Co- and post-magmatic modifications of the oceanic crust: lead isotope evidences

Author : Yongbin SunPublication : Bulletin of the Geological Society of MalaysiaVolume : 39Page : 91-99Year : 1996


Bulletin of the Geological Society of Malaysia, Volume 39, July 1996, pp. 91 - 99


Co- and post-magmatic modifications of the oceanic crust: lead isotope evidences


Geranienstr. 10, 12203 Berlin, Germany


Abstract: The Pb-isotopic analysis of the basalt-/basalt-andesitic rocks of the drill hole CY-4 in the Troodos-Ophiolite in Cyprus shows that the postmagmatic alteration by sea water has produced a 206Pb/ 204Pb-increase, and to a lesser extent, 208Pb/204Pb-raising, especially about the hydrothermal root-zone. The primary origin of the radiogenic Pb must refer to the continental Pb, instead of to the fresh Cretaceous ocean water. The average 206Pb/204Pb (18.589), 207Pb/204Pb (15.642) and 208Pb/204Pb (38.508) are almost identical with the crust-lead of Stacey and Kramers and suggest an island arc environment. The Pb-residence time in the source region comes to 0.62 Ga and the corresponding 232Th/238U = 2.88. This time is probably an intermediate value in a certain variation range with a continuously varying Th/U-ratio. Pb-isotopically, the Troodos-Ophiolite deviates strongly from the oceanic composition and shows an unequivocal affinity to sediments and continental basalts. The radiogenic (208Pb/206Pb)* lie in a range from 0.952 to 0.996 and (207Pb/206Pb)* from 0.549 to 0.607 and, therefore, fall into the typical MORB-domain. The Δ(208Pb/204Pb)-value varies between 10 and 60 and locates immediately at the boundary of DUPAL-zone. The Troodos-Ophiolite originates from the complex primary and secondary as well as binary and ternary mixings of the depleted (DMM), the enriched (EMIl) and the HIMU mantle components. DMM is the main component (up to 70%), which was strongly and non-uniformly contaminated by enriched materials from EMIL HIMU appears not to be the immediate source and had previously mixed with DMM to have formed a secondary source for Troodos-Ophiolite.