VOLCANIC-ERUPTION OF THE MIDOCEAN RIDGE ALONG THE EAST PACIFIC RISE CREST AT 9-DEGREES-45-52'N - DIRECT SUBMERSIBLE OBSERVATIONS OF SEA-FLOOR PHENOMENA ASSOCIATED WITH AN ERUPTION EVENT IN APRIL, 1991

In April, 1991, we witnessed from the submersible Alvin a suite of pre viously undocumented seafloor phenomena accompanying an in-progress er uption of the mid-ocean ridge on the East Pacific Rise crest at 9-degr ees-45'N-52'N. The volume of the eruption could not be precisely deter mined, although comparison of pre- and post-eruption SeaBeam bathymetr y indicate that any changes in ridge crest morphology resulting from t he eruption were < 10 m high. Effects of the eruption included: (1) in creased abundance and redistribution of hydrothermal vents, disappeara nce of numerous vent communities, and changes in characteristics of ve nt fauna and mineral deposits within the eruption area since December, 1989; (2) murkiness of bottom waters up to tens of meters above the s eafloor due to high densities of suspended mineral and biogenic partic ulates; (3) destruction of a vent community by lava flows, mass wastin g, and possible hydrovolcanic explosion at a site known as 'Tubeworm B arbecue' in the axial summit caldera (ASC) at 9-degrees-50.6'N; (4) ne ar-critical temperatures of hydrothermal vent fluids, ranging up to 40 3-degrees-C; (5) temporal variations over a 2 week interval in both te mperatures and chemical/isotopic compositions of hydrothermal fluids; (6) unusual compositions of end-member vent fluids, with pH values ran ging to a record low of 2.5, salinities ranging as low as 0.3 wt% NaCl (one-twelfth that of seawater), and dissolved gases reaching high con centrations (> 65 mmol/l for both CO2 and H2S); (7) venting at tempera tures above 380-degrees-C of visually detectable white vapor that tran sformed to plumes of gray smoke a few centimeters above vent orifices; (8) disorganized venting of both high-temperature fluids (black and g ray smoke) and large volumes of cooler, diffuse hydrothermal fluids di rectly from the basaltic seafloor, rather than from hydrothermal miner al constructions; (9) rapid and extensive growth of flocculent white b acterial mats (species unknown) on and under the seafloor in areas exp eriencing widespread venting of diffuse hydrothermal fluid: and (10) s ubseafloor downslope migration of magma normal to the ridge axis in a network of small-scale (1-5 m diameter) lava tubes and channels to dis tances at least 100-200 m outside the ASC. We suggest that, in April, 1991, intrusion of dikes in the eruption area to < 200 m beneath the A SC floor resulted in phase separation of fluids near the tops of the d ikes and a large flux of vapor-rich hydrothermal fluids through the ov erlying rubbly, cavernous lavas. Low salinities and gas-rich compositi ons of hydrothermal fluids sampled in the eruption area are appropriat e for a vapor phase in a seawater system undergoing subcritical liquid -vapor phase separation (boiling) and phase segregation. Hydrothermal fluids streamed directly from fissures and pits that may have been loc i of lava drainback and/or hydrovolcanic explosions. These fissures an d pits were lined with white mats of a unique fast-growing bacteria th at was the only life associated with the brand-new vents. The prolific bacteria, which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids, may thrive on high level s of gases in the vapor-rich hydrothermal fluids initially escaping th e hydrothermal system. White bacterial particulates swept from the sea floor by hydrothermal vents swirled in an unprecedented biogenic 'bliz zard' up to 50 m above the bottom. The bacterial proliferation of Apri l, 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as ra pid heat loss brings about cessation of boiling, and/or by grazing as other organisms are re-established in the biologically devastated area .