A New View of Ridge Segmentation and Near-Axis Volcanism at the East Pacific Rise, 8˚–12˚N, from EM300 Multibeam Bathymetry

New, high-resolution bathymetry for the East Pacific Rise between 8˚N and 12˚N was collected over a6 km wide swath centered on the ridge axis using the 30 kHz Simrad EM300 multibeam system. Thecoverage area corresponds latitudinally to the designated Ridge2000 Integrated Studies Site (ISS) for fastspreading ridges. The EM300 data, gridded at 30 m latitude by 50 m longitude, represent a greater than 4Ximprovement in horizontal resolution over previously available multibeam data and a 2X improvement indepth resolution. The new bathymetry was used to update the locations and hierarchy of ridge offsets forthis area. Among the many applications for this data, it enables us to tabulate volcanoes half the size thatcould be previously detected. The distribution of near-axis volcanic cones \u3e25 m high suggests that thispopulation of small, near-axis cones results from low effusion rate eruptions of the ridge axis

Basaltic Lava Domes, Lava Lakes, and Volcanic Segmentation on the Southern East Pacific Rise

Meter-scale DSL-120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third-order structural segmentation of the ridge axis on the southern East Pacific Rise, 17o11\u27-18o37\u27S. Near segment ends we observe abundant basaltic lava domes which average 20 rn in height and 200 rn in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third-order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third-order segment boundaries also correlate with local maxima in the seismicaxial magmac hamber reflector depth throughout the study area and decreased across-axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross-axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rocks ampling density is not adequate to constrain the variation across all the third-order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (\u3e 140 km/Myr full rate) mid-ocean ridges

Crustal Fissuring on the Crest of the Southern East Pacific Rise at 17˚15\u27-40\u27S

Fissure densities and widths have been mapped along the axial zone of the superfast spreading southern East Pacific Rise (EPR) at 17_150–400S with the near-bottom DSL-120 and Argo II imaging systems. We observe that the youngest lava flows (on a relative age scale) are sparsely fissured and that there is a cumulative increase in fissure abundance with time that produces a strong positive correlation between fissure density and relative age of lava flows. Average fissure widths were used to estimate fissure depths. In the 17_150–400S area, calculated fissure depths are estimated to extend below the seismic layer 2A/2B boundary, and fissures are widest/deepest where lava flows are youngest. We interpret these wide fissures in relatively young flows to be eruptive fissures. Relatively young lava flows combined with high average fissure widths south of 17_250S suggest that there may have been recent dike propagation along the ridge crest in this area. In comparison to the northern EPR at 9_–10_N the density of fissuring on the southern EPR is significantly higher, due in part to the higher occurrence of relatively older, more areally restricted pillow lava flows

Correlation Between Volcanic and Tectonic Segmentation of Fast-Spreading Ridges: Evidence from Volcanic Structures and Lava Flow Morphology on the East Pacific Rise at 9˚-10˚N

Combined analyses of volcanic features in DSL-120 sonar data and Argo I images along the ridge crest of the East Pacific Rise, 9_090–540N reveal a consistent decrease in inferred lava effusion rate toward the ends of third-order segments. The correlation of tectonic segmentation and volcanic style suggests that third-order segmentation corresponds to the volcanic segmentation of the ridge. Along-axis changes in volcanic structures (from collapse troughs to basaltic lava domes) and lava morphology (from sheet to pillow flows) coincide with the boundaries of morphologically defined third order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry. Pillow lava flows cover 25% of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third-order segment ends. An additional 25% of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough. The remaining terrain consists of lobate lava flows. We interpret the spatial correlations of morphologic, structural, seismic, and petrologic data as evidence that individual volcanic plumbing systems are organized at _20 km spacing along the ridge axis (third-order segment scale) in agreement with the hypothesis that volcanic and tectonic segmentations are correlated. For fast spreading ridges, we estimate that the longevity of volcanic segments is _104–105 years, 1–3 orders of magnitude longer than fourth-order segments (_102–103 years). This implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable

Crustal fissuring and its relationship to magmatic and hydrothermal processes on the East Pacific Rise crest (9°12′ to 54′N)

The deep-towed Argo I optical/acoustical vehicle and a geographic information system (GIS) have been used to establish the abundance, widths, and spatial distribution of fissures, as well as the relative age distribution of lavas along the narrow (<500 m wide) axial zone of the East Pacific Rise (EPR) from 9°12′ to 9°54′N. On a second-order scale (~78 km long), wider but less numerous fissures are found in the northern portion of the survey area; this changes to narrower, more abundant fissures in the south. A profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike (near 9°49′, 9°35′, and 9°15′N), where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle, filling and covering older fissures. On a fourth-order scale (5–15 km long) the mean density of fissuring on a given segment is greater where relative axial lava age is greater. Fissure density also correlates with hydrothermal vent abundance and type. Increased cracking toward segment tips is observed at the second-order scale, whereas fourth-order segments tend to be more cracked in the middle. Cracking on a fourth-order scale may be driven by the propagation of dikes, rather than by the far-field plate stresses. The above relations constrain the model of Haymon et al. [1991] in which individual fourth-order segments are in different phases of a volcanic-hydrothermal-tectonic cycle

High-Resolution Surveys Along the Hot Spot–Affected Galapagos Spreading Center: 1. Distribution of Hydrothermal Activity

The spatial density of hydrothermal activity along most mid-ocean ridges is a robust linear function of spreading rate (or magmatic budget), but extreme crustal properties may alter this relationship. In 2005–2006 we tested the effect of thickened crust on hydrothermal activity using high-resolution mapping of plumes overlying the hot spot–affected Galapagos Spreading Center from 95o to 89o42\u27W (~560 km of ridge crest). Plume mapping discovered only two active, high-temperature vent fields, subsequently confirmed by camera tows, though strong plume evidence indicated minor venting from at least six other locations. Total plume incidence (ph), the fraction of ridge crest overlain by significant plumes, was 0.11 ± 0.014, about half that expected for a non–hot spot mid-ocean ridge with a similar magmatic budget. Plume distributions on the Galapagos Spreading Center were uncorrelated with abrupt variations in the depth of the along-axis melt lens, so these variations are apparently not controlled by hydrothermal cooling differences. We also found no statistical difference (for a significance level of 0.05) in plume incidence between where the seismically imaged melt lens is shallow (2 ± 0.56 km, ph = 0.108 ± 0.045) and where it is deep (3.4 ± 0.7 km, ph = 0.121 ± 0.015). The Galapagos Spreading Center thus joins mid-ocean ridges near the Iceland (Reykjanes Ridge), St. Paul-Amsterdam (South East Indian Ridge), and Ascension (Mid- Atlantic Ridge) hot spots as locations of anomalously scarce high-temperature venting. This scarcity implies that convective cooling along hot spot–affected ridge sections occurs primarily by undetected diffuse flow or is permanently or episodically reduced compared to normal mid-ocean ridges

Crustal fissuring on the crest of the southern East Pacific Rise at 17°15′–40′S

Fissure densities and widths have been mapped along the axial zone of the superfast spreading southern East Pacific Rise (EPR) at 17°15′–40′S with the near-bottom DSL-120 and Argo II imaging systems. We observe that the youngest lava flows (on a relative age scale) are sparsely fissured and that there is a cumulative increase in fissure abundance with time that produces a strong positive correlation between fissure density and relative age of lava flows. Average fissure widths were used to estimate fissure depths. In the 17°15′–40′S area, calculated fissure depths are estimated to extend below the seismic layer 2A/2B boundary, and fissures are widest/deepest where lava flows are youngest. We interpret these wide fissures in relatively young flows to be eruptive fissures. Relatively young lava flows combined with high average fissure widths south of 17°25′S suggest that there may have been recent dike propagation along the ridge crest in this area. In comparison to the northern EPR at 9°–10°N the density of fissuring on the southern EPR is significantly higher, due in part to the higher occurrence of relatively older, more areally restricted pillow lava flows.Keywords: tectonic cycle, fractures, southern East Pacific Rise, mid-ocean ridges, fissure

Caminite: A new magnesium-hydroxide-sulfate-hydrate mineral found in a submarine hydrothermal deposit, East Pacific Rise, 21 °N

As laboratory experiments predict, a Mg-hydroxide-sulfate-hydrate mineral, here named caminite, precipitates in nature from seawater heated in an active submarine hydrothermal system. Caminite is found intergrown with anhydrite in the wall of a black-smoker chimney precipitated around hydrothermal fluids discharging on the East Pacific Rise axis at 2l'N latitude. Caminite is tetragonal (space group I4,/amd) with a = 5.239Å and c = 12.988Å. The five strongest lines appearing in X-ray powder difraction patterns (CuKa radiation) are 3.345 (I/Io = 100; hkl = 103); 3.220 (80; 112); 1.871 (50; I l6); 1.620 (25; 303); 1.609 (20; 224). Bond-strength calculations and experimental results predict that the caminite structure accommodates a range of compositions described by a general formula: MgSO4 . xMg(OH)2 . (1- 2x)H2O, where 0 < x < 0.5. The caminite in our sample has a composition corresponding to a stoichiometry of MgSO4 . 0.4Mg(OH)2 . 0.2H2O. It is soft (H = 2.5) and apparently colorless. Caminite is uniaxial negative and has low birefringence (0.002). Its indices of refraction are omega = 1.534 and epsilon = 1.532. In the recharge zones of submarine hydrothermal systems, large volumes of convecting seawater heated above approximately 240°C rnay precipitate abundant caminite and anhydrite. Formation of abundant caminite can drastically lower the pH of downwelling seawater in such systems, and rapid removal of sulfate into caminite and anhydrite may prevent the reduction of much seawater sulfate to sulfide within the hydrothermal system. Incorporation of seawater sulfate into caminite and anhydrite at elevated temperatures and subsequent recycling of this sulfate into the oceans by dissolution at low temperatures should affect the oxygen-isotope compositionof seawater sulfate and may play a part in maintaining the oxygen-isotope values of oceanic sulfate in disequilibrium with δO18 of seawater