Sampling Strategy | Table of Contents


High-Temperature Fluids
The highest vent temperature measurement at PACMANUS black smokers (strictly "gray smokers") is 280°C; however, the number of such measurements is too limited to ensure this is the true maximum. Indeed, a variety of evidence mentioned above indicates that phase separation (boiling) of the hydrothermal fluids is occurring below the surface. There is no way of knowing how deep below the seafloor this occurs, so for safety purposes we must presume it is relatively shallow, and define the boiling point at the collar of the shallowest water depth site (Site PCM-2A, 1655 m). The end-member salinities of the PACMANUS fluids are variable but fairly close to seawater (650-800 mM, Ishibashi et al, 1996).

According to Appendix 4 of Bischoff and Rosenbauer (1985), the two-phase boundary of normal seawater (3.2% NaCl) at a pressure of 166 bars (= 1655 m seawater depth) lies at 354°C. This temperature is lowered to 349°C for pure H2O, representing the extreme (but unlikely) situation of a very low salinity hydrothermal fluid and the theoretical temperature limit that must not be exceeded in any hydrothermal aquifer drilled. The comparable values for Middle Valley and the Trans-Atlantic Geotraverse (TAG) hydrothermal mound are ~385° and 430°C respectively.

PACMANUS end-member vent fluids are relatively rich in dissolved CO2 (20-40 mM) and methane (20-40 µM) so should gently effervesce before boiling or "flashing." Nevertheless, for Sites PCM-2A and PCM-3A, and probably PCM-1A as well, it will be necessary to adopt a protocol of frequent temperature measurements to ensure that a specified limit (e.g., 340°C), is never exceeded.

Hydrogen Sulfide and Radon Gas
Measured H2S contents of PACMANUS vent fluids range up to 4 mM, and that of the calculated end-member fluid is 7 mM (Shitashima et al., 1997). These values are higher than those at Middle Valley (3 mM) and TAG (0.5 mM), and potentially hazardous levels of H2S gas could potentially be released into the local atmosphere upon opening or cutting samples cored from hydrothermal aquifers. The same precautions of providing an H2S detector, and of allowing relevant samples to degas before storing them in enclosed areas should be adopted as were applied during Legs 136, 158, and 169.

Barite is a significant gangue mineral in PACMANUS chimneys, and it contains detectable radium (Dickson, et al., 1995). The rare earth element abundance patterns of PACMANUS vent fluids indicate that subsurface precipitation of barite also occurs (Douville, et al., 1999). Direct radiation from barite-rich samples is not likely to present a health risk; however, release of radioactive 222Rn gas by decay of 226Ra is a potential hazard. Similar to Leg 169, all mineralized cores should be checked for radioactivity and those identified as hazardous should be marked as requiring proper venting to allow dispersal of radon prior to handling, if they have been stored in sealed containers.

Sampling Strategy | Table of Contents