Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data

Abstract

The TAG active hydrothermal mound, located 2.4 km east of the neovolcanic zone at 26oN, Mid-Atlantic Ridge, is -200 rn in diameter, exhibits 50 rn of relief, and is covered entirely by hydrothermal precipitates. Eight different types of vent solids were recovered from the mound by the submersibles Alvin and Mir in 1986, 1990, and 1991. Detailed petrographic and geochemical studies of samples and their distribution are used to deduce patterns of fluid flowand seawater/hydrothermal fluid interaction. Geochemical modeling calculations using fluid composition data corroborate these interpretations. Current activity includes highly focused flow of 363øC fluid from a chimney cluster on the top of the mound and deposition of a high fS2-jD 2 mineral assemblage that reflects low concentrations of H2S in black smoker fluid. Slow percolation of black smoker fluid pooled beneath the black smoker cluster and entrainment of seawater result in formation of massive sulfide crusts and massive anhydrite. These three sample types are enriched in Co and Se. Blocks of sulfide and white smoker chimneys, enriched in Zn, Au, Ag, $b, Cd, and Pb, are forming on the surface of the mound from black smoker fluid that has been modified by mixing with entrained seawater, precipitation of sulfides and anhydrite, and dissolution of sphalerite within the mound. This is the first time that on-going remobilization, zone refinement, and significant modification of high-temperature fluid in the near surface has been documented in a seafloor hydrothermal system. Deposits of ocherous material and massive sulfide with outer oxidized layers that formed during previous hydrothermal episodes are exposed on the steep outer walls of the mound. Studies of the full range of samples demonstrate that highly focused fluid flow, consequent seawater entrainment, and mixing within the mound can result in formation of a large seafloor hydrothermal deposit exhibiting sample types similar to those observed in Cyprus-type ore bodies. Introduction Detailed studies of active seafloor vent sites allow investigation of the interrelationships among geologic setting, fluid composition, styles of seawater/hydrothermal fluid interaction, mineralogy and texture, metal remobilization, zone refinement, and the overall structure and development of massive sulfide deposits. The TAG active mound, located at 26øN, Mid-Atlantic Ridge (MAR), is particularly useful for investigating such interrelationships. The mound itself is large (-200 m in diameter), distinctly circular in plan view, with all black smoker activity concentrated in one area [Thompson et al., 1988]. This is in contrast to most other active seafloor vent sites where the size of individual mounds is considerably smaller (<10-30 m diameter), multiple Copyright 1995 by the American Geophysical Union. Paper number 95JB00610. 0148-0227/95/95JB-006 ! 0505.00 small deposits are aligned parallel to the ridge axis, and black smoker chimneys are discrete features dispersed on top of small mounds, instead of clustered in one area [...

Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

“…However, felsic-hosted magmatic hydrothermal systems are likely to differ substantially in this respect due to compositional differences in degassed volatiles . Significant modification of high temperature fluids by subsurface mixing (iv) has been recognized in MOR sites, such as the TAG hydrothermal mound on the Mid-Atlantic Ridge (Edmond et al, 1995;Tivey et al, 1995;Gamo et al, 1996b), North Cleft on the Juan de Fuca Ridge and Kairei Field, Central Indian Ridge (Gallant and Von Damm, 2006) and depends on the hydrologic regime in the 'upflow' zone. In the following discussion we examine the evidence for these four processes in the chemistry of Pual Ridge and Vienna Woods vent fields.…”

“…Mixing of seawater and hydrothermal fluids prior to venting has been recognized ever since hydrothermal activity was first discovered in 1977 at the Galapagos Spreading Center (Corliss et al, 1979;Edmond et al, 1979a,b;Edmond et al, 1995;Tivey et al, 1995;Gamo et al, 1996b;Von Damm et al, 1998;Gallant and Von Damm, 2006). The hypothesis that extensive subsurface mixing is 5.14a and 5.14b).…”

“…Our understanding of key geochemical parameters such as fluid redox and pH, and the mobility and behavior of metals in hydrothermal solutions has also advanced (e.g. SEYFRIED, 1987;SEEWALD and SEYFRIED, 1990;SEYFRIED and DING, 1995;TIVEY et al, 1995;ALLEN and SEYFRIED, 2003;CRADDOCK, 2008;KLEIN et al, 2009;MCCOLLOM and BACH, 2009). While these inorganic aspects of vent fluid geochemistry are reasonably well constrained, what is less well understood is the formation and transformation of carbon species in hydrothermal solutions, especially in ultramafic geologic settings where highly reducing fluids contain high concentrations of CH 4 and other hydrocarbons of purported non-biological (abiotic)…”

“…Fluids were collected from three bare-rock hydrothermal systems hosted in basalt, including Lucky Strike (LANGMUIR et al, 1997;VON DAMM et al, 1998) andTAG (THOMPSON et al, 1985;TIVEY et al, 1995) on the Mid-Atlantic Ridge (MAR) and 9°50'N (HAYMON et al, 1993; on the East Pacific Rise. Two bare-rock systems hosted in peridotite were also sampled, including Rainbow (CHARLOU et al, 2002) and Lost City (KELLEY et al, 2001; on the Mid-Atlantic Ridge.…”

“…Our understanding of key geochemical parameters such as fluid redox and pH, and the mobility and behavior of metals in hydrothermal solutions has also advanced (e.g. SEYFRIED, 1987;SEEWALD and SEYFRIED, 1990;SEYFRIED and DING, 1995;TIVEY et al, 1995;ALLEN and SEYFRIED, 2003;CRADDOCK, 2008; KLEIN et al, 2009; MCCOLLOM and BACH, 2009 origin (HOLM and CHARLOU, 2001; KONN et al, 2009). In addition, though water-rock interactions in backarc environments may broadly resemble those of basalt-hosted systems in some respects , our understanding of the role of acidic magma-derived solutions in influencing hydrothermal fluid chemistry in these environments is currently limited by a lack of comprehensive geochemical investigations (GAMO et al, 2006;.…”