Hot spot hydrothermal activity

Bathymetric relief map of Loihi Seamount shows summit and rift zones Bathymetric data courtesy of F. Duennebier

Bathymetric relief map of Loihi Seamount shows summit and rift zones. Bathymetric data courtesy of F. Duennebier

Close-up of summit of Loihi Seamount with newly formed pit crater. White dots mark location of hydrothermal vents.

Close-up of summit of Loihi Seamount with newly formed pit crater. White dots mark location of hydrothermal vents.

Hydrothermal systems at Loihi Seamount and at rift zones

Loihi Seamount, the youngest of the Hawaiian volcanoes, is located 28 km off the SE coast of Hawaii. In July and August 1996, a large swarm of earthquakes (>4000) occurred on Loihi which led to release of intense hydrothermal plumes in the surrounding seawater and formation of a large summit pit crater. For more information about Loihi, visit the web page of The Hawaii Center for Volcanology. Evidence of hydrothermal activity has also been found on the submarine extensions of the rift zones of Kilauea and Haleakala Volcanoes.

Magmatic processes provide the driving mechanism for hydrothermal circulation through oceanic rocks. Interactions between hot rock and seawater give rise to venting of hydrothermal fluids (<200°C to > 400°C) that form sulfide and sulfate precipitates upon mixing with sulfate-rich seawater. Many such vent sites have been observed at mid-ocean ridges where precipitates may form great chimney-like structures. Prior to the seismic events in 1996, Loihi hydrothermal vents emitted only low temperature fluids (<30° C) which precipitated iron oxide and iron-rich clay minerals. After the seismic swarm, high -temperature sulfide minerals were found in the glass sands near the margins of the newly formed pit crater and vent fluids with temperatures up to 200° C were measured. Some glass grains have rounded edges but show hardly any alteration as might be expected from reaction with hot, acidic fluids. This suggests that reactions occurred rapidly, probably while glass shards were ejected into a turbulent current. The small, euhedral crystals do not appear to be pieces of chimney-like structures or stockwork from deeper in the hydrothermal system. Most likely these crystals precipitated from a hot hydrothermal plume, violently ejected through deep fractures and fissures along the walls of the collapsing pit crater.

pyrrhotite wurtziteSulfide minerals include pyrite, pyrrhotite (left), wurzite and sphalerite (right), some with copper, lead, and cadmium-rich inclusions.
Photos © 1998 MBARI

The sulfates consist of anhydrite and barite; the latter has a high strontium content. Chemical compositions are similar to those observed at mid-ocean ridge vent precipitates.

The sulfates consist of anhydrite and barite; the latter has a high strontium content. Chemical compositions are similar to those observed at mid-ocean ridge vent precipitates.

Backscattered electron images (BSE) of rounded glass shard completely surrounded by a shell of delicate sulfide and sulfate crystals. Photos © 1998 MBARI

Backscattered electron images (BSE) of rounded glass shard completely surrounded by a shell of delicate sulfide and sulfate crystals. Photos © 1998 MBARI

Our research on hydrothermal activity at Loihi Seamount

Changes in hydrothermal system on Loihi

LOIHI – A large swarm of >4000 earthquakes occurred at Loihi Seamount in the summer of 1996. Pisces V submersible dives conducted two months after the seismic event discovered that part of the summit had collapsed to form a new pit crater. The sudden collapse of the summit crater resulted in a cataclysmic discharge of hydrothermal fluid, ejecting magmatic gases and sulfide crystals. Hydrothermal sulfide and sulfate minerals were discovered in sediment samples from the new pit crater. The presence of wurtzite, pyrrhotite, and chalcopyrite indicate high-temperature fluids (>250ºC), the first such occurrence documented for an ocean-island volcano. The composition of the sulfide minerals is similar to that of sulfides from black smokers at mid-ocean ridges. On follow-up cruises in 1997 and 1998, barite-rich mounds were discovered to have been built after the initial cataclysmic discharge of the hydrothermal plume. Samples resemble white smoker deposits from mid-ocean ridges. The d34S of the sulfide is lower than that of minerals from massive sulfide deposits formed on oceanic basalt, suggesting that the Loihi basalt has lost sulfur as magmatic SO2. Dissolution and oxidation of the ejected high-temperature sulfides, observed in the years following the event, suggest that these minerals are not easily preserved. Similar crystals forming at deeper levels in the hydrothermal system probably are ejected repeatedly when volcanic and tectonic activity disrupts the hydrothermal system.

Reference: A.S. Davis, D.A. Clague, R.A. Zierenberg, C.G. Wheat, B.L. Cousens (2003) Sulfide formation related to changes in the hydrothermal system on Loihi Seamount, Hawai’i, following the seismic event in 1996, The Canadian Mineralogist, 41: 457-472.

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