Geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field (East Pacific Rise, 18°S)

International audienceAn inactive vent field comprised of dead chimneys was discovered on the ultrafast East Pacific Rise (EPR) at 18 degrees S during the research campaign NAUDUR with the R/V Le Nadir in December 1993. One of these chimneys was sampled, studied and found to be largely composed of silica-mineralized bacterial-like filaments. The filaments are inferred to be the result of microbial activity leading to silica (+/- Fe-oxyhydroxide) precipitation. The chimney grew from the most external layer (precipitated 226 +/- 4 yr. B.P.) towards the central chimney conduit. Hydrothermal activity ceased 154 +/- 13 yr. B.P. and the chimney conduit was completely sealed. Mixing between an end-member hydrothermal fluid and seawater explains the Sr-Nd isotopic composition of the chimney. Seawater was the major source of Sr to the chimney, whereas the dominant Nd source was the local mid-ocean ridge basalt (MORB) leached by the hydrothermal fluids. The mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions. The proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85%. Pb-isotope data indicates that all of the Pb in the chimney was derived from the underlying MORB. The precipitation temperatures of the chimney layers varied between 55 and 71 degrees C, and were a function of the seawater/end-member hydrothermal fluid mixing ratio. delta Si-30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the Si-isotope composition of the chimney. Concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end-member hydrothermal fluid. The inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater/hydrothermal fluid mixing ratio, which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids. The silica chimney is significantly enriched in U, likely a result of bacterial concentration of U from the seawater-dominated vent fluid. The chimney is poor in rare earth elements (REE). It inherited its REE distribution patterns from the parent end-member hydrothermal fluids. The dilution of the hydrothermal fluid with over 85% seawater could not obliterate the particular REE features (positive Eu anomaly) of the hydrothermal fluids

When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift)

At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)mol of the lake water. Increasing (Mg/Ca)mol of the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation

Mn‑carbonate deposition in a seafloor hydrothermal system (CLAM field, Iheya Ridge, Okinawa Trough): Insights from mineralogy, geochemistry and isotope studies

A seafloor hydrothermal system located at the Iheya Ridge (Okinawa Trough), named CLAM, deposits Mn‑carbonate chimneys that have no analogue found so far on the seafloor. The chimneys are composed of Mn-calcite and Ca-rhodochrosite. The crystallographic differences between these carbonates appear to control the rare earth elements (REE) partitioning between them that results in enrichment of the Ca-rhodochrosite in middle and heavy REE, and enrichment of the Mn-calcite in light REE. Chemistry of the CLAM hydrothermal fluids suggests: (1) low water/rock ratio of the hydrothermal system; (2) phase separation and dominance of low-chlorinity vapor phase; (3) sub-seafloor formation of Na-rich alteration minerals during fluid/rock reactions; (4) removal of some elements from the seawater to the host rocks during the seawater/rock interaction; (5) high Mn/Ca ratio of the basement rocks is responsible for the high Mn concentration in the hydrothermal fluids. C-O-isotope compositions of the CLAM Mn‑carbonates suggest they precipitated through binary mixing of end-member hydrothermal fluid and seawater accompanied by progressive degassing and cooling of the fluid. Mn-calcite precipitated from almost pure end-member hydrothermal fluid, whereas Ca-rhodochrosite precipitated from seawater-dominated vent fluid. Mg-isotope fractionation during Mn‑carbonate precipitation is assumed to depend on carbonate growth conditions and resulting carbonate mineralogy. S-isotope composition of the CLAM Mn‑carbonates suggests that the Ca-rhodochrosite precipitated in oxic conditions through rapid mixing of hydrothermal fluid and seawater, whereas the Mn-calcite precipitated in reduced conditions (thermochemical or microbial sulfate reduction) through slow mixing of hydrothermal fluid and seawater. Sr-isotope composition of the CLAM hydrothermal fluids is close to that of Okinawa Trough deep seawater. In contrast, Sr-isotopes in the CLAM Mn‑carbonates are more variable, indicating that Sr was derived from seawater, local lavas and sediments. Nd-isotope composition of the Mn‑carbonates indicates that Nd was derived from the local lavas and sediments. Pb in the majority of the CLAM Mn‑carbonates is of sedimentary origin (Pb isotope data), but involvement of anthropogenic Pb in the hydrothermal system is inferred for some Mn-calcite samples. Stability phase diagram modeling coupled with C-O-S-Sr-isotope data suggest that in the CLAM vent fluid the rhodochrosite is stable in a narrow Eh-pH range (6  0) and in a wide range of [Mn] and [Ca] activities, whereas calcite precipitates from a close to the end-member hydrothermal fluid in reduced conditions (Eh < 0)