Hot spot hydrothermal activity


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.

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.


Sulfide 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.	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

The discussions below are paraphrased from abstracts of papers published by the Submarine Volcanism group.

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.

Pele's Pit hydrothermal deposits

LOIHI - Sulfide and sulfate minerals collected from Pele's Pit on Loihi Seamount in October 1996 with the Pisces V submersible are the first occurrence of high-temperature hydrothermal mineralization documented for ocean island volcanoes. Small talus from the north rim of Pele's Pit consist of aggregates of pyrite, marcasite, pyrrhotite, wurtzite, and barite (found as clear, euhedral barite crystals or as acicular, radial crystal clusters). The minerals and composition of the sulfide phases are similar to those of hydrothermal sulfide from mid-ocean spreading centers and confirm the presence of high-temperature fluids (>250 degrees C) in the Loihi hydrothermal system. The sulfide samples are not pieces of broken chimneys or basal mounds, or stockwork from lower levels in the hydrothermal system. The crystal shapes and composition suggest rapid precipitation from a high-temperature plume injected into cold seawater along fractures and fissures that define the north wall of the new pit crater.

Reference: A.S. Davis and D.A. Clague (1998) Changes in the hydrothermal system at Loihi Seamount after the formation of Pele's Pit in 1996. Geology, 26(5): 399-402. [Abstract]

Hydrothermal mineralization along submarine rift zones

HALEAKALA and PUNA RIDGES - This is the first description of hydrothermal manganese-oxide mineralization of midplate submarine rift zones and volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala (SE of Maui) and Puna (extension of Kilauea's SE rift zone off Hawaii) Ridges. These Mn oxides form two types of deposits, metallic statiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and birnessite. Thin Fe-Mn crusts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of δ-MnO₂. The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material (in Mn-cemented sandstone), and seawater that mixed with the hydrothermal fluids. Mineralization on Haleakala Ridge occurred sometime during the past 200 to 400 thousand years, when the summit was at a water depth of more than 1,000 m. Hydrothermal circulation was probably driven by heat produced by intrusion of dikes, magma reservoirs, and flow of magma through axial and lateral conduits. The supply of seawater to ridge interiors must be extensive because of their high porosity and permeability. Precipitation of Mn oxide below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks.

Reference: J.R. Hein, A.E. Gibbs, D.A. Clague, and M. Torresan (1996) Hydrothermal mineralization along submarine rift zones, Hawaii, Marine Georesources and Geotechnology, 14: 177-203.

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Last updated: Jan. 24, 2008