U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Abstract. Hydrothermal venting occurs on mid-ocean ridge axes with a
diverse array of morphological, structural, and petrological characteristics.
Consequently, it is not clear if certain geological environments are more conducive than
others to hydrothermal activity. This paper uses complementary data sets recently
collected on multiple-tectonic-segment scales along intermediate to superfast spreading
ridge sections to systematically examine this issue. The spatial density of venting is
gauged by plume incidence, the fraction of linear ridge crest overlain by hydrothermal
plumes. Five geological indexes are compared with plume incidence: ridge-axis elevation,
ridge-axis cross-sectional area, the percentage of ridge axis underlain by an axial magma
chamber (AMC) reflector, MgO wt% in basaltic glass, and spreading rate. A mean value for
plume incidence and each index, where available, is calculated for each of 14 second- to
fourth-order tectonic segments from the Juan de Fuca Ridge (44°30 to 48°45
N) and the northern
(9° to 11°50
N) and southern (13°50
to 18°40
S) East Pacific
Rise. Plumes are most common on segments with a cross-sectional area >3.5 km2,
a net elevation >0.35 km, AMC coverage >60%, and MgO >7 wt%. Spreading rate
is not deterministic on the segment scale. The data suggest two classes of ridge segments.
Segments with relatively low geological index values have a uniformly low plume incidence
indicative of a presently feeble magmatic budget. Those with high index values have a
plume incidence ranging from low to complete coverage, apparently a consequence of a magma
supply rate sufficient to produce frequent dike intrusions and highly variable
hydrothermal activity. The best individual predictors of hydrothermal activity are
cross-sectional area and net elevation. Applying these indexes to the East Pacific Rise
between 18°N and 33°S suggests that the unexplored segments with the highest probability
of activity are those at 15°30
16°N, 6°9°N,
5°6°S, 27°28°S, and 30°30
33°S.
Hydrothermal venting occurs along submarine spreading centers of virtually every spreading rate, morphological classification, and crustal structure. Soon after its discovery in the late 1970s, investigators began searching for a systematic relation between ridge crest characteristics and the distribution of hydrothermal sources [e.g., Rona, 1978]. Francheteau and Ballard [1983] and Crane [1985] proposed an early paradigm based on the simplifying assumption that a three-dimensional pattern of melt delivery would create tectonic segments with uniform bathymetric profiles. They proposed that on average, hydrothermal venting on any segment would be greatest at the shallowest bathymetric point and decrease toward the deepening segment ends. They noted that complete hydrothermal surveys of many segments would be useful in testing this paradigm.
Several years later, Macdonald and Fox [1988] categorized ridge morphology and axial magma chamber (AMC) occurrence [Detrick et al., 1987] along the East Pacific Rise (EPR) from 9° to 13°N in an effort to develop indicators of spatial and temporal variations in the axial magmatic budget. They concluded that axial areas with a broad cross section, an axial summit graben (or a graben recently buried by fresh lava flows), and a shallow AMC reflector were enjoying a comparatively robust magmatic budget. They were unable to extend this correlation to the distribution of hydrothermal venting because of a paucity of information about the location of discharge sources. This state persisted until recently, largely because most observations of hydrothermal venting were localized and therefore unrepresentative of segment-scale geological characteristics. Moreover, the expected gross inequality between the timescales of hydrothermal fluctuations and ridge crest structural changes encouraged the assumption that the spatial pattern of hydrothermal venting would be only poorly sensitive to ridge crest structural or thermal variations. Simply put, it seemed that almost any section of intermediate to superfast spreading ridge crest could provide sufficient heat to drive hydrothermal venting.
During the last several years, coordinated and quantitative studies of ridge
morphology, ridge structure, basalt petrology, and hydrothermal venting patterns
have furnished new and detailed data sets. Any skepticism that a close correspondence
could exist between geological and hydrothermal patterns was dispelled by comprehensive
and detailed studies within individual segments [e.g., Embley et al.,
1991; Haymon et al., 1991]. These and similar data now enable
an examination of the geological indexes of hydrothermal venting on intermediate
to superfast spreading ridges at the scale of individual tectonic segments.
Scheirer and Macdonald [1993] have quantified ridge crest morphology
with measurements of cross-axial inflation along nearly the entire EPR from
18°N to 23°S. Multichannel seismic (MCS) investigations of the EPR from 9° to
13°N [Detrick et al., 1987] and 14° to 20°42S
[Detrick et al., 1993] have provided unprecedentedly detailed
views of the seismic structure of the ridge crest. Fine-scale petrographic sampling
of recent lavas in these same areas [e.g., Thompson et al., 1985;
Langmuir et al., 1986; Sinton et al., 1991; Batiza
and Niu, 1992; Perfit et al., 1994] has yielded information
on the temperature and composition of magma beneath the ridge crest. Finally,
multisegment patterns of hydrothermal discharge have been deduced from continuous
mapping of temperature, optical, and chemical anomalies in waters above the
Juan de Fuca Ridge (JDFR, 44°30
to 48°30
N)
[Baker and Hammond, 1992], northern EPR (8°40
to 13°N) [Bougault et al., 1990; Baker et al., 1994]
and southern EPR (13°50
to 18°40
S)
[Urabe et al., 1995; Baker and Urabe, 1996].
Using these and other data, I examine here the correlation between hydrothermal venting patterns and key morphological, structural, and petrological parameters on intermediate, fast, and superfast spreading segments of the mid-ocean ridge. The correlations are examined first at a fine scale of continuous along-axis variations, then on a scale of second- to fourth-order [Macdonald et al., 1991] tectonic segments, and finally at a multisegment, regional scale.
The Juan de Fuca Ridge in the northeast Pacific extends from the Blanco transform
fault at 44°30N to the Sovanco transform fault
at 48°45
N (Figure
1a). The full spreading rate is 55 mm yr1 [Elvers et
al., 1973; DeMets et al., 1994]. The ridge consists of
six tectonic segments that vary considerably in morphology and are bounded by
first- or second-order discontinuities. I consider here only Cleft, Vance, Cobb,
and Endeavour; the Axial Volcano, West Valley, and Middle Valley segments are
tectonically complex and largely unexplored for hydrothermal activity. Along-axis
seismic data are scarce on the JDFR. Morton et al. [1987] ran
three MCS lines over Cleft and Vance in 1981, and McDonald et al.
[1994] used seismic refraction to examine the shallow extrusive layer in
the same area. Rohr et al. [1988] ran a single MCS line across
the bathymetric high of Endeavour. Site-specific seismic refraction experiments
have been conducted by McClain and Lewis [1982] and Christeson
et al. [1993] on Cobb, and by Cudrak and Clowes [1993]
on Endeavour. Karsten et al. [1990], Rhodes et al.
[1990], and Smith et al. [1994] reported chemistry of axial
lavas. Baker and Hammond [1992] summarized the distribution of
hydrothermal plumes.
Figure 1. Location maps of tectonic segments studied along the (a) Juan de Fuca Ridge, (b) northern East Pacific Rise (EPR), and (c) southern EPR. Segments ClippertonOrozco2 and CO3 lie immediately north of segment CO1 on the northern EPR; H, G2, and G1 lie immediately south of I on the southern EPR. Segment labeling on the southern EPR from Sinton et al. [1991].
The northern EPR between 9° and 11°50N includes
three segments bounded by first-, second-, or third-order discontinuities [Macdonald
et al., 1992] (Figure 1b). Full-rate
spreading increases from 95 mm yr1 at 11°50
N
to 104 mm yr1 at 9°N [DeMets et al., 1990,
1994]. Scheirer and Macdonald [1993] calculated axial cross
sections for this area, and Detrick et al. [1987] reported an
MCS survey of the area. Petrological sampling has been dense, particularly between
9° and 10°N [Thompson et al., 1985; Langmuir et al.,
1986; Batiza and Niu, 1992; Perfit et al., 1994;
Batiza et al., 1996]. Baker et al. [1994] mapped
the hydrothermal plume distribution.
The southern EPR between 13°50 and 18°40
S
includes seven segments bounded by first-, second-, third-, and fourth-order
discontinuities (Figure 1c). This ridge section
is segmented by overlapping spreading centers (OSCs) noted by Lonsdale
[1989] and Scheirer et al. [1996], plus large devals at 15°S
and 17°05
S. These OSCs and devals also correspond
to petrologic boundaries [Sinton et al., 1991]. Full-rate spreading
is ~145 mm yr1 [DeMets et al., 1990, 1994].
Scheirer and Macdonald [1993] calculated axial cross sections
for this area, Detrick et al. [1993] obtained MCS profiles from
along and across axis, and Sinton et al. [1991] sampled axial
lavas. Urabe et al. [1995] and Baker and Urabe [1996]
have reported the distribution of along-axis hydrothermal plumes.
As Fornari and Embley [1995] point out in reviewing the tectonic and volcanic controls on submarine hydrothermal processes, the two principal factors are crustal permeability and magmatic heat. A systematic consideration of the effects of permeability is precluded by an almost complete lack of data on its magnitude and distribution in zero-age crust. Moreover, permeability is likely to be highly localized on a scale even smaller than the second- to third-order segments considered here [e.g., Wright et al., 1995]. In contrast, a variety of morphological, structural, and petrological indexes have been suggested as proxies for the along-axis variability of magmatic heat. These indexes are schematically summarized in Figure 2 and explained below.
Figure 2. Schematic cross section of a ridge axis showing relationships between all of the geological indexes considered, except MgO wt% in axial basalts. See text for descriptions.
The height of the ridge axis above a reference depth (at 0.5 Ma) for each first-order segment is taken from Scheirer and Macdonald [1993] for the northern and southern EPR regions and calculated here for the JDFR using their method. Using Enet instead of absolute depth removes the effect of long-wavelength topographic variations not related to the recent magmatic budget at the axis. Enet equals the absolute ridge depth less the reference depths of 2994 and 2938 m for the northern EPR north and south, respectively, of the Clipperton transform fault, 3144 m for the southern EPR, and 2660 m for the JDFR.
Cross-sectional area of the ridge axis above the reference depth out to ±8 km from the spreading axis. Axs is effectively the average volume per unit length along the ridge. Scheirer and Macdonald [1993] calculated Axs at 1-km intervals along the EPR; I use a three-point boxcar-smoothed version of those data. JDFR Axs was calculated by the same method at ~6 km intervals, or at greater intervals where near-axis seamounts were present and would bias the profile.
Layer 2A is the seismic layer identified with volcanic extrusives in the near-axis region. The region between the ridge axis and the end of significant off-axis thickening of layer 2A is commonly defined as the neovolcanic zone [Harding et al., 1993; Christeson et al., 1994; Kent et al., 1994; McDonald et al., 1994].
An AMC reflector is a seismic reflection horizon interpreted as the top of a sill of partial melt lying immediately beneath the layer of sheeted dikes (layer 2B). The percentage of each segment underlain by an AMC reflector was determined from the along-axis data of Detrick et al. [1987, 1993], supplemented by cross-axis lines at the northern EPR [Kent et al., 1993a, b]. No JDFR segments have sufficient data to estimate this index.
The MgO wt% of magma melt varies directly with temperature [Helz and Thomber, 1987] and has been used as a relative index of the magmatic supply rate [e.g., Langmuir et al., 1986; Scheirer and Macdonald, 1993]. I used the instrumental correction factor given by Batiza and Niu [1992] to adjust the glass analyses of normal mid-ocean ridge basalts by various investigators using different analytical systems.
The spreading rate us is the full-rate separation at the spreading axis, corrected according to recent revisions of the geomagnetic reversal timescale [DeMets et al., 1994]. Corrected rates are approximately 96% of those calculated by the NUVEL-1 model of plate motion [DeMets et al., 1990] and earlier estimates [e.g., Elvers et al., 1973].
The distribution of hydrothermal activity along multisegment stretches of ridge axis is most efficiently ascertained by mapping the distribution of hydrothermal thermal, optical, and chemical anomalies in the overlying waters (see Baker et al. [1995] for a global summary). This paper uses optical (light attenuation), rather than thermal or chemical, anomalies to map the extent and location of hydrothermal sources. Many hydrothermal particles settle out of the plume soon after their formation above a vent field, so the optical signal defines discharge locations more precisely than the conservative thermal signal. Chemical data are generally not available in sufficient detail. The spatial density of plumes is quantified as plume incidence ph, the fraction of linear ridge axis overlain by a light attenuation anomaly exceeding the local background value of ~0.005 m1 (Figure 3). The determination of ph assumes that the effect of dispersal by along-axis currents has little effect on plume coverage. This assumption rests on the observation that in places with both detailed plume and seafloor observations, optical plume anomalies are co-located with discharge areas at a resolution scale of better than 10 km [Embley et al., 1991; Lupton et al., 1993; Baker et al., 1994, 1995; Baker and Urabe, 1996].
Figure 3. Comparison of (top) along-axis trends in Enet, or net elevation (heavy solid line), and Axs, or cross-sectional area (dotted line), of the ridge axis with (middle) the presence of an axial magma chamber reflector and (bottom) the distribution of hydrothermal plumes for (a) Juan de Fuca Ridge, (b) northern East Pacific Rise, and (c) southern EPR. The top panel for each area shows reference lines at a net elevation of 0.35 km (solid line) and an area of 3.5 km2 (dashed line), as well as position and order of segment boundaries used here. Data are from Scheirer and Macdonald [1993]. The middle panel (EPR only) shows depth to AMC in terms of two-way travel time (TWTT) between seafloor and the AMC [Detrick et al., 1987, 1993]. The true AMC distribution may be somewhat more extensive, as off-axis ship wander can cause reflector drop-outs [Kent et al., 1993a, b]. Plume distribution is revealed by continuous two-dimensional transects of light attenuation anomalies caused by suspended hydrothermal precipitates. Plume incidence is the fraction of ridge length overlain by plumes with light attenuation anomalies exceeding the background, generally ~0.0050.01 m1. Plume incidence increases from ~0.28 on the JDFR, to ~0.38 on the northern EPR, to ~0.6 on the southern EPR.
Of the indexes given above, Enet, Axs,
and us are available essentially continuously along all
three ridge sections; AMC percent coverage is available only for the EPR. A
comparison of Enet with the plume distribution (Figure
3) allows a test of the Francheteau and Ballard [1983] hypothesis
that hydrothermal venting is favored at bathymetric highs. The agreement between
Enet and plume location is robust on some segments (e.g.,
Endeavour, 9°1710°05
N)
but weak on most. Correlations weaken considerably as the along-axis variability
of Enet decreases with increasing us.
In general, little venting occurs where Enet < ~0.35 km.
Agreement between Axs and plume distribution is better, particularly on faster spreading segments (Figure 3). Axs is more sensitive because at the fastest spreading rates its along-axis dynamic range remains higher than that of Enet. Virtually all significant plumes occur where Axs > ~3.5 km2, especially on the JDFR and the northern EPR.
The correlation between AMC and plume presence is less precise than either of the two bathymetric variables. In general, extensive venting occurs only where an AMC reflector is observed, although an observable AMC does not guarantee hydrothermal venting (Figure 3). Neither is there a clear relation between AMC depth and plumes. Apparent variations in AMC depth often result from slight wander of the ship track away from the narrow region of minimum thickness of layer 2A [Kent et al., 1993a, b].
Structural discontinuities ranging from first-order (transform faults) to fourth-order
(slight devals and offsets) define tectonic segments that determine the pattern
and timing of ocean crust creation [Macdonald et al., 1991].
First-order and second-order (large OSCs with offsets of ~2 to 30 km) discontinuities
typically correspond to petrologic boundaries and thus are thought to represent
distinct spreading cells. The identification of third- and fourth-order discontinuities
is more subjective, and tectonic and petrologic segmentation do not always agree
[Macdonald et al., 1991; Sinton et al., 1991].
Because of these ambiguities, most of the segment boundaries used here are first-
or second-order. Exceptions are a third-order deval identified at 9°17N
[Macdonald et al., 1992], four third-order offsets of >2 km
between 16°30
and 18°37
S
[Lonsdale, 1989; Scheirer et al., 1996], and fourth-order
(?) devals at 15° and 17°05
S. All mark significant
petrological boundaries [Langmuir et al., 1986; Sinton
et al., 1991]. Detailed seafloor mapping can reveal hydrothermal, volcanic,
petrologic, and magmatic variations at the scale of fourth-order segments [e.g.,
Langmuir et al., 1986; Toomey et al., 1990; Haymon
et al., 1991; Wright et al., 1995], but these variations
likely arise from local conditions that require careful geologic mapping to
document (e.g., the scale of individual fissure eruptions). Such mapping is
rare along most of these three study areas.
The three study areas thus include 14 segments: four of intermediate spreading rate, three fast, and seven superfast. For each segment I determined ph and the mean and standard deviation of Enet, Axs, MgO wt%, and us. The percent of each segment underlain by an AMC reflector was calculated only for the northern and southern EPR. Layer 2A variations are not considered as a separate index because data throughout the neovolcanic zone are available from only a few cross-axis MCS lines. Correlations between geological indexes and ph are expressed in a series of scatterplots (Figure 4).
Figure 4. Scatterplots of the mean and standard deviation of (a) Axs, (b) Enet, (c) AMC percent coverage, (d) MgO wt%, and (e) us against plume incidence ph for each of 14 segments. Standard deviations are not applicable for AMC and are negligible for spreading rate.
Axs exhibits the strongest correlation with ph
(Figure 4a). An exponential least squares fit
of Axs versus ph gives an r2
value of 0.54, the only index with r2 > 0.5.
Enet (Figure 4b) displays
a similar trend but weaker correlation (r2 = 0.27).
(There is no correlation between ph and absolute depth of
the ridge axis.) Relationships between these measures of ridge inflation and
the magma supply rate are complex. Along-axis changes in ridge inflation are
evidently controlled both by factors directly related to magma supply (eruption
rates, melt lens inflation) and those more indirectly related (structural variations
in the low-velocity region within the crust) [Harding et al., 1993;
Scheirer and Macdonald, 1993; Kent et al., 1994].
The thickness of layer 2A at the axis is not related to either Axs
or Enet, as it is uniform at ~200 m throughout fast
and superfast spreading EPR study areas [Harding et al., 1993;
Christeson et al., 1994; Kent et al., 1994; Mutter
et al., 1995]. A thickness of 350400 m is common where measured
on the JDFR [Cudrak and Clowes, 1993; McDonald et al.,
1994], but fine-scale variations are ±150 m. Differences in the near-axis
volume of layer 2A can be significant when Axs varies by
a factor of 2 (as between 9°40 and 13°30
N
[Harding et al., 1993]) but can be relatively invariant for smaller
Axs changes (14° to 14°30
S
[Kent et al., 1994]). Scheirer and Macdonald [1993]
have also suggested a temporal difference between Axs and
Enet. They note that while these indexes are generally well
correlated, axial lengths where Enet is high relative to
Axs may indicate segment portions where the magma supply
has only recently begun to increase, as along 14°30
16°30
S
(Figure 3c). At least in that area, however,
the proposed increase in magmatic activity causing an increase in Enet
has not produced an AMC reflector under most of the axis nor substantially enhanced
hydrothermal activity. An explanation of Axs and Enet
in terms of the magmatic budget will require more data on the three-dimensional
structure of the ridge axis than are presently available.
Whereas Axs and Enet are indirect indexes of the magma supply rate, AMC percent coverage is a direct, though simplistic, index of the relative availability of pooled magma to power hydrothermal activity. Other AMC characteristics, such as depth and width, either show little intersegment variation, large variations at the spatial scale of kilometers, or cannot yet be calculated for most segments. Within the limited areas of the northern and southern EPR where AMC width has been determined [Kent et al., 1993a, b, 1994; Mutter et al., 1995], it shows no clear correlation with other geological indexes. Careful analyses of along-axis MCS data on the EPR indicate that although the depth of the AMC reflector decreases with increasing us on a regional basis [Purdy et al., 1992], there is little systematic along-axis variation within a given area [Harding et al., 1993; Detrick et al., 1993; Kent et al., 1993a, b]. Depth variations in the AMC originally thought to correlate with hydrothermal activity on the 9°10°N segment of the EPR [Haymon et al., 1991; Baker et al., 1994] have since been shown to result mostly from a combination of ship wander and the sharp increase in layer 2A thickness along the edges of the neovolcanic zone [Kent et al., 1993a, b].
Figure 5 shows comparisons of AMC width and
depth with plume distributions where AMC data are available from the northern
and southern EPR [Kent et al., 1993a, b; Kent et
al., 1994; Mutter et al., 1995]. No consistent relationships
are apparent in any of the three areas. AMC width is greatest near the 9°03N
OSC (Figure 5a), which Kent et al.
[1993b] attribute not to an increased magma supply rate but to decoupling
between melt supply and the emplacement of extrusives on the seafloor. Even
at 17°25
S on the southern EPR [Mutter
et al., 1995], where a broad and shallow AMC spike underlies a peak
in hydrothermal activity, more and greater hydrothermal maxima occur in adjacent
segments where the AMC depth varies little [Urabe et al., 1995;
Baker and Urabe, 1996].
Figure 5. Detailed comparisons of plume distribution with AMC width (solid line) and depth (dashed line) reveal no systematic correlation. AMC width and depth from cross-axis seismic reflection lines along the (a) northern [Kent et al., 1993a, b] and (b,c) southern [Kent et al., 1994; Mutter et al., 1995] East Pacific Rise. Symbols on each line show location of cross-axis lines; no depth information is yet available for the 14°14.5°S section. "Net AMC depth" is depth to the AMC reflector less the thickness of layer 2A. This calculation accounts for changes in apparent AMC depth caused by ship wander and the rapid off-axis thickening of layer 2A.
There is a slight tendency for MgO wt% to increase with increasing ph on the EPR, but the JDFR trend is nearly the opposite (Figure 4d). Petrologic variations along the JDFR are likely to be complicated by the presence of a mantle thermal anomaly at the site of Axial Volcano [Rhodes et al., 1990], propagating rifts at the southern end of Cleft and the northern end of Cobb segments [Sinton et al., 1983], and the lack of a steady state magma chamber along much of the JDFR [Christeson et al., 1993; Cudrak and Clowes, 1993].
Finally, ph shows extreme variability for all values of us (Figure 4e). The poor predictive value of us on the scale of individual segments is an expected consequence of its slight and uniform along-axis variation within each study area.
Figure 4 makes it clear that no simple functional
relation exists between the distribution of hydrothermal venting on a particular segment
and any of the geological indexes examined. Nevertheless, it is also apparent that plume
incidence is not distributed randomly among the segments studied. In every case, the
probability of observing a higher ph is greatest for high values of
the geological indexes. Segments with Axs > ~3.5 km2,
Enet > ~0.35 km, AMC coverage > ~60%, and
(for the EPR) MgO > ~7 wt% have a much higher mean ph
than other segments. To produce a plot including all the indexes, I calculated for each
observation a normalized value x = (x-
)/
x,
where x is the observed index value for a given segment, with
the mean value and
x the standard deviation of all 14
segments for a particular index. This normalization produces a distribution with a mean of
0 and standard deviation of 1 for each index. Spreading rate us is not
included because it has no significant segment-to-segment variation in a given study area.
For the JDFR, the normalization does not use AMC coverage percent because of a lack of
data, nor MgO wt% because of the complexities mentioned above.
Plotting the normalized indexes of Axs, Enet,
MgO wt%, and AMC percent coverage against ph reveals that the data
fall into two distinct groups (Figure 6). For normalized
values 0.3 (slightly below the mean value of each
index), ph values never exceed 0.4 and have a mean value of
0.25 ± 0.09. For normalized values >0.3, ph values
range from 0.09 to 1 with a mean value of 0.63 ± 0.26. This pattern indicates
that segments do not support a vigorous hydrothermal environment if their geological
indexes indicate a relatively low recent magma supply rate, regardless of the local
long-term magma supply (i.e., us). Conversely, segments with
characteristics of a presently high magma supply rate exhibit a range of hydrothermal
activity from slight to ubiquitous. There is little gradation between the two states in
the available segment population. Any segment whose geological indexes exceed the mean is
a candidate for extensive hydrothermal activity.
Figure 6. Scatterplot of normalized values of geological indexes (except spreading rate) against plume incidence ph for each study area. The distribution defines two populations: low ph when the normalized index <0.3, and a highly variable ph when the normalized index is >0.3. Each area has indexes in both populations.
The observations summarized in this paper suggest that the distribution of
ph shown in Figure 6 can
best be explained as a result of the episodic nature of magmatic and hydrothermal
processes at ridge crests. This hypothesis presupposes that hydrothermal fields,
at least on intermediate to superfast spreading ridges, typically begin, or
are renewed, as a sudden response to a dike injection. An injection instantly
invigorates hydrothermal circulation, producing widespread chronic venting and,
often, event plumes. This effect has been documented at the Cleft [Baker
et al., 1987; Embley and Chadwick, 1994] and CoAxial
[Embley et al., 1995] segments of the JDFR and at 9°50N
on the EPR [Haymon et al., 1993], and modeled by Lowell
and Germanovich [1994, 1995] and Wilcock [1994].
Still unpredictable is the timescale of the invigoration, which might last from
a few years to hundreds of years.
The model proposed here postulates that on magmatically starved segments (normalized
indexes <~0) episodes of dike intrusions are presently uncommon in both space
and time. Consequently, ph values are low and uniform. Segments
with a relatively high rate of magma supply (normalized indexes >~0) have
a greater probability of dike intrusions, leading to ph
values that are higher and more variable. Variability in ph
may arise both from between-segment differences in the magma supply that may
be steady on the order of 1 Myr, and from short-term (on the order of 1 kyr)
waxing and waning of the magma supply at a particular location. For example,
temporal stability in large-scale features such as ocean crustal thickness [Barth
and Mutter, 1996], abyssal hill characteristics [Goff, 1991],
and lava petrology [Batiza et al., 1996] suggests that the average
magma supply rate at some locations changes little over timescales of several
hundred thousand years. Recent high-resolution studies, however, provide evidence
of high-frequency fluctuations not recorded in these features. Careful mapping
of along-axis volcanic, tectonic, and hydrothermal patterns on the 9°1710°05
N
segment clearly indicates that magma is episodically supplied, at intervals
of the order of 1 kyr, as individual intrusions on a fourth-order or smaller
spatial scale [Haymon et al., 1991; Wright et al.,
1995]. Dense petrological sampling at several sites on the northern EPR [Hekinian
et al., 1989; Reynolds et al., 1992; Perfit et
al., 1994; Batiza et al., 1996] has revealed a diversity
of lava types that apparently arises from rapid changes in the chemistry and
temperature of magma lenses that erupt at intervals of no more than 0.11 kyr.
Because of this interplay between low- and high-frequency magmatic fluctuations, some segments (e.g., Cobb and CO1) have suffered a low mean magma supply rate for perhaps several hundred thousand years. As a result, average index and ph values are low even though small sections of these segments are hydrothermally active at present. Other segments with high index values indicative of a high mean magma supply rate have highly variable ph values, depending on whether much (e.g., Cleft and J) or little (e.g., N and K2) of the segment has been recently perturbed by dike intrusions. A presently high magma supply rate is thus a necessary but not deterministic condition for large values of ph.
The indexes described above, together with available data from the EPR, permit some
simple predictions about the likelihood of hydrothermal activity on segments outside these
study areas. Several segments adjacent to the northern and southern EPR study areas have
been examined for all five of the geological indexes discussed here but as yet have no
fine-scale continuous plume data. Segments CO2 and CO3, bounded by OSCs at 11°45, 12°37
, and 12°54
N, lie sequentially north of segment CO1. Segments H, G2, and
G1, bounded by OSCs at 18°37
and 19°, a deval at 19°24
S, and a propagating rift offset at 20°42
S, lie sequentially south of segment I. The degree of
hydrothermal activity expected on each of these segments should be predictable using the
method described by Figure 6. Figure
7 shows a recalculation of the normalized indexes, including these five additional
segments. For each segment, the various indexes have been averaged to give a mean value.
The new segments show a normalized index range of 1.3 to 0.4, indicating the possibility
of a wide range in ph. The model predicts a high probability of
extensive hydrothermal plumes along segments H and CO3, a moderate plume distribution
along G2 and CO2, and meager plumes over G1.
Figure 7. Scatterplot of the mean and standard deviation of normalized values of geological indexes (except spreading rate) against plume incidence ph for each studied segment plus five adjacent segments where all four indexes can be calculated but no continuous plume distributions are available. These segments are plotted below the ph = 0 line. Segment abbreviations are given in Figures 1 and 3.
Some information is available to test this prediction. Bougault et al.
[1990] mapped hydrothermal chemical anomalies along segments CO2 and CO3
using their "dynamic hydrocast" system, which collected an integrated
plume sample approximately every 2 km between 12°10
and 13°10
N. They covered CO3 completely and
observed significant Mn and CH4 anomalies along about three quarters
of the axis. CO2 was covered only partially and had somewhat less plume coverage.
Much of the rest of the EPR between 18°N and 23°S has been quantified in terms of Enet and Axs by Scheirer and Macdonald [1993], who used these data to predict the probability of an AMC. Hey et al. [1995] have recently produced similar data along five segments surrounding a large overlap zone between the Easter and Juan Fernandez microplates. I have averaged these data on a segment-by-segment basis [Lonsdale, 1989; Sinton et al., 1991; Macdonald et al., 1992; Hey et al., 1995] to predict the distribution of hydrothermal activity (Figure 8). Segments with Axs > 3.5 km2 and Enet > 0.35 km have a high probability of present-day hydrothermal activity. On the northern EPR (Figure 8a) a short stretch of ridge just north of the Orozco transform fault is a prime candidate. Also, most of the ridge between the Clipperton transform fault and 6°N exceeds the threshold, but just barely. On the southern EPR (Figure 8b) a small region around the Gofar transform fault, the already discussed 13°19°S region, and the segments adjacent to the large overlap zone at 29°S are the only locations with high values of both Enet and Axs. Much of the ridge between Gofar and Garrett has Axs > 4 km2 but Enet between 0.2 and 0.3 km, so hydrothermal explorations there will reveal much about the relative usefulness of Enet and Axs as hydrothermal indexes.
Figure 8. Segment-by-segment plot of cross-sectional area Axs (dashed line) and net elevation Enet (solid line) for the (a) northern and (b) southern EPR. The horizontal line in each panel marks the 3.5 km2 Axs and 0.35 km Enet values, postulated as the approximate threshold for extensive hydrothermal activity. Locations of transform faults and other major features are also given.
A global review of the distribution of hydrothermal plumes [Baker et al., 1995] found a significant linear correlation between ph and us extending from spreading rates of 20 to 150 mm yr1. Baker et al. [1995] emphasized that because of the highly variable distribution of venting among adjacent segments, this relation depended on averaging hydrothermal activity over many segments, typically over distances of several hundred kilometers. As is apparent from Figure 4e, there is little correlation between ph and us if only individual second-order or smaller segments of different us are considered.
Averaging the Axs and Enet data regionally for the three study areas considered here also yields linear correlations with ph (Figure 9a). The uniformity of the three trends in Figure 9a indicates that over broad spatial scales, and by implication over broad temporal scales as well, both low-frequency (us) and high-frequency (Axs and Enet) indexes of the magmatic budget correlate linearly with the relative incidence of hydrothermal activity in these three areas. Regional means of MgO wt% and AMC percent coverage do not correlate significantly with ph.
Figure 9. (a) Scatterplot showing that at the regional scale, plume incidence ph is linearly correlated with mean values of spreading rate us, cross-sectional area Axs, and net elevation Enet. (b) Plot of Axs versus us. (c) Plot of Enet versus us. The linear relation in Figure 9a exists because for the three areas studied (solid symbols) both Axs and Enet are linearly correlated with us. This relation does not hold for other similar-sized regions of the superfast spreading southern EPR.
How general is the agreement between the low- and high-frequency indexes in Figure 9a? If us, Axs and Enet are correlated all along the EPR, then a prediction of the large-scale distribution of venting along the entire EPR may be straightforwardly made using any of these indexes. To test this possibility, Figures 9b and 9c show mean values of Axs and Enet against us for the JDFR and discrete units of the EPR. The northern EPR, 5°18°N, is quantized into 3° units to match the 9°12°N study area, while the southern EPR, 4°33°S, uses approximately 5° units to match the 13.5°18.5°S study area (the Easter microplate is not considered). For full spreading rates of 60 to 120 mm yr1 the correlation between us and Axs is nearly perfect (r2 = 0.9995). Moreover, the least squares fit for intermediate to fast rates extends precisely through the value for the 13.5°18.5°S superfast area. Elsewhere on the southern EPR, however, the inflation values are well below that predicted from this fit. At the extreme, the 18.5°23°S section, with a spreading rate virtually identical to the 13.5°18.5°S section, has a mean Axs only one-third as great, less than every section except the JDFR. The situation for Enet may be similar, except that the fit is poorer for the northern EPR between us of 80120 mm yr1 (Figure 9c). Alternatively, Figure 9c can be interpreted to demonstrate an effectively constant Enet except at extreme high and low values of us.
Figure 9 demonstrates that at the regional scale we presently have insufficient data to deduce whether a low-frequency (us) or high-frequency (Axs, Enet) index of the magmatic budget is the more general predictor of hydrothermal activity. If us, then virtually all large sections of the southern EPR should have a ph > 0.5. If Axs or Enet, the 13.5°18.5° section will have by far the largest ph, and the superfast section immediately south will have a ph lower than any other section of comparable size on the EPR.
Geologic predictors of hydrothermal activity on slow spreading ridge segments will be much different than those for faster ridges. The indexes Axs and Enet will not have the same interpretation for slow spreading ridges because axial topographic relief changes from positive to negative at spreading rates below ~5580 mm yr1, depending on the axial depth [Malinverno, 1993]. AMC percent coverage is a similarly ineffectual index because a reflector has been reported at only a single isolated location on the Mid-Atlantic Ridge (MAR) [Calvert, 1995]. Finally, not only is there no broad correspondence between hydrothermal observations and detailed petrological sampling along the MAR, but existing data suggests that because lavas on slower spreading ridges have more chemical diversity than those on faster ridges, a general relationship between MgO wt% and axial depth is not likely [e.g., Batiza, 1996].
While no thorough review of geologic indexes for slow spreading ridges will
be attempted here, there are two obvious possibilities for hydrothermally diagnostic
indexes: mantle Bouguer anomalies (MBA) and tectonization of the seafloor. Recent
geophysical surveys along the MAR find that the midpoints of tectonic segments
usually correspond to MBA gravity lows or "bull's eyes," interpreted
as arising from focused accretion of new ocean crust [e.g., Lin and Phipps
Morgan, 1992; Detrick et al., 1995]. Along the northern
MAR, vent fields at 23°12N (Snake Pit) [Ocean
Drilling Program Leg 106 Scientific Party, 1986], 29°10
N
(Broken Spur) [Murton et al., 1994], 36°27
N
(AMAR) [German et al., 1996], 37°17
N
(Lucky Strike) [Fouquet et al., 1994], and 37°50
N
(Menez Gwen) [Fouquet et al., 1994] all occur at segment bathymetric
and MBA minima. Though a gravity survey has not been published around the TAG
field at 26°10
N [Rona et al., 1986],
it too is found near a segment bathymetric minimum [Karson and Rona,
1990]. Crustal accretion at segment centers apparently results from enhanced
magma upwelling, and these areas may be focal points of hydrothermal activity.
A contrasting model was offered by German et al. [1996], who found that five of seven hydrothermal sites identified by plume surveying on the MAR between ~36° and 37°N occurred in highly tectonized areas at or near the ends of second-order ridge discontinuities. They suggest that the dominant control on vent field location is not the crustal thermal field but the presence of cross-cutting fault fabrics at segment termini. These fault zones allow access to the deeper heat sources characteristic of slow spreading ridges [e.g., Purdy et al., 1992]. While faulting is clearly a fundamental control on vent field location on both slow [e.g., Karson and Rona, 1990] and fast [e.g., Wright et al., 1995] spreading ridges, the relative importance of thermal and permeability factors remains uncertain.
The spatial frequency of hydrothermal plumes along second- to fourth-order tectonic segments on the Juan de Fuca Ridge and the East Pacific Rise is positively correlated with ridge axis cross-sectional area and elevation and with the spatial frequency of an axial magma chamber reflector. The correlation holds weakly on the EPR, but not on the JDFR, for the weight concentration of MgO in basalt glasses from the axis. There is no segment-by-segment correlation with spreading rate, an expected consequence of its slight and uniform along-axis variation. Segments with a cross-sectional area >~3.5 km2, a net elevation >~0.35 km, AMC coverage >~60%, and (for the EPR) MgO > ~7 wt% have a much higher mean incidence of hydrothermal plumes than other segments. The best single predictor of hydrothermal activity is the cross-sectional area.
Despite these correlations, there is no simple functional relationship that links plume spatial frequency with any of the geological indexes. Rather, the data suggest two classes of ridge segments. Those segments with index values lower than the mean of the studied population exhibit uniformly low plume incidence. These segments apparently have current magmatic budgets too weak to power widespread hydrothermal activity. Segments with index values above the mean have a plume incidence ranging from low to continuous. Their magma supply rates are sufficient to produce frequent intrusions that create or renew hydrothermal activity. The spatial variation in hydrothermal activity among these segments may be a measure of the relative temporal frequency of hydrothermal activity and thus of magmatic events.
If ridge segments with both Axs > 3.5 km2 and Enet > 0.35 km have the highest probability of present-day hydrothermal activity, then the most active areas on the northern (5°18°N) EPR are expected to be just north of the Orozco transform fault and the area between the Clipperton transform fault and 6°N. On the southern (4°33°S) EPR, active areas should be found around the Gofar transform fault, between the Garrett transform fault and 19°S, and along the segments adjacent to the large overlap zone at 29°S.
On a regional scale, mean values of area, net elevation, and spreading rate for each region are linearly correlated with each other and with plume incidence. We thus cannot conclude from the available data which index is the best predictor of hydrothermal activity on the multisegment scale. A detailed plume survey along the EPR between 19° and 23°S, where the greatest departure is found from the JDFR/EPR trend of spreading rate versus area and depth, will provide the most useful new information for deciphering the relation between ridge characteristics and hydrothermal venting.
The indexes described here for intermediate to superfast ridge segments are not likely to be similarly useful on slow spreading ridges. Alternative indexes that show some degree of correlation with hydrothermal activity are mantle Bouguer (gravity) lows and zones of cross-cutting tectonization.
Acknowledgments. This research was supported by the NOAA VENTS Program. I thank S. Walker for CTD data processing, and colleagues from NOAA, the University of Hawaii, and the Geological Survey of Japan who cooperated in the collection of plume data on the Juan de Fuca Ridge and the East Pacific Rise. D. J. Fornari and R. W. Embley provided helpful comments on the manuscript. D. Scheirer's sharing of detailed cross-section and depth data from the EPR was indispensable, as was similar generosity on the part of R. Detrick, G. Kent, M. Perfit, and J. Sinton. R. Batiza and J. Karsten provided useful and detailed reviews. NOAA Pacific Marine Environmental Laboratory contribution 1692.
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