Nature and Scale of Mantle Heterogeneities in Upper Mantle Reservoirs: Evidence from trace element abundances and Os isotopic compositions in the Oman ophiolite
Final Report Abstract
This study describes the Os isotope systematics and the distribution of highly siderophile elements in mantle peridotites from the 90 Ma-old Oman ophiolite. It reveals a large spread in Os isotopic composition indicating a bottom to top isotope heterogeneity in the upper mantle. The most unradiogenic harzburgites (187Os/188Os(t = 90Ma) = 0.1149 and 0.1169) yield Re depletion ages of ~ 2 Ga. They represent the most depleted members of oceanic lithosphere (e.g. YbN (cpx) - 1 . 1 ; Tin (cpx) ~ 0.3), (Pd/Ir)N ratios of < 1). The rare sulphides (pentlandite ± chalcopyrite) have grain sizes ranging from 10-60 pm and are associated with orthopyroxene or olivine. The remaining residual harzburgites (187Os/188Os(t = 90Ma) ~ 0.124) have broadly chondritic relative PGE abundances with the exception of high Pd/Ir(N) and positive Pd anomalies. The sulphides of these samples occur typically as interstitial, subhedral or anhedral grains which are closely associated with spinel and/or clinopyroxene. The grains sizes of the sulphides frequently range up to 400 pm. A second type of samples belong to a non-residual group comprising clinopyroxene- and plagioclase enriched peridotites, dunites and pyroxenitic dikes. These samples are characterized by melt-type low PGE abundances, high Pd/Ir(N) ratios and high Os isotope ratios (187Os/188Os(t = 90Ma) = 0.1367 to 0.1675). The radiogenic samples have lower Os concentrations (0.13 - 2.41 ppb) than the residual peridotites (3.19 - 5.99 ppb). The positive covariations of the Os isotopes with Pd/Ir ratios and lithophile elements indicate interaction with percolating melts. The primary sulphide assemblage is composed of massive pyrrhotite + chalcopyrite + pentlandite. Large sulfide grains (up to 700 pm) are attached to coarse grained, subhedral clinopyroxene and/or spinel. In a 187Os/188Os vs 1/Os diagram the radiogenic samples define a mixing line having the depleted mantle and radiogenic pyroxenite dykes (187/188Os > 0.21510) as end members. It is suggested that the Os isotope heterogeneity is largely due the mobility of sulphide melts during partial melting and melt percolation. High degrees of melting may leave refractory, high-temperature sulphides or alloys in the mantle residuum that preserve an ancient melt depletion event. Such ancient component may be frequently masked by relative radiogenic sulphides precipitated from percolating pyroxenitic melts 90 Ma ago. The Oman pyroxenites originate from a high-Re/Os reservoir. However, the Nd isotopic composition points to a long-term source depletion of Nd (143Nd/144Nd(t=90Ma)= 0.513299). This suggests a depleted upper mantle origin and rather reject a recycling model of ancient oceanic crust.
Publications
- (2004): Geochemical Heterogeneity of the Upper Mantle: Os Isotope Evidence from the Oman Ophiolite. 13. Goldschmidt Conference, Copenhagen: Geochimica et Cosmachimica Acta 68, no. 11, Suppl., PP. A704
M. Kurth-Velz and G. Brügmann
- (2004): Os Isotopes and PGE Systematics of Peridotites from the Oman Ophiolite: Evidence for Small-Scale Mantle Heterogeneity. 82. Jahrestagung der Deutschen Mineralogischen Gesellschaft, Karlsruhe: European Journal of Mineralogy 16, Suppl., 130, p. 78
M. Kurth-Velz and G. Brügmann
- (2005): Highly Siderophile Element Systematics in Mantle Rocks from the Oman Ophiolite: Evidence for Ancient Crustal Components in the Upper Mantle. Eos Trans. 86, AGU Fall Meeting, San Francisco. Meet. Suppl, Abstract V41D-1501
M. Kurth-Velz, GE Brügmann, H. Palme
- (2005): The Maqsad Diapir (Sumail Massif), Southern Oman Ophiolite: Evidence for Heterogeneous Source Composition. 83. Jahrestagung der Deutschen Mineralogischen Gesellschaft, Aachen: European Journal of Mineralogy 17, Suppl., p. 79
M. Kurth-Velz and G. Brügmann
- (2007): Sulfide mobility during melt percolation: implications for PGE and Os isotopes from the Oman ophiolite. Goldschmidt Conference, Köln: Geochimica et Cosmochimica Acta 71, p. A1062
M. Velz and G. Brügmann
- (2008): PGE and Os and Nd Isotopes from the Oman Ophiolite: Implications for Mantle Heterogeneities. Goldschmidt Conference, Vancouver: Geochimica et Cosmochimica Acta 72, No. 12, Suppl., p. A507
M. Kurth-Velz and G. Brügmann
- (2009) PGE and Os and Nd Isotopes from the Oman Ophiolite: Implications for Mantle Heterogeneities. Chemical Geology
M. Kurth-Velz, G. Brügmann and H. Palme
