J. A. C. Resing

Hydrothermal exploration of the Fonualei Rift and Spreading Center and the Northeast Lau Spreading Center

We report evidence for active hydrothermal venting along two back-arc spreading centers of the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC) and the Northeast Lau Spreading Center (NELSC). The ridge segments investigated here are of particular interest as the potential source of a mid-water hydrothermal plume (1500–2000 m depth) which extends more than 2000 km across the SW Pacific Ocean dispersing away from an apparent origin close to the most northeastern limits of the Lau Basin. Our results indicate the presence of at least four new hydrothermal plume sources, three along the FRSC and one on the NELSC, the latter situated within 150 km of the maximum for the previously identified SW Pacific regional-scale plume. However, TDFe and TDMn concentrations in the southernmost FRSC plume that we have identified only reach values of 19 and 13 nmol/L and dissolved 3He anomalies in the same plume are also small, both in relation to the SW Pacific plume and to local background, which shows evidence for extensive 3He enrichment throughout the entire Lau Basin water column. Our results reveal no evidence for a single major point hydrothermal source anywhere in the NE Lau Basin. Instead, we conclude that the regional-scale SW Pacific hydrothermal plume most probably results from the cumulative hydrothermal output of the entire topographically restricted Lau Basin, discharging via its NE-most corner.

Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth's fastest seafloor spreading system, EPR 27°–32°S

[1]xa0We 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.

Methane dynamics in hydrothermal plumes over a superfast spreading center: East Pacific Rise, 27.5°–32.3°S

[1]xa0Samples 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 systems recovery from a phase separation event. In contrast, methane/manganese ratios on the other segments ranged from 0.077 to 0.091. Methane δ13C values in plume maxima ranged from −27 to −33‰ versus Peedee belemnite; background values were around −40‰. These data are compared with hydrothermal plume methane data from slower spreading ridges and illustrate similarities in hydrothermal processes and sources between these systems.