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The Afar triangle in northeastern Ethiopia is where the Red Sea rift, the Carlsberg Ridge of the Indian Ocean, and the Rift Valley system of east Africa meet. About 20 m.y. ago, the Arabian Peninsula and Africa were joined. Fit of shorelines of Arabia and Africa works out most successfully if the African coast is left intact and the Arabian coast is superposed in two separate sections. In this reconstruction, a corner of Arabia overlaps the Afar triangle, an area that now has some of the characteristics of an ocean floor.
In 1979, J. Welhan and H. Craig reported that hydrothermal vents at 21°N, on the East Pacific Rise, are discharging turbid waters. Mixtures of the plumes with ambient seawater contain significant amounts of dissolved H2 and CH4 as well as mantle-derived 3He-rich helium. The 3He/4He ratios of rock samples obtained earlier by J. Lupton and H. Craig from the Mid-Oceanic Ridge, including the Mid-Atlantic Ridge and the East Pacific Rise, are extremely high at an almost constant value of (1.3 ± 0.2) × 10-5, which they defined as the MOR-type helium. However, the deep brines of the Red Sea contain about 1,000 times more methane than normal seawater does, according to Gold and Soter in 1980.
Much evidence leads us to believe that large amounts of 3He-rich helium-bearing natural gas have been gushing out in many places of the Rift Valley of east Africa for a long time. If waters of some lakes are charged with natural gas from the mantle of the earth, in due time, dissolved-gas deposits will form in the deeper zones of some lakes. If charging continues, the water throughout the lake becomes saturated and then oversaturated by gas. In 1980, Gold and Soter stated that Lake Kivu, which occupies part of the East African rift valley, contains 50 million tons of dissolved methane for which there is no adequate microbial source.
The Japanese Islands began to separate from the Asian continent during the early Miocene. The early Miocene was characterized by intensive volcanic activity that produced large amounts of pyroclastics and other volcanic rocks, generally called "green tuff" in Japan. It has been suggested that oil and gas in "green tuff" is derived from the upper mantle.
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