Lithophile and Siderophile Element Systematics and Isotope Chronology Associated with Melt Depletion and Metasomatism of a Relict Peridotite

Lu-Hf, Sm-Nd, and Re-Os isotope data of garnet peridotite from the Sandvik ultramafic body, Western Gneiss Region (WGR), Norway, records an exceptionally long history spanning 3 billion years. This long history affords a unique view into the response of lithospheric mantle to melting and metasomatic events as expressed in their petrologic and geochemical record. Lu-Hf and Re-Os whole rock isotope data of garnet lherzolite yields model mantle extraction (MA) ages that are 3.311 ± 0.024 Ga (Lu-Hf; εHf(0) = +2165) and 3.363 ± 0.057 and 3.329 ± 0.055 Ga (Re-Os; γOs(0) = – 17.5), indicating that the two isotope systems are very well correlated. The weighted mean melt extraction age is 3.320 ± 0.021 Ga (2ó) and agrees well with melt extraction ages observed in other peridotite bodies of the WGR [1] but is much older than protolith ages recorded in the surrounding host crustal rocks.

Sm-Nd whole rock isotope data of garnet lherzolite yields εNd(0) = -8.8, indicating that the Sm-Nd isotope system is strongly decoupled from the Re-Os and Lu-Hf systems. Although melt extraction would result in correlated depletions in Sm/Nd, Lu/Hf, and Re/Os ratios, the Sm-Nd systematics have been subsequently modified in a way that left the Lu-Hf and Re-Os systems unaffected. Sm-Nd isotope systematics of olivine-garnet pyroxenite and garnet periditite as well as the associated garnet lherzolite indicate that these rocks were metasomatized by LREE-rich fluids during the Sveconorwegian orogeny at 1149 ± 11 Ma [2]. On the basis of a Sm-Nd isochron age of low-pressure kelyphitic rims on garnet, emplacement of these mantle rocks into the crust likely started with Late Proterozoic (~600 Ma) rifting and was complete by the Scandian phase of the Caledonian orogeny (~400 Ma).

[1] Beyer et al. (2004) Geology, 32, 609-612. [2] Lapen et al. (2005) CMP 150, 131-145.