba-Zn-rich precipitates near the surface. Suitable fluid chemistry is required to mobilize the gold and a favorable precipitation mechanism is needed to concentrate it. These observations could explain the occurrence of high gold concentrations in selected ore types of some ancient massive sulfide deposits on land.