Fluid inclusion studies as a guide to the temperature regime within the TAG hydrothermal mound, 26°N, Mid–Atlantic Ridge

Abstract

The active TransAtlantic Geotraverse (TAG) hydrothermal mound is a mature submarine massive sulfide deposit at the slow-spreading Mid-Atlantic Ridge at 26°N. Fluid inclusion measurements were conducted on quartz and anhydrite from six boreholes drilled in different areas of the mound to characterize the fluids responsible for the deposition of sulfide-silica breccias and anhydrite and to investigate the vertical and horizontal temperature zonation within an actively forming hydrothermal system. Fluid inclusions in both host minerals are generally two phase liquid/vapor inclusions that homogenize into the liquid phase. Trapping temperatures for quartz and anhydrite from the TAG mound range from 212° to 390°C. Salinities vary from 1.9 to 6.2 wt% NaCl equivalent for anhydrite and from 4.0 to 6.0 wt% NaCl equivalent for quartz. This salinity variation is probably best explained by supercritical phase separation at temperatures above 450°C with subsequent remixing of the liquid and the vapor phase during ascent. A zone of anhydrite-rich precipitates recovered at 20 to 35 mbsf below the central Black Smoker Complex (TAG-1) is characterized by trapping temperatures averaging 348°C for anhydrite and 358°C for quartz, which is slightly below the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360°−366°C). Breccias in the stockwork zone underlying the anhydrite zone were formed at slightly higher temperatures ranging from 327°− 381°C for quartz and from 349° to 384°C for anhydrite. Trapping temperatures vary strongly between different areas of the mound. Fluid inclusions in quartz and anhydrite from the central Black Smoker Complex are characterized by a narrow range of trapping temperatures, whereas other areas drilled on the mound were influenced by lower temperature hydrothermal fluids percolating through the mound or by local entrainment of seawater into the mound. White smokers venting on the southeastern side of the TAG mound are characterized by exit temperatures of 270°−300°C, (Kremlin area, TAG-2). Fluid inclusion measurements in quartz and anhydrite from this area give trapping temperatures in the range of 266°−375°C with a distinct peak around 340°C, only somewhat lower than results for the Black Smoker Complex. Trapping temperatures in anhydrite-hosted fluid inclusions in this area show a strong vertical temperature increase. The west side of the mound (TAG-4) is characterized by trapping temperatures ranging from 212° to 390°C showing evidence for seawater entrainment or overprinting by lower temperature hydrothermal events at the sulfide/basalt interface. Samples from the northern side of the mound (TAG-5) exhibit trapping temperatures in the range from 258°−383°C with a strong vertical temperature increase, indicating additional hightemperature upflow at the northern margin of the mound outside the central Black Smoker Complex.

On the Sr isotope and REE compositions of anhydrites from the TAG seafloor hydrothermal system

“…Trapping temperatures (T t ) for fluid inclusions in individual crystals of anhydrite from the TAG hydrothermal mound range from 187 to 388°C; however, for the TAG-1 area and for depths Ͼ9 mbsf in the TAG-2 area, the range is much smaller (338 to 388°C) (Petersen et al, 1998;Tivey et al, 1998;Mills and Tivey, 1999). In these areas, there is a general trend of increasing T t from the mound surface, where they are close to the temperatures of the venting fluids, to a maximum at 121 mbsf, suggesting that temperatures deep within the mound are in excess of 390°C (Petersen et al, 1998;Tivey et al, 1998).…”

“…Consideration of changes in REE speciation when hot hydrothermal fluid mixes with conductively heated seawater is prompted by studies of fluid inclusion trapping temperatures in anhydrites suggesting that, throughout much of the mound, anhydrite precipitated at temperatures Ͼ337°C-higher than those estimated from Sr isotope ratios (Petersen et al, 1998;Tivey et al, 1998;Mills and Tivey, 1999;Humphris and Tivey, 2000). In all calculations, mineral precipitation during mixing is suppressed.…”

“…Large blocks of anhydrite have been recovered by submersible (Thompson et al, 1988;Tivey et al, 1995), and drilling during Ocean Drilling Program (ODP) Leg 158 revealed abundant anhydrite in the matrix of breccias, filling vugs, and as multiple, composite veins up to 45 cm thick . A number of studies have analyzed anhydrite recovered during drilling (Mills and Elderfield, 1995;Chiba et al, 1998;Humphris, 1998;Mills et al, 1998;Petersen et al, 1998;Teagle et al, 1998a-c;Tivey et al, 1998), and a variety of data sets (Sr, S and O isotopes, REE, fluid inclusion analyses) from different sample suites have been integrated in these and other papers (e.g., Mills and Tivey, 1999;Humphris and Tivey, 2000) to constrain first-order aspects of subsurface seawater entrainment and fluid evolution within and beneath the TAG hydrothermal mound.…”

Evolving patterns of the fluids within the TAG hydrothermal field

“…This calculated fluid shared characteristics comparable to the white smoker fluid. However, the results [22,23] of the isotopic compositions of the deposit and the trapping temperatures of fluid inclusions in some anhydrite can not be explained by the mixing process of hydrothermal fluids/seawater (2℃) alone within the TAG deposit. The results suggested that seawater should be heated by wall rock before reacting with hydrothermal fluid.…”

Numerical simulation for fluids mixing within seafloor hydrothermal sulfide deposit: taking the Trans-Atlantic Geo-Traverse (TAG) field for example

“…The mixing process between the entraining seawater and hydrothermal fluid largely governs the physical structure and chemical compositions within a sizable hydrothermal sulfide deposit like Trans-Atlantic Geo-Traverse (TAG) hydrothermal field, Mid-Atlantic Ridge Petersen et al, 1998;Tivey, 1998).…”