Pb-isotope systematics of a fossil hydrothermal system from the Troodos ophiolite, Cyprus: Evidence for a polyphased alteration history

Abstract

Abstract-Pb-isotopic ratios have been determined on massive sulphides from the Troodos ophiolite and their inferred source rocks: epidosites and greenschist altered dikes. The latter have distinctly higher Pb-isotope ratios than the sulphides. The high 207 Pb/ 204 Pb ratios for the whole rocks indicate the involvement of an old (sedimentary) Pb-component. Extensive leaching of three epidosite samples shows that the radiogenic Pb can be removed and the resulting trend for the leached samples is consistent with a Pb-Pb age of ϳ95 Ma for these rocks. Inferred initial Pb isotope ratios are similar to those of the sulphides (this study) and fresh glasses from the Akaki Canyon in Cyprus. Therefore the Pb incorporated in the massive sulphides may originate from these greenschist facies hydrothermally altered rocks.The isotopic composition of the leachate Pb points toward a sedimentary origin. Calculation of the Pb-isotopic ratios for 90 Ma old "average" Troodos sediment shows that the "foreign" Pb in the epidosites may very well be derived from this source. This Pb probably does not reside within the mineral lattices of these metamorphic rocks, but rather is adsorbed on mineral surfaces or is present in microscopic particulate matter that may have been filtered from the circulating seawater. Unmodified seawater is considered an unlikely Pb source because of the extremely small dissolved Pb contents of seawater. Furthermore, no seawater or sedimentary signature is evident in the sulphides. Several models of Pb transport from different sources into these rocks may be envisaged, such as dissolution of Miocene evaporites and brine formation, equilibration of pore waters to the Pb isotope ratios of host sediments, and direct infiltration of deep sea particulate matter from the nepheloid layer. With the limited amount of data presently available it is impossible to critically determine the most likely processes of Pb transport.

Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?

“…Pb isotope ratios for the Cypriot massive sulfi des are similar to those of fresh Troodos glasses reported by Rautenschlein et al (1985), and this suggests that the Pb incorporated in the sulfi des is derived from the Troodos basement (Booij et al, 2000), and the above mentioned Pb ratios are primary. Our Pb isotope data are quite similar to those of the Izu-Bonin boninites and Izu-Bonin volcanic front lavas (Figs.…”

Ophiolitic Remnants from the Upper and Intermediate Structural Unit of the Attic-Cycladic Crystalline Belt (Aegean, Greece): Fingerprinting Geochemical Affinities of Magmatic Precursors

“…The analyzed metabasites from the Cyclades (metagabbros, metabasalts, and metabasaltic andesites) display a wide range of lead isotope compositions for a relatively small number of samples, especially regarding 207 (Figure 11a,b). Similarly, high whole-rock 207 Pb/ 204 Pb ratios (Figure 11a) have been reported for the hydrothermally altered parts of the Troodos ophiolite complex (greenschists and epidosites) [80]. In the 207 Pb-206 Pb and 208 Pb-206 Pb diagrams, the Cycladic metabasites display a spread beyond the normal MORB field and Troodos glasses [80] and plot within the field of seawater, attesting the effect of seawater alteration on the Cycladic metabasites compositions.…”

“…Similarly, high whole-rock 207 Pb/ 204 Pb ratios (Figure 11a) have been reported for the hydrothermally altered parts of the Troodos ophiolite complex (greenschists and epidosites) [80]. In the 207 Pb-206 Pb and 208 Pb-206 Pb diagrams, the Cycladic metabasites display a spread beyond the normal MORB field and Troodos glasses [80] and plot within the field of seawater, attesting the effect of seawater alteration on the Cycladic metabasites compositions. Sample NA-14 (serpentinized harzburgite) has a high 206 Pb/ 204 Pb ratio, probably due to time-integrated development of radiogenic lead caused by a higher U/Pb ratio compared to the other samples (not plotted in Figure 11).…”

Lead isotopic analysis within a multiproxy approach to trace pottery sources. The example of White Slip II sherds from Late Bronze Age sites in Cyprus and Syria

“…Indeed, Booij et al (2000) showed that sulfides have lower Pb isotope ratios than their source rocks, in this case greenschist samples from the Troodos Ophiolite. The lower isotopic values of some of the pottery samples analyzed in the present study can be interpreted as the result of the presence of sulfides in the clay derived from the greenschist facies gabbros that were used for White Slip II production.…”