کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6436367 | 1637565 | 2015 | 11 صفحه PDF | دانلود رایگان |
- Helium in most seafloor sulfides is predominantly derived from a mantle source.
- Heavier noble gases are mainly derived from seawater.
- Low-temperature sulfate and opal fluid inclusions are sinks of heavier noble gases.
- Global fluxes of He and heat from vents are at least 500Â kg per year and 1Â ÃÂ 1011Â W.
Studies of noble gases in seafloor sulfide deposits are an important tool for understanding the origin of ore-forming fluids, both enabling the determination of noble gas sources and revealing the degree of fluid-rock interaction and mantle degassing. The noble gas concentrations and isotopic compositions of 27 sulfide, 3 sulfate, and 2 opal mineral aggregate samples have been studied in different hydrothermal fields from the East Pacific Rise, Mid-Atlantic Ridge, Central Indian Ridge, Southwest Indian Ridge, and North Fiji Back-Arc Basin. The helium concentrations and isotopic ratios in the sulfide aggregate samples are variable (4He 0.12 to 22 Ã 10â 8 cm3 STP/g; 3He/4He ~ 0.6 to 10.4 Ra), and most of the sulfide helium concentrations are higher than those in the opal mineral samples (4He 0.017 and 0.028 Ã 10â 8 cm3 STP/g), suggesting that the low-temperature fluids have lost their mantle helium during cooling. The helium of high (> 7 Ra), medium (1-7 Ra), and low (â 1 Ra) 3He/4He ratio samples is derived mainly from the MORB or OIB mantle by magma degassing, from mixing between hydrothermal fluid and seawater during ore-forming processes, and from ambient seawater, respectively. The high 3He/4He ratios (> 7 Ra) of sulfides imply that high-temperature sulfides retain the helium isotopic compositions of the primary hydrothermal fluid, whereas low-temperature sulfides, sulfates, and opal minerals do not.The neon, argon, krypton, and xenon concentrations in the sulfide aggregate samples are also variable; in most of the sulfide aggregates, they are significantly lower than in the sulfate and opal mineral samples. It is known that barite and opal minerals are characteristic of low-temperature (< 200 °C) paragenetic associations, indicating that heavier noble gases (Ne, Ar, Kr and Xe) are enriched under low-temperature conditions. Most of the sulfide, sulfate and opal mineral aggregate samples possess heavy noble gas elemental abundances similar to those of air-saturated seawater and Ne, Ar, and Xe isotopic compositions that span narrow ranges around atmospheric values, an observation which is most easily explained by the dominance of a seawater-derived component.In addition, based on the calculated helium/heat ratios, global helium and heat fluxes to high-temperature hydrothermal vents are approximately 0.05-6 Ã 104 kg per year and 0.1-12 Ã 1012 W, meaning that roughly 0.3% of ocean heat is supplied by seafloor high-temperature hydrothermal activity.
Journal: Chemical Geology - Volume 409, 20 August 2015, Pages 1-11