Computer Science
Scientific paper
Mar 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..67..351h&link_type=abstract
Earth and Planetary Science Letters, Volume 67, Issue 3, p. 351-366.
Computer Science
22
Scientific paper
Samples of eleven ophiolites from the Mediterranean and the Pacific belts have been studied through Pb-Pb systematics.
The ophiolites studied show both isotopic variations within one sequence and differences between the various complexes. The intra-sequence isotopic variations result principally from the in situ decay of uranium and thorium since the formation of the ophiolites. μ ratios (238U/204Pb) higher than 50 in the effusive part of ophiolites are required to explain the 206Pb/204Pb spread within the sequences, and such values are notably higher than those presently measured in the oceanic crust. This uranium enrichment with respect to lead is due to the combined effect of the magmatic fractionations and of chemical exchanges during marine alteration. Pb loss must be an important factor for the U/Pb increase. Since the 207Pb/204Pb ratio is not affected by recent radioactive decay, it represents a very useful tracer for the discussion of the genetic environment of the ophiolites.
Three main types of Pb isotopic characteristics may be distinguished among ophiolite complexes, on the basis of their 207Pb/204Pb:
-|The ophiolite complexes of In Zecca (Corsica), Semail (Oman) and Toba (Japan) show 207Pb/204Pb similar to the least radiogenic present-day MORB. These complexes probably originated in regions geochemically comparable to the present-day depleted mantle. However, the Semail ophiolite has a higher 208Pb/204Pb ratio than MORB, which may be compatible with an origin in an interarc basin or premature arc.
-|The circum-Pacific ophiolites of Papua New Guinea, New Caledonia and Zambales (Philippines) have 207Pb/204Pb comparable to the present-day ``transitional'' portions of the ridges.
-|Finally the circum-Mediterranean ophiolites of Troodos, Vourinos, and Antalya, display 207Pb/204Pb ratios clearly higher than those of the present accretion zones. Thus, their genesis must have involved a significant continental component, and an origin in an island arc is therefore favoured. Some cases of large isotopic heterogeneities are also observed within the Antalya and Troodos complexes.
Allègre Claude J.
Dupré Bernard
Hamelin Bruno
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