HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC RISE, 8-DEGREES-40' TO 11-DEGREES-50'N - PLUME DISTRIBUTION AND RELATIONSHIP TO THE APPARENT MAGMATIC BUDGET
Et. Baker et al., HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC RISE, 8-DEGREES-40' TO 11-DEGREES-50'N - PLUME DISTRIBUTION AND RELATIONSHIP TO THE APPARENT MAGMATIC BUDGET, Earth and planetary science letters, 128(1-2), 1994, pp. 1-17
The interactions between hydrothermal circulation and large-scale geol
ogical and geophysical characteristics of the mid-ocean ridge cannot b
e ascertained without large-scale views of the pattern of hydrothermal
venting. Such multi-ridge-segment surveys of venting are accomplished
most efficiently by mapping the distribution and intensity of hydroth
ermal plumes. In November 1991, we mapped hydrothermal temperature (DE
LTAtheta) and light attenuation (DELTAc) anomalies above the East Paci
fic Rise (EPR) continuously from 8-degrees-40' to 11-degrees-50'N, a f
ast spreading ridge crest portion bisected by the Clipperton Transform
Fault. Plume distributions show a precise correlation with the distri
bution of active vents where video coverage of the axial caldera is ex
haustive (90-degrees-09'-54'N). Elsewhere in the study area the sketch
y knowledge of vent locations gleaned from scattered camera tows predi
cts only poorly the large-scale hydrothermal pattern revealed by our p
lume studies. Plumes were most intense between 9-degrees-42' and 9-deg
rees-54 N', directly over a March/April, 1991, seafloor eruption. Thes
e plumes had exceptionally high DELTAc/DELTAtheta ratios compared to t
he rest of the study area; we suggest that the phase-separated gas-ric
h vent fluids discharging here fertilize an abundant population of bac
teria. Hydrothermal plume distributions define three categories: inten
se and continous (8-degrees-48'-8-degrees-58'N, 9-degrees-29'-10-degre
es-01'N and 11-degrees-05'-11-degrees-27'N), weak and discontinuous (8
-degrees-58'-9-degrees-29'N) and negligible. The location of each cate
gory is virtually congruent with areas that are, respectively, magmati
cally robust, magmatically weak and magmatically starved, as inferred
from previous measurements of axial bathymetric undulations, cross-axi
s inflation and magma chamber depth and continuity. This congruency im
plies a fine-scale spatial and temporal connection between magmatic fl
uctuations and hydrothermal venting. We thus speculate that, at least
along this fast spreading section of the EPR, cyclic replenishment, er
uption and freezing of the thin axial melt lens exerts greater control
over hydrothermal venting than the more enduring zones of crystal mus
h and hot rock. We found intense, and continuous, plumes along 33% of
the surveyed ridge crest, an observation implying that any point on th
e ridge is, on average, hydrothermally active one-third of the time. C
ombining this result with the 20% plume coverage found along the mediu
m-rate Juan de Fuca Ridge suggests that superfast (approximately 150 m
m/yr) spreading ridges should support vigorous venting along approxima
tely 50% of their length, if spreading rate and along-axis plume cover
age are linearly related.