Isotopic composition of lead in volcanic rocks from central Honshu — with regard to basalt genesis

Geochemical Journal
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The isotopic composition of lead and concentrations of lead, uranium, and thorium were determined in tholeiitic and high-alumina basalts, and their calc-alkali rock series, from central Japan. The isotopic composition of lead of high alumina basalts is similar to that of tholeiites from adjacent areas, whereas their silicic differentiates (calc-alkali rock series) are rich in 207Pb and 208Pb. This is interpreted as a result of crustal contamination. The isotopic composition of lead in the primary basalts gradually decreases in radiogenic character from the Pacific Ocean side to the Japan Sea side, whereas the observed 238U/204Pb and 232Th/238U ratios in the basalts increase in the same direction. This inverse correlation can be interpreted as resulting from differentiation of the upper mantle about 3.6b.y. ago, with tholeiite (Pacific side) generated from a shallower zone than the alkali basalt (Japan Sea side). The magma generation is associated with a process which extracts Pb preferentially to U and Th at shallower depth and U and Th preferentially to Pb at deeper depth in the past (multi differentiation for the source region) or at the magma generation stage. An alternative interpretation of this inverse correlation is that the ocean floor is being thrust under the Japanese Island arcs and the isotopic difference is produced by the degree of mixing of mantle material under the arcs with thrust material.

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Publication type Article
Publication Subtype Journal Article
Title Isotopic composition of lead in volcanic rocks from central Honshu — with regard to basalt genesis
Series title Geochemical Journal
DOI 10.2343/geochemj.3.53
Volume 3
Issue 1
Year Published 1969
Language English
Publisher The Geochemical Society of Japan
Description 34 p.
First page 53
Last page 86
Country Japan
State Honshu
Other Geospatial Izu Islands, Mount Amagi, Mount Fuji, Mount Hakone, Mount Omuro-yama, Mount O-Shima, Mount Sukumo, Mount Taga
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