Ken C. Macdonald

East pacific rise: hot springs and geophysical experiments.

Hydrothermal vents jetting out water at 380� � 30�C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Gal�pagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

Hydrothermal vent distribution along the East Pacific Rise crest (9°09′–54′N) and its relationship to magmatic and tectonic processes on fast-spreading mid-ocean ridges

Using the near-bottom ARGO imaging system, we visually and acoustically surveyed the narrow ( < 200 m wide) axial zone of the fast-spreading East Pacific Rise (EPR) along 83 km of its length (9°09′–54′N), discovered the Venture Hydrothermal Fields, and systematically mapped the distribution of hundreds of hydrothermal features relative to other fine-scale volcanic and tectonic features of the ridge crest. The survey encompasses most of a 2nd order ridge segment and includes at least ten 4th order (5–15 km) segments defined by bends or small lateral offsets of the ridge crest or axis (Devals). 4th order segmentation of the ridge crest is clearly expressed in the high-resolution ARGO data by the fine-scale behavior of the ridge axis and by changes in the characteristics of the axial zone (axial lava age, extent of fissuring, axial morphology and structure, etc.) across segment boundaries. The distribution and along-strike variability of hydrothermal features corresponds closely to the morphotectonic/structural segmentation of the ridge. On the 2nd order scale, we find that high T hydrothermal activity correlates with: (1) shallowing of the axial magma chamber (AMC) reflector to depths < 1.7 km beneath the ridge axis; and, (2) with the presence of a well-developed axial summit caldera (ASC). Previous work refers to this feature as an axial summit graben (ASG); however, the extent of volcanic collapse along the ASG revealed by the ARGO survey adds to evidence that on fast-spreading ridges it is an elongate volcanic caldera rather than a tectonic graben, and supports the introduction of “axial summit caldera” as a more accurate descriptor. All but 1 of the 45 active high T vent features identified with ARGO are located within 20 m of the margins of the ASC. Despite the significant extent of our track coverage outside the ASC, no important signs of venting were seen beyond the axial zone. On the 4th order scale, the abundance and distribution of hydrothermal features changes across 4th order segment boundaries. We find that high T vents are most abundant where: (1) the ASC is very narrow (40–70 m), (2) the AMC reflector is most shallow ( < 1.55 km beneath the axial zone), and (3) the axial lavas are youngest and least fissured. To explain the observed distribution of vent activity, a two-layer model of ridge crest hydrothermal flow is proposed in which 3-D circulation at lower T in the volcanic section is superimposed on top of axis-parallel high T circulation through the sheeted dike complex. In the model, circulation parallel to the ridge axis is segmented at the 4th order scale by variations in thermal structure and crustal permeability which are directly associated with the spacing of recent dike intrusions along strike and with cracking down into the sheeted dikes, especially along the margins of the ASC. Based on ratios between numbers of active high T vents and inactive sulfide deposits along particular 4th order segments, and on corresponding volcanic and tectonic characteristics of these segments, we suggest that the individual 4th order segments are in different phases of a volcanic-hydrothermal-tectonic cycle that begins with fissure eruptions, soon followed by magma drainback/drainage and accompanying gravitational collapse, possible development of an ASC, and onset of hydrothermal activity. The hydrothermal activity may wax and continue for up to several hundred years where an ASC is present. The latest phase in the cycle is extensive tectonic fissuring, widening of the ASC by mass wasting along its margins, and waning of hydrothermal activity. In the ARGO area, where full spreading rates are 11 cm/yr, the entire cycle takes less than ∼ 1000 years, and the tectonic phase does not develop where the time interval between eruptions is significantly less than 1000 years.

Mid-ocean ridges: discontinuities, segments and giant cracks.

Geological observations reveal that mid-ocean ridges are segmented by numerous rigid and nonrigid discontinuities. A hierarchy of segmentation, ranging from large, long-lived segments to others that are small, migratory, and transient, determines the pattern and timing of creation of new ocean floor. To the extent that spreading segments behave like giant cracks in a plate, the crack propagation force at segment tips increases with segment length, which may explain why long segments tend to lengthen and prevail over shorter neighboring segments. Partial melting caused by decompression of the upper mantle due to plate separation and changes in the direction of spreading result in the spawning of new short segments so that a balance of long and short segments is maintained.

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

Magmatic processes at superfast spreading mid‐ocean ridges: Glass compositional variations along the East Pacific Rise 13°–23°S

Major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading (150 mm/yr) East Pacific Rise (EPR), 13°–23°S, and near-ridge seamounts comprise 212 chemical groups. We interpret these groups to represent the average composition of individual lava flows or groups of closely related flows. Groups slightly enriched in K2O (T-MORB) are distributed variably along the axis, in contrast to the Galapagos Spreading Center where T-MORB are extremely rare. This result is consistent with the interpretation that T-MORB magmas arise from low-melting temperature, K-rich heterogeneities in the subaxial EPR mantle. The Galapagos Spreading Center, which is migrating to the west in an absolute reference frame, is underlain by mantle previously processed and depleted in the T-MORB component during melting events giving rise to earlier EPR magmas. Excluding T-MORB, there are nearly monotonic, twofold increases in K/Ti and K/P of axial lavas from 23°S to 13°S. From 22°S to 17°S these gradients correlate with isotopic ratios, but north of 17°S there is a reversal of isotopic gradients, indicating (recent?) decoupling of the isotopic and minor element ratios in the subaxial mantle. A strong, southward increase in degree of differentiation for approximately 200 km north of the large offset at 20.7°S correlates with a gradient in bathymetry, consistent with previous interpretations that this offset is propagating to the south. Samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive, evolved fractionation signatures of rift propagation, suggesting that propagating rift tips have been abandoned preferentially to failing rift tips. Glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset, and possibly also south of 22°S; the latter region may be affected by deformation accompanying northward growth of the Easter Microplate. Near-ridge seamounts on the Pacific Plate between 18°–19°S comprise two distinct populations: those aligned approximately parallel to the spreading direction are extremely variable in major element composition, but consistently enriched in Sr relative to nearby axial lavas; smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and Sr relative to axial lavas. The degree of differentiation of axial lavas between 18°–19°S can be related to the structural development of the rift axis and/or vigor of hydrothermal activity of individual segments. Glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area. Primary magmatic segmentation mainly reflects mantle source variations, the boundaries of which correlate with the largest physical offsets in the rise axis between the Easter Microplate and Garrett Transform Zone. A secondary magmatic segmentation, defined by the along-axis continuity of similar parental magma compositions or liquid lines of descent, occurs with a length scale varying from 11 to 185 km, with an average of 69 ±57 (1σ) km. The boundaries of these segments mainly occur at overlapping spreading centers. All first-, second- and third-order physical offsets correspond to secondary magmatic segment boundaries, but some secondary magmatic segment boundaries also occur at small, fourth-order ridge axis discontinuities. The secondary magmatic segments define the length scale of mantle melting variations, mainly variations in extent of melting, but not the scale of melt extraction processes that feed the axis. This scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth-order physical discontinuities along axis. There is a good positive correlation of average secondary magmatic segment length with spreading rate for four well-sampled areas varying from 20 to 150 mm/yr. Secondary magmatic segments also become more variable in axial length with increasing spreading rate. The average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates. Superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature, defined by extent of magmatic differentiation, which bears little systematic relation to physical or other kinds of magmatic segmentation. At 13°–23°S, the length scale of this variation is 217±60 (1σ) km, approximately corresponding to the wavelength of “rolls” in the gravity field observed off-axis. Taken together, the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere, magma supply to the axis, and crustal magmatic temperature reflect independent, regionally decoupled processes.

Hydrothermal heat flux of the “black smoker” vents on the East Pacific Rise

Abstract Active hydrothermal vents have been discovered on the East Pacific Rise at 21°N [1]. The most spectacular of the vents jet out 350°C water at flow rates of several meters per second. The heat loss associated with a single vent of this type is three to six times the total theoretical heat loss for a 1-km segment of ridge out to 1 m.y. age. This underscores the importance of hydrothermal circulation in the heat budget of mid-ocean ridges. It also requires that vent activity of this type be highly episodic rather than steady state.

East Pacific Rise 8°–10°30′N: Evolution of ridge segments and discontinuities from SeaMARC II and three-dimensional magnetic studies

Magnetic, bathymetric, and SeaMARC II side scan sonar data from the ridge flanks adjoining the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR) are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2.4 m.y. A three-dimensional inversion for magnetization shows the discontinuity evolved from one of small offset (2 km) to an 8-km offset OSC within the past approximately 1.0 m.y. The OSC has left a v-shaped discordant zone indicating an average southward migration of 42 km/m.y. since the beginning of anomaly 2 time (1.8 m.y.). The west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations, while the east flank discordant zone is a broad swath of high magnetization, anomalous lineations, and greater depths. Kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm/yr. Ridge tips have been abandoned at intervals of approximately 50,000–100,000 years both by linkage of neighboring ridge segments and by self-decapitation. Magnetic modelling suggests rotations of overlap basins of >25°, consistent with predictions of kinematic modelling. Orientation of magnetic isochrons indicate a 3°–6° counterclockwise change in Pacifie-Cocos plate motion within the past ∼1 m.y., assuming orthogonal spreading throughout this time. Although the discontinuity currently located at 9°03′N existed prior to this change in plate motion, its growth into a right-stepping, 8-km offset OSC may be causally related to this change. The current configuration of intratransform spreading within the Siqueiros has developed since onset of the Brunhes anomaly (0.73 m.y.) possibly in response to this recent counterclockwise change in plate motion. In addition to the OSC, there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time, which have since migrated along the ridge and vanished. An axial magnetization high is observed along the crest of the EPR. Change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area. Magnetization maxima are found at ridge transform intersections and the OSC. These highs may reflect presence of highly fractionated FeTi basalts erupted from magma chambers with restricted supply located adjacent to both large-and short-offset ridge axis discontinuities. Discrete magnetization highs are found off-axis adjacent to Clipperton and Siqueiros fracture zones, indicating that production of FeTi basalts at ridge-transform intersections is episodic. Magnetizations are higher north of Clipperton than south of it, possibly reflecting a long-lived starvation in magma supply north of Clipperton, and more abundant supply to the south. Most seamounts within the area have the same magnetization polarity as surrounding seafloor, indicating that seamount formation occurs near the ridge axis. The exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis. Few pronounced magnetization highs are associated with seamounts, indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid-ocean ridge basalts.

Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center

Along the fast and ultrafast spreading East Pacific Rise, the cross-sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation. Using an automated method, we measure the ridge area (volume per kilometer along axis) where there is complete bathymetric coverage. Along 3500 km of the northern and southern East Pacific Rise, our two study areas encompass numerous transform faults, large overlapping spreading centers, small overlapping spreading centers, and smaller discontinuities (first-, second-, third-, and fourth-order discontinuities). The cross-sectional area variation mimics the undulation of the ridge crest depth; local area maxima occur toward the middle of segments, and the axial area decreases by 40% or more at first- and second-order discontinuities. Third-order discontinuities are generally marked by smaller disruptions in the ridge area, and fourth-order offsets do not systematically correspond with features of the cross-sectional area profile. The correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross-sectional area represents a longer term average of the ridges magmatic state than axial depth. A correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low-density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone. The cross-sectional area of the axial ridge appears to correlate with the width of the axis-centered low-velocity zone in the crust. These observations, as well as the absence of large, relict axial ridges off-axis, indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis. Portions of the northern East Pacific Rise underlain by a magma chamber reflector generally occur where the ridge cross-sectional area is greatest; this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt. Thus, while the axial high on fast spreading centers resembles a constructional volcano in cross section, it is more like a long, narrow balloon whose cross-sectional area is a sensitive indicator of magma supply. Using this relationship, we predict with 75% confidence that at least 45% of the unsurveyed northern East Pacific Rise (18°N to 13°N and 9°N to 5°N) and at least 60% of the southern East Pacific Rise (4°S to 23°S) is underlain by a magma chamber reflector.

Deep-tow studies of the structure of the Mid-Atlantic Ridge crest near lat 37°N

A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

The axial summit graben and cross-sectional shape of the East Pacific Rise as indicators of axial magma chambers and recent volcanic eruptions

Abstract The axis of the East Pacific Rise (EPR) undulates up and down hundreds of meters over distances of 30–200 km along strike, the deep areas occurring at transform faults and other ridge axis discontinuities such as overlapping spreading centers (OSCs). We have suggested that systematic variations in depth and cross-sectional shape of the rise are indicators of the changes in the local axial magmatic budget along a given ridge segment [1]. A comparison of recently collected multichannel seismic (MCS) data [2] with our Sea Beam and SeaMARC II data have allowed us to test and advance this hypothesis. Along the EPR from 9° to 13°N there is an excellent correlation between three parameters that are all directly related to the phase of a magmatic cycle along a given ridge segment: the cross-sectional shape of the rise, the presence or absence of an axial summit graben, and the presence or absence of a shallow axial magma chamber (as interpreted from MCS data). Where the axial magma chamber is present, the cross-sectional shape of the ridge is broad and an axial summit graben is recognized along the axis. In contrast, where the cross-sectional shape of the rise is narrow and triangular, an axial magma chamber is not detected and an axial summit graben is absent. These ridge axis characteristics tend to occur along deeper portions of a given ridge segment, often near ridge axis discontinuities. We suggest that these systematic variations in ridge axis morphology (cross-sectional shape) and structure (presence or absence of an axial graben) reflect spatial and temporal variations in the magmatic budget of the ridge axis. Where the magmatic budget is waxing, shallow-level magma reservoirs in the crust and underlying upper mantle swell, creating a broad axial bulge with a summit graben. Where the magmatic budget is diminished, the crustal magma chamber is small ( This proposed correlation of shape, structure and magmatic parameters fails along only two short portions of the ridge. In these areas there is evidence for an axial magma chamber and the rise has a broad cross-sectional shape, but there is no summit graben. Bottom photographs and submersible results, however, show that in these areas the rise crest is covered with very fresh lavas undisrupted by faulting, suggesting that the summit graben has been recently filled in by lava flows, and the development of a summit graben (or a linear caldera) by volcano-tectonic collapse has not yet occurred.