Reliability of Os model ages in pervasively metasomatized continental mantle lithosphere: a case study of Sidamo spinel peridotite xenoliths (East African Rift, Ethiopia)

Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology

“…As a consequence of elevated 187 Os/ 188 Os, none of the disturbed BMS Re-Os isotopic ratios yielded geologically reasonable T MA or T RD model ages. They attributed these perturbed Re-Os isotope systematics to interaction with fluids originating in a supra-subduction zone environment where dehydration produces a fluid enriched in radiogenic Os (e.g., Brandon et al 1996;Reisberg et al 2004;Walker et al 2002b). Beyer et al (2004) grains from the Slave Craton, Canada by Westerlund et al (2006), sulfides with unsupported radiogenic 187 Os/…”

“…Although unradiogenic 187 Os/ 188 Os in peridotitic residues often can be adequately explained by ancient melt depletion, using bulk-rock Re-Os isotope systematics indiscriminately can have its drawbacks (Reisberg et al 2004;Rudnick and Walker 2009). Like the standard caveats in other radiogenic isotopic systems, the Re-Os geochronology of bulk-rock peridotites has two built-in weaknesses: (i) mixing ages can be generated that are not geologically meaningful model ages, and (ii) potential parent (Re)-daughter (Os) mobility either directly or in sulfide liquids at deep-mantle temperatures lead to problems in interpreting Re-Os isotopic composition of sulfides as an absolute age.…”

“…The movement of Os during melting, recrystallization, or metasomatism may occur independently of Re and there can be separate addition of Re (e.g., Alard et al 2002;, making the interpretation of bulk-rock Re-Os isotope data ambiguous in some samples, in the context of the timing of melt depletion (see reviews in Reisberg et al 2004;Carlson 2005;Rudnick and Walker 2009). For example, in the study of BMS inclusions in the Western Gneiss region of Norway, Beyer et al (2004) identified at least two generations of BMS with model ages that ranged from 4.0 to < 0 Ga.…”

“…Thus, sulfides that have interacted with subduction-like oxidizing conditions are more likely to have low metal/sulfur ratios than those that do not display evidence for unsupported radiogenic Os. However, an alternative to subduction-related transport of radiogenic Os was recently postulated in the study of Gonzáles-Jiménez et al (2014) Os signature (see also Reisberg et al 2004;Lorand et al 2004;Powell and O'Reilly 2007;Alard et al 2011 Os. However, the effect on T MA model age calculations will be to decrease the calculated age-potentially producing a "future" age ( Fig.…”

Geochemical and Sr–Nd–Pb isotopic compositions of lithospheric mantle: Spinel lherzolites in alkaline basalts from the northwestern Ethiopian plateau

“…These tectono-magmatic processes consequently induced substantial variations in the thermal structure and chemical composition of the upper mantle beneath Ethiopia and the surrounding regions. The mantle xenoliths hosted in the alkaline basalts of southern Ethiopia (Mega-Megado and Dilo-Goraye: Morten et al, 1992;Bedini et al, 1997;Bedini and Bodinier, 1999;Conticelli et al, 1999;Lorand et al, 2003;Reisberg et al, 2004;Meshesha et al, 2011), the central Main Ethiopian Rift xenoliths and xenocrysts (Debre Zeyte and Butajira: Rooney et al, 2005) and northwestern Ethiopia xenoliths (Injibara, Dedesa and Simien shield volcano: Roger et al, 1997Roger et al, , 1999Conticelli et al, 1999;Ayalew et al, 2003Ayalew et al, , 2009Ferrando et al, 2008;Beccaluva et al, 2009Beccaluva et al, , 2011Frezzotti et al, 2010) have therefore been the focus of extensive study over the previous two decades (Fig. 1a).…”