No abstract

we conducted detailed mapping and sampling of hydrothermal plumes along six segments of Earth's fasting spreading mid-ocean ridge, 27.5°-32.3°S on the East Pacific Rise. We compared the distribution and chemistry of hydrothermal plumes to geological indicators of long-term (spreading rate) and moderate-term (ridge inflation) variations in magmatic budget. In this large-offset, propagating rift setting, these geological indices span virtually the entire range found along fast spreading ridges worldwide. Hydrothermal plumes overlaid $60% of the length of superfast (>130 km/Myr) spreading axis surveyed and defined at least 14 separate vent fields. We observed no plumes over the slower spreading propagating segments. Finer-scale variations in the magmatic budget also correlated with hydrothermal activity, as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation. Along the entire ridge crest, the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume. Plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years. Plume samples with high volatile/metal ratios, generally indicating recent seafloor volcanism, were scarce. Along-axis trends in both volatile ( 3 He; CH 4 ; ÁpH, a proxy for CO 2 ; and particulate S) and nonvolatile (Fe, Mn) species showed a first-order agreement with the trend of ridge inflation. Nevertheless, a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments.

[1] Samples of metamorphic rock were collected from drilled holes on ODP Leg 195 and in piston/gravity cores collected with the Jason 2 remotely operated vehicle from S. Chamorro Seamount, a serpentinite mud volcano on the Mariana forearc. The recovered muds are approximately 90% serpentinite, including grit-to boulder-sized clasts of serpentinized peridotites, but also contain a wide variety of small fragments of metabasic rocks. These metabasic fragments include high-pressure, low-temperature rocks derived from the subduction zone. Other serpentinite seamounts also have yielded metabasic rock fragments as small clasts in the serpentinite mudflows, but none have as wide a variety of rock types as S. Chamorro Seamount. The sources of the rock clasts, both serpentinized peridotites and metabasic schists, vary with the eruptive episodes of the mud volcanoes. Swath mapping of S. Chamorro Seamount shows that a sector collapse of its southeastern flank has resulted in debris flows from the summit region of the seamount that have traveled more than 70 km eastward toward the trench. These debris flows, however, have a very different morphology from mudflows observed at the summit. High-resolution seafloor mapping of the summit shows both thin, presumably highly fluid-(or gas-) charged serpentinite mudflows and a relatively viscous protrusion that has formed the main summit knoll. By comparison, the summit of Conical Seamount drilled on ODP Leg 125 lacks a distinct summit knoll and has numerous, relatively thin and widespread mudflows covering the flanks of the edifice. The style of eruption at a given seamount probably varies with time and with the amount of fluid or gas incorporated in a given pulse of mud. The greater diversity of metabasic rocks at S. Chamorro Seamount may be a consequence of recycling of forearc materials through tectonic erosion and subduction in the southern part of the forearc. Fryer, P., J. Gharib, K. Ross, I. Savov, and M. J. Mottl (2006), Variability in serpentinite mudflow mechanisms and sources: ODP drilling results on Mariana forearc seamounts, Geochem. Geophys. Geosyst., 7, Q08014,

[1] We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading ($150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

[1] Samples were collected from hydrothermal plumes along the East Pacific Rise (EPR) from 28°to 32°S during the Ridge Axis Plume and Neotectonic Unified Investigation (RAPANUI) cruise (5 March to 12 April 1998). Forty-five vertical casts and tow-yos were conducted: 3 off axis and 42 over the axes of two overlapping propagating ridges, the West and East ridges. These ridges are composed of several nontransform offset ridge segments. Spreading rates range from 149 mm/yr in the segment interiors to 0 mm/yr at the propagating rifts. These maximum spreading rates are considered the fastest on the mid-ocean ridge system and affect the structure of the ridge. Segment-averaged methane plume maxima ranged from 1.7 to 7.2 nM. Mean methane concentrations on the West Ridge were nearly double those of the East Ridge. Westwardly advecting hydrothermal methane persisted to our most distal station, nearly 480 km west of the East Pacific Rise. Background concentrations were less than 1 nM. The highest methane concentration measured was 50 nM in a buoyant plume. Methane did not covary with manganese or any other hydrothermal tracer in plumes over portions of a segment that exhibited recent magmatic activity, possibly as a result of the hydrothermal system's recovery from a phase separation event. In contrast, methane/manganese ratios on the other segments ranged from 0.077 to 0.091. Methane d 13 C values in plume maxima ranged from À27 to À33% versus Peedee belemnite; background values were around À40%. These data are compared with hydrothermal plume methane data from slower spreading ridges and illustrate similarities in hydrothermal processes and sources between these systems.